Geology of the Milton Ranch and Adjacent Area
By Karen Porter
Overview
The Milton Ranch property is located in Musselshell County, Montana, about 14 miles northeast of Roundup and east of U.S. Highway 87 (Fig. 1). Its rolling, dissected grassland has about 316 feet of relief with highest elevations in the southern part. A local drainage divide at about 3,600 feet trends east-northeast across the southern part of the property, separating north-flowing drainage into North Willow Creek from south-flowing drainage into the Musselshell River south of the ranch.
Geologically speaking, the ranch property lies across the south flank of the Central Montana Uplift and on southward into the Bull Mountains Basin (Figs. 1, 2), two prominent geologic features of central Montana. On the distant skyline to the northwest, the Big and Little Snowy Mountains (Snowy Mountains Uplift) form the southwest corner of the Central Montana Uplift. Other geologic features across the Uplift are smaller and more subtle, not visually obvious. They are recognized by the patterns of outcropping rocks. On the Milton Ranch, rock outcrops show the steep south flank of the Central Montana Uplift along the Devils Basin-Big Wall anticlinal trend (Fig. 2), where outcrops are narrow because beds dip steeply south into the Willow Creek Syncline. This syncline is the axis of the Bull Mountains Basin, a large, very asymmetric structural low with a broad north-dipping south flank. The southern half of the Milton Ranch property lies on this broad south flank of the Bull Mountains Basin, and Gage Dome is a small wrinkle on this flank (Fig. 2). Because this basin is a structural low, the youngest sedimentary rocks, the Tongue River Member of the Fort Union Formation, have been preserved here; and because these sandstones are resistant to erosion, they form the Bull Mountains – a case of high topography preserved in a structural low.
The Milton Ranch property is underlain by sedimentary rocks of both marine and nonmarine origin that comprise a complete stratigraphic section of upper Upper Cretaceous through lowermost Tertiary (Paleocene) rocks, spanning some 44,000,000 years of geologic time (Fig. 3). Younger, Quaternary deposits, perhaps associated with the last ice age of 12,000 to 10,000 years ago occur in isolated erosional remnants across the southern part of the ranch property. Modern stream sediments occur discontinuously along east-flowing North Willow Creek in the northernmost part of the ranch property.
Oil production was active on and adjacent to the Central Montana Uplift in the 1940s through
1970s, driven primarily by detailed surface mapping and rudimentary seismic methods. On the Milton Ranch, Gage Field and Big Wall Field were part of that early successful exploration.
Although commercial sub-bituminous-grade coal associated with the Tongue River Member of the Fort Union Formation is a well-known economic resource in southeastern Montana, coal beds in the Tongue River in central Montana are thinner, discontinuous, and generally noneconomic.
Geologic Structures of the Central Montana Uplift-Kinds and Causes
The Central Montana Uplift, also known as the Central Montana Trough, is an elongate, westnorthwest-trending feature stretching approximately from Lewistown on the northwest to Vananda on the southeast, and from the old community of Mosby on the north to the community of Melstone on the south (Fig. 1). The feature lies across Fergus, Petroleum, and Rosebud Counties and adjacent corners of Musselshell and Garfield Counties. It is bounded on the north by the Cat Creek Anticline, and on the south by the Big Snowy-Devils Basin-Big Wall anticlinal trend. Porcupine Dome forms its east end, while the Judith and Moccasin Mountains and the west end of the Big Snowies define its west end.
The Central Montana Uplift is easy to see on a geologic map. It appears as a prominent westnorthwest-elongate feature shown in various shades of green. On the ground, however, this feature is not easily discerned. To find its boundaries we must pay attention to the specific rock units exposed and their structural orientation – that is, the trend (strike) of the beds, the tilt (dip) of the beds, and how their outcrop pattern changes.
The north side of the Central Montana Uplift is formed by the asymmetric Cat Creek Anticline (Fig. 1) whose very steep north flank forms the south flank of the Blood Creek Syncline. While only the steeply dipping beds are observed at the surface, in the subsurface the beds are known to be offset by vertical movement along an ancient fault. Similarly, the south side of the Central Montana Uplift occurs along the south side of the Big Snowy-Devils Basin-Big Wall anticlinal trend. Again, at the surface, we see only steeply south-dipping beds forming the north limb of the Willow Creek Syncline, but a major fault is presumed at depth. The axis of the asymmetric Willow Creek Syncline is the deepest part of the Bull Mountains Basin; south of this axis beds dip gently north except where they are briefly rumpled by small folds such as Gage Dome. The deep-seated ancient faults bounding the Uplift have been periodically reactivated since Proterozoic time.
Many geologic faults have a long history through geologic time, sometimes active, other times quiescent. When active, they sometimes reverse their relative sense of vertical motion -- that is, the relative up or down motion of the rocks on either side of a fault. This reversal of structural motion has been a long-standing feature of the Central Montana Uplift through geologic time, significantly influencing both depositional and erosional patterns across central Montana for more than a billion years. For long periods of time it was a topographic low area, receiving sediments eroded from adjacent uplands; other times it was elevated and under erosion, itself a source of eroded debris. Currently, this structural feature is in an uplifted position, resulting from strong compression from the southwest during the last phase of uplift of the Rocky Mountains (Late Cretaceous-early Paleocene). These compressive forces popped the Central Montana Uplift upward along the old bounding faults and also rumpled its surface into numerous folds, many with underlying fault blocks.
Across the Uplift are a number of anticlines and domes that have been well explored since the early 1900s for their petroleum potential, including Porcupine, Potter Creek, and Big Wall Domes, and the Sumatra, Devils Basin, Willow Creek and Cat Creek Anticlines. Most of these structural features are not obvious to a traveler across the Central Montana Uplift.
It should be noted here that some researchers consider the south margin of the Central Montana Uplift to be farther south, along the northwest-trending Lake Basin Fault Zone that passes just north of Billings. All of these northwest-trending fault zones and lineaments* reflect the same ancient structural grain of the central Montana region and have likely had a similar geologic history of reactivation. In the present report, the Big Snowy-Devils Basin-Big Wall trend is taken as the south margin of the Central Montana Uplift because it appears to coincide with the south margin of an ancient (Proterozoic) sedimentary basin in central Montana. There is often more than one way to integrate existing geologic data.
[* A lineament is a broad, linear band of crustal weakness containing numerous faults and fractures.]
Stories the Rocks Tell
All the rocks underlying and exposed across the Milton Ranch property, indeed across most of the Central Montana Uplift and south into the Bull Mountains Basin are sedimentary in origin (Fig. 4). What we notice most about them are their color, texture (smooth or gritty), the many small details within them including possible fossils, and the relative degree to which they erode. Together, these features indicate a wide range of depositional settings of the original sediments, from terrestrial (land) environments such as stream beds, swamps, and lakes, to marine environments such as tidal flats or an ocean basin.
By thinking of these sedimentary rocks in their time sequence, with their particular physical and biological features that tell where and how the sediment was laid down, geologists are able to read the record of a changing landscape through geologic time. We must remember, though, that the record is incomplete because throughout depositional history many sedimentary layers were eroded away before later ones were laid down. The preserved sedimentary record is a relatively small part of what the original record probably was. It reflects the complex interaction of deposition and erosion controlled by structural motions of Earth’s crust.
Earliest Geologic History in the Region
The earliest geologic setting for which we have a rock record in central Montana is a vast sedimentary basin that occupied western Montana, with a narrow arm extending into central Montana, from about 1,470 million years ago to 1,400 million years ago (or 1.47 to 1.4 billion years ago), during the middle Proterozoic part of the Precambrian Eon (Fig. 3). The basin’s north and south margins, including along the narrow arm, were major vertical fault systems that actively dropped the basin floor allowing sediments eroded from the margins to settle in the depression. The arm’s closed east end is thought to lie on the east side of Porcupine Dome, based on subsurface well log interpretations. Some researchers believe this basin represents a great rift, or tear, in the western North American craton margin, and thus was open to the western ocean. Other researchers believe the basin’s west end was closed, at least for a large part of its history. Resolution of the issue is difficult and ongoing.
The existence of this vast sedimentary basin is recorded in the enormously thick rock sequence called the Belt Supergroup that dominates western Montana geology. As much as 54,000 feet(!) of sediment have been measured at some locations in present western Montana. In central Montana, in the narrow arm of the basin, Belt Supergroup rocks are much thinner. Nonetheless, this very famous and enigmatic rock sequence has been named for the exposures near Belt, Montana, southeast of Great Falls, where they were first described. Belt Supergroup rocks that are nearest to the Milton Ranch are those forming the core of the Big Snowy Uplift.
As the faults bounding the Belt Basin’s north and south margins became quiet toward the end of the Proterozoic Era, the basin filled and was covered by the early Paleozoic seas that encroached onto the North American continent at the start of the Cambrian Period.
Paleozoic History and Rocks
Throughout about the first half of the Paleozoic (Cambrian through middle Mississippian Periods), the whole North American continent was washed by shallow equatorial seas that advanced and retreated across the continental plate margins and into the interior. For most of this long period, most of the continent was submerged beneath these seas, and the deposited sand, mud, and especially limey mud, slowly lithified to sandstones, shales, and carbonates (limestones and dolomites). In western North America, extensive limestone rocks from this time period record shallow, warm marine conditions especially favorable for the formation of lime muds. The calcareous content comes from abundant shells and other skeletal parts of invertebrate animals and calcareous algae, and from the direct precipitation of carbonate from warm seas. In the dry climate of the West, carbonates are resistant rock units. These are the thick limestones that form resistant topography along the flanks of the Little Belt and Big Snowy Mountains.
For a relatively brief period in early Devonian time, crustal upwarping across western North America exposed sedimentary layers to erosion, stripping away all Silurian and most Ordovician rocks. A thin Ordovician section is preserved in the Big Snowy Uplift. Ordovician and Silurian age rocks begin to be recognized in the subsurface in east-central Montana and on eastward into the Dakotas.
During late Devonian through middle Mississippian time, marine waters again flooded the region. Indeed, the Mississippian seas were the most extensive of the Paleozoic seas across the continent, and Mississippian age carbonates represented by the 1,200- to 1,500-foot-thick Madison Group are traceable across most of western North America from the Rockies to beneath the central Plains of the United States and Canada. They serve as primary groundwater aquifers in some regions, and oil-bearing strata in other regions, based upon their later geologic history.
Late Paleozoic (late Mississippian, Pennsylvanian, Permian)
Throughout the late Paleozoic, structural movements on the Central Montana Uplift and of the entire uplift were frequent and complex, exerting strong control on the patterns of erosion and sedimentation across this major geologic feature. Following deposition of the Madison Group, a broad regional uplift exposed these carbonate sedimentary layers to erosion. Significantly, this widespread post-Madison Group exposure allowed fresh (acidic) waters to dissolve the carbonate, forming highly irregular, pitted and cavernous topography that greatly influences today’s ground-water and hydrocarbon occurrences. Marine waters advanced again in late Mississippian time.
During latest Mississippian-early Pennsylvanian time, elevation of the Central Montana Uplift again exposed older rocks to erosion and carving of drainage valleys. Then another subsidence, or sea level rise, allowed marine waters to push into the valleys causing streams to back-fill, leaving deposits we know as the Tyler Formation. Tyler sandstones are a primary hydrocarbon target across the Central Montana Uplift (Sumatra, Stensvad, Keg Coulee, Melstone, Big Wall, and other oil fields). Shallow marine carbonates (Amsden Formation) were deposited over the Tyler, and are also oil-productive (Gage Dome, Big Wall and other fields).
No Permian age rocks are preserved across the Central Montana Uplift, either because of nondeposition or later erosion. They are well known in the subsurface eastward in the Williston Basin.
Mesozoic History and Rocks
During the Jurassic Period, the western margin of North America began experiencing compressive forces caused by the collision of the Farallon oceanic crustal plate that was subducting beneath the continental craton. This compression continued and intensified into the Cretaceous, raising the Rocky Mountains. As land mass was raised and shoved eastward, the new topography underwent continuous erosion, shedding huge volumes of sediment eastward into the continental interior. The great weight of this sediment load actively depressed the crust in front of the rising mountains, allowing marine waters to invade, forming the vast Late Cretaceous Western Interior Seaway that, at its maximum, stretched from the Arctic Ocean to the Gulf of Mexico.
Throughout the Cretaceous Period, this seaway fluctuated in size, based on changing rates of received sediment and rates of basin subsidence. Shorelines shifted and water depths varied. A variety of depositional environments formed, both terrestrial and marine, and innumerable depositional and erosional events occurred over millions of years, finally filling the basin. During Late Cretaceous time, regional volcanic activity associated with mountain building produced significant amounts of volcanic ash. This ash, converted to clays, is preserved as bentonite, appearing as thin white beds primarily within marine shales.
Because central Montana lies close to the axis of the subsiding Cretaceous basin, it was almost continuously under water throughout the Cretaceous. Earliest Cretaceous sediments, contained in the Kootenai Formation, record terrestrial streams, lakes, and flood plains, but are overlain by the Fall River Formation, a thin shoreline sandstone marking the start of the marine invasion. Overlying the Fall River, sequentially, are marine shales of the Thermopolis, Mowry, Belle Fourche, Greenhorn, Carlile, Niobrara, and Telegraph Creek Formations – as much as 2,000 feet of marine sedimentary rocks. Above the Telegraph Creek Formation, influx of sand into the western side of the basin briefly changed the depositional environments to shallow marine and coastal flood plain as recorded by the Eagle Formation and the overlying Judith River Formation. A relatively thin marine shale, the Claggett Formation, separates these two sandstone intervals, attesting again to the fluctuation of sea levels within the Cretaceous sedimentary basin. Rocks exposed on the Milton Ranch begin with the Niobrara Formation that forms the center of Big Wall Dome.
Following Judith River deposition, the seaway expanded again, and marine shale dominated the basin as recorded by the thick Bearpaw Formation (as much as 1,300 feet thick). Above the Bearpaw lies a relatively thin shoreline sandstone called the Fox Hills (or Lennep) Formation, which signals the start of the Cretaceous Seaway’s final retreat from the continental interior. Overlying the Fox Hills are the continental mudstones and sandstones of the Hell Creek
Formation, in turn overlain by the Paleocene age beds of the Fort Union Formation. When the Fox Hills Sandstone is difficult to distinguish or is absent, the entire Fox Hills-Hell Creek interval is called the Lance Formation, as is common in central and southern Montana, including across the Milton Ranch area.
Cenozoic History and Rocks
By earliest Paleocene time (Fig. 3), the compressive forces and resulting uplift were largely over. The interior of North America was a broad plain east of the new Rocky Mountains; basin-filling and regional upwarp of the crust had drained the Cretaceous seaway from the continent for the last time.
Central and eastern Montana were in the western part of a broad alluvial plain, stretching across Wyoming and the Dakotas. Numerous meandering stream channels carried and deposited sand, and cut into channel margin layers of silt, mud, and peat bogs. Today those deposits are the Fort Union Formation, seen as sharp-based, cross-bedded channel sandstones, horizontal layers of siltstone and mudstone, and thin to thick coal seams.
Through many other Cenozoic geologic events, including volcanism, climate variation, and glaciation, the Rocky Mountains have been under erosion. Eastward, the Montana High Plains and the so-called “island mountain ranges” of central Montana have also undergone extensive erosion and re-deposition throughout the Cenozoic. The most recent deposition was during the Ice Age (Pleistocene) when extensive sediments were eroded off central Montana’s island ranges by mountain glaciers, and formed vast, coalescing, braided outwash plains that nearly buried the ranges and extended far out onto the adjoining plains. Today, these thick braid-plain deposits, the unit mapped as Qab (Fig. 2), are now being dissected by modern streams and carried away to the east by major and lesser drainages like McDonald, North Willow, and Willow Creeks and the Musselshell River.
The Ice Age in Central Montana
Throughout the Early Quaternary Pleistocene Epoch (2,600,000 to around 12,000 years ago) (Fig. 3), North America experienced four major glacial advances of the Laurentide Ice Sheet and the intervening interglacial periods. At its greatest extent, late in the fourth (Wisconsin) advance (85,000-11,000 years ago), the ice sheet periodically extended across the Missouri River and reached at least as far south as northern Fergus, Petroleum and Garfield Counties (Montana Bureau of Mines and Geology, 2007). In front of that fluctuating ice margin, the north-flowing Musselshell River was periodically blocked, backing up vast volumes of water and forming Lake Musselshell (Fig. 5). This proglacial lake is one of six ice-dammed lakes recognized across central Montana by geologists in the field (Colton and others, 1961). The other lakes, from west to east, are lakes Cut Bank, Great Falls, Jordan, Circle, and Glendive. All are variously documented as forming in front (to the south or west) of the fluctuating Laurentide Ice Sheet’s southern margin in response to damming of north- or east-flowing streams. It was during this late Wisconsin glacial advance that the Missouri River was diverted from its original northern course across Montana southward and then eastward along its present course. The Missouri’s original northern course is the broad valley now occupied by the “underfit” Milk River, incongruously small for the size of its valley.
Lake Musselshell (Fig. 5) lay generally between Winnett and Sand Springs, Montana, and from the Missouri River south to the Melstone-Musselshell, Montana area, between 20,000 and 11,500 years ago (Davis, 2004). Thus, its presently recognized southwest margin lay only about ten miles east of the Milton Ranch.
Field evidence for Glacial Lake Musselshell is primarily the occurrence of “glacial erratics”, boulders of various sizes composed of granitic and metamorphic rock types matching outcrops far to the north in Canada (Colton and Fullerton, 1986; Davis and others, 2006). These glacier-transported boulders, arriving at the front of the glacier, were iceberg-rafted out into the lake and randomly dropped as their icebergs melted. These boulders are now found scattered, sometimes partially buried, across the Late Cretaceous shale bedrock and grassy knolls of the Musselshell drainage.
Absent from the field data are typical glacial lake features such as varves (rhythmic, seasonally controlled deposits), delta deposits, and shorelines. Absence of these features suggests that Lake Musselshell was not a stable lake, but rather filled to various levels and emptied numerous times as the ice margin fluctuated (Davis, 2004; Davis and others, 2006). The lake probably drained to the north into the Missouri River, either under the ice margin or by breaching its shoreline and carving temporary new channels across or along the ice front. There is some evidence that the lake may have drained southeastward, through breached low hills such as the Larb Hills, into recognized low areas (Davis and others, 2006). Because this lake developed on erodable shales and sandstones, many features may have been, and continue to be, lost.
Although the great continental Laurentide Ice Sheet dominated the north-central Montana plains, all of central Montana experienced extensive mountain glaciation in the island ranges. The deeply scoured bowls (cirques) and ridges (arêtes) in the Big Snowy Mountains and Crazy Mountains are good examples of the shaping of mountain topography by the scouring, grinding, and transport processes of glacial ice. As these mountain glaciers melted, vast volumes of eroded debris were left behind as glacial till (in lateral, ground, and terminal moraines), or washed out onto the mountain flanks and high plains as glacial outwash deposits. This debris
apparently has been subsequently removed by erosion; if moraine deposits remain on cirque floors in the Big Snowies, they lie beneath the extensive modern landslide deposits.
Regional Paleontology and Fossils as a Geologic Tool
The Upper Cretaceous and Tertiary sedimentary rocks distributed across Central Montana have been productive of both marine and terrestrial fossils. Although to date no fossils have been reported on Milton Ranch ranch property, the presence of the regionally fossiliferous Upper Cretaceous Bearpaw Shale (Kb), Upper Cretaceous Lance (= Hell Creek) Formation (Kl), and Paleocene Tongue River Member (Tftr) beds of the Fort Union Formation that underlie the central and southern ranch property suggest fossils could be found (Fig. 2).
Significant fossil localities in the region include: (1) Mississippian-age fish fauna of the Bear Gulch Limestone on the northeast flank of the Little Snowy Mountains. This world-class site is known for its diversity of fish species as well as sponges, worms, starfish, shrimp, brachiopods, and algae. (2) Upper Cretaceous ammonite fauna (coiled cephalopods, ancestors of the modern pearly Nautilus) and shark teeth of the Colorado Shale in the Mosby area. (3) Upper Cretaceous ammonite specimens in the Bearpaw Shale, Golden Valley County. (4) Upper Cretaceous terrestrial dinosaur fauna in the Hell Creek (= Lance) Formation, Garfield County. (5) Late Paleocene mammalian fauna of the Tongue River Member, Fort Union Formation at the Douglass Quarry in the eastern Crazy Mountains Basin, Wheatland County. The reported fossil remains nearest to the Milton Ranch are dinosaur and turtle remains found in the Judith River Formation near Melstone in Musselshell County.
The primary use of these recognizable, in-tact fossil remains of bones and shells is in age-dating the beds in which they are found, and then correlating these beds regionally. But use can be made of fragmentary fossil remains as well.
Shell and bone fragments, pebbles, and missing sedimentary record. Fossils are usually fragmentary, and often not identifiable, especially shells. Shell hash, an accumulation of fragmented shells, often occurs as a thin bed or lens within or at the top of a sandstone bed. It may include bone fragments, although they seem to be uncommon. Commonly, the bed contains or is even dominated by small pebbles, usually composed of black chert (the shelly material may have been dissolved by ground water or never present). The bed or lens may represent accumulation of coarse material in a small depression on the sea floor during a storm, or it may have accumulated over a long period of time when sediment was being winnowed across the sea floor and no new sediment was being added. In this latter case, the lens or bed is often called a lag deposit. It may be intermittently traced over a very wide area, marking a period of non-deposition and reworking of bottom sediments within the sedimentary basin. The great Western Interior Seaway that flooded North America during the Cretaceous fluctuated substantially over time; its sediments (now rocks) contain a number of lag deposits (composed primarily of pebbles) recording winnowing and non-deposition. Geologists recognize these reworked surfaces as breaks in the sedimentary record, recording time passing with only erosion and no deposition to represent it. These breaks are termed unconformities.
In central Montana, one well recognized pebble lag bed occurs at the top of the marginal-marine Eagle Formation, recording reworking of the uppermost sand layers as the Claggett sea was advancing. A similar discontinuous lag deposit, which in this case locally includes shelly fragments, occurs at the top of the terrestrial Judith River Formation, formed as the Bearpaw sea was advancing. Within the Niobrara Formation, which is a wholly marine sequence, at least one pebble/shelly horizon is known (the MacGowan Bed). It records a time when the Cretaceous sea level dropped, allowing storm energy to touch the sea bottom and winnow coarse materials into a lag deposit.
Trace Fossils – Another Geologic Tool
Another type of fossil should also be discussed – the structures and markings made by soft-bodied animals such as shrimp, crabs, and many kinds of worms. These bottom dwellers live on and within the soft sediments on the sea bottom. Their markings and structures are collectively called trace fossils – that is, features that indicate the presence and activity of an animal, but in most cases do not identify the animal itself. Trace fossils record feeding methods, home-building, stabilization, and locomotion activities. We observe them preserved in ancient sediments, now rocks, as tracks, trails, and burrows. Trace fossil science (Ichnology) is very detailed and is a helpful tool as geologists attempt to determine the depositional environment in which the sediment was laid down. Bottom-dwelling animals, like all animals, have specific requirements for successful living. They are influenced by such factors as salinity, water depth, oxygen level, sedimentation rate, and energy level of the environment. We can recognize certain trace fossils, and assemblages of trace fossils, as indicative of specific environmental conditions. Moreover, as a practical matter, modern biology shows us that marine environments have an order of magnitude more diverse and numerous invertebrate bottom dwellers than do fresh waters. Hence, geologists generally equate the presence of trace fossils with a marine or marine-influenced depositional environment.
Fossil Fuel Resource History
Petroleum Resources
Oil exploration across the Central Montana Uplift has a long history, beginning in the 1920s. There have been notable successes and puzzling failures, due in large part to the complex geologic history of the Uplift. Specific conditions are necessary for oil to originate, migrate, or accumulate, and each time the geologic setting is altered those conditions may be enhanced or compromised or terminated. The central Montana Uplift, as a major block of earth’s crust, experienced a number of downwarps and uplifts over a long period of geologic time, which in turn influenced the patterns of erosion and deposition across the Uplift through time. As a result, geologists must first try to understand this history before imagining where the oil-productive beds will be in the subsurface.
Oil exploration was most active on and adjacent to the Central Montana Uplift in the 1940s through 1970s. The early efforts were driven primarily by the nation’s need for large supplies of petroleum in the conduct of WWII, followed by continuing need to supply a growing post-war nation. Throughout those decades, exploration was conducted primarily by detailed surface mapping and rudimentary seismic methods.
On the south side of the Uplift in northern Golden Valley and Musselshell Counties, exploration began with the discovery of oil in the Mississippian Heath Formation at Devil’s Basin Field in 1919. Devil’s Basin Anticline is a large, elongated dome-like structure that can easily be observed in the field – or just driving up US Highway 87. It is one of many “sheepherder anticlines” recognized across the Uplift by their obvious surface structural expression. That is, geologists in the field saw that the orientation (strike and dip) of rock units on the ground defined a closed anticline or a dome-like feature, recognizable even when the anticline or dome has been extensively eroded. In more recent decades, and continuing today, exploration has depended on more sophisticated analytical tools, primarily 3D seismic technology, but even more on the ability of geologists to interpret the subsurface relationships of rock units through time. A primary example of the complexity encountered is the Pennsylvanian Tyler Formation play involving oil accumulation in ancient channel sandstones. For success in the Tyler, both stratigraphic and structural data must be evaluated.
Two currently producing oil fields are located on or immediately adjacent to the Milton Ranch. Both are typical sheepherder anticlines. Gage Dome Field, on the south flank of the Bull Mountains Basin (Fig. 2), was discovered in 1943, and as of November 2013 has produced 686,464 barrels (Bbls) of oil from the Amsden Dolomite (Pennsylvanian), with 2 wells still in production. Big Wall Field, located just north of the ranch, on the Central Montana Uplift (Fig. 2), was discovered in 1948, and as of November 2013, has produced 8,723,175 barrels (Bbls) of oil from the Tyler Formation (Mississippian-Pennsylvanian) and Amsden Formation (Pennsylvanian), with 12 wells still in production. The wells still producing in these two fields are shut-in (not pumping) many days of the year and probably are being operated as “stripper” wells, that is, producing less than 10 barrels of oil per day. Table 1 summarizes the petroleum production (primarily oil) from fields on or near the Milton Ranch property, as of 2013.
Coal Resources
Coal beds are associated with onshore coastal plain and deltaic environments governed by stable, low-energy, fresh-water conditions over a long time period. Although vegetation may be dense along a coastal shoreline (such as a coastal mangrove swamp), the shoreline is too unstable and subject to erosion to permit thick layers of matted vegetation to form. Studies of modern peat-forming environments show us that only where the depositional environment is quiet and stable over a long time period can vegetative matter accumulate and compress into peat. Estimates vary but generally six to ten inches of peat are required to form one inch of coal.
In the Western Interior of North America, both the Upper Cretaceous and lower Tertiary (Paleocene) stratigraphic records contain significant coal deposits. Of these, the Paleocene deposits are by far the most significant.
The Upper Cretaceous Eagle and Judith River Formations (Fig. 4) record two periods when large deltas formed along the western margin of the Cretaceous Interior seaway – that seaway that extended from the Arctic to the Gulf of Mexico. The huge sediment influx reflected the newly rising Rocky Mountains to the west. These deltas prograded far out into the seaway. Behind the fronts of these deltas depositional conditions varied from high-energy stream channels carrying sediment to the delta front to low-energy settings including extensive swamps. Within these swamps, enough peat accumulated to form coal beds upon burial and compaction by overlying sedimentary layers. In both the Eagle and Judith River Formations, the coal beds are at the top of the formation, finally flooded over by the advance of the next seaway (the Claggett and the Bearpaw seas, respectively) (Fig. 4). These Cretaceous coals have never been found in significant volume or with adequate lateral continuity to be economic. However, early settlers in north-central Montana commonly used Judith River coal for domestic heating, and this practice may have been more common than is recorded.
At the very end of Cretaceous time, as the Western Interior basin was filling in and the seaway was draining away, deltaic outbuilding from the seaway’s western margin occurred again, this time for good. A regional deltaic system extended from western Wyoming northeastward into eastern Montana and the Dakotas – today’s Powder River Basin -- depositing the delta-front (marine) Fox Hills Sandstone and overlying delta-plain Hell Creek Formations (Fig 4). Coal beds at the base of the Hell Creek (top of the Fox Hills) are well developed in the region, but do not extend westward into central Montana where these two formations have been mapped as one unit, the Lance Formation (Kl) (Figs 2, 4).
The subtropical, swampy coastal plain advanced across the region uninterrupted into the early Tertiary (Paleocene), recorded by the Fort Union Formation (Fig. 4). Coal beds in the Tongue River Sandstone, upper member of the Fort Union Formation, are a significant economic resource for Montana and Wyoming, centered in the Powder River Basin. These sub-bituminous, low-sulphur coals are ten’s of feet thick, reflecting long periods of quiet, fresh-water growing conditions for Paleocene vegetation. They are interbedded with the sandstones of channels that coursed across the coastal-deltaic plain. As with the coals of the Fox Hills-Hell Creek interval, Tongue River coals do not extend westward into central Montana in anything but thin, discontinuous non-economic beds and stringers.
*Possibly in reference to the favorable elevations from which sheep herders could view their flocks.
Acknowledgements
I am indebted to the Information Services Division of the Montana Bureau of Mines and Geology for providing the staff time and expertise to produce the figures in this document.
References Cited
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Davis, N.K., Locke, III, W.W., Pierce, K.L., and Finkel, R.C., 2006, Glacial Lake Musselshell: Late Wisconsin slackwater on the Laurentide ice margin in central Montana, USA: Geomorphology, v. 75, p. 330-345.
Johnson, W.R., and Smith, H.R.,1964, Geology of the Winnett-Mosby area, Petroleum, Garfield, Rosebud, and Fergus Counties, Montana: U.S. Geological Survey Bulletin 1149, 91 p. Montana Bureau of Mines and Geology, 2007, Geologic Map of Montana: Geologic Map GM 62, map scale
Lindsay, D.A., 1982, Geologic Map and Discussion of Selected Mineral Resources of the North and South Moccasin Mountains, Fergus County, Montana: U.S. Geological Survey Miscellaneous Investigations Map I-1362, scale 1:24,000.
Maughan, Edwin K., 1993, Stratigraphic and Structural Summary for Central Montana, in L.D. Vern Hunter, editor, Energy and Mineral Resources of Central Montana: Montana Geological Society 1993 Field Conference Guidebook, p. 3-24.
Nelson, W. John, 1993, Structural Geology of the Cat Creek Anticline and Related Features, Central Montana: Montana Bureau of Mines and Geology Memoir 64, 44 p.
Porter, K.W. Porter, and Wilde, E.M., 1999, Geologic map of the Musselshell 30’ x 60’ Quadrangle, Central Montana: Montana Bureau of Mines and Geology Open-File Report 386, scale 1:100,000.
Tonnsen, John J., editor, 1985, Montana Oil and Gas Fields Symposium, vols. 1 and 2: Montana Geological Society, p. 231-234, 529-53.
US and Canadian Fossil Sites – Data for MONTANA. Version 0810; current as of OCT 2008. http://www.fossilsites.com/STATES/MT.HTM
Vorland, Rolf O., Big Wall Field, in Donald L. Foster, editor, Central Montana: Billings Geological Society Field Conference Guidebook, pp. 113-115.
Vuke, S.M., and Wilde, E.M., 2004, Geologic Map of the Melstone 30’ x 60’ Quadrangle, Eastern Montana: Montana Bureau of Mines and Geology Open-File Report 513, scale 1:100,000.
APPENDIX 1
Descriptions of Rock Units Exposed on the Milton Ranch Property
Note 1: Geologic mapping and interpretation of geologic history in central Montana has been conducted by many researchers, representing many public and private agencies and institutions, since the early 1900s. This accumulation of data, information and perspective are what underpin all the recent geologic mapping conducted by the Montana Bureau of Mines and Geology in its comprehensive program to provide updated geologic map coverage for the state of Montana. Readers of this report are referred to MBMG’s website for more geologic information at www.mbmg.mtech.edu.
Note 2: The following rock unit descriptions are taken predominantly from Porter and Wilde
(1999, Geologic Map of the Musselshell 30’ x 60’ Quadrangle, Central Montana), modified where appropriate to reflect the occurrence of these rocks on the Milton Ranch property.
Note 3: Shale units are all thinned along the steeply dipping Devils Basin-Big Wall anticlinal trend, having been squeezed and compacted by the compressional forces that last raised the Central Montana Uplift in latest Cretaceous-early Paleocene time. Their original depositional thicknesses are generally much greater than would be measured along the south side of the Uplift and across the Milton Ranch today.
Quaternary Deposits
Qal Alluvium of modern flood plains and channels (Holocene). Tan and gray-tan gravel, composed of cobbles, sand, silt, and clay deposited in channels and on flood plains of modern rivers and streams. May include some modern terrace deposits not separately mapped. Thickness not measured.
Qab Alluvium of former Braid Plains (Pleistocene and/or Holocene). Weathers to light-gray to yellowish-white and gray-brown deposits of uncemented to locally cemented cobbles in a pebble, sand and clay matrix. Cobbles predominantly light-gray, rounded clasts of Madison Group limestone, commonly with powdery white calcareous coatings. Cemented intervals are 15 feet thick with iron-rich calcareous matrix commonly weathering reddish to yellow-orange or rusty brown. Unit includes several levels of thick gravels apparently deposited in broad, coalescing alluvial fans formed on slopes of adjacent mountain ranges and now being incised by modern streams and largely removed. Unit extensively preserved along flanks of Devils Basin Anticline. Age of unit is unknown, but considered to be early Quaternary based on conclusions of Lindsay (1982) for similar gravels flanking the Moccasin Mountains. Unit may be Tertiary in age. Thickness not measured, but probably ranges from a few feet to as much as 40 feet based on descriptions of Johnson and Smith (1964).
Tertiary Rocks
Fort Union Formation (Paleocene)
Tftr Tongue River Member. Yellowish-gray to light gray, fine- to medium-grained, trough cross-bedded, planar-bedded or massive-appearing sandstone interbedded with lesser amounts of brownish-gray carbonaceous shale, yellow-gray siltstone, and coal beds. Contains numerous large channel sandstones and stacked channel sequences. Thicker sandstone units form ledges and rim rocks in the central part of the Bull Mountains basin. Resistance of these sandstones to erosion has preserved them as high topography (“Bull Mountains”) even though they are in a structurally low basin. Unit generally supports good growth of pines. Unit forms floor of Willow Creek Syncline (the axis of the Bull Mountains Basin). Greatest exposed thickness is 805 feet measured along the axis of the basin south of the Musselshell River.
Tfle Lebo Member. Medium- to dark-gray and olive-gray shale that commonly contains swelling clays (bentonite) and carbonaceous material. Interbedded with silty shale, thin, yellowish-gray sandstone and siltstone, and thin, lenticular coal beds. Typically forms gently rolling, grass-covered slopes where dips are low. Thickness not measured in map area; a thickness of 150 feet is reported in the east-adjacent Melstone quadrangle (Vuke and Wilde, 2004).
Tft Tullock Member. Yellowish-gray, fine- to medium-grained, trough cross-bedded, planarbedded or massive-appearing sandstone interbedded with lesser amounts of brownish-gray, greenish-gray claystone or dark gray carbonaceous shale. Sandstone beds, primarily channel and channel-margin deposits, are thinner, more tabular, and more laterally persistent than those in the underlying Lance Formation, but generally thicker than those in the overlying Lebo Member. May support growth of small pines. Thickness not measured in map area; a thickness of about 265 feet is reported in the east-adjacent Melstone quadrangle (Vuke and Wilde, 2004).
Cretaceous Rocks
Kl Lance Formation (Upper Cretaceous). Tan to light brownish-gray, cliff- and ledgeforming, fine-grained, thick-bedded to massive, commonly cross-stratified sandstone interbedded with medium-gray to olive-gray, fissile shale, tan to greenish-gray clays and a few thin coal lenses. Sandstone beds support pine tree growth locally. The basal sandstones are channel deposits from 20 to more than 100 feet thick; locally, these basal channel sandstones have eroded into the underlying Bearpaw Shale. This formation is stratigraphically equivalent to the interval containing the Fox Hills and Hell Creek Formations as mapped farther east. Locally, and of limited extent, typical Fox Hills Formation and Hell Creek Formation lithologies are observed between the lower channel sandstone deposits of the Lance, but were not separately mapped. Sandstone beds are thicker and more lenticular than those of the overlying Tullock Member of the Fort Union Formation. Total formation thickness is from 400 feet to 450 feet.
Kb Bearpaw Formation (Upper Cretaceous). Medium-gray to brown-gray weathering, fissile, nonresistant, marine shale; thin, greenish-white or yellow-white bentonite layers common throughout; uppermost and lowermost beds silty and sandy; large ovoid, reddish-purple weathering concretions common, especially in lower part; light gray weathering calcareous concretions more common in upper part; both concretion types commonly very fossiliferous. Thicknesses of 1,318 feet and 1,100 feet have been reported in the region, but formation is greatly thinned along the flank of the Willow Creek Syncline across the Milton Ranch.
Kjr Judith River Formation (Upper Cretaceous). Composed of three distinct units. Lower unit: Yellow-gray weathering, very fine- or fine-grained, quartzose, massive to poorly bedded, locally cross-stratified, burrowed to bioturbated sandstone; trace fossils include locally abundant Ophiomorpha; uppermost beds light-brown, ferruginous, forming resistant ledge or cap. Middle unit: Green-gray weathering, fine-grained sandstones, siltstones, mudstones and brownish carbonaceous shale; numerous conspicuous rusty-brown to purple-black weathering ironstone concretions. Upper unit: Basal yellow-gray to yellow-brown weathering, fine-grained, quartzose sandstone overlain by a sequence of interbedded sandstone, mudstone and carbonaceous shale with common small ferruginous concretions. Middle and upper units of formation are typically weathered to badlands topography. Formation thickness not measured in map area; reported thicknesses elsewhere in region are from 215 feet to as much as 275 feet.
Kcl Claggett Formation (Upper Cretaceous). Brown-weathering, dark-gray or gray-brown fresh surfaces, blocky to fissile, commonly sandy. Characteristic orange-brown weathering, smooth, ovaoid, calcareous concretions in upper middle part of formation are as much as 3 feet in diameter, commonly highly fractured with yellow calcite vein-filling, and weather into mounds of small, sharp-edged orange-brown fragments. Numerous gray-white bentonite layers 1 to 5 inches thick in lower part of formation are equivalent to the Ardmore Bentonite of Wyoming. Black chert pebbles occasionally observed locally in lower few feet are presumed to be reworked by burrowing organisms into Claggett beds from underlying chert-pebble-bearing beds at top of Eagle Formation. Brown weathered color often distinguishes this formation from the Bearpaw Shale where stratigraphic position is uncertain. Formation is commonly bare to sparsely grassy. Formation thickness not measured in map area; reported thicknesses elsewhere in region are from 350 feet to as much as 430 feet.
Ke Eagle Formation (Upper Cretaceous). Composed of three distinct units. Lower unit (Virgelle Member of some authors): white to yellow-gray weathering, concretionary, fine- and medium-grained, friable to moderately hard, cherty sandstone, generally massive and burrowed to locally cross-stratified, and commonly forms prominent cliff; supports timber. Middle unit:
Poorly exposed thin sandstones and gray-green shale with thin lignite seams. Upper unit: yellow-tan weathering, light-gray, fine-grained, cherty sandstone, commonly cross-stratified, massive, locally cliff-forming where dips are steep, but often nonresistant and poorly exposed where dips are low. A coarse chert-pebble conglomerate commonly occurs in upper few beds and on upper surface of unit; black chert pebbles in soils often indicate presence of unit where exposures are poor. Formation thickness not measured in map area; reported thicknesses elsewhere in region are from 200 feet to as much as 288 feet.
Kt Telegraph Creek Formation (Upper Cretaceous). Medium- to light-gray weathering, noncalcareous, sandy shale. Lower part contains characteristic thin beds of small, dark-red ironstone concretions weathering to angular chips. Upper part becomes silty to sandy, with characteristic bands of calcareous, tan to chocolate-brown weathering, dark-gray siltstone concretions. Upper contact with overlying eagle Formation is transitional and generally placed at base of lowest cliff-forming sandstone; lower contact with Niobrara Formation shale is generally obscure beneath soils and is only approximately located in the area. Formation thickness not measured in map area; reported thickness elsewhere in region is 164 feet.
Kn Niobrara Formation (Upper Cretaceous). Dark gray to medium-olive-gray, fissile shale. Lower part weathers medium-gray and contains numerous thin, orange weathering bentonite beds sometimes forming a banded look. Upper part strongly calcareous, commonly weathers lighter-gray. Gray and light-gray, calcareous, often slightly septarian concretions common in upper beds of lower part. Formation very poorly exposed in center of Big Wall Anticline. Thickness not measured in map area; reported thickness elsewhere in region is 300 to 400 feet.
Figures
The Milton Ranch property is located in Musselshell County, Montana, about 14 miles northeast of Roundup and east of U.S. Highway 87 (Fig. 1). Its rolling, dissected grassland has about 316 feet of relief with highest elevations in the southern part. A local drainage divide at about 3,600 feet trends east-northeast across the southern part of the property, separating north-flowing drainage into North Willow Creek from south-flowing drainage into the Musselshell River south of the ranch.
Geologically speaking, the ranch property lies across the south flank of the Central Montana Uplift and on southward into the Bull Mountains Basin (Figs. 1, 2), two prominent geologic features of central Montana. On the distant skyline to the northwest, the Big and Little Snowy Mountains (Snowy Mountains Uplift) form the southwest corner of the Central Montana Uplift. Other geologic features across the Uplift are smaller and more subtle, not visually obvious. They are recognized by the patterns of outcropping rocks. On the Milton Ranch, rock outcrops show the steep south flank of the Central Montana Uplift along the Devils Basin-Big Wall anticlinal trend (Fig. 2), where outcrops are narrow because beds dip steeply south into the Willow Creek Syncline. This syncline is the axis of the Bull Mountains Basin, a large, very asymmetric structural low with a broad north-dipping south flank. The southern half of the Milton Ranch property lies on this broad south flank of the Bull Mountains Basin, and Gage Dome is a small wrinkle on this flank (Fig. 2). Because this basin is a structural low, the youngest sedimentary rocks, the Tongue River Member of the Fort Union Formation, have been preserved here; and because these sandstones are resistant to erosion, they form the Bull Mountains – a case of high topography preserved in a structural low.
The Milton Ranch property is underlain by sedimentary rocks of both marine and nonmarine origin that comprise a complete stratigraphic section of upper Upper Cretaceous through lowermost Tertiary (Paleocene) rocks, spanning some 44,000,000 years of geologic time (Fig. 3). Younger, Quaternary deposits, perhaps associated with the last ice age of 12,000 to 10,000 years ago occur in isolated erosional remnants across the southern part of the ranch property. Modern stream sediments occur discontinuously along east-flowing North Willow Creek in the northernmost part of the ranch property.
Oil production was active on and adjacent to the Central Montana Uplift in the 1940s through
1970s, driven primarily by detailed surface mapping and rudimentary seismic methods. On the Milton Ranch, Gage Field and Big Wall Field were part of that early successful exploration.
Although commercial sub-bituminous-grade coal associated with the Tongue River Member of the Fort Union Formation is a well-known economic resource in southeastern Montana, coal beds in the Tongue River in central Montana are thinner, discontinuous, and generally noneconomic.
Geologic Structures of the Central Montana Uplift-Kinds and Causes
The Central Montana Uplift, also known as the Central Montana Trough, is an elongate, westnorthwest-trending feature stretching approximately from Lewistown on the northwest to Vananda on the southeast, and from the old community of Mosby on the north to the community of Melstone on the south (Fig. 1). The feature lies across Fergus, Petroleum, and Rosebud Counties and adjacent corners of Musselshell and Garfield Counties. It is bounded on the north by the Cat Creek Anticline, and on the south by the Big Snowy-Devils Basin-Big Wall anticlinal trend. Porcupine Dome forms its east end, while the Judith and Moccasin Mountains and the west end of the Big Snowies define its west end.
The Central Montana Uplift is easy to see on a geologic map. It appears as a prominent westnorthwest-elongate feature shown in various shades of green. On the ground, however, this feature is not easily discerned. To find its boundaries we must pay attention to the specific rock units exposed and their structural orientation – that is, the trend (strike) of the beds, the tilt (dip) of the beds, and how their outcrop pattern changes.
The north side of the Central Montana Uplift is formed by the asymmetric Cat Creek Anticline (Fig. 1) whose very steep north flank forms the south flank of the Blood Creek Syncline. While only the steeply dipping beds are observed at the surface, in the subsurface the beds are known to be offset by vertical movement along an ancient fault. Similarly, the south side of the Central Montana Uplift occurs along the south side of the Big Snowy-Devils Basin-Big Wall anticlinal trend. Again, at the surface, we see only steeply south-dipping beds forming the north limb of the Willow Creek Syncline, but a major fault is presumed at depth. The axis of the asymmetric Willow Creek Syncline is the deepest part of the Bull Mountains Basin; south of this axis beds dip gently north except where they are briefly rumpled by small folds such as Gage Dome. The deep-seated ancient faults bounding the Uplift have been periodically reactivated since Proterozoic time.
Many geologic faults have a long history through geologic time, sometimes active, other times quiescent. When active, they sometimes reverse their relative sense of vertical motion -- that is, the relative up or down motion of the rocks on either side of a fault. This reversal of structural motion has been a long-standing feature of the Central Montana Uplift through geologic time, significantly influencing both depositional and erosional patterns across central Montana for more than a billion years. For long periods of time it was a topographic low area, receiving sediments eroded from adjacent uplands; other times it was elevated and under erosion, itself a source of eroded debris. Currently, this structural feature is in an uplifted position, resulting from strong compression from the southwest during the last phase of uplift of the Rocky Mountains (Late Cretaceous-early Paleocene). These compressive forces popped the Central Montana Uplift upward along the old bounding faults and also rumpled its surface into numerous folds, many with underlying fault blocks.
Across the Uplift are a number of anticlines and domes that have been well explored since the early 1900s for their petroleum potential, including Porcupine, Potter Creek, and Big Wall Domes, and the Sumatra, Devils Basin, Willow Creek and Cat Creek Anticlines. Most of these structural features are not obvious to a traveler across the Central Montana Uplift.
It should be noted here that some researchers consider the south margin of the Central Montana Uplift to be farther south, along the northwest-trending Lake Basin Fault Zone that passes just north of Billings. All of these northwest-trending fault zones and lineaments* reflect the same ancient structural grain of the central Montana region and have likely had a similar geologic history of reactivation. In the present report, the Big Snowy-Devils Basin-Big Wall trend is taken as the south margin of the Central Montana Uplift because it appears to coincide with the south margin of an ancient (Proterozoic) sedimentary basin in central Montana. There is often more than one way to integrate existing geologic data.
[* A lineament is a broad, linear band of crustal weakness containing numerous faults and fractures.]
Stories the Rocks Tell
All the rocks underlying and exposed across the Milton Ranch property, indeed across most of the Central Montana Uplift and south into the Bull Mountains Basin are sedimentary in origin (Fig. 4). What we notice most about them are their color, texture (smooth or gritty), the many small details within them including possible fossils, and the relative degree to which they erode. Together, these features indicate a wide range of depositional settings of the original sediments, from terrestrial (land) environments such as stream beds, swamps, and lakes, to marine environments such as tidal flats or an ocean basin.
By thinking of these sedimentary rocks in their time sequence, with their particular physical and biological features that tell where and how the sediment was laid down, geologists are able to read the record of a changing landscape through geologic time. We must remember, though, that the record is incomplete because throughout depositional history many sedimentary layers were eroded away before later ones were laid down. The preserved sedimentary record is a relatively small part of what the original record probably was. It reflects the complex interaction of deposition and erosion controlled by structural motions of Earth’s crust.
Earliest Geologic History in the Region
The earliest geologic setting for which we have a rock record in central Montana is a vast sedimentary basin that occupied western Montana, with a narrow arm extending into central Montana, from about 1,470 million years ago to 1,400 million years ago (or 1.47 to 1.4 billion years ago), during the middle Proterozoic part of the Precambrian Eon (Fig. 3). The basin’s north and south margins, including along the narrow arm, were major vertical fault systems that actively dropped the basin floor allowing sediments eroded from the margins to settle in the depression. The arm’s closed east end is thought to lie on the east side of Porcupine Dome, based on subsurface well log interpretations. Some researchers believe this basin represents a great rift, or tear, in the western North American craton margin, and thus was open to the western ocean. Other researchers believe the basin’s west end was closed, at least for a large part of its history. Resolution of the issue is difficult and ongoing.
The existence of this vast sedimentary basin is recorded in the enormously thick rock sequence called the Belt Supergroup that dominates western Montana geology. As much as 54,000 feet(!) of sediment have been measured at some locations in present western Montana. In central Montana, in the narrow arm of the basin, Belt Supergroup rocks are much thinner. Nonetheless, this very famous and enigmatic rock sequence has been named for the exposures near Belt, Montana, southeast of Great Falls, where they were first described. Belt Supergroup rocks that are nearest to the Milton Ranch are those forming the core of the Big Snowy Uplift.
As the faults bounding the Belt Basin’s north and south margins became quiet toward the end of the Proterozoic Era, the basin filled and was covered by the early Paleozoic seas that encroached onto the North American continent at the start of the Cambrian Period.
Paleozoic History and Rocks
Throughout about the first half of the Paleozoic (Cambrian through middle Mississippian Periods), the whole North American continent was washed by shallow equatorial seas that advanced and retreated across the continental plate margins and into the interior. For most of this long period, most of the continent was submerged beneath these seas, and the deposited sand, mud, and especially limey mud, slowly lithified to sandstones, shales, and carbonates (limestones and dolomites). In western North America, extensive limestone rocks from this time period record shallow, warm marine conditions especially favorable for the formation of lime muds. The calcareous content comes from abundant shells and other skeletal parts of invertebrate animals and calcareous algae, and from the direct precipitation of carbonate from warm seas. In the dry climate of the West, carbonates are resistant rock units. These are the thick limestones that form resistant topography along the flanks of the Little Belt and Big Snowy Mountains.
For a relatively brief period in early Devonian time, crustal upwarping across western North America exposed sedimentary layers to erosion, stripping away all Silurian and most Ordovician rocks. A thin Ordovician section is preserved in the Big Snowy Uplift. Ordovician and Silurian age rocks begin to be recognized in the subsurface in east-central Montana and on eastward into the Dakotas.
During late Devonian through middle Mississippian time, marine waters again flooded the region. Indeed, the Mississippian seas were the most extensive of the Paleozoic seas across the continent, and Mississippian age carbonates represented by the 1,200- to 1,500-foot-thick Madison Group are traceable across most of western North America from the Rockies to beneath the central Plains of the United States and Canada. They serve as primary groundwater aquifers in some regions, and oil-bearing strata in other regions, based upon their later geologic history.
Late Paleozoic (late Mississippian, Pennsylvanian, Permian)
Throughout the late Paleozoic, structural movements on the Central Montana Uplift and of the entire uplift were frequent and complex, exerting strong control on the patterns of erosion and sedimentation across this major geologic feature. Following deposition of the Madison Group, a broad regional uplift exposed these carbonate sedimentary layers to erosion. Significantly, this widespread post-Madison Group exposure allowed fresh (acidic) waters to dissolve the carbonate, forming highly irregular, pitted and cavernous topography that greatly influences today’s ground-water and hydrocarbon occurrences. Marine waters advanced again in late Mississippian time.
During latest Mississippian-early Pennsylvanian time, elevation of the Central Montana Uplift again exposed older rocks to erosion and carving of drainage valleys. Then another subsidence, or sea level rise, allowed marine waters to push into the valleys causing streams to back-fill, leaving deposits we know as the Tyler Formation. Tyler sandstones are a primary hydrocarbon target across the Central Montana Uplift (Sumatra, Stensvad, Keg Coulee, Melstone, Big Wall, and other oil fields). Shallow marine carbonates (Amsden Formation) were deposited over the Tyler, and are also oil-productive (Gage Dome, Big Wall and other fields).
No Permian age rocks are preserved across the Central Montana Uplift, either because of nondeposition or later erosion. They are well known in the subsurface eastward in the Williston Basin.
Mesozoic History and Rocks
During the Jurassic Period, the western margin of North America began experiencing compressive forces caused by the collision of the Farallon oceanic crustal plate that was subducting beneath the continental craton. This compression continued and intensified into the Cretaceous, raising the Rocky Mountains. As land mass was raised and shoved eastward, the new topography underwent continuous erosion, shedding huge volumes of sediment eastward into the continental interior. The great weight of this sediment load actively depressed the crust in front of the rising mountains, allowing marine waters to invade, forming the vast Late Cretaceous Western Interior Seaway that, at its maximum, stretched from the Arctic Ocean to the Gulf of Mexico.
Throughout the Cretaceous Period, this seaway fluctuated in size, based on changing rates of received sediment and rates of basin subsidence. Shorelines shifted and water depths varied. A variety of depositional environments formed, both terrestrial and marine, and innumerable depositional and erosional events occurred over millions of years, finally filling the basin. During Late Cretaceous time, regional volcanic activity associated with mountain building produced significant amounts of volcanic ash. This ash, converted to clays, is preserved as bentonite, appearing as thin white beds primarily within marine shales.
Because central Montana lies close to the axis of the subsiding Cretaceous basin, it was almost continuously under water throughout the Cretaceous. Earliest Cretaceous sediments, contained in the Kootenai Formation, record terrestrial streams, lakes, and flood plains, but are overlain by the Fall River Formation, a thin shoreline sandstone marking the start of the marine invasion. Overlying the Fall River, sequentially, are marine shales of the Thermopolis, Mowry, Belle Fourche, Greenhorn, Carlile, Niobrara, and Telegraph Creek Formations – as much as 2,000 feet of marine sedimentary rocks. Above the Telegraph Creek Formation, influx of sand into the western side of the basin briefly changed the depositional environments to shallow marine and coastal flood plain as recorded by the Eagle Formation and the overlying Judith River Formation. A relatively thin marine shale, the Claggett Formation, separates these two sandstone intervals, attesting again to the fluctuation of sea levels within the Cretaceous sedimentary basin. Rocks exposed on the Milton Ranch begin with the Niobrara Formation that forms the center of Big Wall Dome.
Following Judith River deposition, the seaway expanded again, and marine shale dominated the basin as recorded by the thick Bearpaw Formation (as much as 1,300 feet thick). Above the Bearpaw lies a relatively thin shoreline sandstone called the Fox Hills (or Lennep) Formation, which signals the start of the Cretaceous Seaway’s final retreat from the continental interior. Overlying the Fox Hills are the continental mudstones and sandstones of the Hell Creek
Formation, in turn overlain by the Paleocene age beds of the Fort Union Formation. When the Fox Hills Sandstone is difficult to distinguish or is absent, the entire Fox Hills-Hell Creek interval is called the Lance Formation, as is common in central and southern Montana, including across the Milton Ranch area.
Cenozoic History and Rocks
By earliest Paleocene time (Fig. 3), the compressive forces and resulting uplift were largely over. The interior of North America was a broad plain east of the new Rocky Mountains; basin-filling and regional upwarp of the crust had drained the Cretaceous seaway from the continent for the last time.
Central and eastern Montana were in the western part of a broad alluvial plain, stretching across Wyoming and the Dakotas. Numerous meandering stream channels carried and deposited sand, and cut into channel margin layers of silt, mud, and peat bogs. Today those deposits are the Fort Union Formation, seen as sharp-based, cross-bedded channel sandstones, horizontal layers of siltstone and mudstone, and thin to thick coal seams.
Through many other Cenozoic geologic events, including volcanism, climate variation, and glaciation, the Rocky Mountains have been under erosion. Eastward, the Montana High Plains and the so-called “island mountain ranges” of central Montana have also undergone extensive erosion and re-deposition throughout the Cenozoic. The most recent deposition was during the Ice Age (Pleistocene) when extensive sediments were eroded off central Montana’s island ranges by mountain glaciers, and formed vast, coalescing, braided outwash plains that nearly buried the ranges and extended far out onto the adjoining plains. Today, these thick braid-plain deposits, the unit mapped as Qab (Fig. 2), are now being dissected by modern streams and carried away to the east by major and lesser drainages like McDonald, North Willow, and Willow Creeks and the Musselshell River.
The Ice Age in Central Montana
Throughout the Early Quaternary Pleistocene Epoch (2,600,000 to around 12,000 years ago) (Fig. 3), North America experienced four major glacial advances of the Laurentide Ice Sheet and the intervening interglacial periods. At its greatest extent, late in the fourth (Wisconsin) advance (85,000-11,000 years ago), the ice sheet periodically extended across the Missouri River and reached at least as far south as northern Fergus, Petroleum and Garfield Counties (Montana Bureau of Mines and Geology, 2007). In front of that fluctuating ice margin, the north-flowing Musselshell River was periodically blocked, backing up vast volumes of water and forming Lake Musselshell (Fig. 5). This proglacial lake is one of six ice-dammed lakes recognized across central Montana by geologists in the field (Colton and others, 1961). The other lakes, from west to east, are lakes Cut Bank, Great Falls, Jordan, Circle, and Glendive. All are variously documented as forming in front (to the south or west) of the fluctuating Laurentide Ice Sheet’s southern margin in response to damming of north- or east-flowing streams. It was during this late Wisconsin glacial advance that the Missouri River was diverted from its original northern course across Montana southward and then eastward along its present course. The Missouri’s original northern course is the broad valley now occupied by the “underfit” Milk River, incongruously small for the size of its valley.
Lake Musselshell (Fig. 5) lay generally between Winnett and Sand Springs, Montana, and from the Missouri River south to the Melstone-Musselshell, Montana area, between 20,000 and 11,500 years ago (Davis, 2004). Thus, its presently recognized southwest margin lay only about ten miles east of the Milton Ranch.
Field evidence for Glacial Lake Musselshell is primarily the occurrence of “glacial erratics”, boulders of various sizes composed of granitic and metamorphic rock types matching outcrops far to the north in Canada (Colton and Fullerton, 1986; Davis and others, 2006). These glacier-transported boulders, arriving at the front of the glacier, were iceberg-rafted out into the lake and randomly dropped as their icebergs melted. These boulders are now found scattered, sometimes partially buried, across the Late Cretaceous shale bedrock and grassy knolls of the Musselshell drainage.
Absent from the field data are typical glacial lake features such as varves (rhythmic, seasonally controlled deposits), delta deposits, and shorelines. Absence of these features suggests that Lake Musselshell was not a stable lake, but rather filled to various levels and emptied numerous times as the ice margin fluctuated (Davis, 2004; Davis and others, 2006). The lake probably drained to the north into the Missouri River, either under the ice margin or by breaching its shoreline and carving temporary new channels across or along the ice front. There is some evidence that the lake may have drained southeastward, through breached low hills such as the Larb Hills, into recognized low areas (Davis and others, 2006). Because this lake developed on erodable shales and sandstones, many features may have been, and continue to be, lost.
Although the great continental Laurentide Ice Sheet dominated the north-central Montana plains, all of central Montana experienced extensive mountain glaciation in the island ranges. The deeply scoured bowls (cirques) and ridges (arêtes) in the Big Snowy Mountains and Crazy Mountains are good examples of the shaping of mountain topography by the scouring, grinding, and transport processes of glacial ice. As these mountain glaciers melted, vast volumes of eroded debris were left behind as glacial till (in lateral, ground, and terminal moraines), or washed out onto the mountain flanks and high plains as glacial outwash deposits. This debris
apparently has been subsequently removed by erosion; if moraine deposits remain on cirque floors in the Big Snowies, they lie beneath the extensive modern landslide deposits.
Regional Paleontology and Fossils as a Geologic Tool
The Upper Cretaceous and Tertiary sedimentary rocks distributed across Central Montana have been productive of both marine and terrestrial fossils. Although to date no fossils have been reported on Milton Ranch ranch property, the presence of the regionally fossiliferous Upper Cretaceous Bearpaw Shale (Kb), Upper Cretaceous Lance (= Hell Creek) Formation (Kl), and Paleocene Tongue River Member (Tftr) beds of the Fort Union Formation that underlie the central and southern ranch property suggest fossils could be found (Fig. 2).
Significant fossil localities in the region include: (1) Mississippian-age fish fauna of the Bear Gulch Limestone on the northeast flank of the Little Snowy Mountains. This world-class site is known for its diversity of fish species as well as sponges, worms, starfish, shrimp, brachiopods, and algae. (2) Upper Cretaceous ammonite fauna (coiled cephalopods, ancestors of the modern pearly Nautilus) and shark teeth of the Colorado Shale in the Mosby area. (3) Upper Cretaceous ammonite specimens in the Bearpaw Shale, Golden Valley County. (4) Upper Cretaceous terrestrial dinosaur fauna in the Hell Creek (= Lance) Formation, Garfield County. (5) Late Paleocene mammalian fauna of the Tongue River Member, Fort Union Formation at the Douglass Quarry in the eastern Crazy Mountains Basin, Wheatland County. The reported fossil remains nearest to the Milton Ranch are dinosaur and turtle remains found in the Judith River Formation near Melstone in Musselshell County.
The primary use of these recognizable, in-tact fossil remains of bones and shells is in age-dating the beds in which they are found, and then correlating these beds regionally. But use can be made of fragmentary fossil remains as well.
Shell and bone fragments, pebbles, and missing sedimentary record. Fossils are usually fragmentary, and often not identifiable, especially shells. Shell hash, an accumulation of fragmented shells, often occurs as a thin bed or lens within or at the top of a sandstone bed. It may include bone fragments, although they seem to be uncommon. Commonly, the bed contains or is even dominated by small pebbles, usually composed of black chert (the shelly material may have been dissolved by ground water or never present). The bed or lens may represent accumulation of coarse material in a small depression on the sea floor during a storm, or it may have accumulated over a long period of time when sediment was being winnowed across the sea floor and no new sediment was being added. In this latter case, the lens or bed is often called a lag deposit. It may be intermittently traced over a very wide area, marking a period of non-deposition and reworking of bottom sediments within the sedimentary basin. The great Western Interior Seaway that flooded North America during the Cretaceous fluctuated substantially over time; its sediments (now rocks) contain a number of lag deposits (composed primarily of pebbles) recording winnowing and non-deposition. Geologists recognize these reworked surfaces as breaks in the sedimentary record, recording time passing with only erosion and no deposition to represent it. These breaks are termed unconformities.
In central Montana, one well recognized pebble lag bed occurs at the top of the marginal-marine Eagle Formation, recording reworking of the uppermost sand layers as the Claggett sea was advancing. A similar discontinuous lag deposit, which in this case locally includes shelly fragments, occurs at the top of the terrestrial Judith River Formation, formed as the Bearpaw sea was advancing. Within the Niobrara Formation, which is a wholly marine sequence, at least one pebble/shelly horizon is known (the MacGowan Bed). It records a time when the Cretaceous sea level dropped, allowing storm energy to touch the sea bottom and winnow coarse materials into a lag deposit.
Trace Fossils – Another Geologic Tool
Another type of fossil should also be discussed – the structures and markings made by soft-bodied animals such as shrimp, crabs, and many kinds of worms. These bottom dwellers live on and within the soft sediments on the sea bottom. Their markings and structures are collectively called trace fossils – that is, features that indicate the presence and activity of an animal, but in most cases do not identify the animal itself. Trace fossils record feeding methods, home-building, stabilization, and locomotion activities. We observe them preserved in ancient sediments, now rocks, as tracks, trails, and burrows. Trace fossil science (Ichnology) is very detailed and is a helpful tool as geologists attempt to determine the depositional environment in which the sediment was laid down. Bottom-dwelling animals, like all animals, have specific requirements for successful living. They are influenced by such factors as salinity, water depth, oxygen level, sedimentation rate, and energy level of the environment. We can recognize certain trace fossils, and assemblages of trace fossils, as indicative of specific environmental conditions. Moreover, as a practical matter, modern biology shows us that marine environments have an order of magnitude more diverse and numerous invertebrate bottom dwellers than do fresh waters. Hence, geologists generally equate the presence of trace fossils with a marine or marine-influenced depositional environment.
Fossil Fuel Resource History
Petroleum Resources
Oil exploration across the Central Montana Uplift has a long history, beginning in the 1920s. There have been notable successes and puzzling failures, due in large part to the complex geologic history of the Uplift. Specific conditions are necessary for oil to originate, migrate, or accumulate, and each time the geologic setting is altered those conditions may be enhanced or compromised or terminated. The central Montana Uplift, as a major block of earth’s crust, experienced a number of downwarps and uplifts over a long period of geologic time, which in turn influenced the patterns of erosion and deposition across the Uplift through time. As a result, geologists must first try to understand this history before imagining where the oil-productive beds will be in the subsurface.
Oil exploration was most active on and adjacent to the Central Montana Uplift in the 1940s through 1970s. The early efforts were driven primarily by the nation’s need for large supplies of petroleum in the conduct of WWII, followed by continuing need to supply a growing post-war nation. Throughout those decades, exploration was conducted primarily by detailed surface mapping and rudimentary seismic methods.
On the south side of the Uplift in northern Golden Valley and Musselshell Counties, exploration began with the discovery of oil in the Mississippian Heath Formation at Devil’s Basin Field in 1919. Devil’s Basin Anticline is a large, elongated dome-like structure that can easily be observed in the field – or just driving up US Highway 87. It is one of many “sheepherder anticlines” recognized across the Uplift by their obvious surface structural expression. That is, geologists in the field saw that the orientation (strike and dip) of rock units on the ground defined a closed anticline or a dome-like feature, recognizable even when the anticline or dome has been extensively eroded. In more recent decades, and continuing today, exploration has depended on more sophisticated analytical tools, primarily 3D seismic technology, but even more on the ability of geologists to interpret the subsurface relationships of rock units through time. A primary example of the complexity encountered is the Pennsylvanian Tyler Formation play involving oil accumulation in ancient channel sandstones. For success in the Tyler, both stratigraphic and structural data must be evaluated.
Two currently producing oil fields are located on or immediately adjacent to the Milton Ranch. Both are typical sheepherder anticlines. Gage Dome Field, on the south flank of the Bull Mountains Basin (Fig. 2), was discovered in 1943, and as of November 2013 has produced 686,464 barrels (Bbls) of oil from the Amsden Dolomite (Pennsylvanian), with 2 wells still in production. Big Wall Field, located just north of the ranch, on the Central Montana Uplift (Fig. 2), was discovered in 1948, and as of November 2013, has produced 8,723,175 barrels (Bbls) of oil from the Tyler Formation (Mississippian-Pennsylvanian) and Amsden Formation (Pennsylvanian), with 12 wells still in production. The wells still producing in these two fields are shut-in (not pumping) many days of the year and probably are being operated as “stripper” wells, that is, producing less than 10 barrels of oil per day. Table 1 summarizes the petroleum production (primarily oil) from fields on or near the Milton Ranch property, as of 2013.
Coal Resources
Coal beds are associated with onshore coastal plain and deltaic environments governed by stable, low-energy, fresh-water conditions over a long time period. Although vegetation may be dense along a coastal shoreline (such as a coastal mangrove swamp), the shoreline is too unstable and subject to erosion to permit thick layers of matted vegetation to form. Studies of modern peat-forming environments show us that only where the depositional environment is quiet and stable over a long time period can vegetative matter accumulate and compress into peat. Estimates vary but generally six to ten inches of peat are required to form one inch of coal.
In the Western Interior of North America, both the Upper Cretaceous and lower Tertiary (Paleocene) stratigraphic records contain significant coal deposits. Of these, the Paleocene deposits are by far the most significant.
The Upper Cretaceous Eagle and Judith River Formations (Fig. 4) record two periods when large deltas formed along the western margin of the Cretaceous Interior seaway – that seaway that extended from the Arctic to the Gulf of Mexico. The huge sediment influx reflected the newly rising Rocky Mountains to the west. These deltas prograded far out into the seaway. Behind the fronts of these deltas depositional conditions varied from high-energy stream channels carrying sediment to the delta front to low-energy settings including extensive swamps. Within these swamps, enough peat accumulated to form coal beds upon burial and compaction by overlying sedimentary layers. In both the Eagle and Judith River Formations, the coal beds are at the top of the formation, finally flooded over by the advance of the next seaway (the Claggett and the Bearpaw seas, respectively) (Fig. 4). These Cretaceous coals have never been found in significant volume or with adequate lateral continuity to be economic. However, early settlers in north-central Montana commonly used Judith River coal for domestic heating, and this practice may have been more common than is recorded.
At the very end of Cretaceous time, as the Western Interior basin was filling in and the seaway was draining away, deltaic outbuilding from the seaway’s western margin occurred again, this time for good. A regional deltaic system extended from western Wyoming northeastward into eastern Montana and the Dakotas – today’s Powder River Basin -- depositing the delta-front (marine) Fox Hills Sandstone and overlying delta-plain Hell Creek Formations (Fig 4). Coal beds at the base of the Hell Creek (top of the Fox Hills) are well developed in the region, but do not extend westward into central Montana where these two formations have been mapped as one unit, the Lance Formation (Kl) (Figs 2, 4).
The subtropical, swampy coastal plain advanced across the region uninterrupted into the early Tertiary (Paleocene), recorded by the Fort Union Formation (Fig. 4). Coal beds in the Tongue River Sandstone, upper member of the Fort Union Formation, are a significant economic resource for Montana and Wyoming, centered in the Powder River Basin. These sub-bituminous, low-sulphur coals are ten’s of feet thick, reflecting long periods of quiet, fresh-water growing conditions for Paleocene vegetation. They are interbedded with the sandstones of channels that coursed across the coastal-deltaic plain. As with the coals of the Fox Hills-Hell Creek interval, Tongue River coals do not extend westward into central Montana in anything but thin, discontinuous non-economic beds and stringers.
*Possibly in reference to the favorable elevations from which sheep herders could view their flocks.
Acknowledgements
I am indebted to the Information Services Division of the Montana Bureau of Mines and Geology for providing the staff time and expertise to produce the figures in this document.
References Cited
Alden, W.C., 1932, Physiography and glacial geology of eastern Montana: U.S. Geological Survey Professional Paper 174, 133 p., 1 sheet.
Colton, R.B., and Fullerton, D.S., 1986, Proglacial lakes along the Laurentide ice sheet margin in Montana: Geological Society of America Abstracts with Programs, v. 18, n. 5, p. 347.
Colton, R.B., Lemke, R.W., and Lindvall, R.M., 1961, Glacial Map of Montana East of the Rocky Mountains: US Geological Survey Miscellaneous Investigations Series Map I-327, scale 1:500,000, 1 sheet.
Darrow, George, Gage Field, Musselshell County, Montana, in Donald L. Foster, editor, Central Montana: Billings Geological Society Field Conference Guidebook, p. 104-107.
Davis, N.K., 2004, Extent and timing of Laurentide Glacial Lake Musselshell, central Montana, M.S. Thesis, Montana State University, Bozeman, MT, 202 p.
Davis, N.K., Locke, III, W.W., Pierce, K.L., and Finkel, R.C., 2006, Glacial Lake Musselshell: Late Wisconsin slackwater on the Laurentide ice margin in central Montana, USA: Geomorphology, v. 75, p. 330-345.
Johnson, W.R., and Smith, H.R.,1964, Geology of the Winnett-Mosby area, Petroleum, Garfield, Rosebud, and Fergus Counties, Montana: U.S. Geological Survey Bulletin 1149, 91 p. Montana Bureau of Mines and Geology, 2007, Geologic Map of Montana: Geologic Map GM 62, map scale
Lindsay, D.A., 1982, Geologic Map and Discussion of Selected Mineral Resources of the North and South Moccasin Mountains, Fergus County, Montana: U.S. Geological Survey Miscellaneous Investigations Map I-1362, scale 1:24,000.
Maughan, Edwin K., 1993, Stratigraphic and Structural Summary for Central Montana, in L.D. Vern Hunter, editor, Energy and Mineral Resources of Central Montana: Montana Geological Society 1993 Field Conference Guidebook, p. 3-24.
Nelson, W. John, 1993, Structural Geology of the Cat Creek Anticline and Related Features, Central Montana: Montana Bureau of Mines and Geology Memoir 64, 44 p.
Porter, K.W. Porter, and Wilde, E.M., 1999, Geologic map of the Musselshell 30’ x 60’ Quadrangle, Central Montana: Montana Bureau of Mines and Geology Open-File Report 386, scale 1:100,000.
Tonnsen, John J., editor, 1985, Montana Oil and Gas Fields Symposium, vols. 1 and 2: Montana Geological Society, p. 231-234, 529-53.
US and Canadian Fossil Sites – Data for MONTANA. Version 0810; current as of OCT 2008. http://www.fossilsites.com/STATES/MT.HTM
Vorland, Rolf O., Big Wall Field, in Donald L. Foster, editor, Central Montana: Billings Geological Society Field Conference Guidebook, pp. 113-115.
Vuke, S.M., and Wilde, E.M., 2004, Geologic Map of the Melstone 30’ x 60’ Quadrangle, Eastern Montana: Montana Bureau of Mines and Geology Open-File Report 513, scale 1:100,000.
APPENDIX 1
Descriptions of Rock Units Exposed on the Milton Ranch Property
Note 1: Geologic mapping and interpretation of geologic history in central Montana has been conducted by many researchers, representing many public and private agencies and institutions, since the early 1900s. This accumulation of data, information and perspective are what underpin all the recent geologic mapping conducted by the Montana Bureau of Mines and Geology in its comprehensive program to provide updated geologic map coverage for the state of Montana. Readers of this report are referred to MBMG’s website for more geologic information at www.mbmg.mtech.edu.
Note 2: The following rock unit descriptions are taken predominantly from Porter and Wilde
(1999, Geologic Map of the Musselshell 30’ x 60’ Quadrangle, Central Montana), modified where appropriate to reflect the occurrence of these rocks on the Milton Ranch property.
Note 3: Shale units are all thinned along the steeply dipping Devils Basin-Big Wall anticlinal trend, having been squeezed and compacted by the compressional forces that last raised the Central Montana Uplift in latest Cretaceous-early Paleocene time. Their original depositional thicknesses are generally much greater than would be measured along the south side of the Uplift and across the Milton Ranch today.
Quaternary Deposits
Qal Alluvium of modern flood plains and channels (Holocene). Tan and gray-tan gravel, composed of cobbles, sand, silt, and clay deposited in channels and on flood plains of modern rivers and streams. May include some modern terrace deposits not separately mapped. Thickness not measured.
Qab Alluvium of former Braid Plains (Pleistocene and/or Holocene). Weathers to light-gray to yellowish-white and gray-brown deposits of uncemented to locally cemented cobbles in a pebble, sand and clay matrix. Cobbles predominantly light-gray, rounded clasts of Madison Group limestone, commonly with powdery white calcareous coatings. Cemented intervals are 15 feet thick with iron-rich calcareous matrix commonly weathering reddish to yellow-orange or rusty brown. Unit includes several levels of thick gravels apparently deposited in broad, coalescing alluvial fans formed on slopes of adjacent mountain ranges and now being incised by modern streams and largely removed. Unit extensively preserved along flanks of Devils Basin Anticline. Age of unit is unknown, but considered to be early Quaternary based on conclusions of Lindsay (1982) for similar gravels flanking the Moccasin Mountains. Unit may be Tertiary in age. Thickness not measured, but probably ranges from a few feet to as much as 40 feet based on descriptions of Johnson and Smith (1964).
Tertiary Rocks
Fort Union Formation (Paleocene)
Tftr Tongue River Member. Yellowish-gray to light gray, fine- to medium-grained, trough cross-bedded, planar-bedded or massive-appearing sandstone interbedded with lesser amounts of brownish-gray carbonaceous shale, yellow-gray siltstone, and coal beds. Contains numerous large channel sandstones and stacked channel sequences. Thicker sandstone units form ledges and rim rocks in the central part of the Bull Mountains basin. Resistance of these sandstones to erosion has preserved them as high topography (“Bull Mountains”) even though they are in a structurally low basin. Unit generally supports good growth of pines. Unit forms floor of Willow Creek Syncline (the axis of the Bull Mountains Basin). Greatest exposed thickness is 805 feet measured along the axis of the basin south of the Musselshell River.
Tfle Lebo Member. Medium- to dark-gray and olive-gray shale that commonly contains swelling clays (bentonite) and carbonaceous material. Interbedded with silty shale, thin, yellowish-gray sandstone and siltstone, and thin, lenticular coal beds. Typically forms gently rolling, grass-covered slopes where dips are low. Thickness not measured in map area; a thickness of 150 feet is reported in the east-adjacent Melstone quadrangle (Vuke and Wilde, 2004).
Tft Tullock Member. Yellowish-gray, fine- to medium-grained, trough cross-bedded, planarbedded or massive-appearing sandstone interbedded with lesser amounts of brownish-gray, greenish-gray claystone or dark gray carbonaceous shale. Sandstone beds, primarily channel and channel-margin deposits, are thinner, more tabular, and more laterally persistent than those in the underlying Lance Formation, but generally thicker than those in the overlying Lebo Member. May support growth of small pines. Thickness not measured in map area; a thickness of about 265 feet is reported in the east-adjacent Melstone quadrangle (Vuke and Wilde, 2004).
Cretaceous Rocks
Kl Lance Formation (Upper Cretaceous). Tan to light brownish-gray, cliff- and ledgeforming, fine-grained, thick-bedded to massive, commonly cross-stratified sandstone interbedded with medium-gray to olive-gray, fissile shale, tan to greenish-gray clays and a few thin coal lenses. Sandstone beds support pine tree growth locally. The basal sandstones are channel deposits from 20 to more than 100 feet thick; locally, these basal channel sandstones have eroded into the underlying Bearpaw Shale. This formation is stratigraphically equivalent to the interval containing the Fox Hills and Hell Creek Formations as mapped farther east. Locally, and of limited extent, typical Fox Hills Formation and Hell Creek Formation lithologies are observed between the lower channel sandstone deposits of the Lance, but were not separately mapped. Sandstone beds are thicker and more lenticular than those of the overlying Tullock Member of the Fort Union Formation. Total formation thickness is from 400 feet to 450 feet.
Kb Bearpaw Formation (Upper Cretaceous). Medium-gray to brown-gray weathering, fissile, nonresistant, marine shale; thin, greenish-white or yellow-white bentonite layers common throughout; uppermost and lowermost beds silty and sandy; large ovoid, reddish-purple weathering concretions common, especially in lower part; light gray weathering calcareous concretions more common in upper part; both concretion types commonly very fossiliferous. Thicknesses of 1,318 feet and 1,100 feet have been reported in the region, but formation is greatly thinned along the flank of the Willow Creek Syncline across the Milton Ranch.
Kjr Judith River Formation (Upper Cretaceous). Composed of three distinct units. Lower unit: Yellow-gray weathering, very fine- or fine-grained, quartzose, massive to poorly bedded, locally cross-stratified, burrowed to bioturbated sandstone; trace fossils include locally abundant Ophiomorpha; uppermost beds light-brown, ferruginous, forming resistant ledge or cap. Middle unit: Green-gray weathering, fine-grained sandstones, siltstones, mudstones and brownish carbonaceous shale; numerous conspicuous rusty-brown to purple-black weathering ironstone concretions. Upper unit: Basal yellow-gray to yellow-brown weathering, fine-grained, quartzose sandstone overlain by a sequence of interbedded sandstone, mudstone and carbonaceous shale with common small ferruginous concretions. Middle and upper units of formation are typically weathered to badlands topography. Formation thickness not measured in map area; reported thicknesses elsewhere in region are from 215 feet to as much as 275 feet.
Kcl Claggett Formation (Upper Cretaceous). Brown-weathering, dark-gray or gray-brown fresh surfaces, blocky to fissile, commonly sandy. Characteristic orange-brown weathering, smooth, ovaoid, calcareous concretions in upper middle part of formation are as much as 3 feet in diameter, commonly highly fractured with yellow calcite vein-filling, and weather into mounds of small, sharp-edged orange-brown fragments. Numerous gray-white bentonite layers 1 to 5 inches thick in lower part of formation are equivalent to the Ardmore Bentonite of Wyoming. Black chert pebbles occasionally observed locally in lower few feet are presumed to be reworked by burrowing organisms into Claggett beds from underlying chert-pebble-bearing beds at top of Eagle Formation. Brown weathered color often distinguishes this formation from the Bearpaw Shale where stratigraphic position is uncertain. Formation is commonly bare to sparsely grassy. Formation thickness not measured in map area; reported thicknesses elsewhere in region are from 350 feet to as much as 430 feet.
Ke Eagle Formation (Upper Cretaceous). Composed of three distinct units. Lower unit (Virgelle Member of some authors): white to yellow-gray weathering, concretionary, fine- and medium-grained, friable to moderately hard, cherty sandstone, generally massive and burrowed to locally cross-stratified, and commonly forms prominent cliff; supports timber. Middle unit:
Poorly exposed thin sandstones and gray-green shale with thin lignite seams. Upper unit: yellow-tan weathering, light-gray, fine-grained, cherty sandstone, commonly cross-stratified, massive, locally cliff-forming where dips are steep, but often nonresistant and poorly exposed where dips are low. A coarse chert-pebble conglomerate commonly occurs in upper few beds and on upper surface of unit; black chert pebbles in soils often indicate presence of unit where exposures are poor. Formation thickness not measured in map area; reported thicknesses elsewhere in region are from 200 feet to as much as 288 feet.
Kt Telegraph Creek Formation (Upper Cretaceous). Medium- to light-gray weathering, noncalcareous, sandy shale. Lower part contains characteristic thin beds of small, dark-red ironstone concretions weathering to angular chips. Upper part becomes silty to sandy, with characteristic bands of calcareous, tan to chocolate-brown weathering, dark-gray siltstone concretions. Upper contact with overlying eagle Formation is transitional and generally placed at base of lowest cliff-forming sandstone; lower contact with Niobrara Formation shale is generally obscure beneath soils and is only approximately located in the area. Formation thickness not measured in map area; reported thickness elsewhere in region is 164 feet.
Kn Niobrara Formation (Upper Cretaceous). Dark gray to medium-olive-gray, fissile shale. Lower part weathers medium-gray and contains numerous thin, orange weathering bentonite beds sometimes forming a banded look. Upper part strongly calcareous, commonly weathers lighter-gray. Gray and light-gray, calcareous, often slightly septarian concretions common in upper beds of lower part. Formation very poorly exposed in center of Big Wall Anticline. Thickness not measured in map area; reported thickness elsewhere in region is 300 to 400 feet.
Figures
figure_1_milton_ranch.pdf |
figure_2_geology_of_milton_ranch.pdf |
figure_3_geologic_time_scale.pdf |