Regional Paleontology and Fossils as a Geologic Tool
By Karen Porter
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 – a 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.
US and Canadian Fossil Sites – Data for MONTANA. Version 0810; current as of OCT 2008. http://www.fossilsites.com/STATES/MT.HTM