Geology

Marias River Shale

• 100 – 85 mya

• Kevin member (outcropped in Teton county) around 189 to 213 m thick

Depositional environment

• This was a part of the Western Interior Seaway

  • Intertidal zone/foreshore (?)

Composition

• The majority of this local region’s outcrops are the Kevin Member

  • Dark gray siltstone and mudstone with calcareous concretions and bentonite beds

  • Occasionally some concretionary limestone and dolostone with phosphatic pellets as well as gray to black chert pebbles

Fossils

• Invertebrates

  • Baculites

  • Ammonites

  • Bivalves

Telegraph Creek Formation

• 85 – 80 mya

• Up to 200 m thick (?)

Depositional environment

• Shallow water and shoreline environment

Composition

• Very fine- to fine-grained calcareous sandstone interbedded with yellowish-gray-weathering silty mudstone and light- to dark-gray-weathering fissile (easy to split) shale

Virgelle Sandstone

• 80 mya

• 90 m thick (?)

Depositional environment

• Coast/beach environment

• Delta

  • Easterly flow

Composition

• Intermittent siltstone and sandstone

  • Very fine- to medium-grained, thinly bedded, cross-bedded sandstone (planar and trough)

• As this forms many of the buttes in the area, the thin black cap (cliff forming) has a high concentration of ilmenite and magnetite (up to 1 m thick)

  • These are the deposits from a delta, all of the heavy minerals concentrated at the delta while the rest of the water drained into the seaway

  • Fun fact – the magnetite, if you sit on the cap of these buttes – can mess up phone signal, the signal of a compass, and deactivate credit cards

• This member is thinner and thicker in some areas

  • This is because the thicker areas were places where the river was slowing down, thinner areas are places where the river sped up

• Fun fact – there are dinosaur footprints in this formation located at the bottom of Bynum Reservoir

Two Medicine Formation

80.5 – 74 mya Dated with the various bentonite ash layers in the area 800 m thick Depositional environment Terrestrial Flat, expansive coastal plain with bush like fields The Rockies were located about 100 miles further west than where they are now Possible that this caused a rain shadow Semi-arid seasonal climate

Composition

Bentonitic siltstones and mudstones with occasional gray and greenish gray coarse-grained sandstone lenses Contains smectite Swelling clay (name for a group of phyllosilicate minerals) Red and green sediment – paleosols Fossil soil horizons Red beds and caliche horizons

Fossils (Groups/Genera)

• Ankylosaurids

  • Edmontonia

  • Euoplocephalus

  • Oohkotokia

  • Scolosaurus

• Avialans

  • Gettyia

• Ceratopsians

  • Achelousaurus

  • Brachyceratops

  • Cerasinops

  • Einiosaurus

  • Prenoceratops

  • Stellasaurus

  • Styracosaurus

• Non-avalian eumaniraptorans

  • Bambiraptor

  • Dromaeosaurus

  • Richardoestesia

  • Saurornitholestes

  • Troodontidae

• Ornithopods

  • Acristavus

  • Glishades

  • Gryposaurus

  • Hypacrosaurus

  • Maiasaura

  • Orodromeus

  • Prosaurolophus

• Oviraptorosaurs

  • Chirostenotes

• Tyrannosaurids

  • Daspletosaurus

  • Gorgosaurus

• Egg species (Parents i.e., not the ootaxa name)

  • Troodon formosus

  • Maiasaura peeblesorum

  • Hypacrosaurus stebingeri

This is the end of the seaway regression, now it started to transgress

Bearpaw Shale

• 74 – 72 mya

• 350 m

Depositional environment

• “Overbank deposits that resulted from lateral stream migration, which brought the floodplain environment over the former channel”

• “Overbank…” to “...brackish-water estuary, lagoonal, or delta plain sedimentation”

• “....levee followed by overbank swamps that were occasionally covered by crevasse splay sedimentation…” “...coastal or delta plain swamp deposits…”

Composition

• Fissile silt shale, clay stone, and poorly sorted sandstone occasionally

• Iron and calcareous concretions

• Thin bentonite beds

Fossils

• Invertebrates

  • Bivalves

  • Ammonites

  • Baculties

• Plesiosaurs

• Mosasaurs

• Turtles

Horsethief Sandstone

• 72 mya

• 60 m thick

Depositional environment

• Brackish and shallow water

• Lower shoreface → upper shoreface and tidal inlet channel → beach foreshore and backshore → lagoon, marsh, tidal-flat → flood-tidal delta (??)

Composition

• Interbedded mudstone, siltstone and sandstone

• Trough cross-bedded sandstone; horizontally stratified sandstone Carbonaceous shale, siltstone, and sandstone

• Wedge-planar cross-stratified sandstone

Fossils

• Some but no distinguishable dinosaur remains

• Gastropods

• Bivalves

• Ichnofossils

  • Burrows

  • Mudballs

  • Plant rootlet traces

St. Mary’s River Formation

• 71.9 – 67 mya

• 300 – 400 m thick

Depositional environment

• Brackish water (estuary) (?)

• Freshwater fluvial and floodplain environments(?)

• Swamp(?) Anastomosing river system

  • Main river channel

  • Splay channels

  • Crevasse splays

  • Overbank fines or floodplains

Composition

• Large and small sandstone lenses, sandstone/siltstone sheets, shales, coquinoid beds (sedimentary rocks of shells of mollusks, brachiopods, and other invertebrates)

• Coal

Fossils

• Partial tree trunks

• Mammals

• Dinosaurs

  • Tracks

  • Anchiceratops

  • Edmontonia

  • Montanoceratops

  • Pachyrhinosaurus

  • Regaliceratops

  • Albertosaurus

  • Saurornitholestes

  • Troodon

• Invertebrates

  • Clams

  • Snails