Tropic Shale
Tropic Shale | |
---|---|
Stratigraphic range: Cenomanian to Turonian | |
Type | Geological formation |
Unit of | Kaiparowits Plateau |
Underlies | Straight Cliffs Formation |
Overlies | Dakota Formation |
Thickness | Maximum 1,450 feet (440 m), average 600 feet (180 m) |
Lithology | |
Primary | Shale |
Location | |
Coordinates | 37°37′44″N 112°04′34″W / 37.629°N 112.076°W |
Region | Utah |
Country | United States |
Type section | |
Named for | Exposures near Tropic, Garfield County, Utah |
Named by | Gregory and Moore, 1931[1] |
The Tropic Shale is a
Geology
The Tropic Shale is predominantly marine
The Tropic Shale conformity overlies the Dakota Formation and underlies the Straight Cliffs Formation. The top of the Dakota Formation is known for its sandier coarsening up sequences and estuarine shell beds. The distinction between the Tropic Shale and underlying Dakota is marked by the appearance of marine mudstones. In some localities there is a sharp non conformable contact between the Dakota Formation and Tropic Shale. The contact with the overlying Straight Cliffs is gradational with the distinction between the two units defined as the point where sandstone becomes more abundant than shale.
The Tropic Shale has two dominate lithologies, with the lower two thirds of the formation consisting of a bluish gray calcareous mudstone that encompasses eleven
Stratigraphy and age
The Tropic Shale has been correlated temporally with the Tununk Member of the Mancos Shale in central Utah, the Allen Valley Shale of the western Wasatch Range in Utah,[4] the Mancos Shale exposed at Black Mesa, Arizona, and additionally the Bridge Creek Member of the Greenhorn Limestone at Pueblo, Colorado. Bentonite layers present in all these formations have been correlated throughout deposits associated with the Western Interior Seaway.
Solid and
The bentonites of the Tropic Shale form
Radioisotopically dated beds:[5]
Bentonite | Date | Error +/- | Correlated Ammonoid Zone |
---|---|---|---|
"A" | 93.49 | 0.89 | Upper Cenomanian biozone Euomphaloceras septemseriatum |
"B" | 93.59 | 0.58 | upper Cenomanian biozone of Neocardioceras juddii |
"C" | 93.25 | 0.55 | Lower Turonia biozone of Vascoceras birchbyi |
"D" | 93.40 | 0.63 | - |
"E" | - | - | - |
Genus | Species | Date | Error +/- | Stage |
---|---|---|---|---|
Prionocyclus | hyatti | 92.46 | 0.58 | Middle Turonian |
Collignoniceras | praecox | - | - | Middle Turonian |
Collignoniceras | woollgari | - | - | Middle Turonian |
Mammites | nodosoides | - | - | Lower Turonian |
Vascoceras | birchbyi | 93.48 | 0.58 | Lower Turonian |
Pseudoaspidoceras | flexuosum | 93.1 | 0.42 | Lower Turonian |
Watinoceras | devonense | - | - | Lower Turonian |
Nigericeras | scotti | - | - | Upper Cenomanian |
Neocardioceras | juddii | 93.32 / 93.82 | .38 / .3 | Upper Cenomanian |
Burroceras | clydense | - | - | Upper Cenomanian |
Euomphaloceras | septemseriatum | 93.68 | 0.5 | Upper Cenomanian |
Vascoceras | diartianum | 93.99 | 0.72 | Upper Cenomanian |
Paleontology
The Tropic Shale is known for a wide assortment of marine vertebrates with minor contributions from terrestrial vertebrates. Recovered fossils include sharks, fishes, marine reptiles, turtles and dinosaurs. The marine deposition of vertebrates such as dinosaurs is interpreted as animals being washed out to sea while still alive in a storm event that then drowned or decomposing animals that were washed out to sea in a bloat and float model of transportation.[7]
Reptiles
Dinosaurs
Dinosaurs reported from the Tropic Shale | |||||
---|---|---|---|---|---|
Genus | Species | Presence | Material | Notes | Images |
Nothronychus | N. graffami | Kaiparowits Basin, Kane County, Utah.[8]
|
UMNH VP 16420 (nearly complete postcranial skeleton).[7][8] | A therizinosaur. |
Mosasaurs
Mosasaurs reported from the Tropic Shale | |||||
---|---|---|---|---|---|
Genus | Species | Presence | Material | Notes | Images |
Sarabosaurus | S. dahli | GLCA site 327, Glen Canyon National Recreation Area.[9] | Fragments of VP21800). | A plioplatecarpine. |
Plesiosaurs
Plesiosaurs reported from the Tropic Shale | |||||
---|---|---|---|---|---|
Genus | Species | Presence | Material | Notes | Images |
Brachauchenius | B. lucasi | Partial skeleton (MNA V9433).[7] | A pliosaurid. | ||
Eopolycotylus | E. rankini | Partial skeleton (MNA V9445).[7] | A polycotylid. | ||
Palmulasaurus | P. quadratus | Partial skeleton (MNA V9442).[7] | A polycotylid. | ||
Scalamagnus | S. tropicensis | Nearly complete specimen with associated gastroliths (MNA V10046).[7] | A polycotylid. | ||
Trinacromerum | T. ?bentonianum | Multiple specimens.[7] | A polycotylid. |
Turtles
Turtles reported from the Tropic Shale | |||||
---|---|---|---|---|---|
Genus | Species | Presence | Material | Notes | Images |
Desmatochelys | D. lowi | Partial skeleton (MNA V9446).[7] | A protostegid. | ||
Naomichelys | N. sp. | Fragmentary carapace & plastron with a limb fragment (MNA V9461).[7] | A helochelydrid. | ||
Protostegidae Genus et sp. indet. | Indeterminate | MNA V9458.[7] | Provisionally identified as a possible new genus.[7] |
Fish
Bony fish
Bony fish reported from the Tropic Shale
| |||||
---|---|---|---|---|---|
Genus | Species | Presence | Material | Notes | Images |
Gillicus | G. arcuatus | Nearly complete articulated skeleton (MNA V10081).[7] | An ichthyodectiform. | ||
Ichthyodectes | I. ctenodon | A specimen with dentaries, 6 vertebrae & skull fragments (MNA V9467).[7]
|
An ichthyodectid .
|
||
I. sp., cf. I. ctenodon | Fragmentary lower jaw (MNA V9483).[7] | An ichthyodectid .
|
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Pachyrhizodus | P. leptopsis | Grand Staircase–Escalante National Monument[10] | A disarticulated specimen (MNA V10651).[10] | A crossognathiform. | |
Pycnodontoidei | Genus & species undetermined | Premaxillae with dentition (MNA V10076).[7] | A pycnodont. | ||
Xiphactinus | X. sp., cf. X. audax | Fin, vertebral & skull elements.[7] | An ichthyodectid .
|
Cartilaginous fish
Cartilaginous fish reported from the Tropic Shale
| |||||
---|---|---|---|---|---|
Genus | Species | Presence | Material | Notes | Images |
Cretalamna | C. appendiculata | Teeth.[7] | A megatooth shark .
|
||
Cretoxyrhina | C. mantelli | 7 teeth.[7] | A mackerel shark .
|
||
Ptychodus | P. anonymus | 16 teeth.[7] | A ptychodontid .
|
||
P. decurrens | Vertebrae & hundreds of teeth.[7] | A ptychodontid .
|
|||
P. occidentalis | 4 teeth.[7] | A ptychodontid .
|
|||
P. sp. cf. P. mammillaris | Numerous teeth.[7] | A ptychodontid .
|
|||
P. sp. indet. | A tooth (MNA V9982).[7] | A ptychodontid. | |||
P. whipplei | Multiple teeth.[7] | A ptychodontid .
|
|||
Ptychotrygon | cf. P. sp. | Partial tooth (MNA V10097).[7] | A sawskate .
|
||
Scapanorhynchus | S. raphiodon | Teeth.[7] | A mitsukurinid. | ||
Squalicorax | S. curvatus | Multiple teeth.[7] | An anacoracid. |
Invertebrates
The Tropic Shale is known for its large invertebrate assemblage.
Genus | Species | Common Name |
---|---|---|
Callianassa | ?sp. | Mud Shrimp |
Turritella | ?sp | Gastropod |
Goniocylichna | ?sp | Gastropod |
Paleopsephaea | ?sp | Gastropod |
Toruatellaea | ?sp | Gastropod |
Preissoptera | prolabiata | Gastropod |
Mytiloides | hattini | Bivalve |
Nymphalucina | cf. linearia | Bivalve |
Solemyid | ?sp | Bivalve |
Arcoid | ?sp | Bivalve |
Inoceramus | pictus | Bivalve |
Rudistid |
Bivalve | |
Pycnodonte | newberryi | Oyster |
Prionocyclus | hyatti | Ammonite |
Collignonicras | praecox | Ammonite |
Collignonicras | woollgari | Ammonite |
Mammites | nodosoides | Ammonite |
Vascoceras | birchbyi | Ammonite |
Pseudaspidoceras | flexuosum | Ammonite |
Watinoceras | devonense | Ammonite |
Nigericeras | scotti | Ammonite |
Neocardioceras | juddii | Ammonite |
Burroceras | clydense | Ammonite |
Euomphaloceras | septemseriatum | Ammonite |
Vascoceras | diartianum | Ammonite |
Sciponoceras | gracile | Ammonite |
Paleobotany
Limited occurrences of petrified wood have been reported in the Tropic Shale. These are interpreted predominately as drift wood that settled to the bottom of the inland seaway.[12]
Paleoecology
During the late Cretaceous the Western Interior Seaway was occupied by a sea that is regressing by the Turonian. There was a brief transgression as the estuary like Dakota Formation was replaced by deeper marine shelf deposits. This transgression/regression (named the Greenhorn) cycle lasted about four million years and correlates to an oceanic anoxic event. Evidence of the change is characterized by massive deposits of calcium carbonate in the marine mudstones that can be seen in the upper third of the Tropic Shale when calcium carbonate is absent.
During the late Cretaceous widespread conditions of oceanic anoxia occurred across the Cenomanian–Turonian (C-T) stage boundary between about 94.2 and 93.5 million years ago (Oceanic Anoxic Event II, OAE II).[12] This Cenomanian–Turonian Boundary Event is reflected by one of the most extreme carbon cycle perturbations in Earth's history. Studies have been done on the marine reptiles to determine the impact of OAE II on the biodiversity of the group in the Western Interior Seaway. Results from that study seem to suggest that at least locally the OAE II had little to no effect on marine reptile diversity.[13]
Cold
References
- ^ Geolex — Unit Summary, USGS
- ^ Weishampel, et al. (2004). "Dinosaur distribution." Pp. 517-607.
- ^ Gregory, H.E. and Moore, R.C., 1931, The Kaiparowits region, a geographic and geologic reconnaissance of parts of Utah and Arizona: U.S. Geological Survey Professional Paper, 164, 161 p.
- ^ Hintze, L.F., 1988. Geologic History of Utah. Frigham Young University Geology Studies, Special Publication 7.
- ^ Obradovich, D., 1993. A Cretaceous time scale. W.G.E. Caldwell, E.G. Kauffman (Eds.), Evolution of the Western Interior Basin, Geological Association of Canada (1993), Special Paper 39 pp. 379-396
- ^ Cobban, W.A., Dyman, T.S., Pollock, G.L., Takahashi, K.I., Davis, L.E., & Riggin, D.B., 2000. Inventory of Dominantly Marine and Brackish-Water Fossils from Late Cretaceous Rocks in and near Grand Staircase–Escalante National Monument, Utah. Geology of Utah's Parks and Monuments, Utah Geological Association, 28
- ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa Albright, L.B., Gillette, D.D., Titus, A.L., 2013. Fossil vertebrates from the Tropic Shale (Upper Cretaceous), southern Utah. In: Titus, A.L., Loewen, M.A. (Eds.), At the Top of the Grand Staircaes, The Late Cretaceous of Southern Utah. Indiana University Press.
- ^ PMID 19605396.
- .
- ^ )
- ^ Titus, A.L., Roberts, E.M., & Albright, L.B., 2013. Geologic overview. In: Titus, A.L., Loewen, M.A. (Eds.), At the Top of the Grand Staircase, The Late Cretaceous of Southern Utah. Indiana University Press.
- ^ a b Dean, W.E., Kauffman, E.G. & Arthur, M.A. 2013. Accumulation of Organic Carbon-Rich Strata along the Western Margin and in the Center of the North American Western Interior Seaway during the Cenomanian-Turonian Transgression. At the top of the Grand Staircase (42-56)
- ^ Schmeisser McKean, R.L. & Gillette, D.D. 2015. Taphonomy of large marine vertebrates in the Upper Cretaceous (Cenomanian-Turonian) Tropic Shale of southern Utah. Cretaceous Research, 56(278-292)
- Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. 861 pp. ISBN 0-520-24209-2.