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″N112°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 Mesozoic geologic formation. Dinosaur remains are among the fossils that have been recovered from the formation, [2] including Nothronychus graffami . The Tropic Shale is a stratigraphic unit of the Kaiparowits Plateau of south central Utah. The Tropic Shale was first named in 1931 after the town of Tropic where the Type section is located. [3] The Tropic Shale outcrops in Kane and Garfield counties, with large sections of exposure found in the Grand Staircase–Escalante National Monument.
The Tropic Shale is predominantly marine mudstone and claystone, with several radioisotopically-dated bentonite marker beds, and occasional sandstone layers deposited during the late Cretaceous Period during the Upper Cenomanian through the Middle Turonian (95-92 Ma). The Tropic Shale has an average thickness range from 183–274 m.
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 ammonoid biozones, and the upper third that is a darker gray and non-calcareous that encompasses only one or two ammonoid biozones. Additionally the upper portion, hummocky cross stratified and turbiditic sandstone beds become more common.
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 septarian carbonate concretionary nodule horizons are characteristic of the lower and middle parts of the formation informally named as concretionary layer 1-4. The statigraphically[ check spelling ] lowest is layer one with the stratigraphically highest being layer 4. Layers 1 and 2 seem to be in isolated sections while layers 3 and 4 seem to have a wide distribution and act as marker beds between Bentonite "A" and "B". The ammonites Sciponoceras gracile and Euomphaloceras septemseriatum are commonly preserved in these concretionary nodules.
The bentonites of the Tropic Shale form erosional benches that can be easily traced throughout the formation. These bentonites have been correlated with other formations that are interpreted as part of the Western Interior Seaway. They are white to light grey when freshly exposed or can have a yellowish discoloration when weathered. The average thickness of these bentonite beds is 1–6 mm. They are organized using a lettered system (A-E) with the lowest stratigraphically positioned bentonite being "A" and the highest stratigraphically positioned bentonite being "E". Several of these bentonites have also been related to known ammonoid biozones. Bentonites "A" and "B" are associated with massive accumulations of clam fossils.
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 |
Fossils have been found throughout the entire section of the Tropic Shale. Invertebrates such as ammonites and innoceramid clams seem to dominate. Shark remains consist almost entirely of tooth remains while marine reptiles vary in preservation from isolated fragments to articulated specimens.
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]
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 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 cranium, mandible, and vertebrae (UMNH VP21800). | A plioplatecarpine. | |
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 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] | ||
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. | |||
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 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. | ||
The Tropic Shale is known for its large invertebrate assemblage. Ammonites seem to be major contributors to the ecosystem with oysters and gastropods rounding out the ecosystem. Cold hydrocarbon seeps seem to have their own invertebrate biozone located at the bottom of the formation. Rudists and solitary corals seem to be quite rare and have not been studied due to their lack of presence in the Tropic Shale as they are recorded from other formations associated with the Western Interior Seaway. [11]
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 |
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]
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 hydrocarbon seep bioherms in the lower portion of the Tropic Shale during the Cenomanian give glimpses of different ecosystems to the marine shelf deposits. These bioherms tend to be around one meter tall and up to three meters wide with large concentrations of invertebrates surrounding the seeps.
The exposed geology of the Capitol Reef area presents a record of mostly Mesozoic-aged sedimentation in an area of North America in and around Capitol Reef National Park, on the Colorado Plateau in southeastern Utah.
Polycotylidae is a family of plesiosaurs from the Cretaceous, a sister group to Leptocleididae. They are known as false pliosaurs. Polycotylids first appeared during the Albian stage of the Early Cretaceous, before becoming abundant and widespread during the early Late Cretaceous. Several species survived into the final stage of the Cretaceous, the early Maastrichtian around 72 million years ago. The possible latest surviving member Rarosaurus from the late Maastrichtian is more likely a crocodylomorph.
Dolichorhynchops is an extinct genus of polycotylid plesiosaur from the Late Cretaceous of North America, containing the species D. osborni and D. herschelensis, with two previous species having been assigned to new genera. Definitive specimens of D. osborni have been found in the late Coniacian to early Campanian rocks, while those of D. herschelensis have been found in the late Campanian to early Maastrichtian rocks. Dolichorhynchops was a prehistoric marine reptile measuring around 3 metres (9.8 ft) long. Its Greek generic name means "long-nosed face".
The Cedar Mountain Formation is the name given to a distinctive sedimentary geologic formation in eastern Utah, spanning most of the early and mid-Cretaceous. The formation was named for Cedar Mountain in northern Emery County, Utah, where William Lee Stokes first studied the exposures in 1944.
The Dakota is a sedimentary geologic unit name of formation and group rank in Midwestern North America. The Dakota units are generally composed of sandstones, mudstones, clays, and shales deposited in the Mid-Cretaceous opening of the Western Interior Seaway. The usage of the name Dakota for this particular Albian-Cenomanian strata is exceptionally widespread; from British Columbia and Alberta to Montana and Wisconsin to Colorado and Kansas to Utah and Arizona. It is famous for producing massive colorful rock formations in the Rocky Mountains and the Great Plains of the United States, and for preserving both dinosaur footprints and early deciduous tree leaves.
Terminonaris is a genus of extinct pholidosaurid crocodyliforms that lived during the Late Cretaceous epoch. The name means: “enlarged snout or nose” at the front of the skull. Terminonaris is an early crocodile, within a subgroup called Mesoeucrocodylia. Its remains have only been found in North America and Europe. Originally known under the generic name Teleorhinus, it was once believed to be a teleosaurid. Both prehistoric crocodiles such as Terminonaris, as well as modern crocodiles, belong to the same group called crocodyliformes, although modern crocodiles have specific features that indicate they are distant relatives of this species and members of the subgroup Eusuchia.
Polycotylus is a genus of plesiosaur within the family Polycotylidae. The type species is P. latippinis and was named by American paleontologist Edward Drinker Cope in 1869. Eleven other species have been identified. The name means 'much-cupped vertebrae', referring to the shape of the vertebrae. It lived in the Western Interior Seaway of North America toward the end of the Cretaceous. One fossil preserves an adult with a single large fetus inside of it, indicating that Polycotylus gave live birth, an unusual adaptation among reptiles.
Eopolycotylus is a genus of polycotylid plesiosaur known from the Cenomanian-age Tropic Shale of Utah.
Palmulasaurus is a genus of polycotylid plesiosaur from the Turonian Tropic Shale of Utah. It was originally described as Palmula, but the name was occupied by a genus of Cretaceous foraminifer first described in 1833.
The Tres Hermanos Formation is a geologic formation in central and west-central New Mexico. It contains fossils characteristic of the Turonian Age of the late Cretaceous.
The Mowry Shale is an Early Cretaceous geologic formation. The formation was named for Mowrie Creek, northwest of Buffalo in Johnson County, Wyoming.
The Straight Cliffs Formation is a stratigraphic unit in the Kaiparowits Plateau of south central Utah. It is Late Cretaceous in age and contains fluvial, paralic, and marginal marine (shoreline) siliciclastic strata. It is well exposed around the margin of the Kaiparowits Plateau in the Grand Staircase – Escalante National Monument in south central Utah. The formation is named after the Straight Cliffs, a long band of cliffs creating the topographic feature Fiftymile Mountain.
Palmula is an extinct genus of foraminifera which is known from a number of species found in rocks dating from near the beginning of the Jurassic to the end of the Cretaceous, in Africa, Asia, Europe, and New Zealand. A genus of polycotylid plesiosaur was named Palmula in 2007, but because the name was already in use, the plesiosaur was renamed, becoming Palmulasaurus.
Megacephalosaurus is an extinct genus of short-necked pliosaur that inhabited the Western Interior Seaway of North America about 94 to 93 million years ago during the Turonian stage of the Late Cretaceous, containing the single species M. eulerti. It is named after its large head, which is the largest of any plesiosaur in the continent and measures up to 1.75 meters (5.7 ft) in length. Megacephalosaurus was one of the largest marine reptiles of its time with an estimated length of 6–9 meters (20–30 ft). Its long snout and consistently sized teeth suggest that it preferred a diet of smaller-sized prey.
The Graneros Shale is a geologic formation in the United States identified in the Great Plains as well as New Mexico that dates to the Cenomanian Age of the Cretaceous Period. It is defined as the finely sandy argillaceous or clayey near-shore/marginal-marine shale that lies above the older, non-marine Dakota sand and mud, but below the younger, chalky open-marine shale of the Greenhorn. This definition was made in Colorado by G. K. Gilbert and has been adopted in other states that use Gilbert's division of the Benton's shales into Carlile, Greenhorn, and Graneros. These states include Kansas, Texas, Oklahoma, Nebraska, and New Mexico as well as corners of Minnesota and Iowa. North Dakota, South Dakota, Wyoming, and Montana have somewhat different usages — in particular, north and west of the Black Hills, the same rock and fossil layer is named Belle Fourche Shale.
The Greenhorn Limestone or Greenhorn Formation is a geologic formation in the Great Plains Region of the United States, dating to the Cenomanian and Turonian ages of the Late Cretaceous period. The formation gives its name to the Greenhorn cycle of the Western Interior Seaway.
The Thermopolis Shale is a geologic formation which formed in west-central North America in the Albian age of the Late Cretaceous period. Surface outcroppings occur in central Canada, and the U.S. states of Montana and Wyoming. The rock formation was laid down over about 7 million years by sediment flowing into the Western Interior Seaway. The formation's boundaries and members are not well-defined by geologists, which has led to different definitions of the formation. Some geologists conclude the formation should not have a designation independent of the formations above and below it. A range of invertebrate and small and large vertebrate fossils and coprolites are found in the formation.
The Favel Formation is a stratigraphic unit of Late Cretaceous age. It is present in southern Manitoba and southeastern Saskatchewan, and consists primarily of calcareous shale. It was named for the Favel River near Minitonas, Manitoba, by R.T.D. Wickenden in 1945.
Juana Lopez refers to both the uppermost member of the Carlile Shale formation and to the environment that caused it to form. The Juana Lopez Member is calcareous sandstone dated to the Turonian age of the Upper Cretaceous and is exposed in the southern and western Colorado, northern and central New Mexico, and northeastern Utah. The unit has been described as "the most enigmatic" member of the Carlile Shale.
Scalamagnus is an extinct genus of polycotylid plesiosaur from the Late Cretaceous Tropic Shale Formation of the United States. The genus contains a single species S. tropicensis, known from a skull and two partial skeletons. Scalamagnus was historically considered to represent a species of the genus Dolichorhynchops before it was moved to its own genus.
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