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22-626: 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

44-700: A further increase to the upper limit for the matrix size in order to bring it into line with the upper limit for silt (63 μm). On December 13, 2016, NASA reported further evidence supporting habitability on the planet Mars as the Curiosity rover climbed higher, studying younger layers, on Mount Sharp . Also reported, the very soluble element boron was detected for the first time on Mars. In June 2018 , NASA reported that Curiosity had detected kerogen and other complex organic compounds from mudstone rocks approximately 3.5 billion years old. Turtle Too Many Requests If you report this error to

66-414: 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: Fossils have been found throughout the entire section of

88-491: A variety of accessory minerals . In the Dunham classification (Dunham, 1962 ) system of limestones , a mudstone is defined as a mud-supported carbonate rock that contains less than 10% grains. Most recently, this definition has been clarified as a matrix-supported carbonate-dominated rock composed of more than 90% carbonate mud (<63 μm) component . A recent study by Lokier and Al Junaibi (2016) has highlighted that

110-547: 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). This Cenomanian–Turonian Boundary Event

132-412: Is clay particles. Another definition is that mudstone is a sedimentary rock in which neither silt, clay, nor coarser grains is predominant. Rock of this composition that does show laminations or fissility is sometimes described as mudshale rather than mudstone . The lack of fissility or layering in mudstone may be due to either original texture or the disruption of layering by burrowing organisms in

154-473: Is distinguished from shale by its lack of fissility . The term mudstone is also used to describe carbonate rocks ( limestone or dolomite ) that are composed predominantly of carbonate mud. However, in most contexts, the term refers to siliciclastic mudstone, composed mostly of silicate minerals. The NASA Curiosity rover has found deposits of mudstone on Mars that contain organic substances such as propane , benzene and toluene . There

176-441: 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

198-465: 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. 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

220-556: Is not a single definition of mudstone that has gained general acceptance, though there is wide agreement that mudstones are fine-grained sedimentary rocks, composed mostly of silicate grains with a grain size less than 0.063 millimetres (0.0025 in). Individual grains this size are too small to be distinguished without a microscope, which means that most classifications emphasize texture rather than mineral composition, and mudstones have historically received less attention from petrologists than have sandstones . The simplest definition

242-580: 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. Cold hydrocarbon seep bioherms in

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264-420: Is that a mudstone is a fine-grained clastic sedimentary rock that is not laminated or fissile . Most definitions also include a requirement that the rock contain significant amounts of both silt- and clay-sized grains. One common requirement is that a mudstone is a mudrock (a rock containing more than 50% silt- to clay-sized particles) in which between a third and two-thirds of the mud (silt and clay) fraction

286-684: 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 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

308-482: 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

330-405: 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

352-523: 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

374-406: 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. 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

396-519: The inland seaway. 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

418-480: 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. Mudstone Mudstone , a type of mudrock , is a fine-grained sedimentary rock whose original constituents were clays or muds . Mudstone

440-454: The most common problems encountered when describing a mudstone is to incorrectly estimate the volume of 'grains' in the sample - in consequence, misidentifying mudstone as wackestone and vice versa. The original Dunham classification (1962) defined the matrix as clay and fine-silt size sediment <20 μm in diameter. This definition was redefined by Embry & Klovan (1971 ) to a grain size of less than or equal to 30 μm. Wright (1992 ) proposed

462-486: The sediment prior to lithification . Mudstone looks like hardened clay and, depending upon the circumstances under which it was formed, it may show cracks or fissures, like a sun-baked clay deposit. When the mineral composition of mudstones has been determined, using such techniques as scanning electron microscopy , electron probe microanalysis , or X-ray diffraction analysis , they have been found to be composed primarily of clay minerals , quartz , and feldspars , with

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484-969: 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, 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

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