Моэнкапи формациясы: геологиялық құрылым және қабаттары

Geologic formation in the southwestern United States

The Moenkopi Formation is a geological formation that is spread across the U. S. states of New Mexico, northern Arizona, Nevada, southeastern California, eastern Utah and western Colorado. This unit is considered to be a group in Arizona. Part of the Colorado Plateau and Basin and Range, this red sandstone was laid down in the Lower Triassic and possibly part of the Middle Triassic, around 240 million years ago.

There is no designated type locality for this formation. It was named for a development at the mouth of Moencopie Wash in the Grand Canyon area by Ward in 1901. In 1917 a 'substitute' type locality was located by Gregory in the wall of the Little Colorado Canyon, about 5 miles below Tanner Crossing in Coconino County, Arizona. While in the Great Basin, Bassler and Reeside characterized and named the Rock Canyon Conglomerate, Virgin Limestone, and Shnabkaib Shale members in 1921. Salt Creek (later replaced by Wupatki and Moqui Members) and the Holbrook Member were found and named in the Black Mesa basin by Hager in 1922. The Sinbad Limestone Member was named in the Paradox Basin by Gilluly and Reeside in 1928. Williams and Gregory named the Timpoweap Member in the Plateau sedimentary province in 1947. The Wupatki Member was first used in the Plateau Sedimentary Province and its age was modified to Early and Middle(?) Triassic by McKee in 1951. Contacts were revised by Robeck in 1956 and Cooley in 1958. The Tenderfoot, Ali Baba, Sewemup, and Pariott Members were named in the Piceance and Uinta Basins by Shoemaker and Newman in 1959. The Hoskinnini Member was assigned in the Black Mesa and Paradox basins by Stewart in 1959. Contacts were revised again by Schell and Yochelson in 1966. Blakey named the Black Dragon, Torrey, and Moody Canyon members in the Paradox Basin and Plateau Sedimentary Province in 1974. Contacts were revised yet again by Welsh and others in 1979. Kietzke modified the age to Early and Middle Triassic using biostratigraphic dating in 1988. The Anton Chico Member was assigned in the Palo Duro Basin and areal limits set by Lucas and Hunt in 1989. In 1991 areal limits were set again by Lucas and Hayden. An overview was completed by Lucas in 1991, Sprinkel in 1994, Hintze and Axen in 1995 and later, Huntoon and others.

The Moenkopi consists of thinly bedded sandstone, mudstone, and shale, with some limestone in the Capitol Reef area. It has a characteristic deep red color and tends to form slopes and benches. The depositional environment varies from fluvial channel and floodplain deposits in the eastern exposures to tidal mudflats in the Cedar Mesa area to deltaic sandstones and shallow marine limestones at Capitol Reef. In eastern Nevada and northwestern Utah, it thickens dramatically, then transitions to the Woodside, Thaynes, and Mahogany formations. The general deposition setting was sluggish rivers traversing a flat, featureless coastal plain to the sea. The low relief meant that the shoreline moved great distances with changes of sea level or even with the tides. Thickness varies from a feather edge against the Uncompahgre highlands to the east to over in southwestern Utah. The thickness varies greatly in the Paradox Basin, where the Moenkopi is thin to nonexistent on the crests of salt anticlines and over thick in the corresponding synclines. The Moenkopi rests unconformably on Paleozoic beds and the Chinle Formation in turn rests unconformably on the Moenkopi. Both unconformities are locally angular unconformities. The lower unconformity corresponds to the regional Tr 1 unconformity and the upper to the regional Tr 3 unconformity. The Tr 1 unconformity represents a hiatus of at least 20 million years while Tr 2 represents a hiatus of about 10 million years.

Pteriomorphs reported from the Moenkopi Formation Genus Species Location Member Material Notes ImagesBakevelliaB. cf. exporrectaSouth western Utah Virgin Limestone MemberNumerous specimens. B. costataSouth western Utah Virgin Limestone MemberNumerous left valves. The absence of right valves may be caused by the "hydrodynamic separation of the differentially vaulted valves or to weaker calcification". EumorphotisE. venetianaSouth western Utah Virgin Limestone MemberOne left valve with shell over 18.3 mm long and 21.7 mm high. E. ericiusSouth western Utah Virgin Limestone MemberSeveral specimens with shells. The Anisian Pseudomonotis beneckei is a possibly related species. E. virginensisSouth western Utah Virgin Limestone MemberSeveral fragmental specimens. E. cf. multiformisSouth western Utah Virgin Limestone MemberOne incomplete left valve. LeptochondriaL. nuetzeliSouth western Utah Virgin Limestone Member"One articulated specimen and approximately 50 isolated left valves", all with external shell layer preserved. L. curtocardinalisSouth western Utah Virgin Limestone MemberSeveral specimens. ModiolusM. sp. South western Utah Virgin Limestone MemberOne right valve (USNM 543477) 6.5 mm long and 4.3 mm high. Shell interior is unknown. USNM 543477 have similarities with "Modiolus" sp. from the late Griesbachian of Russia, and Promytilus homevalensis from the early Permian of Australia. ParallelodonP.? aff. beyrichiiSouth western Utah Virgin Limestone MemberTwo right valves. PromyalinaP. putiatinensisSouth western Utah Virgin Limestone MemberNumerous medium sized specimens, mostly with outer shell layer but eroded to varying degrees. PernopectenP.? sp. South western Utah Virgin Limestone MemberOne left valve 27 mm long and 27.6 mm high.

Heteroconchs reported from the Moenkopi Formation Genus Species Location Member Material Notes ImagesArcomyaA.? sp. South western Utah Virgin Limestone MemberTwo steinkerns. HeminajasH.? cf. balatonisSouth western Utah Virgin Limestone MemberFew specimens. Possibly belong to Neoschizodus thaynesianus. MyoconchaM. cf. planaSouth western Utah Virgin Limestone MemberSeveral specimens. Slightly differs from the holotype by greater divergence of the dorsal and ventral margins. NeoschizodusN. laevigatusSouth western Utah Virgin Limestone MemberOne left valve. N. praeorbicularisSouth western Utah Virgin Limestone MemberOne steinkern. PermophorusP. triassicusSouth western Utah Virgin Limestone MemberSeveral specimens (steinkerns). PleuromyaP. primaSouth western Utah Virgin Limestone MemberOne steinkern. Tre earliest specimen of the genus. ProtopisP.? aff. waageniSouth western Utah Virgin Limestone MemberOne left valve over 10 mm long and 10 mm high. SementiconchaS. recuperatorSouth western Utah Virgin Limestone Member119 steinkerns. TrigonodusT. cf. orientalisSouth western Utah Virgin Limestone MemberSeveral specimens. T. cf. sandbergeriSouth western Utah Virgin Limestone MemberSeveral specimens. UnicaridumU.? sp. South western Utah Virgin Limestone MemberOne left valve. Assigned to Unicaridum due to subelliptical shape and slightly concave central part of the umbo. UnionitesU.? fassaensisSouth western Utah Virgin Limestone MemberThree steinkerns. U.? canalensisSouth western Utah Virgin Limestone MemberFour steinkerns. U.? cf. borealisSouth western Utah Virgin Limestone MemberNumerous specimens. Specimens look similar to the Griesbachian U. borealis from East Greenland, but the type material is poorly preserved, which makes comparison with published specimens difficult.

A diverse fossil vertebrate fauna has been described from the Moenkopi Formation, mainly from the Wupatki Member and Holbrook Member of northern Arizona. Described basal vertebrates include freshwater hybodont sharks, coelacanths, and lungfish. Temnospondyl amphibians are a common component of the fauna. Temnospondyli include Eocyclotosaurus, Quasicyclotosaurus, Wellesaurus, Vigilius, and Cosgriffius. The rhynchosaur Ammorhynchus is known, but rare. Anisodontosaurus is an enigmatic reptile only known from a few tooth bearing jaws. The poposauroid archosaur Arizonasaurus is known from one relatively complete skeleton and a significant amount of other isolated material. Footprints and several fragmentary body fossils are known from dicynodonts. The footprints of Cheirotherium and Rhynchosauroides are common in the Wupatki Member.