Moenkopi formation

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Layers of the Moenkopi Formation, Zion National Park , Utah.

The Moenkopi Formation is a rock formation ( formation of sedimentary origin) that can be found in large parts of the US states of New Mexico , Arizona (north), Nevada , California (southeast), Utah and Colorado (west). In Arizona, the Rock Association is granted group status. Geologically, the Moenkopi Formation belongs to the Colorado Plateau and the Basin and Range . Their red-colored shales and sandstones were deposited during the Lower Triassic and possibly even during the Middle Triassic around 240 million years ago.

stratigraphy

Shift members (members)

The Moenkopi Formation is divided into a myriad of members (in alphabetical order) because of its widespread distribution and its different facial formation:

Local profiles

For a better illustration of the complex stratigraphic relationships, some local profiles follow (sequence from young to old):

Northern Utah ( Uinta Basin ):

  • Pariott member
  • Sewemup member
  • Ali Baba member
  • Tenderfoot member

Southeast Utah ( Canyonlands ) - 300 meters:

  • Moody Canyon member
  • Torrey member
  • Sinbad Limestone Member
  • Black Dragon member
  • Hoskinnini member

Northeastern Arizona (near Holbrook ) - 100 to 150 meters:

  • Holbrook member (identical to the Anton Chico member in New Mexico)
  • Moqui member
  • Wupatki member (upper part identical to Shnabkaib member in SW-Utah)

Southwest Utah ( Virgin River ) - 700 meters:

  • Upper Red Member
  • Shnabkaib member
  • Middle Red Member
  • Virgin Limestone Member
  • Lower Red Members
  • Timpoweap member

Note: The Timpoweap member will lately cease to exist as it is identical to the Rock Canyon Conglomerate member and the subsequent Sinbad Limestone member.

The southwest of Utah is a facial transitional area in which the marine limestone layers of the Thaynes Formation or Thaynes Group interlock with the continental red sediments of the actual Moenkopi Formation. The Thaynes group includes the Sinbad Limestone Member, the Virgin Limestone Member and the Shnabkaib Member.

Equivalent formations

The predominantly siliciclastic Moenkopi Formation is equivalent to the following formations:

History

The Moenkopi Formation has no type locality . Its first describer Ward named it in 1901 after a settlement at the mouth of the "Moencopie Wash" in the Grand Canyon area. As a replacement, Gregory managed in 1917 with a profile in the rock wall of Little Colorado Canyon , about 8 kilometers below Tanner Crossing in Coconino County .

During their stay in the Great Basin in 1921, Bassler and Reeside divided the formation for the first time, they eliminated the Rock Canyon Conglomerate Member , the Virgin Limestone Member and the Shnabkaib Member . In 1922, Hager was able to separate the Salt Creek Member (later replaced by the Wupatki Member and the Moqui Member ) and the Holbrook Sandstone Member in the Black Mesa Basin . In 1928 , James Gilluly and John Bernard Reeside established the Sinbad Limestone Member in the Paradox Basin . Gregory first described the Timpoweap member in 1948 . The Wupatki member was introduced by McKee in 1951, who also carried out an age revision on the lower and middle Triassic.

Layer boundaries were first revised by Robeck in 1956 and Cooley in 1958. Shoemaker and Newmann were then able to separate the Tenderfoot Member , the Ali Baba Member , the Sewemup Member and the Pariott Member in the Piceance Basin and Uinta Basin in 1959 . The Hoskinnini member was introduced by Stewart in 1959 to the Black Mesa Basin and the Paradox Basin .

In 1966, Schell and Ellis Yochelson undertook another revision of the stratum boundaries. In 1974 Blakey named the Black Dragon Member , the Torrey Member and the Moody Canyon Member in the Paradox Basin . In 1979 there was another revision of the stratum boundaries. In 1988 Kietzke confirmed a Lower or Middle Triassic age using biostratigraphic methods. In 1989 the Anton Chico member was finally introduced by Lucas and Hunt in the Palo Duro Basin , at the same time they revised the geographical expansion. Lucas and Hayden also worked on the geographical conditions in 1991. Summarizing overviews were given by Lucas in 1991, Sprinkel in 1994, Hintze and Axen in 1995, and then Huntoon et al. a. released.

General overview

The deposit area of ​​the Moenkopi Formation was a very extensive sedimentation basin that stretched from Wyoming, Montana and Idaho through Colorado and Utah to New Mexico, Arizona and Nevada. It was a ramp-shaped craton rim that gradually descended to the northwest. It had a length of 650 kilometers and was between 160 and 500 kilometers wide. An inland sea transgressed overhead, at its peak it reached from southern Utah to British Columbia . In three larger pulses, it interlocked with continental siliciclastic red sediments, whose areas of origin were high areas such as the Uncompahgre Plateau , the Defiance Uplift , the Mogollon Rim and a tectonic barrier in western Nevada, from which northwest-flowing rivers carried the sediment load into deeper basins .

The formation generally shows discordant contact with its Paleozoic subsurface, for example it reaches down to the Coconino Sandstone in northeastern Arizona , in the Grand Canyon to the Kaibab Limestone and in the Canyonlands to the Cutler Formation . It ends with an erosive discordance and is in turn covered by the Shinarump Conglomerate of the Chinle Formation . The thicknesses of the Moenkopi Formation vary greatly, in the east it is only very thin (around 100 meters), towards the west it grows gradually to reach up to 700 meters on its west and north-west edge. Analogous to the fluctuations in thickness, there are also facial changes. In the eastern part of its deposit area, the Moenkopi Formation has a fluvial character, in the central part it becomes increasingly intertidal to tidal, and finally takes on a completely marine character on the western edge. For example, in the northwest and west it changes into the cephalopod- bearing Thaynes Formation .

Age

On the basis of the fossil content, the Moenkopi Formation can be assigned a Lower Triassic age, which includes the stages Indusian , Olenekian and Lower Anisian , i. H. roughly the period from 250 to 245 million years BP. Their vertebrate fauna and especially their trace fossils point to an equivalence with the German Middle and Upper Buntsandstein ( Volpriehausen Formation ) and sediments from the eastern massif of Central France.

Occurrence

The Moenkopi Formation occurs in the following sedimentation spaces:

It can also be found in the following national parks:

Furthermore in:

Fossil content

Eocyclotosaurus
Arizonasaurus babbitti

A very diversified vertebrate fauna originates from the Moenkopi formation , in particular the Wupatki-Member and the Holbrook-Member in northeastern Arizona are known for their rich vertebrate finds. Among the invertebrates to be mentioned are ammonites , conchostraca , gastropods , tubefoots , ostracods , echinoderms , stromatolites and spirorbid worms. Charophyta , green algae (Codiaceae) and remains of plants are also present.

The basal vertebrate groups include Hybodontiformes ( shark-like ), coelacanth and lungfish . Temnospondyle amphibians are also very common. Among the Temnospondyli there are Eocyclotosaurus , Quasicyclotosaurus , Wellesaurus , Vigilius and Cosgriffius . Rhynchosaurs also occur occasionally . Anisodontosaurus is a puzzling reptile of which only a few tooth-encrusted jaw fragments are present. An almost complete skull and other isolated bone remains are available from Arizonasaurus , a poposaurid archosaur .

The finds in detail:

Vertebrates :

Archosaurs

Temnospondyli

Invertebrates

Ammonites :

Ostracods :

Trace fossils

The Moenkopi Formation is very rich in trace fossils. In addition to numerous vertebrate animal tracks, there are also traces of invertebrates such as horseshoe crabs . It is also worth mentioning the presence of coprolites .

From Dicynodontiern next few bone fragments and their footprints were discovered. Traces of Chirotherium and Rhynchosauroides are particularly common in Wupatki members .

The following Ichnotaxa occur in the Moenkopi Formation:

swell

Individual evidence

  1. ↑ a b GEOLEX database
  2. ↑ Lucas, SG, Krainer, K. and Milner, ARC (2007). The type section and age of the Timpoweap Member and stratigraphic nomenclature of the Triassic Moenkopi Group in southwestern Utah.
    New Mexico Museum of Natural History and Science Bulletin 40     PDF file; 6.32 MB
  3. ↑ For the entire section: GEOLEX database, bibliography
  4. ^ Blakey, RC 1974. Stratigraphic and depositional analysis of the Moenkopi Formation, southeastern Utah: Utah Geological and Mineralogical Survey, Bulletin 104, 81 p.
  5. ↑ Paull, RK and Paull, RA, 1994, Lower Triassic transgressive-regressive sequences in the Rocky Mountains, eastern Great Basin and Colorado Plateau, USA; in Caputo, MV, Peterson, JA and Franczyk, KJ, eds., Mesozoic systems of the Rocky Mountain region, USA: Denver: Rocky Mountain Section, Society for Sedimentary Geology, p. 169-180.
  6. ↑ Lucas, SG, Goodspeed, TH and Estep, JW (2007). Ammonoid biostratigraphy of the Lower Triassic Sinbad Formation, east-central Utah. New Mexico Museum of Natural History and Science Bulletin 40 (PDF; 6.4 MB)
  7. ^ Demathieu, GR (1985). Trace Fossil Assemblages in Middle Triassic Marginal Marine Deposits, Eastern Border of the Massif Central, France. In: Biogenic Structures: Their Use in Interpreting Depositional Environments. Curran, HA ed. SEPM Special Publication No. 35

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