Wheeler Shale

The Wheeler Shale, often called the Wheeler Formation, is a rock formation in the U.S. state of Utah. It originated in the mid- Cambrian period about 507 million years and contains a concentrate reservoir with exceptionally well-preserved fossils.

World fame of the Wheeler Shale by finds excellent preserved trilobites, for example Elrathia kingii ( State Fossil of Utah), as well as various representatives of Agnostid. Organisms without mineral skeleton, about Naraoia, Wiwaxia and Hallucigenia are obtained in Wheeler Shale. The maintenance mode ( carbon-rich, film-like coating ) of these " soft-bodied fauna " recalls the Konservatlagerstätten of Burgess Shale - type.

Naming

The Wheeler Shale was named in 1908 by Charles Walcott after its type locality, the Wheeler Amphitheater, a semi-circular depression in the terrain north-eastern House Range. The Wheeler Amphitheater in turn, had obtained as many other places of the American Southwest named in honor of the officer and surveyor George Montague Wheeler.

Occurrence and geological framework

The Wheeler Formation in western Utah extends from the ridge of the House Range ( in Millard County) about 40 km to the northeast to the Drum Mountains ( in Juab County). The Drum Mountains perform similar fossils with similar condition. A smaller deposits is still in the Canyon Mountains Zentralutahs. In the aforementioned ridges are prominent clumps belonging geologically to the eastern Basin and Range. Add them now occur otherwise covered Cambrian sedimentary sequences largely to days. Through the Neogene extensional tectonics of the layers association was tilted in part - in the House Range, for example, the sediments therefore usually underline North to northeast and fall flat (10 to 20 ° ) to the east.

Facies

The sediments of the Wheeler Shales are tropical marine origin and were at about 300 kilometers wide, passive continental margin Laurentias in a bay of the House Range Embayment (HRE ) is deposited. The gradually increasing by the subsidence of the continental margin sea level is documented by sinking δ13C values. The relatively oxygen-poor basins of the House Range Embayment was very likely traced out by faults and was very close to the wide Laurentide Karbonatschelfs. In the Wheeler Shale are therefore simultaneously encountered several sedimentary facies: In the Drum Mountains, lay their deposition area relatively close to the shelf edge, a carbonate-rich shelf and Schelfrandfazies came to be deposited, whereas in the House Range dominates the basinal.

Lithology and stratigraphy

The usually olive to dark -colored Wheeler Shale consists mostly of homogeneous, fine-grained shales ( black shale sediments with grain sizes below 50 microns and a carbonate content of 17-47 percent by weight), can be switched on in the dünnbankige Karbonatlagen. He came below the wave base as low -energy sediment under anoxic conditions for deposition. The Karbonatkomponente can be explained by the nearby shelf margin.

The profile of the type locality in the House Range consists of heterogeneous deposits of calcareous clay, Mergelkalken, mudstones and dünnbankigen Solnhofen. In the Drum Mountains are found as shelf basin sediments patchy, thin-layer Sparite, the open shelf area rhythmites and in subsequent lower slopes mudstones.

On Marjum pass 39.25- 113.33333333333Koordinaten: 39 ° 15 '0 "N, 113 ° 20' 0 " W in the House Range of Wheeler Shale reaches a thickness of 190 meters, in the 40 kilometers to the north-east located Drum Mountains rise to the cardinality to 306 meters. Stratigraphically it follows the resistant limestone formation of the Swasey Limestones, of which he clearly sets itself apart with its much gentler terrain. Together with the overlying Marjum formation and the subsequent lower Weeks lineup he represents one of the most powerful, well digested and richest fossil mittelkambrischen sequences of North America

Biostratigraphically the Wheeler Shale is within the Laurentide Trilobitenzone Bolaspidella. In him also documented 62 meters above its base, the first occurrence of the trilobite Acidusus atavus ( Ptychagnostus atavus ), which replaces Ptychagnostus gibbus (start of Drumiums ). Tens of meters higher a negative carbon isotope anomaly associated with a Transgressionshöchststand.

Fauna

The discovered fauna is composed of algae, annelids, arthropods diverse (eg Anomalocariden, Emeraldella, Phyllocariden ), brachiopods, Carpoiden, Chan Cello Riden, Eocrinoideen, Priapuliden, jellyfish, sponges, primitive echinoderms, trilobites and Trilobitenähnlichen.

Rare microfossils are acritarchs, conodonts and radiolarians.

Taphonomy and biofacies

Strangely, contact Hart Schaler as trilobites and organisms with soft body preservation never together in the same layer auf.Gaines and Droser (2005) explain this fact with the oxygen content of the sea water near the ground and with the degree of bioturbation. Thus, the soft-bodied preservation took place in deeper, anaerobic pelvic sections with minimal bioturbation. The hard Schaler, however, are found in higher dysaeroben areas that were more ransacked. The occurrence of Elrathia kingi (i ) denotes the transition. An exception in this scheme represent the Agnostid that can occur anywhere as planktonic organisms.

Specifically, the following biofacies can be distinguished (in order of increasing water depth or decreasing oxygen content or reduced Durchwühlung ):

Fauna ordered by tribes

Kingdom Protista ( algae)

  • Margaretia dorus
  • Marpolia spissa - cyanobacterium
  • Morania fragmenta - cyanobacterium
  • Yuknessia simplex - green algae?

Master Arthropoda ( Arthropods )

  • Branchiocaris pretiosa - Crustacea
  • Branchiocaris sp.
  • Canadaspis cf perfecta
  • Dicerocaris opisthoeces
  • Pahvantia hastata
  • Perspicaris dilatus
  • Proboscicaris agnosta
  • Pseudoarctolepis sharpi - Phyllocarida
  • Tuzoia? peterseni
  • Cambropodus gracilis - Uniramia

Master Arthropoda - Arachnomorpha

  • Alalcomenaeus cf cambrius
  • Dicranocaris guntherorum
  • Molli Sonia symmetrica
  • Naraoia
  • Sydneyia sp.

Master Arthropoda - Class Trilobita

  • Baltagnostus eurypyx - order Agnostida
  • Hypagnostus parvifrans
  • Peronopsis amplaxis
  • Peronopsis bidens
  • Peronopsis fallax
  • Peronopsis gaspensis
  • Peronopsis intermedius
  • Peronopsis interstrictus
  • Peronopsis montis
  • Peronopsis segmentis
  • Ptychagnostus atavus = Acidusus atavus
  • Ptychagnostus germanus
  • Ptychagnostus gibbus
  • Ptychagnostus intermedius
  • Ptychagnostus michaeli
  • Ptychagnostus occultatus
  • Ptychagnostus seminula
  • Glyphaspis concavus - order Asaphida
  • Bathyuriscus fimbriatus - order Corynexochida
  • Kootenia species - very rare!
  • Olenoides expansus
  • Olenoides nevadensis
  • Olenoides serratus
  • Tonkinella breviceps
  • Zacanthoides divergens
  • Zacanthoides sp.
  • Altiocculus harrisi - order Ptychopariida
  • Alokistocare sp.
  • Asaphiscus wheeleri
  • Bathyocos housensis
  • Bathyuriscus sp.
  • Bolaspidella drumensis
  • Bolaspidella housensis
  • Bolaspidella sp.
  • Bolaspidella wellsvillensis
  • Brachyaspidion microps
  • Brachyaspidion sulcatum
  • Cedaria minor
  • Elrathia kingi ( i)
  • Elrathia sp.
  • Elrathina wheeleri = Ptychoparella wheeleri
  • Jenkinsonia varga
  • Modocia brevispina
  • Modocia laevinucha
  • Modocia typicalis
  • Ptychoparella sp.
  • Ptychoparella wheeleri
  • Spencella sp.

Master Brachiopoda ( brachiopods )

  • Acrothele subsidua

Phyla Chordata ( chordates )

  • Hertzina - Conodonta

Master Cnidaria ( cnidarians )

  • Cambromedusa

Master Lobopodia ( Lobopoden )

Master Mollusca ( molluscs)

  • Pelagiella

Master Echinodermata ( echinoderms )

  • Castericystis sp. - Carpo Idea
  • Castericystis sprinklei
  • Cothurnocystis sp.
  • Ctenocystis sp. - Ctenocystoidea
  • Gogia spiralis - Eocrinoidea

Phylum Porifera (sponges )

  • Chancelloria pentacta - Chancelloriidae
  • Choia carteri
  • Choia utahensis
  • Crumillospongia
  • Diagonella

Master Priapulida ( Priapswürmer )

  • Selkirkia sp.
  • Selkirkia willoughbyi

Taxa with uncertain assignment

  • Anomalocaris nathorsti - Anomalocaride
  • Anomalocaris sp.
  • Eldonia
  • Hylolithellus sp - worm -like
  • Wiwaxia

Gallery

Model of Anomalocaris

Aysheaia

Branchiocaris

Hallucigenia

Ottoia

Wiwaxia

Millard County, Utah Juab County, Utah Basin and Range Province Claystone Bed (geology) Anoxic waters Marl Folk classification Drumian Crustacean Emu Bay Shale Sirius Passet Digital Object Identifier
819109
de