Thursday, December 5, 2013

Early Triassic Moenkopi/Thaynes - Ammonoid Biostratigraphy, Correlation and Paleogeography

Smithian ammonoid faunas from Utah: implications for Early Triassic biostratigraphy, correlation and basinal paleogeography

Arnaud Brayard, Kevin G. Bylund, James F. Jenks, Daniel A. Stephen, Nicolas Olivier, Gilles Escarguel, Emmanuel Fara, Emmanuelle Vennin, Swiss Journal of Palaeontology, 2013, 132:141-219


Intensive sampling of the lower portion of the Thaynes and Moenkopi Groups (Lower Triassic) at separate localities within the Confusion Range, Pahvant Range, Mineral Mountains, Star Range, Kanarraville, Cedar City, Torrey and San Rafael Swell areas (mainly central and southern Utah, USA) leads to the recognition of a new key regional Smithian ammonoid succession. The new biostratigraphical sequence, which is more precise than the long-recognized Meekoceras gracilitatis and Anasibirites kingianus Zones, comprises twelve subdivisions, thus resulting in a sequence with much higher resolution that can be correlated not only with other western USA sites, but also with major worldwide localities as well. Middle and late Smithian faunas contain many taxa with wide geographic distribution, thus enabling long-distance correlation with faunal successions from other regions (e.g., British Columbia, Canadian Arctic, South China, Spiti and Oman). New assemblages from the lowermost beds are the least diversified and poorest preserved; they represent the earliest early/middle Smithian ammonoid faunas reported from the western North American basin. They highlight (a) the sudden Smithian advancement of the marine transgression within this epicontinental sea, (b) that this event is diachronous, and (c) that the paleotopography of the basin most likely was highly irregular. The newly obtained ammonoid succession also allows us to date and follow the transgression from the northern and central part of the basin to the southwesternmost and southeasternmost parts, which were reached during the late Smithian (Anasibirites kingianus beds). In addition, we briefly discuss the now-limited previous regional biozonation in the light of these new results. One new genus (Minersvillites) and nine new species (Kashmirites utahensis, Kashmirites confusionensis, Kashmirites stepheni, ?Xiaoqiaoceras americanum, Minersvillites farai, Inyoites beaverensis, Meekoceras olivieri, Meekoceras millardense, Vercherites undulatus) are also described.

The ammonoids are mostly all the old Early Triassic ones described by C.A.White from collections made by the early geologic surveys of the american west, and by J.P.Smith in the early part of the last century.  A more precise and revised biostratigraphy is proposed.  Below is one of the newly described ammonoids, this one from the Minersville area, one of the earliest Smithian ammonoids found in the western USA.
Vercherites undulatus Brayard Bylund and Jenks 2013
The rocks we have been working in represent the Permian and Early Triassic transition beds, usually some Permian limestones followed by an erosive surface, conglomerate, then a transgressive sequence with marginal marine redbeds, inner shelf, outer shelf and basinal deposits following.
Moenkopi and Thaynes Groups as exposed near Minersville

Permian rocks at the bottom, then the red Black Dragon Formation, and capped with the Sinbad Formation.  As exposed just west of Capitol Reef National Park.
     As you can see, some very wonderful country to explore and work in.  More papers are in the works covering the paleontology and geology of these rocks, I'm really looking forward to the adventure.