Discussion

Time:

about 248 million years ago, Anasibirites Beds (Tardus zone), Late Smithian Stage, Early Triassic Period,  Mesozoic Era



Place:

The West Coast of North America in what is presently West Central Utah.

Map from Dr. Ron Blakey


Outcrop:

Rocks of the Thaynes Formation are some of the only remaining mesozoic rocks preserved in western Utah.  They occur in a syncline formed during later mesozoic mountain building (tectonic) events.  The rocks under consideration outcrop in the central part of the Confusion Range, around Cowboy Pass on Old US Highway 6, in an area about 15 Km north-south, by 4 Km east-west, or about 102 square Km.  Everywhere in the study area the Anasibirites beds outcrop, they have the same general thickness and fossil content.
Thaynes Formation Outcrop
Outcrop of the Anasibirites Beds in Western Utah.
The base of the anasibirites beds is about the middle
of the rock hammer handle, the head of the hammer
rests on a thin shale layer that would be at or below the
base of the cross section. The beds below the hammer
are all part of the Meekoceras Beds.




Paleogeography:

The fissile shales above and below the top section are basinal deposits, >50m water depth, not much current to stir the beds up.
The fossils of the Anasibirites Beds were deposited in beds that are believed to be part of the outer shelf facies, <50m water depth, below normal wave depth, but possibly subject to storm currents.  The rocks consist of whole fossil wackestones & packstones.
Inner Shelf rocks consist of more silt and sand, with broken shells of bivalves and ammonoids.  The sandstones, siltstones and limestones of this facies were deposited above wave base.  The Sinbad Limestone of Eastern Utah preserves the Anasibirites beds in this facies.
The shore is preserved as sandstones and siltstones of intertidal mudfats in the Moenkopi Formation.  Characteristics of this formation include the red color, ripple marks, and gypsum deposits, some reptile trackways have also been observed.



Anasibirites age rocks in Eastern Utah
Anasibirites age rocks in Eastern Utah. Tm is the upper part of the
Moenkopi Fm., Tms, is the Sinbad Limestone Member, The lower
part of the Moenkopi is oil-bleached inside the San Rafael Swell, but
in other parts of Southeastern Utah it is red like the upper part here.




Paleoenvironment:

The Anasibirites beds were deposited about 3 million years after the End Permian Mass Extinction Event, the most devastating event in the history of the earth. Periods of local anoxia on the shelfs, elevated carbon dioxide levels, or a combination of both (Woods & Bottjer 2000), made for a pretty harsh environment.  Upwelling from the basin in the good times though, meant favorable conditions for ammonoids.  All this also meant a fluctuating aragonite compensation depth (ACD) meaning all aragonitic shells might dissolve depending on the depth of deposition.





A few personal observations:




Body Chamber Casts
Casts of body Chambers from the shale below the top section.  Note the form
of the last septum at the apical end of the casts
.




Small Gastropods
A close-up of rocks from the Meekoceras Beds, The arrow points to one of many small gastropods,
there is another larger one (2mm long) just above the Juvenites (20mm dia.) ammonoid





References:

Most of the information about paleogeography came from

Carr, T. R. and Paull, R. K., 1983
, Early Triassic stratigraphy and paleogeography of the Cordilleran Miogeocline, in Reynolds and Dolly, eds., Mesozoic paleogeography of the west-central United States: Denver, Rocky Mountain Section of the Society of Economic Paleontologists and Mineralogists, Rocky Mountain Paleogeography Symposium 2, p. 39-55

All of the information about the Paleoenvironment and some on the Meekoceras beds came from

Woods A. D. and Bottjer, D. J., 2000, Distribution of Ammonoids in the Lower Triassic Union Wash Formation (Eastern California): Evidence for Paleoceanographic Conditions During Recovery from the End-Permian Mass Extinction: PALAIOS, V 15, p.535-545

and again, some of the studies that have come out of these beds

Brayard, A., Bylund, K. G., Jenks, J., Stephen, D. A., Olivier, N., Escarguel, G., Fara, E. & Vennin, E., 2013. Smithian ammonoid faunas from Utah: implications for Early Triassic biostratigraphy, correlations and basinal paleogeography. Swiss Journal of Paleontology 132:141-219.

Daniel A. Stephen, Kevin G. Bylund, Paul J. Bybee and Wesley J. Ream, 2010, Ammonoid Beds in the Lower Triassic Thaynes Formation of western Utah, USA, in: Cephalopods Present and Past, edited by K. Tanabe, Y. Shigeta  and T. Sasaki & H. Hirano. Tokai UniversityPress, Tokyo, p. 243-252




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