about 248 million years
ago, Anasibirites Beds (Tardus zone), Late Smithian Stage, Early
Period, Mesozoic Era
The West Coast of North
America in what is presently West Central Utah.
Map from Dr. Ron
Thaynes Formation are some of the only remaining mesozoic rocks
in western Utah. They occur in a syncline formed during later
mountain building (tectonic) events. The rocks under
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
or about 102 square Km. Everywhere in the study area the
beds outcrop, they have the same general thickness and fossil content.
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.
above and below the top section are basinal deposits, >50m water
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
of this facies were deposited above wave base. The Sinbad
Limestone of Eastern Utah preserves the Anasibirites beds in this
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. 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,
in other parts of Southeastern Utah it is red like the upper part
beds were deposited about 3 million years after the End Permian
Extinction Event, the most devastating event in the history of the
Periods of local anoxia on the shelfs, elevated carbon dioxide
or a combination of both (Woods & Bottjer 2000), made for a
harsh environment. Upwelling from the basin in the good
though, meant favorable conditions for ammonoids. All this
meant a fluctuating aragonite compensation depth (ACD) meaning all
aragonitic shells might
dissolve depending on the depth of deposition.
- The Shale above
the top of the Anasibirites Beds is barren of fossils. Probably
below the ACD, shells that were deposited, if any, dissolved before
lithification took place.
- The Ammonoids in
the top section are imbricated. The current flowed towards the
?NNW. This agrees with the chart showing preferred orientation,
this means the current was toward the basin. This bed was below
the ACD, the shells were replaced with spar (crystalline calcite)
the original aragonite dissolved, and while the matrix was being
lithified. Most chambers in the phragmocones are spar filled,
filled some damaged chambers.
below the top section is filled with concretionary internal molds
of ammonoid body chambers. The shell dissolved after the
in the body chamber was lithified, some damaged chambers are
but most were either crushed and/or dissolved, there is no trace of
part of the shell now.
- Almost all fossil
cephalopods (>99%) in the upper two sections are the platyconic
Prionitids, Anasibirites kingianus (Waagen) 1895, and Wasatchites
Mathews 1929, occasionally a serpenticonic Wyomingites sp.
or an orthoconic nautiloid are seen.
- The fossils in the two
middle sections are molds of body chambers, some with a few chambers
preserved, all are fragmentary and/or truncated. Some camera
filled, some shell visible. Some look like the shell was
dissolved and then truncated by the current.
- The bottom
section, and the section above it, belong to the Meekoceras Beds (The
Beds and Meekoceras Beds of my Smithian page are combined here).
- Fossil ammonoids
are jumbled (no preferred orientation, most are horizontal) in the
section. Woods and Bottjer (2000) considered the meekoceras beds
in california as a condensed bed, where upwelling currents provided
conditions for ammonoids. The combination of a slow
rate and steady rain of ammonoid shells produced beds with a wide
of ammonoid types.
- Ammonoid types in
the Meekoceras Beds include the oxycones Aspenites,
the spherocone Juvenites, the platycones Inyoites, Clypeoceras,
and Arctoceras, and the serpenticones Kashmirites, Xenoceltites and Wyomingites.
- Small gastropods
are numerous in the lowest section and occur in lenses in the upper
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
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
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
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
Tokyo, p. 243-252