Discussion

Time:

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

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Place:

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

Map from Dr. Ron Blakey

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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.

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.

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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. 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.

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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.

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A few personal observations:

• 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 any 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) after the original aragonite dissolved, and while the matrix was being lithified.  Most chambers in the phragmocones are spar filled, sediment has filled some damaged chambers.
• The shale below the top section is filled with concretionary internal molds of ammonoid body chambers.  The shell dissolved after the sediment in the body chamber was lithified, some damaged chambers are preserved, but most were either crushed and/or dissolved, there is no trace of any 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 perrini 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 spar filled, some shell visible.  Some look like the shell was partially dissolved and then truncated by the current.
• The bottom section, and the section above it, belong to the Meekoceras Beds (The Inyoites Beds and Meekoceras Beds of my Smithian page are combined here).
• Fossil ammonoids are jumbled (no preferred orientation, most are horizontal) in the bottom section.  Woods and Bottjer (2000) considered the meekoceras beds in california as a condensed bed, where upwelling currents provided favorable conditions for ammonoids.  The combination of a slow sedimentation rate and steady rain of ammonoid shells produced beds with a wide variety of ammonoid types.
• Ammonoid types in the Meekoceras Beds include the oxycones Aspenites, Pseudosageceras and Lanceolites, 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 sections.

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

References: