Ammonoids of the Lower Triassic Thaynes Group in the Pahvant Range, Utah

Kevin G. Bylund1, Daniel A. Stephen2, Arnaud Brayard3, Hugo Bucher4, Jim Jenks5, and RaNae Dawn McShinsky2

(1) 140 South 700 East, Spanish Fork, UT 84660, kevin@ammonoid.com, (2) Earth Science Department, Utah Valley University, 800 West University Parkway, Orem, UT 84058, (3) UMR-5561 Biogéosciences, Université de

Bourgogne, Dijon, 21000, France, (4) Paleontological Institute and Museum, University of Zurich, Karl Schmid-Strasse 4, Zurich, 8006, Switzerland, (5) 1134 Johnson Ridge Lane, West Jordan, UT 84084

The Thaynes Group is a marine unit that thins from northwest to

southeast across Utah, where it interfingers with the terrestrial

Moenkopi Group. The limestones and shales of the Thaynes

Group reflect deposition within the Sonoma Foreland Basin, and

outcrops of these Lower Triassic rocks can be seen now in the

Pahvant Range of central Utah.

ConclusionsIntroduction Methods

Two sections were measured through a portion of the Lower

Thaynes Group at separate localities in the Pahvant Range.

Fossils were systematically collected from the limestone beds;

fossils are extremely rare in the intercalated shales. Ammonoids,

though generally poorly preserved, are abundant in the upper

beds in both measured sections.

This report focuses on ammonoids from the upper beds, 7

through 10.

We recognize ammonoids in bed 10 as Anasibirites kingianus.

In the underlying beds 7, 8 and 9, the ammonoid fauna is much

more diverse and includes: Inyoites oweni, Wyomingites arnoldi,

Churkites noblei, Guodunites hooveri, Owenites koeneni,

Lanceolites bicarinatus, Juvenites septentrionalis, ?Xenoceltites

intermontanus, an ussuriid, and an unidentified proptychitid.

The upper bed, 10, is assignable to the informal Anasibirites

kingianus Zone, while the underlying beds, 7-9, belong to the

upper part of the Meekoceras gracilitatis Zone. Furthermore, the

A. kingianus Zone correlates with the A. multiformis Zone (Upper

Smithian), while the upper part of the M. gracilitatis Zone

correlates with the Owenites Beds (Middle Smithian) of the

Tethyan Paleoequatorial Zonation. An Inyoites Horizon is also

recognized just below the Anasibirites beds which is also

recognized in Tethyan faunas.

Results

References

Our results complement recent reports of the occurrence of the

Upper Smithian Anasibirites fauna in the Sinbad Formation

(Thaynes Group) at several sites in southern and southeastern

Utah. The Pahvant Range assemblages described herein

provide an important geographic link between localities to the

east and south and localities to the west and north. Interestingly,

ammonoids of the Middle Smithian Meekoceras fauna appear to

be absent in the correlative Sinbad Formation localities to the

south and east. Therefore, the occurrence of the Meekoceras

fauna in the Pahvant Range represents the farthest southeast

this fauna has been reported in Utah. The lower beds of the

Thaynes Group in the Pahvant Range also contain ammonoids,

which continue to be investigated and may soon yield more

significant discoveries.

Fauna

Measured Sections

Guodunites hooveri

x3

Wyomingites arnoldi

x3Inyoites oweni

x2

Owenites koeneni

x2 Juvenites septentrionalis

x2

Churkites noblei

x1

Lanceolites bicarinatus

x1.5

?Xenoceltites intermontanus

x1.5

Anasibirites kingianus

x2

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(Allochthonous

Terrane)