Ammonoids of the Lower Triassic Thaynes Group in the Pahvant

Ammonoids of the Lower Triassic Thaynes Group in the Pahvant Range, Utah
Kevin G.
1
Bylund ,
Daniel A.
2
Stephen ,
Arnaud
3
Brayard ,
Hugo
4
Bucher ,
Jim
5
Jenks ,
and RaNae Dawn
2
McShinsky
(1) 140 South 700 East, Spanish Fork, UT 84660, [email protected], (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
Introduction
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.
Measured Sections
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.
Results
Conclusions
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.
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.
(Allochthonous
Terrane)
Anasibirites kingianus
x2
Wyomingites arnoldi
x3
Guodunites hooveri
x3
Owenites koeneni
x2
Lanceolites bicarinatus
x1.5
Churkites noblei
x1
Fauna
Inyoites oweni
x2
Juvenites septentrionalis
x2
?Xenoceltites intermontanus
x1.5
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