AMMONOIDS OF THE LOWER TRIASSIC THAYNES GROUP IN THE PAHVANT RANGE, UTAH

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.

Two sections were measured through a portion of the 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 are abundant, though generally poorly preserved, in the upper beds in both measured sections. We recognize these upper layers as the Anasibirites Beds, which are dominated by the eponymous taxon. In the underlying beds, which we refer to as the Meekoceras Beds, the ammonoid fauna is much more diverse and includes: Inyoites, Wyomingites, Churkites, Guodunites, Owenites, an ussuriid (?Lanceolites), and an unidentified proptychitid. The upper beds are assignable to the informal Anasibirites kingianus Zone, while the underlying beds 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.

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.