During the Early Triassic rocks of the Thaynes Formation were deposited in the Sonoma Foreland Basin. Fossil Cephalopods are common in these rocks in western Utah, eastern Nevada and southeastern Idaho. Late in the Smithian the sea spread eastward onto the Moenkopi mudflats and the Sinbad Limestone Member of the Moenkopi Formation was deposited.
Asa A. L., 1929, The Lower Triassic Cephalopod Fauna of the
Fort Douglas Area, Utah, Walker Museum Memoirs Vol.1 No.1 University of
Chicago Press; Hose, R. K., and Repenning, C. A., 1959, Stratigraphy of
Pennsylvanian, Permian, and Lower Triassic Rocks of the Confusion
Utah, A.A.P.G. Bulletin vol. 43, no. 9; Tozer, E. T., 1971, Triassic
and Ammonoids: Problems and Proposals, Canadian Journal of Earth
8; Tozer, E. T., 1994, Canadian Triassic Ammonoid Faunas, GSC Bulletin
Stephen, K. G. Bylund, P. J.
W. J. Ream, 2010, Ammonoid
Beds in the Lower Triassic Thaynes Formation of western Utah, USA, in: Cephalopods – Present and Past,
edited by K.
and T. Sasaki & H. Hirano. Tokai University
Press, Tokyo, p. 243-252
Thaynes Formation of the Confusion Range in western Utah are three
of Smithian "Ayaxian" ammonoids. The lower two probably represent the
Hedenstroemia bosphorensis / "romunderi" Zone with Meekoceras,
Arctoceras and Aspenites in the lower beds, and Inyoites, Lanceolites
and Pseudosageceras above. The upper assemblage contains Anasibirites
and Wasatchites, representative of the Anasibirites nevolini / "tardus"
Zone. These are just assemblages as I see them in the beds in
west-central Utah and are by no means considered biozones.
Meekoceras beds Inyoites beds Anasibirites beds