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Parcell, William C.1, Leah Kasten1, Kimberly Minks1, Monica K. Williams1 
(1) Wichita State University, Wichita, KS

ABSTRACT: Mapping Middle Jurassic Microbial Buildups, Cody, Wyoming: Implications for Stratigraphic Correlation of Middle Jurassic Units across the Bighorn Basin

The recognition of a laterally continuous microbial horizon provides a potential tool for correlation of Middle Jurassic units in the Bighorn Basin of Wyoming and Montana. During the summer of 2002, students and faculty from Wichita State University discovered outcrops of thrombolite patch reefs in Middle Jurassic strata (Bajocian- to Bathonian-age) in the Indian Pass Quadrangle (1:24,000 scale), near Cody, Wyoming. These are among of the earliest known worldwide occurrence of Jurassic thrombolite reefs. During the following 2003 field season, the distribution of these reefs was mapped as part of a USGS EDMAP mapping project of the Indian Pass Quadrangle. 
The microbial buildups occur within the Gypsum Spring Formation. This unit, along with the Piper and Sawtooth Formations of Montana, represent the first two of four transgressions to inundate the Western Interior during the Jurassic. The Gypsum Spring and Piper Formations are composed of mixed carbonate and evaporite deposits formed in restricted inner ramp settings. Mixed carbonate and siliciclastic lithologies represent tectonically-influenced deposition in the Sawtooth Formation. 
The microbial buildups are found in the second regional transgressive-regressive cycle. These thrombolites develop along a widespread, laterally-continuous microbial horizon within the middle member of the Gypsum Spring Formation. This distinct microbial layer may be a useful stratigraphic marker across the Bighorn Basin for it develops in association with the maximum flooding surface. Microbialite appears at the same apparent stratigraphic level within the Gypsum Spring Formation in central Wyoming and Piper Formation of southern Montana.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.