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Williams, Monica K.1, William C. Parcell1 
(1) Wichita State University, Wichita, KS

ABSTRACT: Stratigraphic Framework for Microbial Buildups in the Middle Jurassic (Bajocian and Bathonian), Bighorn Basin, Wyoming

Middle Jurassic thrombolite buildups recently discovered near Cody, Wyoming are examined within a sequence stratigraphic framework to determine timing of development. Geologic mapping of the Indian Pass Quadrangle near Cody indicates that Middle Jurassic rocks outcrop along the limbs of a series of modern-day anticlines and synclines that trend approximately N-S through the map area. The thickness of the Middle Jurassic section is irregular and appears to be affected by local paleohighs of the Triassic Chugwater Formation that existed during Middle Jurassic time. During the 2002 field season, well-exposed outcrops of thrombolite buildups were discovered several miles east-southeast of Cedar Mountain in the Middle Jurassic strata of Bajocian and Bathonian age. These are some 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 (1:24,000 scale). 
The buildups can be found on the upper surface of a rippled limestone bed. Below this horizon are found interbedded calcareous, olive-gray shales and thin limestone beds containing bivalve, gastropod, and Pentacrinus shell hash. Above the thrombolite horizon are more olive-gray shales with nodular pink to white gypsum in the upper part of the unit. These rocks have been considered to be either near the top of the Gypsum Spring Formation or the bottom of the “Lower Sundance” Formation. The focus of this study is to clarify the stratigraphic position of the buildups and determine their depositional environment.


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