STRATIGRAPHIC RELATIONSHIPS WITHIN THE MIDDLE JURASSIC (BAJOCIAN AND BATHONIAN), BIGHORN BASIN, WYOMING AND MONTANA

Stratigraphic relationships within the Middle Jurassic section are examined in the Bighorn Basin of Wyoming and Montana. Of primary interest is the depositional history and lithostratigraphy of the Bajocian and Bathonian Piper and Gypsum Spring Formations. The Piper and Gypsum Spring Formations are lithologically similar and both contain Middle Jurassic fauna; however, physical correlation of these units has not been clarified.

The Piper Formation is divided into three principal lithofacies: 1) the upper Bowes Member composed of red shale with interbedded gypsum and limestone; 2) the middle Firemoon Limestone Member composed of gray shale and interbedded limestone; and 3) the lower Tampico Shale Member composed of red shale interbedded with dolomitic limestone and thick masses of gypsum. The Gypsum Spring Formation is divided into two principal lithofacies: 1) an upper shale and carbonate series composed of gray and brown limestone, red and green shale, and occasional gypsum; and 2) a lower gypsum interval composed primarily of interbedded red shale and thick gypsum masses.

The lower interval of the Gypsum Spring Formation may equate to the Tampico Member of the Piper and the upper section of the Gypsum Spring Formation may equate to the Firemoon and Bowes Members. Lithofacies within both formations are laterally continuous; however, locally the basal gypsum facies is completely replaced by a chert and limestone breccia that has been used to mark regional unconformities in the Jurassic section. Paleohighs and local tectonic movements in the Jurassic Wyoming-Montana shelf area probably caused the absence of the thick gypsum facies allowing its replacement by the chert/limestone breccia.