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

Lithostratigraphic, biostratigraphic and sequence stratigraphic relationships within the Middle Jurassic section in the Bighorn Basin are examined in order to clarify the stratigraphic occurrence and significance of regional unconformities in the Middle Jurassic. Of particular interest is the depositional history and relationship between the Bajocian and Bathonian Piper and Gypsum Spring Formations in the Bighorn Basin. These units are lithologically similar, share a comparable sequence of strata and both contain Middle Jurassic fauna, but physical correlation of these units has never been demonstrated. This initial study focuses on the lithofacies in the Piper Formation along the western margin of the Bighorn Basin.

The Piper Formation in its type area of central Montana is divided into 1) a succession of red shale and gypsum (lower Tampico Member), 2) an interval of bioclastic carbonate and green calcareous shales (middle Firemoon Member) and 3) a succession of red and variagated shales (upper Bowes Member). This same sequence of lithofacies occurs along the western margin of the Bighorn Basin. From outcrop and subsurface data, six principal lithofacies are recognized within the Piper Formation. These include: 1) red shale and mudstone, 2) gypsum and interbedded dolomite, 3) peloidal and fossiliferous mudstone to wackestone, 4) oolitic packstone to grainstone, 5) fossiliferous and peloidal packstone, and 6) green to black glauconitic carbonate mudstone. All lithofacies within the Piper Formation, except for the basal gypsum and interbedded dolomite facies, are laterally continuous. In places, the gypsum/dolomite facies is completely replaced by a chert and limestone breccia that has been used to mark the regional unconformities in the Jurassic section.