Isopachs and lithofacies of the Big Snowy Group in the Bridger Range of southwestern Montana reflect subtle shifting of tectonic elements along an ancient structural lineament. The group comprises a transgressive series informally divided into the Kibbey Formation (with two members) and the Lombard facies, which is equivalent to Otter and Heath Formations of the Big Snowy Group farther east in central Montana.

The lower Kibbey member was deposited in a sabkha environment and is composed of algal laminated dolostone with desiccation features and evaporite solution breccias deposited at the leading edge of the transgressing sea. Siliciclastic intertidal channels that developed on the sabkha were restricted to a low-lying area that developed in the central part of the range. The upper Kibbey includes a regressive shoreface deposit composed of sandstone at the northern and southern ends of the range in contrast to mudstone and siltstone that dominate in the center of the range where deeper water and lower energy conditions prevailed. Ultimately, the Kibbey sabkha and shoreface transgressed out of the area leaving a partially restricted shelf lagoon. Shale and lime mudstone of the Lombard facies were deposited in quiet water at the center of the range. To the north and south, bioclastic wackestone, packstone, and grainstone were deposited under shoaling, higher energy conditions within the lagoon.

All three units of the Big Snowy are thickest where deeper water lithofacies occur, indicating that subsidence was greatest in the central part of the Bridger Range and that this subsidence was an important factor on the control of lithofacies distribution. Similar tectonic influence on sedimentation in this area is evident in other Paleozoic formations. The depositional changes in the central part of the range are coincident with the southern margin of the Precambrian Belt embayment along the Ross Pass fault zone, and seem to indicate a zone of deep crustal weakness along the transition from the fold and thrust belt to the Wyoming foreland deformation province.

End_of_Article - Last_Page 850------------