STRATIGRAPHIC RELATIONSHIPS WITHIN THE LOWER ELLIS GROUP, SOUTHERN MARGIN OF BELT ISLAND, SOUTHERN MONTANA

Lithostratigraphic correlation of outcrop and well data aid in clarifying the transition between siliclastic, carbonate and evaporate sediments in the Middle Jurassic section along the southern margin of Belt Island, southern Montana. The Middle Jurassic deposits of Wyoming and Montana represent mixed deposition in a retro-arc foreland basin. This depositional setting was influenced by two major syndepositional positive-trending structures: Belt Island in central Montana and the Sheridan Arch in northern Wyoming. This positive trend probably dominated the restricted marine conditions during the early and late Middle Jurassic time. The domination of marine conditions affected the sediments, both the type and amount found. Due to rapid facies changes and apparent lithologic similarities comprising the Sawtooth and Rierdon Formations, correlation of these units has been inconsistent. The formations within the Ellis Group along Belt Island have not been distinguished from each other in a systematic study. Well-exposed outcrops and dense well data help in clarifying the transition between siliclastic, carbonate and evaporate sediments in the Middle Jurassic Sawtooth and Piper Formations. This information will help to resolve the stratigraphic relationships between the Sawtooth and Rierdon Formations, better differentiate between the formations, and aid in reconstruction of the depositional history along Belt Island.