Late Oligocene-early Miocene facies and lacustrine sedimentation, upper Ruby River basin, southwestern Montana

Sediments deposited in and around a closed, shallow, perennial lake (Lake Passamari) constitute the late Oligocene-early Miocene Passamari Member of the Renova Formation. Three sedimentary facies are recognized: 1) lacustrine-nearshore and open lacustrine environments; 2) marginal lacustrine-deltaic and fluvial-lacustrine environments; and 3) alluvial-meandering stream, braided delta-distributary system, and allivial fan environments. Within the basin, a central core of limestones and fine-grained clastics of the lacustrine facies is flanked on the north and west by coarse-grained deposits of the marginal lacustrine and alluvial facies. Along the north margin of Lake Passamari, extensive intertonguing of facies occurs, while on the west facies intertonguing is much more restricted. Vertical lithologic repetitions in the lacustrine facies are attributed to lake-level fluctuations in response to climatic changes. Marginal lacustrine and alluvial facies lithologic repetitions were also controlled, in part, by lake-level fluctuations. However, they were also sensitive to rapid depositional episodes largely unrelated to overall climatic conditions.