Depositional History of the Paleocene-Eocene Colton Formation, North-Central Utah

Abstract The late Paleocene to Eocene Colton Formation of north-central Utah is a fluvial tongue extending to the north and west into the Lake Flagstaff and Uinta basins. Along the south side of the Uinta basin, the Colton is composed of a mud-dominated and a sand-dominated facies. The mud-dominated facies consists of discrete channel sandstone bodies set in a matrix of mudstone and overbank flood deposits. These rocks were deposited by a moderate-sinuosity stream system with low-sinuosity distributaries that prograded over the mudflats of the Lake Uinta margin. The sand-dominated facies consists of thick, laterally-continuous sandstone bodies and less abundant mudstone and overbank deposits. This facies was deposited by a laterally-migrating stream system which prograded over the more distal mud-dominated alluvial plain in the eastern end of the basin. Colton deposits in the Sanpete Valley area consist of alternating lacustrine and mud-dominated fluvial units. Lacustrine beds are gray-green mudstone and limestone; limestones exhibit mud cracks and contain an abundant vertebrate and invertebrate fauna. These sediments were deposited in shallow, relatively fresh lakes with marginal mudflats, developed on the floodplains of Colton streams. Colton sandstones in the Sanpete Valley area contain abundant feldspar, crystalline and sedimentary rock fragments and some volcanic rock fragments. Source areas probably include the Monument Upwarp and other Laramide structures in southeastern Utah and adjacent areas. The minor volcanic component may have originated in southern Arizona. Colton sandstones on the south side of the Uinta basin contain less mica, fewer crystalline rock fragments and volcanic rock fragments. Source areas probably include the San Juan Mountains and older sandstones of the San Rafael Swell and other sedimentary terranes traversed by Colton rivers.