Paleocene lakes of the central rocky mountains, Western United States

Abstract

Lacustrine environments are well developed in the Paleocene to early Eocene sedimentary sequences in the intermontane basins of the central Rocky Mountains. The available evidence points toward a widespread episode of lake formation in the middle-late Paleocene (Torrejonian-Tiffanian) followed by a second major period during the latest Paleocene to early Eocene (Clarkforkian-Wasatchian). The earlier period of lake formation is dominated by relatively shallow, although often areally extensive, freshwater lakes associated with fluvial and paludal environments. In basins in northern Wyoming and Montana, a high clastic input prevailed. Thich mudstone, often structureless, marks the deep-water facies of these lakes, with tabular sandstone, lenticular carbonate and lignite occurring along the fluviodeltaic margins. Farther south and west, in Colorado and Wyoming, alkaline lakes dominated by carbonate deposition were prevalent. The appearance and disappearance of standing water in both northern and southern basins may be more closely controlled by tectonics than by climate: most evidence points toward prevailing humid sub-tropical conditions until the very end of the Paleocene.