Abstract

Sedimentary rocks associated with Cenozoic volcanic centers in the Rocky Mountains can provide an important key to the history and paleogeography of the region. Specific examples of sedimentologic investigations from three major volcanic fields illustrate the potential of integrated studies. Sedimentary facies of the Absaroka Volcanic Supergroup in northwestern Wyoming, the Clarno volcanic field of eastern Oregon, and the Challis volcanic field of central Idaho demonstrate analogies with modern high-relief andesitic terranes such as Mount St. Helens and volcanic centers in Guatemala. These volcanic fields contain associated sedimentary facies deposited by debris and sediment flows, high-energy braided streams, and temporary lakes. Sedimentary facies grade from chaotic boulder conglomerate with few fossils near the paleovents to well-bedded fossiliferous cobble conglomerate, tuffaceous sandstone and siltstone from alluvial, lacustrine, and overbank deposition far from the vents. All three volcanic fields have well-preserved “forests” of petrified logs and stumps, leaf and pollen floras, and vertebrate fossils in fine-grained facies. Plant paleoecology and comparison with modern equivalent environments indicate extremely rapid deposition in valleys, in a terrane where andesitic volcanic mountains produced high relief. Paleocurrent studies based on imbricated clasts in the northern Absaroka region are helping to determine the frequency and distribution of simultaneously erupting vents. Contributions to the structural history of the region can also come from investigations of volcaniclastic deposits. In the Challis field, sedimentologic studies and radiometric dating of volcanic strata demonstrate that normal faulting and basin formation began no later than Eocene time. This integrated approach to basin analysis provides a geographic context for paleontological studies and illustrates the importance of delineating paleotopography to properly interpret paleobotanical and paleoclimatic data.