Cenozoic Evolution of the Montana Cordillera: Evidence From Paleovalleys
نویسندگان
چکیده
Paleovalleys are components of regional unconformities that provide important constraints on the tectonic evolution of orogenic systems. Some paleovalleys represent canyons cut through deformed strata. Others comprise linear rift-valleys filled with lakes, streams, or alluvium. Locally preserved fluvial and alluvial gravel may directly link source regions to depositional basins, providing timelines for the synergetic uplift, denudation, and subsidence of different parts of an orogenic system. Well-preserved paleovalley networks can provide a direct reference datum for the tectonic stability of individual drainage basins and divides. Remnants of paleovalleys in western Montana and Idaho record four main tectonic phases in the Cenozoic evolution of the Montana Cordillera. In this paper, we map changes in regional drainage in western Montana that occurred during Cenozoic time, and discuss tectonic implications of the evolving paleodrainage patterns. Figure 1 shows the region of our investigation. Our interpretations are based mostly on the distribution of lithologically distinctive, stream-rounded pebbles and cobbles preserved in dated paleovalley conglomerates. Particularly distinctive clast lithologies that can be uniquely matched to bedrock source regions include Mesoproterozoic and Ordovician quartzites, Devonian black chert, Cretaceous andesite, and Eocene volcanics. This overview is intended to Cenozoic Evolution of the Montana Cordillera: Evidence From Paleovalleys
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