New insights into the provenance and regional tectonics of a middle Cretaceous retroforeland basin, southwestern Montana, USA

A primary goal of modern geology is looking at how mountain ranges have developed through time. The Rocky Mountains have been continually forming for the past ~145 million years. Examining the geologic record to produce a continuous timeline of continental development is crucial to understanding continental formation processes, how they relate to the planet as a whole, and how this impacts climate. Previous work in southwestern Montana shows that 100 million years ago (Ma), rivers drained from mountains in Idaho to an inland sea that flooded the middle of the continent. Erosion and volcanic activity have since changed the Idaho landscape. In order to examine what this area was like in the past, we must look at the river sediments that drained these ancient mountains. Examining the ages of multiple individual sand grains from ancient river samples, in comparison with the ages of rocks preserved across Idaho, allows us to link specific Idaho geographic regions that were eroding sediment into Montana. Our work shows two source regions – one in western Idaho, with volcanoes and being exhumed via strike-slip faulting, and one in eastern Idaho with young rocks being via thrusting associated with plate subduction, with increased motion beginning at ~105 Ma. Both of these regions contributed sediment to the basin in southwestern Montana. This is consistent with previous work indicating this sort of accelerating tectonic activity also occurred at ~105 Ma throughout the Rocky Mountains.