In the lowest part of the passive margin sediments you see rocks like this: coarse, poorly sorted, angular to well rounded sedimentary clasts supported by a fine grain matrix. Sedimentary rocks with this texture are know as a ‘Diamictite.’ ------------------- The interesting thing about this diamictite is that some of the clasts have scratches or shallow linear groves cut into them. These groves are indicative of glacial transport. When the glacier is sliding down slope on the basal bedrock, there are often clasts that are imbedded in the ice that are dragging across the underlying rock. This dragging creates the groves or ‘striations’ on the clasts. -------------------- There are no dateable ash horizons or other opportunities for direct dating. But, if the texture and striations demonstrate that this deposit is indeed glacial, then we have a good sense of the age of the initiation of sedimentation on the passive margin of North America. If these deposits belong to the well-studied, globally synchronous glacial period in the late Proterozoic, then we can bracket the initiation of sedimentation somewhere between 650 and 600 ma. (this image is from canada)
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