Friday Field Foto #8: Cretaceous shoreline deposits

I’ve missed the last few Fridays…

The Book Cliffs are in central Utah and western Colorado and are famous for their spectacular outcrops of Cretaceous shoreline deposits. The seaway, which once stretched across the whole of North America from the Gulf of Mexico to the Arctic Ocean, left a fantastic record of relative sea level changes in these sedimentary rocks. The landward or basinward movement of the shoreline can be mapped along the Book Cliffs by identifying the various sedimentary environments (coastal plain, beach, shoreface, shelf, etc.) in the record.

The map above illustrates this paleogeography nicely. Note how much of Colorado is part of this seaway at this time. Eventually, the rivers and shorelines (as recorded by the stratigraphy) march across Colorado too. This image taken from a fantastic website by geologist Ron Blakey at the University of Northern Arizona. Check it out.

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