The Mancos Shale is an important source of natural gas in the western United States. This rock
unit formed during the Late Cretaceous (100 to 66 million years ago) from sediment deposited
in a vast seaway (the Cretaceous Interior Seaway) that spanned from the Gulf of Mexico to
Canada. Why does shale form in deep ocean environments? What other facies (specific
features) may help us to determine the shale is marine?