Depositional Environment Of Upper Cretaceous Black Sandstones Of The Western Interior

Cretaceous black sandstone deposits of Western Interior have stratigraphic, structural, textural, and mineralogical characteristics helpful in reconstructing local and regional depositional environments.

The geology of six geographically separated but geneti- cally related heavy-mineral placer deposits are investigated along 40 miles of coastal-marine sandstone outcrops comprising the McCourt Sandstone Tongue. The heavy- mineral placer deposits consist of dark-brown to black, fine- to medium-grained, ferruginous sandstones that occur in elongated lenses that are as much as 6.6 ft thick and 2,000 ft long. The placer deposits are mostly intercalated with light-gray or tan quartzose shoreline sandstones that offlapped southeastward across the study area and formed a strand plain during a regression of the interior Cretaceous sea. The placers were deposited along a single shoreline during one stage of the regression. The heavy-mineral placer deposits are composed of about 85 percent opaque iron minerals, mostly magnetite, hematite, and ilmenite, and about 15 percent nonopaque minerals, mostly zircon, with minor amounts of tourmaline, rutile, garnet, sphene, hornblende, apatite, and traces of other minerals. The depositional settings are river mouth, berm, forebeach, surf, and middle shoreface, where the segregation of light and heavy minerals took place by fluvial, marine, and eolian processes. A plutonic source for most of the heavy minerals was probably the Sevier orogenic belt located 150-250 miles west of the study area. The heavy-mineral deposits in the McCourt Tongue are analogous in origin to that of heavy-mineral deposits that are presently forming along the southeast Atlantic and gulf coasts of the United States.