ABSTRACT: Upper Cretaceous "Stacked" Storm Deposited Shelf Sandstones

TILLMAN, RODERICK W., Consulting Sedimentologist/Stratigrapher, Tulsa, OK, and DAG NUMMEDAL, Louisiana State University, Baton Rouge, LA

"Stacked" predominantly current (rather than wave) deposited shelf sandstones separated by relatively thick shale sequences occur in three relatively widely separated areas in Wyoming and Colorado.

In northwest Colorado, three stacked, predominantly storm generated, generally upward coarsening "Gulch" shelf sandstones were deposited in the Upper Campanian (Baculites perplexus ammonite zone) during one or more relative sea level rises. The suite of facies is identical to that observed in the Shannon Sandstone in Wyoming. The "Gulch" sandstones occur in a 500 ft thick sequence that is predominantly shale but includes two, and locally three, up to 70 ft thick, at least 4 mi wide shelf sandstones.

In the lower part of all three ridges are bioturbated siltstones and sandstones, which are very fine grained sandstones that are (1) transitional with the underlying shales, (2) range up to 34 ft thick, and (3) form the "platform" on which the remainder of the ridges are formed. The overlying coarser grained (175-225 microns), cross-stratified to rippled facies lie in apparent conformity with the underlying bioturbated sandstones.

Three highly cross-laminated facies are observed. An up to 34 ft thick southwest flow oriented, totally cross-laminated, glauconitic sandstone with subhorizontal bedding surfaces forms most of the volume of the ridges (Central Ridge Facies). Some of the ridge margins were periodically eroded by storm currents and draped by clay by tidal or other fairweather processes.

Probably correlatable shoreface deposits occur 3 mi to the west of the Wise Gulch Sandstone, suggesting that all three ridge sandstones were formed on the inner shelf but below storm wave base.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)