Submarine Debris-Flow, Slump-Block, and Turbidite Deposits in Mancos Shale (Cretaceous) of Northwestern Colorado
John C. Lorenz, Chris A. Muhr
The resistant ledges within the Mancos Shale on the slopes of Mount Garfield near Grand Junction, Colorado, have long been ignored or explained as geomorphic features. However, sedimentologic study has shown that there are three significantly different types of Cretaceous submarine deposits interbedded with the Mancos in this part of the Book Cliffs. (1) The lowest ledges consist of millimeter to meter-scale unsorted rip-up clasts of Mancos mudstone and shallow marine sandstone that overlie planar contacts marked by shear-zone lineations. Lenses of this intraclast conglomerate are up to 20 m thick and several hundred meters wide, have steep, erosional margins, and are interpreted as submarine debris flows. (2) A 14-m thick interval just below the upper ledge consists of a amalgamation of sandstone-shale laminations, each a few centimeters thick and each laterally extensive and heavily burrowed. This is a distal turbidite fan. (3) The upper ledge, at least 500 m in extent and 20 m thick, consists of large (tens of meters) slump blocks of Mancos Shale that are rotated to dip toward the paleoshoreline. These overlie a planar base that is marked with lineations similar to that of the debris flows. Offset bedding along internal shear planes records reconfiguration within the large blocks as they conformed to the flat basal contact during and/or after slumping. Although turbidites are now commonly recognized in the Cretaceous shales of the Western Interior, debris-flow intraclast conglomerates and large-scale slump blocks have not been widely reported. These d posits have implications for water depths, submarine slope angles, and seabed configuration with the Western Interior seaway. Some may represent the sedimentary expression of the shelf-edge break in slope.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.