Lowstand Submarine Bypass Surfaces and their Implication for Detached Lowstand Deposition, Mancos Shale, Book Cliffs, Utah

Diane L. Kamola and Steven M. Holland

An erosional surface within the offshore marine Upper Cretaceous Mancos Shale is interpreted as a lowstand submarine bypass channel scour, filled with shallow marine deposits. The scour event represents a lowstand in sea level, where the shoreline shifted far enough basinward that fluvially induced scouring occurred in the offshore deposits. The scour surface can be traced for over a mile in outcrop, and served as a major sediment bypass surface. The lowstand deposit is yet further basinward and not exposed in the (Book Cliff) region. The scour fill occurred in at least 3 stages, and is composed of transgressive systems tract sediments. Each stage of fill is interpreted as a parasequence and collectively indicates progressive deepening as sea level began to rise. Scour fi l is dominated by coarser deposits than the surrounding Mancos Shale and contains graded bedding, unidirectional wave-modified current ripples and Inoceramus in growth position. The amount of separation between the highstand shoreline and lowstand deposit is at least 30 km, and great enough that it is unlikely for both deposits to be physically correlated, or to be interpreted as related deposits. The significance of the lowstand submarine bypass system is that, in contrast to more recent ideas of shelf-attached lowstands, some lowstands occur many 10's of km basinward of their highstand shoreline. Where have all the beaches gone? In this case the evidence is present that lowstand shorelines do occur, not as an attached lowstand, but as detached deposits many, many km basinward rom the highstand shoreline.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California