ABSTRACT: The Muddy Sandstone: A Complex Product of Fluctuating Sea Level and Syndepositional Tectonics

MARTINSEN, RANDI S., University of Wyoming, Laramie, Wyoming

A lowstand surface of erosion (LSE) that corresponds with the 98 Ma sea level fall is widely recognized within the Lower Cretaceous Muddy Sandstone. Evidence for this LSE is both clear and abundant and consists of regional truncation of underlying deposits that exhibits more than 30 m of local relief, a well-developed soil profile in marine deposits beneath the surface, and onlap of overlying alluvial valley fill and transgressive marine deposits. Locally, evidence including truncation, incision, basinward shifts in lithofacies, and the (rare) occurrence of roots suggest the presence of additional unconformities in the marine deposits underlying the 98 Ma LSE. Much controversy exists among workers as to whether or not these older unconformities are LSEs because they cannot be regional y delineated and because they don't have well-developed soil profiles similar to the 98 Ma LSE. However, delineation of unconformities in rocks deposited during periods of active basement block movements (such as the Muddy) should be difficult because the characteristics of such erosional surfaces are highly variable. Uplift of basement blocks without an accompanying drop in regional base level could create unconformities that occur over a wide region (on all the blocks high at one particular time), yet not be continuous. In conjunction with a regional drop in base level, differential block uplift would cause the characteristics of the unconformity to vary (i.e., degree of valley incision, soil formation and/or preservation, etc.) and locally remove and/or obscure evidence of older uncon ormities, while down-dropped blocks could contain conformable sequences.

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