Implications for Large-Scale Shoreline Translation in the Turonian Western Interior Seaway: Evidence From the Codell Sandstone, Colorado

Abstract

The Codell Sandstone Member of the Carlile Shale is a complex stratigraphic succession preserved within and south of the Denver Basin. Evidence for large-scale basinward migration of the shoreline in the Turonian is interpreted from facies and isotope study of this unit. The Codell Sandstone Member is the terminal unit in the well-studied Greenhorn regressive hemicyclothem of Kauffman, and records an overall upward-shallowing depositional facies trend. This unit was revisited to determine the magnitude of sea level fall during this regressive event, and the extent of strandline migration associated with the fall. This was done through facies interpretation of key stratigraphic sections (outcrop and subsurface) and strontium analysis of the strata bracketing this interval. Outcrops and core studied were deposited near the axis of the Cretaceous Western Interior Seaway. Two distinct facies are interpreted within the Codell Sandstone Member: an upward–coarsening unit interpreted as distal lower shoreface facies at the base of the member and an estuarine facies association at the top of the member. The estuarine facies association is composed of complex relationships of bioturbated, heterolithic, channelized, and deltaic facies. The surface separating the lower shoreface facies and estuarine facies association is interpreted as a sequence boundary and the base of a 10 meter thick incised valley fill exposed near Pueblo, Colorado. 87 Sr/86Sr ratio analyses from unaltered shell material show an isotopic excursion within the estuarine facies, which is attributed to freshwater input. Facies interpretation and isotopic analysis places the lowstand shoreline for the Codell Sandstone Member west of Pueblo, Colorado. The time equivalent shoreline for the highstand (lower shoreface) strata of the Codell Sandstone Member is placed between central Kansas and central Missouri, 700 to 1100 km east of the study area. Brackish-water strata of the incised valley fill would require a minimum of 700 km of lateral translation of the eastern margin of the Cretaceous Western Interior Seaway during deposition of the Codell Sandstone Member. This westward shift in the eastern shoreline resulted from a sea level fall that could have been as little as 30–60 meters. Such a significant seaward translation in shoreline position associated with a sea level fall of this magnitude suggests that the sea floor along the eastern margin of the Seaway was of low gradient over large distances.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015