Cobban and Kennedy posed the possibility that the Western Interior seaway connected briefly across the Transcontinental arch during the latest Albian/earliest Cenomanian between the Albian Kiowa-Skull Creek cycle and the Cenomanian Greenhorn cycle based on mixing of Tethyan endemic (Neogastroplites) with cosmopolitan (Metengonoceras) ammonites in the Boreal Mowry Shale. This hypothesis is here evaluated utilizing biofacies and sequence-stratigraphic surfaces recognized in outcrops, well logs, and cores. Precise age relationships are determined by graphic correlation of selected sections utilizing palynomorphs, ammonites and foraminifers.

Where the Muddy Sandstone is conformable with the underlying Thermopolis Shale and the overlying Shell Creek Shale near the Wyoming-Montana border, its age is 100.05 to 99.82 Ma. The top of the Shell Creek is 98.40 Ma, and the top of the Mowry is dated by the Clay Spur bentonite as 97.17 ± 0.69 Ma. The Mowry section correlates southward into a regional (<35,000 km²), diachronous (99.4 and 98.2 Ma) sequence boundary that separates estuarine from fluvial valley-fill strata of the Muddy/Dakota throughout southeastern Colorado and northeastern New Mexico. The unconformity represents a major northward excursion and retreat of the southern/Tethyan arm of the Western Interior sea toward the northern/Boreal arm during Mowry deposition.

These data support the supposition that the northern and southern arms of the Western Interior sea joined during Mowry deposition. It also suggests the possibility that the Mowry transgressive/regressive cycle rivaled the traditional five Western Interior cyclothems of Kauffman both in duration and regional extent. Fully marine facies that would connect the maximum flooding Mowry in the north with the latest Albian sequence in Texas are possibly represented by this newly mapped intra-Muddy unconformity.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah