WESTERN INTERIOR SEAWAY KIOWA-SKULL CREEK DELTAIC SHORELINE: INVESTIGATING THE ROLE OF SEA LEVEL CHANGES AND AUTOGENIC PROCESSES

The influence of allogenic drivers vs. autogenic self-organization on clastic sediment-routing systems (SRS) is becoming more well-known and accepted in modern systems but remains poorly known for ancient strata. This poster discusses the mid to late Cretaceous upper Dakota group of the Colorado Front Range, which is in the center of a >800 km north-to-south oriented deltaic shoreline that represents the western shoreline of the Western Interior Seaway during the Albian-Cenomanian Kiowa-Skull Creek cycle. The deltaic component of the upper Dakota is referred to as the Ft. Collins member of the Muddy sandstone, whereas the overlying fluvial channel-belt sandstones are referred to as the Horsetooth member: Horsetooth sandstones are incised into this shoreline system and have produced an erosion surface that has been interpreted as unconformable, a classical sequence boundary that formed in response to relative sea-level fall (allogenic process). However, a study from Nazworth (2019) yielded younger MDAs for the Horsetooth than previously interpreted at ~98-100 Ma, raising the question of whether fluvial incision of the deltaic shoreline was actually autogenic and is actually conformable. To better understand the role of allogenic vs. autogenic processes in the stratigraphic record of the Western Interior Seaway, this study will focus on obtaining detrital-zircon U-Pb maximum depositional ages (MDAs) for the Ft. Collins member. Thirteen sandstone samples were obtained from Ft. Collins sandstones at three locations along strike on the paleo shoreline and from an outcrop of fluvial deposits in a delta plain setting, located 80-100 km updip and to the west. If Ft. Collins MDAs at each location statistically overlap with each other and with the overlying Horsetooth MDAs, then the composite scour surface between the Ft. Collins and Horsetooth will be considered a good candidate for autogenic processes. Additional methods for testing the hypothesis of autogenic processes that produced this scour surface will be implemented as this study progresses. The results of this study will aid in the development of a new interpretive model for ancient low-accommodation fluvial deltaic environments.