THE IDENTIFICATION OF A TECTONICALLY CONTROLLED SEQUENCE BOUNDARY AT THE CONTACT BETWEEN THE UPPER CRETACEOUS WAHWEAP AND KAIPAROWITS FORMATIONS

In Grand Staircase-Escalante National Monument, Utah the Wahweap Formation and overlying Kaiparowits Formation of the Late Cretaceous Cordilleran foreland basin record fluvial sedimentation linked to the initial Laramide Orogeny, coincident with well-documented eustatic sea level changes of the Western Interior Seaway. The contact is defined by a shift in source area, indicated by a transition from quartzose to lithic sandstone and by a significant change in paleocurrent direction. Detailed study of the Wahweap-Kaiparowits contact, specifically the recognition of tectonic influences superimposed on accommodation space related to sea level change, has allowed for the identification of an unconformity, and provided evidence that this contact is a tectonically controlled sequence boundary.

The Wahweap-Kaiparowits unconformity/boundary has been identified through the documentation of: 1) erosion features along the contact, 2) preserved paleosols, 3) lithology and paleocurrent changes, including the recognition of a local transitional unit between the Wahweap and Kaiparowits, 4) variations in fluvial styles, and 5) seismogenic features. Up to 8-meter-deep channels are eroded into the capping sandstone, the youngest member of the Wahweap Formation. Preserved paleosols are present in the upper meter of the capping sandstone member indicating a long period of exposure and non-deposition. The capping sandstone member was deposited by southeastern flowing braided rivers. Northeastern flowing meandering fluvial systems were responsible for the deposition of the Kaiparowits. The transitional unit consists of braided river deposits, but contain paleocurrents that suggest a paleoslope similar to that of the Kaiparowits Formation. Specialized techniques were developed to recognize and analyze seismites; both local and regional faulting have been recognized throughout the capping sandstone member and in the lower Kaiparowits, affirming tectonic activity associated with the initial Laramide movement.

The above evidence indicates an unconformity at the Wahweap-Kaiparowits contact, and implies that this is a significant, tectonically controlled boundary.