COMBINING TEPHROSTRATIGRAPHY WITH SANDSTONE PROVENANCE AND DETRITAL ZIRCON GEOCHRONOLOGY TO ASSESS ALONG-STRIKE CONTINUITY OF ALLUVIAL SEQUENCE STRATIGRAPHIC PATTERNS IN THE CAMPANIAN WESTERN INTERIOR BASIN, FROM WESTERN MONTANA TO SOUTHERN UTAH

The utility of alluvial sequence stratigraphy still lags significantly behind that of the marine realm. The most significant issues affecting the testability of sequence stratigraphic models in the alluvial realm include the often appreciable lack of age control necessary for correlation, the difficulty of distinguishing localized vs. diagnostic surfaces (e.g., sequence boundaries), and challenges associated with recognizing facies tract migration in fully continental settings. The Cretaceous (Campanian-Maastrichtian) Western Interior Basin (WIB) is an ideal natural laboratory for investigating these problems, yet even traditionally well-studied areas in the WIB, including the Book Cliffs in central Utah, are limited by the paucity of absolute age control (e.g., datable tuffs) and related problems associated with facies tract migration through time and space. Our new results from southern Utah and western Montana demonstrate the potential of combining tephrostratigraphy with sandstone provenance and detrital zircon geochronology as a tool for distinguishing local (autocyclic or lateral variation) from regional (allocyclic) changes in alluvial architecture. We utilize existing tephrochronology, detrital zircon geochronology, and sandstone provenance data in combination with new data sets from coeval Campanian continental deposits along strike within the WIB, including sites from the Kaiparowits Basin (Straight Cliffs-Wahweap-Kaiparowits-Canaan Peak succession) and west-central Montana (Virgelle-Two Medicine-Horsethief-Saint Mary River succession), to improve the precision of stratigraphic control on alluvial architecture across the WIB. We focus in particular on along-strike variation in alluvial response to two well-known third order stratigraphic sequences in the WIB: 1) the Claggett (R7-T8) and 2) Bearpaw (R8-T9) cyclothems. Results indicate broad basin-wide synchronicity of alluvial response within the Claggett cyclothem, but variable local response suggesting a dominantly eustatic origin with superimposed tectonic influence. Similarly, the alluvial response to the Bearpaw cyclothem appears to be highly variable across the basin. This event is asynchronous, generally younging to the north, thereby suggesting significant tectonic influence.