STRATIGRAPHIC RECORD OF MIDDLE CAMPANIAN SEVIER-LARAMIDE TECTONIC AND CLIMATIC TRANSITION, SOUTHERN CORDILLERAN FORELAND BASIN AND SOUTHWESTERN LARAMIDE PROVINCE

Continental strata in the southern part of the Cordilleran foreland basin and intermontane basins of southern Arizona and New Mexico record sweeping changes in tectonic and climatic patterns in the southern Cordillera at the end of middle Campanian time. Coniacian to middle Campanian foreland-basin fill in southern Utah consists of two upward-coarsening fluvial successions capped by large-scale megafan systems whose rivers were fed by monsoonal precipitation. Stratigraphic cycles are 300-400-m intervals of fluvial strata distributed initially by axial rivers that drained topography south of the basin and subsequently by compositionally mature fluvial megafans, containing diverse detrital-zircon ages, shed directly from the Sevier orogenic belt. The younger megafan deposit is the capping sandstone member of the Wahweap Formation, which connected eastward with the Tarantula Mesa Sandstone in the Henry Basin, then part of the broader foreland basin, but now a Laramide structural basin on the west flank of the Monument upwarp. This voluminous younger megafan correlates with the Castlegate Sandstone, a comparable megafan deposit in central Utah.

Lithic sandstones of the overlying Kaiparowits Formation (~76-74 Ma) containing Mesoproterozoic and Mesozoic zircons were derived from the Mogollon highlands and arc terranes to the south and deposited in a Laramide basin formed during early partitioning of the foreland. Contemporary intermontane basins near the US-Mexico border in SE Arizona and SW New Mexico accumulated thick lacustrine deposits (Fort Crittenden and Ringbone formations) and attest to onset of significant topographic relief by late middle Campanian time.

Stratigraphic patterns indicate not only a shift from foreland basin to broken foreland but also waning influence of monsoonal precipitation in the Western Interior. We postulate that regional precipitation patterns which developed upon connection of the Gulf of Mexico and interior seaway near the end of Early Cretaceous time were altered by newly-formed southwestern Laramide uplifts, such that monsoonal moisture, previously delivered to Utah by north-flowing summertime air masses and backstopped by eastward-advancing thrust-belt topography, was instead partially captured in Laramide intermontane basins of the Southwest.