COMPOSITE MECHANISMS FOR TERRESTRIAL ENVIRONMENTAL VARIABILITY AND THEIR RELATIONSHIP TO ALLUVIAL SEQUENCE STRATIGRAPHY: THE K/T SUCCESSION IN WEST TEXAS, USA, AND SOUTH-CENTRAL ALBERTA, CANADA

Analysis of time-equivalent, paleosol-bearing alluvial successions spanning the Cretaceous-Tertiary (K-T) boundary in southern and central Alberta, Canada, and west Texas, USA, indicate cyclic sediment accumulation and pedogenesis during corresponding episodes of climatic variability. Two hectometer-scale, fining-upward alluvial sequences are recognized in both Texas and Alberta, and correlate with two intense greenhouse maxima of pCO2 and temperature as documented by paleosol carbonate stable C and O isotopes. In west Texas, greenhouse maxima coincide with the transition between the most immature and poorly-drained paleosols (transgressive-equivalent), and increasingly mature and better-drained paleosols (early highstand-equivalent). In central Alberta, greenhouse maxima correspond with highstand-equivalent strata near the top of each sequence and immediately preceded deposition of thick coals. The covariation between greenhouse episodes and alluvial sequences in Texas and Alberta may reflect a systematic record of environmental change produced by the composite and interrelated effects of tectonism, sea level change, and atmospheric composition. Previous work in Alberta by Catuneanu et al. (2000) provide evidence that K-T sequences are related to foreland tectonism, whereas Atchley et al. (2004) argue that age-equivalent sequences in west Texas record rising and falling sea level. In both cases, greenhouse maxima and the accumulation of alluvial sequences during the K-T transition may ultimately relate to third-order variations in oceanic crust production, i.e., accelerating rates of seafloor spreading coincident with: (1) rising sea level; (2) increased volcanic and tectonic activity along convergent plate boundaries; (3) increasing atmospheric pCO2 and temperature; and (4) mid-latitudinal atmospheric shifts in the position of the subpolar low and corresponding patterns of global precipitation.