EUSTASY, CLIMATIC INSTABILITY, AND AUTOCYCLICITY: MECHANISMS FOR A FLUVIAL CYCLIC HIERARCHY PRESERVED WITHIN THE K/T TRANSITION OF BIG BEND NATIONAL PARK, TEXAS, USA

Stacking pattern analysis of a continuously exposed, relatively conformable alluvial succession of latest Cretaceous and earliest Tertiary age within the Tornillo Basin of Big Bend National Park, Texas, reveals the composite effects of autogenic and allogenic processes during a 7 Ma period of sediment accumulation. Three scales of stratigraphic cyclicity are observed within the study interval. Forty-nine meter-scale fluvial aggradational cycles (FACs) are recognized, and consist of fining-upward deposits that are most frequently capped by a paleosol, or sharply overlain by the coarser-grained base of the succeeding FAC. FACs occur within six, decameter-scale, fluvial aggradational cycle sets (FAC-sets) that from base to top fine-upward, and within the succession of component FACs, exhibit either a gradual increase in paleosol maturity and paleosol drainage, or a quasi-symmetrical pattern of increasing and decreasing paleosol maturity and drainage. FAC-sets occur within four (two completely and two partially exposed) hectometer-scale fluvial sequences that are each characterized by an upsection, asymmetric increase and decrease in FAC thickness, increase in the proportion of overbank mudrock, and increase in paleosol maturity and better paleosol drainage. FACs are interpreted to record individual episodes of channel avulsion and ensuing channel stability, whereas FAC-sets are interpreted to have been produced by a succession of avulsion events that caused the channel axis to drift away from and back towards a fixed position within the alluvial valley through time. The equivalent marine shoreline along the southwestern margin of the North American Western Interior Seaway was within 25 to 500 km of the study area during the latest Cretaceous and earliest Tertiary, and oscillated with a third-order frequency. Hectometer-scale fluvial sequences coincide with both third-order changes in eustatic sea level (TR 8-10), and cyclic variations in atmospheric pCO₂ as reconstructed from the composition of pedogenic carbonate nodules. Fluvial sequences, therefore, may record the long-term adjustment of fluvial equilibrium to both eustasy and greenhouse-coolhouse cycles of climatic instability.