Eustatic Control on Alluvial Sequence Stratigraphy: A Possible Example from the Cretaceous-Tertiary Transition of the Tornillo Basin, Big Bend National Park, West Texas, U.S.A.

Abstract

Paleosol-bearing alluvial strata of latest Cretaceous and earliest Tertiary age are continuously exposed along Dawson Creek, in Big Bend National Park, west Texas, U.S.A., and exhibit a three-tier hierarchy of depositional cyclicity. Meter-scale, fluvial aggradational cycles (FACs) occur as fining-upward successions that are gradation- ally overlain by paleosols or are sharply overlain by the coarser- grained base of the succeeding FAC without an intervening paleosol. FACs stack into decameter-scale, fluvial aggradational cycle sets (FAC sets) that also fine upward, and from base to top contain either a grad- ual upsection increase in soil maturity and soil drainage or a somewhat symmetrical pattern of increasing and decreasing paleosol maturity. Longer-period trends of FAC thickness, lithologic proportions, paleosol maturity, and paleosol drainage indicate that two complete, and two partial, hectometer-scale fluvial sequences occur within the study in- terval. From base to top, each sequence is characterized by an asym- metric increase and decrease in FAC thickness, a decrease in the pro- portion of sand-prone fluvial facies, an increase in paleosol maturity, and better paleosol drainage. Whereas FACs and FAC sets are interpreted to record cyclic epi- sodes of channel avulsion and stability, and longer-term avulsive chan- nel drift within the alluvial valley, respectively, fluvial sequences may coincide with third-order sea-level changes within the North American Western Interior Seaway. As such, the Cretaceous-Tertiary (K-T) transition within the Tornillo Basin may provide an example of me- gascale stratigraphic cyclicity that is controlled by eustatic sea level within a fully fluvial succession. Thickening and thinning successions of FACs record a third-order period of accelerating (transgressive- equivalent) and decelerating (highstand-equivalent) base-level rise, and subsequent base-level fall (falling stage- to lowstand-equivalent). Se- quence boundaries are placed at the sharp inflection between thinning and thickening FACs. Sand-prone facies and immature, more poorly- drained paleosols are associated with the transgressive-equivalent por- tion of each sequence, and mudrock-dominated overbank facies and their associated mature, well-drained paleosols are associated with the highstand- and falling stage-equivalent.