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We present a terrestrial record of stable carbon and oxygen isotopes from paleosol carbonate for climate interpretations between ca. 71.0 and 63.6 Ma. Isotopic ratios point to covarying and elevated atmospheric CO 2 pressures and temperatures between ca. 70.0 and 69.0 Ma and ca. 65.5 and 65.0 Ma. These two greenhouse episodes were characterized by atmospheric CO 2 levels between 1000 and 1400 ppmV (V = volume) and by mean annual temperatures in west Texas between 21 and 23 °C (~35°N paleo-latitude). Atmospheric CO 2 and temperature relations indicate that a doubling of pCO 2 was accompanied by an ~0.6 °C increase in temperature. A temperature gradient of ~0.4 °C per degree of latitude is proposed for North America across the Cretaceous-Tertiary boundary when comparing temperature proxies from west Texas with paleobotanical work in North Dakota. Our data demonstrate strong coupling between terrestrial climates and ocean temperatures that were possibly forced by Deccan trap volcanic degassing, leading to dramatic global climate changes.

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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 gradationally overlain by paleosols or are sharply overlain by the coarsergrained 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 gradual 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 interval. From base to top, each sequence is characterized by an asymmetric increase and decrease in FAC thickness, a decrease in the proportion of sand-prone fluvial facies, an increase in paleosol maturity, and better paleosol drainage. Whereas FACs and FAC sets are interpreted to record cyclic episodes of channel avulsion and stability, and longer-term avulsive channel 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 megascale 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 (transgressiveequivalent) and decelerating (highstand-equivalent) base-level rise, and subsequent base-level fall (falling stage-to lowstand-equivalent). Sequence boundaries are placed at the sharp inflection between thinning and thickening FACs. Sand-prone facies and immature, more poorlydrained paleosols are associated with the transgressive-equivalent portion of each sequence, and mudrock-dominated overbank facies and their associated mature, well-drained paleosols are associated with the highstand-and falling stage-equivalent.

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