HAS EARTH EVER BEEN ICE-FREE? IMPLICATIONS FOR GLACIO-EUSTASY IN THE WARMEST CRETACEOUS GREENHOUSE AGE USING HIGH-RESOLUTION SEQUENCE STRATIGRAPHY

Controls on high-frequency sequences in the Greenhouse condition of the Cretaceous Western Interior Seaway remain equivocal as a result of the active foreland basin tectonic setting and lack of direct evidence of glaciation. Recent work has hypothesized glacio-eustatic mechanism in Cretaceous time that challenges the paradigm of an ice-free Greenhouse world. Magnitudes and frequency of sea-level changes can be used to interpret the existence of glaciation in the absence of oxygen isotopic data. Estimation of the eustatic component of sea-level changes requires removal of tectonic influence, which can be evaluated through backstripping techniques. This paper estimates the magnitude and frequency of sea-level changes from sequence stratigraphic analysis of shallow marine deposits to investigate dominant controls on the origin of high-frequency sequences and to seek evidence of glacio-eustasy in Cretaceous time. Sixty-five parasequences, constituting twenty-nine parasequence sets and twelve sequences are identified in the approximate 1.2 Myr of the Late Cretaceous Gallup system in NW New Mexico. The new ⁴⁰Ar/³⁹Ar age dating of bentonites supports that the durations of the individual parasequences, parasequence sets, and sequences match the eccentricity, obliquity, and precession durations of Milankovitch cycles respectively, suggesting an orbital control. The estimated magnitudes of sea-level changes of individual parasequences range between -28 m and +22 m; the values of the sea-level changes presented in this paper coincide with the reported sea-level changes that are hypothesized to be attributed to the ephemeral Antarctica ice sheets. We interpret that the high-frequency sequence stratigraphy of the Gallup system is controlled by glacio-eustatic sea-level changes resulting from growth and decay of ice caps. This study provides critical evidence from shallow marine sequence stratigraphic records for glaciation-dominated high-frequency sea-level variations in Greenhouse worlds.