Has Earth ever been ice-free? Implications for glacio-eustasy in the Cretaceous greenhouse age using high-resolution sequence stratigraphy

Controls on high-frequency sequences formed during super-greenhouse conditions in the Late Cretaceous Western Interior Seaway remain equivocal because of the ac­tive foreland basin tectonic setting and the lack of direct evidence of polar glaciations to support a glacio-eustatic origin. This paper quantifies eustatic sea-level changes based on high-resolution sequence stratigraphic analysis and improved chronometry of shal­low marine deposits of the Late Cretaceous Gallup Sandstone in New Mexico, USA. Backstripping techniques remove tectonic and compactional subsidence and enable quantification of the magnitude of eustatic sea-level change, that allow evaluation of the dominant controls on the high-frequency se­quences to resolve the role of orbitally con­trolled, climate-driven eustasy versus tecton­ics. Sixty-five parasequences, constituting 29 parasequence sets and 12 sequences are iden­tified in the ∼1.2 m.y. duration of the Late Cretaceous Gallup system. New 40Ar/39Ar dating of bentonites constrains the dura­tions of the individual parasequences, para­sequence sets, and sequences, and that these match Milankovitch periodicities, indicating an orbital climate control. The magnitudes of sea-level changes between parasequences range between −28 m and +22 m, which are compatible with hypotheses of both aquifer and glacio-eustasy. Aquifer-eustasy predicts a reciprocal relationship between floodplain cycles and shallow marine sequences, such that aquifer drawdown and falling water tables should correlate to rising sea levels (highstands), whereas increased aquifer stor­age and rising water tables should correlate to falling sea levels (lowstands). Our prelimi­nary observations show synchronous, versus reciprocal, relationships that may be more compatible with a glacio-eustatic origin. The results of this study support the hypothesis that the Cretaceous greenhouse was marked by high-frequency, low-amplitude glaciations driven by orbital climate cycles, but further work is required to evaluate the contribution of aquifer-eustasy.