High-resolution sequence stratigraphy and implications for Cretaceous glacioeustasy of the Late Cretaceous Gallup System, New Mexico, U.S.A.

Deciphering the relationships between traditional lithostratigraphy and sequence stratigraphy is the key to correctly understanding time-stratigraphic relationships and basin evolution. Both insufficient sample spacing, which induces ‘‘aliasing,’’ and lack of datums are problematic issues for sequence stratigraphic correlation. High- resolution sequence stratigraphic analysis of the Cretaceous Gallup Formation, deposited during the late Turonian to early Coniacian, documents the high-frequency depositional cyclicity using detailed facies analysis in extensively exposed outcrops in northwestern New Mexico, USA. Seventy-one sedimentological sections were measured along the depositional-dip-oriented outcrop belt, with less than 1 km average distance between measured sections. Laterally extensive bentonite layers, deposited on nearly horizontal seabeds, provide chronological control and are used as datums for accurate correlations with minimal stratigraphic distortion. Based on the integration of key bounding surfaces with detailed facies analysis, twelve stratigraphic sequences, consisting of twenty-six parasequence sets composed of sixty-one parasequences, were identified. Four classes of shoreline trajectory are quantified: ascending regressive with accelerating and decelerating rates, descending regressive, and transgressive. The maximum shoreline transgression and regression of individual parasequence sets are up to 60 km and 48 km respectively. The maximum estimated relative sea-level fall and rise of individual parasequence sets are 34 m and 38 m. Parasequence sets show aggradational–progradational, retrogradational, progradational–aggradational, and degradational stacking patterns. These accommodation successions help to define systems tracts. The results show that the previously mapped sandstone tongues are equivalent to high-frequency sequence sets. Chronostratigraphic analysis shows that the Dilco Member is diachronous and correlated to different sequences. The Torrivio Member fluvial sandstone was probably deposited diachronously, and part of it may be Gallup age. The matching magnitudes of relative sea-level falls and rises, and an absence of evidence of tectonism, suggest a eustatic control. Autogenic processes may have operated during deposition but were not the dominant controls. The estimated depositional durations of sequence, parasequence set, and parasequence are 100 kyr, 46.2 kyr, and 19.7 kyr, respectively, implying that Milankovitch- cycle-dominated glacio-eustasy may be the key control on the high-frequency sequence stratigraphy of the Gallup system in greenhouse age.