Reconstruction of the Cretaceous Gallup Shoreline

Abstract

The parasequence-thickness-to-sandstone-fraction ratio (TSF) is a recently-developed sequence stratigraphic approach to estimate internal stacking pattern and associated systems tract for siliciclastic paralic sequences. The well-exposed Gallup shoreline system and Ferron Notom deltaic complex along the western margin of the Cretaceous Western Interior Seaway (WIS) provide an excellent platform to explore the application of the TSF method to shallow marine deposits. The conducted TSF analyses on these two coastal-margin systems show a good correlation with the preexisting architectural-facies studies with regard to the interpretations of sequence division, shoreline trajectory, and progradation distance. Accordingly, the TSF method proves to be a small but effective tool in sequence stratigraphic analyses of allogenic-dominated clastic systems. Deciphering controlling mechanisms for sequence development under the Late Cretaceous ultragreenhouse climate is the key to evaluate possibilities of ephemeral ice sheets in the contemporaneous polar areas. Two-dimensional airy back-stripping results derived from the Milankovitch-scaled coastal-margin deposits including the Gallup Formation, the Notom and the Last Chance deltaic complexes of the Ferron Sandstone, and the Cardium Formation in the Western Interior Foreland Basin. Our back-stripping analyses show the magnitudes of the sea-level changes along the 1500 km west coastline of the Western Interior Seaway ranged from -10 to +42 m. In light of the depositional duration of the coastal systems as well as their associated sea-level variation, it is deduced that the orbital-forcing glacioeustasy plays the most significant role in modulating accommodation space for the sequence development. Other factors such as tectonics, aquifer- and thermal- eustasy, autoretreat show much less important influence on sea-level change and shoreline migration for the coastal depositional systems. The prevalent Turonian paralic sequences with the glacioeustatic origin indicate the existence of polar glaciation during one of the warmest ages in the earth history. Depositional history of the Gallup Formation and its genetically-related Torrivio and Dilco members, also termed as the Gallup system, remains ambiguous due to paucity of basinwide sequence stratigraphic analysis and associated paleogeographic reconstruction. The subsurface and outcrop sequence stratigraphic correlation based on 990 well logs and three outcrop belts unequivocally demonstrate the Milankovitch-scaled depositional cycles within the marine-dominated Gallup system. As a comparison, nonmarine Torrivio and Dilco Members are attributed to coastal plain deposits with the development of incised valley systems and multiple fluvial aggradational cycles, respectively. The reconstructed paleogeography of the Gallup shoreline system shows wave-dominated beach-to-offshore deposits with episodic fluvial influence. Overall, the newly-established three-dimensional sequence stratigraphic framework show an excellent correspondence with the previous outcrop-based counterpart, which adds more evidence for the dominated glacio-eustatic controls on the Turonian coastline of the WIS.