Integrating sandstone petrology and nonmarine sequence stratigraphy: application to the Late Cretaceous fluvial systems of southwestern Utah, U.S.A.

Lawton, T.F.; Pollock, S.L.; Robinson, R.A.J.

Petrographic and dispersal data are essential to correct interpretation of mechanisms that create continental sequence-stratigraphic architecture. A case study from southern Utah demonstrates that Upper Cretaceous (upper Santonian-Campanian) alluvial successions in the southern most part of the Cordilleran foreland basin were deposited by fluvial systems of contrasting drainage directions and provenance, and suggests that different mechanisms governed their sequence architecture. Most of the rivers flowed northeast, subparallel to the basin foredeep. Less common fluvial systems flowed to the east-southeast. The fluvial sandstones fall naturally into four petrofacies: quartzofeldspatholithic, feldspatholithic, quartzolithic, and quartzose. Petrofacies 1 and 2 were derived from mixed supracrustal and basement sources to the southwest and south, respectively, whereas petrofacies 3 and 4 were derived from uplifted thrust sheets of the Sevier orogenic belt to the southwest and west, respectively. Only the east-southeast-flowing rivers transported the quartzose petrofacies. The fluvial strata, which include the uppermost Straight Cliffs, Wahweap, and Kaiparowits formations, form two large-scale stratigraphic successions typically interpreted as continental sequences hundreds of meters thick. Each succession begins with an amalgamated braided-fluvial deposit, grades to mudstone-rich strata with low sandstone-body connectivity, and culminates in highly connected sandstone bodies with multistory stacking. For the first sequence, the foredeep axis controlled the progradation direction, which was likely driven by climatically induced sediment influx, a eustatic fall, or both. For the second sequence, increased sediment supply permitted the rivers to cross the foredeep. Temporal association of the upper amalgamated deposit with active structures in the thrust belt and foreland basin indicates that syntectonic thrust uplift, not isostatic uplift or climate, caused the influx of quartz.

Integrating sandstone petrology and nonmarine sequence stratigraphy: application to the Late Cretaceous fluvial systems of southwestern Utah, U.S.A.

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