--> No Evidence for Significant Sea Level Fall in the Cretaceous Strata of the Book Cliffs of Eastern Utah

AAPG ACE 2018

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No Evidence for Significant Sea Level Fall in the Cretaceous Strata of the Book Cliffs of Eastern Utah

Abstract

The Book Cliffs of Central Utah have been a key testing ground for the concepts of high resolution sequence stratigraphy since the early 1990’s. In that period over 100 scientific articles have been published on the outcrops and thousands of people have visited the region on academic and commercial fieldtrips. The Book Cliffs are considered central to our understanding of the sequence stratigraphic paradigm. A core component of the sequence stratigraphic model is the implicit assumption of a rising and falling relative sea-level curve, with the occurrence of “sequence boundaries”, formed during intervals of sea-level fall. These are primarily recognized by the presence of incised valleys and associated lowstand deposits basinward of the highstand shoreline.

The commonly accepted sequence stratigraphic model for the Santonian-Campanian section in the Book Cliffs recognizes up to ten high frequency sequence boundaries. Critical re-analysis of these indicates no conclusive stratigraphic evidence for any relative sea-level falls within these strata. The majority of “valley-fill” successions are filled with very large, single-storey tidal macro-forms, that suggest a higher tidal range than is normally implied for the region but no evidence for falling base-level. The rare multi-storey valley fills are interpreted to have formed during the vertical aggradation of raised peat-mires on the coastal plain during normal regression rather than stepped sea-level rise within a valley during transgression after sea-level fall. Previously documented “forced regressions” in the Book Cliffs are associated with a demonstrably rising shoreline trajectory. The major “sequence boundary” associated with the base of the Castlegate Formation is reinterpreted as the progradation of proximal facies belt in a major distributive fluvial system, enhance by upstream uplift and tectonics, rather than sea-level fall. Furthermore, convolution of the known rates of subsidence with recent sea-level curves for the Campanian suggest that no relative sea-level falls would be expected, because the magnitude of falling sea-level was outpaced by the basin subsidence.

These observations indicate that a key aspect of the sequence stratigraphic model is not applicable in outcrops that are widely considered to be one of the type areas for sequence stratigraphic teaching and research. This also has important implications for the use of the sequence stratigraphic approach during greenhouse times.