ABSTRACT: Sequence Stacking and Architecture, Cretaceous Foreland Basin, Utah

Peter Schwans

Tectonic and eustatic accommodation determined the stacking patterns of third-order sequences at the subsiding Cordilleran basin margin in Utah. Tectonic subsidence provided the necessary accommodation, whereas relative sea level changes determined the facies stacking patterns. Strata beneath sequence boundaries are progressively truncated toward the thrust front, whereas strata above onlap the surfaces in a landward direction, indicating a significant hiatus. At the basin margin, 100-600-m-thick fluvial and deltaic stratal packages disconformably overlie 100-150-m-thick marine mudstones deposited at water depths of up to 70 m. At the thrust front, correlative fluvial strata are stacked into 150-350-m-thick, upward-fining packages bounded by unconformities. Within sequenc s alluvial architecture changes from laterally amalgamated channel-form conglomerates deposited by braided

streams to nested sandstone channel forms set within thick overbank strata and representing an overbank-prone, multichannel, anastamosed system. On the basis of regional well log and outcrop correlations, the deltaic and shoreline strata in the basin represent the lowstand through transgressive systems tracts, while the fluvial strata at the thrust front form transgressive through early highstand systems tracts. The changes in depositional architecture within sequences are a direct response to foreland basin accommodation changes occurring on a 1- to 3-m.y. scale.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990