Brian J. Willis¹, Sharon L. Gabel¹, Christopher D. White²

(1) State University of New York at Oswego, Oswego, NY
(2) Louisiana State University, Baton Rouge, LA

ABSTRACT: Sharp-based, tide-dominated, delta deposits of the Sego Sandstone, Book Cliffs, Utah

Detailed architecture studies of the Cretaceous lower Sego Sandstone Member of the Mancos Shale in east-central Utah allows interpretation of depositional processes within tide-dominated deltas and quantification of the dimensions and character stratal heterogeneities that can influence reservoir behavior. Lobate tidal sandstones (~30 km wide and 10-20 m thick) have sharp erosional bases that formed as falling sea level allowed tides to scour offshore marine deposits. The tidal sandstones formed as ebb-dominated tidal bars migrated down prograding delta fronts. Proximal parts of these deltaic-sandstones contain an upward-coarsening succession of late highstand tidal bar deposits locally cut by fluvial valleys, or tide-eroded estuaries, formed during relative sea-level lowstand or early stages of a subsequent transgression. An upward fining of tidal sandstones at the distal end of prograding lobes records the landward shift in deposition following maximum lowstand. Estuary fills are highly variable, reflecting local depositional processes and variable rates of sediment supply along the coastline. Lateral juxtaposition of regressive deltaic deposits and delta-front incisions filled with transgressive estuarine deposits produced marked facies changes in sandstone layers along strike. Estuarine fills cut into the forward-stepped deltaic sandstone tend to be more deeply incised and richer in sandstone than those cut into the backward-stepped deltaic sandstone. Most delta top deposits were stripped during transgression. Tidal currents strongly influenced deposition during both forced regression and subsequent transgression of shorelines. This contrasts with sandstones in similar basinal settings elsewhere, which have been interpreted as tidally influenced only in transgressive parts of depositional successions.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado