ABSTRACT: Tectonic and Sea-Level Controls on Net/Gross Ratios of Cretaceous Nonmarine Strata

GUSTASON, EDMUND R., BP Exploration (Alaska), Anchorage, AK

High net/gross fluvial deposits within the foredeep region of the Cretaceous Western Interior foreland basin accumulated during periods of base-level lowering, whereas high net/gross fluvial deposits accumulated along the eastern stable margins of the foreland basin during periods of base-level rise. This "out of phase" relationship between high net/gross fluvial deposits is interpreted as an interaction between fluctuating rates of foredeep-basin subsidence and eustatic sea-level.

Nonmarine deposits from the western foredeep region display a peculiar alteration of cliff-forming, high net/gross and slope- and ledge-forming, low net/gross intervals. High net/gross intervals consist predominantly of bedload and mixed-load meanderbelt and braidbelt deposits with rare preserved floodplain and overbank fines, whereas low net/gross intervals consist of mixed-load and suspended load meanderbelt and floodplain deposits. Except for the lowermost and uppermost intervals, all of these nonmarine deposits merge basinward into (interfinger with) marginal marine strandplain and deltaplain deposits. High net/gross nonmarine deposits correlate to progradational marginal marine deposits whereas low net/gross non-marine deposits correlate to aggradational and possibly (?) retrogra ational marginal marine deposits. This relationship occurs on parasequence, parasequence set, and sequence scales.

In contrast to the western foredeep region, stratigraphic relationships from the eastern stable region of the foreland basin indicate that valley incision occurred during periods of base-level lowering, high net/gross fluvial deposits aggraded within paleovalleys during periods of base-level rise, and low net/gross fluvial deposits were deposited during periods of stable base level.

Although the timing of deposition of high net/gross fluvial strata appears to be "out of phase" across foreland basins, high net/gross fluvial deposits are associated with base-level lowering.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)