Edwards, Chris M.¹, David Hodgson¹, Stephen Flint¹, John Howell²
(1) University of Liverpool, Liverpool, United Kingdom
(2) Geological Institute, Bergen, Norway

ABSTRACT: Turbidite Deposition During Forced Regression in a Ramp Margin: Turonian Lower Ferron Sandstone Member of the Mancos Shale, Central Utah

Down dip correlations of the lower Ferron Sandstone demonstrate that significant turbidite successions can be generated in ramp settings with rapid sea-level fall. These Turonian-age strata consist of shallow marine to continental successions that were deposited in the Western Interior retroarc foreland basin and now form the principal coalbed methane reservoirs in northern Castle Valley. Around the northern tip of the San Rafael Uplift (SRU) lower Ferron strata comprise a series of southeastward-stepping sharp-based shoreface parasequences deposited within a falling stage systems tract. The uppermost parasequence, constituting the lowstand systems tract, can be traced down dip into slumped sandstones before tapering into Mancos Shale siltstones. Coeval channelised, coarse-grained, locally conglomeratic, sandstone turbidites and adjacent fine-grained overbank facies are found up to 70 km down dip. This relationship indicates a direct linkage between forced regressive shorefaces and turbiditic successions in a ramp margin. In the absence of direct evidence, we speculate that intrabasinal structures may have created local and transient “shelf breaks” and/or rapid sedimentation rates during falling sea-level generated slump and turbidity current events. In addition, turbidites are confined to the eastern side of the north-south trending (i.e. parallel to palaeoflow) SRU implying structural influence on turbidite routing. The observations have ramifications for predicting the geometries and distribution of reservoirs deposited during relative sea level fall in basins lacking a distinct shelf break physiography.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.