Alan R. Carroll1,
J. A. Toni Simo2,
Kevin M. Bohacs3,
Yow Yuh Chen4,
Ronald J. Hill5
(1) University of Wisconsin, Madison, WI
(2) University of Wisconsin
(3) Exxon Production Research Company, Houston, TX
(4) Exxon Exlporation Company, Houston, TX
(5) Exxon Production Research Company
Abstract: Deep Water Organic Facies and Marine Transgression, Cherry Canyon Formation, West Texas
Organic-rich siltstone facies (ORS) preserved within the lower Cherry Canyon Formation (below the “genetic top” of the Brushy Canyon Formation) contain up to 4.53% total organic carbon (TOC), with the highest values generally occurring in three informal subunits ORS-1, ORS-2, and ORS-3. These may be readily correlated between proximal outcrops near El Capitan Peak and distal outcrops 20 km south at Delaware Peak, based on sedimentary facies, %TOC, and gamma radiation. Based on visual examination of kerogens and biomarker analysis of bitumens, aquatic organisms dominated organic matter input to the siltstone facies, with only minor advection of terrestrial organic matter. Brown algae (kelp) is also abundant in some samples. These observations, together with the presence of minor bisaccate (conifer) pollen, are consistent with a proposed eolian origin for the silt rather than a fluvial origin. Organic enrichments appear to be in part a function of varying input of inorganic clastic sediments.
High C35/C32 homohopane ratios in proximal outcrops of ORS-1 (and in the underlying Brushy Canyon Formation) record reducing conditions, and correlate with elevated gammacerane. The proximal facies may have been deposited within an oxygen minimum zone (OMZ), whereas low homohopane ratios in distal outcrops record better oxegenated conditions. This contrasts with previous interpretations of basin-wide anoxic bottom water. Low homohopane ratios characterize ORS-3 at both locations, indicating that relatively well-oxygenated conditions prevailed. We propose that the base of the proximal OMZ rose relative to the depositional surface, in response to deepening during a 3rd order transgression.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana