ABSTRACT: Styles of Sequence Development within Latest Leonardian through Guadalupian Strata of the Guadalupe Mountains
KERANS, C., W. M. FITCHEN, and M. H. GARDNER, Bureau of Economic Geology, University of Texas at Austin, Austin, TX, M. D. SONNENFELD, Colorado School of Mines, Golden, CO, S. W. TINKER, Marathon Research, Littleton, CO, and B. R. WARDLAW, U.S. Geological Survey, Reston, VA
The current generation of field-oriented research in the classic Guadalupe Mountains has been stimulated in large part by the significant potential for incremental recovery of remaining hydrocarbons within the Permian basin. Multiple depositional sequences were previously recognized within latest Leonardian-Guadalupian strata; however, refined shelf-to-basin correlations highlight four phases within the 12 Ma evolution of the platform and its associated deep-water basin. Each phase contains one or more depositional sequences, yet is distinguished by the nature of its facies associations and shelf-to-basin dynamics.
Latest Leonardian to earliest Guadalupian Phase I is characterized by platform sequences of an initially landward-stepping to aggradational nature, followed by 15 km of shelf-margin progradation with a highstand progradation to aggradation ratio equal to 70:1. Inner-shelf to toe-of-slope carbonate facies tracts form broad (18 km), low angle (less than or equal to 2 degrees) sigmoidal clinoforms. Platform top sediments within this phase involve skeletal to peloidal sands, whereas slope and basin facies are micrite-rich. Rare slope and basin megabreccias are slope-derived and involve carbonate mud matrix.
On the platform, early Guadalupian Phase II is characterized by widespread karstification and siliciclastic bypass across a single surface followed by relatively minor shelf-margin progradation of 2.5 km yielding a progradation-aggradation ratio of 80:1. In the basin, Phase II is represented by deposition of 300 m of deep-water sandstone and siltstone that forms a symmetrical cycle with abundant higher-energy features and a paucity of limestone beds.
Mid-late Guadalupian platform sequences in Phase III are strongly progradational (15 km) with a progradation to aggradation ratio of 136:1. Phase III is characterized by multiple platform surfaces of moderate karstification and siliciclastic bypass. High-frequency alternation between preservation of shelf siliciclastics and bypass to the deep-water basin records the onset of "coupled" shelf and basin depositional systems. Inner shelf to toe-of-slope profiles form contracted (5 km), moderately steep (less than or equal to 17 degrees) sigmoid to oblique clinoforms. Relative to Phase II, the 300 m thick basinal strata of Phase III form an asymmetric cycle with an increase in sandstone relative to limestone, an increased diversity of lithofacies, and more poorly organized sediment dispers l patterns.
Late Guadalupian platform sequences in Phase IV are distinguished by a steady increase in slope inclination up to 50 degrees together with a progressive decrease in the progradation/aggradation ratio from 41:1 to 5:1. Inner shelf to toe-of-slope profiles contracted even further (1 km) in connection with the onset of a reef-rimmed platform with associated slope megabreccias with carbonate and siliciclastic matrix. Phase IV is also marked by an abundance of basinal hemipelagic limestones. High-frequency, asymmetric, eolian-derived turbidite cycles are increasingly well resolved within line-feed basinal strata.
The long-term trend from sequences dominated by transgressive systems tracts and siliciclastic bypass across a single surface (Phase I, II) to sequences dominated by highstand systems tracts with siliciclastic bypass across multiple surfaces (Phases III, IV) reflects a progressive decrease in platform accommodation and an increase in sediment flux to the basin. This platform evolution reflects a latest Leonardian to early Guadalupian relative sea level rise, followed by stillstand or fall. Extrinsic accommodation controls on platform geometries are modified by compound effects from underlying basin margin configurations.
AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)