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Stratigraphic Architecture and Eustatic Setting of Slope Grainstones Associated with Peak Icehouse Conditions

El-Azzi, Joseph 1; Kerans, Charles 2; Wahlman, Gregory P.3
1 RCRL, Bureau of Economic Geology, Austin, TX.
2 Jackson School of Geosciences, University of Texas at Austin, Austin, TX.
3 BP America, Houston, TX.

High-latitude Gondwanan glaciation is widely recognized during Late Pennsylvanian and Early Permian (Wolfcampian) time. Accompanying high-amplitude glacioeustatic fluctuations (up to120m) have been documented. Recognition of major episodes of subaerial exposure and erosion are common during this period, but documenting the magnitude of eustatic sealevel fall, requiring recognition of major downward shifts in coastal onlap, presents a special case. Challenges include locating shore-proximal facies perched on pre-existing slopes, and differentiating autochthonous lowstand prograding shallow shelf deposits from deeper-water allochthonous slope facies. Exposures of the Hueco Group (Wolfcampian) in the Southern Outlier of the Hueco Mountains, El Paso County, Texas present a potential example of such a downward shift in coastal onlap. Data collected for this study include 1) photopanels and ground-based lidar of an oblique-dip-oriented exposure, 2) 10 RTK-GPS-calibrated measured sections, 3) 216 hand samples, including core plug transects across key surfaces and 129 petrographic thin sections. These data allow high-resolution 2- and 3-D mapping of stratigraphic surface geometries and facies correlations in both dip and strike views. The Lidar-constrained surfaces also aided reconstruction of eustatic pinning points associated with proposed subaerial exposure events.This Hueco Mts. outlier shelf-margin facies complex includes: basinal, thin bedded, spiculitic wackestone; toe-of-slope, brachiopod-crinoid-fusulinid mud-dominated packstone (mdp) to grain-dominated packstone (gdp); lower slope debris flow complexes of lithoclastic-fusulinid-skeletal packstones; mid-upper slope fusulinid-crinoid-Tubiphytes-skeletal mdp to gdp; shelf-margin reef-mound sponge-Tubiphytes-phylloid-red algae bafflestones and boundstones. Within this framework, a pair of shingled wedge-shaped accumulations composed of steeply dipping crinoid-fusulinid gdp’s with lesser debris flows record deposition on the middle slope. The upper surface of the first of these wedges is capped by an erosion surface with 4m of local relief and displaying low areas infilled by siliciclastic siltstones. Two possible interpretations for these wedges are as (1) allochthonous falling-stage middle slope wedges or (2) lowstand shoreline complexes. Independent of the exact model, using the in-situ reef-mound facies as a proxy for sea-level, a minimum eustatic shift of 55m is associated with these wedges.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009