ABSTRACT: Sequence Stratigraphic Analysis of the Lower Permian Hueco Group, Northern Sierra Diablo, West Texas
STARCHER, MICHAEL A., University of Texas at Austin, Austin, TX
The upper Wolfcamp Hueco Group in the northern Sierra Diablo of west Texas was deposited in a shelf setting on the western margin of the Delaware basin. This outcrop study subdivides the Hueco Group, including the Powwow Formation, into a sequence stratigraphic framework. Two depositional sequences and component system tracts within the Hueco have been defined on the basis of cycle stacking patterns and the recognition of unconformities. This study serves as a framework for cycle and sequence recognition in the remainder of the Delaware basin for the upper Wolfcamp.
The Powwow Formation (up to 75 m thick) is a basal clastic unit overlying a pre-Permian angular unconformity. This unconformity marks the cessation of Marathon deformation, and Powwow deposits record initial transgression of Permian seas over a tectonically deformed region. The Powwow represents a tectonically enhanced lowstand system tract. Gradationally, overlying the nonmarine deposits are lime-rich sand and siltstones that are transitional to open-marine deposits.
The remainder of the Hueco Group (up to 410 m thick) is shallow-water platform limestones and dolomites. This unit is subdivided into three divisions, A, B and C, based on cycle stacking patterns, with respect to thickness and subfacies. Approximately 36 cycles are recognized in the limestones and dolomites of the A division (200 m). These rocks are thick bedded, subtidal, crinoidal-peloidal packstones shoaling to fusulinid grainstones and packstones, and is interpreted as a transgressive system tract.
The B subdivision (120 m) contains approximately 50 well-defined cycles. This unit contains thin beds of limestone and dolomite made of burrowed, subtidal, fusulinid-peloid packstones capped by intertidal to supratidal fenestral laminites. This part of the section is interpreted as a highstand system tract because of the meter-scale cycle stacks and the shallow-water to supratidal depositional environment. Further evidence for highstand deposition is found in the upper boundary of this division, which is a minor unconformity marked by low-angle truncation of the underlying rocks.
The C subdivision (90 m) contains approximately 25 cycles. The unit contains predominantly thick beds of limestone and dolomite. These cycles consist of burrowed, subtidal packstones shoaling up to fusulinid grainstones that occasionally shoal to laminite caps. This unit is interpreted as another transgressive system tract, and the underlying boundary is defined as a sequence boundary.
The Hueco is cut by a major regional unconformity, and much of the Hueco, including any shelf margin, is eroded. This unconformity is a type I sequence boundary that cuts down basinward into the lowermost transgressive unit (A division), truncating 250-300 m of Hueco section. Updip the unconformity is marked by extensive topography with karst development. Downdip, the unconformity is characterized by high-angle (up to 15 degrees) truncation of Hueco strata overlain by slope deposits of the Leonardian Victorio Peak Formation. The unconformity flattens out basinward and separates the A division from basinal deposits of the Bone Spring Formation.
AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)