Strike Variability of Carbonate Platform Margin Stratal Architecture and Cycle Stacking Patterns: Outcrop and Seismic Examples from Lower Permian Depositional Sequences of the Permian Basin, U.S.A.

William M. Fitchen

Strike variability in stratal architecture, cycle stacking patterns and facies differentiation within sequences exemplifies the potential for differential stratigraphic response of platforms to eustasy. Variability exists within base-level cycles of several scales within a compound stratigraphic hierarchy, though the causes may vary among different scales. Cross-sections from outcrops in the Sierra Diablo document the framework and stacking pattern of 3rd-order sequences (HFS), including: two progradational middle Wolfcampian HFS (mW1-2), one backstepped upper Wolfcampian HFS (uW1), one progradational lower Leonardian HFS (L1), aggradational middle (L2) and upper Leonardian HFS (L3), three progradational upper Leonardian HFS (L4-6), and two backstepped, aggradational upper Leonardian HFS (L7-8). Seismic lines from the northern Delaware and Midland Basins and San Simon Channel area document the regional consistency of 3rd-order sequence stacking patterns (a response to eustasy), but show local variability related to local subsidence, antecedent topography (owing to deeper structures and platform margin erosion), windward vs. leeward facing, and siliciclastic sediment supply. Sequences L2 and L3 appear to exhibit the greatest variability in stacking pattern.

Strike variability in 4th/5th-order cycle stacking patterns within 3rd-order sequences as studied in outcrop is greatest in sequences L2 and L3, in which headland-bight margin trends are developed on a lateral scale of 1-2 mi. Aggradational to backstepping reef-margin facies with steep (<=35°) foreslopes developed along headlands. Mudstones abut these margin facies abruptly along headlands and may contain megabreccias at the toe-of-slope. More gently sloping (10-15°) "ramp" margin strata composed of fusulinid packstones characterize bights. Progradational ramp margin cycles compose cycle sets with well-developed mudstone bases that may extend over a mile into the platform. Ramp margin cycle sets stack landward in the TST and seaward in the HST. Headland-bight trends (set up by differential subsidence and antecedent topography) lead to strike variability in marine energy, wave base depth, and substrate type. The greater variability in L2 and L3 cycle and sequence attributes relative to enclosing progradational sequences suggests that the potential for strike variability is greatest during composite eustatic sea-level rise.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California