Print this pageDynamic Facies Models Linked To Stratigraphic Hierarchy: Grayburg Formation (Guadalupian), Brokeoff Mountains, New Mexico
WARD, W. BRUCE, and ROGER J. BARNABY
Dynamic facies models tied to stratigraphic hierarchy, shelf position and antecedent topography have been established from outcrop data for the Grayburg Formation (Guadalupian) in the Brokeoff Mountains, New Mexico. The Grayburg is part of a composite sequence of at least 4 high-frequency sequences (HFS): (1) initial transgressive, (2) transgressive-to-turn-around, (3) maximum-flood and progradational, and (4) progradational. HFS-1 has 18+ high-frequency cycles, and HFS-2 has 13 cycles grouped into 4 composite cycles. Stratigraphic hierarchy depends on shelf position and is a product of 4 scales of sea-level fluctuations acting on antecedent topography and shelf profile.
The initial Grayburg onlaps antecedent topography of the locally karsted ramp of the San Andres Formation HFS-1 isopach maps reveal the study area to be landward of the steep margin, with a local high protruding basinward. Quartz-sandstone-dominated HFS-1 deposits filled topographic lows and moved the slope break basinward. There was no well-developed ramp crest. During the initial HFS-2, grainstone and fenestral bodies developed over the local high. Tidal channels and ridges parallel this feature. By the time of deposition of the last HFS-2 transgressive cycle, antecedent topography was obscured. During the HST of HFS-2, basinward-migrating ooid sand shoals piled up to form a well-developed ramp crest. Tepee-fenestral facies developed on the landward side of the crest. These relationships show that for the transgressive portion of the Grayburg (HFS-1 and 2), major influences on facies geometries evolved from preexisting topography, to tidal ridges and channel cutting, to dominantly constructional topography (e.g., ooid shoals). Understanding the depositional dynamics that produce facies juxtapositions is required for predicting facies distributions.