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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.