Autochthonous outer ramp sedimentation: The Alamogordo Member of the Lake Valley Formation,New Mexico |
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Authors: | David L Jeffery Dr Robert J Stanton Jr |
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Institution: | (1) Department of Geology and Geophysics, Texas A& M University, 77843-3115 College Station, TX, USA |
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Abstract: | Summary Paleozoic carbonate ramp sedimentation has generally been described in terms of downlapping clinoforms composed of allochthonous
sediment derived from shallower environments. However, during transgressive episodes when carbonate sediment production is
low and down slope sediment transport by gravity becomes inactive, autochthonous carbonate sediment accumulates in vertical
stacks of essentially in situ sediment. Autochthonous outer ramp deposition is probably a part of many Paleozoic ramp strata,
but has heretofore not been recognized because of the general absence of adequate exposures.
Evidence of autochthonous, in situ deposition and preservation of sediments in a starved setting is well displayed in the
Alamogordo Member of the Lake Valley Formation in south central New Mexico. This evidence includes: 1) beds and bed sets that
are individually continuous and traceable along ramp slope for 32 km, 2) down-ramp sequential distributions of depth-sensitive
organisms and assemblages but patchy distribution of rock types, 3) lack of sedimentary structures indicative of transport,
4) well preserved, unabraded fossils, 5) the common occurrence of fossils in life position, 6) beds traceable into and through
mounds 7) bed thickness trends ascribed to biotic productivity, and 8) geopetal structures in original position.
Integrated paleontologic, sedimentologic, and stratigraphic data provide information about depositional processes and setting.
The depositional slope was approximately 0.5o based on the distribution of fossil algae; this is comparable to dips reported for other Mississippian homoclinal ramps.
An oxygen minimum zone may have impinged on the ramp during a major flooding event. Shifts in biotic gradients from bed to
bed reveal transgressive-regressive patterns that would not be resolvable without detailed paleontological evidence.
The Alamogordo Member formed as a result of transgressive and early highstand starved carbonate sedimentation along a narrow,
homoclinal outer ramp. The surface of maximum flooding and the boundary between the TST and HST are within the Alamogordo
Member. |
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Keywords: | Carbonates Ramp Sequence Stratigraphy Microfacies Autochthonous Sediment Production New Mexico Mississippian (Waulsortian) |
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