Growth history of Lower Mississippian Waulsortian mounds: Distribution,stratal patterns,and geometries,New Mexico |
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Authors: | Dr David L Jeffery Prof 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 The factors controlling the localization and growth of Lower Mississippian Waulsortian mounds have been difficult to establish
because of limited exposure of individual mounds and mound-bearing platforms in western Europe, where the Waulsortian facies
have been studied most intensively. Mounds on the Lower Mississippian homoclinal ramp of the Lake Valley Formation in the
Sacramento Mountains, however, are exposed exceptionally well at platform, outcrop, and mound scales in an area roughly 5
km by 20 km, and provide the opportunity to better understand these aspects of Waulsortian mounds.
Mounds occur in the northern 2/3 of the essentially continuous 32 km dip transect of the ramp. Mounds grew in an outer ramp
setting below wave base, predominantly in the deeper part of the photic zone. Mounds range from broad composites of laterally
back-stepping subunits on the shallow part of the ramp to taller and more vertically stacked composite structures down-ramp.
The composite nature of the mounds is documented by distinct stratal units that have characteristic facies and geometries
common to mounds throughout the transect. As a result, mound growth and form can be described in terms of several primary
controlling parameters—submarine topography, water circulation (upwelling of nutrients and oxygen rich waters; oxygen deficient
bottom waters), light penetration and the distribution of phototrophic microorganisms, and fluctuations in accommodation.
Episodic mound growth is documented by diastems bounding the stratal units within the mounds as well as by the long-established
useage of Alamogordo, Nunn, and Tierra Blanca phases of mound growth, correlative with the contemporaneous level-bottom units.
However, mound growth that has been correlated with the level-bottom Nunn Member in reality took place during the late stage
of deposition of the Alamogordo Member, and nondeposition or erosion occurred on the mounds during deposition of the Nunn
Member.
Mounds in the shallower (northern) part of the ramp grew primarily on the margins of a broad, low, intra-ramp topographic
high, which had been defined previously from facies and isopach trends in underlying strata. Both the margins and the irregular
topography of the high are reflected in the distribution, growth geometries, and facies patterns of the mounds, and by the
facies and thickness trends of the strata enclosing the mounds. The siting of individual mounds on the shallower part of the
ramp was controlled by local topography on and along the margins of the intra-ramp high. Mound growth along the margin began
at or just behind local highs, retrograded onto the intra-ramp high, and then prograded onto the basinward side of the initial
mound.
The lesser height and more pronounced backstepping of mounds on the shallower part of the ramp, in contrast to mounds that
grew more vertically and with less back-stepping down ramp suggest that growth and overall morphology were also controlled
by accommodation. |
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Keywords: | Waulsortian Mounds New Mexico Ramp Reef Reef Localization Reef Growth Sequence Stratigraphy Upwelling Mississippian |
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