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Controls and Physiographic Limits from Cyclostratigraphy: Upper Cambrian and Lower Ordovician Examples from the Great Basin

Evans, Kevin R.1
1 Department of Geography, Geology, and Planning, Missouri State University, Springfield, MO.

In west-central Utah, conspicuous carbonate cycles, typically 1.5-3.0 m thick, were deposited during a major eustatic rise that began during the Upper Cambrian Elvinia biozone and continued well into the Ordovician (Ibexian Series). These cycles span depositional environments and lithofacies ranging from deep-subtidal spiculitic lime mudstone to subtidal-supratidal dolomite settings. Key marker beds in the deep-subtidal succession of the Sneakover Limestone Member of the Orr Formation can be correlated laterally up to 115 km from the southern House Range to the Fish Springs Range. More than 200 cycles comprise a thick succession of shallow marine dolomitic beds in the overlying Hellnmaria Member of the Notch Peak Formation. Cycles in the uppermost Lower Ordovician in the Confusion Range accumulated in an intermediate depositional setting. Measured stratigraphic sections and outcrop gamma-ray profiles of the deep and intermediate successions can be correlated confidently with a gamma-ray log from the American Quasar Horse Heaven - State 16-21A well on the crest of the Confusion Range.

Spectral analysis of the gamma-ray log shows secular variation in frequencies that can be interpreted as precessional and obliquity-dominated cyclic successions. Previous investigators have argued for an allogenic origin of meter-scale cycles in Cambrian and Ordovician strata of the Great Basin based on cumulative aggradational plots (Fisher plots) and gamma analysis; these generally have lumped cycles together as precessional successions or bundled eccentricity-dominated cycles. Spectral analyses of gamma-ray log data in this study show that although precessional frequencies dominate the signal in most intervals, frequency responses within the obliquity and eccentricity ranges strengthen a case for astronomical forcing, and the occurrence of short intervals of obliquity-dominated cycles has not been recognized previously.

The preservation of these astronomical signals is related to major sea-level rises in a region that experienced both rapid subsidence and high sedimentation rates. In contrast to the cyclic succession in Utah, in east-central Nevada, beds at the top of the Barton Canyon Limestone Member of the Windfall Formation (equivalent to the lower Sneakover Limestone Member) record a marked relative rise of sea level and development of a drowning unconformity.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009