Comparison of Carbonate Cycles in the Cordilleran Region with Midcontinental Cyclothems Suggests a Common Eustatic Origin
Ralph L. Langenheim, Jr., 1994. "Comparison of Carbonate Cycles in the Cordilleran Region with Midcontinental Cyclothems Suggests a Common Eustatic Origin", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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Cyclic carbonates from the middle of zone 21 through the Zone of Fusulina at Arrow Canyon, Nevada are asymmetric. They begin with an abrupt initiation of deeper-water deposition and a relatively restricted benthonic fauna. These sediments are gradationally followed by more coarsely grained sediment containing a more varied fauna. Some of the cycles shallow to subaerial exposure. Using current 'best' radiogenic dates for the base of the Bashkirian, Moscovian, and Kasimovian (Harland and others, 1989), calculated cycle duration ranges from 224,000 to 328,000 years. Carbonate rocks below zones 19, 20, and the lower half of 21, are less uniformly cyclical and the cycles reflect a more or less symmetrical rise and fall of sea level. The change to asymmetrical cycles occurs in the near vicinity of the onset of southern hemisphere glaciation, about 315 my (Harland and others, 1989). Asymmetric cycles, similar in character and length to those at Arrow Canyon characterize contemporaneous passive margin deposits of central Utah, through southern Arizona, into New Mexico.
The Arrow Canyon cycles are compatible in asymmetry and apparent duration with the well-documented cyclothems of the Midcontinent. Although it is impossible to biostratigraphically distinguish and correlate western and middlewestern cycles, their occurrence in similar numbers within zonal units and their similar length strongly suggests contemporaneity of individual cycles, continuity within sedimentary basins and a common, eustatic causal process.
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The collected volume begins with a brief perspective by one of the conveners, followed by articles in order of increasing stratigraphic age. Eustatic sea-level changes and tectonic warpings of basins are competing mechanisms for explaining many stratigraphic patterns. The model for sea-level changes should be developed first for a basin, since it is allocyclic and leads to a series of time bands in the strata. The residual effects should then be modeled for tectonic patterns affecting the depositional processes. Doing the reverse limits time constraints on the tectonic warping models and will blur the resolution of detailed time surfaces in the strata. Case histories of situations with both tectonic warping and time surfaces marked by sea-level events will lead to improved interpretations of earth history.