Depth Determination and Quantitative Distinction of the Influence of Tectonic Subsidence and Climate on Changing Sea Level During Deposition of Midcontinent Pennsylvanian Cyclothems
George D. Klein, 1994. "Depth Determination and Quantitative Distinction of the Influence of Tectonic Subsidence and Climate on Changing Sea Level During Deposition of Midcontinent Pennsylvanian Cyclothems", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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New sedimentological determinations of the water depth and associated sea-level change of midcontinent Pennsylvanian cyclothems shows that they accumulated in water depths ranging from as low as 32 m to as high as 160 m, depending on which model is used to establish the deepest water facies. These depth determinations also indicate that, regardless of model, depth variations existed for different cyclothems both laterally and in time. Average water-depth determinations and sea-level change for models of Heckel (1977) and Gerhard (1991) are 96.4 m and 86.0 m respectively.
Analysis of tectonic subsidence permits calculation of the magnitude of tectonic processes and associated climatic effects, which controlled changes in sea level during deposition of Pennsylvanian cyclothems. Far-field tectonic effects, in response to regional orogenic movements, partially influenced Pennsylvanian sea-level change in the midcontinent. Organization of Virgilian and Missourian midcontinent cyclothems into four-to fivefold bundles shows that sea-level changes in midcontinent platform areas were influenced both by Milankovitch orbital parameters and longer-term climate change, whereas Desmoinesian sea-level change apparently was influenced more strongly by tectonic subsidence controlled by foreland-basin tectonism.
The magnitude of tectonically-contributed change in sea level varied laterally. In the midcontinent, tectonic subsidence accounts for approximately 5 to 20% of the total sea-level change in platform areas, and perhaps as much as 20% in basin depocenters. The remaining change in sea level is controlled by both short-term glacial eustasy (Milankovitch orbital forcing; approximately 70% of sea-level change) and long-term climate change (approximately 15% of sea-level change). These findings suggest that, away from orogenic belts, climatic change is the principal driving mechanism controlling sea level change, whereas within orogenic belts, climate becomes somewhat more subordinate as a driving mechanism for Pennsylvanian sea-level change, even though indicators of climatic change itself are preserved.
Methods discussed herein permit calculation of magnitudes of both tectonic and climatic-eustatic components of sea-level change influencing Pennsylvanian cyclothem deposition, and may be applicable to other cyclic sequences.
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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.