Distinguishing Tectonic and Eustatic Signals in a Cyclic Stratigraphic Record
P. W. Goodwin, E. J. Anderson, 1994. "Distinguishing Tectonic and Eustatic Signals in a Cyclic Stratigraphic Record", Tectonic and Eustatic Controls on Sedimentary Cycles, John M. Dennison, Frank R. Ettensohn
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In foreland basins tectonic processes tend to act continuously and leave a record of gradual facies changes (both laterally and vertically). In marked contrast, eustatic processes appear to be organized in a hierarchy in which sea-level rise in the shortest term cycle (the precessional cycle) is so rapid that it is recorded as a stratigraphic surface. Such surfaces mark the boundaries of meter-scale cycles or PACs (6th order sequences), the basic building blocks of the stratigraphic record. Indeed, all actual surfaces at cycle boundaries that are not unconformities (including the boundaries of the 5th or 4th order sequences driven by eccentricity cycles) are a product of this process. Eccentricity (at both scales) produces trends by modifying the magnitude of sea-level change in the basic 20-ky cycle and by producing longer term progressive patterns of deepening and shallowing. However, these larger-scale processes are not directly responsible for surfaces at cycle boundaries.
While eustatic processes seem to be responsible for small-scale cyclic packaging of the stratigraphic record, tectonic processes play different roles. Tectonic subsidence provides the long-term accommodation space for the stratigraphic record and, on a large scale, determines its general composition and degree of completeness. Also tectonically driven differential subsidence leads to lateral thickness and depth variations in facies. Correlations of the 6th and 5th order sequences in the Silurian-Devonian carbonates of the Appalachian basin indicate that differential subsidence is a significant contributor at the scale of 4th order sequences and perhaps at the 5th order scale. These observations suggest that eustatic and tectonic processes both play significant roles in the development of the stratigraphic record, but that these roles are noncompeting.
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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.