Application to Outcrops
Although sequence stratigraphy was originally designed for seismic sections, sequence principles can be readily applied to outcrop, core, and well logs . The first step in this approach is to interpret individual beds in terms of depositional events, including an evaluation of the shear stress in the environment, the type of flow (currents, waves, tides, combined flow), bioturbation and trace fossils, etc. This information is critical for the next step, to recognize bedsets, that is groups of beds that record similar depositional processes, and to interpret those bedsets as facies, the records of particular depositional environments. These steps are critical because errors at this point may cause errors in interpretations of relative depth, which in turn affects the recognition of parasequences and stacking patterns. Solid facies work is essential for a solid sequence analysis.
From successions of facies in an outcrop, shallowing-upward successions can be recognized as well as flooding surfaces such that parasequences can be delimited. Vertical trends in the range of water depths present in successive parasequences can be used to identify stacking patterns and to recognize surfaces that mark the turnarounds from one parasequence set to the next. Potential sequence boundaries should be identified at this step based on one or more of the following criteria: clearly defined erosional truncation, direct evidence of subaerial exposure, or abrupt basinward shifts of facies. Likewise, potential condensed sections should be recognized on the basis of unusual burrowed surfaces, abundant diagenetic materials, fossil concentrations, closely spaced bentonite beds, or radioactive shales. Condensed sections may, but do not necessarily, lie along the maximum flooding surface.
From the recognition of parasequence sets and potential sequence boundaries and condensed sections, systems tracts and major stratal surfaces (sequence boundary, transgressive surface, and maximum flooding surface) can be recognized. It is important to stress that not all of these surfaces or systems tracts may be present within any given sequence in an outcrop. The absence of one or more surfaces or systems tracts may provide important clues as to the relative position of the outcrop within the basin. For example, lowstand systems tracts are commonly absent in updip areas where the transgressive surface and sequence boundary are merged as one surface. In such areas, significant portions of the highstand systems tract may have eroded away and the sequence boundary is marked by the beginning of retrogradational stacking. In downdip areas, the transgressive and highstand systems tracts may be thin and relatively mud-rich, whereas the lowstand systems tract may be characterized by the abrupt appearance of thick sandy facies. Many more variations are possible and many basins are characterized by a typical pattern of sequence architecture.
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