Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
AAPG Annual Convention, Denver, Colorado, June 36, 2001
Predicting Tidal-Bar Architecture within a Sequence Stratigraphic Framework: Examples from Ancient and Modern Systems
Lesli J. Wood
Tidal bars and amalgamated tidal ridges form important elements within tidally influenced shoreline and deltaic reservoir systems and commonly contain the best reservoir-quality sands. Tidal element dimension, shape, and distribution are influenced by sediment type and amount, water depth, tidal velocity and magnitude, and nuclei type and occurrence. These factors vary within a cycle of shoreline rise and fall, and that change is reflected in tidal-bar architecture. Tidal-bar architectural elements and amalgamated tidal-ridge complexes within the Sego Sandstone (Upper Cretaceous) of eastern Utah show dimensions that vary systematically within a cycle of relative shoreline change. Falling-stage, open-shelf tidal bars show large width:height ratios, open-marine bioturbation, few mud clasts, and more unidirectional crossbeds. Bars deposited during the early falling stages of the third-order Sego cycle show narrower geometries than those deposited during the late falling stage of the maximum lowstand, owing to increased sand supply and shallowing of water depths at maximum lowstand. Falling-stage tidal bars in the eastern (more basinward) outcrop are taller and narrower. Transgressive, estuarine tidal bars show small width:height ratios, brackish-marine bioturbation, and bidirectional crossbeds. The dimensions of tidal bars in the Sego closely resemble those characterizing modern bars in the Bay of Bengal, offshore India (Off, 1963). However, in general tidal ridges from the Sego exhibit much larger width:height ratios than the majority of those modern elements mapped by Off (1963). This difference may be a function of the Segos depositional setting in a shallow epicontinental seaway, where tidal velocities during deposition may have been somewhat dampened (~1.2 to 1.4 knots; 70 to 80 cm/s) and tidal ranges rather low (1 to 2 m).
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