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Rappeport, M.L., 1982

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Rappeport, M.L., 1982, Studies of tidally-dominated shallow marine bed forms; lower Cook trough, Cook Inlet, Alaska: Palo Alto, California, Stanford University, Ph.D. dissertation, 336 p., illust.

Abstract

The scarcity of modern shallow-marine, tidally-dominated environment analogs has contributed to interpretation problems of ancient tidal sedimentary environments. Characteristic sedimentary microtopography of each environment is then an important discrimination aid. Cook Inlet is a high-energy, tidally-dominated environment containing a spectrum of bed form types. This thesis incorporates three interrelated studies of the sedimentary microtopography of a field of complex micro-, meso- and macro-scale clastic bed forms at the mouth of Cook Trough, Cook Inlet, Alaska. The first paper addresses the depositional environments and Quaternary sedimentary units in lower Cook Inlet. Six major depositional environments were delineated: (1) high-energy shoreface, (2) trough-edge platform, (3) trough slope, (4) trough floor, (5) trough-mouth plateau, and (6) seaward progradational ramp. Four seismic stratigraphic units were recognized. Unit D represents unsorted glacial sediments that overlie glaciated basement. Unit C is a thin layer of glacial outwash material. Unit B locally overlies Unit C and is composed of large-scale sand wave complexes. The uppermost unit, Unit A, consists of intercalated sand and silt layers. The second study assesses the scale, distribution, and morphology of bedform types in the study region. Primary bedform size increased with water depth. This reflects a favorable depositional setting resulting from the channeling of tidal currents by topographic highs. Large bedforms were ebb-oriented in the north and flood-oriented in the south. Finer sand is accumulating in the troughs of the large bedforms. Here, micro-scale forms are smaller and more poorly developed. Last, tidal current characteristics, sediment transport, drag partitioning, and flow interactions in the vicinity of the large sand waves was assessed. Near-bottom current speeds ranged from 4-5 cm/sec to 60-70 cm/sec. Mean peak ebb speeds slightly exceeded mean peak flood speeds measured at a height of 1 m. Near-bottom current speeds were higher at crests of large sand waves than in troughs by 20-30%. Spatial control of tidal currents, flow-sheltering effects, and variations in ripple crest trends related to these effects were observed.

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