Graduate Thesis Or Dissertation
 

Holocene sedimentation and potential placer deposits on the continental shelf off the Rogue River, Oregon

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  • Changes in sea level during the past 20,000 years are recorded in sediments taken from the continental shelf off the Rogue River, Oregon. Sea level has risen approximately 125 m. during the Holocene (Curray, 1965) and the general transgression has been interrupted by several stillstands and minor regressions. Box core samples taken in the area of investigation depict the nature of sedimentation during the last rise of sea level as well as present-day equilibrium sediment relationships. Three sediment facies, a recent fine-grained mud, a basal transgressive sand, and an intermediate sediment, believed to be a mixture of the other two, are defined on the shelf on the basis of textural parameters. The mud facies dominates surface sediment on the central shelf while the sand facies is the most commonly exposed on both the inner and outer shelf. The percentage of sand generally increases with depth in the box cores, often producing a change in sediment facies with depth and demonstrating the transgressive nature of the sediments. Grain-size analysis of the sand fraction of the offshore sediments reveals that offshore sands are finer-grained and better sorted than those occurring on the present beaches. The offshore sands most likely represent relict nearshore deposits and not ancient beach sands which would occur lower in the sediment sequence. High concentrations of heavy minerals are found in the sand fraction of the offshore sediments as well as in beach sands in the area. Brief stillstands of sea level may be reflected in depths where the offshore sands contain anomalously high percentages of heavy minerals. Opaque minerals, chiefly magnetite, occur in placer accumulations on the present beaches and high percentages of these minerals in the offshore sands may be indicative of submerged beach environments associated with stillstands of sea level. The magnetite may be concentrated in sufficient quantity in the placers to produce detectable magnetic anomalies, several of which have been recorded in the area. Other lines of evidence used to determine the depths of probable stillstands of sea level are bathymetric relief, the distribution of shallow water fauna in sediments from deep water, and the distribution of rounded gravels on the shelf. A compilation of the several lines of evidence suggests several stillstands of sea level associated with the Holocene transgression occurring at depths of 18, 29, 47, 71, 84, 102, and 150 meters.
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