Marine geology of the continental margin off southern Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2r36v139f

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  • The continental margin off southern Oregon, which includes the shelf and slope from Cape Blanco to the Oregon-California border, exhibits a distinctive marginal-plateau structural pattern which divides the margin into the continental shelf, the upper continental slope and its associated benches, and the lower continental slope. Lutum transport and deposition have dominated the sedimentary processes on the margin since the start of Holocene time. The structure of the southern Oregon margin is characterized by north-south trending compressional folds, and near-vertical faults which have been down-dropped to the west. Large-scale folds on the upper slope have ponded sediments behind them resulting in the formation of the Klamath Plateau, Cape Blanco Bench, and other bench-like features. Development of the structural pattern is most likely a result of the compressive underthrusting of the oceanic lithospheric plate beneath the southernmost Oregon-northern California margin and the crustal extension which exists throughout the nearby continent and ocean basin. Useful stratigraphic horizons within the late Pleistocene and Holocene margin deposits include Mazama Ash (6600 years B.P.) and several recognizable shifts in the abundance of Radiolaria and planktonic Foraminifera, particularly one dating from 5000-4000 years B.P. Holocene sedimentation rates vary from an average of 10 cm/ 1000 years on the upper slope to an average of 50 cm/1000 years on the lower slope, indicating that the lower slope is out-building and up-building more rapidly than the upper slope. The paleo-depth range of Pliocene fauna in sedimentary rocks from the margin suggests that subsequent to their deposition both uplift and subsidence occurred on the southern Oregon margin. Sediments from the southern Oregon margin consist primarily of olive gray lutite, gray lutite, and sand-silt layers. Olive gray lutite is Holocene in age and is ubiquitous on the margin, with the thickest accumulation (10 m average) found on the lower slope, while the distribution of Holocene lutite on the upper slope is thin and patchy (3-4 m or less). The gray lutite appears to be a late Pleistocene deposit, and the sand-silt layers reflect both ages. The surface sediment distribution pattern on the shelf consists of modern inner shelf sand, modern central shelf mud, and mixed deposits of both types. Relict deposits are present at the shelf edge. The lower slope consists entirely of modern mud, but the surface sediment on the upper slope and benches consists of both modern and relict deposits, and mixtures of the two. The mineralogy of the unconsolidated and consolidated sediments from the margin indicates that the Klamath Mountains have been the dominant source for these deposits since early Tertiary time. This is reflected in the abundance of blue-green hornblende and other heavy minerals indicative of the Mesozoic rocks of the Klamath Mountains; the same source is suggested for the abundant chlorite found in the clay fraction of margin sediments and rocks. There are indications in the mineralogy of lower slope sediments which suggest that the Tertiary strata of the southern Oregon Coast Ranges may be a secondary source for the deposits in this environment. When compared to the upper slope sediments, those from the lower slope have a higher feldspar content, a higher pyroxene-to-amphibole ratio, and an apparently higher illite content. As a result of the Holocene rise in sea level, the deposition of coarse clastics on the southern Oregon margin has been restricted to the inner shelf. Consequently, only the fine-grained lutum discharged from rivers is deposited on the outer margin environments. Submarine topography, oceanographic conditions, and gravity are important factors which effect transport and deposition of lutum on the margin. A model of modern lutum transport by bottom turbid layers and fine-particle suspensate is proposed for the southern Oregon margin. Long-period swell is believed to be responsible for much of the formation of bottom turbid layers on the shelf. Once formed, these turbid layers move north and west over the shelf under the influence of shelf currents, alternating tidal action, and gravity; upon reaching the slope they are funneled into submarine valleys and deposited on the lower slope and adjacent deep sea. Lutum deposited on the upper slope is eventually re-suspended and transported by southerly bottom currents into down-slope valleys; very little lutum remains behind on the upper slope. Deposition of the fine-particle suspensate as well as slumping and other gravitational processes contribute to the lower slope sediments. The end result of modern lutum transport is the continual up-building and out-building of the lower slope.
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