Sedimentation in Loch Earn and Loch Lubnaig, Scotland (reply)
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Cascadia Channel is the most extensive deep-sea channel known in the Pacific Ocean and extends across Cascadia Basin, through Blanco Fracture Zone, and onto Tufts Abyssal Plain. The channel is believed to be more than 2200 km in length and has a gradually decreasing gradient averaging 1:1000. Maximum channel relief reaches 300 m on the abyssal plain and 1100 m in the mountains of the fracture zone. The right (north and west) bank is consistently about 30 m higher than the left (south and east). Turbidity currents have deposited thick, olive-green silt sequences throughout upper and lower Cascadia Channel during Holocene time. The sediment is derived primarily from the Columbia River and is transported to the channel through Willapa Canyon. A cyclic alternation of the silt sequences and thin layers of hemipelagic gray clay extends at least 650 km along the channel axis. Similar Holocene sequences which are thinner and finer grained, occur on the walls and levees of the upper channel and indicate that turbidity currents have risen high above the channel floor to deposit their characteristic sediments. A thin surficial covering of Holocene sediment along the middle channel demonstrates the erosional or non-depositional nature of the turbidity currents in this area. The Holocene turbidity current deposits are graded texturally and compositionally, and contain Foraminifera from neritic, bathyal, and abyssal depths which have been size-sorted. A sequence of sedimentary structures occurs in the deposits similar to that found by Bouma in turbidites exposed on the continent. There is a sharp break in the textural and compositional properties of each graded bed. The coarser grained, basal zone of each bed represents deposition from the traction load; the finer grained, organic-rich, upper portion of each graded bed represents deposition from the suspension load. Individual turbidity current sequences are thinnest in the upper and thickest in the lower channel. Recurrence intervals between flows range from 400 years in the upper to 1500 years along portions of the lower channel. Evidently each flow recorded near shore did not extend its entire length. Turbidity currents have reached heights of at least 117 m and spread laterally 17 km from the channel axis. Calculated flow velocities range from 5.8 m/sec along the upper channel to 3.3 m/sec along the lower portion. Pleistocene turbidity currents within Cascadia Basin were much more extensive areally than the Holocene flows, and they deposited sediment which was coarser and cleaner. Pronounced levees which border the upper channel are due chiefly to Pleistocene overflow. Coarse gravels and ice-rafted pebbly clays were also deposited along Cascadia Channel during Pleistocene time.
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Clastic sands and silts transported and deposited by turbidity currents have created the vast abyssal plains of the ocean basins and have constructed the abyssal cones and natural levee systems of the continental rise. Clastic silts and lutites, largely transported by ocean currents, have created much of the continental rise and the outer ridges which parallel the continental margins. Biogenous sediment, resulting from near-surface productivity, more or less redistributed by currents, has created the rolling abyssal swales of productive mid-oceanic areas and has contributed to the continental rise and marginal trench sediments. Although turbidity currents are responsible for the leveling of the abyssal plains, turbidites constitute less than one-third of the sediments beneath the plains. Horizontal size grading in turbidites away from source areas is detectable but small. Apparently a much more important control on size is imposed by the lower courses of the major rivers. Turbidity currents originate near the mouths of several major rivers at the rate of 50 per century but in many other likely areas none have occurred for thousands of years. End_of_Article - Last_Page 344------------
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ing the latest Tertiary overlie and are controlled by fans (deltas).Slump scars, gullies, zones of nondeposition due to currents, debris flows, and levees are identified.The Skralinge, Christian, Pining canyons, and other significant features are mapped in greater detail.Paleo-shelves and slopes of the continental margin are defined in seismic profiles.Differential erosion by the canyons reveals lithologic differences which further delineate the older structures.Implications for structural traps and resource deposits are encouraging if technology becomes available for the challenge.
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Examination of long cores collected by deep-sea drilling shows that, at least during the Cenozoic, oceanic sediments accumulated at rates which varied widely in space and time, and that there are many gaps in the sedimentary record. Locally, sedimentation may be extensively controlled by ocean circulation and chemistry. Comparison of data from different regions, however, reveals broad, globally synchronous fluctuations in rate of sediment accumulation, the oceans apparently oscillating between periods of high (middle Eocene, early Miocene) and low (Oligocene, Paleocene) accumulation. Hiatuses in the record are common during periods of generally low accumulation. Such global changes in the rate of deep-sea sediment accumulation can be related to both sea-level fluctuations and global climatic changes, and their influence on sediment supply and ocean circulation. End_of_Article - Last_Page 438------------
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The thrust belt in central Utah can be divided geometrically into four major thrust systems, from west to east: the Canyon Range, the Pavant, the Gunnison, and the Wasatch thrust systems, Biostratigraphic correlations together with constraints imposed by the geometry indicate the following ages for thrusting events: late Albian for the Pavant 1 thrust, late Santonian-early Campanian for the Pavant 2 thrust, middle to late Campanian for the late Canyon Range thrust, late Maestrichtian for the Gunnison thrust system, and late Paleocene for the Wasatch thrust system. In the hinterland, a combination of structural, stratigraphic, and chronologic evidence indicates that shortening was accommodated by the development of a backbreaking (overstep) thrust sequence: Pavant 1 thrust, Pavant 2 thrust, (late) Canyon thrust. This led to the formation of successive overlapping unconformities of late Cenomanian, early-middle Campanian, and late Campanian age. In the foreland, the Gunnison thrust system has a ramp-flat geometry; a series of blind, splay, imbricate faults are associated with a major ramp beneath Sevier and Sanpete Valleys. Late Cretaceous and Paleocene unconformities coincide with the development of an imbricate fan, which was subsequently deformed during the late Paleocene by formation of a deeper duplex structure within the Wasatch thrust syst m. Associated back thrusts accommodated shortening toward the surface at the west side of the Wasatch Plateau. The times of superimposed thrusting phases, when compared with eustatic episodes recorded in the Cretaceous seaway, indicate that episodes of continental tectonism were approximately synchronous with eustatic rises in central Utah. End_of_Article - Last_Page 869------------
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