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Ancient Submarine Slope-Fan Systems
Depositional Processes and Fluid Mechanics of Upper Jurassic Conglomerate Accumulations, British North Sea
Upper Jurassic fault-controlled fan-deltas and submarine fans which have been drilled and cored along the western edge of the South Viking Graben in the British North Sea contain three major lithofacies: (1) interbedded sandstone and shale; (2) massive to bedded sandstone; and (3) pebble to boulder conglomerate. These lithofacies are indicative of depositional setting and processes within individual fan systems. Depending on a subaerial or submarine setting, lithofacies 1 represents fan-delta front and submarine channel overbank deposition; lithofacies 2 represents tractive deposition in a fan-delta channel and high-density turbidity current activity in a submarine fan channel. Lithofacies 3, the pebble to boulder conglomerate, was largely deposited as debris flows in subaerial fan-delta and submarine fan channels. Subaerial debris flows were differentiated from subaqueous flows by examination of fabric order and degree of correlation between maximum clast size and flow thickness. Better ordered fabric, matrix support, and poor correlation between maximum clast size and flow thickness were indicative of subaqueous debris flows with the converse true for subaerial flows.
Assuming debris flows to be Bingham substances, yield strengths were calculated for 14 selected flows from various cores in the study area. All of the calculated values using both largest boulder and flow unit parameters fell in 103-104 dynes/cm2 range, and correlated well with yield strengths computed by other workers for modern debris flows and experimental laboratory slurries. Calculation of rigid plug flow velocities gave results which also compared favorably with velocity measurements in natural and experimental debris flows.
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