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Trench sedimentation in the Lower Palaeozoic of the Southern Uplands

SIRS,—This letter draws attention to similarities in the sedimentology of the Southern Uplands in the Lower Palaeozoic with that of an active present-day trench—the eastern Aleutian Trench off southern Alaska (von Huene and Kulm 1971.) The comparison supports Dewey's (1971) contention that the Southern Uplands marked the site of an active trench. The eastern Aleutian Trench off southern Alaska terminates to the east in the Queen Charlotte Islands Transform Fault. This transform fault and the trench separate the Pacific Plate from the North American Plate. To the west, the topographic trench is interrupted by Kodiak Seamount, which prevents transfer of sediment to the deeper part of the trench south of the Aleutian arc. The mean depth of the Pacific Ocean floor south of the trench is about 4 km, but near the trench this abyssal plain rock sequence plunges rapidly, reaching a depth of about 6 km in the trench axis. The axial part of the trench is filled with a wedge of horizontally-bedded sediment 25 km wide and up to 1 km thick, and is characterised by a very high rate of sedimentation; it gradually becomes deeper westwards with increasing distance from the transform fault. The abyssal plain rock sequence consists of almost 1 km of pelagic sediment and interbedded turbidites, transported across the Queen Charlotte Islands Transform Fault from mountainous areas on the American Plate. The trench fill consists almost entirely of turbidites, with a minor contribution from slumps. Sands are restricted to a zone near the trench wall: most of the trench fill is silt and mud. Most of this sediment is derived from the mountains of southern Alaska, and is transported directly down the trench wall. Some may be transported axially from the shallow eastern end of the trench. Axial flow gives a prominent turbidity current channel at the foot of the trench wall, which appears to rework submarine fans built up by lateral supply. Lateral turbidites may swing round to an axial direction on reaching the trench floor.