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Keywords:

  • Debris flow;
  • fan delta;
  • Gilbert foresets;
  • gravelly lakeshore;
  • Nevada;
  • spits;
  • Walker Lake

Abstract

The 3·2 km long Rose Creek fan delta of west-central Nevada is prograding from an active rift margin into the 32 m deep Walker Lake. A case study of the forms, processes and facies of this fan delta reveals that the proximal and medial zones mainly are of sub-aerial origin, and the distal zone is of lacustrine origin. Pebbly to bouldery rock-avalanche mounds >100 m thick (Facies A) and muddy to bouldery debris flow levées 0·5 to 2·0 m thick (Facies B) dominate the proximal zone, whereas mostly matrix-supported cobbly pebbly debris flow lobes 0·1 to 1·0 m thick (Facies C) typify the medial zone. Surficial pebble lags and gully fills (Facies D) are widespread in both zones but, in exposures, comprise only partings or lenticles between debris flow units. The distal fan delta mainly consists of lakeshore to lake-bottom tracts formed by extensive wave reworking of debris flow facies. Nearshore deposits include erosional cobbly boulder lag beaches (Facies E), pebbly constructional beaches attached at headcuts or on barrier spits (Facies F), pebbly upper shoreface (Facies G) and sandy lower shoreface (Facies H) tracts positioned lakeward of the beach, and pebbly landward-dipping foresets (Facies I) and backshore-pond sand and mud (Facies J) present landward of the spits.

Erosional lag beaches fringe the windward north side of the fan-delta front, attached constructional beaches characterize the central zone, and southward-elongating barrier spits typify the leeward south side, extending from the zone of greatest projection of the fan delta into the lake. Shoreline facies asymmetry results from largely unidirectional longshore drift caused by high fetch to the north and minimal fetch to the south, combined with the arcuate shape of the fan-delta front. The spits overlie a platform deposited below common wave base consisting of south-east-trending cones of pebbly Gilbert foresets (Facies K) and sandy toesets (Facies L). Typically slumped silt and mud (Facies M) fringe both this platform and lower shoreface sand in deeper water. This case demonstrates facies types and patterns that are inconsistent with the widely promoted fan-delta facies model having a front consisting of an apron of radially directed Gilbert foresets deposited where sub-aerial flows enter the lake. The Rose Creek fan-delta front instead features a sharp contact between sub-aerial and lakeshore facies formed where waves erode, sort and redistribute heterogeneous debris flow sediment into the various shallow-to-deep lake facies. Gilbert foresets are present only in the lee of the fan delta where sediment moving by longshore drift reaches the brink of the spit front. This facies scenario results from the infrequency of fan-building events versus nearly constant wind-induced waves, a scenario that, in contrast to the popular Gilbert model, probably is the norm for fan deltas. The level of Walker Lake, and thus the position of wave reworking on the Rose Creek fan delta, fluctuated over a range of ∼157 m during the last 18 kyr, producing complex interfingering between sub-aerial and lakeshore facies across a 1700 m wide radial belt, typifying a wave-modified, freestand lacustrine fan delta.