Sedimentary Fabrics of Debris Flow-Dominated, Stream-Modified Alluvial Fan, Saline Valley, California: ABSTRACT

Although depositional sequences in alluvial-fan deposits may reflect tectonic activity, they are difficult to distinguish from those produced by changes of climate or by local aggradation. This presentation deals with an alluvial fan depositional style that occurs in arid conditions in which rare, large-scale debris flows are modified by more frequent, but small-scale, stream floods. Recognition of this type of deposition in an alluvial-fan succession could indicate change of climate and aggradational sequences may be seen by changes in its fabric. A small alluvial fan in Saline valley, California, was described from apex to toe to determine the types of fabrics produced and their relative distributions. Debris flows produce levees and lobes which appear as 1 to 3 m high mounds of poorly sorted, matrix-supported conglomerates. These mounds become lower, less regularly distributed downfan and disappear before the fan toe. The tops and outer edges of levees and lobes have the largest boulders, which are tightly packed in a near-vertical, imbricatelike pattern. Fan-toe debris-flow fabric is 1 to 10-cm thick pebbly mud sheets with irregular lines of isolated cobbles. The small streams are deeply incised between levees at the fan apex, shallow and anastomosing around levees and lobes at mid-fan, and braided at the fan toe. Deep chann ls are floored by step patterns produced by partly or completely exhumed debris-flow boulders with fine gravel or sand deposited on the upstream side. Shallow channels have two major fabrics: (1) 5 to 20-cm high hummocks of poorly sorted, muddy gravel surrounded by fine gravel and sand with better sorting, and (2) muddy, moderately well-sorted gravel in which the long axes are horizontal and oriented parallel with flow. The braided channels at the fan toe are dominated by horizontally discontinuous lamination and abundant mud intraclasts. Windblown sand commonly accumulates in channels, preserving mud curls and mud drapes from the last flood. All fabrics observed in the small fan in Saline valley were also seen in larger fans there and in several other Mojave basins. These fabrics were also found in some Triassic alluvial-fan deposits in New Jersey. End_of_Article - Last_Page 633------------