Sedimentology and progressive tectonic unconformities of the sheetflood-dominated Hell's Gate alluvial fan, Death Valley, California

The Hell's Gate alluvial fan of northern Death Valley has an area of 49.5 km(2), a radius of 1 1.8 km, and a smooth 5-3 degrees sloping surface interrupted by shallow (<0.5 m), radially aligned gullies 1-4 m wide. Facies analysis of 1-14 m high exposures at 45 sites reveals that the fan is built almost entirely by water-flow processes. Two facies deposited by sheetflooding dominate the exposures from apex to toe. The main one (Facies A), comprising 80-95% of the cuts, consists of sandy, granular, fine to medium pebble gravel that alternates with cobbly, coarse to very coarse pebble gravel in planar-stratified couplets 5-20 cm thick that are aligned parallel to the fan surface. Facies B, comprising 0-10% of the exposures, consists of 10-40 cm thick wedge-planar beds of sandy pebble gravel in backsets dipping 5 to 24 degrees. Both Facies A and B are produced by infrequent sediment-charged flash floods from the catchment, and accumulate on the fan from supercritical standing waves of an expanding sheetflood. Antidune backsets are deposited during the buildup stage of the standing-wave cycle, and the couplers during the washout stage. The autocyclic growth and destruction of standing waves during a single sheetflood produces 50-25 cm thick sequences of multiple couplers with backsets. Couplers prevail over antidunes due to the selective preservation of deposits of the standing wave washout phase. Three minor facies comprise 5-20% of the fan exposures. The most common one (Facies D) is pebbly cobble gravel in lenticular beds typically 5-25 cm thick that overlie erosional scours into sheetflood deposits. Tt comprises gravel concentrated in gullies by fine-fraction winnowing of sheetflood units during recessional flood or by secondary overland flows. Though common on the fan surface, this facies is stratigraphically limited. Facies C consists of medium- to very fine-grained eolian sandsheet deposits 5-30 cm thick present on the distal fan in association with gully-fill gravel. It forms by wind reworking of the fan surface, and by sand transport from the adjoining erg. Facies C and D gully-fill and eolian deposits together comprise bounding beds that divide successive sheetflood sequences. They record secondary processes active on the surface between infrequent sheetfloods that mainly aggrade the fan. The fifth fan facies (Facies E) consists of lakeshore gravel deposited along the distal fan when it was transgressed by Lake Manly during latest Pleistocene time. The medial 3.8 km part of the Hell's Gate fan is uplifted and backtilted 1 to 16 degrees into a tectonic ridge formed during strike-slip motion along the North Death Valley fault. Progressive intrafan unconformities, each likely initiated during one large earthquake, are common in these deposits, Two to eight sheetflood units capped by gully-fill or eolian facies are exposed within unconformity-bounded intervals, indicating that fan-aggrading catastrophic sheetfloods on a given part of fan are 2 to greater than or equal to 8 times more frequent than earthquakes that cause backtilting. (C) 2000 Elsevier science B.V. All rights reserved.