Pulsed iceberg delivery driven by Sturtian ice sheet dynamics:An example from Death Valley, California

The Kingston Peak Formation is a Cryogenian sedimentary succession that crops out in the Death Valley area, California, USA. It is widely accepted to record pre-glacial conditions (KP1), followed by two glaciations of pan-global extent, the older Sturtian (KP2 to KP3) and younger Marinoan glaciation (KP4). In the type area (the Kingston Range), detailed facies analysis of the Sturtian succession reveals that a basal diamictite unit and an upper boulder conglomerate were deposited by proglacial subaqueous sediment gravity flows. An olistostrome unit punctuating the succession is interpreted to result from tectonically induced downslope mobilization during isostatic rebound, triggered by significant ice-meltback. Focussing on strata onlapping the olistostrome, this article provides insight into the processes of glacial re-advance following an intra-Sturtian glacial minimum. The first 50 m of strata above the olistostrome are thinly bedded turbidites that are devoid of lonestones. A trend towards thicker graded beds upsection, in concert with the gradual appearance and then abundance of lonestones, testifies to the influence of ice rafting and to the resumption of a direct ice sheet influence upon sedimentation. Stratigraphic organization into thickening and coarsening upward bedsets over a multi-metre scale reveals a subaqueous gravity flow-dominated succession composed of a spectrum of high to low density turbidites, with thick graded boulder-conglomerates at intervals. The finer-grained facies assemblage is heterolithic: current ripple cross-laminated sandstones intercalated with shales that bear delicate granule to pebble-sized dropstones in abundance. Intervals of dropstone-bearing and dropstone-free strata are attributable to dynamic oscillation of the ice margin in the hinterland. Integrating palaeocurrent data with observations from neighbouring outcrop belts allows a detailed palaeogeographic map of the eastern Death Valley area to be compiled for the first time.