ABSTRACT

A turbidite sequence in the lower Mesozoic Mineral King roof pendant (southern Sierra Nevada, California) is interpreted to be a small submarine fan, informally named the Bullfrog fan, which formed in a continental arc setting. Abundant sand-sized sediment was supplied to the fan by rapid downslope reworking of rhyolitic pyroclastic debris from a silicic volcanic source that lay to the southeast in an area now occupied by younger plutons. The Bullfrog fan accumulated in a segment of an epicontinental silicic volcanic are in which caldera subsidence structures have been identified.

The Bullfrog fan is a 480-m-thick progradational sequence. Bed thickness, average grain size, and abundance of channelized mass-flow deposits all increase toward the southeast where the most proximal part of the fan and the inferred source area have been intruded by a large, Cretaceous granitic pluton. The Bullfrog fan prograded onto starved basin-plain sediments consisting of very thin bedded and laminated silt and pyritic mud. Fan-fringe and lobe-fringe deposits compose only the basal tenth of the section, resulting in abrupt lateral pinch-out of fan sandstones into starved basin-plain deposits. The overlying outer-fan depositional lobes compose over one-third of the section, have high sandstone to shale ratios, and commonly lack fine-grained interlobe deposits, suggesting that depo itionally the lobes laterally interfinger and are vertically stacked. These are overlain by midfan sandstones with abundant channel deposits that form over half of the Bullfrog fan. The midfan deposits consist of a lower sequence of lobe-apical channel cycles and an upper sequence of braided channel cycles. Each lobe-apical channel cycle is characterized by an unchannelized upward coarsening and thickening sequence overlain by a channelized upward-fining and thinning sequence. Channelized deposits form about 40 percent of the lobe-apical channel cycles, and each cycle records the extension of a channel over the depositional lobe that it fed. Pebbly sandstones characterize both the lobe-apical channels and the braided-channel cycles, and interchannel material is rare. Pebbly mudstone debr s-flow deposits are locally interstratified with the braided-channel deposits and overlie the midfan in a nearly continuous sheet less than 8 m thick. The debris flows locally pass upward into laminated and rippled shallow-marine siltstones, coquinites, limestones, and cross-bedded sandstones bearing Late Triassic megafossils, although faulting has obscured this contact in most areas. Nonetheless, the shallow-marine sedimentary rocks apparently directly overlie midfan deposits with no inner-fan or slope deposits intervening.

Without sufficient clay-sized sediment, highly mobile turbidity currents capable of depositing basin-plain, fan-fringe, lobe-fringe, and interchannel turbidites apparently could not develop. Instead, high-density turbidity currents and fluidized flows deposited sandand pebble-sized pyroclastic debris on smooth and channeled lobes that filled a relatively shallow basin within a submarine caldera complex.