Abstract

Three conglomeratic thinning- and fining-upward sequences in the Upper Cretaceous at Wheeler Gorge, California have previously been assigned to submarine-channel environments. There is continuing controversy as to whether the channels represent inner to midfan environments, or whether they are incised into basin-plain facies. To shed further light on the problem, the mudstones and turbidites below and above the conglomerates are described here in detail. Below the conglomerates, the sequence is dominated by mudstones interbedded with very fine grained turbidite sandstones in beds up to 5 cm thick. There is only one slumped horizon, about 240 m below the conglomerates. By contrast, the mudstones above the conglomerates contain more and thicker turbidites (beds up to 47 cm), which occur in at least three thinning- and fining-upward sequences. Bouma sequences (A, AC, BC) are common, whereas below the conglomerates, the thin turbidites show only graded bedding (very fine sandstone to mudstone) or ripple cross lamination. Slumping is much more common above the conglomerates; at least eight distinct slumped horizons are scattered through about 33 m. The slumped turbidites are interpreted as channel-margin or levee facies, and the thinning- and fining-upward sequences of relatively thin, unchannelized turbidites suggest a levee environment with gradual lateral migration of the channel away from the depositional site, yielding the thinning and fining upward. Below the conglomerate complex, the thinness of the turbidites (average 1 cm, maximum 5 cm), the absence of thinning- or thickening-upward sequences, and the dominantly muddy nature of the rocks, suggest a depositional environment on a basin floor far from any channel influence. The overall sequence of units records the incision of a channel complex (the conglomerates) into quiet basin-floor mudstones. Following deposition of all of the conglomerates, there was lateral shifting of the channel, with concurrent deposition of a channel-levee complex. The channel-levee complex is a variant of the more general lobate submarine-fan model. In the fan model, the fan is fed by a single channel: the channel-levee complex replaces that feeder channel by a group of channels. Similarly, the few branching channels on the lobate midfan may be replaced by extensive channel-levee complexes with little development of smooth lobes.

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