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ABSTRACT: A Structurally Controlled Fan-Delta Complex at the Southern Margin of the Peninsular Range Forearc Basin Complex (Baja California)

William R. Morris, Cathy Busby-Spera

A confined trunk fan delta and its structurally controlled tributary fan deltas are extremely well exposed along the southern margin of the Rosario embayment. This fan-delta complex consists of nonmarine to deep marine deposits almost continuously exposed for over 20 km in the down-paleoslope direction. Facies and stratal patterns in the fan-delta complex were controlled by local faulting, climatic variation, and eustasy, resulting in relative sea level fluctuations.

Basin bathymetry and drainage patterns were controlled by a series of half-grabens that formed along north-south-trending faults that lay along the northern margin of an east-west-trending depression. Breccias were initially shed into the north-south-trending half-grabens; axial drainage systems were later established within the grabens, making up the tributary fan deltas. These tributary fan deltas fed a voluminous trunk fan delta confined to the east-west-trending depression.

The trunk and tributary fan delta deposits show two major progradational to retrogradational cycles that record relative sea level fluctuations. Progradation in the shallow-marine environment is represented by conglomerate channels cut into ripple-laminated or bioturbated siltstone and HCS sandstones, overlain by conglomerate mouth bar deposits interstratified with nearshore sandstone deposits. Retrogradation in the shallow marine environment is recorded by either a vertical clastic facies transition or a clastic-carbonate facies transition. The clastic facies transition consists of nearshore sandstone deposits overlain by offshore bioturbated siltstones. The clastic-carbonate facies transition consists of the development of red algal patch reefs and rhodoliths on top of fan-delta con lomeratic lobes, mudstone and sandstone bank channel margins, or paleobasement highs. The clastic-carbonate facies transition reflects low sediment supply controlled by climatic conditions. The progradational-retrogradational cycle in the deep-marine environment consists of interbedded mudstone and sandstone overlain by lobate sandstone deposits in turn overlain by slumped mudstone and conglomerate channelized into the mudstone; these in turn are draped by nonchannelized fine-grained mudstone and sandstone. Progradation in the deep-marine environment apparently began as soon as subaqueous channels in the fan delta reached the shelf-slope break.

Tectonic events provided the major control on the configuration of the fan-delta complex and availability of clastic sediment supply, while progradational-retrogradational cycles were controlled by the interaction of climate, tectonics, and eustasy.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990