RAPID SUBSIDENCE AND STACKED GILBERT-TYPE FAN DELTAS, PLIOCENE LORETOBASIN, BAJA-CALIFORNIA-SUR, MEXICO

Pliocene nonmarine to marine sedimentary rocks exposed in the Loreto b asin, Baja California Sur, provide a record of syntectonic subsidence and sedimentation in a transform-rift basin that developed along the w estern margin of the Gulf of California, A thick sequence of twelve Gi lbert-type fan deltas, having a total measured thickness of about 615 m, accumulated near the fault-bounded southwestern margin of this basi n. Based on stratal geometries and lithofacies associations, sedimenta ry rocks are divided into Gilbert-delta topset, foreset and bottomset strata, shell beds and background shallow-marine shelf deposits. Topse t strata of each Gilbert-type delta cycle are capped by laterally pers istent molluscan shell beds containing diverse assemblages of bivalves , pectens, oysters, gastropods and echinoids. These shell beds are int erpreted to be condensed intervals that record sediment starvation dur ing abandonment of the fan-delta plain. Delta abandonment may have bee n caused by large episodic faulting events, which submerged each pre-e xisting fan-delta plain, substantially slowed detrital input by drowni ng of alluvial feeder channels, and created new accommodation space fo r each new Gilbert-type fan delta. Alternatively, it is possible that delta-plain abandonment was caused by upstream avulsions and autocycli c lateral switching of fan-delta lobes during relatively uniform rates of slip along the basin-bounding fault. Two contrasting, plausible ba sin models are proposed for the Loreto basin: (1) asymmetric subsidenc e along a high-angle oblique-slip normal fault, producing a classic ha lf-graben basin geometry with vertically stacked Gilbert-type fan delt as; or (2) lateral stacking and horizontal displacement of strata away from a relatively fixed depocenter due to fault movement in the relea sing bend of a listric strike-slip fault. We favor the first model bec ause field relations and simple geometric constraints suggest that mos t of the total measured section represents a true vertical stratigraph ic profile, Assuming vertical sediment accumulation and using ages of interbedded tuffs obtained from high-precision Ar-40/Ar-39 dating of p lagioclase and biotite, quantitative decompaction and geohistory analy sis was carried out for the Loreto basin sequence. Tuff ages range fro m 2.61 +/- 0.01 Ma in the lower part of the basinal sequence to 1.97 /- 0.02 Ma near the top, with two intermediate tuffs dated at 2.46 +/- 0.06 and 2.36 +/- 0.02 Ma that are separated by 782 m of measured sec tion. Basin subsidence initially took place at moderate rates of 0.43 +/- 0.17 mm/yr and accelerated dramatically at 2.46 Ma to 8.1 +/- 5.1 mm/yr. This phase of extremely rapid subsidence lasted for only about 100 ka, and it produced much of the total accommodation space and sedi mentary thickness in the basin, Accumulation of Gilbert-type fan delta s took place only during the short pulse of very rapid subsidence, bet ween 2.46 and 2.36 Ma. Prior to this time interval, alluvial-fan and s helf-type fan-delta depositional systems prevailed; afterwards no fan deltas of any kind were deposited, and the basin evolved to a slowly s ubsiding low-energy carbonate shelf setting. This suggests that very r apid subsidence, combined with rapid sediment input, may be required t o maintain steep basin-margin slopes and continually create new accomm odation space, conditions that seem necessary for the development of t hick sequences of stacked Gilbert-type fan deltas.