Mixed carbonate-siliciclastic sedimentation on a tectonically active margin: Example from the pliocene of Baja California Sur, Mexico

Bioclast-rich, coarse-grained deposits in the Pliocene Loreto basin provide a record of mixed carbonate and siliciclastic sedimentation at the steep h anging-wall margin of this small, fault-controlled basin. Sedimentary facie s consist of sand- to gravel-sized carbonate debris mixed with volcaniclast ic sand and gravel in a proximal to distal facies tract that includes matri x-rich and matrix-poor shelly conglomerate, impure calcirudite and calcaren ite, mixed-composition turbidites, and bioturbated calcarenitic sandstone. Carbonate material was produced by mollusks and other benthic organisms on a narrow, high-energy shelf and mixed with volcaniclastic sand and gravel i n cross-shelf channels. These mixtures were transported down a steep subaqu eous slope by debris flows, grain flows, and turbidity currents, forming fo resets and bottomsets of marine Gilbert-type deltas. This style of mixed ca rbonate-siliciclastic sedimentation has not been documented in detail elsew here but should be locally abundant in the stratigraphic record of fault-bo unded basins, particularly those with cool or nutrient-rich waters that sup port relatively few binding and framework-building faunas, Recognition of s imilar facies in other settings can provide useful insights into ancient co nditions of carbonate production, oceanography, climate, and tectonics.