Late neogene stratigraphy and tectonic control on facies evolution in the Laguna Salada Basin, northern Baja California, Mexico

The Laguna Salada Basin (LSB) in northeastern Baja California records late- Neogene marine incursions in the Salton Trough and progradation of the Colo rado River delta. Early subsidence and subsequent tectonic erosion are rela ted to evolution of the Sierra El Mayor detachment fault during late Miocen e time (< 12 Ma). The stratigraphy of uplifted blocks on the east-central m argin of the Laguna Salada Basin and from three exploratory wells allows re construction of the main sedimentary and tectonic events. Marine mudstone a nd sandstone, and subordinate conglomerate of the Imperial Formation tecton ically overlie metamorphic and granitic basement. Microfossils, lithology, and sedimentary structures in the Imperial Formation define Upper Miocene ( <6 Ma) outer-shelf facies that grade up-section into inner-shelf and tide-d ominated delta plain deposits of the ancient Colorado River. Lower Pliocene (similar to4-2 Ma) reddish, sub-arkosic fluvial sandstone and siltstone of the Palm Spring Formation defines progradation of non-marine fluvio-deltai c deposits over the marine Imperial Formation. Continuous outcrops of the P alm Spring are less than 170-m thick, but correlative deposits are more tha n 570 m thick in the lower part of a 2400-m deep geothermal exploratory wel l on the eastern margin of LSB. Interfingering fluvial-sandstone deposits a nd prograding alluvial fanglomerates with coarse debris-flow and rock-avala nche deposits crudely mark the onset of vertical slip along the Laguna Sala da fault and rapid uplift of Sierra Cucapa and Sierra El Mayor. Up to 2 km of Quaternary alluvial-fan and lacustrine deposits accumulated along the ea stern margin of LSB, whereas lower subsidence rates produced a thinner sedi mentary wedge over a ramp-like crystalline basement along the western margi n. In early Pleistocene time (similar to2-1 Ma), the Laguna Salada became p rogressively isolated from the Colorado River delta complex, and the Salton Trough by activity on the Elsinore and Laguna Salada fault zones. (C) 2001 Elsevier Science B.V. All rights reserved.