Tectonic and eustatic controls on sequence stratigraphy of the Pliocene Loreto basin, Baja California Sur, Mexico

The Loreto basin formed by rapid westward tilting and asymmetric subsidence within a broad releasing bend of the Loreto fault during transtensional de formation along the western margin of the active Gulf of California plate b oundary. Sedimentary rocks range in age from similar to 5(?) to 2.0 Ma and consist of siliciclastic and carbonate deposits that accumulated in nonmari ne, deltaic, and marine settings, The basin is divided into the central and southeast subbasins, which have distinctly different subsidence histories and stratigraphic evolution. Sedimentary rocks of the Loreto basin are divi ded into four stratigraphic sequences that record discrete phases of fault- controlled subsidence and basin filling. Sequence boundaries record major c hanges in tilting geometries and sediment dispersal that were caused by reo rganization of basin-bounding faults. Sequence 1 consists of nonmarine cong lomerate and sandstone that accumulated in alluvial fans and braided stream s. The sequence 1-2 boundary is a marine flooding surface in both subbasins , and parasequences within sequence 2 consist of progradational Gilbert del tas that are capped by transgressive marine shell concentrations and floodi ng surfaces. The sequence 2-3 boundary is a low-angle erosional unconformit y in the southeast subbasin and a thin interval of downlap in the central s ubbasin, Sequence 3 is characterized by bioclastic Limestones that were der ived from the uplifted portion of the hanging-wall tilt block, The sequence 3-4 boundary is an angular unconformity in the southeast subbasin and an a brupt marine flooding surface in the central subbasin, Sequence 4 consists dominantly of in situ shallow-marine carbonate deposits. By comparing parasequences of sequence 2 with marine oxygen-isotope curves, we can discriminate between eustatic and tectonic controls on stratigraphi c evolution, In the central subbasin, sequence 2 accumulated during a short phase of extremely rapid subsidence (8 mm/yr); it contains 14 paracycles t hat do not match the O-isotope curve, and there are no unconformities. In t he southeast subbasin, sequence 2 accumulated at a rate of similar to 1.5 m m/yr; it contains 4 paracycles that appear to match the O-isotope curve, an d sequence boundaries are unconformities, Thus, we conclude that during seq uence 2 deposition: (1) extremely rapid subsidence in the central subbasin outpaced eustatic sea-level changes, and Gilbert delta paracycles were prod uced by episodic fault-controlled subsidence; and (2) subsidence in the sou theast subbasin was slower than the rate of eustatic sea-level changes, and the internal stratigraphic cyclicity preserves a record of eustatic rather than tectonic events.