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,2
1 Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403-1272
2 Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011
The Loreto basin formed by rapid westward tilting and asymmetric subsidence within a broad releasing bend of the Loreto fault during transtensional deformation along the western margin of the active Gulf of California plate boundary. Sedimentary rocks range in age from 
5(?) to 2.0 Ma and consist of siliciclastic and carbonate deposits that accumulated in nonmarine, 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 divided into four stratigraphic sequences that record discrete phases of fault-controlled subsidence and basin filling. Sequence boundaries record major changes in tilting geometries and sediment dispersal that were caused by reorganization of basin-bounding faults. Sequence 1 consists of nonmarine conglomerate and sandstone that accumulated in alluvial fans and braided streams. The sequence 1–2 boundary is a marine flooding surface in both subbasins, and parasequences within sequence 2 consist of progradational Gilbert deltas that are capped by transgressive marine shell concentrations and flooding surfaces. The sequence 2–3 boundary is a low-angle erosional unconformity in the southeast subbasin and a thin interval of downlap in the central subbasin. Sequence 3 is characterized by bioclastic limestones that were derived 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 abrupt 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 stratigraphic evolution. In the central subbasin, sequence 2 accumulated during a short phase of extremely rapid subsidence (8 mm/yr); it contains 14 paracycles that do not match the O-isotope curve, and there are no unconformities. In the southeast subbasin, sequence 2 accumulated at a rate of 1.5 mm/yr; it contains 4 paracycles that appear to match the O-isotope curve, and sequence boundaries are unconformities. Thus, we conclude that during sequence 2 deposition: (1) extremely rapid subsidence in the central subbasin outpaced eustatic sea-level changes, and Gilbert delta paracycles were produced by episodic fault-controlled subsidence; and (2) subsidence in the southeast 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.
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