Stratigraphy, sedimentology, and tectonic development of the southeastern Pliocene Loreto Basin, Baja California Sur, Mexico

The Pliocene Loreto basin formed as a westward-tilting asymmetric half graben that subsided rapidly in response to oblique dextral-normal slip on the Loreto fault during late Pliocene time. We recognize four stratigraphic sequences throughout the Loreto basin that record different phases and rates of fault-controlled subsidence, uplift, and input of siliciclastic sediment. The southeastern Loreto basin formed at the southern end of the eastern structural high, which is the uplifted portion of the hanging-wall tilt block of the Loreto fault. Sedimentary rocks of sequence 2 in the southeastern Loreto basin are divided into eight lithofacies associations that represent different depositional environments (in parentheses): (1) shelly sandstone and pebbly sandstone (siliciclastic shallow marine shelf), (2) sandy and pebbly bioclastic limestone (mixed bioclastic-siliciclastic shelf), (3) laterally continuous shell beds (sediment-starved shelf), (4) megaforesets and associated fan-delta facies (Gilbert-type fan deltas), (5) heterolithic facies (gravelly fandelta plain), (6) cross-bedded conglomerate and sandstone (gravelly braid stream), (7) planar-stratified sandstone (siliciclastic shoreface), and (8) conglomerate and bioclastic limestone (rocky shoreline). Detailed measured sections combined with 1:10,000-scale structural and facies mapping permit correlation of strata across numerous normal and strike-slip faults. A reconstructed north-south facies panel reveals complex stratigraphic architecture in sequence 2, which has a total thickness of 340 m and is organized into six parasequences bounded by hiatal marine shell beds. A prominent buttress unconformity on the northern margin of the basin records progressive subsidence of the basin relative to the southern flank of the eastern structural high during sequence 2 time. Paleocurrent data and lateral distribution of lithofacies show that gravelly and sandy siliciclastic sediments were shed into the southeastern Loreto basin from the footwall of the Loreto fault located south and west of the study area. The uplifted portion of the hanging wall tilt block, although acting as a structural high, remained mostly submerged below sea level during sequence 2 time and therefore was not a source of siliciclastic sediment. The base of sequence 2 is a prominent, widespread marine flooding surface that coincides stratigraphically with a pronounced shift in paleocurrent directions. This sequence boundary records a rapid subsidence event and major structural reorganization within the basin that probably was controlled by a change in kinematic behavior of the Loreto fault. The lower half of sequence 2 was dominated by shelf-type fan deltas that prograded northward into the sandy siliciclastic shelf setting. The upper half of sequence 2 consists of four gravel-rich Gilbert-type fan deltas, 3 of which were derived from the footwall uplift of the Loreto fault to the southwest, and one of which consists of shelly debris derived from the ESH to the north. The upper boundary of sequence 2 is a low-angle unconformity marked by an abrupt shift to bioclastic carbonate deposition. This records a second major change in the structural behavior of the basin, in which the hanging wall tilt block experienced renewed uplift and erosion of carbonate shoals and reefs. The stratigraphic evolution of the southeastern Loreto basin thus provides insights into important details of the tectonic and structural evolution of this active basin and its margins.