Sequence Stratigraphy and Structural Evolution of the Pliocene Loreto Basin, Baja California Sur, Mexico

Rebecca J. Dorsey and Paul J. Umhoefer

The Loreto basin is a west-tilted half-graben basin that formed by asymmetric subsidence along the Loreto fault, on the eastern margin of Baja California. Application of standard sequence-stratigraphic terminology is hindered in this basin by its small size, steep gradients, and lack of continuity of unconformities that bound distinct formation-scale lithosomes. Thus we group Pliocene sedimentary rocks into four parasequence sets (PSS's), which provide a record of structurally controlled subsidence and uplift in the basin. PSS-1 consists of nonmarine alluvial-fan and braid-stream conglomerate. It is overlain along a sharp marine flooding surface by PSS-2, which consists of shallow marine sandstone and stacked footwall-derived Gilbert-type fan deltas. Structural and stratigraphic relations indicate that the basin formed in a broad rollover structure during PSS-2 time. PSS-3 overlies a hinge-line unconformity that passes westward into a conformable section in the basin center; it consists of hangingwall-derived bioclastic fan deltas and footwall-derived volcaniclastic fan deltas that prograded toward each other across the basin axis. This architecture cannot be explained by simple models for half-graben subsidence, and is interpreted to record slowing of slip on the Loreto fault and initiation of a new fault to the east which cut and initiated uplift in the flat limb of the pre- existing rollover. The PSS 3-4 boundary is an angular unconformity in the southeastern sub-basin. PSS-4 contains shallow-marine carbonate facies that record substantial slowing of basin subsidence and slowing of clastic input from both footwall and hangingwall sources. The PSS 3-4 boundary may represent a break-up unconformity, but evidence for Quaternary faulting indicates that classic rift models may not be applicable to this obliquely rifted margin.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California