Tectonic Controls on Sedimentation in the Pliocene-Quaternary 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 on the eastern margin of Baja California during oblique rifting and opening of the Gulf of California. Subsidence and alluvial sedimentation were initiated at ~3.5 Ma, which is about the time that Pacific-North America plate motion was first located fully within the Gulf of California. Earliest marine incursion occurred at 2.6 Ma and was followed by a short phase of extremely rapid subsidence (8.1 ± 4.3 mm/yr) between 2.46 and 2.36 Ma (ages from ⁴⁰Ar-³⁹Ar dating of interbedded tuffs). About 770 m of section consisting of stacked Gilbert-type fan deltas, alluvial-fan, and shallow-marine clastic facies accumulated during this time. Faulting stepped to the east at ~2.4 Ma, and initiate the eastern coastal fault one. This resulted in: (1) intiation of hangingwall uplift and erosion of the flat limb of a pre-existing rollover structure, (2) slowing of subsidence along the Loreto fault, and (3) progradation of fan deltas and alluvial gravels into and northward along the axis of the basin. The southern Loreto basin has been uplifted at least 400 m in the hanging wall of the coastal fault zone since ~2.4 Ma, at a rate of 0.17 mm/yr. This is consistent with uplift rates determined from Quaternary marine terraces. Uplift in the south is related to development of an accommodation zone at the southern termination of the basin; this resulted in truncation of a large Pliocene drainage system that supplied much of the sediment to the basin during the earlier phase of very rapid subsidence. Preliminary geomorphic data suggest Quaternary deformation in the south. The northern Loreto basin continues to subside due to inferred northward and westward tilting; this has produced an undissected modern alluvial plain in the north which is locally cut by fault scarps along the reactivated (?) Loreto fault.

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