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Description
The Late Cretaceous deep-sea fan and related Upper Cretaceous rocks of the Point Loma and Cabrillo Formations in coastal San Diego were mapped in detail to determine distribution of lithofacies, paleogeographic setting, and to provide a control for determining the nature of the Rose Canyon fault zone in La Jolla. The eight Cretaceous map units were: 1) shallow-marine sandstone; 2) slope and basin-plain mudstone; 3) outer-fan; 4) channelized mid-fan; 5) planar-bedded mid-fan; 6) inner-fan sandstone and conglomerate; 7) interchannel overbank; and 8) interchannel mudstone. Map data was integrated with measured sections, petrography, sedimentology, paleontology, and paleomagnetism. Identification and mapping of lithofacies was accomplished using depositional models which take into account characteristics such as bed form, thickness, grain size, and the recognition of thickening-or thinning-upward mega-sequences. The principal mechanisms of sedimentation were submarine fluidized flows, grain flows, turbidity flows, and debris flows. In sea cliff exposures these characteristics are easily identified and they provided stratigraphic control when mapping inland. The results of mapping indicate that these terrigenous sediments were deposited in a complex lateral and vertical arrangement of environments in a local fore-arc basin. Folding and faulting occurred after the deposition of the Eocene strata which unconformably overlies the Upper Cretaceous sedimentary rocks. Field data suggest that after the onset of right-lateral, strike-slip movement along the Rose Canyon fault, a westward-stepping of the faulting occurred as a result of the left-stepping bend in the Rose Canyon fault. Subsequently, uplift and clockwise rotation of the fault blocks occurred which is analogous in style and geometry to the western Transverse Ranges and the San Andreas fault. Preliminary data from Olivenhain suggest that Point Loma Formation underlies a section of Upper Cretaceous conglomerates. Poorly preserved fragments of Baculites anceps pacificus Matsumoto and Obata or B. Lomaensis Anderson, considered late Campanian-early Maestrichtian in age occur stratigraphically below the conglomeratic unit. This suggests that the age of the conglomerates is post early Maestrichtian, and may be closely related to the Cabrillo Formation.
