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Description
The continental slope due west of Ensenada. Baja California, Mexico, is transected by an east-west trending submarine valley characterized by a flat floor and steep-walled, constricted mouth. Calcareous olive-green silts, containing abundant sand-size glauconitic grains, were deposited during Late Pleistocene time from turbid suspensions flowing across the valley floor. Geomorphic lineaments and sub-bottom acoustic relection profiles suggest that the submarine valley is a wedge-shaped graben that has undergone recurrent faulting along its northern boundary. Based on a microfossil assemblage, including foraminifers and calcareous nannoplankton, a calcareous ooze from the top of the continental slope is correlated with the Miocene Globorotalia “fohsi” Zone (s.l.) of Caribbean stratigraphy. Discrete layers of red radiolarian chert fragments occur in the ooze and are lithologically similar to chert from the Franciscan Group (s.l.) of California. Sedimentary aprons and fans at the base of the continental slope are characterized by Late Pleistocene gray silts and clays, including turbidity current deposits, that are overlain by postglacial, tan calcareous silts and clays. Climatic changes indicated by Globigerina pachyderma coiling ratios in these sediments agree well with established glacial, sea level, and lake level fluctuations during Main Wisconsin and post-glacial time in middle latitudes of North America. Olive-green clayey silts were deposited at sill depth along the northern end of Long Basin during post-glacial time at a rate approximately one-fourth that evidenced on the basin floor. Areal sedimentation in the study area during the Pleistocene Epoch was characterized by non-deposition and/or erosion on Patton Ridge, and fault-triggered sediment slumps and slides in the submarine valley. Sediment suspensions, associated with mass-movements, flowed across and down valley depositing glauconite-rich sand, silt, and clay along the valley floor and forming an alluvial fan at the valley mouth. An ESP record across a faulted nose of Patton Ridge suggests that the ridge is the crest of a broad fold, now truncated by faults on both flanks. Thin, layered sediments now blanket the seaward flank of the Ridge suggesting a low accumulation rate on the continental slope.
