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Description
In a portion of northern San Diego County Eocene depositional history is reconstructed from the spatial and temporal distribution of nine lithofacies and three subfacies, and Pleistocene marine terraces are defined by their distinctive topographic expression. The middle Eocene transgressive-regressive sedimentation cycle preserved in the La Jolla Group of the San Diego metropolitan area is also present in the adjacent coastal area to the north. A conformable section is exposed along the coast. Moving up-section from south to north, lagoonal Delmar Formation claystone and sandstone is overlain by Torrey Sandstone tidal channel and barrier bar sandstone, which in turn is overlain by a nearshore Ardath Shale equivalent. The Ardath is overlain along a scour surface by Scripps Formation tidal channel sandstone. One poor exposure at the far north end suggests the presence of Delmar/Friars Formation lagoonal sandstone. Inland, Delmar/Friars muddy tidal flat claystone is channeled by, and interfingers with, coarse, fluvial/deltaic sandstones in a narrow, three-dimensional band which parallels the basement contact. Westward of this band are sub-parallel zones of Torrey and Scripps sandy tidal flat sandstone and tidal channel and bar sandstone. This landward-to-seaward sequence represents the compression of lagoonal environments against a steep basement topography during the middle Eocene sea level transgression and high stand. A pronounced buttress unconformity between Eocene strata and Jurassic basement, and a steep depositional contact between muddy inner and sandy outer lagoonal strata are the result. Sandy tidal environments were at times invaded by coarse, elastic deposits. In the south, a narrow, northward-thinning lobe of Poway-suite conglomerate stringers records the flushing of flood gravels into the lagoon and corresponds to the northward depositional limit of Poway-suite conglomerate. In the north, a wedge of coarse, pebbly sandstone pinches out to the south within Scripps tidal sandstone and records lagoonal delta progradation in later middle Eocene time. Structurally, the area is relatively undisturbed except in the northwest, where middle Pleistocene and older deformation appears to have dropped and tilted rocks to the west. At least nine Pleistocene marine terraces are incised into Eocene strata along the coastal strip. The Bird Rock and Nestor terraces of the San Diego area appear to continue north through the field area. Withdrawal of the sea after each sea level high stand left gravel lag, beach and eolian dune deposits mantling each terrace. Eolian beach ridges coincide with topographic breaks between terraces. Continuous uplift during the Pleistocene has lifted each terrace clear of wave attack during subsequent sea level high stands.
