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Description
Fossiliferous marine strata of late Pleistocene age are exposed around the margins of Point Loma. These strata were deposited on the Nestor terrace, which was formed about 120,000 years before present by a sea level stillstand 2 to 10 m higher than at present. Subsequent deformation has elevated the terrace to its present position 20 to 55 m above sea level. Late Pleistocene Point Loma took the form of two barrier islands separated by a marshy channel. Portal Island was the small northern island and Loma Island was the large southern one. Four paleoenvironments are inferred to have been present in the nearshore waters surrounding the islands--a slightly hypersaline salt marsh, a tidal flat, a sheltered coastline, and an exposed coastline. Surrounding Portal Island and extending down one half of the eastern shore of Loma Island was a slightly hypersaline salt marsh. Five species of foraminifera and two species of ostracods are considered characteristic of this salt marsh microfauna. The foraminifera are Cellanthus gunteri, Cribrononion frigidum, Cribrononion lene, Cribroelphidium spinatum translucens, and Gavelinopsis campanulata; the characteristic ostracods are Aurila conradi californica and Loxocorniculum sculptoides. The microfauna suggests that the mean water salinity was 39 o/oo plus or minus 3 o/oo. An annual temperature range of 17° to 33°C is suggested, with a mean annual paleotemperature of about 23°C. Sediments from the five localities sampled were primarily poorly sorted, coarse-skewed very fine sand. The fossiliferous stratum representing the sheltered coast paleoenvironment is a siltstone cobble conglomerate. Two species of foraminifera characterize the two distinct substrate-specific faunas of this environment. Quinqueloculina vulgaris is characteristic of the fauna living on the cobbles, and Buccela parkerae characterizes the fine sand substrate fauna. Normal ocean salinity is suggested by the composition of the microfauna. A mean annual temperature of l5°C, with an annual temperature range of 12° to 20°C, is suggested; occurrences of warm and cool water species in this fauna suggest that the species exploited the extremes of annual temperature in colonizing this environment. Sediments are well sorted, coarse-skewed fine sand. The west coast of Loma Island was an exposed coastline similar to modern western Point Loma. Six species of foraminifera are characteristic of this paleoenvironment--Bolivina pseudoplicata, Brizalina vaughani, Dyocibicides biserialis, Elphidium crispum, Elphidiella hannai, and Rosalina columbiensis. The fauna suggests a mean annual nearshore ocean temperature of 15°C, an annual range of temperature between 12° and 20°C, and normal ocean salinities. Sediments are poorly sorted, fine-skewed fine sand. Adjacent to the eastern margin of the salt marsh, and northeast of the sheltered coast paleoenvironment was an intertidal mudflat. The record of it is preserved in the upper portion of the stratigraphic section near salt marsh locality 2583. The salt marsh surrounded Portal Island and extended down one half of the eastern shore of Loma Island. Immediately adjacent to its eastern shore was an intertidal mudflat. A sheltered coastline comprised the southern one half of Loma Island's eastern shore. The western margin of Loma Island formed an exposed coastline. Barrier bars were present in the positions of Crown Point and Coronado. Zuniga Shoal formed a composite inlet delta between Loma Island and the Coronado barrier bar. This suggests that late Pleistocene Point Loma was a portion of a larger coastal lagoon system encompassing the coastal portion of southern San Diego County. Mugu Lagoon, Southern California, might best be considered as a modern analog.
