Quaternary shorelines along the northeastern Gulf of California; Geochronological data and neotectonìc implications

A general reconnaissance of the remnants of Pleistocene high sea-level stands was conducted along the Sonora coast, between the Rio Colorado delta and the Guaymas area, to determine the neotectonic comportment of the mainland margin of the Gulf of California plate boundary. The generally low relief of this coastal region and the extensive cover of the late Quaternary eolian sands (in the northern gulf) and alluvium (in the eastern gulf) somewhat limit the study of Pleistocene shorelines, but coastal deposits corresponding to several episodes of high sea level are preserved at low elevations all along the coastline. The morphestratigraphic interpretations and lateral correlations of these deposits support the conclusion that, unlike the Baja California peninsula and coastal California, the northeastern Gulf of California remained vertically stable, at least during the late Quaternary. Pleistocene vertical motions have been insignificant on the edge of the North American plate, except in the Rio Colorado delta area and along the Cerro Prieto fault zone, which links the San Andreas fault system and the northernmost Gulf of California. This chapter emphasizes the problems of correlation and age determination of the terrace remnants and particularly those met in the identification of the last interglacial maximum (isotopic substage 5e, about 125,000 yr ago). Uranium-series, radiocarbon, and amino-acid stereochemistry data from fossil pelecypod shells provide useful geochronologic information, but in some cases proved to be inaccurate (large spread of Th/U ages from a single locality), or unreliable (radiocarbon), because of contamination and diagenetic alteration. The Holocene coastal deposits are generally well developed along the Sonoran shores; several lines of evidence indicate that the sea level nearly reached its present position about 4,000 yr ago. MARINE TERRACES, QUATERNARY CLIMATOSTRATIGRAPHY, AND RECENT VERTICAL MOTIONS A marine terrace is a coastal landform resulting from littoral erosion produced when sea level remains long enough at a given position relative to the land. In the “stable” and slowly uplifted regions, emerged marine terraces generally correspond to the highest sea stands coeval with the Quaternary warmest episodes (global glacial minima). It is now widely accepted that the Quaternary was characterized by glacialhterglacial climatic cycles, identified in the deep-sea-core oxygen-isotope records. The l80 isotopic variations measured in foraminifers of deep oceanic cores register the basis of the Quaternary chronostratigraphy (Shackleton and Opdyke, 1973, 1976). The episodes of eustatic high sea-level stands, *Present address: Misión ORSTOM en el PerÚ, Apartado 18-1209, Lima Ortlieb, L., 1991, Quaternary shorelines along the northeastern Gulf of California; Geochronological data and neotectonic implications, in Pérez-Segura, E., and 18, Peru. i ’ Jacques-Ayala, C., eds., Studies of Sonoran geology: Geological Society of America Special Paper 254.