Varve Formation in the Central Gulf of CaliforniaA Reconsideration of the Origin of the Dark Laminae from the 20th Century Varve Record

Abstract
Formation of the dark “summer” laminae of the varved sediments from the anaerobic slopes of the central Gulf of California has previously been considered a response to the summer-fall peak in runoff from the major rivers of Sonora. Reconstruction of a 20th century varve chronology for an area off Guaymas, Sonora now provides a well constrained temporal framework for direct comparison of river discharge data to the mass accumulation of material in the dark laminae. A continuous record of dark lamina deposition from 1934 through 1966 shows no indication that construction of dams across the major Sonoran rivers, begun during the 1940s, has had any effect on the accumulation of these laminae. No relationship exists between the mass flux values and the “controlled” discharge of the rivers below the dams. However, a weak relationship was found to exist between mass flux and the “free,” undammed summer-fall river discharge of the Sonora, Yaqui and Mayo Rivers compiled from stations above the dams or for periods prior to dam construction. Based on these results we argue that the transfer of terrigenous material from the mainland occurs principally through eolian processes by removal and transport of desert dust associated with convective summer thunderstorms. The Gulf of California is an excellent laboratory for studying sedimentary processes on time scales that are not resolvable in the open ocean. The high biological productivity and the unique physical character of the gulf combine to produce sedimentological processes that preserve annual phenomena. This volume is organized into six sections. Part 1 covers historical exploration of the area. Part 2 includes 5 chapters detailing information contained on the 5 fold-out maps that accompany the volume. Part 3 consists of chapters on regional geophysics and geology. Part 4 covers satellite geodesy. Part 5's seven chapters discuss physical oceanograpy, primary productivity, and sedimentology. Part 6 covers hydrothermal processes.

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