Geographic gradients in the formation of shell concentrations: Plio-pleistocene marine deposits, Gulf of California

Abstract Shell bed (coarse concentrations of invertebrate bioclastic material) are rich sources of paleontologic and stratigraphic information. Studies often emphasize the relationship between shell bed formation and bathymetric gradients, because many sedimentologic and ecologic factors that influence shell-concentrating processes vary with water depth. However, shell bed formation is also influenced by factors that are not related to water depth. Shell beds in Plio-Pleistocene marine sediments of the Gulf of California (Mexico) formed in less than 10–15 m water depth, but under varying conditions of coastal topographic relief and tidal range. Sampling of shell beds focused on two regions: the northern mainland coast and the southern peninsular coast. Shell beds in the northern mainland region (13 total) formed along a macrotidal, low-relief coastline, whereas shell beds in the southern peninsular region (27 total) formed along a microtidal, high-relief coastline. Shell beds represent community beds (11); storm beds (3); beach berm beds (6); tidal channel bed (1); or current/wave-winnowed beds (19). The community beds represent biogenic concentrations; the other four types represent sedimentologic concentrations. Sedimentologic concentrations formed either through short-term concentrating events (storm beds), or long-term dynamic bypassing (beach berm beds, tidal channel beds, and current/wave-winnowed beds). Concentrations formed through dynamic bypassing are far more common (26 beds) than storm concentrations (3 beds). Of the 26 concentrations formed through dynamic bypassing, 10 cap major stratigraphic discontinuities, representing temporal breaks ranging from 103 to 108 years. The abundance and distribution of the five types of shell beds is significantly different between the two sampling areas. The northern mainland coast is dominated by allochthonous beach berm beds capping linear beach ridges. The formation of beach ridges in the north is favored by the wide shallow shelf (representing a large reservoir for beach shells), and sediment starved conditions resulting from the low topographic relief and periodic reductions in sediment supply from the Colorado River delta. Beach berm beds are absent along the southern peninsular coast, where coarse terrigenous sediment shed from adjacent highlands overwhelms shell input to the beach, and the narrow steep shelf supplies the beach with few shells. The southern peninsular coast is dominated by parautochthonous current/wave-winnowed beds and autochthonous community beds. Southern shell beds have undergone significantly less reworking, winnowing and transportation than northern shell beds. This is a function of both the relative weakness of tidal currents in the south, and the relatively higher rates of terrigenous sedimentation along the rugged southern coastline, which can bury shell concentrations relatively rapidly. Geographic variation in topographic relief and tidal range results in significant variation in the distribution and abundance of shell beds within these shallow marine sediments. These variables have root tectonic cause (relief reflects tectonic activity; tidal range reflects basin configuration), underscoring the potential utility of fossil concentrations for reconstructing earth history.