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Pleistocene Marginal Marine Sediments Near Mouth of Mad River, Humboldt County, California
Study of late Pleistocene sediments near the mouth of the Mad River revealed a sequence of nearshore marine and shallow bay deposits. This sequence, bounded by unconformities, is informally named the Mouth of Mad unit. The Mouth of Mad unit can be divided into four distinct depositional facies at the study site. The lowest facies is a nearshore sand that lies unconformably on a paleosol. The sand grades upward into a high-energy, interbeded sand-and-gravel facies containing storm and rip-channel deposits. Above the sand and gravel is a strand-plain sand facies. This sand is overlain by a laterally variable sequence of shell-rich bay sediments. The bay deposits can be further divided into five subfacies: 1) a bioturbated backshore sand, 2) a lower tidal-flat mud, 3) a mixed sand and mud, 4) an oyster-rich bay mud, and 5) an upper tidal-flat mud. The bay sequence is overlain unconformably by younger late Pleistocene terrace deposits. The Mouth of Mad unit represents a rapid transgression followed by a more gradual regression, with development and migration of a protective spit. Sediments within the Mouth of Mad unit have been dated at 176+/-33 ka, placing them in the late Pleistocene. Correlation between the Mouth of Mad unit and other formations in the region is difficult owing to extensive folding and faulting and lack of continuous exposure. None the less, amino acid racemization and thermoluminescence dates can be used to establish a rough relationship between the Mouth of Mad unit and other nearby Pleistocene fossil bearing deposits. The Mouth of Mad unit appears to be younger than the fossiliferous deposits at Elk Head, Cranell junction, Trinidad Head, and Moonstone Beach, and possibly time equivalent with portions of the Hookton Formation.
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