The middle Pleistocene Merced-2 and -3 sequences from Ocean Beach, San Francisco (original) (raw)

Carter, R.M., Abbott, S.T., Graham, I.J., Naish, T.R., and Gammon, P.R.(2002)The middle Pleistocene Merced-2 and -3 sequences from Ocean Beach, San Francisco. Sedimentary Geology, 153 (1-2). pp. 23-41.

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Abstract

The Merced Formation comprises a 2-km-thick shallow marine and non-marine succession that was deposited in a small transtensional basin along the San Andreas Fault during the late Pliocene to middle Pleistocene. The sediments dip between 10° and 80° to the northeast, and are locally disrupted by small faults. During tilting, the beds have been rotated into subparallelism with the San Andreas Fault zone, splays of which bound the outcrop belt of Merced sediments to both the southwest and northeast. The Merced Formation contains more than 20 transgressive–regressive sedimentary rhythms (cyclothems, or sequences) that are generally between 40 and 120 m thick, and which were deposited mostly during interglacial time, under the influence of rising, highstand and early falling sea levels. Sequences Merced-2 [units M1–N of Soc. Econ. Paleontol. Mineral., Field Guidebook 3 (1984) 1] and Merced-3 (units O–P), though in fault contact, comprise typical Plio-Pleistocene shallow water cyclothems. The Merced-2 Sequence is 22+ m thick, and comprises a sandy and shelly transgressive systems tract, including a basal Type A shellbed, an in situ Type B mid-cycle shellbed, and a highstand systems tract of massive siltstone. The Merced-3 Sequence is 47 m thick, and comprises a basal compound shellbed, a thin highstand systems tract siltstone, and a sand-rich regressive systems tract. The RST comprises distal shoreface sands with an abundant in situ molluscan fauna, and upper shoreface and back-beach trough cross-bedded sands and pebbly sands. The top of the Merced-3 cycle comprises a beach sand capped by a palaeosol and lignite (the “Beetle Bed”), which together mark the subaerial exposure of the site during the ensuing glacial sea-level lowstand. Analysis of 10Be across the Merced-3 Sequence shows major peaks, indicative of sedimentation starvation, in the basal transgressive systems tract shellbed and in the capping lignite of the Beetle Bed. Smaller 10Be peaks are associated with a shellbed that is inferred to represent winnowing at the foot of the shoreface, and with a minor exposure surface that delimits a small paracycle in the top of the sequence. Otherwise, 10Be abundances decline regularly across the Merced-3 Sequence, consistent with an increasing sedimentation rate as shoreface progradation, and regression, progressed. The character of cycles Merced-2 and Merced-3 respectively resembles the Seafield and Rangitikei sequence motifs described from similar Plio-Pleistocene sediments in New Zealand. The cycles are of mid-Pleistocene age, and were probably deposited during interglacial oxygen isotope stages 21 and 19, respectively.

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