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Correlation of Volcanic Ashes in the Monterey Formation between Naples Beach and Gaviota Beach, California
The major and trace element composition of seventy whole-rock samples of volcanic ash, tuff, and bentonite from beach outcrops of the Monterey Formation west of Santa Barbara were analyzed to determine if these tephra layers could be used as a stratigraphic correlation tool. Five broad groups of ashes can be distinguished based on the abundance of seven elements that are immobile during alteration of the ashes to bentonites (La, Sm, Yb, Nb, Th, Ti, Zr). Lower to middle Miocene ash beds that are correlative with the Tranquillon, Obispo, and possibly the Santa Cruz Island Volcanics are found in the lower Monterey Formation. Upper Miocene ashes that are correlative with ash beds in the type section for the Monterey Formation in Monterey County, as well as ashes from a nonmarine sequence north of the Garlock fault, are found in the upper Monterey Formation at Naples Beach. An ash bed zonation was developed for the Monterey Formation which was used to correlate between stratigraphic sections along the Santa Barbara coastline. The concurrent range zonation developed for these ashes has a resolution that is comparable to the resolution achievable with benthic foraminifera in this part of the time scale.
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