Luisian and Mohnian Biostratigraphy of Monterey Shale at Newport Lagoon, Orange County, California: ABSTRACT

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Sediments of Luisian and Mohnian age are exposed almost continuously in a 1,400-ft section of Monterey Shale along the margin of Newport Lagoon. The lowermost 200 ft appears to be barren of fossils, consequently, no definite age assignment can be demonstrated for this part of the section. Overlying this barren interval is approximately 265 ft assigned to the Luisian Stage on the basis of occurrence of Brizalina advena striatella, Brizalina imbricata, Pullenia miocenica, Siphogenerina spp. and Valvulineria californica californica. Approximately 40 ft of section directly above the Luisian sediments is assigned to the Brizalina modeloensis Zone of the lower Mohnian Stage as defined by the restricted occurrence of the nominate species (sensu stricto); also present, but not restricted to the zone, are Bulimina unigerinaformis, Cassidulina monicana, and Concavella gyroidinaformis. Overlying the Brizalina modeloensis Zone is 325 ft of sediments End_Page 438------------------------------ which is assigned to the Bulimina uvigerinaformis Zone of the lower Mohnian Stage. The Bulimina uvigerinaformis Zone can be readily divided into an older subzone designated the Concavella gyroidinaformis Subzone and a younger subzone here designated the Brizalina woodringi Subzone. The top of the older subzone is marked by the last occurence of Concavella gyroidinaformis; the Concavella gyroidinaformis Subzone is approximately 210 ft thick at this locality. The overlying Brizalina woodringi Subzone is based on the continued occurrence of Bulmina uvigerinaformis after the extinction of Concavella gyroidinaformis. This subzone is approximately 115 ft thick with its top, and the top of the lower Mohnian Stage, marked by the last occurrence of Bulimina uvigerinaformis. Above the lower Mohnian Stage at Newport Lagoon is approximately 470 ft of foraminiferal Monterey Shale assigned to the upper Mohnian Stage. Among the many species present are Bolivina hughesi, Brizalina benedictensis, Brizalina decurtata, Brizalina giradensis, Brizalina granti, Cassidulinella renulinaformis and Discorbinella valmonteensis. No foraminifers were found in the uppermost 100 ft of Monterey Shale which overlies the sediments of demonstrable Mohnian age. Cyclococcolithina neogammation, a fossil coccolith, was found to have its last occurrence coincident with the top of the lower Mohnian Concavella gyroidinaformis Subzone at Newport Lagoon. Another coccolith, Reticulofenestra pseudoumbilica, was observed to have its first occurrence at the base of the lower Mohnian Brizalina modeloensis Zone. The last occurrence of Sphenolithus heteromorphus was confirmed to be in the uppermost Luisian Stage. End_of_Article - Last_Page 439------------