Abstract
Geomagnetic reversals and magnetic polarity chrons provide an important chronostratigraphic tool for global correlation. An integrated Tithonian-Berriasian biostratigraphic and magnetic polarity time scale for the Tethyan faunal realm for the Tithonian and Berriasian stages is compiled from 17 independent biomagnetostratigraphic sections. This time scale incorporates zones and first/last appearance datums from ammonites, calpionellids, calcareous nannofossils, and dinoflagellates. This database provides an estimate of the range of observed appearance datums or zonal boundaries relative to polarity chrons; such apparent “diachroneity” probably results from a combination of preservation of species and of paleontological methodology, rather than from migration. The lithologic transition from “Rosso Ammonitico” red marly limestone to “Maiolica” white limestone occurs at different times during the Tithonian among the various sections, ranging from polarity zone M22n (mid-Early Tithonian) in some Spanish “slope” and Italian basinal-facies sections to polarity zone M19n (mid-Late Tithonian) in the central Atlantic and some plateau-facies Italian sections. This widespread lithologic change is, therefore, probably a result of shifting local patterns of fertility overprinted on the main regional trend. Magnetostratigraphy from the Purbeck Limestone Formation in the English Boreal faunal realm was obtained from the “classic” section at Durlston Bay in Dorset. The Dorset section displays predominantly normal polarity with a minimum of three reversed-polarity zones, but distortion of the magnetic polarity pattern by variable rates of sedimentation in this marginal clastic environment coupled with lack of independent correlation methods currently precludes a unique correlation to the Tithonian-Berriasian magnetic polarity time scale. The Tithonian-Berriasian magnetic polarity time scale may eventually provide a global chronostratigraphic definition of the Jurassic-Cretaceous boundary.
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