Gamma-ray spectrometry across the Aalenian-Bajocian boundary in the Cape Mondego section (Bajocian GSSP, Portugal): preliminary results

Abstract

The Aalenian-Bajocian boundary at the Cape Mondego section (Lusitanian Basin, Portugal) is represented by a prograding succession of greyish marl and limestone alternations, which provides rich and diversified fossil (ammonoids, brachiopods) and microfossil (benthic foraminifera, calcareous nannoplacton) record. The ammonite assemblages, the calcareous nannofossils record and the magnetostratigraphic reversal based the formal definition of the Bajocian GSSP in 1996, the first stage boundary established for the Jurassic System by the IUGS.

Different bioevents have been already identified across the Concavum Zone (upper Aalenian) - Discites Zone (lower Bajocian) transition, namely among the ammonites, brachiopods, calcareous nannofossils and especially among the foraminiferal assemblages, which record a remarkable decrease on abundance and diversity, also detected in other coeval sections of different basins located at the northern hemisphere and probably related to a significant warming episode extending up to the earliest part of the Discites Biochron.

The gamma-ray data across this global reference boundary shows generally low values and variability (12 to 45 nGy/h), reflecting the stable depositional conditions of typical external marine shelf, also pointed by the composition of the foraminiferal assemblages. Moreover, the Th/U ratio is generally higher than 2 throughout the section suggesting well-oxygenated environmental conditions (also documented by the composition of the foraminiferal assemblages), which would have prevented significant organic matter accumulation; some levels displaying low Th/U ratio may reflect depletion in thorium (typical of many marine carbonates) rather than an increase in authigenic uranium, that usually is lower than 1 ppm.

Before and after the faunal impoverishment bioevent of Late Concavum-Early Discites Biochron, the Th/U ratio and K% display a relative increase, probably related to an increment in the detrital supply, and therefore nutrient supply, which has enabled the faunal recovery, as well as the raise of deep infaunal foraminifers recorded at the latest part of the Discites Biochron.

In basin analysis of carbonate platforms the integration of major biotic turnovers and gamma-ray spectrometry data can be a useful tool in the improvement of correlation between wells and outcrops.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015