Microfacies and Geochemistry of a Shallow-Water Lower Aptian Carbonate Platform Sequence from the Madotz Section (Basco-Cantabrian Basin, Aralar, Spain), Its Relation to Cretaceous Anoxic Event 1a (Oae-1a)
High-resolution analyses of microfacies, inorganic carbon (TIC, weight % CaCO3), organic carbon (TOC, weight % C), and stable carbon isotope measurements (δ13Corg) of Lower Aptian sequence at the Madotz section (NE Spain) were carried out in order to identify lithologic and geochemical variations related to Oceanic Anoxic Event 1a (OAE-1a). The lowermost part of the section consists of shale known as Unit 1, with TIC <10%, TOC <0.2%. δ13Corg values at the top of unit 1 vary between -23.52 and 23.18‰. In suprajacent subunits 2a and 2b, called the “Madoz Limestone”, subunit 2a comprises 20m of biocalcirudite; its characteristic microfacies are packed and sparse biomicrites (calcarenites, calcirudites) with coral, non-rudist bivalve, and echinoid and bryozoan fragments. TIC varies between 75.97 and 98.81%, and TOC is nearly 0%; carbon isotope composition was not measured. Subunit 2b includes 20 m of interbedded sparse fossiliferous and packed marlstones, clayey fossiliferous micrites, as well as softer calcareous shale, and shale. Fossils of this subunit are mainly orbitolinids, with fewer echinoids, non-rudist bivalves, calcareous algae and calcispheres. TIC varies from 8.16 to 68.87%, while TOC values increase relative to infrajacent subunit 2a, with fluctuation from 0.03 to 0.97%, with maximum in a 1.8m thick shale interval, 36.08m above the base of the measured section. Higher TOC values are associated with shales and clayey micrites, characterized by the absence of orbitolinids and bioturbation and scarcity of other benthic faunas. However, they contain scattered calcispheres and up to 2.5% pyrite as isolated framboids, infilling fossil cavities, and micronodules. Most of δ13Corg values in subunit 2b remain constant between -23.64 and -21.93‰ immediately following a negative value of -26.19‰.
The absence of benthic foraminifers and bioturbation, increased pyrite and TOC in the shales and calcareous shales of subunit 2b at Madotz coincide with an interval of increased δ13Corg values throughout most of the subunit, immediately succeeding a significant negative δ13Corg. We associate this interval of subunit 2b with conditions of Oceanic Anoxic Event 1a. Ammonites present in the middle part of subunit 2b include taxa such as Deshayesites cf. forbesi, Pseudosaynella bicurvata and Pseudosaynella undulata that suggest a stratigraphic position correlative with at least the upper part of Deshayesites weissi zone of the standard Mediterranean ammonite zonation.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009