Mathematics – Logic
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmpp24a..02g&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #PP24A-02
Mathematics
Logic
0460 Marine Systems (4800), 0473 Paleoclimatology And Paleoceanography (3344, 4900), 3030 Micropaleontology (0459, 4944), 4243 Marginal And Semi-Enclosed Seas, 4999 General Or Miscellaneous
Scientific paper
The Aptian stage at Madoz in the Sierra de Aralar, NE Spain, shows contrasting lithological succession of intercalated clastic-rich intervals and rudist-rich limestone beds varying between Medium Olive Gray (5Y 5/1) and Olive Gray (5Y 4/1). They are subdivided into different sub-units (Duvernois et al., 1972; Cherchi & Schroeder, 1998) with unit 1, as well as subunits 2a and 2b of "Madoz Limestone" are placed within the Lower Aptian Palorbitolina lenticularis Zone. Their stratigraphic level corresponds at least to the Deshayesites deshayesi ammonite zone, based on the presence of the nominate taxon in clastic Unit 1. Sub-unit 2b includes a distinct 180-cm thick black (Medium to Dark Gray, -N4 to N3) shale layer toward the close of the upper Lower Aptian. Detailed microfacies analysis was carried out on the Lower Aptian interval in order to characterize the different lithofacies and their possible relationship to Cretaceous OAE-1a. Subunit 2a is 20m thick and its microfacies consists of sparse and packed biomicrites, moderate to poorly sorted fine calcirudites and calcarenites composed of 40 - 50% remains of corals, predominantly non-rudist bivalves, echinoids, bryozoans, benthic foraminifera and calcareous algae indicative of well oxygenated conditions. Matrix and bioclasts are highly affected by neomorphism and growing of micritic envelopes is frequent. Superjacent subunit 2b is also 20m thick, but is lithologically very variable and consists of interbeds of indurated biomicrite with 30 - 50 % fossil fragments dominated by orbitolinids, and echinoids, non-rudist bivalves, benthic foraminifers, and algae as secondary components. These beds also contain 15 - 25 % mostly silt-size quartz grains, and clays. Other indurated biomicrite beds within subunit 2b contain 1 - 20 % fragments of non-rudist bivalves, echinoids, other benthic foraminifers, and algae, but orbitolinids are scarce. Terrigenous components make up 10 - 25 % of the matrix. Subunit 2b also includes soft levels of fossiliferous marls (bioclasts < 10%), calcareous shales and pyritic black shales (bioclasts < 5%) that contain non-rudist bivalves, algae, echinoids and scarce benthic foraminifers. Microfacies of the prominent black shale bed, which occurs about between 14.5 and 16.3 m within subunit 2b, show a matrix composed essentially of clays (> 50%), framboidal pyrite (1-2.5%), and organic matter (5-7 %). Benthic foraminifers and bioturbation are totally absent within this interval, but there are occasional algal fragments, and very rare ostracods. In contrast to the adjacent levels, the absence of benthic foraminifers lack of bioturbation, and increased OM suggest that the black shale level clearly implies development of anoxic to dysoxic conditions within that interval. Based on the presence of D. desahyesi in subunit 1 its relationship to OAE- 1a remains ambiguous, and its occurrence may be related to a possible regional event due to the complexity of the paleogeography of that region during its tectonic development in the Early Cretaceous. Duvernois, Ch., Floquet, M., Humbel, B. 1972. La Sierra d'Aralar (Pyrénées basques Espagnoles): stratigraphie, structure. Thèse de Doctorat de Troisième cycle, Université de Dijon, 240 pp. Cherchi, A., Schroeder, R. 1998. Aptian and Albian large foraminifera at Madoz. 24° Coloquio Europeo de Micropaleontología. Libro Guía. Lamolda, M.A. Ed. 71-73p.
Gaona-Narvaez T.
Lamolda M. A.
Maurrasse F. J.
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