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Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Microfacies and Nannofacies Analyses in Aalenian Deposits from Western Tethys (Iberian Range, Spain) 306 A. García-Frank 1 , N. Perilli 2 , S. Ureta 1 1. Departamento and UEI de Paleontología, Facultad C.C. Geológicas e Instituto de Geología Económica, (CSIC-UCM), José Antonio Novais 2. 28040, Madrid, Spain (E-mail: agfrank@geo.ucm.es; solureta@geo.ucm.es) 2. Dipartimento di Scienze della Terra, Via S. Maria 53, 56100 Pisa, Italy (E-mail: perilli@dst.unipi.it) This report addresses new data regarding the contribution of calcite microparticles to the input of carbonate sediments. The study of the fine-fraction of limestones and marls in Muro de Aguas, a relatively expanded Aalenian section at the NW Iberian Range (northern Spain), has shown that a combined analysis of nannofacies in ultra-thin sections and smear slides using the classical microfacies approach increases notably the information about the origin of this fine- fraction. It has been revealed as a good tool in order to improve inferences made by field (facies) observations in monotonous sections, that can overlook factors concerning carbonate sedimentology, because allows to distinguish provenance tendencies and palaeoenviron- mental changes. Previous studies of this section (García-Frank, 2007; García-Frank et al., 2008) dis- played a quite monotonous alternation of micrite- rich microfacies within the calcareous sediments, organized in twelve coarsening-upwards stratigraphic sequences (s1 to s12), where three major tectonic pulses were distinguished. Unfortunately, the microfacies alternation does not seem to show special connexion to the main tectonic pulses inferred for the area (Fig.1). Since the last decades the use of nannofacies analysis as a tool to elucidate the origin of the fine-carbonate fraction or to reconstruct paleoenvironmental changes has been reported in several studies (e.g. Minoletti et al., 2005; Bour et al., 2007; Beltran et al., 2009; Giraud, 2009; Carcel et al., 2010). The first studies concerning nannofacies (term defined by Nöel, 1969) used SEM in order to identify the nannofossil contribution to micrite (Nöel and Melgen, 1978). More recent works (e.g. Bour et al., 2007) have also been carried under SEM, to investi- gated nannofacies but since this is technique that involves more time and expenses, and can be only applied to small samples and to very restricted areas, we decided to use the same methodology as in the microfacies, with the difference of scale (10x-40x vs. 1000x-1500x). The entire set of characters (fossil and mineral abundance and textures) observed on different samples by the microfacies approach was used to define the nannofacies, studying not only the limestone levels, but also the marly ones, with the combination of ultra-thin section and smear slide observations. As aforementioned, the use of ultra-thin sections, allows to see fabric relationships (fossil and mineral abundance and textures), but medium- to small-sized and rare to very rare calcareous nannofossil taxa could be difficult to identify. As this is not a biostratigraphic study, we have selected some taxa that are easily recognizable in the petrographic microscopy, i.e. Schizosphaerella spp. In contrast, due to their preparation, the smear slides lack of textures and original fossil abundance, but are very useful for isolating the fine fraction and re- cognizing all the calcareous nannofossil taxa and mineralogies that can be scattered in the thin/ultrathin sections (such as quartz, phyllosilicates and zircon). The nannofacies analysis reveals that Muro de Aguas micrite contains a mixture of abraded skeletal components that were deposited with larger skeletons and hence is an allomicrite (Wolf, 1965). The allomicrite (senso Flügel, 2004) can be subdivided into bioclastic micrites, formed by disintegration of benthic invertebrate skeletons, and nannomicrites, formed by the disintegration of pelagic biota (accumulation of calcareous plankton, i.e. foraminifers, pteropods and calcareous nannofossils). The bioclastic micrites in- clude mainly what we have referred as carbonate large fragments, that consists of broken fragments >2 μm of easily recognizable benthic foraminifera tests and other invertebrate groups (mainly Bositra and echinoderms). The nannomicrites includes specimens of Schizo- spherella, “Schizosphaerella-debris” (i.e. tiny broken fragments of Schizosphaerella shells), specimens of coccoliths (e.g. Crepidolithaceae, Watznaueriaceae) and of nannoliths (e.g. Hexalithus), as well as indeter- minate micarb. Micarb are calcite crystals
Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321 Subbiochron), the decrease of the bioclastic micrite is correlated to the increase of “Schizosphaerella-debris” + micarb, though the contribution of Schizosphaerella is also recognizable. Within the Murchisonae Biochron, nannomicrites are dominant, and mainly composed of “Schizosphaerella-debris” + micarb. This event is related to the prevailing marly levels (s4 and s5 sequences) and coincides with an increase of the percentage of hialine foraminifera in the microfacies. The subsequent decrease in the bioclastic micrite content (Gigantea-Concavum Subbiochrones) is related to the sharp increase of nannomicrite which also shows a significant contribution of Schizosphaerella. This episode also coincides with the maximum flooding of the basin during the Aalenian. Dominance of nannomicrites could be related with several causes, e.g. 1) tectonics: restructure of the basin after the ‘mid- Cimmerian’ Unconformity event (García-Frank et al., 2008) related with the first tectonic pulse (chance for Schizosphaerella to fill empty niches formed during the palaeogeographic changes) or the run out of other allomicrites in nearby areas after the tectonic pulses; 2) palaeoecology: the planktonic rain (proliferation of nannoplankton, i.e. dominance of Schizosphaerella) was the most effective source at these moments. More detailed works about the calcareous nannofossil assem- blages are needed in order to see the real impact of palaeoecology. Fig.1 Muro de Aguas section (NW Iberian Range, Spain) Comparisson of the micro- and nannofacies components (allochems vs. micrite). Data for microfacies and abundance of allochems from García-Frank, 2007. Another remarkably fact, when comparing micro- and nannofacies, is the higher accuracy of the fluctua- tions of the different detrital fractions behaviour, which in both cases are a minor components of the nannofacies. Though quartz values in the microfacies shown the same general trend as in the nannofacies, with this new approach the general knowledge improves, due to a more complete record of different sized particles and also new levels sampled. Besides, the nanno-zircon crystals abundance display similarities with quartz, whereas the phyllosilicates shown opposite behaviour: They generally peak when quartz and zircon decrease, and although all of them are within the silt-sized particles range, they settled differently (flocculation vs. disperse grain deposition). So the provenance factors are clearly enhanced, and show a neat correlation with the sedimentological evolution of the basin, linked both with eustatic and tectonic controls determining the accommodation space. Acknowledgements: This work was funded by projects CGL2008-03112, CGL2005-04574 and Grupo 910431 UCM-BSCH; by ‘Estancia breve para pro- fesorado U.C.M en el extranjero (conv. 2008)’, and by ‘Ministerio de Educación (Estancias de movilidad “José Castillejo”, conv. 2009)’. Key words: Microfacies; Nannofacies; Lime- stone-marl alternations; Early-Middle Jurassic; Iberian Basin; Spain 307
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