Facies distribution patterns and environment interpretation of the Upper Láncara Formation in the Esla Nappe (Cantabrian zone, NW Spain) by quantitative microfacies analysis

by Thomas Wotte, Freiberg; Olaf Elicki, Freiberg; Antonio Perejón, Madrid & Elena Moreno-Eiris, Madrid

WOTTE, TH.; ELICKI, O.; PEREJÓN, A. & MORENO-EIRIS, E. (2004): Facies distribution patterns and environment interpretation of the Upper Láncara Formation in the Esla Nappe (Cantabrian zone, NW Spain) by quantitative microfacies analysis. Freiberger Forschungshefte, C 502: Paläontologie, Stratigraphie, Fazies (12): 101-118; Freiberg.

Abstract

The microfauna of the carbonatic Upper Láncara Formation in the Esla Nappe (Cantabrian zone, NW-Spain) is described in detail and discussed in respect to biofacies and palaeoecology for the first time. For it, thin-sections were undergone bio- and lithofacial investigations. The here represented data on point-counting exhibit the first quantitative biofacies values for the vertical succession of the Upper Láncara Formation. The curves of faunal distribution show clear trends in the development of the biofacies. The microfauna reveal three biofacies associations which allow a palaeoecological interpretation.
The deposition of the Lower Láncara Formation took place on a southwest-tilted, carbonate-dominated homoclinal ramp, influenced by siliciclastic input from the northeast. Oolitic-bioclastic packstones indicate the existence of an oolitic shoal in the northeast. This shoal supplied oolites to the distal environment, and protected the distal areas from siliciclastic input. Due to a rising sea-level during the Upper Láncara time, the sedimentation belts shifted to proximal areas of the shelf and the oolitic shoal were drowned. Subsequently, the drowned shoal was invaded by echinoderms. The high abundance of echinoderm ossicles created the so-called Beleño facies. Toward the top of the Upper Láncara Formation, a gradual change from the echinoderm-dominated fauna to a trilobite and brachiopod dominated fauna can be observed. The here occurring lithotypes are typic for the so-called Barrios facies. The constant thickness of that facies over the whole working area and the same faunal succession in the investigated sections suggest a deposition under similar bathymetric conditions. An increase of siliciclastic material leads to the development of a mixed carbonate-siliciclastic ramp. This sedimentary turnover mirrors the general drowning process of the depositional environment.

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