Facies distribution patterns and environment reconstruction of the upper member of the Láncara Formation in the Somiedo-Correcilla unit (Lower-Middle Cambrian, Cantabrian zone, NW Spain) with special respect to biofacial investigations

摘要

The microfauna of the upper member of the Láncara Formation in the Somiedo-Correcilla unit (Lower-Middle Cambrian, Cantabrian zone, northwestern Spain) is described in detail and discussed in respect to biofacies and palaeoecology. The study of about 140 thin sections allows to reconstruct a sedimentological model based on bio- and lithofacial trends. A vertical faunal differentiation is detected in the three studied outcrops. The base of the upper member of the Láncara Formation (Beleño facies) is characterised by a high amount of sessile suspension feeders (echinoderms) which indicate a low or absent siliciclastic influx. The amount of trilobites (mobile deposit feeders) and sessile filter feeders (brachiopods) is clearly lower. This echinoderm-rich facies characterises the lower few meters and is called Faunal Assemblage A. The transition to Faunal Assemblage B is characterised by a decrease of sessile suspension feeders, whereas the mobile organisms increase in percentage. This evolution continues in “Faunal Assemblage C”, which is dominated by mobile suspension- and sessile filter-feeders. The continuous lithological and faunal succession, but overall low faunal and facial gradients of the upper member of the Láncara Formation are representative for a ramp environment. The deposition is interpreted as a turnover from an eastward-tilted homoclinal carbonate ramp (lower member of the Láncara Formation) to a mixed carbonate-siliciclastic ramp (Beleño and Barrios facies, respectively Faunal Assemblages A, B, C, D). The abrupt appearance of the Faunal Assemblage A, points to a sudden subsidence of the environment. A continuous increase in the siliciclastic content is accompanied with a successive change in the faunal composition from a fauna dominated by sessil suspension feeders to a mobile deposit feeder-dominated one. The increase of siliciclastic material indicates a more or less constant rise of sea-level that caused on onshore shift of the facies belts, and mirrors the general drowning of the ramp.