Use of charophyte carbonates as proxy indicators of subtle hydrological and chemical changes in marl lakes: Example from the Miocene Bicorb Basin, eastern Spain

摘要

The lower, alluvial unit in the Miocene Bicorb Basin contains several metric-scale limestone intervals which record episodic shallow lacustrine environments in an alluvial setting developed during the early stage of the basin's evolution. Five main carbonate facies have been differentiated in the lacustrine limestones, although calcite charophyte incrustations predominate and constitute the most striking features of these deposits. The thinnest limestone intervals correspond to deposits from charophyte meadows in ponded shallow depressions in floodplains. The thickest limestone intervals are mainly formed by banded limestones and usually correspond to diverse types of regressive sequences that have been interpreted as resulting from the infill of shallow lakes. The sedimentological features and sequences show noticeable differences in the gradient of the littoral zones and the amount of palustrine deposits with models proposed for marl lakes. Charophytic carbonates from the best-preserved facies show similar microtextures to those from recent charophyte incrustations. The variations in stable isotopes (δ13C, δ18O) for these primary carbonates occur in parallel with luminescence variations and correspond to hydrological changes and variations in solute composition and Eh-pH status in the lake waters. The carbonates that display moderate to strong diagenetic modifications show a diverse degree of compaction, aggrading neomorphism, strong cementation and nodulization. The isotopic values for these are arranged in diverse clusters. There is a correlation between the degree of luminescence and the δ13C. This suggests that hydrological and hydrochemical variations both in the lacustrine and diagenetic environments are being recorded in parallel. We emphasize the need for further comparative studies between recent and ancient charophytic carbonates. As these carbonates have been used in palaeoenvironmental reconstructions, special attention must be paid to the diagenetic changes in ancient charophytic marls. (C) 2000 Elsevier Science B.V. All rights reserved.