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

The lower, alluvial unit in the Miocene Bicorb Basin contains several metri c-scale limestone intervals which record episodic shallow lacustrine enviro nments in an alluvial setting developed during the early stage of the basin 's evolution. Five main carbonate facies have been differentiated in the la custrine limestones, although calcite charophyte incrustations predominate and constitute the most striking features of these deposits. The thinnest L imestone intervals correspond to deposits from charophyte meadows in ponded shallow depressions in floodplains. The thickest limestone intervals are m ainly formed by banded limestones and usually correspond to diverse types o f regressive sequences that have been interpreted as resulting from the inf ill of shallow lakes. The sedimentological features and sequences show noti ceable differences in the gradient of the littoral zones and the amount of palustrine deposits with models proposed for marl lakes. Charophytic carbon ates from the best-preserved facies show similar microtextures to those fro m recent charophyte Incrustations. The variations in stable isotopes (delta (13)C, delta(18)O) for these primary carbonates occur in parallel with lumi nescence variations and correspond to hydrological changes and variations i n solute composition and Eh-pH status in the lake waters. The carbonates th at display moderate to strong diagenetic modifications show a diverse degre e 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 delta(13)C. This s uggests that hydrological and hydrochemical variations both in the lacustri ne and diagenetic environments are being recorded in parallel. We emphasize the need for further comparative studies between recent and ancient charop hytic 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 res erved.