Palaeogene lacustrine record in Mallorca (NW Mediterranean, Spain): depositional, palaeogeographic and palaeoclimatic implications for the ancient southeastern Iberian margin

Abstract

Middle Bartonian (Middl1e Eocene) and Chattian (Upper Oligocene) lacustrine and alluvial-lacustrine successions exist in Mallorca (northwestern Mediterranean). They yield a significant palaeobiological record comprising fossil land plants and vertebrates and constitute the best available depositional record for a preliminary palaeogeographic and palaeoclimatic analysis of the northeastern part of the Palaeogene southern Iberian Margin (western Tethys). Both lacustrine systems developed under similar warm and humid climatic conditions, increasingly affected by seasonal and/or pluriannual rhythms marked by changing precipitation patterns. These climatic conditions were mainly caused by the low palaeolatitude where the lacustrine systems evolved, the still probable although decreasing influence of Indo-Pacific warm surface currents, and the occurrence of very extensive marine water masses adjacent to wide, emerged continental zones. The Palaeogene lacustrine systems were shallow, and characterised by low gradient depositional conditions. They were perennial, open freshwater-oligosaline carbonate lakes where eutrophy, thermal meromixis and anoxic bottom conditions could develop, allowing the accumulation and preservation of macrophytic plant remains of the circum-lacustrine marginal belt and neighbouring terrestrial mesophytic zones. The higher plant/algal-bacterial organic contribution balance favoured the generation of mineable huminitic coal seams, carbominerites and of gas-prone, kerogene type III and IIb source rocks. The hydrogen-bicarbonate-rich waters led to high biogenic and inorganic bio-induced carbonate sediment production. Biogenic muds, cyanobacterial incrustations and travertine and tufa precipitation constituted the major carbonate products. Moreover, the high pH conditions in the lacustrine water bodies and in the lacustrine sediment pore waters favoured bacterial sulphate reduction, contributing to organic sulphur enrichment in coals. These highly alkaline conditions also resulted, in some cases, in the exceptionally good preservation of chlorophyllinite, a rare liptinite maceral recorded in the Eocene coal deposits. The Middle Eocene lacustrine system did not evolve in a very well defined tectonic setting, characterised by the generation of low uplifts and gently subsiding basins. Terrigenous contributions from the source areas and clastic sedimentation were restricted to river mouths. High autochthonous carbonate production and accumulation characterised most of the remaining lacustrine zones. A significant spring recharge of the lacustrine zones is suggested by the well developed spring-related travertine deposits. By contrast, the Oligocene lacustrine zones developed both in fluvial-related floodplains and in the distal zones of a major alluvial system, which was fed from tectonically active source areas located to the NW of the island. A more significant water runoff contribution is suggested for this system although underground recharge could also be substantial.