CARBONATE POND DEPOSITS RELATED TO SEMIARID ALLUVIAL SYSTEMS - EXAMPLES FROM THE TERTIARY MADRID BASIN, SPAIN

Carbonate pond deposits occur associated with alluvial sediments in Mi ocene sequences of the Madrid Basin, central Spain. The ponds develope d near the basin margins, either in floodplain environments (north) or mud-flat settings (south). Three main facies assemblages are recogniz ed: (1) floodplain/mud-flat, (2) palaeosols and (3) pond deposits. In the northern part of the basin, ponds developed on the floodplain of t erminal fluvial systems. The floodplain facies are typically red mudst ones with interbedded sandstones and siltstones. Palaeosols associated with the ponds show a pedofacies relationship, the maturity of soils increasing with distance from the main channel. Carbonate pond deposit s consist mainly of limestones, which display typical 'palustrine' fea tures. The formation and further accumulation of carbonate in the pond s took place in periods of reduced elastic sediment input and it is su ggested that recharge into the pond areas was mainly from groundwater. In the south, ponds developed on mud-flats located between sheet-floo d-dominated alluvial fans and evaporite lakes. Mud-flat facies consist of red mudstone that exhibits evidence of progressive soil developmen t near both edges and beneath the carbonate pond lenses. Carbonate in the ponds is mainly dolomite and comprises two subfacies, mottled and laminated dolomicrites. This mineralogy, together with the presence of gypsum crusts below and in the lower part of the carbonate body, sugg ests higher evaporation rates and/or more saline waters filling the po nds in this part of the basin. In spite of differences in depositional setting and, to some extent, climatic conditions between the two area s of the basin, both facies associations and the sequential arrangemen t of the ponds show strong similarities that allow the proposal of a f acies model for carbonate pond deposits related to semi-arid alluvial systems. The sequences recognized from the pond deposits record a set of facies clearly different to those forming in swampy lakes associate d with many permanent fluvial systems developed in more humid climates .