Estratigrafía y sedimentología de la Fm La Banda (Cuenca de Loja, Mioceno, Sur de Ecuador)

Zusammenfassung

The La Banda Formation (middle/upper Miocene) is a thin (up to 20 m thick) but laterally persistent carbonate unit that crops out in the western sector of the Loja Basin (southern Ecuador). It is composed of laminated limestones and dolostones with intercalations of marls, siltstones, fine-grained sandstones, bedded-cherts and abundant secondary gypsum veins. The laminated lithofacies could form domal structures (up to 80 cm high and 105 cm wide) and commonly exhibit ghost of erect micritic filaments (sometimes bifurcated), wrinkle structures and polygonal desiccation cracks. The unit is interpreted as a shallowing upwards or regressive sequence, deposited in a tidal flat environment under arid or semi-arid climate. Lithofacies grade from subtidal deposits at the base (yielding ostracoda and foraminifera) to intertidal to supratidal deposits at the top. Dolomitization is interpreted as a synsedimentary diagenetic process related to intense evaporation (sabkha model). Climatic conditions favoured gypsum precipitation that increased the Mg/Ca ratio of the brine and resulted in the direct precipitaion of dolomite or dolomitization (replacement processes).