Depositional history and diagenesis of quartz-sand bars and lime-mud environments in the Devonian Basibé Formation (Central Pyrenees, Spain)

The Basibé Formation, of Lower Devonian age (Emsian) according to conodonts, consists, in the area between the Esera River and Mañanet River, of nodular weathering limestones, dolomites, silty to sandy argillaceous dolomites, quartzites and limestones. Thickness variations of the lower member (nodular limestones) and the upper member (limestones) are of minor importance, whereas the middle member (quartzite-dolomite alternation) is wedge-shaped: its thickness decreases over a distance of about 35 km from about 100 m (50 m of which are quartzites) in the west to 0 m in the east. In the surrounding areas the Basibé Formation consists solely of limestones. Carbonates were deposited as lime muds in shallow, open marine environments. Mature quartz sands, probably brought into the area by longshore currents, were accumulated by wave and current action as rather stationary bars or barrier islands, with a NW-SE direction. Leeward of bar complexes, highly bioturbated silty to sandy argillaceous lime muds were deposited. These back-bar deposits are, however, open marine sediments. Bars were buried by shallow, open marine lime mud sediments, due to subsidence and/or lack of clastic supply. The carbonates of the lower and middle member were originally fossiliferous micrites or biomicrites and were changed by diagenetic processes into nodular weathering limestones. This nodular appearance is caused by numerous stylolites and solution stringers which originated after lithification. Mg-enriched interstitial water, driven out of the quartz sandstones, during cementation, caused the limestones interfingering with these quartz sandstones to be replaced by dolomites. The pure quartz sandstones underwent quartz cementation, while pressure solution led to various types of contacts and to high pressolution of the quartz grains. Burial was not the only cause, but also tectonic pressure and a rise in temperatures due to the intrusion of the Maladeta batholith. Replacement of quartz grains and secondary quartz by dolomite is a rather important late diagenetic process. The limestones of the upper member show recrystallization and stylolitization, while some original pelmicrite beds were preferentially dolomitized.