Sedimentology and diagenesis of sandstones of the Barro Duro formation (albian), western part of the Barreirinhas Basin, Brazil

Abstract

Located ín the Barreírinhas Basin, Northoostern Brazil, the Albian Barro Duro Formation compríses a siliciclastic sequence up to 2 000 m thick, constítuted mainly of sandstones and, secondarily, of shales_ On the basis of sedímentologícal descriptions of cores, seven líthofacies associations were distinguished, corresponding to the following environments: distributary channel fill, interdistributary bay, crevasse splay, distributary mouth bar, distal bar, prodefta, and shaffow marine. Petrographic evidence indicates that Barro Duro sandstones are predominantly arkoses that were subject to the following sequence of main diagenetic events: (1) emplacement of clays by infiltration and bioturbation; (2) precipitation of early calcite; (3) forma­tion of quartz and feldspar overgrowths,- (4) precipitation of chlorite; (5) development of ferroan calcite and dolomite; (6) dissolution of grains and carbonate cements; (7) formation of late cements and replacements, including kaolinite, chlorite, quartz, feldspar, ferroan dolomite/ankerite, siderite, pyrite, and titaniferous minerals; and (8) development of films of íron oxide on sideríte and late ferroan dolomite, as well as partial dissolution of these carbonate minerais. Porosities observed ín thin sections of Barro Duro sandstones range from less than 1% to 41% and are mainly of secondary origin_ Values higher than 17% are dominantly retated to the distributary channel fill and mouth bar facies associations, which probably displayed high primary poro­sities. However, even in these deposits, lower porosities may occur, due mainly to higher cement content.