Depositional, diagenetic and stratigraphic aspects of microfacies from Riachuelo Formation, Albian, Sergipe Basin, Brazil

Abstract

The rocks of the Riachuelo Formation, Sergipe Basin, Brazil, represent an example of carbonate sedimentation related to the drift phase during the opening of the South Atlantic Ocean. The Carapeba and Brejo quarries exhibit the best onshore outcrops of the drift carbonate section along the Brazilian continental edge. Field studies and microfacies analysis of the outcropped sedimentary section showed six sedimentary deposits related to the physiography of a carbonate shelf. Proximal mixed deposits are represented by the rich-terrigenous dolostone. Levels with alternate layers of fine grained sandstones and siltstones are here related to distal facies of submarine fans deposits. Mudstones with miliolids and textularids represent a lagoonal environment in a semi-restricted middle shelf. Packstones, grainstones and occasionally wackestones with oncoids, intraclasts and peloids represent sedimentary deposits related to the back of shallow sandy bars and environments at the interface with the lagoon. Grainstones whit ooliths, oncoids, intraclasts and bioclasts, with trough cross-bedding, represent a shallower shoreface environment over the shallow carbonate back on outer shelf. Cements and other post-depositional features suggest four different diagenetic environments: a) marine phreatic diagenetic environment with active water circulation; b) marine phreatic diagenetic environment with stagnant water; c) freshwater phreatic diagenetic environment; d) burial diagenetic environment. The sedimentary succession is formed by shallowing upward cycles overlain by a possible transgressive surface, which may indicate the passage of a lowstand to a transgressive system tract.