Devonian Storm- and Tide-Dominated Shelf Deposits, Parnaiba Basin, Brazil: ABSTRACT

Abstract

Storm- and tide-dominated shelf deposits crop out in the Parnaiba basin, Picos region, State of Piaui, Brazil, belonging to the Pimenteiras and Cabecas Formations. The Pimenteiras is dominantly Middle Devonian whereas the Cabecas Formation is Late Devonian in age. The Pimenteiras Formation is organized into coarsening-upward cycles, with the top marked by bar-like sand bodies, bearing hummocky cross-stratification. Locally, channel-like depressions, carpeted with shale clasts, transect or are nested between adjacent bars. The overall environment is envisaged as a storm-dominated open shelf. The Cabecas Formation consists of a thick quartzarenite interval composed of apparently elongate bodies with radial accretionary wedges in a complex imbricated pattern. Double-tangential--or sigmoidal--bedding is the main sedimentary structure, followed by slumping and dewatering features and locally by lenticular-wavy-flaser bedding. This unit, in this part of the Parnaiba basin, can be interpreted as formed in a tide-dominated environment, probably of the shoal-retreat massif type, with subordinate tidal flats. In terms of vertical succession, the Pimenteiras and Cabecas Formations depict a cycle of relative rise of sea level (or of coastal onlap) with an increasing terrigenous influx toward the upper end of the cycle. The dominance of storm-wave action in the base of the cycle (Pimenteiras paracycles) points to a narrower shelf or steeper basinal slope. Later, the high terrigenous influx would decrease such a slope, preclude storm-wave action, and possibly amplify the tidal range--by shelf widening--in such a way that deposition would become tide-dominated. Oil shows have been reported in the Cabecas Formation, which presents remarkable reservoir characteristics, but apparently its high sand/shale ratio would not favor the formation of stratigraphic traps. Conversely, storm-dominated deposits appear to form excellent stratigraphic traps despite their poorer reservoir characteristics. End_of_Article - Last_Page 562------------