HOLOCENE STRATIGRAPHY IN THE PARANAGUA BAY ESTUARY, SOUTHERN BRAZIL

The Paranagua Bay estuary is a 50 km long microtidal barrier estuary t hat is still far from infilled even after 3.5 m of continuous sea-leve l fall during the last 5000 years. Data derived from several independe nt studies, including surface sediment samples, jet-probe cores, vibra cores, and seismic records, were compiled in order to provide an asses sment of the Quaternary stratigraphy of the estuary, The estuary displ ays the longitudinal tripartite zonation of surface sedimentary facies (marine sand-estuarine mud-fluvial sand) that is characteristic of ma ny coastal-plain estuaries. Five sedimentary facies were identified ov erlying the bedrock. Pre-Holocene fluvial and continental deposits wit hin the paleo-valley form the substrate for the more recent sedimentat ion. A transgressive mud facies marks the initial stages of estuary in undation. This deposit is observed only in the lower half of the estua ry, where greater accommodation space existed and less intensive tidal scouring associated with the subsequent deposition of the transgressi ve sand facies occurred, The onset of the regressive stage is marked b y the deposition of the regressive mud facies, which is presently the most extensive sedimentary deposit within the estuary, The regressive sand facies, which is composed of fluvially derived sediments and is r estricted to the head of the estuary, is the least developed sedimenta ry facies, The vertical succession of the sedimentary facies shows an almost complete stratigraphic sequence and the presence of several bou nding surfaces: a transgressive surface, a maximum flooding surface, a tidal ravinement surface, and a tidal diastem. A comparison of the Pa ranagua Bay, Gironde, and James estuaries, all of similar size, illust rates various stages of sequence development in modern incised-valley systems.