A Planície Costeira Bragantina (NE do Pará): influência das variações do nível do mar na morfoestratigrafia costeira durante o holoceno

Abstract

The Bragantina Coastal Plain, with an approximate area of ​​1,570 Km2, is located in the Cretaceous Coastal Basin of Bragança-Viseu (NE of Pará). The architecture of the basin and its paleotopography, associated with recent tectonic movements, have controlled the distribution and thickness of tertiary and quaternary deposits. This coastal plain is a depositional system called macrotidal (6m), developed under conditions of a hot and humid equatorial climate, with a well-defined rainy and dry season and an average annual precipitation equivalent to 3,000 mm. The geomorphology of the area is compartmentalized into three morphological patterns: (1) alluvial plain, with fluvial channel, levees and floodplain; (2) estuarine plain, with estuarine channel compartmentalized in estuarine funnel, straight and meandering segment and upper course channel, tidal streams and floodplain; (3) coastal plain, with saline marsh environments (internal and external), tidal plain (supratidal, intertidal and sandy plain with tidal flats), chenier, coastal dunes and beaches. The study of morphostratigraphy allowed the characterization of 11 morphostratigraphic units: (1) floodplain, (2) levees, (3) channel bar, (4) supratidal mangrove, (5) saline marsh, (6) intertidal mangrove , (7) sandy plain, (8) bar in point, (9) coastal dunes, (10) chenier and (11) beaches and 4 stratigraphic facies: (1) river sand, (2) marine/estuarine sand and mud, (3) sand and mud with heterolithic bedding and (4) mottled muddy sand. The stratigraphic analysis allowed the individualization of three stratigraphic sequences: (1) basal transgressive marine (S1), with fluvial, marshy and shoreface environments; (2) regressive marine (S2), with fluvial, tidal flat, chenier and saline marsh environments and; (3) current transgressive marine (S3) with estuarine and coastal environment. The evolution of this depositional system in the Bragantina Coastal Plain is associated with the sea level oscillations during the Late Holocene. At the height of the Holocene transgression (6,000 years B.P.), a rise in sea level invaded coastal areas, eroding the Coastal Plateau (Barreiras Group). After this event, the muddy plain prograded, under regressive or stable sea level conditions, on the basal transgressive sandy sheet. Therefore, erosive periods, probably related to rapid sea level rises or climate change, affected the discharge of rivers, allowing the formation of cheniers. Currently, new sandy deposits are migrating over the muddy plain.