OS23D-01
The Morphologic Evolution of the Amazon Coastal Plain, Cabo Norte, Amapa, Brazil: The Need for Integrated Investigation on the Internal Continental Shelf.
The north brazilian coastal region is submitted to the conjunction of three major forcing: 1) Atmospheric, related to the wind and precipitation regimen and controlling the climatic seasonality and the local rivers hydrology; 2) Oceanic, result of the general oceanic circulation (North Equatorial Current-NEC) and it modulation with the atmospheric forcing (North Brazilian Coastal Current reflection), and 3) Amazonic, which is a result of the sedimentary transport from Amazon river, being itself under the influence of the atmospheric and oceanic forcing. Their main characteristic are the proper periodicities and also variabilities expressed according multiples and differentiated scales of time and space, determining the structure and functioning of the coastal space, giving to the Amazon coast line a considerable environmental instability. The Amapa Coastal Plain shows important part of this environmental instability. Semidiurnal macro tides, strong currents and tidal bores acts over a low gradient coastal plain which evolutionary process are intrinsically related with tectonic and geological settings. Two main areas can be distinguished in this scenario. The first one is the Amapa Lakes Region, developed over meanders of ancient drainages building a mosaic of low relief forms, with varied shapes, linked throughout communication channels. Two lacustrine belts showing alignments with the main tectonic traces were recognized beginning at the internal limit of the coastal plain, flowing southeast through the Tabaco Creek until reach the Araguari River. In it way it conform the Occidental and Meridional Belts represented by shallow lakes, totally dependent of Tartarugal River discharge and surrounded by enormous areas of peat highly sensitive to fire during the regional low discharge. Light blue clays, peat and a package of tidal deposits with fluidization structures were recognized close to Araguari River, suggesting deposition during high sea level. The second area refers to the coastal zone between the Amapa Grande River and Araguari River, including Maraca Island and the Oriental lacustrine Belt. Westward the island, at least three paleolevels of clays with roots in life position suggests regressive/transgressive events. Extraordinaty paleodrainage network beginnig at the continent and recognized at the insular portion suggests links with the paleochannels found at the continental shelf. The Oriental Belt of lakes is located close to the coastline, at Cabo Norte. It main feature is a mud lump approximately 10 Km ratio, well recognized at the remote sensing. It shows similar evolutionary processes with Araguari River, dating from XIX century, when this river had two mouths defined by the Carpori Island. The reasons of the deactivation are still unknowed, but, this rapid morphological evolution indicates short time colmatation processes that can be linked to tectonic regional processes. On the other hand, the Cabo Norte feature consolidation may impose changes in the sedimentation processes yielding space reduction over the coastal plain accumulation, diminishing of the solid and liquid fluvial discharge and promoting the availability of the local sediment transport over the littoral. The investigation of these processes requires an integrated coastal plain-continental shelf morphological study applying adequate techniques for modification studies and dating ages over short geological time frame, in century scale level.
OS23D-02
An approach to the coastal water circulation in the Piratuba Lake Biological Reservation, Northeast of Amapa State, Brazil
This study shows the pioneer results of the water quality characterization of a lake region, including the Piratuba lake (within the limits of the Piratuba Lake Biological Reservation) and the Sucuriju river, localized at the northeast portion of the Amapa State, Brazil, and left margin of the Amazon River mouth. Due to the influence of the Amazon river and another important river, the Araguari river, the northeast coast of Amapa State receive little impact of salty water from the Atlantic ocean. The highest salinity values detected on this coastal area is 20 psu. The Piratuba Lake region which can be described as an unique wetland system formed by recent geological processes (Quaternary), it constitutes a very fragile environment and possesses a number of shallow water lakes distributed into a mixed mangrove and "varzea" type of vegetation and it is considered very important looking at the biological point of view. The borderline between this lake system with the coastal waters is a narrow portion of mangrove containing species of Rizhophora and Avicennia parallel to the coast line. A preliminary water circulation could be accessed through the detection of variation in water quality parameters throughout three field studies conducted on March, 2004, June 2005 and November 2005. Surface water sampling points spatially distributed on the study area with distances less than 2 km were set, covering almost 800 square kilometers. Among the parameters studied (pH, electrical conductivity, turbidity, concentration of suspended solids, depth, temperature, chloride, dissolved oxygen, nitrate, nitrite and phosphate) the turbidity, electrical conductivity and pH were the most important for identifying the entering of coastal waters into the lake region. Mainly, three points of direct contact were identified; one of them is a manmade illegal entrance to the Biological Reservation. The seasonal variation was also very important factor and as expected, during the dry season (July to November, represented by the study on November 2006), we could measured the highest values of conductivity, pH and turbidity in many of the lakes sampled. The implications of understanding, even preliminarily the water circulation in the study area is related to the influence onto the freshwater living species, environemtnal conservation management purposes, geomorphology process indicator and the possible biogeochemical exchanges dynamics, once the Amazon waters can be nutrient rich and the lake waters apparently have high dissolved organic carbon content.
OS23D-03
High-Resolution Subsurface Imaging and Stratigraphy of Quaternary Deposits, Marapanim Estuary, Northern Brazil
The Marapanim estuary is situated in the Para Coastal Plain, North Brazil. It is characterized by an embayed coastline developed on Neogene and Quaternary sediments of the Barreiras and Pos-Barreiras Group. This system is strongly influenced by macrotidal regimes with semidiurnal tides and by humid tropical climate conditions. The interpretation of GPR-reflections presented in this paper is based on correlation of the GPR signal with stratigraphic data acquired on the coastal plain through five cores that were taken along GPR survey lines from the recent deposits and outcrops observed along to the coastal area. The profiles were obtained using a Geophysical Survey Systems Inc., Model YR-2 GPR, with monostatic 700 MHz antenna that permitted to get records of subsurface deposits at 20m depth. Were collected 54 radar sections completing a total of 4.360m. The field data were analyzed using a RADAN software and applying different filters. The interpretation of radar facies following the principles of seismic stratigraphy that permitted analyze the sedimentary facies and facies architecture in order to understand the lithology, depositional environments and stratigraphic evolution of this sedimentary succession as well as to leading to a more precise stratigraphic framework for the Neogene to Quaternary deposits at Marapanim coastal plain. Facies characteristics and sedimentologic analysis (i.e., texture, composition and structure aspects) were investigated from five cores collected through a Rammkernsonde system. The locations were determined using a Global Positioning System. Remote sensing images (Landsat-7 ETM+ and RADARSAT-1 Wide) and SRTM elevation data were used to identify and define the distribution of the different morphologic units. The Coastal Plain extends west-east of the mouth of the Marapanim River, where were identified six morphologic units: paleodune, strand plain, recent coastal dune, macrotidal sandy beach, mangrove and salt marsh. The integration of GPR profiles and stratigraphy data allowed for the recognition of paleochannel geometry, with width of 150m and depth of 20m, developed on Barreiras Group, two discontinuity surfaces and three facies associations organized into sedimentary facies: (i) Tidal channel with mottled sand, Conglomerate with clay pebble and Ophiomorpha/linear Skolithos, channel-fill and tabular cross-bedding sand and sand/mud interlayer facies. (ii) Dune/interdune with wavy bedding and cross-bedding sand and planar bedding and tabular cross-bedding sand facies. (iii) infilled tidal channel with mottled sand, planar/flaser bedding sand, lenticular bedding clay and sand/mud interlayer facies. The present study demonstrates that some facies associations occur restricts to tidal paleochannels and shows features well preserved that are very important to reconstruction of the relative sea-level history in the Marapanim Estuary.
OS23D-04
The Amazon Mangrove Coast: The Role of Geological Factors in its Evolution During the Quaternary
The Amazon mangrove coast considered in this work includes the northeast Para and northwest Maranhao states. This coast is extremely irregular and jagged with numerous bays and estuaries. The main goal of the present work is to recognize the principal geological factors responsible for the development of the mangrove system. The integration of remote sensing data with other available studies indicates the existence of four main geomorphological sectors along the mangrove coast as described below. Sector 1 extends from Marajo to Pirabas Bay and is developed over the Para platform. The coastal plateau reaches the shoreline forming terraces and active cliffs. Sector 2 extends between the Pirabas and Gurupi bays and is structured over the Bragança- Viseu basin. In this sector, the coastal sedimentary environments widen considerably towards the east from Pirabas Bay and the coastal plateaus stretch out southward to constitute inactive cliffs. Mangroves developed seaward, reaching currently a width of 30 km. Sector 3 extends from Gurupi to Turiaçu Bay and is developed over the Gurupi horst. The latter represents a stratigraphic window where proterozoic rocks outcrop near the coast. Compared with the other sectors, mangrove deposits here reach their maximum extension with up to 40 km wide. Sector 4 extends between the Turiacu and Cuma bays and is structured in the Sao Luis basin. This coastal basin is also developed on land and represents a gravimetric low along a NE-SE direction.Mangroves are narrower, with a maximum width of 26 km. This analysis of the coastal geomorphology by considering neotectonic activity allows the identification of five sectors. Sector 1, with a positive gravimetric anomaly and poorly influenced by peripheral bulge. These characteristics suggest a relative tectonic stability of this sector, where the coastal plateau reaches the shoreline and mangroves are poorly developed. Sector 2 is marked by low gravimetric anomalies and normal faults reactived by peripheral bulge. In this sector, the location of these inactive cliffs is spatially coincident with the peripheral bulge. Hence, we suggest that the inactive cliffs are a result of the flexural reactivation of ancient normal faults, which is supported by studies of in the northeastern Brazilian coast. Sector 3 is also marked by normal faults and peripheral bulge influence, presenting geomorphological characteristics similar to Sector 2. In Sectors 2 and 3 the retreated coastal plateau and inundation deposits of the estuaries allowed the development of wide tidal flats where the largest mangrove belt is established. In Sector 4 there is a great mangrove development. This area is characterized by a gravimetric high, with little influenced by peripheral bulge and is structurally controlled by normal faults limited by the Cururupu arch. The interaction of regional framework and flexural deformation explains the reactivation of ancient faults responsible for the geomorphology of the North Brazilian mangrove coast. However, further structural and geodetic monitoring from interferometric SAR data are needed for a more detailed knowledge of the Quaternary tectonics of this region. This may provide elements for a better comprehension of wetland evolution in the moist tropics, particularly regarding their response to coastal subsidence and relative sea level changes in time of global changes.