The eastside of the Marajó Island (Pará State) shows a diversity of morphological
features produced by sea level changes, neotectonic and coast dynamics, during Late
Cenozoic. The sea level changes, from Miocene to Holocene, controled the deposition of
Barreiras Formation and Pós-Barreiras Sediments that form the coastal upland. The
framework neotectonic structures controll the NE-SW strip-slipe fault systems and NW-SE
normal faults, influencing the distribution of relief units and the fluvial and shoreline
morphology.
The coastal morphology shows two main units: coastal upland and coastal plain.
Coastal upland represents a flated relief with low ondulations and elevations between 5 and
15 meters. The contact with the coastal plain is abrupt, forming “dead” and active cliffs.
The coastal plain has a flat relief with low gradients, with portions below 5 meters in hight,
which favors sea inundations and the sedimentary dynamic.
Long term coastal changes, during the last 5.000 years, resulted in shoreline
accretion in response to regressive or stable sea level conditions, with the development of
tide flats and mangroves, with migration of barrier-beaches and dunes over mangrove
deposits. The accretional and regressive successions of the Soure coastal plain is agreeable
to the regressional succession (S2) and transgressive succession (S3) as proposed to coastal
plains of Bragança, Salinópolis, Marapanim and São João de Pirabas.
The medium term coastal dynamic (1986/2001) is represented by morphological
changes in response to interactive action of wave, current, tide and wind process, which has
produced changes in the shoreline position. Soure and Salvaterra coast has been submited,
in the last 15 years, to erosional processes characterized by shoreline retreat. The total of
erosioned lands reached from 0.89 km2 (1986/1995), to 0.38 km2 (1995/1999) and 0.75
km2 (1999/2001). While the accretional areas represent a total of 0.21 km2 (1986/1995),
0.32 km2 (1995/1999) and 0.08 km2 (1999/2001).
The short term coastal changes involve the morphological and textural variability
from beach topographic profiles in the rain and dry periods seazon in 2001. The seasonal
changes are reflected in answer from the beach profiles in respose to variations of tides,
ranges of currents, wave energy, availability of sediments and coastal morphology. In Soure, the erosional phase (february to april rainy period with highest tidal range)
when it is possible to observe the shoreline retreat (21 m), backshore reduction (13 m),
parallel dislocation of foreshore, sedimentary loses, granulometric coarsing (2.81 to 2.94 ϕ,
thin sand)) and better selection (0.24 to 0.33, very well selected). The accretionary phase
(july to nevember, dry period with stronger winds) showed: hight tide line extension (82
m), backshore enlargement (48 m), sedimentary gain (+339.25 m3), granulometric finning (2.86 a 3.10 ϕ, thin to very thin sand) and worse selection (0.28 to 0.40, very well to well
selected). In Salvaterra, the accretional phase (february to april) showed high tide line
extension (29 m), backshore enlargement (13 m) and raise of beach volume. In profile 1,
there was a granulometric coarsing (0.84 ϕ, thick sand) and worse selection (0.51,
moderatelly selected). In profile 2, the grain became thinner (1.49 f medium sand) and
better selection (0.44 well selected). The erosional phase (july to november) showed high
tide line retreat (25 m), diminution backshore (8 m), sedimentary loses (-22.69 m3),
exchange of material between the higher amd lower part of the profiles, the grain became
thinner (1.39 ϕ medium sand) and worse selection (0.52 moderatelly selected).
The vulnerability of the coast zone to natural risks occur in response to erosive
processes in the last 15 years. The environmental zonement resulted in an integration of
morphological dates with analysis of coastal changes geoindicators and human interference
levels. Different sectors of the coastal zone were classified in : permanent preservation
areas (mangroves, beaches and dunes), adequated to occupation areas (coastal upland),
areas with risk to occupation (cliff sides) and environmental damage areas (deforested
mangroves, beach-dune ridges occupied). The recomendations of preservation, use and
future occupation from the coast should subsidize the planning and coastal management.
The use of remote sensing and of Geographic Information System, in the several
stages of this thesis development, represented important tools to data aquisitions, spatial
analysis and synthesis, understanding of coastal relief distribution and caracteristics,
observation and quantification of the changes, thematic mapping. Thus this information
presents a large use in coastal studies.
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