Sedimentary evolution of northeastern Guanabara Bay, RJ, revealed by sismoestratigraphic analysis

Guanabara Bay (GB) is described by Amador (1997) as an ancient Pleistocene riverine basin originated during Guanabarina Transgression, beginning at Pleistocene-Holocene limit and had its maximum transgressive flooding between 6,000 and 5,000 years BP, when the Holocene optimal climate occurred. Its present configuration is linked to sea-level fluctuation during this period in addition to sediment accumulation. In order to understand the sedimentary and stratigraphic evolution of the northeastern portion of GB, approximately 250 km of high resolution seismic lines was acquired and sedimentary column was cored for validation of seismic interpretations and age dating. A continuous sedimentary column of approximately 13m was collected and carbonate shells was age dated with radiocarbon, AMS-C14 technique. Sediment was analyzed for grain size, calcium carbonate and organic matter contents. Using the sedimentary and seismic data, it was possible to define three main sedimentary units: pleistocenic sands deposits (Unit 3), fluvial-marine drowning deposits (Unit 2) and late estuarine sedimentation (Unit 1) which are comprised between four main reflectors; R4, R3, R2 and R1. Reflector R4 is the basal reflector. It has very irregular topography resembling the geology of the granite-gneiss mountain chain surrounding GB, It represents the local basement. Above reflector R4 is Unit 3 consisting mainly of pleistocenic arcosean sands, with no fossils, poorly sorted and highly compacted at the base. The presence of feldspar and mica plates indicates proximity of the crystalline basement. Reflector R3 is the base of a fluvial-marine sequence, Unit 2. At 11,25m below ocean bottom occurs the first marine carbonate shells, dated as 8,890 calibrated years BP, representing the leading edge of the marine transgression. Sediment of this unit has small grain-size, high mud content and some internal reflectors related to possible sea-level oscillations. The environment is interpreted as a low energy estuary. Others age dating inside this unit was 7,506 and 2,853 calibrated years BP. Unit 1, defined by R2 at the base and R1 at the top (seabed) is also constituted of fluid silty mud. During the deposition of this unit, relative sea level showed a slow and gradual fall, allowing a calm and continuous deposition, without formation of internal reflectors. The obtained ages dating allowed the inference of an average accumulation rate over the last 9,000 years of 0.126cm.year-1, well below the current accumulation rates.