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46696-AC8
Strand Plain Geometry, Stratigraphy, and Evolution: Tijucas, Santa Catarina, Brazil

Duncan M. FitzGerald, Boston University

Utilizing new and improved technologies and dating methods, we are studying the Tijucas strandplain in Santa Catarina, Brazil to advance our understanding of strandplain development and ultimately to provide a new depositional model for strandplains forming in humid climates in a regime of falling sea level. To date, we have gathered approximately 20 km of ground penetrating radar (GPR) profiles, 23 sediment cores, including four 20-m cores, and RTK topographic survey data. Chronology is based on radiometric dating of 19 organic samples. Our initial findings demonstrate that the Tijucas plain is essentially flat rising gently westward from Tijucas Bay 5 km towards a Holocene high stand shoreline defined by a dune ridge system. Landward of the highstand ridge is a topographically flat region that is interpreted as a former muddy lagoon. A core through this sequence shows that the mud is 8.6 m thick and is underlain by a sandy unit. Four hundred meters landward of the highstand ridge a 4 m-high scarp, which is believed to be the oxygen isotope 5e Pleistocene highstand shoreline, punctuates the plain. This interpretation is based on the following: 1. Ridge morphology consisting of a fining upwards sequence culminating in a fine dune sand facies, 2. Presence of marine bivalve shells, 3. GPR transects exhibiting sigmoidally-shaped, en echelon seaward-dipping reflectors, and 4. Dating of similar features reported at along the rear of other southern Brazilian strandplains.

The mid-Holocene highstand ridge in Tijucas is manifested by a semi-continuous dune ridge system sitting 1.0 to 1.5 m above the adjacent plain. GPR reflector geometry and the facies mapping using three closely-spaced (~ 50 m apart) sediment cores along a shore-normal transect suggest that the dune ridge represents the juxtaposition of retrogradational and progradational facies. Low angle, landward dipping reflectors produced by coarse sand layers (identified in hand augers and a core) front the ridge and prograde into muddy sediments. This sequence is believed to have developed by washover sands deposited into a muddy lagoon that formed between the Pleistocene scarp and the Holocene highstand transgressive barrier. We interpret the washover sands as representing the leading edge of the mid-Holocene transgression. The GPR transect represents the first time that the mid-Holocene transgression has been imaged geophysically. A core taken through the lagoonal mud and washover deposits bottoms in a coarse sand unit containing numerous shell fragments and a razor clam (Ensis directus) in growth position.  The shell dates at 5957 ± 58 cal BP, which provides an oldest date for the mid-Holocene highstand. The 3.0 m elevation above mean sea level of the washover sand provides an estimate of highstand sea level position. This elevation and timing compare favorably with estimates of the highstand at locations in southern Santa Catarina (~ 2.1 m at 5400 cal BP) and in the adjacent State of Parana (3.5 - 3.6 m at 4800 - 5300 cal BP).

Immediately east of the Holocene highstand a GPR transect reveals repetitive seaward dipping reflectors that depict beach, shoreface, and foreshore accretion. Sediment cores in this region indicate that the progradational sequence consists of medium to coarse-grained sand. The dip of the reflectors (5 to 12 degrees) and coarse nature of the sediment suggest that deposition took place in a moderately high-energy setting that is quite different from today's low energy coastal setting. Toward the present shore, sand thickness gradually decreases from 7-8 m in the upper plain to 1 to 3 m near coast. The middle plain is periodically interrupted by cheniers 1 to 3 m in thickness, 5 to 10 m apart, and underlain by more than 6 m of mud. The lower third of the plain is dominated by mud containing widely-spaced chenier ridges. Although the beach is sandy, the nearshore and offshore consist of a liquid mud extending at least one kilometer offshore. It is apparent that temporal changes in sediment discharge of the Tijucas and/or the extent of wave-reworking of the fluivial sediment have profoundly affected the depositional history of the strandplain. The reason for this alteration in the sedimentation regime within Tijucas Bay may be related to changes in the fluvial bedload/suspended load ratio caused by climate change, which in turn produced modifications in vegetation patterns, bedrock weathering and soil formation processes, and ultimately sediment contribution in the drainage basin. Alternatively, basinal infilling due to mud deposition by the Tijucas River may have shoaled the bay resulting in greater attenuation of wave energy and less reworking of nearshore sediment. Deep cores are needed to test these hypotheses.

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