标题：Rochas de Praia (Beachrocks) da Ilha do Cabo Frio, Arraial do Cabo: Registro Geológico Ímpar da Transição Pleistoceno – Holoceno no Estado do Rio de Janeiro
其他标题：Beachrocks from the Cabo Frio Island: Unique Geological Record of the Pleistocene - Holocene Transition in Rio de Janeiro State
摘要：Rochas de praia (beachrocks) marcam a linha de costa pretérita e a constitui..o sedimentar de paleopraias. Em ambiente de micro-marés definem com precis.o a posi..o do nível médio do mar na ocasi.o do processo deposicional. O presente trabalho tem como objetivo estudar as oscila..es do nível do mar entre o final do Pleistoceno e o início do Holoceno a partir da investiga..o de 4 (quatro) afloramentos emersos e submersos de rochas de praia na ilha do Cabo Frio, Estado do Rio de Janeiro. O método de trabalho constou de coleta de material, análise petrográfica e data..o ao radiocarbono. Resultados obtidos sugerem que entre 13.130 a 11.149 cal AP, ocorreu rápida mudan.a do nível relativo do mar em torno de 6,0 m com oscila..o negativa de - 4,5 m e positivas de + 1,5 m. Os registros geológicos aqui estudados demarcam claramente a passagem entre o Pleistoceno e o Holoceno no Estado do Rio de Janeiro.
其他摘要：The Cabo Frio Island, Rio de Janeiro State, southeast Brazil shows four bodies of beachrocks, emerged up to +1.5 m and submerged down to - 4.5 m. These bodies are constituted of discontinuous strips, parallel to the coastline. Beachrock is beach sediment that has been cemented within the intertidal zone. Like the beach itself, it represents a transition between the marine and meteoric environments, where it is commonly elected by processes from each. Because beachrock is lithified within the intertidal zone and because it commonly forms in a few years, its potential as an indicator of past sea level is important. Four beachrocks samples were analyzed by polarizing microscope. This study indicated that carbonate elements that constitute most of the samples were at least partly incorporated within the intertidal zone. The adequate method for radiocarbon dating (total sample or cement) was decided according to these observations. The dates obtained from Cabo Frio island beachrocks indicate 4 separate sea level stands: the first one at about + 1.5 m around 13.000 BC, the second one at about 0.0 around 12.500 BC, the third at about - 3.0 m around 11.000 BC and finally the � fourthsea level at about - 4.5 m around 10.000 BC. The results suggests that between 13.000 to 11.000 yr BP, a sea level fluctuation was around 6,0 m, corresponding probably to negative fluctuation of - 4.5 m and positive fluctuation of + 1.5 m. On the coast of Rio de Janeiro State, Brazil several relative sea level positions are recognized. The more important was the Malhada mash to the north area, a great marine transgression around 5.000 years B.C.
关键词：rochas de praia; varia..es do nível relativo do mar; data..o ao radiocarbono..
其他关键词：beachrocks; sea level position; radiocarbon dating
