Gênese dos arenitos de praia no litoral de Pernambuco interpretada através de estudos isotópicos

Abstract

This study is a paleoenvironmental and paleoclimatic analysis of beachrocks through oxygen and carbon isotopes on its cement. The beachrocks have from 60% to 85% of its basis formed by detrital grains of quartz and its carbonate cement is composed by Magnesium-rich calcite. Isotopic values from -1.1‰ to 3.5‰ were obtained for δ 13 C and from -0.9‰ to 0.5‰ for δ 18 O VPDB. Those values are typical of those from sea carbonate cement deposited on shallow marine environment and under the influence of fresh water (meteoric vadose environment), which causes cementation on the foreshore zone. Paleotemperature of the beachrock cement ranged from 22.9ºC to 28.2ºC. This variation is due to the rise of the precipitation depth of the cement and indicates that the water temperature was fairly constant during the process of deposition of the carbonate cement.

Beachrocks of the northeast of Brazil: local effects of sea level fluctuations in a far-field during in Holocene

“…Petrological analysis permits classifying the samples as beachrocks, reinforcing some previous studies (Dominguez et al, 1990;Laborel, 1970;van Andel and Laborel, 1964). These samples have similar characteristics to others in northeastern Brazil: magnesian calcite cement, quartz predominance, and presence of marine bioclasts, signs of compaction, fractures, and dissolution (Caldas et al, 2006;Guerra et al, 2005;Ferreira Júnior et al, 2011;Vieira et al, 2007). The occurrence of plagioclase with iron hydroxide associated with feldspar grains can be attributed to the Barreiras Formation, commonly outcropping on the Pernambuco coast.…”