Genesis and age of beachrocks on the Rio de Janeiro coastline, Southeast - Brazil

Abstract

Beachrocks are relatively common along the intertropical zone of the Brazilian coastline. These deposits have the potential to yield important information about the character of paleo-beaches, implications for paleo sea-level, and paleoenvironmental change related to their genesis. In this manuscript, we present new data on the genesis and age of beachrocks from Rio de Janeiro's coastline, which have not been described in scientific literature, using detailed analyses of petrographic, stable isotope δ13C and δ18O, and cathodoluminescence. Two types of cement were verified in most samples, independent of depth and latitude. The primary cement is composed of a fibrous rim of magnesian calcite and the secondary cement consists of microcrystalline calcite. The maximum value of δ13C was +3.503‰ and minimum - 4.731‰. δ18O values vary between −1.584‰ and + 1.049‰. The paleotemperature was calculated to be between 21 °C and 32 °C during lithification. In general, three identified categories of beachrocks show: (a) precipitation of magnesian calcite from seawater due to high temperature; (b) precipitation of magnesian calcite resulting from the mixture of groundwater and seawater; but mostly (c) precipitation of magnesian calcite from pore water in sediments. Data obtained from 14C radiocarbon dating suggests that beachrocks were lithified between 11,940–11,240 to 3130–2860 cal yrs. BP. As a result, we considered these rocks to be predominantly associated with the shallow marine environments, due to high evaporation.