Carbonate cements in contemporaneous beachrocks, Jaguaribe beach, Itamaracá island, northeastern Brazil: petrographic, geochemical and isotopic aspects

Abstract

Holocene beachrocks of the Jaguaribe beach, State of Pernambuco, northeastern Brazil, consist of horizontal, cemented layers approximately 40 cm thick. The cement shows three textural varieties: (a) calciferous, surrounding siliciclastic grains, (b) micritic, with an acicular fringe; and (c) cryptocrystalline calcite in pores. Early cementation took place at the water table below beach ridges, where geochemical, hydrodynamic and, perhaps, also microbiological conditions favored rapid precipitation of aragonite and/or high-Mg calcite. δ 13 C values range from -1.8 to +1.5 • / •• for dissolved carbonate in interstitial water and from +0.2 to +2.1 • / •• for bioclastic components. δ 18 O values range from -2.8 to +0.5 • / •• for seawater, freshwater and interstitial water. δ 13 C values and diagenetic features suggest that cementation occurred in meteoric-vadose and/or marine-phreatic water by loss of CO 2 during evaporation of the interstitial water. Locally, superimposed lowMg calcite cements point to subsequent freshwater influence. Total-rock cement composition of vertically stacked beachrock beds at the Jaguaribe beach shows that the highest beachrock bed is older than the one (of same petrographic composition) situated at the current groundwater level. This implies a downward progression of cementation, which probably followed the sea-level fall after a local high stand.

Geomorphology of the continental shelf of Tavolara Island (Marine Protected Area ‘Tavolara-Punta Coda Cavallo’ – Sardinia NE)

“…An explanation may be found in the synsedimentary cementation processes that follow the transgression (Guerra, Kiang, & Sial, 2005;Kitano & Hood, 1965;Taylor & Illing, 1969).…”

Geodiversidade na praia de Maracaípe: um estudo de caso no litoral sul de Pernambuco

“…Segundo Guerra et al (2005), esses aglomerados constituem-se por depósitos de arenitos cimentados por carbonato de cálcio e são indicadores de paleoníveis marinhos, sendo que os mais próximos à costa, são os mais recentes de diagênese fraca, portanto, friáveis.…”

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

“…Dados de δ 13 C e δ 18 O apresentados por Guerra et al (2005), para cimento em arenitos de praia na Ilha de Itamaracá (de -2,0‰ a 3,6‰ para ä 13 C e de -2,1‰ a 1,3‰ para δ 18 O VPDB ) se aproximam aos valores apresentados nesta pesquisa. Guerra et al (2005) sugerem que a cimentação ocorreu em água meteórica-vadosa e/ou marinha-freática, por perda de CO 2 durante a evaporação da água intersticial.…”

“…Guerra et al (2005) sugerem que a cimentação ocorreu em água meteórica-vadosa e/ou marinha-freática, por perda de CO 2 durante a evaporação da água intersticial.…”