Mixed Oceanic and Freshwater Depositional Conditions for Beachrocks of Northeast Brazil: Evidence from Carbon and Oxygen Isotopes

Holocene beachrocks of Northeast Brazil are composed predominantly of quartz (90%) with minor carbonate fragments (6% algal detritus) and feldspars (4%). The cement shows three textural varieties: (1) calciferous, surrounding siliciclastic grains; (2) micritic, with an acicular fringe; and (3) cryptocrystalline calcite in pores. Sandstone structures and composition show evidence of submerged and low-energy beaches. Cement is formed by ～20 mol% MgCO₃; the δ¹³C in cement ranges from ?1.3‰ to +3.5‰ PDB and δ¹⁸O varies from ?2.1 to +1.2‰_PDB. The cement was precipitated under high CO₂ pressure, as a result of the interaction of CaCO_3^? saturated seawater and nonsaturated groundwater, in a beach environment.