MIXED OCEANIC, AND FRESH-WATER DEPOSITIONAL CONDITIONS FOR BEACHROCKSOF 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 fe ldspars (4%). The cement shows three textural varieties: (1) calcifero us, surrounding siliciclastic grains; (2) micritic, with an acicular f ringe; and (3) cryptoclystallinr calcite in pores. Sandstone structure s and composition shon evidence of submerged and low-energg beaches. C ement is formed by similar to 20 mol% MgCO3; the delta(13)C in cement ranges from -1.3 parts per thousand to +3.5 parts per thousand PDB and delta(18)O varies from -2.1 to +1.2 parts per thousand(PDB). The ceme nt was precipitated under high CO2 pressure, as a result of the intera ction of CaCO3-saturated seawater and nonsaturated groundwater, in a b each environment.