Cementation patterns and genetic implications of Holocene beachrocks from northeastern Brazil

This paper describes the complex patterns of diagenesis in Holocene beachrock ledges located along both the eastern and northern coasts of the state of Rio Grande do Norte, northeastern Brazil. These beachrocks show highly variable diagenetic processes, fabrics and textures. Carbonate cements are exclusively composed by high-Mg calcite and occur in seven main morphologies: (1) cryptocrystalline coatings; (2) isopachous prismatic rims; (3) equant spar; (4) cryptocrystalline or micritic pore-filling; (5) pseudo-peloidal aggregates; (6) radial aggregates; (7) isolated and randomly oriented clumps of scalenohedral crystals. Other diagenetic features include: (a) carbonate-siliciclastic mixed sediment; (b) infiltrated micrite; (c) authigenic smectitic clay minerals; (d) organic films; (e) chalcedony pore-filling cement; (f) vadose silty internal sediment. The texture and composition of the cements are indicative of precipitation in the active marine phreatic zone. The presence of vadose features and planktonic foraminifers within the infiltrated materials suggest that they were introduced in the backshore sands by muddy waters dislocated from the shelf during storms. A general succession of diagenetic phases can be recognized, comprising the precipitation of authigenic smectite, micritic coatings, radial aggregates, isopachous rims of prismatic crystals or equant spar, cryptocrystalline and pseudo-peloidal pore-filling cements, and vadose infiltration of micritic, carbonate-siliciclastic mixed sediments or silty materials. The absence of organic structures, such as microbial (bacteria or fungi) filaments and bodies within the cements suggests that the mechanism behind beachrock cementation is essentially inorganic, most probably the evaporation of entrapped seawater in response to the dry climatic conditions.