Journal of Coastal Research 23 6 1535–1548 West Palm Beach, Florida November 2007 Lithofaciology and Palaeoenvironmental Analysis of Holocene Beachrocks in Northeastern Brazil Marcela Marques Vieira † , Luiz Fernando De Ros ‡ , and Francisco Hila ´rio Rego Bezerra † † Departamento de Geologia Centro de Cie ˆncias Exatas e da Terra Universidade Federal do Rio Grande do Norte Caixa Postal 1639, CEP 59072-970, Natal, RN, Brazil marcela@geologia.ufrn.br ‡ Instituto de Geocie ˆncias Universidade Federal do Rio Grande do Sul Av. Bento Gonc ¸alves 9500, CEP 91501-970, Porto Alegre, RS, Brazil ABSTRACT VIEIRA, M.M.; DE ROS, L.F., and BEZERRA, F.H.R., 2007. Lithofaciology and palaeoenvironmental analysis of Ho- locene beachrocks in northeastern Brazil. Journal of Coastal Research, 23(6), 1535–1548. West Palm Beach (Florida), ISSN 0749-0208. Five lithofacies were identified in beachrock ledges distributed along the eastern and northern coasts of the state of Rio Grande do Norte, northeastern Brazil: (1) massive to weakly stratified conglomerate, (2) low-angle cross-stratified conglomerate and sandstone, (3) medium-scale tabular-planar and trough cross-stratified sandstone, (4) Skolithos- bioturbated conglomeratic sandstone, and (5) massive sandstones. Most of the described beachrocks (53% relative to total thickness) were deposited on the upper shoreface zone, represented by lithofacies 3 and 4. Lithofacies 2, which was deposited on the foreshore zone, represents 31% of the sections described. The remaining 16% are attributed to collapse of overlying material as a result of sea cliff undercutting (massive conglomerate, lithofacies 1), transport as a traction carpet deposit (weakly stratified conglomerate, lithofacies 1), and a high degree of alteration (lithofacies 5). The distribution of 14 C-dated beachrock samples in a sea level envelope curve shows that, along the northern coast, framework constituents were predominantly deposited between approximately 4140 and 2190 cal. yr. BP, when sea level dropped, and during low sea level stillstand. On the northern coast, the upper shoreface sedimentation zone is preferentially distributed during sea level drop and low sea level stillstand phases, whereas the foreshore sedimen- tation zone can be found along the whole sea level curve. Conversely, the beach deposits distributed along the eastern coast were preferentially formed between approximately 7460 and 4240 cal. yr. BP during rapid sea level rise and high sea level stillstand. In the case of the eastern coast, both zones are equally concentrated in the first stages of rapid sea level rise and high sea level stillstand. ADDITIONAL INDEX WORDS: Sea level, shoreface, foreshore, Brazil, radiocarbon dating. INTRODUCTION Beachrocks are common along many littoral zones through- out the world. They are conventionally defined as friable to well-cemented sedimentary rocks, formed in the intertidal or adjacent subtidal zones in tropical or subtropical regions, con- sisting of sand or gravel (detrital and/or skeletal) cemented with calcium carbonate (BATES and JACKSON, 1987). Beach- rocks form very rapidly (BRICKER, 1971), as attested by the common presence of objects from present or recent times ce- mented into the surrounding beach materials (FRANKEL, 1968). The literature is replete with descriptions of beachrocks; nevertheless, their origin is not fully understood, even after many decades of investigation. Mixing of marine and mete- oric waters (MOORE, 1973; SCHMALZ, 1971), evaporation (GINSBURG, 1953; MOORE and BILLINGS, 1971; TAYLOR and ILLING, 1969), CO 2 degassing of shallow groundwater (BINK- DOI: 10.2112/05-0562.1 received 22 July 2005; accepted in revision 24 August 2006. Funding was provided by a scholarship granted by CAPES, an organ of the Ministry of Education and Culture of Brazil. LEY,WILKINSON, and OWEN, 1980; GISCHLER and LOMAN- DO, 1997; HANOR, 1978), and direct or indirect activity of organisms (MAXWELL, 1962; NEUMEIER, 1999; WEBB,JELL, and BAKER, 1999) have been pointed out as the most common processes responsible for the lithification of these rocks. Two other aspects of beachrock formation have been the centre of controversy. First, the exact portion of the beach profile in which the original sands have been for the most part deposited (intertidal or adjacent supratidal and subtidal zones). Second, there are some uncertainties between sea lev- el changes and beachrock cementation. A few authors suggest that beachrocks are formed during sea level rise (ALEXAN- DERSSON, 1972), high sea level stillstand (COORAY, 1968), sea level drop (SIESSER, 1974), and low sea level stillstand (KIN- DLER and BAIN, 1993). To give some contribution to the elucidation of these two last points, this work attempts to (1) make an accurate de- scription of beachrock lithofacies from several locations in northeastern Brazil (Figure 1), (2) interpret their palaeoen- vironmental setting, and (3) relate beachrock deposition and cementation to sea level changes. The study area serves very well for these purposes because several beachrock ledges, some with great continuous extension, are encountered along