SciELO - Scientific Electronic Library Online

vol.4 número1Calcretas de aguas subterráneas y pedogénicas: génesis de los depósitos carbonáticos de la Cuenca de Santa Lucía, sur del Uruguay (Cretácico Superior?-Paleógeno) índice de autoresíndice de assuntospesquisa de artigos
Home Pagelista alfabética de periódicos  

Serviços Personalizados




  • Não possue artigos citadosCitado por SciELO

Links relacionados

  • Não possue artigos similaresSimilares em SciELO


Revista de la Asociación Argentina de Sedimentología

versão impressa ISSN 1853-6360


PALMA, Ricardo M  e  LAPRIDA, Cecilia. Cementación marina reciente y contenido micropaleontológico en arenitas de playa "beachrock" en la costa de Salvador (Praia da Paciência), Bahía, Brasil. Rev. Asoc. Argent. Sedimentol. [online]. 1997, vol.4, n.1, pp.37-48. ISSN 1853-6360.

Along the coast of Salvador city, northeastern of the State of Bahia, Brazil, lie several and discontinuous beachrock deposits (Ferreira Yeda, 1969) similar to others, widespread in tropical and subtropical areas of the world (Scoffin & Stoddart, 1983; Kendall et al. 1994). This study is restricted to the occurrence of some beachrock located at rio Vermelho (Fig. 1) which is exposed on the "Praia da Paciência" beach. The "Praia da Paciência" beachrock was investigated in detail to find out different types of cements, related diagenetic microfacies and micropalentological content. The beachrock which comprises up to 1,70m thick of calcareous sandstone, lies paralell to the actual shoreline; its base is only exposed at low-tide. The length of beachrock deposit varies between 30 and 45 meters. Between the sea and landward side there are small pools protected by beachrock blocks which show a low-angle cross-bedding (Fig. 2a). On the top of the beachrock some solution features (Fig. 3) of different sizes are recognized. The beachrock at Rio Vermelho consists of well sorted medium-fine-grained calcareous sandstones. Most characteristic are a low-angle cross-bedding and small scale trough cross-bedding. The set exposed on the top of the beachrock attains lengths up to 50 cm and thickness between 15 and 20 cm. The sandstone commonly comprises up to 70% of monocrystalline and polycrystalline quartz grains, feldspar, metamorphic rocks fragments and carbonate allochems. Allochems consist mainly of skeletal fragments which are made up of foraminifers, gastropods, bivalves and echinodermae fragments as well as corrallinae algae. The main accesory minerals are zircon, hyperstene, garnet, hornblende, and tourmaline, which together with the dominant grains came from a metamorphic source, exposed around Salvador city. Beachrock contains three distinct cement types that according to their precipitation order are the following: 1. a dark brown rim of cryptocrystalline calcite at the grain contact (micrite envelope); 2. this cement is overlain by a fringe of acicular high magnesian calcite crystalls that grow perpendicular to grain surfaces (Figs. 5a y 5b), and 3. intergranular microcrystalline calcite crystals. X-ray analysis indicates that high magnesian calcite is the dominant mineral in the last two cement phases. The beachrock locally overlays a vermiform gastropod bioherm or the Tertiary conglomerates of the Barreiras Formation (Mabessone, 1964) which is only exposed in few places at low-tide. The contact with the bioherm is irregular to transitional and unmistakable because of the difference in lithification and the contrast in color. The dominant skeletal framework of the bioherm consists of vermiform gastropods (Pentaloconchus sp.) and corallinae algae (Lithophylum sp.). Other organisms including bryozoa, serpulids, bivalves and forams were found in minor amounts. Details of petrographic and scanning electron microscopy on the gastropod structure (Fig. 4b) reveal  the  replacement of aragonite by calcite crystals as a consequence of neomorphism process. In the gastropod shells a widespread spectrum of unaltered aragonite and calcite aragonite crystals were observed. Aragonite crystals are not only recognized by fibrous fringes in the shell structure, but may also be intergrown among adjacent shells. This type of vermiform gastropods bioherm and diagenetic processes is similar to those found in recent peritidal tropical and subtropical environments (Ginsburg & Schroeder, 1973; Macintyre et al. 1996; among others) as well as in the Lower Carboniferous of England (Burchette & Riding, 1977). The most abundant microfauna in the beachrock is represented by twenty seven species of foraminifera and two of ostracods. The foraminiferal assemblage is dominated by Archaias angulatus Fitchel y Moll, Amphistegina lessoni d'Orbigny, Quinqueloculina parkerii (Brady) var. bowdenensis, and Peneroplis proteus d'Orbigny. On the other hand ostracods are represented by Bairdia genus. The most important taphonomic attributes were restricted to forams, including abrasion and dissolution. Even though test are not extensively altered by mechanical processes, plenty of them show biological alteration induced by bioeroding (fungi, algae) or the action of encrusting (corallinae algae) organisms which are very common in marine environments (Kidwell & Bosence, 1991). The mechanical features of tests such as abrasion is an effect of physical processes (currents and waves generated by tides and storms) responsible for movements and collisions of tests with other sedimentary particles. It is important to point out, however, that many tests show deep and irregular pits induced probably by bioeroding organisms, but the irregularity in shape, may be attributable to continued taphonomic or diagenetic alteration (Schroba, 1993). Such feature is associated with altered to scaloped margin preservation. SEM analysis of test (Fig. 6) provided strong evidence of dissolution processes which may occur in the environment if the water were undersaturated with respect to carbonate phase (Alexandersson, 1972a). Nevertheless, dissolution can also be caused by meteoric waters. Rain and groundwater are two important factors to take into account in the study area. Evidence of intergranular microcrystalline calcite precipitation in diagenetic microfacies is related to this kind of influence. The diagenetic features suggest the beachrock passed through marine-phreatic (microfacies 1 and 2) to mixing diagenetic environments (microfacies 3). Microfacies sequence is related to response to Holocene sea level rise (Mabessone, 1964).

Palavras-chave : Beachrock; Carbonate; Diagenesis; Paleontology; Taphonomy; Brazil.

        · resumo em Espanhol     · texto em Espanhol     · Espanhol ( pdf )


Creative Commons License Todo o conteúdo deste periódico, exceto onde está identificado, está licenciado sob uma Licença Creative Commons