Hola.
Ahí va algo que puede aclarar un poco las cosas:
" Beach rock is here taken to be a beach sand that was cemented while it was still in the intertidal zone.
(...)
The typical beach rock consists of a layer of cemented calcarenite forming a hard surface on or in the beach, underlain by unconsolidated lime sand. The lamination and attitude of the rock are the same as those of the local unconsolidated beach sediment, to which the grains of the rock correspond also in composition and texture: it may be a calcarenite, a conglomerate or a breccia. The upper surface of the beach rock, if exposed, is typically indurated, pitted (as limestone pools, p.362), abraded and bored by penetrative algae and stained. The base of the rock, where it overlies unconsolidated sediment, is commonly sharp and plane. The degree of cementation varies throughout the rock.
(...)
That beach cementation is a contemporary process is clear from the inclusion in the rock of man-made articles such as beer bottles in the Bahamas and objects only seven to eight years old on Eniwetok (EMERY et al., 1954).
(...)
The chemical process leading to the formation of beach rock has been tacitly assumed, by many authors, to be a simple matter of an increase in the level of supersaturation for CaCO3, as a result of evaporation and loss of C02 during subaerial exposure of the beach at low tide (as DANA, 1851, p.368). Experimental or field tests have scarcely been attempted. SCHMALZ's, (in press) work is a rare exception. Nowadays, however, thinking has become more sophisticated, as for example in the approach to the formation of submarine hardgrounds (p.373) and grapestone (p.316). We know that aragonite and high-magnesian calcite are widely precipitated from shallow subtropical and tropical sea water (p.364). Thus, rather than ask why grains are cemented on (or in) beaches, we should ask why they are not cemented everywhere. The answer is probably the same as that given for cementation of submarine hardgrounds in the Persian Gulf (p.374). Sand grains near the beach surface are too mobile to be cemented. Farther down, the pore water is relatively stagnant, nearly in equilibrium with the grains and isolated from fresh supplies of sea water. In between these two zones is one in which the grains are mechanically stable, but are bathed in supersaturated water frequently replenished from above by the pumping action of waves and tides. Here the grains will be cemented to make a hard layer."
BATHURST, Robin G.C.
"Carbonate sediments and their diagenesis", pp 367-370 Amsterdam, Elsevier, 1975, 2 ed. aum., 658 p., grf. Developments in sedimentology, n, 12.
Para resumir (y simplificando mucho): la roca de playa se forma en la zona intermareal, habitualmente (aunque no exclusivamente) en zonas tropicales y subtropicales, a partir de los mismos componentes que el resto de la arena, solo que las condiciones de la zona permiten que se cemente y resalte como una roca frente a la arena que la rodea. Como se forma a lo largo de toda la playa, pero su anchura es muy limitada, porque sólo se da en la zona intermareal, la roca de playa tiene una gran longitud en relación a su anchura: es como un camino. (Esto no viene en este artículo) la erosión subsiguiente provoca que la roca de playa sea laminada perpendicularmente a su longitud, por lo que adquiere la forma de grandes baldosas. La alta velocidad (geológica) de formación de la roca de playa se demuestra, por ejemplo, por el hecho de que se pueden encontrar incluidas botellas de cerveza en la roca de playa de Bahamas.
Saludos
Mario
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