PALEOECOLOGÍA, COMUNIDADES BENTÓNICAS Y SUCESIÓN PALEOAMBIENTAL EN EL GRUPO RÍO CACHIRI, DEVÓNICO, SIERRA DE PERIJA, VENEZUELA

Teresa M. Sanchez, Juan L. Benedetto

Abstract


PALAEOECOLOG,BENTHIC COMMUNITIES AND PALAEOENVIRONMENTAL SUCCESSION IN THE RIO CACHIRÍ GROUP, DEVONIAN, SIERRA DE PERIJÁ, VENEZUELA.- The Río Cachirí Group provides a good example of a marine transgressive sequence followed by a main regressive cycle produced by a prograding delta complex. The transgressive sequence begins with unfossiliferous black shales and interbedded quartzitic sandstones with many coquinoid horizons. This facies of shallow water, wave agitated deposits is dominated by the Leptocoelia-Acrospirifer Community. The climax of the transgression, which was reached during Emsian-Early Eifelian times, includes very fossilíferous dark argillaceous limestones and calcareous siltstones deposíted in an open marine platform with moderate circulation. This environment was inhabited by two distinct bryozoan communities dominated by fenestrate and ramose forms, brachíopods, crinoids and scattered solitary and colonial Rugosa corals. The prograding deltaic sequence at Eifelian-Civetian time is related with a regional uplift of the region to the southeast of the ancient shoreline. The Leptocoelia, "Devonochonetes" -Mediospirifer, Longispina and Mediospirifer-Atrypa-Camarotoechia communities fluorished in a shallow subtídal-prodelta environment. The upper part of the regressive marine sequence is characterized by influx of terrigenous sediments which consist of coarse sandstones, green siltstones and carbonaceous shales with much vegetal debris, suggesting a delta platform environment. Thís zone was inhabited by Tropidoleptus and "Devonochonetes"-Tylothyris-pelecypods communities, in which the percentage and díversíty of molluscan fauna reached a maximum. The Devonian sedimentary cycle culminated with a thick alluvial plants-bearíng sequence.

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