Nuevos antecedentes estratigraficos y sedimentologicos de la Formacion Zorritas, Devonico-Carbonifero de Sierra Almeida, Region de Antofagasta, Chile

Abstract

RESUMEN. El estudio detallado de seis columnas estratigraficas en el lugar-tipo de la Formacion Zorritas, en Sierra Almeida, alta cordillera de Antofagasta, permite dividir esta sucesion en tres miembros. El Miembro Inferior consiste en cuarzoarenitas con abundante estratificacion cruzada planar, estratificacion cruzada en esqueleto de arenque y otras estructuras indicativas de ambiente de alta energia. Son interpretadas como depositos de mareas de mar somero. La aparicion de Tropidoleptus sp. y Australocoelia indican la presencia del Devonico Medio (Emsiano-Givetiano) e incluso Frasniano. E Miembro Medio consiste en intercalaciones de pelitas y areniscas muy finas, altamente fosiliferas, con ondulitas y estratificacion cruzada del tipo 'hummocky'. Se interpreta como representativo de una sedimentacion controlada por tormentas y olas en un ambiente 'nearshore'. En la parte basal contiene fauna fosil del Carbonifero Inferior. El Miembro Superior (Carbonifero Inferior), se encuentra materializado por areniscas de grano fino con estratificacion cruzada planar y en artesa, las cuales contienen pequenos canales de erosion. Se le atribuye una deposicion en ambiente fluvial contiguo a la linea de costa, marcando niveles regresivos dentro de la Formacion Zorritas. La columna estratigrafica total de la Formacion Zorritas (3.040 m) representa un ciclo sedimentario de transgresion-regresion completo, que abarca desde el Devonico Medio al Carbonifero Inferior, sin mediar discordancia alguna o quiebre de la sedimentacion que registre la ocurrencia de 13 Fase tectonica 'chanica'. Los sedimentos que integran la Formacion Zorritas se depositaron dentro de una cuenca silicoclastica de plataforma estable, cuyo borde oriental estuvo marcado por el 'Arco Puneno' y cuya extension occidental permanece aun incierta. El suministro detritico a partir del 'Arco Puneno' desde el sureste fue continuo, y estuvo formado, esencialmente, por rocas metasedimentarias clasticas asociadas a escasas metamorfitas de bajo grado, plutonitas y volcanitas acidas e intermedias. Se sugieren algunas correlaciones con afloramientos de similar edad de la Puna Argentina y de la precordillera de Antofagasta y Atacama. Finalmente, se agregan algunas consideraciones paleogeograficas y geodinamicas para estos territorios durante el Devonico-Carbonifero Inferior. ABSTRACT. New stratigraphic and sedimentological data on the Zorritas Formation, Devonian-Carboniferous of Sierra Almeida, Antofagasta Region, Chile. Detailed stratigraphical analysis based on six stratigraphical columns in the type section of Formacion Zorritas, at Sierra Almeida, High Cordillera of Antofagasta, allows its division into three members. By mean s of detailed facial analysis of each of these members, environmental conditions were reconstructed. The Lower Member is formed by coarse grained quartz arenites in which planar cross bedding and herring-bone cross bedding are common. They are interpreted as tidal deposits in a shallow marine environment. Presence of Tropidoleptus sp. and Australocoelia allows to date this member as Middle Devonian (Emsian-Givetian) even Frasnian. The Middle Member is composed by shales and very fine grained sandstones, with ripple marks and hummocky cross bedding. It represents sedimentation controlled by storms and waves in a nearshore environment. In its lower section, it contains Wocklumeria, an Upper Devonian form of goniatite. In its middle to upper section a faunal association diagnostic of Lower Carboniferous age was found. The Upper Member (Lower Carboniferous) is composed by fine-grained sandstones with planar cross bedding. Small erosional canals can be also observed. This member is interpreted as formed in a near shore fluvial environment. It has a regressive character. Considering the whole section of Zorritas Formation (3,040 m thickness) a complete transgressive-regressive cycle is apparent. No stratigraphical discontinuity, nor discordance, was observed in the Devonian-Carboniferous boundary. Thus, in the precordillera of Region de Antofagasta there is no evidence of 'Chanic' Orogeny. The deposition of Zorritas Formation took place in a stable siliciclastic platform basin that in its eastern side was bounded by the 'Arco Puneno'. This was a positive relief that supplied from the southeast most of the detrital material to the basin. Detrital analysis of sandstone from Zorritas Formation suggests that the 'Arco Puneno' was formed mainly by metasedimentary clastic sequences associated with minor metamorphic rocks, plutonites and acidic to intermediate volcanic rocks. Detailed measurements of sedimentary structures made in all three members of Zorritas Formation, allow establishing a northwestern direction for detrital supplies, coming from the 'Arco Puneno'. Lithological and facial correlations of Zorritas Formation with other outcrops of the same age along the Argentinian-Chilean border are suggested. Paleogeographical and geotectonic comments about this realm during Devonian-Lower Carboniferous time are al so suggested.