Significado del complejo esquistograuváquico en relación con otras series "pre-arenig" de España central

Abstract

In Central and Northern Portugal all pre-Ordovician terraines are known under the denomination "Complexo Xisto-Grauváquico". This broad definition describes only the predominance of schists and greywackes in a thick and monotonous sequence resting unconformably under the Arenigian "Armorican" quartzite levels. Further precisions about its Cambrian or Precambrian age and its lithostratigraphic relationships have remained obscure.

New insights on paleontological and sedimentological data in similar series of Central Spain have lead the authors to define three different units below the ubiquitous Armorican Quartzite. The upper unit is formed by shallowwater sediments of red-beds affinities. An Upper-Middle Tremadocian age has been found for this clastic formation that is separated by a widespread unconformity from the underlaying formations. In these formations it can be deffined an unit whose age ranges from the uppermost Lower Cambrian to the lowest fossiliferous levels of the Lower Cambrian. (Los Cortijos, Los Navalucillos and Tamames faunas). This Cambrian unit shows clearly shelf-type features in its upper levels, whilst its base is constituted by a more Deep shale sedimentation. Below this Cambrian unit the sedimentation becom clarly tubiditic with a mixtite-like level of olistostromes and paraconglomerates (including dolomite levels). The upper limit of this mixtite level is here used by convenience as the begining of the Precambrian unit. In this sense, there have been considered the position of the lowest Cambrian fauna and the first main sedimentary break-up.

Taking into account these new data and the last assumption, an attempt has been made in this paper to delimite and correlate the different rock types of the "Complexo Xisto-Grauváquico" in its own area of definition. It can be assumed that the paraconglomerate levels and their associate discontinous limestones of the Complexo, are equivalent to the Latest Precambrian limestone bearing mixtites of Central Spain. Thus the underlaying turbidite sediments (mainly schists and greywackes) may represent the Precambrian. The convergent deeper basal facies of the Lower Cambrian is also represented between the Ordovician unconformity and the paraconglomerate levels. Nevertheless the Lower Cambrian limestones and their overlaying levels are not represented, and the absence of Middle-Upper Cambrian is also inferred. Moreover the Tremadocian elastics are less relevant towards Northern Portugal and they often disappear.