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Resumen:
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The Bashkirian-lower Moscovian Valdeteja Formation crops out in the Cantabrian Zone (NW Spain). It is composed of pale grey limestone with a diverse fossil content, calcareous breccias and massive limestone composed of algal and microbial mounds accumulated in a high relief carbonate platform. Outstanding outcrops of that formation appear near the village of Truébano (León Province, North Spain) at the old coal mine “Mina Rosario.” The mine is a peculiarity in the formation as it shows interbedded siltstones and coal beds containing coal balls. The studied succession above the coal seam is lower Bashkirian and 20.2 m thick. Dark grey, massive to wellbedded limestones interbedded with thin marly beds are dominant in that interval. The main component of the limestones is the algospongia Donezella that is found in two different facies: packstone of resedimented Donezella, which appears in the lower beds of the section, and boundstone of Donezella, Girvanella and chaetetids, in the upper beds. Additional components are highly diverse including foraminifera, other calcified microbes, rhodophyta, sponges, echinoderms, arthropods, brachiopods, bryozoans and scarce corals and molluscs. Organic matter is abundant in the marly beds, but palynomorphs are poorly preserved. An interbedded layer of quartz sandstones lacking fossil content occurs in the upper part of the sequence.
The depositional environment of the facies is part of a carbonate platform top near the fair-weather wave base, within subtidal zone, with development of “algal” mounds and sedimentation of debris from the same buildups. The composition and components distribution of both microfacies fit well with the mounds previously described in other outcrops of the Valdeteja Formation, with the exception of the participationof chaetetids as a main building component in some beds.
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