Microfacies and paleoenvironment of Donezella accumulations across an Upper Carboniferous high-rising carbonate platform (Asturias, NW Spain).

Donezella is a problematic organism that during the mid-Carboniferous (latest Serpukhovian to Moscovian) characterized carbonate depositional systems in Europe, North Africa, Russia, Kazakhstan, and North America. Though Donezella is generally included in the green calcareous algae, it has been attributed to different systematic groups and its classification and paleoecology still remain controversial. This work focuses on the distribution of Donezella across a carbonate platform (Sierra de Cuera) of Upper Carboniferous (lower Bashkirian-lower Moscovian) age located in the Cantabrian Mountains (Asturias, NW Spain). Sierra de Cuera exhibits a well-exposed cross-section from the horizontal platform through a steep slope (30°) to the basin floor. This unique feature allows reliable estimates of paleo-water depth and distance from the platform margin. Donezella specimens are interpreted as in situ when they form a network supporting cement-filled primary cavities and the ramified skeletons are surrounded by micrite coatings, often with a peloidal fabric, or by early marine cement. In the platform interior, Donezella is associated with phylloid algae and occurs in mud-rich low-relief bioherms. Towards the platform margin, massive units of boundstone are characterized by clotted peloidal micrite and radial fibrous cement-filled primary cavities. They contain Donezella and a diverse fossil assemblage of calcareous algae, bryozoans, and foraminifers. Donezella's delicate network appears fortified by the in situ precipitation of peloidal micrite. On the upper slope in situ precipitated peloidal micrite, abundant radial fibrous cement, and fenestellid bryozoans are the major components of the boundstone facies, along with Donezella and a skeletal community similar to the outer platform one. On the slope, in situ Donezella were observed down to paleo-water depths up to 200 m. Sedimentologic, petrographic, and microfacies analysis of Donezella accumulations in the different facies belts of Sierra de Cuera strongly suggest that this problematic organism was able to thrive over a large depth range, in low-energy but also in moderately agitated environments or in settings with temporary increase in current action, and in organic, physical-chemical and oceanographic conditions that enhanced the precipitation of peloidal micrite. The interval of water depth inferred from the well-exposed slope geometry of Sierra de Cuera suggests that either Donezella might not have belonged to the green calcareous algae or, alternatively, this depositional system was influenced by particular paleo-oceanographic conditions that extended the euphotic zone below the average depth. The morphology of Donezella's skeleton and its sedimentological occurrences are not exclusively indicative of an affinity with Chlorophyta. Therefore, it is suggested that Donezella should be considered as a microproblematicum organism. The data presented in this study contribute to the interpretation of comparable Donezella accumulations in carbonate depositional systems where limited outcrop exposures do not allow correct evaluation of the geometry and facies distribution.