Trilobite traces and the age of the lowest part of the Ordovician reference section for NW Spain

A quartzite-shale sequence forming the lowest part of the Ordovician reference section for NW Spain in the Cantabrian zone at Cabo Peñas, lacks body fossils and has hitherto been of uncertain age. This sequence has, however, yielded a variety of trilobite-produced trace fossils. The lowest exposed beds have Cruziana breadstoni and Rusophycus sp. while a few tens of metres higher C. furcifera, C. goldfussi, C. rugosa and Rusophycus asturiensis sp.nov., are also present. The highest beds have yielded only C. furcifera. The traces suggest that while the top of the sequence is at least as high as Arenig the lowest beds are of Tremadoc (post-Dictyonema flabelliforme Zone) age. This implies a thickness for the Arenig of no more than 400 m and probably considerably less. In contrast, thicknesses in excess of 1000 m occur to the W in the West Asturian–Leonese Zone. This, together with evidence of tidal to intertidal deposition at Cabo Peñas, tends to confirm that the area was the shelf to an Ordovician trough situated in the West Asturian–Leonese zone.