THE ORDOVICIAN OF THEARMORICAN MASSIF (FRANCE)

. A review is presented of recent progress in knowledge of the Ordovician of the Armorican Massif. Attention is drawn to the confusion that has resulted from indiscriminate use of lithological units which have previously been considered as chronostratigraphical units. In order to avoid confusion, the use of formations is introduced with the intention of translating local successions into lithostratigraphical terms. The lithological successions and the faunas collected are outlined to provide a clear statement of the degree of correlation at present possible between successions in different areas of the Massif Armoricain, with comments on elements of ambiguity that still remain. The results are expressed in thirteen representative columns and a correlation chart. Following the regional studies, a comprehensive picture (excluding the southern area) is proposed, outlining the development of the Ordovician transgression; the main features of sediments which are of shallow-water type and comprise sandstones, siltstones, mudstones and shales, limestones being almost entirely absent; and the elements of uncertainty within the upper part of the sequence (in relation to the problem of the Ordovician-Silurian boundary) due to the fact that typical Ashgill faunas are not known and that no diagnostic basal Llandovery fossils have been found. Correlations with Ordovician successions outside the area are considered. Correlations with the British Isles involve considerable difficulties on account of the poorness of Ordovician graptolite faunas in the Massif Armoricain. Shelly faunal relationships between the Massif Armoricain and other areas in the 'Mediterranean Province' (Tethyan region), which have been demonstrated in previous studies, are more or less confirmed. Thus intra-Mediterranean Province correlations seem to involve fewer difficulties, but a progressive approach to these questions remains necessary, beginning with the Iberian Peninsula which presents the nearest relationships with the Armorican area.