Biostratigraphical calibration of third order Ordovician sequences on the northern Gondwana platform

This study illustrates bathymetric trends across the northern Gondwana platform during the Ordovician. The sedimentary records of the Algerian Sahara, central Morocco, and western France are investigated to document related environmental changes through the period. Outcrop and subcrop sections have been selected with regard to their completeness and to the precision of the available biostratigraphical data, based on chitinozoans. This group provides the most continuous biostratigraphical record in the terrigenous Ordovician successions of northern Gondwana. For each area, the chitinozoan data have been assembled, updated and referred to one of the 26 Ordovician chitinozoan biozones defined in northern Gondwana regions. ential analysis based on the identification of genetic units is presented. The stacking pattern of these units is established for each locality, and the resulting curves express variations in sedimentary environments partly driven by relative sea-level changes. From successive smoothings of these curves, we define 16 main cycles at lower frequencies. ainties concerning the precise chronostratigraphic position of some of these cycles are discussed. The proposed northern Gondwana relative sea-level variation curves are compared to the recently-published global Ordovician sea-level curve and to other regional curves available for the major Ordovician palaeoplates (Laurentia, Baltica, South China, north-eastern Gondwana).