Ordovician sedimentation and basin development in the North Armorican Massif, NW France: Field evidence from the northern Cotentin Peninsula

Field evidence from the northern Cotentin Peninsula and regional data are used to construct a tectono-stratigraphic model for the Ordovician which characterizes basin development in the North Armorican Massif. In La Hague, 15 m of transgressive marine sandstone belongs to the Dapingian age Grès Armoricain Formation which onlaps lower Cambrian, rift-fill deposits via an unconformity. Approximately 450 m of overlying Darriwilian strata are dominated by shallow marine sandstone showing hummocky cross-stratification with subordinate shales containing trace and body fossils. Together, these facies support an interpretation of offshore shallow marine strata overlying a break-up unconformity. Regional analysis indicates the time gap at the unconformity is 20–40 Ma and formed from crustal upwarping, which was greatest in the north of the Armorican Massif. Dapingian strata (Grès Armoricain) thins irregularly to the north (0–94 m), interpreted to reflect passive onlap onto residual relief associated with the uplift and the initiation of thermal subsidence on the margin. The succeeding Darriwilian strata (Schistes de Beaumont to Grès de May) conversely display a steady thickening (161–623 m) to the north, the stratal patterns suggesting that from the Darriwilian onwards, the ocean basin to the north was firmly established as the main locus of subsidence on the continental margin.