LINGULID BEDS AT THE UPPER LIMIT OF THE ARMORICAN QUARTZITE (ORDOVICIAN, ARENIG, SW EUROPE)

Within the Lower Ordovician of SW Europe, lingulid beds are frequently recorded in the uppermost part of the Armorican Quartzite Formation o r at the base of the overlying unit of dark shales. They have a broad geographical extension, particularly in the Hesperian and Armorican Ma ssifs. The lingulid horizons occur within littoral fine to medium sand s deposited on flat and relatively stable areas, without tide, however under storm influence. The large lingulids which characterize these f acies were living in conditions similar to those of the extant linguli des. Two types of lingulid beds are described. Type A forms lags with lamination, from some millimetres to several centimetres thick, at the base of or within quartzo-sandstone strata. It originated through sed imentary floods of coarse particles, probably transported by rivers du ring periods of heavy rains which also induced large salinity decrease s. Type B, the most common, consists of conglomeratic coquinoid beds, associated with sandstone and quartzitic layers, containing phosphatic and sideritic pebbles and heavy minerals, embedded in a matrix with a huge concentration of fragments of lingulid valves and sometimes of s ome other fossils, i.e. bivalves and microarthropods. The coquinas are related to erosive discontinuities at the base of sandstone sequences with hummocky cross stratification. They originated during very short catastrophic events which induced unconformable deposits of valve fra gments in the littoral zone. Most of the most widespread beds of type B were probably deposited during hurricanes or tsunamis, the latter re lated to explosive volcanism on the perigondwanan shallow shelves. Num erous evidences of acid volcanism of Explosive type at the end of the Arenig are documented in the upper part of the Armorican Quartzite in the Iberian Peninsula. Nevertheless, at least one of the thickest depo sits of type B does not belong to a tempestite but could have formed t hrough a coastal upwelling of phosphate-rich waters in which large lin gulid populations could develop.