NEOPROTEROZOIC EARLY CAMBRIAN GEOLOGY AND PALEONTOLOGY OF IBERIA

Neoproterozoic-early Cambrian successions in Iberia are reexamined. A gradual transition across the Neoproterozoic-Cambrian boundary is pres ent in Central Iberia, whereas in the Cantabrian region and the Iberia n Chains Lower Cambrian arenaceous successions rest with profound angu lar unconformity on Neoproterozoic turbidites. In Central Iberia, the Neoproterozoic sedimentary succession is referred to the informal Demo Extremeno group, representing mostly basinal facies, and the overlyin g Rio Huso group consisting of slope deposits and proximal turbidites that grade into shallower marine deposits. The latter is inferred to r epresent distal slope to outer platform depositional conditions and co ntains widespread carbonate olistostromic units. The position of the P recambrian-Cambrian boundary is within the Pusa shale of the Rio Huso group and can be correlated at the regional level by the occurrence of trace fossils, acritarchs, and in particular the abundant shelly meta zoan Cloudina. The succession also yielded megascopic carbonaceous fos sils, such as vendotaenids and Beltanelloides ? sp. ind., and Sabellid ites. Contrary to former interpretations assuming transport of older p latform carbonates from the Ibor region into 'younger' olistostromic b eds of the Rio Huso group, we interpret sedimentary and fossil evidenc e to suggest that shallower platform deposits of the informal Ibor gro up were penecontemporaneously incorporated in the olistostromic lower part of the Rio Huso group. Hence, the olistostromes are not believed to mark a major erosive unconformity. Based on the ichnofossil record and recent U-Pb age determinations, we argue that a proposed disconfor mity between Lower and Upper 'Alcudian' strata is neither regional nor does it mark a significant hiatus. Neoproterozoic-early Cambrian depo sition in Central Iberia can be accommodated in a model that implies a generalized stretching of the crust during an extensional event which closely followed the Cadomian phase of the Pan-African Orogeny and wh ich eventually could have included transcurrent components. An extensi onal phase with transcurrent components during the deposition of the I bor and lower Rio Huso groups is regarded as a probable cause of wides pread pending resulting in the juxtaposition of platform and basinal s uccessions, eventually leading to anoxic conditions in Pusa shale depo sition times. A possible cause for repeated collapse events developing olistostromes and intra-sequential folding could be sought in this te ctonic context.