Neoproterozoic-early Cambrian successions in Iberia are reexamined. A gradual transition across the Neoproterozoic-Cambrian boundary is present in Central Iberia, whereas in the Cantabrian region and the Iberian Chains Lower Cambrian arenaceous successions rest with profound angular unconformity on Neoproterozoic turbidites. In Central Iberia, the Neoproterozoic sedimentary succession is referred to the informal Domo Extremeño group, representing mostly basinal facies, and the overlying Rio Huso group consisting of slope deposits and proximal turbidites that grade into shallower marine deposits. The latter is inferred to represent distal slope to outer platform depositional conditions and contains widespread carbonate olistostromic units. The position of the Precambrian-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 metazoan Cloudina. The succession also yielded megascopic carbonaceous fossils, such as vendotaenids and Beltanelioides? sp. ind., and Sabellidites. Contrary to former interpretations assuming transport of older platform carbonates from the Ibor region into ‘younger’ olistostromic beds of the Rio Huso group, we interpret sedimentary and fossil evidence to suggest that shallower platform deposits of the informal Ibor group 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 disconformity between Lower and Upper ‘Alcudian’ strata is neither regional nor does it mark a significant hiatus. Neoproterozoic-early Cambrian deposition 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 which eventually could have included transcurrent components. An extensional phase with transcurrent components during the deposition of the Ibor and lower Rio Huso groups is regarded as a probable cause of widespread ponding resulting in the juxtaposition of platform and basinal successions, eventually leading to anoxic conditions in Pusa shale deposition times. A possible cause for repeated collapse events developing olistostromes and intra-sequential folding could be sought in this tectonic context.
The degradation-resistant organic-walled cell envelopes of acritarchs are the most abundant microfossils in Proterozoic and Cambrian rocks. These microfossils reveal diversity fluctuations that illuminate the nature of the record of primary producers near the Proterozoic/Phanerozoic boundary. Neoproterozoic radiations, some 1000–542 m.y. ago, reached levels comparable to those observed in the Cambrian Period. The microbiotas from rock successions from 13 Cambrian biochrons display significant fluctuations in the total number of microfossil taxa belonging to discrete microfossil assemblages. The assemblages reveal that Cambrian protist assemblages evolved over relatively short time spans, apparently out of low-diversity remnant populations after gradual declines in diversity. The characteristic microbiotas of the terminal Neoproterozoic and the Early, Middle, and Late Cambrian blossomed over relatively narrow time ranges, subsequently collapsing to nearly the initial levels. By virtue of the decreasing time spans involved in the late Vendian, Early, Middle, and Late Cambrian respectively, the tempo of specific turnover appears to have varied considerably. Speciation levels gradually decreased during Early and Middle Cambrian times and during Early Cambrian times were accompanied by rising levels of extinction. This latter feature seems to have reversed during Middle Cambrian times, lasting well into Late Cambrian times. Acritarchs were at the base of the marine trophic chain together with bacteria and other protists that are largely unrepresented in the fossil record. For this reason, the rise of diverse Cambrian protistan plankton must have been essential for early marine metazoan differentiation. Indeed, patterns of total diversity, speciation, and extinction of Cambrian acritarchs clearly mirror those of contemporaneous marine invertebrate faunas at the generic level.
Acritarchs (organic-walled phytoplankton) are described from the siliciclastic rocks of the Buen Formation. A total of 52 form taxa are reported from a small number of extremely fossiliferous samples from th deeper shelf portion of the formation.
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