What do latest Famennian and Mississippian miospores from South American diamictites tell us?

摘要

Cores from shallow boreholes penetrating the Mississippian Poti Formation, in the western margin of the Parnaíba Basin, contain dark grey diamictites which are extremely rich in well-preserved palynomorphs. Eighty-eight miospore taxa have been identified, and almost half of these are obviously reworked. The presence of these early Late Viséan-age diamictites might possibly contradict the accepted climatic implications of the Paraca Flora, which is also recorded in the Poti Formation. However, a time span of ca. 4 Ma, corresponding to almost the entire Late Viséan, probably allowed the warmer-climate Paraca Flora to exist between the early Late Viséan and Serpukhovian ice ages. Cores from a deep borehole penetrating the upper Cabeças strata of latest Famennian age, in central Parnaíba Basin, contain tillites and varve-like rhythmites, usually laminated siltstones and sandstones, with scattered clasts. Forty-one miospore taxa have been identified from these diamictites and associated siltstones, most of which (70 %) were reworked from Middle and Upper Devonian sediments. An 18-m-thick diamictite section in the lower portion of the Itacua Formation at Bermejo, southeast Bolivia, was reported to display the three successive Strunian miospore zones (LL–LE–LN) established in Western Europe, and thus interpreted as a composite that records several deglaciation events occurring over 3 million years. However, we challenge the presence of the three successive Strunian miospore zones in the Bolivian diamictites which for us correspond only to parts of the LE and LN zones. In Western Europe, the same shorter interval of the miospore zonation corresponds to a period of lower sea-surface palaeotemperatures based on oxygen isotopes from conodont apatite (δ18Ophosph) as well as a conspicuous sea-level change. Conodont data suggest a much shorter time span (100,000 years) for the highest LE and the LN interval encompassing the Hangenberg and Drewer Sandstones. On the other hand, the Itacua Formation (Bolivia), sampled 33 m and 58 m above the base of the formation, more likely testifies to multiple glacial–interglacial events featuring a superposition of latest Famennian and Mississippian diamictites.