What do latest Famennian and Mississippian miospores from South American diamictites tell us?
What do latest Famennian and Mississippian miospores from South American diamictites tell us?
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来自南美混积岩的最新法门期和密西西比期小孢子告诉我们什么？
DOI：
10.1007/s12549-012-0109-1
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发表时间：
2013
影响因子：
1.4
通讯作者：
Miguel Perez
中科院分区：
文献类型：
--
作者：
M. Streel;M. Caputo;José Henrique G. Melo;Miguel Perez
在帕尔纳巴盆地西缘穿过密西西比波提组的浅钻孔中，岩心含有深灰色的硅质岩，其中保存完好的孢粉形态极其丰富。已鉴定出88个小孢子分类群，其中近一半显然是经过修改的。这些早期、晚期的辉绿岩的存在可能与公认的Paraca植物群的气候含义相矛盾，后者也被记录在波提组中。然而，大约4 Ma的时间跨度，几乎相当于整个晚期Viséan，可能允许在Viséan早期晚期和Serpukhoian冰期之间存在气候较温暖的Paraca植物群。在帕尔尼亚巴盆地中部，穿过法门纪晚期的上Cabeças地层的深井中的岩心含有碎屑岩和火山口状韵律岩，通常是层状粉砂岩和砂岩，零星分布着碎屑。从这些硅质岩和伴生粉砂岩中已鉴定出41个小孢子分类群，其中大部分(70%)来自中、上泥盆统沉积物。据报道，玻利维亚东南部贝尔梅霍伊塔库亚组下部的一个18米厚的透辉岩剖面显示了西欧建立的三个连续的斯特鲁尼亚微孢子带(LL-LE-LN)，因此被解释为记录了300万年来发生的几次冰川消融事件的复合体。然而，我们质疑玻利维亚斜晶石中三个连续的Strunian小孢子带的存在，对我们来说，这只对应于LE和LN带的一部分。在西欧，根据牙形刺磷灰石的氧同位素(δ18Ophoph)，同样较短的微孢子分带间隔对应着一个较低的海面古温度时期，以及一个明显的海平面变化。牙形刺数据表明，包括汉根堡砂岩和德鲁尔砂岩在内的最高LE和LN间隔的时间跨度要短得多(10万年)。另一方面，伊塔库亚组(玻利维亚)在地层底部上方33米和58米处取样，更有可能证明了多个冰川-间冰期事件，其特征是最新的法门期和密西西比期的硅质岩叠加。
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.