Chemostratigraphy of the lower Bambuí Group, southwestern São Francisco Craton, Brazil: insights on Gondwana paleoenvironments

ABSTRACT: The Bambuí Group, the most extensive carbonate-siliciclastic cover on the São Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern São Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambuí Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ13C (c . -4‰), and a large oscillation of the δ18O (-6 to -15‰). The Interval II shows a striking homogeneity in δ13C and δ18O, around 1‰ and -7‰, respectively. At the top, Interval III shows a large positive excursion of the δ13C (up to 8‰) and δ18O (-8 to -3‰) values. Unaltered 86Sr/87Sr ratios range from 0.7075 to 0.7077, mainly at the top of the section. The geochemistry of the carbonates is controlled by their terrigenous content (mostly quartz and clay minerals) which is concentrated in the lower units. Samples free of terrigenous contamination show Y/Ho ratios ranging from 25 to 50, suggesting a freshwater input during carbonate deposition. It is concluded that the diamictite has a glaciogenic origin and is covered by a cap carbonate. This pair has been identified along the basin and is related to one of the main Neoproterozoic glaciations. Discrepancy between the 86Sr/87Sr values and the global variation curves can be related to freshwater input during the carbonate deposition. Based on the regional tectonic context, the Bambuí Basin may have been a restricted marine basin, totally or partially surrounded by mountain ranges within Gondwana, in the Neoproterozoic/Paleozoic boundary. In its early stages, the sedimentation was influenced by a global glacial event, whose melting phase was responsible by freshwater input in the basin. The gradual rise of the temperature was followed by an increase of the biological activity. Finally, a sudden increase in the biological activity could have been driven by paleogeographic changes caused by the active tectonic.

摘要：班布伊群（Bambuí Group）是圣弗朗西斯科克拉通（São Francisco craton）分布最广泛的碳酸盐-硅质碎屑盖层，因其与全球冰川事件存在潜在关联而长期备受争议。遗憾的是，现有多数化学地层学数据均采自巴西东南部典型强化学风化环境下的地表岩石露头，极易受到风化作用的影响。对此，本研究基于圣弗朗西斯科克拉通西南部一处无风化连续沉积序列的53件样品的碳、氧、锶同位素数据，开展了高分辨率化学地层学研究。该沉积序列厚度达175米，通过钻孔钻探获取，揭露了卡兰卡斯组（Carrancas formations）与塞特拉戈阿斯组（Sete Lagoas formations）——班布伊群的最下部地层。岩心数据显示，基底冰碛岩与太古宙片理状花岗闪长岩基底之间的接触关系极不规则。年代学与沉积学数据强烈表明，该冰碛岩属于驻留冰碛物。这套冰成沉积之上覆盖有石灰岩序列，底部为含文石假像扇与薄层黑色页岩的不纯碳酸盐岩。石灰岩序列逐渐过渡为泥灰岩-泥岩段，继而变为含生物组分的碳酸盐岩，顶部则发育叠层石白云岩。以维也纳佩德博尔标准（V-PDB）为参照的碳、氧同位素特征可将下部碳酸盐-泥屑段划分为三个区间：第一同位素区间对应帽碳酸盐岩，其δ¹³C值约为-4‰的负值，δ¹⁸O值则呈现-6‰至-15‰的大幅波动；第二区间的δ¹³C与δ¹⁸O值均表现出显著的均一性，分别约为1‰与-7‰；顶部的第三区间则出现δ¹³C（最高可达8‰）与δ¹⁸O（-8‰至-3‰）的大幅正偏移。未蚀变的⁸⁶Sr/⁸⁷Sr比值介于0.7075至0.7077之间，该比值主要集中于剖面顶部。碳酸盐岩的地球化学特征受其陆源组分（主要为石英与黏土矿物）控制，这些陆源组分富集于下部地层。无陆源污染的样品的钇/钬（Y/Ho）比值介于25至50之间，表明碳酸盐岩沉积时期存在淡水输入。研究认为，该冰碛岩属于冰成成因，其上覆的帽碳酸盐岩可在整个盆地内被识别，二者与新元古代主要冰川事件之一相关。⁸⁶Sr/⁸⁷Sr比值与全球变化曲线之间的差异，可能与碳酸盐岩沉积时期的淡水输入有关。结合区域构造背景，班布伊盆地在新元古代/古生代界线时期可能为受冈瓦纳大陆（Gondwana）内部山脉完全或部分围限的局限海盆。沉积早期受全球冰川事件影响，冰川消融阶段为盆地注入淡水。随着温度逐渐升高，生物活动随之增强；最终，活跃构造引发的古地理变化可能驱动了生物活动的突发性激增。