Paleoproterozoic high δ13C dolomites from the Lastoursville and Franceville basins (SE Gabon): Stratigraphic and synsedimentary subsidence implications
Paleoproterozoic high δ13C dolomites from the Lastoursville and Franceville basins (SE Gabon): Stratigraphic and synsedimentary subsidence implications
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Lastoursville 和 Franceville 盆地（加蓬东南部）的古元古代高 δ13C 白云岩：地层和同沉积沉降的影响
DOI：
10.1016/j.precamres.2011.05.013
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发表时间：
2011
影响因子：
3.8
通讯作者：
F. Delpomdor
中科院分区：
文献类型：
--
作者：
A. Préat;P. Bouton;D. Thiéblemont;Jean;S. S. Ndounze;F. Delpomdor
加蓬东南部（Franceville和Lastoursville次盆地）浅海沉积碳酸盐碳同位素组成的新数据表明，这些沉积物中海水δ 13 C值在2.1Ga左右经历了一次大的正偏移。这些碳酸盐岩属于Francevillian群的FC组，沿着太古代Chaillu盆地边缘，在开阔的海洋陆棚和蒸发岩环境中沉积，局部经历了下绿片岩相变质作用。四个微相（MF 1-MF 4）识别出从潮下蓝藻dolobindstones间和潮上塌陷角砾岩（MF 4）。花岗岩的~（13）C值相对于VPDB值大多高于0.50‰，最高值为9.74‰，平均值接近4‰，与2.22 ~ 2.06Ga全球性大幅度正漂移期间的沉积相一致。这些正δ 13 C值与微相类型无关。这种高的13 C值也记录在嵌入在下伏FB组的黑色页岩中的粗粒砂岩和砾岩间层中的改造碳酸盐（碳酸盐岩）中，这表明碳酸盐岩至少在地层学上与黑色页岩矿床相当。早期构造发育与沉积物充填的相互作用，形成了沉降量和沉降时间差异明显的几个次级盆地。富13 C的辉长岩可能与高埋藏率的有机碳开始与沉积的FB沉积物在这种伸展大地构造背景。因此，硅质岩和硅质岩FC组不再被视为标志层段。正偏移的末端位于Francevillian统（即FE）的顶部附近，可以在未来的研究中作为古元古代潜在的化学地层学工具。
New data for the isotopic composition of carbon in shallow-marine sedimentary carbonates in southeastern Gabon (Franceville and Lastoursville sub-basins) indicate that the seawater value of δ13C in these sediments underwent a large positive excursion around 2.1Ga. These carbonate rocks belong to the FC Formation of the Francevillian Group deposited along the margin of the Archean Chaillu Craton in open-marine shelf and evaporite settings and experienced, locally, lower greenschist facies metamorphism. Four microfacies (MF1–MF4) are recognized from subtidal cyanobacterial dolobindstones to inter- and supratidal collapse breccias (MF4). The dolomites exhibit elevated13C values, most values are above 0.50‰, the highest value is 9.74‰ and the average value is near 4‰ vs. VPDB, consistent with deposition during the large worldwide positive excursion between 2.22 and 2.06Ga. These positive δ13C values are independent of the microfacies type. Such high13C values are also recorded in reworked carbonates (dolomites) embedded in coarse-grained sandstones and conglomerates interstratified in the black shales of the underlying FB Formation suggesting that the dolomites were at least stratigraphically equivalent to the black shale deposits. Interaction between early structural development and sediment fill lead to the formation of several sub-basins with distinct differences in terms of amount and timing of subsidence. The13C-enriched dolomites are probably linked to high burial rate of organic carbon starting with the deposition of the FB sediments in this extensional geotectonic context. As a consequence the cherty and dolomitic FC Formation cannot longer be considered as a marker interval. The end of the positive excursion is located near the top of the Francevillian Series (i.e. FE) and could be used in future studies as a potential chemostratigraphic tool in the Paleoproterozoic.