Turbidites in the Upper Carboniferous Ross Formation, western Ireland: reconstruction of a channel and spillover system
Turbidites in the Upper Carboniferous Ross Formation, western Ireland: reconstruction of a channel and spillover system
复制标题
爱尔兰西部上石炭统罗斯组的浊积岩：水道和溢出系统的重建
DOI：
--
复制
发表时间：
2003
期刊：
影响因子：
--
通讯作者：
O. Martinsen
中科院分区：
文献类型：
--
作者：
T. Lien;R. Walker;O. Martinsen
摘要香农群的上石炭统深水岩石沉积在爱尔兰西部克莱尔郡的香农盆地中，并在香农河口沿着的海崖中极好地出露。石炭纪石灰岩为盆地底部，盆地充填序列从深水克莱尔页岩开始。这些页岩被罗斯组（460 m厚）的各种浊积岩相覆盖。 浊积岩系统的类型、浊积岩砂岩体的规模和地层序列的总体特征使罗斯组非常适合作为世界各地被动边缘盆地中富砂浊积岩成藏组合的类比。罗斯组下部170米处包含无通道的板状浊积岩，总体趋势是向上变砂，尽管没有小规模的向上增厚或变薄的连续性。罗斯组上部290 m由浊积岩和混合浊积岩组成，浊积岩通常排列在向上增厚的包裹体中，混合浊积岩形成通道填充物，每个通道填充物的厚度可达10 m。 罗斯河上游的一些河道有一个最初的横向加积阶段，有砂岩和泥岩夹层沉积，随后有一个垂直加积阶段，有厚层混合浊积岩。本文认为，随着河道的充填，越来越多的浊流溢出越远越远。出色的露头显示，当河道最初溢出泥浆和薄层浊积岩时，向上增厚的包裹体形成，溢出距离可达1 km，随后是厚层混合浊积岩，溢出距离接近河道边缘。 古水流方向与合并通道填充建议低通道弯曲度。25-40 m厚的河道和溢出包叠层可能在相同的地层间隔但在不同的位置显示出显著的古水流变化。 这表明，单个通道和溢出包堆叠成通道溢出带，并且带也遵循弯曲模式。罗斯系统的储集层要素包括板状浊积岩、河道充填沉积、作为溢流瓣形成的向上增厚的包裹体，以及在更大的尺度上，宽2.5 -5 km的弯曲河道带。 带的边缘可以粗略地定义为包装良好的溢出沉积物横向进入较泥泞，包装不良的板状浊积岩。低弯曲度河道带被解释为向下游进入未形成河道的板状浊积岩，相当于罗斯组下部相。
ABSTRACT The Upper Carboniferous deep‐water rocks of the Shannon Group were deposited in the extensional Shannon Basin of County Clare in western Ireland and are superbly exposed in sea cliffs along the Shannon estuary. Carboniferous limestone floors the basin, and the basin‐fill succession begins with the deep‐water Clare Shales. These shales are overlain by various turbidite facies of the Ross Formation (460 m thick). The type of turbidite system, scale of turbidite sandstone bodies and the overall character of the stratigraphic succession make the Ross Formation well suited as an analogue for sand‐rich turbidite plays in passive margin basins around the world. The lower 170 m of the Ross Formation contains tabular turbidites with no channels, with an overall tendency to become sandier upwards, although there are no small‐scale thickening‐ or thinning‐upward successions. The upper 290 m of the Ross Formation consists of turbidites, commonly arranged in thickening‐upward packages, and amalgamated turbidites that form channel fills that are individually up to 10 m thick. A few of the upper Ross channels have an initial lateral accretion phase with interbedded sandstone and mudstone deposits and a subsequent vertical aggradation phase with thick‐bedded amalgamated turbidites. This paper proposes that, as the channels filled, more and more turbidites spilled further and further overbank. Superb outcrops show that thickening‐upward packages developed when channels initially spilled muds and thin‐bedded turbidites up to 1 km overbank, followed by thick‐bedded amalgamated turbidites that spilled close to the channel margins. The palaeocurrent directions associated with the amalgamated channel fills suggest a low channel sinuosity. Stacks of channels and spillover packages 25–40 m thick may show significant palaeocurrent variability at the same stratigraphic interval but at different locations. This suggests that individual channels and spillover packages were stacked into channel‐spillover belts, and that the belts also followed a sinuous pattern. Reservoir elements of the Ross system include tabular turbidites, channel‐fill deposits, thickening‐upward packages that formed as spillover lobes and, on a larger scale, sinuous channel belts 2·5–5 km wide. The edges of the belts can be roughly defined where well‐packaged spillover deposits pass laterally into muddier, poorly packaged tabular turbidites. The low‐sinuosity channel belts are interpreted to pass downstream into unchannellized tabular turbidites, equivalent to lower Ross Formation facies.