The lower Lias Group of the Hebrides Basin
The lower Lias Group of the Hebrides Basin
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赫布里底盆地下利亚斯群
DOI：
10.1144/sjg34010023
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发表时间：
1998
影响因子：
0.7
通讯作者：
H. Jenkyns
中科院分区：
文献类型：
--
作者：
S. Hesselbo;M. Oates;H. Jenkyns
赫布里底盆地Lias群的下部和较大部分由浅海至边缘海相泥岩、粉砂岩、砂岩和石灰岩组成，排列在明确的海进-海退相旋回中。在盆地的北方地区，地层沉积在一系列的半地堑中，其西部边缘以主要在三叠纪期间发育的主要东倾正断层为界。解释下利阿斯的古地理环境是复杂的，因为暴露的序列发生在附近的转移区，随后作为焦点的古新世火成岩活动。在这项研究中，主要暴露量已被测量或重新测量，详细的沉积记录，包括以前未发表的菊石测定。在内赫布里底海槽的北方部分（Skye-Pabay-Raasay地区），识别出两个地层：布罗德福德地层和Pabay页岩地层。本研究中所用的布罗德福德组仅相当于先前作者的“下布罗德福德层”。Pabay页岩包括以前使用的“上布罗德福德层”和“Pabba页岩”。正式确定了北方地区Pabay页岩组的两个新的主要砂质段：Hallaig砂岩段（下Sinemurian; Semicostatum-Turneri带）产于下Pabay页岩内，其模式产地在Raasay的Hallaig瀑布; Suisnish砂岩段（上西内穆里亚-下普莱恩斯巴赫阶; Raricostatum - Jamesoni区）位于Pabay页岩组的中部，其模式产地在Rubha Suisnish和洛赫Eishort南部海岸沿着的悬崖中，关于斯凯在南部地区（穆尔-莫文），使用“下布罗德福德层”或“上布罗德福德层”也是不合适的。在阿德纳穆尔昌，以及在更北的暴露面中的有限程度上，布罗德福德组与其更南的近海对应物Blue Lias组相互交错。阿德纳穆尔昌-莫尔文-穆尔地区的帕拜页岩一直延伸到与蓝利亚的交界处。砂岩单元在下西内穆里亚-上西内穆里亚边界附近发育良好：这里将其命名为帕拜页岩组的托罗赛砂岩段，并且在穆尔东南海岸的几个地方不完全出露。虽然在砂岩中没有发现年龄诊断化石，但它必须位于上钝角带至下Raricostatum带的区间内。总的来说，总的趋势是从北部和东部较近的沉积环境向南部和西部较远的沉积环境转移。这种区域模式受到假定的转换带附近富砂、非常接近的继承性局部发生的调制，例如Camasunary断层北方终端东南部的区域。来自西北Skye的Upper Glen-1钻孔的记录表明，位于赫布里底海-小明奇盆地中心的扩大的下Lias层序可以以与内赫布里底海槽暴露的相似的方式进行细分;例如，它表明在下Sinemurian至上Sinemurian边界附近发育了更多的桑迪地层。与Ardnamurchan剖面一样，它也显示了布罗德福德组和Blue Lias的夹层。Pabay页岩的上半部分似乎在钻孔中被截断：这是否是由于Scalpa砂岩沉积前的侵蚀、沉积物缺乏或钻孔与正断层的交叉造成的，这是有争议的。
Synopsis The lower and greater part of the Lias Group of the Hebrides Basin comprises shallow-marine to marginal-marine mudstones, siltstones, sandstones and limestones arranged in unambiguous transgressive-regressive facies cycles. In the northern region of the basin, the strata were deposited in a series of half graben bounded on their western margins by major eastward-dipping normal faults developed principally during the Triassic. Interpretation of the palaeogeographic setting of the lower Lias is complicated because exposure of the sequences occurs in the vicinity of transfer zones that have subsequently acted as foci for Palaeocene igneous activity. The principal exposures have been measured or remeasured in this study, and detailed sedimentary logs are presented that incorporate previously unpublished ammonite determinations. In the northern part of the Inner Hebrides Trough (Skye–Pabay–Raasay area), two formations are recognized: the Broadford Formation and the Pabay Shale Formation. The Broadford Formation, as used in this study, is equivalent only to the ‘Lower Broadford Beds’ of previous authors. The Pabay Shale incorporates the former ‘Upper Broadford Beds’ and ‘Pabba Shale’ of previous usage. Two new predominantly arenaceous members of the Pabay Shale Formation are formally defined for the northern area: the Hallaig Sandstone Member (Lower Sinemurian; Semicostatum–Turneri Zones) occurs within the lower Pabay Shale and has its type locality at Hallaig Waterfall on Raasay; the Suisnish Sandstone Member (Upper Sinemurian–Lower Pliensbachian; Raricostatum – Jamesoni zones) occupies the middle part of the Pabay Shale Formation and has its type locality at Rubha Suisnish and in the cliffs along the southern shore of Loch Eishort, on Skye. In the southern area (Mull–Morvern) use of the terms ‘Lower Broadford Beds’ or ‘Upper Broadford Beds’ is also inappropriate. In Ardnamurchan, and to a limited extent in more northerly exposures, the Broadford Formation interdigitates with its more southerly offshore equivalent, the Blue Lias Formation. The Pabay Shale of the Ardnamurchan–Morvern–Mull area is recognized down to the junction with the Blue Lias. A sandstone unit is well developed at about the Lower Sinemurian–Upper Sinemurian boundary: it is here named the Torosay Sandstone Member of the Pabay Shale Formation and is incompletely exposed at several localities on the southeastern coast of Mull. Although no age-diagnostic fossils have been found within the sandstone, it must lie within the upper Obtusum Zone to lower Raricostatum Zone interval. Overall, there is a general trend from more proximal environments of deposition in the north and east, towards more distal environments in the south and west. This regional pattern is modulated by the localized occurrence of sand-rich, very proximal successions in the vicinity of the supposed transfer zones, such as the area immediately SE of the northern termination of the Camasunary Fault. Records from the Upper Glen-1 borehole, in NW Skye, show that the expanded lower Lias sequence in the centre of the Sea of the Hebrides–Little Minch Basin can be subdivided in a similar manner to that exposed in the Inner Hebrides Trough; for example, it shows the development of more sandy strata at about the Lower Sinemurian to Upper Sinemurian boundary. Like the Ardnamurchan section, it also shows an intercalation of the Broadford Formation and Blue Lias. The upper half of the Pabay Shale appears to be truncated in the borehole: it is debatable whether this resulted from erosion prior to deposition of the Scalpa Sandstone, from sediment starvation or, alternatively, intersection of the borehole by a normal fault.