A highstand oolitic sequence and associated facies from a Late Triassic lake basin, south‐west England
A highstand oolitic sequence and associated facies from a Late Triassic lake basin, south‐west England
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DOI：
10.1046/j.1365-3091.2000.00288.x
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发表时间：
2000-02
期刊：
影响因子：
3.5
通讯作者：
P. Milroy;V. Wright
中科院分区：
文献类型：
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作者：
P. Milroy;V. Wright
高能湖岸碳酸盐序列很少记录在地质记录中。然而，一个例子发生在英国南部的上三叠统麦西亚泥岩群 (MMG)。 MMG 是与北大西洋中生代裂谷盆地相关的众多厚层非化石泥岩矿床之一。 MMG 泥岩的起源是当前争论的主题，提出了海洋、普拉亚-湖泊和冲积-风成模型。盆地北缘已记录了海岸线特征，但在 MMG 其他地方此类特征很少见，导致许多工作人员怀疑泥岩的湖泊起源。在英格兰西南部布里斯托尔海峡克利夫登地区南部盆地边缘的几个部分中发现了波浪主导的湖岸线沉积物。这些沉积物为 MMG 泥岩堆积的大型常年至半常年超盐湖的发育提供了证据。海岸线沉积物覆盖在冲积河流和席状洪水沉积物上，并从受海岸正常波浪影响（在半湿润条件下沉积）的带有生物碎屑的海侵砾石-砾岩海滩单元，过渡到受更多变化的波浪途径影响的低梯度、高位鲕粒砂（在逐渐更加干旱的条件下沉积），最终形成被普拉亚泥滩覆盖的低位鲕粒状平原沉积物。单位。海岸线沉积物记录了一个简单的浅浅-向上海侵-高位-低位序列。然而，从反射（海侵）海岸线到耗散（高位）海岸线的变化被认为代表了气候引起的盛行风向的变化。 MMG 中识别的海岸线特征与美国西部最近的普拉亚湖盆地的海岸线特征相似。除了滩岩组构之外，鲕粒还表现出各种尺寸、断裂和溶解特征，表明它们最初由放射状霰石组成，类似于来自犹他州大盐湖的现代鲕粒。
High energy, lake‐shoreline carbonate sequences are rarely documented in the geological record. However, one example occurs in the Upper Triassic Mercia Mudstone Group (MMG) of southern Britain. The MMG is one of a number of thick, non‐fossiliferous mudstone deposits associated with North Atlantic Mesozoic rift basins. The origin of the MMG mudstones is the subject of current debate, with marine, playa‐lacustrine and alluvial–aeolian models having been proposed. Shoreline features have been documented from the northern margin of the basin, but the rarity of such features elsewhere in the MMG has led many workers to doubt a lacustrine origin for the mudstones. Wave‐dominated, lake‐shoreline deposits have been recognized in several sections from the southern basin margin in the Clevedon area of the Bristol Channel in south‐west England. These deposits provide evidence for the development of a sizeable perennial to semi‐perennial hypersaline lake in which the MMG mudstones accumulated. Shoreline sediments overlie alluvial stream and sheet‐flood deposits, and pass from transgressive gravel–conglomerate beach units with bioclasts, influenced by shore‐normal waves (deposited under semi‐humid conditions), to lower gradient, highstand oolitic sands affected by more varied wave approach (deposited under progressively more arid conditions), which culminated in lowstand, oolitic strand‐plain deposits overlain by a playa‐mudflat unit. Shoreline deposits record a simple shallowing‐upward transgressive–highstand–lowstand sequence. However, a change from a reflective (transgressive) to dissipative (highstand) shoreline is believed to represent a climatically induced change in prevailing wind direction. Shoreline features recognized in the MMG are similar to those of recent playa‐lacustrine basins of the western United States. Ooids display a variety of size, fracture and dissolution features in addition to beachrock fabrics, suggesting that they were originally composed of radial aragonite, similar to modern ooids from the Great Salt Lake, Utah.