Paleogeography of the Horse Spring Formation in relation to the Lake Mead fault system, Virgin Mountains, Nevada and Arizona
Paleogeography of the Horse Spring Formation in relation to the Lake Mead fault system, Virgin Mountains, Nevada and Arizona
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与米德湖断层系统、维尔京山脉、内华达州和亚利桑那州相关的马泉地层的古地理
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发表时间：
1996
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通讯作者：
L. Beard
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作者：
L. Beard
内华达州东南部第三系马泉组记录了走滑和伸展构造的发育，形成了米德湖左旋断裂体系。在维尔京山脉和南维尔京山脉进行的地层学、地质年代学和制图研究阐明了这种演化的细节。处女山地区马泉组的暴露仅包括其下方的两个成员，按升序排列，彩虹花园和拇指成员。马泉矿床受正滑断层和斜滑断层的断裂和不同程度的倾斜。彩虹花园段（约26 ~ 18 Ma）沉积于西北和东南为正区、较宽的浅水盆地；相变化是渐进的，不受活动断裂的影响。相比之下，拇指段复杂的相关系是米德湖断裂系统沿运动连接的走滑和伸展断层的复杂混合变形沉积的结果，约为16-14 Ma。第三纪和更老的岩石在水平轴上的倾斜明显发生在伸展期晚期，大约在14ma或更晚。彩虹花园成员由三个单元组成，每个单元代表独立的前伸展沉积体系。基底砾岩为东北古流沉积的辫状平原或山墙。中间的单元是凝灰质河流和湖泊沉积物的混合物，是西南向彩虹花园湖排水逆转的结果。古流动方向的变化是由于北部广泛的火山活动阻断了早期的东北流动。上部单元由成土蚀变的湖相碳酸盐岩组成，形成于湖相沉积停止和北部火山活动减弱期间。以前未被认识到的不整合记录了断层的开始，将彩虹花园成员与上覆的拇指成员分开，并且可以在整个处女山脉地区追踪。不整合面将下伏的蚀变碳酸盐岩与上覆的层状良好的湖相灰岩分开。局部的不整合以砾岩和巨角砾岩为标志，这些砾岩来自下方的彩虹花园碳酸盐岩，横跨附近的断裂带。不整合面以上的碳酸盐岩在横向和纵向上演化为湖相石膏和拇指段细粒砂岩。这些岩石在横向和纵向上依次与边缘湖相和冲积扇相交错。巨角砾岩和粗砾岩从金丘和石灰岭断裂向北突入湖相矿床。比尔德，L. S., 1996，马泉组古地理与米德湖断裂系统的关系，维珍山脉，内华达州和亚利桑那州，在Beratan， K. K.，编，用沉积学和地层学重建盆地和范围扩展的历史：Boulder， Colorado，美国地质学会特别论文303。27 SP303-03。QXD 4•1•96下午2:06页27
The Tertiary Horse Spring Formation in southeast Nevada records the development of strike-slip and extensional tectonism that formed the Lake Mead left-lateral fault system. Stratigraphic, geochronologic, and mapping studies in the Virgin and south Virgin Mountains clarify the details of that evolution. Exposures of the Horse Spring Formation in the Virgin Mountains area consist of only its lower two members, in ascending order, the Rainbow Gardens and Thumb Members. The Horse Spring deposits are faulted and variably tilted by normal and oblique-slip faults. The Rainbow Gardens Member (ca. 26–18 Ma) was deposited in a broad shallow basin with positive areas to the northwest and southeast; facies changes are gradual and show no influence of active faulting. In contrast, complex facies relations in the Thumb Member are the result of deposition during complex mixed-mode deformation along kinematically linked strike-slip and extensional faults of the Lake Mead fault system, about 16–14 Ma. Tilting of Tertiary and older rocks about horizontal axes apparently occurred late in the extensional episode, at about 14 Ma or later. The Rainbow Gardens Member consists of three units that each represent separate pre-extensional depositional systems. The basal conglomerate is a braid plain or pediment deposited by northeast paleoflow. The middle unit is a mixture of tuffaceous fluvial and lacustrine deposits that are the result of a reversal of drainage to the southwest, into the Rainbow Gardens lake. The change in paleoflow direction is attributed to extensive volcanism to the north that blocked the earlier northeastward flow. The upper unit consists of pedogenically altered, palustrine carbonate rocks that formed during cessation of lacustrine deposition and waning of volcanic activity to the north. A previously unrecognized unconformity that records the onset of faulting separates the Rainbow Gardens Member from the overlying Thumb Member and can be traced throughout the Virgin Mountains area. The unconformity separates the underlying pedogenically altered carbonate rocks from overlying well-bedded lacustrine limestones. Locally the unconformity is marked by conglomerate and megabreccia deposits derived from the underlying Rainbow Gardens carbonate rocks across nearby fault scarps. Carbonate rocks above the unconformity grade laterally and vertically into lacustrine gypsum and fine-grained sandstone of the Thumb Member. These rocks in turn intertongue laterally and vertically with marginal lacustrine and alluvial fan facies. Abrupt influx of megabreccia and coarse conglomerate into lacustrine deposits occurred northward from both the Gold Butte and Lime Ridge faults. At approximately the same time, megabreccia and coarse conglomerate were shed Beard, L. S., 1996, Paleogeography of the Horse Spring Formation in relation to the Lake Mead fault system, Virgin Mountains, Nevada and Arizona, in Beratan, K. K., ed., Reconstructing the History of Basin and Range Extension Using Sedimentology and Stratigraphy: Boulder, Colorado, Geological Society of America Special Paper 303. 27 SP303-03.QXD 4•1•96 2:06 pm Page 27