CAMBRIAN DEPOSITIONAL HISTORY OF THE ZANSKAR VALLEY REGION OF THE INDIAN HIMALAYA: TECTONIC IMPLICATIONS
CAMBRIAN DEPOSITIONAL HISTORY OF THE ZANSKAR VALLEY REGION OF THE INDIAN HIMALAYA: TECTONIC IMPLICATIONS
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DOI：
10.2110/jsr.2006.020
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发表时间：
2006-02
影响因子：
2
通讯作者：
P. Myrow;K. Snell;N. Hughes;T. Paulsen;N. Heim;S. K. Parcha
中科院分区：
文献类型：
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作者：
P. Myrow;K. Snell;N. Hughes;T. Paulsen;N. Heim;S. K. Parcha
印度北部赞斯卡河谷保存完好的寒武纪剖面此前被解释为记录了与寒武纪-奥陶纪造山作用相关的从被动构造边缘到主动构造边缘的转变。这种解释已被用来支持特提斯喜马拉雅山其他层序的构造地层解释。我们详细的古环境分析极大地修正了这些寒武纪沉积物的构造和沉积历史：在这些中寒武世晚期的岩石中没有记录即将发生的寒武纪-奥陶纪造山作用的明确记录。从厚度为 125 米、含叠层石的碳酸盐矿床 Karsha 组到 Kurgiakh 组的页岩和砂岩的关键转变被解释为代表了构造引起的碳酸盐台地的淹没。库尔吉赫组的硅质碎屑地层被认为记录了靠近弧沟系统的构造活动前陆盆地中的深水复理石沉积。这种解释是基于具有经典布玛序列的砂岩层。我们发现库尔贾赫的这些事件床包含丘状交叉层理、准平面层理和组合流波纹层理，所有这些都反映了浅海、风暴影响环境中的沉积。因此，尽管卡沙碳酸盐台地可能已被淹没，但它并未最终形成深海复理石沉积，这反过来又消除了将库尔贾赫沉积与寒武纪-奥陶纪造山作用开始联系起来的主要证据。印度北部寒武纪-奥陶纪沉积物的其他方面也对库尔贾赫沉积与寒武纪-奥陶纪造山事件之间的联系提出了质疑。首先，我们改进的生物地层数据库表明，根据上覆奥陶纪磨拉石的记录，从 Karsha 碳酸盐岩到 Kurgiakh 组的过渡可能早于寒武纪-奥陶纪造山作用的主要阶段，早了 20-30 My。其次，来自奥陶纪磨拉石的已发表数据表明存在向北的古水流，这与卡沙以下Parahio组硅质碎屑沉积物记录的古水流平行，因此与寒武纪-奥陶纪事件的前陆盆地发育的标准模型不一致。我们对 Parahio 组沉积旋回的沉积学分析表明，这些地层记录了从近海海洋到岸面再到河流环境的风暴影响环境。这与之前从深海复理石到潮间带沉积物的古环境解释不一致。 Zanskar 和南部 Spiti 山谷的 Parahio 组海相和河流相的古水流数据表明存在东北沉积物迁移。这支持了这样的观点，即Parahio和上覆的Karsha组碳酸盐记录了寒武纪时期印度古老的北部被动边缘，并且这些地层可能是小喜马拉雅山较年轻的寒武纪沉积物的远端等价物。
A well-preserved Cambrian section in the Zanskar Valley of northern India has previously been interpreted to record the transition from a passive to an active tectonic margin related to Cambrian-Ordovician orogenesis. This interpretation has been used to support the tectonostratigraphic interpretation of other successions across the Tethyan Himalaya. Our detailed paleoenvironmental analysis significantly revises the tectonic and depositional history of these Cambrian deposits: no definitive record of impending Cambrian-Ordovician orogenesis is recorded in these late Middle Cambrian rocks. A critical transition from an , 125-m-thick, stromatolite-bearing carbonate deposit, the Karsha Formation, into shale and sandstone of the Kurgiakh Formation, was interpreted to represent tectonically induced drowning of a carbonate platform. Siliciclastic strata of the Kurgiakh Formation were thought to record deep-water flysch deposition in a tectonically active foreland basin next to an arc-trench system. This interpretation was based on sandstone beds with classic Bouma sequences. We show that these event beds in the Kurgiakh instead contain hummocky cross-stratification, quasi-planar lamination, and combined-flow ripple stratification, all of which reflect deposition in shallow-marine, storm-influenced environments. Thus, although the Karsha carbonate platform may have been drowned, it did not culminate in deep-sea flysch deposition, and this in turn eliminates a major line of evidence linking Kurgiakh deposition to the onset of Cambrian-Ordovician orogenesis. Other aspects of Cambrian-Ordovician deposits of northern India also shed doubt on the proposed link between Kurgiakh sedimentation and the Cambrian-Ordovician orogenic event. First, our improved biostratigraphic database suggests that the transition from the Karsha carbonate to the Kurgiakh Formation may have predated the main phase of Cambrian-Ordovician orogenesis, as recorded by overlying Ordovician molasse, by as much as 20-30 My. Second, published data from the Ordovician molasse indicate northward paleocurrents, which are parallel to those recorded by siliciclastic deposits of the Parahio Formation below the Karsha, and thus are at odds with standard models of foreland basin development for the Cambrian-Ordovician event. Our sedimentological analysis of depositional cycles of the Parahio Formation indicates that these strata record storm- influenced environments from offshore marine to shoreface to fluvial settings. This is at odds with previous paleoenvironmental interpretations that ranged from deep-sea flysch to intertidal deposits. Paleocurrent data for marine and fluvial facies of the Parahio Formation in both Zanskar and the Spiti Valley to the south indicate northeast sediment transport. This supports the view that the Parahio and overlying carbonate of the Karsha Formation record the ancient northern passive margin of India during the Cambrian and that these strata may be distal equivalents of the younger Cambrian deposits of the Lesser Himalaya.