Abstract

Terminal Ediacaran–late early Cambrian deposition, faunas and passive margin evolution of the north Indian margin are recorded in the Nigali Dhar syncline succession. Restudy of the upper Tal Group (upper lower Cambrian Koti Dhaman Formation, KDF) ichnofauna from the Khud-Drabil section reduces it to 18 confidently named forms. The lower KDF (Lower Quartzite Member) Cruziana-Rusophycus assemblage is in subtidal (not intertidal) sandsheet facies. The overlying black Shale Member (SM) records trans-East Gondwanan deepening, not intertidal facies, in the Palaeolenus Zone. The SM, with low diversity Planolites-Palaeophycus assemblage, is overlain by subtidal (not intertidal) sandsheet facies of the middle KDF (Arkosic Sandstone Member, ASM) with shallow burrowers and furrowers (Gordia marina assemblage, new; Cruziana ichnofacies). KDF faunas with Cruziana and Rusophycus are similar to coeval, shallow marine associations elsewhere in Gondwana and NW Laurentia. Interpretation of a second KDF section 20 km from Khud-Drabil has confused an understanding of Lesser Himalaya geologic evolution as it claims Ordovician Cruziana species in the ASM and an angular SM–ASM unconformity caused by the Cambrian–Ordovician boundary Kurgiakh orogeny. However, upper lower Cambrian microfaunas occur in and above the ASM, while the angular SM–ASM unconformity is consistent with submarine sliding. KDF-type ichnofaunas do not show a tropical location of Avalonia, which has the distinctive lithofacies and biotas of a high-latitude continent unrelated to Gondwana.

摘要

在 Nigali Dhar 向斜序列中记录了北印度边缘的埃迪卡拉纪末期-晚早寒武纪沉积、动物群和被动边缘演化。对来自 Khud-Drabil 剖面的上塔尔群（上下寒武纪 Koti Dhaman 组，KDF）鱼类动物群的重新研究将其减少到 18 种有把握地命名的形式。较低的 KDF（下石英岩成员）Cruziana - Rusophycus组合位于潮下（而非潮间）砂板相中。上覆的黑色页岩段 (SM) 记录了古列努斯带中横贯冈瓦纳东部的加深相，而不是潮间带相。SM，具有低多样性Planolites-Palaeophycus组合，被中层 KDF（Arkosic Sandstone Member，ASM）的潮下（非潮间带）砂岩相覆盖，具有浅挖洞和开沟（Gordia marina组合，新；Cruziana ichnfacies）。带有Cruziana和Rusophycus 的KDF 动物群类似于冈瓦纳和西北劳伦西亚其他地方同时期的浅层海洋协会。对距离 Khud-Drabil 20 公里的第二个 KDF 部分的解释混淆了对小喜马拉雅地质演化的理解，因为它声称奥陶纪克鲁齐亚纳ASM 中的物种和由寒武纪-奥陶纪边界库尔贾赫造山运动引起的有角度的 SM-ASM 不整合。然而，上、下寒武统微型动物群出现在 ASM 内部和上方，而角度 SM-ASM 不整合与海底滑动一致。KDF 型鱼类动物群没有显示阿瓦隆尼亚的热带地区，该地区具有与冈瓦纳大陆无关的高纬度大陆的独特岩相和生物群。