| 浙江桐庐晚奥陶世晚期沉积层序和沉积环境分析 |
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| 引用本文： | 范代读,李从先,蔡进功,邓兵,罗璋.浙江桐庐晚奥陶世晚期沉积层序和沉积环境分析[J].沉积学报,2003,21(2):247-254. |
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| 作者姓名： | 范代读 李从先 蔡进功 邓兵 罗璋 |
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| 作者单位： | 1.海洋地质教育部重点实验室同济大学海洋地质与地球物理系 上海 200092; |
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| 基金项目： | 国家自然科学基金项目 (批准号 :40 1760 2 2,499760 16)资助 |
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| 摘 要： | 文昌组上段顶部是一套潮汐层理非常发育的泥质砂岩或砂质泥岩,存在双向交错层理,层面有雨痕,应为潮坪沉积。潮坪沉积由小型层序构成,小型层序又是由砂、泥质单层组成。砂质单层底部通常为岩性突变面或侵蚀面,砂质纹层较厚,其中可见对称波痕或泥砾;向上砂质纹层变薄,过渡到泥质单层。砂质单层形成于暴风浪时期,泥质单层是风浪衰减后恢复正常的潮汐沉积。因此,小型层序从成因上说是一风暴层序。碎屑成份、砾石成份分析表明沉积物均来自华夏古陆的沉积岩和变质岩基底。物源一致,岩层产状变化不大,反映文昌组沉积环境稳定。岩性、粒度分析表明文昌组是一向上变细、由浅海高能环境向近岸低能环境过渡的沉积层序。文昌组下段为浅海砂岩沉积,上段顶部为潮坪沉积。二者之间是一套夹砾岩透镜体的泥质粉细砂岩,其沉积环境应介于浅海和滨岸之间,为水下岸坡沉积。砾岩层只是大的沉积旋回中出现的事件性水下冲积物。
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| 关 键 词： | 潮坪沉积 风暴层序 砾岩 沉积层序 晚奥陶世 浙皖边缘海盆 |
| 文章编号： | 1000-0550(2003)02-0247-08 |
| 收稿时间： | 2002-03-05 |
| 修稿时间： | 2002年3月5日 |
Sedimentary Successions and Environments of the Late Ordovician Wenchang Formation in Tonglu, Zhejiang |
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| FAN Dai-du,LI Cong-xian,CAI Jin-gong,DENG Bing,LUO Zhang.Sedimentary Successions and Environments of the Late Ordovician Wenchang Formation in Tonglu, Zhejiang[J].Acta Sedimentologica Sinica,2003,21(2):247-254. |
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| Authors: | FAN Dai-du LI Cong-xian CAI Jin-gong DENG Bing LUO Zhang |
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| Institution: | 1.Laboratory of Marine Geology, Tongji University, Shanghai 200092;2.Hangzhou Institute of Petroleum Geology, Hangzhou 310023 |
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| Abstract: | The Late Ordovician Wenchang Fm. is subdivided into two members. Member I (M I, the lower section), characterized by presence of thick massive sandstone layers intercalated with thin mudstone layers and brachiopodous, bryozoan and crinoidea assemblage, is interpreted as nearshore to shallow-marine deposits. Member Ⅱ(M Ⅱ, the upper section), is further subdivided two parts: M Ⅱ-1 (the lower one), consisting of sandstone intercalated with conglomerate lenses; M Ⅱ-2 (the upper section), composed of muddy sandstone or sandy mudstone with well-developed tidal bedding. There are two opposite opinions concerning the sedimentary environment of M Ⅱ. Some consider that M Ⅱ-1 is debris-flow deposits on the basin slope owing to biological assemblage containing graptolite, and M Ⅱ-2 is deep-water internal-tide deposits. Others interpret M Ⅱ-1 as near-shore subaqueous channel deposits based on its biological assemblage dominated by shallow-water species and conformity contact with the underlying M Ⅰ in the region, and M Ⅱ-2 as tidal-flat deposits. M Ⅱ-2 is characterized by alternations of sand and mud laminae, and of sand- and mud-dominated layers. In this study, raindrop imprints were observed on some bedding-planes, exhibiting the periodic subaerial exposure of sedimentation facies. The presence of symmetrical oscillation ripples shows wave activities were common at the study area. Mud pebbles are inferred to form on the intertidal flats during storms in comparison with studies on the modern tidal flats margining the Changjiang Delta. M Ⅱ-2 is, therefore, not deep-water internal-tide deposits but normal tidal-flat deposits with strong storm influence. The sand-dominated layers (SDL), several to several tens of cm thick, are interpreted as storm deposits based on the presence of: 1) scour features, 2) numerous intraformation mud-pebbles, 3) oscillation ripples, 4) relatively thick sand laminae at the lower part with an upward trend of thinning toward the adjacent mud-dominated layer. The mud-dominated layers (MDL), consisting of thin sand/mud alternating laminae, deposit after storms when sedimentary environments return to tidal regime. A small sequence, consisting of a SDL and a MDL, is thus a storm succession. Facies transition is gradual among the three units of the formation (M I, M Ⅱ-1,M Ⅱ-2). Dip directions and angles are close to each other. Detrital composition analyses show that they have similar provenance from the Cathaysian Block. It is inferred that sedimentary environments should have no much change during deposition of three different units. Sedimentological study suggested a shallowing-upward progradational succession from high-energy shallow-marine deposits to low-energy tidal-flat deposits. Upward increases of clay content indicate that sedimentary environments get closer to the shoreline. M Ⅱ-1, interlayered between neritic facies and tidal-flat deposits, is thus interpreted nearshore facies with conglomerate layers of event subaqueous channel deposits. |
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| Keywords: | tidal rhythmites storm deposits conglomerates sedimentary succession Late Ordovician Zhe-Wan Marginal Sea Basin |
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