A DEEP WATER SEDIMENTARY CRITERIA——"KNOTTY NODULE" AND ORIGIN
摘要: “瘤状结核”是由富灰硅质结核和富粘土质“外壳”组成.从结构和成因可划分瘤状结核(不连续)、连续,瘤状结核和压扁瘤状结核(或压扁构造).据研究“瘤状结核”是在海水较深的外陆棚至盆地环境中形成的.“瘤状结核”有三种不同的成因,即(1)瘤状结核是在底流和洋流作用下,促使沉积作用中断,于沉积物—水界面附近,由海底早期成岩胶结作用形成;(2)连续瘤状结核是在较长的沉积作用中断和成岩作用相结合,致使结核体发育和生长以及互相连结,由岩化作用形成和(3)压扁瘤状结核是一种次生沉淀成因,它是由晚期埋藏和溶解、压实联合作用下形成的.Abstract: The " knotty nodule" of Dalong Formation of Late Permian in Northern Sichuan and Sorthern Shaanxi Province are composed of siliceous-rich nodules and clay-rich matrix. They may have two differ sedimentary sequence: One is type of Northern Sichuan, which is mainly distributed to Northern Jiangyou, Guangyuan, and Western Wangcang region in Northern Sichuan: The other is type of Sorthern Shaanri, which is distributed to Xixiang region in Sorthern Shaanxi (Fig.l). The "knotty nodule" and the surrounding rocks are dark colored thin to medium-thin bedding micritic limestone, siliceous micritic limestone, radiolarias siliceous rock, siliceous rock, argillaceous limestone and shale (or mudstone) etc. with horizontal and rhythms bedding, part rich in organic ous materials and pyrites (or nodule). In the "knotty nodule, " the biota is characterized as abundance planktonic faunas such as radiolarias, ammonoids, condonts and thin-shelled brachiopods and a few foraminifers, ostracods, calcisphers and sponge- spiculite etc. According to sedimentary characterxs and faunal studies, the" knotty nodule" and the surrounding rocks may suggest deposition in outer shelf to basin environment, with waterdepth in the 100 to 200m or 300 to 500m range and under normal salinities. In accordance with origin and texture, the"knotty nodule" may be divided into three types, e.g. knotty nodule (discontinuous), continuous knotty nodule and flaser knotty nodule (or flaser structures). The knotty nodule is mainly distributed to upper part of Dalong Formation in Xixiang, Shaanxi Province.It is composed of dispersion nodules and rich in lime-muddy matrix. These nodules arc dark grey siliceous-bering micritic limestone, and usually are irregular and lenticle nodules. The continuous knotty nodule is distributed to upper and lower part of Dalong Formation in Guangyuan, Sichuan Province. They are silica-rich siliceous micritic limestone, radiolarias siliceous rock, in which the nodules preceded to the interlocking coalescence to nodules layer of semi-continuous or continuous. In general, the continuous knotty nodules inerleaved with more distortional siliceous shale layers and exhibit a certain rhythm structure. The flaser knotty nodule consist of small lime-siliceous ellipsoidal bodies or lenses (or flaser nodule) surrounded by siliceous- rich solution seams. The most flaser nodules are the comparatively high degree of preferred orientation, and nearly parallel to bedding. In many cases, both flaser nodules and the adjacent solution seams have abundent radiolarias. Within these nodules, the radiolarias are random bioturbated fabric, that it was locked in place by early cementation, whereas in the surrounding solution seams of nodules have the preferred alignment of radiolarias elongate grams. These are formed by the surrounding matrix remained unlithification during burial and may even have flowe plasticully around the hardened nodule during compaction. The solution seams may cut across primary depositional features such as bedding planes and omission surface, and they cut and truncate burrows. In many flaser knotty nodules, the solution seams form a secondary matrix to nodules, and seams are distorted around or cemented nodules. Carbon and oxygen isotope data (table 2, Fig. 4) indicated that the " knotty nodule" and the surrounding rocks were formed by submarine diagenesis cementation, which were in the direction of equilibrium with ambient colder bottom water than the surrounding sediment. This data is very similar to carbon and oxygen isotopes of the nodule, chalk nodule and hardground by the Cretaceous-Tertiary of the Caribbean (Deep Sea Drilling Project) and by the Cretaceous of the Northwest-European (Fig. 5) As noted above, from the characters of sedimentary, carbon and oxygen isotopes in the "knotty nodule" lead to the following conclusions about their origin: 1, The knotty nodule produced by sea floor early diagentic cementation, in which the pause in pelagic sedimention and closer to the sediment-water interface. 2, The continuous knott
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