豫西太原组、山西组中潮道沉积的类型和特征

豫西太原组中的潮道为陆表海滨岸潮道类型,其特点为:向上变细的层序,冲刷切割下伏的灰岩层,具大型板状、槽状交错层理和双向交错层理,潮道曲流砂坝发育。山西组下段的潮道为障壁后潮道,位于障壁砂坝后的潮坪区;潮道砂岩泥质杂基较高,正粒序,砂体自海向陆分叉尖灭;潮坪、潮道等亚环境组合影响了其上发育的煤层的厚度变化。山西组上段的潮道为下三角洲平原潮道类型,多位于分流河口,砂岩为正粒序,双(单)粘土层和潮汐周期层序发育。

THE TYPES AND CHARACTERISTICS OF TIDAL CHANNEL DEPOSITS IN WESTERN HENAN PROVINCE

There are different types of tidal channel deposits in Taiyuan Formation (C32-3) and Shanxi Formation (P11) in western Henan Province. Taiyuan Formation consists mainly of alternated carbonate and clastic deposits and Shanxi Formation of coal-bearing clastic deposits. Most of the tidal channel deposits are elastics and a few are carbonates in Taiyuan Formation. The main characteristics of clastic tidal channel deposits are as follows: 1. Consist of quartz sandstones fining upwards. 2. With erosion surface and lag deposits at the bottom. 3. With large-scale tabular and trough cross bedding and a few herringbone and longitudinal cross bedding. 4. Tidal channel point bars formed by lateral aggredation. 5. Sand bodies spread widely owing to the migration of tidal channels and their thickness varied laterally. 6. Tidal channel sandstones associate with fine clastic deposits of shorezone laterally and overlain the bioclastic limestones bearing normal marine fossils assemblage. Carbonate tidal channel deposits consist of flat angular conglomerates of intraclast limestones, large-scale tabular and herringbone cross bedding may be found occassionally, and associated with thin bedded quartz sandstones of shore-zone in vertical and lateral sequence. Both clastic and carbonate tidal channel deposits are all developed on the shore zone of epeiric sea, therefore, they may called shore-zone tidal channel of epeiric sea. Depositional characteristics of the clastic tidal channels in the lower part of Shanxi Formation are similar to those of Taiyttan Formation. The differences are as follows. 1. Sandstones comprise more muddy matrix and plant fragments. 2. Coexist with tidal flat fine graindeposits in vertical and lateral which contain abundant siderite nodules. 3. Sandstones are more thick and distribute as narrow banded in plane view and lenses in cross section, gradually they were change into tidal flat deposits to lateral fanks. 4. The main coal bed was developed continuously over the tidal flat and tidal channel sand stone deposits. The thickness of the coal bed decreases sharply away from tidal and the quality becomes poor. This type of tidal channels is distributed on the back-barrier of mesotidal coast, therefore may be named tidal channels of back-barrier. The upper part of Shanxi Formation developed in the tidal channel deposits of the distal lower delta plain. The main characteristics are. 1. Composed of sublith-arenites, and comprises numerous carbonaceous and plant fragments, muscovites concentrated on the surfaces of the bedding, mudclasts. 2. With large-scale tabular and trough cross bedding, especially rare of double-mud-layers, single-mud-layers and tidal cyclic sequences formed by tidal currents. 3. Sandstones fining upwards, coexist with mouth bar coarsing upwards and lower delta plain deposits in lateral. These channels distributed in the distal of the distributary channels of lower delta plain, the fluvial and tidal currents used the same channel, and the distributary channel mouths were also tidal channels. Other tidal channels developed on tidal flats of interdistributary bays. In ancient deposits, it is difficult to distinguish them only by the depositional indicators, so may be called by a joint name - tidal channels of the distal lower delta plain. Tidal channels underlying the main coal bed all characterized by herringbone cross bedding and tidal point bars, indicating the existance of symmetric tidal currents. While tidal channel deposits overlying the main coal seam lack of herringbone cross bedding, but occur asymmetric tidal current depositional indicators such as tidal bundle, double-mud-layers and single-mud-layers. Paleocurrent pointed to the south (seaward), thus the channels were ebb tide dominated tidal channels. Different types of tidal channel deposits occur in regular order in the coal-bearing strata: from bottom to top, the shore zone tidal channels of epeiric sea occurred earlier, tidal channels associated of back-barrier subsequently, and tidal channels developed in distal of the lower d