海南岛排浦更新世沉积的白云岩化作用
DOLOMITIZATION OF PLEISTOCENE SEDIMENTS ON HAINAN ISLAND, CHINA
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摘要: 1981年5月.笔者在海南岛儋县排浦公社海滨发现了更新世白云岩,1982年3月—4月又率学生去现场实习,接着作了各项室内分析研究工作。本文将着重探讨这套更新世沉积的白云岩化作用。Abstract: The Pleistocene dolomites occurred in the abrasion flat and coast of the inter-tidal zone of the fringing rear reef in Paipu area，Zuan County,Hainan Island，Chi-na. The abrasion flat is made of sediments of the littoral-river sandy-conglome-ratic facies, including abundant mould of marine mollusca. On the flat the sea cliff consists of littoral lagoonal mudy siltfine sand facies. Dolomites(dolostone)consist of dolomite and clastic grains. It is confirmed by the identifications of thin section, staining techniques, thermocouple analysis，X-ray- diffraction techniques，scanning electron microscopy. electron probe microanaly-sis, chemical quantitative analysis etc. There are two kinds of dolomites: 1) dolomite of euhedral rhombic crystal witii tonal tenure; 2) clear, limpid dolomite formed by fissure filling and shell moalding. The origin of dolomites is of replacement in the early diagenesis stage, Thereare three patterns of occureaces of dolomites: 1) Dolomitic sandy conglomerateformed by- coarse clas'tic deposits and dolomite has a bedding structure, 40-50cm inthickness, and consists of gravels, several cm to 30cm in major diameter and do-lomite, having the forms of matrix and cement, It has been known now as the coar-sest elastic materials associated with dolomite, 2) Sandy dolomite of lenticular orbedding structure 8-30cm, in thickness，interlayers with elastic depositss it consi-sts of 75% dolomite and about 20-25% elastic materiels and authigenic minerals, 3)Dolomite crystals and dolomitized infiltration tubules distribute in muddy sand-siltbed. The latter has circle and oval cross section and concentric stratoid structure.The loose and rough central area and outer stratum consist of 10-40% dolomite.The fine inner stratum dolomitized intensely consists of 900u dolomite and only 5%elastic materials and 5% pore space. The silts- elastic particles have relict structure,caused by replacement (Plat. I, 3，II，5-7，Table 4) Thcre arc three kinds of dolomitiza tion, which may be sure in this region: 1. Dolomitization of the miring of fresh ground water and sea water in inter-tidal zone of coast and in the mouth of small river. These conditions favour thereplacement of dolomite because of the drastical dropping of salinity and the Mg/Caratio remaining less variance. 2. Dolomitization of schizahaline pore- water in the intertidal zone .It forms clear, limpid dolomite, replacing shell of mollusca and filling the tension crack with a lamellar arrangement. 3, Dolomitization of plants forms dolomitized in(il'tralion tubule, It is di-rectly- controlled by the physiological action of plants around their roots. In the in-tertidal zone of the coast, because the transpiration and respiration of plants elec-tip-ely suck up moisture and cationes and drain off the C02 and organic acid, thesalinity and Mg Ca ratio become higher and higher,and the microenvironment nearthe root becomes favourable of the replacement, Thus，the dissolution，precipitationand metasomatism of carbonate take place and dolomitized infiltration tubules areformed around the root system, This local intense dolomitization forces the peo-ple to realize the possibility of forming dolomite(dolostone) by replacing elastic rocks on gig en conditions
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