贵州拉丁晚期扬子台地边缘地表暴露带的特征及裂隙中微型钟乳石的成因
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摘要
贵州中三叠世拉丁晚期扬子台地边缘地表暴露带 ,经历的时间长 ,约 1Ma(2 33~ 2 32Ma) ;形成的厚度大 (一般 10 0~15 0m左右 ,最厚可达 180m左右 )。暴露特征清楚 ,渗流豆石、窗格 (鸟眼 )构造、中 大型帐篷构造、喀斯特化角砾岩非常发育 ,这显然是潮湿 半潮湿气候条件下 ,在古隆起区形成的钙质风化壳。暴露带裂隙中的微型钟乳石 ,有覆水石、滴水石、流水石 3种类型。其18O同位素组成一般在 - 10‰ (PDB)左右 ,13 C的同位素组成一般在 1.5‰~ 2‰ (PDB)左右。18O同位素组成表明 ,钟乳石是淡水成因的 ,但13 C的同位素组成为略高于淡水成因的沉积物的值 ,这可能是不饱和的大气降水溶解了基岩 ,而获得13 C的缘故。上述 3种形态的微型钟乳石的岩石学特征也表明 ,是典型的淡水成因形成。由此可见 ,暴露带裂隙中的微型钟乳石为淡水成因的产物。
The superficial exposure belt experienced a long evolutionary time(about 1 Ma ),and formed a great thickness of sidiments(usually between 100~150 m,with the largest thickness being some 180 m) during Late Ladinian period of Middle Triassic. There exist very clear exposure features,such as vadose pisolite,fenestral(birdeye) structure,middle-large type teepee structure and karst breccia. The belt is therefore evidently a calcareous weathering crust under humid-semihumid climate condition in an ancient upwarped region. There are three types of micro-stalactite in fissures of the exposed belt,namely,drapestone,dripstone,and flostone. Their 18 O isotope composition is usually about -10‰(PDB),and their 13 C isotope values are generally between 1.5‰-2‰(PDB).The 18 O isotope composition indicates that micro-stalactites are of fresh-water origin,but their 13 C values are slightly higher than the values of the freshwater sedimentary. This is probably attributed to the fact that meteoric water dissolved pre-formed limestone (bedrock) and obtained 13 C.The three shapes of the micro-stalactite are petrologically typical of fresh-watter sediments .It is thus considered that the micro-stalactites are fresh water sediments.
The superficial exposure belt experienced a long evolutionary time(about 1 Ma ),and formed a great thickness of sidiments(usually between 100~150 m,with the largest thickness being some 180 m) during Late Ladinian period of Middle Triassic. There exist very clear exposure features,such as vadose pisolite,fenestral(birdeye) structure,middle-large type teepee structure and karst breccia. The belt is therefore evidently a calcareous weathering crust under humid-semihumid climate condition in an ancient upwarped region. There are three types of micro-stalactite in fissures of the exposed belt,namely,drapestone,dripstone,and flostone. Their 18 O isotope composition is usually about -10‰(PDB),and their 13 C isotope values are generally between 1.5‰-2‰(PDB).The 18 O isotope composition indicates that micro-stalactites are of fresh-water origin,but their 13 C values are slightly higher than the values of the freshwater sedimentary. This is probably attributed to the fact that meteoric water dissolved pre-formed limestone (bedrock) and obtained 13 C.The three shapes of the micro-stalactite are petrologically typical of fresh-watter sediments .It is thus considered that the micro-stalactites are fresh water sediments.
引文
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EstebanM ,KlappaCF .1983.Subaerialexposureenvironment.InSchollePA ,BeboutDCandMooreCHeds.CarbonateDeposi tionalEnvironment.AAPCmenoir33.
GuizhouBureauofGeologyandMineralResources.1988.Regionalge ologyofGuizhouprovince.Beijing:GeologicalPublishingHouse(inChinese).
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