全球二叠系顶部白云岩与沉积相关系统计及其白云石化机制的意义

Sedimentary facies and genetic mechanisms of the dolostones in the world-wide end-Permian strata

关键词:  上二叠统  长兴阶  沉积相  白云岩  川东北  白云石化机制

Key words:  Upper Permian  Changxingian  sedimentary facies  dolostone  northeastern Sichuan  genetic mechanism of dolomitization

摘要:

本文对全球范围内23个典型的研究程度较高的晚长兴期露头进行了研究,逐一检查了晚长兴期白云岩前身沉积的沉积相,结果发现所有白云岩的前身都是浅水相的(除汉中梁山一个剖面外)。相反,目前已经研究的深水相上二叠统碳酸盐沉积物都没有发生白云岩化。这一结果表明,世界范围内上二叠统顶部白云岩的形成机制可能都与海平面下降有关,很可能是蒸发成因的浓缩卤水使沉积物发生白云岩化。据此推断,四川盆地东北部长兴组白云岩也应当是此种成因。

Abstract:

The statistics and correlation in the present paper are involved into 23 world-wide representative and well-documented Permian-Triassic (late Changxingian) boundary sections and the precursory sedimentary facies of the late Changxingian dolostones. Except from one section from the Liangshan in Hanzhong, Shaanxi, all the other previous deposits of the late Changxingian dolostones belong to the shallow-water sedimentary facies. On the contrary, the known deep-water Upper Permian carbonate deposits have not been dolomitized up to now. This implies that the genetic mechanisms of the world-wide dolostones from the topmost parts of the Upper Permian strata may be related to the rapid global sea-level falls, which led to the exhumation of the shallow-water carbonate deposits to the intertidal and/or supratidal environments. The concentrated brines in these evaporation environments penetrated into the pre-existing carbonate deposits and gave rise to the dolomitization and finally to the formation of the dolostones. A similar example is provided by the Changxing Formation dolostones as excellent reservoir rocks in northeastern Sichuan Basin, where the pervasive dolomitization took place in the pre-existing organic reef facies and shallow-water open platform carbonate deposits, and the dolomitization mechanisms may well be controlled by the same sea-level falling event as the other parts of the world.