P-T事件后特提斯洋从停滞到动荡的岩石学证据

The petrological evidences of sluggish to oscillating Tethys ocean after the P-T Event

上扬子地区下三叠统飞仙关组下部薄层状、纹层状微晶灰岩、泥质灰岩之上普遍发育与风暴作用有关的角砾状灰岩、"错时相"扁平砾屑灰岩、丘状交错层理颗粒灰岩或粉砂岩,这一现象在南阿尔卑斯、伊朗、克什米尔等地的早三叠世早期地层中也广泛存在,其等时性有待深入研究。从薄层状、纹层状灰岩到砾屑灰岩的岩性转换表明,特提斯洋沿岸陆架地区的古海洋水动力条件在早三叠世早期经历了从停滞到动荡的显著变化。岩石学特征显示纹层状、薄层状灰岩泥质含量较高且普遍缺乏反映浅水暴露的沉积构造标志,表明其形成水体相对较深,为低能环境的产物,指示了当时静止、停滞的海洋环境;角砾状灰岩、扁平砾屑灰岩中丘状交错层理以及"菊花状"构造的大量出现则表明其形成是风暴引起的高度搅动的水体对弱固结的碳酸盐沉积物进行打碎、撕裂、搬运等再改造作用的结果,代表了动荡的海洋环境。薄层灰岩、纹层灰岩中生物扰动较弱,反映了P-T事件后萧条的海洋生态系统对底栖生物垂直钻孔作用的限制,生物扰动作用的缺乏有利于薄层、纹层灰岩的发育和保存;而触发大范围砾屑沉积的风暴活动则可能与当时全球CO2增多导致的全球变暖、海水温度升高,特提斯洋所处的古地理位置以及当时盛行的巨型季风气候等多种因素有关。

In the upper Yangzte region,laminated limestone,thin-bedded pelmicrite,argillaceous limestone are widely covered by storm-generated brecciform limestone,anachronistic flat-pebble conglomerate calcirudyte,hummocky cross-bedding grain limestone or siltstone.This shows in the first member of Lower Triassic Feixianguan Formation in the Guangyuan and Chongqing areas.This phenomenon is widespread in the strata of the Early Triassic in southern Alps,Kashmir,and Iran.The isochronism of this event needs further investigation.The petrologic change from laminated sediments to grain limestone and calcirudyte indicates that the palaeo-oceanographical environment of the coastal shelf area in Tethys has changed significantly.The petrology features suggest that the limestone is rich in mud and lack of which can reflect the exposed shallow water sedimentary structures and that the laminated limestone and thin bedded limstone are the products of low energy and formed at deep water,indicating the quiet and stagnant marine environments.The HCS and chrysanthemum structure in the brecciform limestone and flat-pebble conglomerate calcirudyte mark the oscillating environment and show that they formed by the weakly solidified carbonate which were broken,torn up and re-worked by the oscillating water resulting from the storm.The weak biodisruption in the thin bedded and laminate limestone indicates that the depressed marine ecosystem after the P-T Event set a limit to the drilling of benthos.The storm activities resulted in the development of the calcirudyte that may be associated with diversified factors including global warming and higher temperature sea water caused by the increase of CO2,the paleogeography of the Tethys and the prevailing megamonsoon in Early Triassic.