江西修水二叠纪·三叠纪界线地层海平面下降的岩石学证据

在江西修水四都镇东岭村发现了一个连续的、出露完好的二叠纪-三叠纪界线地层剖面。在剖面中有一段厚约2.1m 的地层具有指示海平面下降的岩石组构。这段地层的底部为红色的碳酸盐颗粒岩。红色是颗粒表面的红色铁染,成分为褐铁矿,是出露地表风化形成的。这层红色颗粒岩之上有约2.0 m 的去白云石化灰岩,其中保留了原来粉晶白云石的晶形。去白云石化作用是白云岩受淡水影响而发生的,指示海平面下降事件。这段地层之上是厚1m 的颗粒岩和灰泥岩,代表浅水潮下环境。其上是二叠系-三叠系的界线。这段地层之下的地层中生物丰富多样,并且有复(蜒)、钙藻等大绝灭前的生物。这段地层及其以上的地层中生物种类很少,(蜓)、钙藻等已经消失,表明集群绝灭事件开始于这段地层之下,生物大绝灭的发生与海平面下降事件的发生是基本同时的。由此认为,可能是海平面下降有关的环境剧变事件引发了二叠纪末的集群绝灭事件。

Petrologic evidence for sea-level drop in latest Permian in Jiangxi province, China and its meanings for the mass extinction

A new-found Permian-Triassic boundary section in Xiushui,Jiangxi Province,China shows evidence for sea-level drop during the latest Permian.Evidence for sea-level drop was preserved in a 2.1 m thick interval of limestone,and is 1 m from the P-T boundary.The basal 0.1 m of this interval is a reddish sparry-cemented grainstone.The grains are rounded,dolomitized,all having a black coating.Microprobe analysis indicates that the coating is limonite,which indicates a weathering event in subaerial environments after a sea-level drop event.The internal structure of most grains has been destroyed by diagenesis.Some remained structures of foraminifers and fusulinids.So,this bed is formed before the mass extinction.The 2.0 m thick bed overlying the reddish grainstone is a dedolomitized limestone.The shapes of some original finely-crystalline dolomite rhombus were preserved.The occurrence of dedolomitization is generally related to meteoric water.So,this bed recorded a sea-level drop event.Fossils in this bed are simple: fusulinids and large foraminifers such as Colaniella are absent.It is clear that mass extinction began before this bed.The 1 m thick limestone between the dedolomitized bed and the P-T boundary is composed of grainstone and lime mudstone.Study of this section reached two points:(1)sea-level drop occurred in latest Permian;(2)The end-Permian mass extinction began in the earliest episode of the sea-level drop event.Thus,we cannot exclude the possibility that the mass extinction was caused by a dramatic environmental change which triggered the end-Permian sea-level drop.