Sedimentation and Environmental Development of Late Proterozoic Luoyu Group , Western Henan
Men Qingren, Hu Jianmin (Xi'an College of Geology)
Widely exposed in the western Henan,the Late Proterozoic Luoyu Group consists of sili-ciclastic rocks in the lower section and stromatolite dolostones and dolo-mudstones in the upper section, recording a complete transgressive-regressive depositional sequence. The transgression is demonstrated by the fact that high energy coastal deposits are gradually overlapped by shelf facies,with amalgmated glauconitic sandstones as condensed layers. Trans-gressive section ,hower ,makes up only 10-15 percent of the Luoyu Group. In contrast,the subsequent regression brought about striking coastal prpgradtion ,resulting in a thick vertical stack of facies of outer shelf-inner shelf-beach/barrier islands-lagoon environments in a ascending order. In terms of internal sedimentary structures ,siliciclastic rocks are divided into several different facies. Hummocky stratified sandatones,swalley stratified sandstones and thin-bedded Bouma-type fine-grained sandstones are clearly of storm origin. The cross-bedded and parallel-laminated sandstones may be also generated by storm-related ourrents. These distinctive facies and facies associations are widespread and dominate the Luoyu Group, indicative of costal-shelf environments which were influenced mainly by storm events. While sand trn-sport and deposition in inner shelf might be controlled by storm-surf! ebbs,the formation of thin fine-grained Boumatype layers in outer shelf are related to shelf water density (turbidity)currents. The carbonate deposition of Upper Luoyu Group is characterized by development of stromatolites which can be grouped,according to their growth forms,into different types,i. e. .spheroid,dome,columnar and stratiform. A shallowing-upward depositional trend is inferred from the vertical variations of the stromatolite geometry. Thin-layered dolomudstones directly capping stromatolite facies are distingguished by desiccation, solution surfaces, cauliflower chert nodules and tepee structures which provide convincing evidence of subaerial exposure and further attest the shallowing-upword nature of the stromatolite to thin-layered dolomudstone sequence. Alagoon setting for the Upper Luoyu Group mudstone and carbonate deposition is suggested because of its immediate superposition over the beach or barrier islands in a progradational sense. The depositional evolution of Luoyu Group was closely linked to the rifting of southern margin of North China Plateform,and the whole sequence is the products of thermal subsidence phase of a rifted basin in western Henan. The progradation-dominated facies sequence certainly resulted from regression due to low rate of subsidence and relatively rapid fillling of the rifted basin.