云南施甸铺门前组灰岩的微相特征及其环境意义

<正> 保山地块位于怒江和澜沧江断裂带之间,西为西缅地块及怒江—龙陵—瑞丽缝合带,东为昌宁—双江缝合带,海进自泥盆纪开始,早石炭世发生海退,下石炭统香山组及铺门前组碳酸盐沉积仅局限分布于由保山—施甸呈袋形向南延伸的区域内(图1)。沉积间断发生于早石炭世末、晚石炭世初之间。而后,海进扩大,上石炭统丁家寨组覆于铺门前组灰岩之上。

Microfacies characteristics and environmental significance of the Pumenqian Formation limestone in Shidian of Yunnan

The study of carbonate microfacies and sedimentary environments of the Pumenqian Formation is important not only for the interpretation of the origin of pebble-hearing layers at the lower part of Dinjiazhai Formation, but also for the understanding of palaeogeo-graphy and tectonics in the Early Carboniferous of the Baoshan Block. The limestones of the Pumenqian Formation consist mainly of ooids, pellets, oncoids, intraclasts and bioclasts. Genesis of micrite calcite patches or strips in ocids can be assigned to the activity of blue-green algae or fungi. They cut through the concentric layers of coids and are the same as the genesis of micrite envelops. Oncoids are formed by Ortonella which took in micrite calcite when they grew arround the nuclei. The asymmetry of oncoids gives a hint for interpreting depositional environments of oncoids. They were formed in the lower-energy environments. Algal debris is typical of intraclasts. There are a large number of bird's-eye, worm-shaped and filamentous pores filled by sparry calcite in algal debris. Three main diage-netic environments can be recognized from the limestones of the Pumenqian Formation. They vadose silt and pisoids can be acted as the diagenetic products of the freshwater vado-se zone. The freshwater phreatic zone contains two subzones: CaCO3-undersaturated subzone and CaCO3-saturated subzone. The meteoric water in the former dissolved the intragranular micrite calcite or intergranular micrite calcite matrix and produced moldic porosities or enlarged interparticle porosities which were filled by granular calcite later. The cements infilling the original interparticle porosities in the latter "are characterized by the blocky calcite or calcite syntaxial overgrowth on echinoderms or foraminifers. Two generations of sparite calcites can be regarded as the signs of diagenetic products of the marine phreatic zone. According to the characteristics of grains, diagenesis, textures and structures, the limestones of the Pumenqian Formation can be divided into 12 main microfacies types: echi-noderm-foraminifer-mollusc sparite, oolitic sparite, intrasparite, superficial ooid sparite, in-trasparrudite, echinoderm-brachiopod-foraminifer micrite, algal debris micrite, pelsparite, intramicrite, lamellar micrite, ostracod-bearing micrite, oncomicrite. They were deposited in the intertidal flat and channel, the subtidal lower-energy and high-energy zones. The change of depositional environments from the subtidal high-energy zone to the intertidal flat which reflects a regressive course took place in Shidian, western Yunnan in the Early Carboniferous Visean.