云南保山地区丁家寨组沉积环境及下部含砾层成因的再认识

Restudies on the Depositional Environments of the Dingjiazhai Formation in Shidian of Yunnan and the Genesis of Its Bottom Pebble-bearing Layer

丁家寨组的沉积特征表明，自二叠纪初阿赛尔期至萨克马尔期在云南保山地区为一持续的海进过程，其下都含砾层可分成两种成因类型：滨海相成因的下部砾岩层和碎屑流成因的上部角砾状灰岩层。文章就了家寨组在横向上的展布特点和纵向上岩相变化规律以及其中所含的海相化石等，对含砾层的各种冰川成因说提出了质疑，并用翔实的资料对区内冰川成因说的重要证据（条痕石、落石、三重序列、冰绿海生物群）提出否定意见。

The Dingjiazhai Fm. aged Asselian-Sakmarian pseudoconformably overlles the Pumenqian Fm. aged the Lower Carboniferous. The Dingjiazhai Fm. in the Dingjiazhai section at Shidian County mainly comprises 3 parts in ascending order: 1. the pebble-bearing layer at the lower part; 2. siltstones and black shales at the middle part in which trace fossils PalaeoPhycus sp., Taenidium serpentinum, have been found, 3. muddy siltstones and biocalcarenites at the upper part. A thin purple mudstone occurs at its topmost in which the trace fossil Dendrotichnium sp., is present. According to the palaeontological, petrological and ichnological features, six main sedimentological facies are distinguished from the upper part of Pumenqian Fm. to the Dingjiazhai Fm. They are tidal flat facies, margin facies and lagoon facies which were located in the restricted platform,sand bar or shoal facies, basin facies of shallow water and open marine platform which belonged to open marine environments. The Dingjiazhai Fm. in the Jinji section of Baoshan region consists of the pebble-bearing layer at its lower part and mudstones and muddy siltstones at its middle and upper part. Palaeo phycus sp. B and Tisou jinjiensis are main trace fossils present in the mudstones and muddy siltstones overlying the pebble-bearing layer. The vertical U-shaped structures of Tisoa jopnsis are similar to those of Areni colites. The trace fossil assemblage can be attributed to the Skolithos ichnofacies. As indicated by Frey & Pemberton (1984), the environtnents which the Skolithos ichnofacies represents are characterized by shifting substrates, moderate to high-energy conditions and episodic erosion or deposition. On the basis of petrological studies on the pebble-bearing layer, it can be divided into two beds: the lower pebble bed and the upper brecciated limestone. The pebbles with better rounding in the former are elliptical and flat and mainly consist ofsiltstones, quartzites, silicolltes and lithic sandstones. The breccia in the latter are briefly composed of llmestones whose carbonate microfacies are similar to those of the underlying Pumenqian Ftn..The cementation of the brecciated limestone is much better than that of the lower pebble bed. The authors consider that the origin of the pebble-bearing layer at the lower part of the Dingjiazhai Fm. is not related to glaciation. The Iower pebble bed was deposited in the normal littoral environments and the upper brecciated llmestone was derived from the debris flow. The palaeoecologic features of biota (corals, fusulinids, spores) also indicate that the organisms lived in warm marine environments. Many geologists have regarded striated clasts, dropstones,three-fold sequence and periglacial marine biota as main evidences of glacial origin of the Dingjiazhai Fm.This paper puts forward different viewpoints against thelr conclusions. The striated clasts are not only products of glacial process. They can be produced by many other geological processes such as faults, debris flow,landslip, landfall, mountain flood and even wind action. The photos of striated clasts (Fan & Fang, 1992) do not show any typical characters generated by glaciatlon. They have better roundness and lack evident worn planes. The direction of striates is different from that of long axis of clastsSome Chinese and foreign geologists misunderstand that the dropstones are able to stab the stratification when they fall down. In fact, when the dropstones fall down and come into contact with the surface of deposits, the power of dropstones is not strong enough to crash the stratification and only can bend it. The local concentration of pebbles and coarse clasts and the occurrence of tin Pellets can be regarded as products of iceberg raft. Un fortunately, these phenomena have not been found in the Dingjiazhai Fm.Wopfner (1994) suggest that the three-fold sequence consisting of 1. diamictites and coarse clasticsl 2. pebbly mudstones and laminites and 3.black pyritic mudstones respectively correspond to the deposits in glacial advance, glacial retreat and deglacia