CAUSE OF THE PARTIAL MISSING OF GRAPTOLITE ZONES IN WUFENG AND KAOCHIAPIEN FORMATIONS OF WELL GD-1, CHAOHU AREA, LOWER YANGTZE REGION

The well GD-1, located in Gushan, Chaohu of Anhui Province, is a well initiated from the Kaochiapien Formation. From 770.5 m to 1234.5 m, there occur black graptolite shale of the Kaochiapien Formation and Wufeng Formation. Graptolite biostratigraphy reveals that there are four graptolite biozones in an ascending order, say Dicellograptus complexus-Paraorthograptus pacificus (WF2-WF3) zones, Akidograptus ascensus (LM2?) zone, Coronograptus cyphus (LM5) and Demirastrites triangulatus (LM6) zones (No apical in LM6). Compared with the Wufeng and Lungmachi (Kaochiapien) Formations in the Yangtze Region, the graptolite biozones of Metabolograptus extraordinarius-Metabolograptus persculptus (WF4-LM1) zones and Parakidograptus acuminatus-Cystograptus vesiculosus(LM3-LM4)zones are lacking in the well GD-1, but found in other holes nearby. It is suspected that the WF4-LM1 zones may not have been deposited in the area surrounding the well GD-1 due to the expansion of ‘Jiangnan Oldland’. The fact that the LM3-LM4 zones can be seen in the adjacent area but absence in well GD-1 may suggest the influence of the decollement structure, which also leads to the thinning of the black shale. The LM5 and LM6 zones are relatively thicker than the others in the Yangtze region, possibly due to the strata repetition affected by the thrust nappe tectonics.