Reef and Gravity Flow Sedimentation on the Permian Slopes in...

8.10.1　Introduction

There were the five types of sedimentary basins of the Sichuan-Yunnan-Guizhou-Guangxi region during Permian.Each types of basins had slopes.Sediments on these slopes are mainly heterochthonous sediments,which include reefs and gravity flow sediments such as turbidites,clast flow sediments and grain flow sediments.Stratigraphical sequences made of these sediments are different from those in other types of sedimentary areas such as platforms and basins.Especially their reefs and gravity flow sediments are important for generating and reserving of oil-gas（James,1992;Cook,1992;Schlager et al.,1992;Milim et al.,1995）.This book aimed at summarizing general characteristics of these reefs and gravity flow sediments on the basis of sequence analyses.

8.10.2　Slopes

The slopes are difficult to be defined.In the Upper Yangtze cratonic basin there were epicontinental slopes and various intraplatform slopes.For the four types of Permian basins in Guangxi and Guizhou,each had its marginal internal slopes.Again,these slopes had different characteristics,depending on basins where they were located.In most cases we can not accurately locate slopes,but we have known indeed that reefs and gravity flow sediments were accumulation on slopes.Fig.8.40 gives sedimentary models for slopes by water level.

Fig.8.40　Sedimentary models for slopes

A.High water level,B.Marine progression and C.Low water level

8.10.3　Reefs

General characteristics of organic reefs on various slopes in the study region are summarized by system tract in Table 8.8.

The LST reefs occurred in the bottom of some sequences with progressive reefs and their typical examples have been seen in Dachang,Hongxian,Guilin（Tangjiawan）,Debao（Qinjia）,Guixian（Mengguan）,and Tianlin（Langping）of Guangxi Province and Puan and Dushan of Guizhou Province.Some of them are Stromatopara reefs.

The TST reefs occurred chiefly in the lower of Sequences 5,6,9,10 and 11 and their typical examples have been seen in Kexian（Honghua）,Fengdu（Taiyuan）,Huayingshan,Liangping and Zhongxian ofeastern Sichuan Province,Lichuan of western Hubei Province,Ziyun of southern Guizhou Province and Guilin（Longlin）and Lingyun of Guangxi.

The HST reefs occurred chiefly in the upper of Sequences 4,5,6,10 and 11 and their typical examples have been seen in Fengdu（Taiyuan）,Huayingshan and Liangping of eastern Sichuan,Guilin（Longlin）,Tianlin,Ningming and Lingyun of Guangxi and Ziyun of southern Guizhou.

Table 8.8　General characteristics of organic reefs on various slopes in the study region

8.10.4　Gravity Flow Sediments

General characteristics of gravity flow sediments on various slopes in the study region are summarized in Table 8.9.

Table 8.9　General characteristics of gravity flow sediments on various slopes in the study region

The LST gravity flow sediments occurred in Sequences 6,7 and 11 and their typical examples have been seen in Badu and Langping（Tianlin）,Shipao（Xilin）,Jiangdongguo（Ziyun）,Ceheng,Zhebao（Longlin）,Xiaoxinzhai（Qiubei）,Haiwai（Guangnan）,Xinje（Xichou）,Lingyun（Nandan）and Wuyu（Hechi）.

The TST gravity flow sediments occurred chiefly in Sequences 5,6,9,10 and 11 and their typical examples have been seen in Badu and Langping（Tianlin）,Xiaoxinzhai（Qiubei）,Shipao（Xilin）,Xinje（Xichuo）and Wuyu（Hechi）.

The HST gravity flow sediments occurred chiefly in Sequences 4,5,6,7,8,9,10 and 11 and their typical examples have been seen in Ziyun,Huishui,Luodan,Xilin,Longlin,Tianlin,Qiubei,Fuquan,Lingyun and Guangnan.

8.10.5　Discussions and Conclusions

In principle,the reefs and gravity flow sediments mentioned above should have occurred on slopes of all basins of different types and ages in the study region（Zeng Yunfu et al.,1995;Huo Fanghao et al.,1984;Research Division of Carbonate Rocks,1982;Chen Hongde et al.,1990）.In the light of our investigation,they typically occur on slopes of the Youjiang passive epicontinental rift basin of Early Permian and the Youjiang back-arc rift basin of Late Permian.Of these slopes some were:epicontinental slopes and others were intrabasinal slopes,i.e.slopes between isolated platform and platform basin.These slopes can be divided into three types mainly according to their sedimentary characteristics:drop-depositional slopes,slip-depositional slopes and depositional slopes.

The drop-depositional slope was a slope where there deposited avalanche or ablation breccia and clast-flow brecciated limestone intercalated with high-density calcareous clast flow,grain flow and fluidized flow sediments and other transitional sediments,which are inverse graded and have various truncate structures.Fluidized flow or low-density turbidity current sediments towards the basin replaced them.Their source rocks were reef-hank framestone,bafflestone and bioclastic grain limestone from the platform margin.Such slopes are similar to the slope-basal apron and carbonate submarine fan proposed by Coniglio.They were chiefly in some sites of the Youjiang Late Permian back-arc basin and secondly in some sites of the Youjiang Early Permian passive epicontinental rift basin and in the Qinfang Early Permian passive epicontinental strike-slip basin.

The slump-depositional slope was a slope where sediments are characterized by olistostrome,including wrinkled marl and limestone,pseudobreccia-lithic limestone,high-density calcareous debris turbidite,normal-graded debris flow breccia limestone and low density calcareous debris turbidite.Such slopes can be compared to the slope apron of the rimmed platform proposed by Coniglio.They were mainly in the north side of the Youjiang Early Passive epicontinental rift basin,i.e.,in the Lower Permian of southern Guizhou.

The depositional slope was a slope where there were deposited high and low-density turbidite,lime mudstone,silt-clastic wackestone,pseudobreccia calcilith intercalated with tempestite,organic reef and contourite,which contain autochthonous fossils.Such slopes are similar to the slope of the open platform proposed by Conigilo.They were in some sites of the Youjiang Early Permian passive epicontinental rift basin.

Reefs and gravity flow deposited on these slopes are the most favorable for oil-gas storing in all Permian sediments in the study region.As to their storing capability,reefs and gravity flow sediments of HST is the first,those of TST the second and those of LST the third.

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