8.9.1 IntroductionEleven third-order sequences were already recognized in Permian strata of the Sichuan-Yunnan-Guizhou-Guangxi region（Chen Hongde et al"s another paper in the issue）.We have noted that the same sequence indifferent areas are synchronous,but its characteristics are varying from area to area.There are many factors to influence sequences.This book attempts to interpret this difference in terms of basin type and then establish the development models in different types of basins and the sedimentary models of different ages throughout the study region.Finally,we make correlation of changes of sea level in the study region with global changes of sea level.8.9.2 Types of BasinsAccording to analyses of 11 sequences and their system tracts and considering other geological data,especially data of tectonic setting,we recognized 5 types of Permian basins in the study region:cratonic basin,foreland basin,passive epicontinental,rift basin,passive epicontinental strike-slip basin and back-arc rift basin.8.9.2.1 Cratonic basinsCratonic basins are in fact a large-area settling platforms or gentle slope.A typical cratonic basin in the study region was the Upper Yangtze cratonic basin.This basin covered central and southeastern Sichuan and northern Guizhou.It showed three environments in different times of Permian.It appeared as the Upper Yangtze carbonate platform in the Maokou Age,as the Upper Yangtze terrigenous clast-carbonate mixed platform in the Qixia Age,and as the Upper Yangtze carbonate gentle slope in Late Permian.The Upper Yangtze carbonate gentle slope was determined on the basis of Sequences 1,2 and 3;the Upper Yangtze carbonate platform was determined on the basis of Sequences 4,5 and 6;and the Upper Yangtze mixed platform was determined on the basis of Sequences 7,8,9,10 and 11.Carbonate platforms and mixed platforms have characteristics below：①sequences are regressive ones consisting of weakly retrograding,aggrading and weakly prograding parasequence sets,show obviously transverse changes,and usually have top unconformity；②depositional agents are mainly tide,wave and gravity flow;and③there are organic mound-reef-bank complexes which were formed by keeping up-catching-up type aggrading,prograding or mix-grading and appeared in various system tracts.Carbonate slopes have characteristics below：①sequences are progressive ones consisting of strongly prograding or strongly retrograding sedimentary wedges,and show no obviously transverse changes,and usually have a lot of terrigenous clast sediments in their lower parts；②depositional agents are mainly wave,storm wave and storm flow;and③there are deep water catching-up reef-mound complexes.8.9.2.2 Foreland basinsA typical foreland basin appeared in the Shiwan Dashan Mountains area of Guangxi Province in Late Permian.It was determined mainly on the basis of Sequences of 7,8,9,10 and 11.In such basins LST sediments are relatively poorer developed and TST and HST sediments are prosperous.The former has a weakly-prograding-weakly-retrograding parasequence set consisting of turbidite fan facies and fan-delta facies;but the latter has a strongly-prograding parasequence set consisting of fan-delta facies and alluvial fan-plain facies.In addition,TST and HST sediments are coarse in grain size;large in thickness,and not easily identified and correlated,sea level showing a high amplitude and low frequency change.8.9.2.3 Passive epicontinental rift basinsA typical passive epicontinental rift basin appeared in the Youjiang area of Guangxi in Early Permian.It was determined on the basis of Sequences 1,2,3,4,5 and 6.Such basins had an（isolated）platform alternating with（platform）basin framework.In isolated platforms,water was relatively shallower;sequences was formed by catching-up and keeping-up deposition,showing characteristics of retrograding,aggrading and prograding parasequence sets.Platform basins were compensative,in which water was shallower and sequences were formed by calcareous debris-gravity flow,showing characteristics of aggrading-prograding parasequence sets.8.9.2.4 Passive epicontinental strike-slip basinsA typical passive epicontinental strike-slip basin appeared in the Qinfang area of Guangxi in Early Permian.It was determined on the basis of Sequences 1,2,3,4,5 and 6.Such basins were controlled by strike-slip faults.They were usually poor-compensative trough-like deep water ones.Sequences filling in them are made chiefly of silicolites and are not easy to be identified but easy to be correlated.Such sequences may indicate global changes of sea level characterized by high-amplitude and low frequency.A typical back-arc rift basin appeared in the Youjiang area in Late Permian.It was determined on the basis of Sequences 7,8,9,10 and 11.Such basins had a（platform）basin-surrounding-（isolated）platform framework.Platform basins were usually non-compensative deep water ones,which were in brooding and sequences filling in them,were made of turbites and silicolites.Isolated platforms in which water was shallower were in contraction;prograding,filling-up and spilling-up,showing characteristics of aggrading-prograding parasequence sets formed sequences filling in them.It is the five types of basins that essentially decided the main characteristics of individual sequences and even individual system tracts.As to details of some sequences or system tracts,special intrabasinal environments or outbasinal conditions may interpret them.8.9.3 Regional Sedimentary ModelsRegional sedimentary models can be set up according to the distribution of the five types of basins and their sequences and system tracts in the study region.Figs.8.36,8.37and 8.38 are three regional sedimentary models we set up.From the three models we can read out the Permian sedimentary evolution of the study region.Again,these models help predicting areas favorable for generating,storing and covering of oil-gas.The Caledonian Movement made the Yangtze Landmass and the Cathaysian Landmass have been jointed the South China Plate to a large extent.From Devonian on,with opening of Paleotethys,passive continental margins appeared in the south,west and north sides of the South China Plate,and thus the Upper Yangtze area and adjacent areas were in a tensile tectonic stress state throughout the Hercynian Movement.It is in the stress state that Permian sedimentary basins in the Sichuan-Yunnan-Guizhou-Guangxi formed.These basins had obvious inheritance in the Late Paleozoic depositional history,but owing to their locations and peripheral environments as well as synsedimentary faulting and the Dongwu Movement at the end of Early Permian,the sedimentary pictures of the study region were varying both from stage to stage and from basin to basin to some extent,distinct from their late sedimentation.The west part of the study region,i.e.,areas east of the Sichuan-Yunnan Old Land,changed from a carbonate platform through a mixed platform to a carbonate gentle slope and received very thick shallow marine carbonate sediments.The southeast part of the study region changed from a strike-slip basin to a foreland basin,owing to its special tectonic location,and the sediments showed a certain complexity.The vast area between both changed from a passive epicontinental rift basin which had a platform alternating with basin framework to back-arc rift basin which had a basin surrounding platform framework,and the sediments showed considerable diversity.8.9.4 Global Correlatability of Sea Level ChangesThe changes of sea level are also an important factor to influence sequences.The eleven third-order sequences identified in Permian strata of the study region represent 11 times of third-order sea level cycles.The curve of sea level cycles drawn by means of the 11 sequences had a certain correlatability with Vail et al.（1977）and Ross curve（Fig.8.39）.Vail et al.regarded the Leonardian and Guadalupian of Lower Permian in Europe and North America as the lower progressive sequence of a second-order sea level cycle and the Upper Permian there as the upper regressive sequence of this cycle.The Leonardian and Guadalupian in Europe and North America correspond to Sequences 1,2,3,4,5 and 6 in the study region,i.e.the Qixia Formation and the Maokou Formation of Lower Permian.Ross（1988）regarded the whole Permian as the upper regressive sequence of the second second-order sea level cycle of Upper Paleozoic.According to Zeng Yunfu et al.（1993）,the Lower Permian in the Youjiang compound basin corresponds to the upper regressive sequence of the Hercynian sea level cycle and the upper Permian does to the lower progressive sequence of the Indo-Chinese sea level cycle.Considering that the study region and even southern China are lack of more or less of Devonian and Carboniferous strata and that the top unconformity of the Maokou Formation is widespread,we consider the Lower Permian to be a relatively independent second-order sequence,in which the Qixia Formation corresponds to TST sediments,and most of the Maokou Formation does to HST sediments,and the bottom of the Maokou Formation does to CS sediments.The Qixia Formation and the Maokou Formation in the study region and even southern China have 6 third-order sequences,and compared with most of Europe and North America,and they are short of one sequence or two sequences.This is because the bottom of the Qixia Formation is short of one sequence and the top of the Maokou Formation suffered denudation.In the Youjiang basin area where Carboniferous and Permian are continuous deposition.However,the bottom of the Qixia Formation has a sequence corresponding to that absent sequence,but we incorporated it into Sequence I in order to correlate throughout the study region.Fig.8.36 Regional sedimentary model of the Qixia Age of Permian in the study regionFig.8.37 Regional sedimentary model of Maokou Age of Permian in the study regionFig.8.38 Regional sedimentary model of Late Permian in the study regionOn the other hand,the curve of sea level change for the Upper Permian of the study region has considerable differences from Vail et al."s curve and Ross curve.The upper Permian of the study region represents the early progressive stage of a second-order sea level cycle occurred during Late Permian and Early Triassic and is a progressive Carbonate sedimentary sequence,which is similar those in Alps,Outer Caucasus,Iran,northern Vietnam,Japan and so on.The second-order progressive sequence corresponds to 4 third-order sequences identified by Dennison（1984）in the Upper Permian of Tethys,of which the two sequences of the Kazanian correspond to the three sequences of the Wujiaping Formation in the study region,i.e.,Sequences 7,8 and 9,and the two sequences of the Tatarian correspond to the two sequences of the Changxing,i.e.,Sequences 10 and 11.Obviously,the Upper Permian of the study region is typical in Tethys.Vail et al."s curve and Ross" curve are too simple in describing medium-short cycles because data they used were chiefly from North America,West Europe,Russia and the Gondwana Land whose Late Permian sedimentary records are not typical:either dominated by continental facies in some areas or lack of some or all of the strata corresponding to the Changxing Formation.Yin Hongfu et al.（1994）thought that these differences may be due to that the Cimmerides separating Paleotethys from Mesotethys fast moved towards Eurasia continent and that Paleotethys closed from east to west.It follows that changes of sea level reflected by the sequences of Late Permian in the study region are typical in the world.Therefore we recommend that the segment of Late Permian in the curve of global sea level cycles of Phanerozoic Eon should be corrected using the curve for the Upper Permian in the Sichuan-Yunnan－Guizhou-Guangxi region.Fig.8.39 Changes of sea level in the study region and global changes of sea level during Permian
