| The Majiaoba fieldwork base is located on the edge of front orogen of the middle Longmen Mountain. From Devonian to Triassic, carbonate rocks are widely exposed in the region,except Permian. Devonian is divided into Guanwushan Formation, Shawozi Formation,Maoba Formation. The Guanwushan Formation and Feixianguan Formation of early Triassic were separated by fault. Carboniferous is divided into Majiaoba Fromation, Zongchanggou Formation and Huanglong Formation from bottom to top,in which Zongchanggou Formation and Huanglong Formation are carbonate depositions. The first section of early Triassic Feixianguan Formation and the middle Triassic Leikoupo Formation are still carbonate.In this paper, the researcher measured 4 sections,surveyed 108 geologic sopts, which controlled the whole strata. Meanwhile 4 wells datas and 1︰200,000 Regional Geological Survey Report ( Guangyuan map sheet ) were collected. On the base of field outcrops and slices,and through analysing the feature of facies,and controlling factors of carbonate strata are summarized. Main conclusions are the followings.1. In the Mjiaoba fieldwork base,the lithology of the carbonate rocks exposed can be divided into 4 kinds: grain–lime mud limestone,grain–lime mud dolomite, crystalline limestone,crystalline dolomite. The grain limestone and crystalline dolomite are more than crystalline limestone,grain–lime mud dolomite. The grains include spherulite,intraclast,oncolite and bioclast. Horizontal bedding and parallel bedding are the main sedimentary structures,and ripple and lenticular body are fewer. Sed??tary environment contains restricted platform,open platform,platform evaporite facies and carbonate gentle slope,which can be divided into 10 microfacies.2. The Majiaoba area in Devonian was located in semi-enclosed lagoon - the coastal environment,and its depositional environment is stable,mainly in the intertidal zone - the subtidal zone, partly in the lagoon. The evolution of the main depositional environment is mainly affected by the relative sea-level change,the tectonic movements of Micangshan– Dabashan and Chuanzhong ancient land. Minor environment is mainly affected by evaporation and tidal,leading a lithofacies combination of evaporation flat in subtidal and bioclast bank in open platform with little micrite within it.3. In the Carboniferous,Majiaoba area was located in semi-closed environment,and its depositional environment is stable. The sedimentary environment was controlled mained by the relative sea-level change,tectonic action and tidal action. Most of the carbonate is bioclastic limestone and the bottom of the Carboniferous strata is reef limestone,which are the feature of the whole strata. Oncolite limestone,ooid limestone and calcirudite deposited in few layers,and most of them are sediments of carbonate gentle slope and oper platform. Interbank limestone and supratidal limy dolomite,dolomite deposited in few layers.4. Triassic in eastern Sichuan was a stable platform-type construction. In the Early Triassic,Feixianguan period first transgression happened,with the sea water intrusion from east to west. The Yangtze sea was separated by Daba mountain and Longmen mountain,which lead the sea deep in the east while shallow in the west. Mud tide flat facies was developed in the researched area in the front of Longmen ancient land. Triassic strata are mainly controlled by relative sea-level change,wave action and tidal control,in which shoal deposition??n platform and restricted platform depositon are developed.5. At the beginning of the Triassic Leikoupo period, as uplift of the east side of the Chinese ancient land, the sea became shallow in the east while deep in the west. As the result the Upper Yangtze epicontinental sea transformed into restricted sea basin. In this period a set of gypsum-salt and dolomite strata deposited in the researched area. The depositions are controlled mainly by climate, relative sea level and tide actions. Dolomite facies forming in evaporation flat, dolomite and limy dolomite in lagoon are developed, and partly karst, limestone in tidal channel and bioclast shoal in the infra... |
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