四川龙门山甘溪泥盆纪生态地层、层序地层与海平面变化

DEVONIAN ECOSTRATIGRAPHY,SEQUENCE STRATIGRAPHY AND SEA-LEVEL CHANGES IN GANXI,LONGMEN MOUNTAIN AREA,SICHUAN

关键词:  龙门山  泥盆纪  生态群落  生物事件  沉积层序  海平面变化

Key words:  Longmen Mountain  Devonian  ecological community  biological event  depositional sequence  sea-level change

摘要:

龙门山甘溪剖面是我国泥盆系重要典型剖面之一,倍受中外同行关注。本文着重对生态地层、事件地层、层序地层进行研究,为研究全球海平面变化提供区域背景资料。龙门山甘溪剖面含十分丰富的底栖生物化石,从洛赫柯夫阶-弗拉斯阶自下而上可识别出24个腕足动物群落,另外还建立了若干礁复体群落和浮游群落
本文对以上群落的特征、性质、分异度、成分、底栖组合及其与沉积环境的关系作了分析和讨论,并识别了11个海进海退事件(生物的或非生物的)
本区泥盆系是加里东构造旋回后的第一个沉积盖层,属海平面主体上升和海侵同步条件下的旋回超覆地层,由砂质海岸环境向碳酸盐台地环境推进,构成了区内泥盆纪沉积层序序列的组合特征。根据海平面的变化及其相旋回的变迁,划分出6个三级沉积层序。6个沉积层序代表6次海平面的相对升降周期,大致相当于Vail,P.R.(1977)划分的三级地层旋回的海平面变化周期(延续时限1-12Ma),包括4个较大的海平面上升周期与2个较大的海平面下降周期,即洛赫柯夫期、布拉格晚期至埃姆斯早期,吉维中期及弗拉斯早、中期的海平面上升期与艾费尔期中晚期至法门期的海平面下隆期。
上述沉积层序特征和海平面变化,说明区内泥盆纪台缘断陷盆地的形成经历了由陆向海转化和盆地发展演化过程,反映了上扬子地台西缘陆架沉积发展的历史大致可划分为:盆地的雏形阶段(碎屑岩陆架的形成阶段),盆地的发展阶段(碎屑岩与碳酸盐岩混积陆架的形成阶段)和盆地形成与消亡不同性质的三个阶段。它们的形成与演化主要是构造断陷活动和龙门山海水不断向东侵进、古特提斯海北支向东扩展的结果,展示了由滨岸陆架转变为碳酸盐台地,由陆源碎屑充填转化为碳酸盐岩沉积的发展史。

Abstract:

The Ganxi section in the Longmen Mountain area, Sichuan is a key Devonian section in China. The study is intended to examine the ecostratigraphy, sequence stratigraphy and event stratigraphy for the purpose of delineating the sea-level changes in the study area during Devonian time and providing the regional background data for the study of the global sea-level changes.
The Ganxi section abounds in benthic fossils which may be categorized into twenty-four brachiopod communities in the ascending order from the Lochkovian to Frasnian strata. They are:1. Lingula community, 2. Strophochonetes-Howellella Community, and 3. Orientospirifer-Howellella Community in the Lochkovian-Pragian strata, 4. Strophochonetes-Orientospirifer Community, 5. Howittia-Athyrisina Community, 6. Rostrospirifer,r-Athyrisina Community, 7. Dicoelostrophia-Rostrospirifer Community, 8. Euryspirifer Community, 9. Otospirtifer Xiejiawanensis Community, 10. "Vagrania" -Parachonetes Community, 11. Mesodouvillina-Megastrophia Community, 12. "Otospirifer" -Athyisina Community, 13. Otospirifer-Lttanquenlla Community, 14. Dexquamatia-Chuanostrophia Community, and 15. Neocoelia Community in the Emsian strata; 16.Zdimir Community and 17. Atrypids-Schizophoria Community in the Eifelian strata:18. Stringocephahts Community, 19. Independatrvpa-Uncintthts Community, and 20. Leiorhynchus-Emantlella Community in the Giuetian strata; 21. Zhonghuacoelia-Leiorhuvynchus Community, 22, Leiorhynchus Community, 23. Schizophoria-Gvpidula-,Auypa Community, and 24. Cyrtospirifer Community in the Frasnian strata.
In addition, a number of compound reefal com.munities and planktonic communities have been established as well.
The characteristics, diversities, compositions, benthic associations and their bearings on sedimentary environments are discussed in the present paper. The brachiopod fossils were less developed, with a very low diversity and benthic associations of BAI and BA2 during the Lochkovian stage. Till the Emsian stage, the brachiopods flourished unprecedentedly, with a diversity ranging between 15 and 16, and benthic associations of BA2 and BA3. Marked by the Zdimir Community, the culmination of diversity initiated in the latest Emsian stage might be lasted to the earliest Eifelian stage. The rapid drop of diversity in the middle and late Eifelian stages marked the beginning of the regression after the Emsian stage. There was another culmination of diversily in the middle Givetian. The benthic associations varied between BA2 and BA3, as indicated by the occurrence of the Stringocephahts Community and the hldependatrypa-Ememtella fauna. Therc was a gradation from the benthic associations BA3 to BA4 in the late Frasian. indicating the effect of the world-wide transgression on the study area. The Famennian stage is characterized by the remarkable drop of diversity, the prevalence of the benthic association BA2 and the appearance of lagoon and oolitic shoal, facies indicative of the beginning of the large-scale regression.
On the basis of the community chat'acteristics and diversities, benthic associations, sedimenlary environments and conodont zones, eleven transgression-regression events (biogcnetic or non-biogenetic) are separated in the study area, including:1. the Silurian/Devonian boundary event, 2, the Gangou event (tentatively), 3. the basal transgressive event in the Bailiuping Formation, 4. the sea-level rise event in the dehiscens zone (IB), 5. the sea-level rise event in the perbomts zone, 6. the regression event in the lower serotinus zonc (the uppermost of IB), 7. the sea-level rise event at the Middle-Lower Devonian boundary, 8. the late Eifelian sea-level fall event, 9. the early-middle Givetian sea-level rise event, 10. the early Frasnian sea-level rise event, and 11. the Frasnian/Famennian extinction event.
The petrological and biological indicators and time duration for the events outlined above are herein "described and compared with those in South China. The emphasis is on the effect of the Frasnian/Famennian extinction event on the study area. For instance, a great variety of fossils are identified in the late Middle Devonian (Givetian) and the early Late Devonian (Frasnlian) strata of the Ganxi section, including ten famil:ies and twerlty-sik genera of rugose corals, ten families and twenty genera of Stromatopora, and thirteen families and twenty-five genera of bfachiopodsl However, the sea water began to invade this area during the Middle Devonian (Givetian), and then gradually retreated during the Frasnian stage (in the middle part of the Tuqiaozi Formation corresponding roughly to the upper part of the asymmetricus zone). The further regression in the Anc. trangularis zone (i.e. the uppermost of the Tuqiaozi Formation) and the variations in sedimentary environments caused the common genera and species of brachiopods (about 56%) to be extinguished successively. Afterwards, the regression was maintained, and frequent rise and fall of sea level occurred. The-more enclosed and restricted circumstances, and higher salinity of sea water resulted in the severe disruption of the marine ecological systems and the further extinction of organisms on a large scale. Till the late Frasnian (the upper part of the gigas zone, i.e. the bottom Of the Shawozi Formation), large quantities of rugose corals and stromatopora rapidly died and were even extinguished, with an extinction rate of 70% (seven families) and 80% (twenty-one genera) for rugose corals; 60% (six families) and 70% (fourteen genera) for Stromatopora. Except minor amount of brachiopods, rugose corals and Stromatopota. most of the macrofossils were almost extinguished at the end of the Frasnian stage (i.e. the middle part of the Shawozi Formation, corresponding to the uppermost of the gigas zone). The organic reefs which were once flourishing from the Middle Devonian onwards disappeared without exception. It was till the Late Famennian stage (corresponding to the costatus zone) when the corals and Stromatopora became thriving again.
The Devonian strata in the study area as the sedimentary cover following the Caledonian tectonic cycles are attributed to the progradation of the sandy coasts towards, the carbonate platforms during the overall rise of sea level and transgression. Six third-order depositioal sequences have been divided according to sea-level and facies cycle changes.
The first depositional sequence (BI-B30) with a time interval>12.2 Ma covers the Lochkovian strata and part of the Pragian terrigenous elastic strata (Pingyipu Group), consisting of the transgressive systems tract and highstand systems tract onlapped upon the pre-Devonian strata.
The second depositional sequence (B31-B88),with a time interval <10.3 Ma and type 2 unconformity boundary, represents the depositional cycles from the Pragian retrogradational sediments to the middle and late Erosion progradational sediments corresponding to the Bailiuping, Ganxi, Xiejiawan and Ertaizi Formations and the middle part of the Yangmaba Formation. The sequence comprises a series of mixed terrigenous clastic and carbonate sediments consisting of the shelf-margin systems tract, transgressive systems tract and highstand systems tract.
The third depositional sequence with a time interval of about 5.6 Ma is arranged into the late Emsian (B89-B92) and early Givetian sediments corresponding to the upper part of the Yangmaba Formation and Jinbaoshi Formation. The sediments in this sequence consist of mixed terrigenous elastic and carbonate sediments including the transgressive systems tract and highstand systems tract onlapped immediately upon the basal unconformity boundary.
The fourth depositional sequence (B116-B127), with a time interval of about 3 Ma and corresponding to the Guanwushan Formation, is assigned to the middle and late Givetian and early Frasnian sediments consisting of the transg ressive systems tract and highstand systems tract overlain immediately upon the basal unconformity boundary. The sequence was replaced later by the marine carbonate rocks which serve as good sites of reef development due to the decrease in the terrigenous clasts during the rapidly rising of sea level.
The fifth depositional sequence (B128-B163), with a time interval of about 10 Ma and corresponding to the Tuqiaozi Formation and the lower part of the Shawozi Formation, represents the Frasnian sediments assembled dominantly by muddy micritic limestones. bioclastic micritic limestones and calcirudite in the basin-margin, platform foreslope and lagoon facies. The transgressive systems tract and highstand systems tract are noted in this sequence.
The sixth depositional sequence (B164-B186) represents the Famennian carbonate sediments with a time interval of about 4.5 Ma and corresponding to the upper part of the Shawozi Formation and Changtanzi Formation. The sequence is made up of the grey and light grey bioclastic micritic limestones, ooidal and pelletal micritic or spar bioclastic limestones and fine crystalline dolostones. The transgressive systems tract and highstand system tract may be observed in this sequence.
The preceding depositional sequences approximately correspond to the third-order stratigraphic cycles and sea-level cycles by Vail (1977) with a time duration ranging between 1 and 12 Ma, of which four major rising stages of sea levek i.e, the Lochkovian, late Pragian-early Emsian, middle Givetian, and early-middle Frasnian, and two major falling stages of sea levek i.e. the Eifelian and middle-late Frasnian to Famennian have been distinguished.
Three stages are also suggested in this paper for the formation and evolution of the passive marginal fault basins on the western margin of the Upper Yangtze platform, including the basin initiation stage (formation of the siliciclastic shelf), basin development (formation of the mixed siliciclastic and carbonate shelf, and basin formation and termination.