On the Frasnian-Famennian Mass Extinction Event in SouthChina
The Upper Devonian in South China are widelydistributed and well developed in a distinct pattern of"Basin-platform" or "Intrashelf trough-platform"alternate distrihition (Fig. 1). Six excerllent sections,representing the nearshore (inner shelf) argill.aceous-carbonate facies, the carbonate ptatform facies, theouter shelf margin or slope facies and the intrashelfbasin facies, have been studied for theFrasnian-Famennian extinction event, the Laojiangchongsection, the Guilin Deteroent Plant section, the Yangtisection, the Maanshan section, the Xiangtiaa section andthe Liujing section.
The Laojiangchong section is situated in Xikuangshanneaer the Lengshuijiang City of central Hunan, where theShetianqiao Stage and the Xikuanoshan Stage of UpperDevonian in China were estabt ished. The Frasnian-Famennian boundary beds in this section arewell exposed and developed with rich benthonic organisms,such as brachipods, corals and stromatoporids. They canbe subdivided into 3 lithological units (in descendingorder), the Tuzitang Formation, the ChanglongjieFormation and the Laojiangchong Formation. The TuzitangFormation, about 40 m in thickness, is composed of greyto dark grey thin-or medium-bedded argillaceouslimestones and marlstones with a distinct brachiopodafauna—the "Yunnanella" fauna. It is noted thatYunnnanellina is extinct in the lower part of theTuzitang Formation. The Changlongjie Formation, alsoabout 40 m in thiskness, consists of yellowish grey togrey silty or calcareous shares alternated with greythin-bedded argillaceous limestones, yielding a specialbrachiopoda fauna—the "Yunnanellina" fauna. Inthis section, Yunnanellina first occurs in the lowerpart of the Changlongjie Formation (about 4 m high abovethe base). The Laojiangchong Formation, about 25 m inthickness, is composed of grey to yellowish greythin-bedded to medium-bedded argillaceous and bioclasticlimestones intercalated with yellowish grey calcareous shales. Very rich benthonic Organisms bave been foundfrom this Formation. It is worth pointing out that inthe Lacjiangchong Formation, a bed of shelly Limestoneis developed at the top of this Formation, in whichsome "Frasnian" brachicpods such as Atrypa sp.,Spinatrypa sp. and Gypidula sp. have been discovered.Just beneath this shelly limestone, a bed of blackshales and siliceous rocks are developed, at the base ofwhich almost all Frasnian benthonic reef-buildingorganisms are suddenly died out (Fig.3).
The Guilin Detergent Plant section is exposed at asmall hill near the Guilin Detergent Plant of Guilin,representing a typical carbonate platform facies. TheFrasnian-Famennian boundary beds are traditionallysubdivided into 2 lithological units, the GuilinFormation below and the Rongxian Formation abecve. TheGuilin Formation is more than 100 m thick and composedof grey to dark grey thick-bedded micrites andbioclastic limestones with rich stromatoporids and rarebivalves and gastropods The Rongxian Formation ismore than 150 m in thickness and composed of grey tolight grey thick-bedded to massive dolomitic detrital Limestones with terribly rare fossils. Traditionally,the lithological, boundary between the Guilin Formationand the Rongxian Formation has been considered as theFrasnian-Famennian boundary for a long time in China.Few reliable evidence, however, had been got to prove itbefore, At present, some conodonts have been discoveredfrom the upper part of the Guilin Formation and thelower part of the Rongxian Formation. On the basis ofthe first appearance of Icriodus deformatus, theFrasnian-Famennian boundary can be defined at a levelwithin the uppermost part of the Guilin Formation (Fig.4)
The Yangti section is continuously exposed alongthe highway from Baoan to Yangti, about 40 Km scuth ofGuilin City, in which the transitional beds between theFrasnian and the Famennian can be subdivided into 3formations, the Wuzishan Formation, the XiangtianFormation and the Lazhutai Formation. The WuzishanFormation is characterized by a sequence of greythick-bedded nodular limestones, from which nomegafossils have been found, yet. The XiangtianFormation is a new lithological unit, about 20 m thickand composed of grey to dark grey argillaceous and detrital limestones, in the uppermost part of whichtentaculites become finally extinct. The LazbutaiFormation is about 30 m in thickess and composed of aseries of black, dark grey, dark brown siliceous rocksand skales with rick tentaculites. As showing in Fig.5,some important conodonts have been found from thetransitional beds between Frasnian and Famennian in theYangti section. Palmatolepis linguiformis first appearsin sample YT-33 in the upper part of the XiangtianFormation. Pa. triangularis first occurs in sample YT-37at the base of the Wuzishan Formation. Pa.del.delicatulafirst occurs in sample YT-39 in the lower part of theWuzishan Formation. It is clear that the Frasnian-Famennian boundary would be placed between sample YT-36and sample YT-37 (namely between the Xiangtian Formationand the Wuzishan Formation).
The Maanshan section lies about 7 Km south of theDale village, 4 Km northwest of the Zhongping village inXiangzhou county, Guangxi which was first studied byR.L.Xu in 1938. In this section, the Frasnian-Famennianboundary beds can be subdivided into 3 formations, theJuntian Formation, the Luoxiuhe Formation and the Rongxian Formation. The Juntian Formation consists ofgrey thick-bedded detritat limestones alternated withdark grey thin-bedded to medium-bedded micrites,yielding rich brachiopods Atrypa sp., Gypidula sp.,Cyrtospirifer sp. and rare tentaculites. The LuoxiuheFormation is newly established in this section, about 10m in thisckeness. It is mainly composed of dark grey togrey medium- to thick-bedded micrites and bioclasticlimestones. Rare tentaculites such as Homoctenus arefinally disappeared near the top of this Formation. TheRongxian Formation is composed of light grey to greythick-bedded detrital, argillacecus and dolomiticlimestones, in which no megafossils have been found. Atpresent, one extinction level have been recognized, theuppermost part of the linguiformis Zone. In addition,some Frasnian types of brachiopods found from the top ofthe Luoxiuhe Formation (the lower triangularis Zone) areconsidered to be reworked. As showing in Fig. 6, acontinuous conodont zonation can be easily made, and theFrasaian-Famennian boundary would be placed between Bed18 and Bed 19 near the top of the Luoxiuhe Formation inthe Maanshan section.
The Xianotian section is exposed on the side of arivulet near the Xiangtian village, about 2 Km west ofthe Luoxiu Town in the Xiangzhou county, Guangxi. TheFrasnian-Famennian boundary of this section was firststudied by Ji et al. (1986). In this section, theFrasnian-Famennian boundary beds can be subdivided into 3formations: the Wuzishan Formation, the XiangtianFormation and the Lazhutai Formation. The WuzishanFormation consists of a sequence of grey to light greynodular and bandded limestiones, from which nomegafossils have been found, yet. The XiangtianFormation is newly established in this section, about 10m in thickness, it comprises 5 lithological members. Themember lisdeveloped in the lower part of this formationand composed of dark grey thin-bedded micritesintercalated with dark brown or black thin-beddedsiliceous rocks (about 3.2 m in thickness). The membner2 is about 2.7 m in thickness and composed of greythick-bedded to massive breccia limestones, which isgenerally considered to be slumping or talus deposits ofslope facies. The member 3 is about 2 m in thicknessand composed of grey thick bedded bioclastic limestones with cherts and rare small brachiopods and corals. Themember 4 is about 2.2 m in thickness and composed ofdark grey massive micrites and dark grey thick-beddedmicritic limestones surrounded with black shales. Themember 5 is only 0.2 m in thickness and composed ofbrown, silty calcareous clay. The Lazhutal Formationconsists of a series of greyish brown to black siliceousrocks and shales. As showing in Fig.7, the conodontzones of the Frasnian-Famennian boundary beds in thissection can be completely recognized, and the Frasnian-Famennian boundary would be placed at the base of theWuzishan Formation, on the basis of the first appearanceof Palmatolepis triangularis.
The liujing section is one of the famous Devoniansections in China, and a tot of work on stratigraphy andsedimentology of this section have been done for a longtime. In this section, the Frasnian-Famennian boundarybeds can be subdivided into 3 formations, the RongxianFormation, the Luoxiuhe Formation and the Gubi Formation.The Rongxian Formation is composed of light greythick-bedded dolomitic, detrital limestones. TheLuoxiuhe Formation is about 22 m in thickness and comprises 4 lithological members. The member 1 isdeveloped in the lower part of this formation andcomposed of grey thick-bedded to massive breccialimestones with some brachiopods Hypothyridina sp.,Gypiduls sp. and Atrypa sp., corals Peneckiella sp.,Temnophyllum sp. and Pseudozaphrentis sp., and rarestromatoporids, which is generally interpreted as talusor colluvial deposits of slope facies. The member 2 isabout 2.5 m in thickness and composed of greythickhedded micrites and bioclastic limestones with rarebrachiopods. The member 3 is only 0.6 m in thickness andcomposed of dark grey thin-bedded micrites without anybentho: reef-building organisms. The member 4 isdeveloped in the upper part of this formation andcomposed of grey massive algae-reef limestones (or mudmound)with very rich algae Renalcis spp. and rare brachiopods.It is a pity that some strata just between the RongxianFormation and the Luoxiuhe Formation are not exposed inthis section. The Gubi Formation consists of a series ofdark grey thin-bedded or nodular limestones with rarebrachiopods. As showing in Fig.8, the Frasnian-Famennianboundary would be placed between Bed 5 and Bed 6 in the upper part of the Lucxiuhe Formation. Morever, it isalso noted that all Frasnian benthomic organismssuddenly disappear below Bed 5, maybe which isequivalent to the Upper Kellqasser Katk of West Germanyin stratigraphic level.
According to the data available in this paper, theFrasnian-Famennian mass extinction is certainlyrecognizable in South China. The following remarks areworth mentioning:
1. A main extinction lever near the Frasnian-Famennian boundary can be recognized in South China,but no lithological marks have been found for therecognization of the extinction event in South China. Itis in the upper part of the tinguiformis zone that mostbenthonic reef-buitding organisms (e. g. corals, bryozoaand stromatoporids), almost all elements of tentaculitessuddenty become extinct. Some Frasnian types ofbrachiopods (e. g.Pentameridae and Atrypidae) and a fewconodonts found in the Lower triangularis Zone aregenerally considered to be reworked or survivors (Fig.15).As a kind of organisms, conodonts were atso affected bythe Frasnian-Famennian extinction event inevitable. The conodonts near the Frasnian-Famennian boundary can begrouped into 5 types (Fig. 14), A. the "event speciality".B. the opportunists, C. the strong survivors, D. the weaksurvivors, and E. the extinct forms. The type A includesthose, which Lived only in the extinction beds of anextremely short dnration (e. g. Ancyrognathus ubiquitus).The type B contains those, which could continuefortunately through the Frasnian-Famennian extinctionhorizon as a :root species" of the Famennian radiation(e. g. Palmatolepis praetriangularis). The Type O includesthose, which seem to be of a strong vitality and not tobe or weakly be affected by the Frasnian-Famennian event(e.g. Icriodus alternatus and pelekysgnathus planus).The type D contains those, which were heavily affectedby the Frasnian-Famennian event and finally becomeextinct in the Lower triangularis Zone (e. g. somespecies of Mehlina and Branmehla). The type E containsthose, which were terribly affected by the Frasnian-Famennian event and become abruptly extinct in theuppermost part of the linguiformis Zone (e.g.all elementsof the manticolepid group).
2. In the offshore deep-warer shelf, slope and basin facies areas, large numbers of pelagic palmatolepidelements, together with tess polygnathid elements,dominate the linguiformis Zone, which can be defined asthe palmatolepid-polygnathid biofacies (Figs.9-13).
3. Anotable increase of Icriodus occurs in the Lowertriangularis Zone of the studied sections in South China(Figs.9-13). For exmaple, in the Laojiangchong sectionof central Hunan, the polygnathid-icriodid biofaciesoccurs in the Lower triangularis Zone directlypostdating the Frasnian-Famennian mass extinction, with13-50％Icriodus, 5-11％Polygnathus and 0-7％Palmatolepis(Fig.9). An obviously mixed biofacies, however, alwaysoccurs in the Lower triangularis Zone in the outer shelfmargin or slope locatities in South China, containing10-32％Palmatolepis and 30-50％Icriodus. Such a mixedassociation may have most possibly been caused by thetransport of Icriodus-dominated faunas from thenearshore shallow-water shelf inte offshore areas. Thisimplies perhaps current activities by tsunami or stormsgeneraled by strong physicat conditions by whichIcriodus-dominated conodont faunas (and other biota)might have been flushed or washed away from the nearshore shelf to slope or basin settings (Figs. 10-11).
4. It seems that a marked change in conodontbiofacies near the Frasnian-Famennian boundary iscoincided with the T-R Cycle F-d and with the Frasnian-Famennian mass extinction. It is suggested that asustained eustatic shallowing of sea Level takes placein the Frasnian-Famennian extinction horizon.
5. Four Frasnian T-R cycles are recognized in SouthChina, T-P, Cycle F-a (the MiddLe varcus Zone to An.triangularis Zone) T-R Cycle F-b (An. triangularis Zoneto the Upper gigas Zone), T-P, Cycle F-c (the Upper gigasZone to tinguiformis Zone), and T-R Cycle F-d(linguiformis Zone to the Lower triangularis Zone).These T-R Cycles, in turn, form a major Frasnian T-RCycle (Fig.17).
6. According to the data available in South China,the pattern of the Frasnian-Famennian extinction eventappears clearly developed as follows (in descendingorder):