Decis沙 on 嚓 Lochkovian A-W }j 一.rragian rDounciary Stratotype (Low er D evon ia n ) 妙 Ivo Chlup k and W illiam A . O liver, Jr. T he Global Stratotype Section and Point (G SSP) for the represent a p r o g r e s sio n from low to high energy environ- Lochkovian-Pragian Stage (m iddle Low er D evonian) has now m e n t s. C e rna ro kle h a s the best m egafauna w hile C ik5nka been agreed upon. The boundary is defined at the first Q uarry h a s t h e m o s t a b u n d a n t c o n o d o n t fauna. Velkg occurrence of the zone conodont Eognathodus sulcatus C h u c h le is inte rm e d ia te a nd w as s e le c te d as stratotype sulcatus at the base of bed 12 in the V elkfi C huchle Q uarry, b e c a u se of the good interm ixture of conodonts and m ega- 下丽万而rthwest part of Prague, Czechoslovakia. fo s s ils. R anges key species a t th e t hre e lo ca litie s a r e v e ry In tro du ctio n of sim ilar; a L each locality the 翌竺丝些 boundary is a sh o rt The Low er D evonian stages, from oldest to youngest, are d ist a n c e b e lo w t h e p reviously accepted m egafossil Lochkovian, P ragian, and Em sian. The base of the Loch- bounda四． ChlupA6 and others (1985) published e x t e n s iv e ko via n co inc ide s w ith the ba se o f th e L o w e r D evo n ian S eries range charts and lists of fossils from these a n d o t h e r and of the D evonian System and has been form ally defined boundary sections, together w ith discussions of the as the Silurian-D evonian boundary (M artinsson, ed., 1977). stratigraphic significance and usefulness of the m ajor groups The boundary betw een the Lochkovian and the Pragian is the of fossils of the Lochkovian-Pragian interval. subject of this paper. That betw een the Pragian and Em sian is undefined and is the subject of ongoing discussions w ithin T he boundary is closely related to an event-stratigraphy level of probable global im portance, nam ely the "Loch- the International Subcom m ission on D evonian Stratigraphy kovian-P ragian Boundary E vent," w hich, how ever, lies close (SD S), a unit of the International C om m ission on Stratig- above the Pragian base. T his event-boundary (m ost likely a raphy (ICS) of IU GS. The nam es of the Lower D evonian e u s ta t ic fall in sea level) m ay be useful i n identifying the Stages w ere adopted by SD S in 1983, recom m ended to IC S in w o rld w id e b ro a d e r boundary interval on ． scale (co m pare 1984, and ratified by IU G S in 1985 (Bassett, 1985). ，a In Ia Johnson, M apper and the interval before Cycle In． R egional Setting Sandberg 1985, and applications the stratotype area, in C hlupA6 and K ukal 1986, 1988). Paleom agnetic studies a r e T he boundary beds have been studied in detail at several in progress by D r. M . K rs in Prague. P relim inary results localities in the B arrandian area, southw est of Prague su gg est t hat tne ve tKa 七h uc hle sec tio n ha s b ee n (F ig. 1). Three localities w ere described in detail by both rem agnetized. T he color alteration index of VelkA C buchle W eddigQ.0 987) and ChlupA6, Lukeg and W eddige (1988). The conodonts is 3 (Sch6nlaub, w ritten com m unication, 1989). three, C ernA rokle, V elkA C huchle and C ikfinka Q uarry, 乙少户 Prior to the nam ing of the Pragian Stage in 1958 and its －‘甲－厂一厂N ～es 尸‘es 一一－厂－ 七产只代 了－又 acceptance by SD S in 1983, the "Siegenian Stage" w as used 芙 by m any w orkers for the m iddle stage o f the Low er 一 P R A H A D evonian. This w as based on benthic, near-shore fossils, p rincipally brachiopods, and w as sufficiently useful up to about 1950 w hen pelagic fossils such as conodonts began to V elk6 C hu chle predom inate in long-range correlations. W ith continuing I 伙a R B E R O U N 降 refinem ent of intercontinental correlations, it becam e clear 洲 马、 that the I'Siegenian Stage" could not be satisfactorily 冲创 defined in its type area (W est G erm any) and the shift to ｝1 P ragian resulted. R ecent studies in northern Spain, B rittany and W est G erm any show that the base of the Pragian is nearly correl- ative w ith the base of the "Siegenian" in the sense of C arls 。一}1_1 I (1987). O ne option w ould have been to select a G SSP for the R O K YC A N Y ｝！ I 称舒 "Siegenian" in a pelagic facies (e.g., C zechoslovakia), but the SD S decision to use the term "Pragian" reflects the feeling that the term "Siegenian" is too closely regarded in 邓 term s of a R henish, near-shore clastic, benthic facies, 1 0 2 0 k m ‘ 一一一」 w hereas "P ragian" signifies a diverse fossil assem blage that includes w idespread pelagic fossils. 1田11 2F刀 3区次｝注一墓 I I二口 6压 团 7国 。口 B oundary Stratotype (G SSP ) Figure 1 : Geographic and geologic setting of the stratotype section in the Barrandian area of cen tral T he position of the Lochkovian-Pragian boundary and the B oh em ia . 1 一 Pr o ter o zo ic , 2 - Cam b ria n , 3- O rd ov ic i an , G SSP for it w ere accepted by the International Subcom m is- 4- Silurian and Devonian, 5- m etamorphic Proterozoic sion on D evonian Stratigrap hy in A ugust-O ctober 1988, and and lower Paleozoic, 6- granitoids, 7- continental approved by the International C om m ission on Stratigraphy Upper Carboniferous, 8- 〔孕per Cretaceous and and ratified by the Executive C om m ittee of IU G S during Te rti a ry s ed im en ta ry r ock s . Januarv-F ebruary 1989. T he stratotype is to be m arked by E PISO D ES, Vol. 12, N o. 2, June 1989 1 0 9 a plaque w ith explanation, and a perm anent m etal line is to planes are knobby; the characteristic nodular structure is be inserted in the rock face at the boundary. M arking and developed in m icrites and biom icrites with Chondrites m aintenance w ill be done by the G eological Survey of burrow s that start 2 to 3 m above the stage boundary. C zechoslovakia, in collaboration w ith the C enter for Natural R eserves, also in Prague. B oundary L evel (base of Pragian) T he G SS P of the sulcatus boundary is in southw estern T he stage boundary w as chosen by SD S in 1988 to coincide Prague, in the secti而不r e_IkA C huchle (Fig. 1). This is an w ith the low er boundary of the Eognathodus sulcatus cono- old quarry 。。 ihp rnqri from Praha-VeIkA C huchle to dont Zone defined by the first occurrence of 互.一s ulca生us S liv e n e c o n V dolich Street V fdoli), 8 km southw est of the sulca生矍 P hilip, 1965. In som e discussions this has been city center. T he quarry is on the nature reserve called referred to as the sulcatus boundary. The E . sulcatus Z one "H om olka," in the district of Prague 5; the distance from a w as fi rst defined by Fahraeus, 1971, and has since been city bus term inus is about 400 m (Figs. 2 and 3). In the accepted as part of the standard conodont zonal sequence (K Iapper and Johnson, 1982). T he nam e taxon and zone are VeIk9 C huchle section, the stage boundary is defined as the base of B ed 12, at the first occurrence of E. suicatus now recognized in C zechoslovakia, G erm any, A ustria, su lc a tu s. C hina, A ustralia, U .S.A . (N evada, A laska) and C anada (see W eddige, 1987, p. 484-5, for details). A lthough nearly w orld-w ide in distribution, it has not yet been recorded or 一‘} M C I6 ／“ recognized in the Soviet U nion or eastern N o rth A m erica. ／下，‘，Ch些}I T hese are m ajor geograp hic gaps, but it seem s likely that 匕二，’ the form er, at least, w ill be filled. M axim um provincialism ＼ and inappropriate lithofacies in eastern N orth A m erica 、 爪 个 小个 小‘｛ m ake it less likely that the zone w ill be recognized in this 。 ＿＿． ）， area, but other criteria perm it approxim ate correlations.