GEOLOGICA BALCANICA, 24. 6, Sofia, Decem. 1994, p. 23-30 Numerical criteria of precise delimitation of the calpionellid Crassicollaria and Calpionella Zones in relation to the Jurassic/Cretaceous system boundary Iskra Lakova Geological !nstitttfl', Bulaarian Acadl'lll!f of Sciences, 1113 Sofia (Received 12 . 06. 1994; accrpted 21. 09. 1994) /1 . JlaKooa- Ko .w~ecmoeHHble Kpumepuu OA R 1110</HOZO onpeiJe.11'HUfl zpaHU t(bt Ate.?JCiJy Ka Ab nuoHe AAUO· HbtAtu 30IWAW Cra ssicollaria u Calpionella n C!lfi'JU c zpaHtt l( eii Ate:JKO!f 10 po t1 tt .III'AO-It. 8 CTaTbe npellCTaB­ neHbl CTaTH CTii'leCKHe .'laHHble 0 sepn!KaJJbHOM pacnpocrpaHelfloflf ll OTIIOC liTeJJbHOH 4<1CTOTe KaJibllHO· HeJJJJHil B .llBYX\feTpOBO~I HHTepBaJJe B fJJO)f(eHCKOH CBHTe B 3ana)lHbiX 5aJJKilHII.ll3X Ha rpaHMUe Me)f(.ll.Y 30HaMif Crassicollaria If Calpionella . n epeOIOTpeHbl lf3BeCTHble JlO CIIX nop KpMrepHH llJI!l npoBe.D.CHII!l Hlf)f(HeH rpaHMUbl 30 Hbl Ca/pionella. npeJtJJaraeTC!l onpeJleJJHTb 3TY rpaliHUbl Ha OCHOBaHHH MOpQJOJJO· rH4ecKoro M3~teBeHHH Calpionella alpina, 3KcnJJ03HH ccjlepH4ecKoi-i, 6oJtee MeJIKoif pa3IIOBH.'I.HOCTH 3TO· fO BH,!la, If yna,!lKe Y.ll•1HHeiiHOH, 6onee Kp y nHOii pa3HOBII.'I.HOCTH, KaK H Ha HC'Ie3HOBaHHH fOMeOMOpcpa Calpionella elli plica. noTsep)f(.D.aercH. 'ITO Crassicollaria brevis 11 Crassicollaria massutiniana TO)f(e HC- 4e3 a!OT npH 3TOi't rpaHIIUe HJIH He~IH OrO HH)f(e ee, HO ,.3KCHJI03HH" Bll,!la Ca/pione/la a[pina B UeJJOM B rp aHH4HOM HHTepsaJJe Be Ha6JJIOJiaeTC!l. Abstract. In thi s paper. detailed stati stic d ata on the calpionellid occurrence and relative abun­ dance are presented being obtained whitin a 2 m thick interval o f the Glozhene Formation , Western Balkanides. at the boundary between Cra ssicollaria and Calpionella Zones. Existing cri teri a of defi­ ni lion the lower boundary o f Calpionella Zone have been revi se d. It has bee n proposed to deter mine this boundar y on the base of the morphol ogical transition in Calpionella alpina, the explosion of the medium-sized variety and decline of lagre elongated variety o f this species, and the disappearance of homeomorph Calpionella elliplica. It has been confirmed that Crassicollaria brevis and Crassicollaria massutiniana have their last occurrences at or a little lower the boundary but the "explosion" of the species Calpionella alpina hns not bePn corroborated lit this l evel. Introduction In two sections of the Upper Jurassic- Lower Cretaceous carbonate succession of nodular and micritic pelagic limestones (Ginci and Glozhene Formations) in the Wes­ tern Balkanides, Bulgaria, a calpionellid zonation is recently established. Among others, the standard calpionellid Crassicollaria and Calpionella Zones have been pro­ ved in the sections of Barlj a and Komshtica (La k ova, 1993) . Commonly, the boun­ dary between these two zones is defined by the so-called "explosion" of Calpionella alpina and the morphologic <l l transition from th e large somewhat elongated form to the medium-sized spherical form of this species, CJS well <ls by the disappearance of most species of genus Crassicollaria (R e 111 a n c, 1968; R c m a n e ct al.. 1968; B o r­ z a, 1984; Pop, 1986). 23 The interest in detailed study <1nd reli<1hlc exact definition is c<1u scd by thE' in­ creasing use of the Crassicollaria! Calpionclla (i. e. A/ B) boundary as a limit point beh\·een the Jurass ic and Cret<lceous, bec<1use it coincides approximately with the base of combined Berriasella jacobi - - Pseudosubp!anites grandis ammonite Zone. which itself is the main candidate for the .Jurass ic:' Cretacous boundary following the decision of the Lyon-Neuchatel Colloque in 1973. Although the classical Tithoni an - Berriasian st<tge bound<try is still defined beh\een the B . jacobi <l!ld P. grandis Zones. i.e. within the Calpionella Zon e, in most present-d<Jy publications the JmCJss iciCrct<lcous boun­ d<try has been placed between Crassim!laria and Calpionella (A ;mel B) Zones (C h an­ ne 11 et al.. 1987); C e c c a et al.. 1089: L e He gar ;1 t. Fer r v, 1990; 0 n cl r e- j i c k ova et al.. 1993). - In Bulgaria, the Tithonian - Berriasian bouncl ary is placed at the boundary between Paraulacos'phinctes transitorius ( Malbosiccras chaperi Subzo ne) and Pseudos­ ubplanitcs grandis ammonite Zones (Sap uno v, 1977; N i k o I o v. 1982), the Bul­ garian P. transitorius Zon e being equivalent to the total of P. transilnrius and BNria­ sella (Bcrriascla) jacobi Zones from Southeastern France. It is also important to est<tb li sh preci se and proper numericcll equiv<l lents to the widely used criteria of defi nion the Crassico!lariai Calpionella boundary, such as "mor­ phological transition" and "explosion" of C. alpina, <IS \vel! as to check if these changes <1re simultaneous to the dec! ine of genus Crassicollaria. Statistic app roach in determi­ n<ttion of rel<ltive abundance of genera and species is highly recommended by R em a­ n e (1985, p. 564) especially concerning the boundaries of Ca!pionella Zone. EXPLATIONS TO THE PLATES PLATE I The thin-section are ~ tor ed in the colleclions of Department of P aleontol ogy and Slr~ligr:1phy, Geo­ logical Institute, Sofia. All specimens fr om Barlja section, Glozhene Formation, West Bulg11ria, X 400. I, 2. Tinlinnopsella carpalhica (M urg e an u & F i I i p es c u); I - Crassirollaria Zone, sam­ ple 310; 2 - Calpionel/a Zone, sa mple 3 11 , 3, 4 . Crassicollaria mnssutiniana (Co I o m). Crassicollarin Zonr ; 3- sample 309-G; 4- sample 309-8. 5, 6. Crassicollaria brevis R e man r, Crnssicollariu Zone; 5- S(t mple :l09; G- sa mple 3 10 -4. 7, 8. Crassicolluria parvula Rl'manC', Crassicollaria Zone; 7 - Si!mp le 30!1 -5; 8- s:1m pl e 309-8. 9, I 0, II . Calpionella alp ina L o r e n z (I arge variety), Crassicollaria Zone; 9 - s:1mplr 309-6; 10 - sample 309-8; 11- sample 310- ! . 12, 16 . homeomorph Calpionella elliplico Cadi s c h. Crassicolloria Zone; 12 - ·sample' 3 10 ; lG -sample 310-4. 13, 14, 15. Calpione/la alpina Lorenz (medium-sized variety); 13- Crossicollarin Zone, sam­ ple 309-3; 14, 15- Calpionella Zone, sa mple 310-6. PLATE II Light-grey micritic limestones of the Glozhene Formation in the section of Barljn, Western Dal­ kani des, at the Crassico llaria/ Calpionel/a boundarv interval. I. General view of the 2m thick studied interval (sa mples 309 to 3 11) . 2 . The place of Crassicollaria/ Culpionellu boundary between samples 310-4 and 310-5. 24 PLATE I 1 2 Geologica Balcan ica , 24. 6 - I. L a k o v a Numerical criteria of prec ise .. PLATE II 1 2 ThE' ri ch calpionellicl fl~so ciations r ~ ti-l b li ~ h e d in the Glozhene Formation "t Bar­ lja :-:l'd ion JIIO\\'(XI to carr y out a dct:lil ed ~: tud y of" 2 11 1 thick critica l interv al across the Crassic(}l/aria.'Calpinnclla bound a r~· behn'cn s<~ mpl es 309 and 31 I (L a k o ,. "· 1993) . This interval has bee n s;J mpl ed ~ 1t c;;ch 0. 1 m ::ncl counts haye bee n m;Jde in <11l thin-section to determine the relat ive abund~111c e of th e c:ilpionel licl specie's and vari c­ t ie ~. Previous definitions of A/B boundary Diffrrent criteria wcr<.' propose d to Mfinr the Crassicnl/aria!Calpionclla zonal boun­ dary. P <1rt of them me easy to underst and nnd npp ly in any kind of biostr ;-t tigr<lphic study, for <'X<1mplc "the Calpionella Zon e is characterized by the predominance of ge­ nus Calpioncila" (A I I em a nn et ;i\ .. 197!) . Quantit<:tively. it mc<ms that the Cal­ pionella Zone represents body of strata in \\·hich the number of <; pecimrns of ;-til Cal­ pionclla species is over 50 1)iJ of the tot:il fmmas. The widely accepted standard. however. to fix the bilse of the Calpionclla 7.one is the morphological tr<msition and "cxplcsion" of C. alpina (Borza. 198-l. R e m <1 n e. 1986; R em n ne, ct al., 1986). C e c c a et <1 !. (1989) called attrntion to the sudden increase in Calpionclla alpina and Crossicollaria parvula and the l <1s t occurrence of Crassicollnria intermedin <md Crassicollaria brevis at the Ai B bound <1 rv in SE Fr<1nce. On d r c j i c k ova et ;:J. (1993) in Wes trrn Carpathians <1 nd L e He g-:1 rat. F r r r y (1990) in SE France ch <~ ract e rized the c:ilpi onellid <~ssoc i a tions of the Crnssi­ collaria <1nd Calpionflla Zones but did not di scu ss thr cri teri a of definition the bounclary bet\\'ee n them. A common recent pr:Ktice in determination the A/B bounclary is to assert the morphological transi tion <1 nd ex plosion of C. nlpina \dthout <1ny proof or illustration. In some c <~ s cs. additional criteria. such <1s di sappNiranrE' of C. brevis, were usrd to define the A/ B bound arv (C h ann c I I C't <il., 1987; C h <1 n n r l I, G r a n d e s s o, 1987).