Acta Geologica Polonica, Vol. 48 (1998), No.4, pp. 495-507
The Turonian - Coniacian boundary in the United States Western Interior
IRENEUSZ WALASZCZYKI & WILLIAM A. COBBAN2
I Institute of Geology, University of Warsaw, AI. Zwirki i Wigury 93, PL-02-089 Warszawa, Poland E-mail: [email protected] 270 Estes Street, Lakewood, Colorado 80226, USA
ABSTRACT:
WALASZCZYK, I. & COBBAN, W.A. 1998. The Turonian - Coniacian boundary in the United States Western Interior. Acta Geologica Polonica, 48 (4),495-507. Warszawa.
The Turonian/Coniacian boundary succession in the United States Western Interior is characterized by the same inoceramid faunas as recognized in Europe, allowing the application of the same zonal scheme in both regions; Mytiloides scupini and Cremnoceramus waltersdorfensis waltersdorfensis zones in the topmost Turonian and Cremnoceramus deformis erectus Zone in the lowermost Coniacian. The correla tion with Europe is enhanced, moreover, by a set of boundary events recognized originally in Europe and well represented in the Western Interior: Didymotis I Event and waltersdorfensis Event in the topmost Turonian, and erectus I, II and ?III events in the Lower Coniacian. First "Coniacian" ammonite, Forresteria peruana, appears in the indisputable Turonian, in the zone of M. scupini, and the reference to Forresteria in the boundary definition should be rejected. None of the North American sections, pro posed during the Brussels Symposium as the potential boundary stratotypes, i.e. Wagon Mound and Pueblo sections, appears better than the voted section of the Salzgitter-Salder. The Pueblo section is relatively complete but markedly condensed in comparison with the German one, but it may be used as a very convenient reference section for the Turonian/Coniacian boundary in the Western Interior. The Wagon Mound section was mis-interpreted in respect of its biostratigraphical position and is entirely of Late Turonian age.
INTRODUCTION the traditional C. erectus and C. deformis zones. In a number of his "European papers" (KAUFFMAN-' Since COBBAN & REESIDE'S (1952) publication, 1978a, b; WIEDMANN & KAUFFMAN 1978, and in the Turonian/Coniacian (T/C) boundary in the U.S. HERM & al. 1979) he showed also the applicabili Western Interior was placed at the base of the ty of his scheme to the European succession. It Cremnoceramus erectus Zone (see also SCOTT & was a great step foreword in the knowledge of the COBBAN 1964). It is a horizon approximating the Turonian/Coniacian boundary faunas as well as in appearance level of Coniacian ammonites, and the correlation between the European and North moreover, marking a radical change within inoce American Turonian/Coniacian boundary succes ramid faunas. Subsequently KAUFFMAN (1977 and sions. The problem began, however, when very in KAUFFMAN & al. 1978) introduced a markedly refined studies on the boundary succession were refined zonation for the T /C boundary interval undertaken in Europe. These were initiated with with a series of zones distinguished by him within Gundolf ERNST'S project "Eventstratigraphie" in 496 IRENEUSZ WALASZCZYK & WILLIAM A. COBBAN
106' and/or hiatus at the Turonian/Coniacian boundary c-----__~1Vl1J'_'L ____._--'-- __ I I COLORADO ! in the Salzgitter-Salder section (WALASZCZYK & ! I!NEBRASCA :r: 1 WOOD, this volume, see also discussion in WIESE, biostratigraphy 4rkdllsas /) 8' , ~ of the Upper Turonian and Lower Coniacian of the ~ LaJunta I Western Interior as used here are published else I' • Walsenburg Ii where (WALASZCZYK & COBBAN, in press). The --T-----N-E-\-V-M~k-XI-C-O------~OKLAHOMA interpretation of the Cremnoceramus clade fol • Springerr------) • ! lows that presented in W ALASZCZYK & WOOD "* ( / Wagon Mound (1998). One of the main results of our taxonomic df/. ; i '/) Santa Fe I study, in reference to the Turonian/Coniacian ! \,-~ i + i boundary, was demonstration of the conspecifity "J Albuquerque ~~ of the basal Coniacian marker, Cremnoceramus 2ookn/ ! rotundatus (sensu TRaGER, 1967 non FlEGE 1930) and the North American species Cremnoceramus Fig. 1. Locality map for a part of the Western Interior showing erectus (MEEK, 1877) which, consequently, the localities mentioned in the text becomes a basal boundary marker of the Coniacian stage.
Germany, and publication of WOOD & al. (1984) on the Salzgitter-Salder section. WOOD & al.'s MATERIAL AND DATA paper, as well as a series of successive reports from Europe (CECH 1989, KOCHLER & ERNST This study is based on recent field work in the 1989, WALASZCZYK 1992, MARCINOWSKI & al. Pueblo and La Junta sections in SE Colorado, and 1996), revealed serious difficulties with a precise the Springer and Wagon Mound sections in NE correlation of the boundary interval between New Mexico (Text-fig. 1). Of great help were Europe and North America when using numerous collections from the T/C boundary KAUFFMAN'S scheme (as published e.g. in interval housed in the Geological Survey, Denver. KAUFFMAN & al. 1993). Firstly, KAUFFMAN'S bio Because the Pueblo and La Junta sections have zonation was built to a large extent using inoce much in common in regard to the T/C boundary, ramid taxa left in open nomenclature and second the following discussion will concentrate on the ly, mutual vertical ranges of particular inoceramid Pueblo section. species were, according to his scheme, markedly different from those recognized in Europe. The main objectives of our study were: (i) the Pueblo section comparison of the T/C boundary interval of the Western Interior and Europe applying the same The Turonian - Coniacian boundary near taxonomic concept to inoceramid fauna, (ii) to Pueblo lies within the Fort Hays Limestone, the determine if the refined event scheme, as worked lower member of the Niobrara Formation (Text out in Europe, is recognizable in the Western fig. 2) (see SCOTT & COBBAN 1964 for description Interior, (iii) to document a precise correlation of the entire Niobrara succession in the Pueblo between the inoceramid and ammonite schemes area). The Member is well exposed over a large in the boundary interval, and finally, (iv) to eval area of the Rock Canyon anticline, west of Pueblo, uate the Wagon Mound section as a possible T/C where it caps the hills surrounding the Arkansas boundary stratotype. The Wagon Mound section River Valley (Text-fig. 3a). It is represented by a was the only other section, besides Salzgitter 13.5 m thick succession of evenly bedded, light to Salder, formally proposed as a potential strato medium gray, pelagic/hemipelagic limestones type section during the Brussels Symposium on interbedded with thin calcareous shales (SCOTT & the Cretaceous Stage Boundaries (KAUFFMAN & COBBAN 1964). The limestone-shale rhythmites as al. 1996). The recent statement of condensation well as the higher order cyclicity, composed of 4- TOURONIAN - CONIACIAN BOUNDARY IN THE UNITED STATES 497
Inoceramid zonation Pueblo section and chronostrati graphy after Mouth of Peck Creek in Kauffman & Pratt SWI!4 SWli4 sec. 33. T.20 S., R.66W 1985
Cremnaceramus brownei
", 1~~ '" 0) 0) 6' l eom wen ulnas?' ~6~ 65 I defarmis ~==ll .~ E: .~ ~ 63 ~ ~ ~ ~ n. ssp. \ (Late form) 61 ~ ~'R'-' 59 ~ § ~ ~ 57 ~ 6'~ ij 55 ~ • ~ b, ~ 'R ~ 53 .~'" ~j '"~ 51 ~ " ~ ~:: 139) ; ~ ~ ~ 45 ~::::::~~:merlw~tuSTIrelllle'Yement~~~~~~~-[:i~·8t-~11. defarmisw ,",'0'1 :::':;~ defarmis 5 ~llii ~_:~: -==:=~ " "=,,,rr,,,", I ~ ~ ~ t U +J:2 35 33 erectus I Event I ~ ~
j I j,n"m.eAn,.(m".;< Event ~~.'" Wliller. 1. 'v?' _I!! :33;29 ( ",ohe'm'; fOH"o ~ i ,vee, "'''''0. 1 I 27 ~~~~---t1~tf4"""'".. '"""•• w...... ~,,'._~ '" defarmis n. subsp. I 1<1: !l-i:: 29 ) nat surface ~ (Small early form) I I:::; '-=====:. 1. erectus, n. ssp. Ie ,~2I 27 f- ~ ~ (Late form) , :~ ~::::t Didymofis I Event + "" :;.;: Ir- ~ 19 ~ hel'bichi Event ~ ~ ~I ~ f,i:, ~ 25):g.; %,-. j ~ 1. erectus erectus .S 17 S g.-" "";::. .': Farresteria sp. 6,.;:]::C:E ~ ~ ~ § 15 j ~:;':J< ~ 0 e.. Barroisiceras sp. 1:: N - ~ - ., t) Durania cd !3 ti §'~ ~ \j V'J austinensis -? " ~ ~ ~ '-' i;; 1. rotundatus 9 • ....,;... ~ ~ g I ~ ~~ v '" 1.(~~~~~!~iY~~~,~' I--+~ __ -c~:,~~ ______~~ ______~IIJ_!_' __~ ______~~ ____ f-_lkf~lkf~d~'f~;~~~~::~ls~~lkf lusatiae __ ~35 III~~ r ..... I~ I ~ ~ " ·...... · ..•.• D Lim"ton, Ii2
I ~ l:g .~ Marl I~ 18 ~ I1.Slll Shale Sandstone
Fig. 2. Geological column, inoceramid distribution and biostratigraphy of the upper Turonian - lower Coniacian part of the Pueblo section; for comparison the zonation applied to this section by KAUFFMAN & PRATT (1985) is shown to the right; numbers on the left side of the geological column are after SCOTT & COBBAN (1964) 498 IRENEUSZ WALASZCZYK & WILLIAM A. COBBAN
7 limestone-shale couplets, ascribed to climatical Wagon Mound and Springer sections ly induced phenomena (see LAFERRIERE & al. 1987), are clearly recorded in the Fort Hays Both sections are located in the Raton Basin in Member near Pueblo (Text-fig. 3b). NE New Mexico, and represent the lower part of The Fort Hays Limestone overlies either the the Niobrara Formation (see SCOTT & al. 1986 for basal Upper Turonian shale of the Juana Lopez description of the Niobrara succession in the area). Member or the slightly older, Middle Turonian The Wagon Mound, section is situated about 0.5 Codell Sandstone Member of the Carlile Shale. km north of the town of Wagon Mound and is rep The associated stratigraphical gap comprises an resented by a cut along the U.S. Interstate Highway interval from the topmost Inoceramus dimidius 25. The Springer section is represented by railroad Zone, or even from the older I. howelli Zone to the and road cuts south of the town of Springer at the basal Mytiloides scupini Zone, and represents a southern exit from Interstate 25 to the town (see regionally recognized Carlile-Niobrara unconfor Text-fig. 6) (measured section 1 of SCOTT & al. mity (see HATTIN 1975). 1986). Both sections represent the same succes The basal three beds of the Fort Hays Member sion, although the Springer section ranges strati (beds 1, 3, 5) are fairly fossiliferous containing graphically much higher, into the lowermost inoceramids represented by Mytilodes incertus Coniacian (see Text-figs 4, 6). (JIMBO), M. ratonensis W ALASZCZYK & COBBAN, The lower part of the succession is very well M. scupini (HEINZ), and M. labiatoidiformis exposed in the Wagon Mound section (Text-fig. (TROGER), an association indicative of the lower 6b), where it is represented by the Fort Hays part of the scupini Zone (W ALASZCZYK & Limestone followed by the shale & limestone unit, COBBAN, in press). This interval is followed by an the lowermost, informal unit of the Smoky Hill approximately 2.5 m thick succession referred to Member (Text-fig. 4). KAUFFMAN (1995, referring as the "barren interval" (Text-fig. 2), character to COLLOM 1991) referred the whole succession as ized by extremely rare fossils. It corresponds in exposed here to the Fort Hays Limestone. part to the barren interval of HATTIN (1975) and However, although the basal shale & limestone KING (1972 in HATTIN 1975) recognized in unit resembles the underlying Fort Hays in having Colorado, New Mexico and Kansas, although its several beds of limestone but these are less resis upper boundary is lowered here to the first tant and more marly than those in the typical Fort Mytiloides horizon below the TIC boundary. Its Hays. The Fort Hays Member consists here of 7 equivalent is also well represented in Europe limestone beds interbedded with relatively thick (WOOD & al. 1984), it is an intercontinental phe calcareous shales (Text-figs 4, 6b). The limestones nomenon that had a profound impact on the are richly fossiliferous, although the fauna is rep inoceramid faunas. Inoceramid faunas in the resented mostly by inoceramids. Rarely noted are overying TuronianlConiacian boundary interval ammonites (Prionocyclus germari) and nautiloids became identical to that occurring in Europe. (Eutrephoceras) , and oyster bivalves (SCOTT & al. Inoceramids reappear in bed 25, in the upper 1986). The fauna is relatively common in the low most scupini Zone, marking the base of the inter ermost part of the successive shale & limestone val referred here to the boundary interval. They unit, but it quickly becomes sparse higher in the occur in abundance also higher, up to the top of succession. the Fort Hays Member, dated as the basal crass us The entire Wagon Mound section represents Zone (Text-fig. 2; see also WALASZCZYK & the Upper Turonian scupini Zone, corresponding COBBAN in press). to its lower part at Pueblo, but still below the "bar The TIC boundary, defined by the FAD of ren interval" . Cremnoceramus deformis erectus MEEK [=c. The Fort Hays Limestone and ca. 12 m of the rotundatus (sensu TROGER non FIEGE)] lies in the shale & limestone unit crop out in the Springer upper part of hed 31 (see Text-figs 2-3b) approxi section, about 20 km north of Wagon Mound. The mating well the boundary position as recognized highest part of the succession exposed here, ranges by SCOTT & COBBAN (1964, text-fig. 3). Some dis to the basal Coniacian, where it is accessible in the tance below the boundary, in bed 23, were found highest part of the small hill east of the railroad the first ammonites of the genus Forresteria. The cut (Text-fig. 6a). first horizon with flood occurrence of inoceramids Most of the shaJe & limestone unit represents occurs in bed 26. the "barren interval" with fauna re-appearing ACTA GEOLOGICA POLONICA, VOL. 48 1. W ALASZCZYK & W A. COBBAN, FIG. 3
Pueblo section; a - general view of the Rock Canyon Anticline area, with the yellow-colored Fort Hays Limestone capping the hills surrounding the Arkansas River valley; b - Fort Hays Limestone section in the Recreation Centre, northern side of the Arkansas River Valley (the photograph is taken in the same location as in SCOTT & COBBAN 1964, Fig. 3) TOURONIAN - CONIACIAN BOUNDARY IN THE UNITED STATES 499 about 3 m below the top of the exposed succes Didymotis I Event and still higher by waltersdor sion. A rich fauna begins at the horizon with fensis and erectus I events (Text-fig. 4). small-sized inoceramids (M. scupini and M. her The first Cremnoceramus deformis erectus bichi and I. longealatus Trager), referred here to (MEEK) was found slightly below the erectus I the basal Mytiloides horizon. It is followed by the Event, which places the Turonian/Coniacian
SPRINGER
J)idyp!!!tis IEysnt herbichi.Eve,n,t
41
39 37
36
35 34
33
30
28 Z 26 0 24 »'"oj ::r:: 1::: 0 ~ Carlile Shale Fig. 4. Geological column, inoceramid distribution and biostratigraphy of the upper Turonian - lowermost Coniacian succession in the Springer - Wagon Mound area; numbers of the limestone beds on the left side of the Wagon Mound section (LS 1 through LS 10) are after COLLOM (1991) and KAUFFMAN (1995), and beds LS 11 and 12 are added after our own observations 500 IRENEUSZ WALASZCZYK & WILLIAM A. COBBAN boundary between the waitersdorfensis and erec BOUNDARY SUCCESSION tus I events. Most of the shale & limestone unit in the The interval close to the TIC boundary is char Springer-Wagon Mound region belongs to the acterized by an abundance of inoceramids concen Upper Turonian and represents the middle part of trated in distinct horizons and being relatively rare the scupini Zone ("barren interval"). The Fort or apparently absent in the intervals between. Hays Member is also entirely Upper Turonian with Their succession, composition and stratigraphical its upper boundary lying relatively low in the scu location are the same as recognized in Europe pini Zone and coinciding with the base of the "bar (ERNST & ai. 1983; WOOD & ai. 1984, ren interval" (see text-fig. 4). It shows a marked W ALASZCZYK & WOOD 1998) indicating their diachroneity of the upper boundary of the Fort equivalency and allowing the application of the Hays Limestone between SE Colorado (Pueblo same terminology as used in the Old Continent section) and NE New Mexico (Text-fig. 5). (Text-fig. 7). s N PUEBLO SPRINGER Smoky Hill Member Fort Hays Member Carlile Shale 3m Fig. 5. Litho and chronostratgraphical correlation of the Springer-Wagon Mound and Pueblo section ACTA GEOLOGICA POLONICA, VOL. 48 I. W ALASZCZYK & W A. COBBAN, FIG. 6 Springer and Wagon Mound sections; a - Springer section: in foreground visible topmost part of the Fort Hays Limestone, in background - middle and upper parts of the shale & limestone unit; the Turonian/Coniacian boundary lies in the topmost part of the hill in the upper right corner of the photograph; b - Road-cut section at Wagon Mound; the main part of the photographed section comprises limestone beds 1-7 of the Fort Hays Limestone; at the bottom - topmost part of the Juana Lopez Member of the Carlile Shale, above - lowermost part of the shale & limestone unit of the Smoky Hill Member TOURONIAN - CONIACIAN BOUNDARY IN THE UNITED STATES 501 Basal Mytiloides horizon Central Poland, in the horizon just below the wal tersdorfensis Event. No further investigations, The lowest horizon with inoceramids above however, were made. the "barren interval" was found about 1 m below the Didymotis I Event in the Springer section waltersdOljensis Event (Text-fig. 4). Near Pueblo it is most probably fused in the same shale bed in which the In the lower third of bed 31 in the Pueblo sec Didymotis I Event was recognized. The basal tion, and about 1 m above the Didymotis I Event Mytiloides horizon is characterized by small in the Springer section occurs a level with flood sized mytiloids (M. herbichi and M. scupini), occurrence of small-sized, geniculated, highly Inoceramus longealatus, and rare, small-sized inflated representatives of Cremnoceramus wal Didymotis bivalves. tersdorfensis waltersdorfensis (ANDERT). Two closely-spaced waltersdorensis events were rec Didymotis I Event ognized in the Salzgitter-Salder section (walters dorfensis I and waltersdorfensis II events - see This event is very well represented both in the WALASZCZYK & WOOD 1998). In most of the sec Pueblo and Springer sections. Near Pueblo it tions, however, these two events, as well as the occurs in bed 26 and is characterized by flood intervening Didymotis II Event, are fused into a occurrence of small- to medium-sized representa single event and referred to waltersdorfensis tives of Mytiloides herbichi (ATABEKIAN), M. Event (e.g. WALASZCZYK 1992). As demonstrated scupini, and common Didymotis sp. The latter is by the numerous collections of the Geological represented by poorly elongated and weakly orna Survey in Denver, it is a well recorded event over mented forms lacking radial ribbing. The co all the Western Interior. occurrence of this Didymotis morphotype and In most of the European sections the walters Mytiloides herbichi enables a very convincing dorfensis Event is characterized usually by the co correlation of this horizon with the Didymotis I occurring Didymotis bivalves. They were not Event of Europe. found by us in any of the studied sections. However, the elongated, radially-ribbed morpho Flat Surface type of Didymotis associated with C. waltersdor fensis and early C. deformis erectus it present in In Pueblo bed 29 is characterized by a smooth some collections from the T/C boundary interval upper surface (see SCOTT & COBBAN 1964) which of Texas. is, moreover, characterized by a relatively com mon Mytiloides fauna, most of which may be erectus I Event referred to M. herbichi (ATABEKIAN). This level, marking possibly an erosional event, is easily The erectus Event, characterized by a flood identifiable and represents a very convenient abundance of the C. deformis erectus (MEEK) is marker for local correlation. It may correspond to well documented in both sections. The first forms the herbichi Event of the European succession referable to C. erectus (MEEK) were found, how (see WALASZCZYK & WOOD 1998). ever, both in Pueblo and in Springer, already below this event (topmost part of bed 31 in "websteri fauna" Pueblo), marking the Turonian/Coniacian bound ary in the interval between the waltersdorfensis The bottom part of bed 31 of the Pueblo sec and erectus I events. tion is characterized by very characteristic inoce ramids referred here to the "websteri fauna" erectus II Event owing to the resemblance of at least some forms found here to Woods' concept of Inoceramus The second horizon with flood occurrence of websteri (WOODS 1912, pl. 53, figs 1-2). This Cremnoceramus erectus (MEEK), observed only very peculiar fauna is very poorly known in in the Pueblo section (the succession in the respect of its taxonomy, phylogeny as well as Springer section does not range as high), is even stratigraphical occurrence. Recently, websteri better developed than the erectus I Event and is like forms were found in the Vistula section, well visible in the lower part of bed 35. 502 IRENEUSZ W ALASZCZYK & WILLIAM A. COBBAN 53, CD hanno- SALZGITTER- 52d 52c vrensis hal1J101'rel1Sis 52b SALDER 52a 51 50c 50b 50a ~ PUEBLO u~ 49 39 35 33 31 walters- 29 27 Didymotis I Event 25 23 ~ First Forresteria ~ 21 Z scupini 0 ~ I~ 1m IE-i 41 40 39 38h Fig. 7. Bio- and eventratigraphical correlation of the Pueblo and Salzgitter-Salder section; for details see Text-fig. 2 (in the case of the Pueblo section) and WALASZCZYK & WOOD (this volume, Text-fig. 2); bed numbers on left side of the Pueblo sections are after SCOTT & COBBAN (1964), those on the right side of the Salzgitter-Salder section are after WOOD & al. (1984) (see also WALASZCZYK & WOOD, this volume TOURONIAN - CONIACIAN BOUNDARY IN THE UNITED STATES 503 ? erectus III Event erous part (limestone beds 1 through 5) and the successive interval, characterized by a significant The upper surface of bed 37 near Pueblo, a loss in inoceramid diversity, interpreted by very thin limestone intercalation between shales of KAUFFMAN (1995, and in KAUFFMAN & al. 1996) beds 36 and 38, is crowded with inoceramids of in terms of the Turonian/Coniacian boundary, rep the defarmis lineage, represented in part by C. resents actually a horizon well below the bound defarmis erectus (MEEK) and in part by forms ary. It corresponds to the boundary between the which may represent already C. defarmis dabro inoceramid-rich basal scupini Zone and the suc gensis (SZASZ). The interpretation of this horizon, cessive "barren interval" of the middle scupini in terms of the European event succession, is very Zone, characterized by an almost complete uncertain. In the approximately equivalent interval absence of macrofauna, a phenomenon known in Europe, there occur two well developed events; widely in the Western Interior (see e.g. HATTIN the hannavrensis Event, marking the base of the 1975), and also reported from Europe (WOOD & hannavrensis Zone, and the erectus III Event, al. 1984). located slightly higher (see WALASZCZYK & WOOD 1998) (Text-fig. 7). The former event is character ized by a flood occurrence of C. waltersdorfensis COMPARISON OF THE FAUNAL hannavrensis (HEINZ). The latter event has a more SUCCESSION AT THE TURONIAN variable fauna with the most characteristic C. wal - CONIACIAN BOUNDARY tersdarfensis hannavrensis and large representa INTERV AL BETWEEN THE WESTERN tives of the genus Tethyaceramus SORNAY (non INTERIOR AND EUROPE HEINZ). Both C. waltersdorfensis hannavrensis and tethyoceramids are, however, extremely rare Beginning at the top of the "barren interval" up in the Western Interior, where the succession is through the erectus Zone, the inoceramid succes characterized almost entirely by representatives of sion in the Western Interior is identical to that in the C. defarmis lineage (see WALASZCZYK & Europe (Text-fig. 7). It concerns both the taxo COBBAN in press). We compare our event with the nomic composition of the inoceramid assemblages European erectus III event, basing it on the as well as the mode of their occurrence, with appearance there of early forms comparable to C. inoceramids concentrated in distinct horizons defarmis dobrogensis, which are present in that instead of being randomly distributed (see Text level also in the Western Interior, but the correla figs 2 and 4). Of importance is also the same mode tion is very uncertain. of occurrence of Didymatis. Although only Didymotis I Event was recognized in the Pueblo and Springer sections, some unpublished data EVALUATION OF THE WAGON MOUND from southwestern Texas prove its presence also SECTION in the interval equivalent to the waltersdarfensis Event corresponding to the Didymatis II Event. As shown in the previous chapter, the Wagon Similarly as in Europe, these two horizons are rep Mound section (Text-figs 4, 6b), proposed by resented by two distinct morphotypes of KAUFFMAN (1995, and in KAUFFMAN & al. 1996) Didymatis; the lower event characterized by more as a candidate for the Turonian - Coniacian rounded, weakly ornamented or smooth forms, boundary stratotype, was misinterpreted in respect while the upper level yields elongated forms with of its biostratigraphical position and, consequent well developed radial ornament. ly, can not be further regarded as a potential stra The first Cremnaceramus defarmis erectus totype section for the Turonian/Coniacian bound (MEEK) appears slightly below the erectus I Event ary. The exposed part, composed of the Fort Hays (=ratundatus Event) and simultaneously above Limestone (limestone beds 1 through 7 in the waltersdorfensis Event. In the following inter KAUFFMAN 1995) and of the shale & limestone val, up to the appearance level of Cremnaceramus unit of the Smoky Hill Member (limestone beds 8- crassus crass us (PETRASCHECK), the succession is 10 in KAUFFMAN 1995, and higher part of the suc characterized almost exclusively by representa cession), belong entirely to the Turonian (lower tives of the Cremnaceramus deformis lineage (see and middle parts of the scupini Zone) (see Text WALASZCZYK & WOOD 1998 for definition); C. fig. 6). The boundary between the lower, fossilif- defannis erectus below and C. defannis dabro- 504 IRENEUSZ WALASZCZYK & WILLIAM A. COBBAN gensis above. The waltersdorfensis lineage (sub any specimens from much of this part of the suc species waltersdorfensis and hannovrensis - see cession. The forms like C. waltersdorfensis wal WALASZCZYK & WOOD, this volume), dominating tersdorfensis (ANDERT) and C. waltersdorfensis much of the Lower Coniacian succession in hannovrensis (HEINZ), recognized by KAUFFMAN Europe, is almost absent in the Western Interior, (see in KAUFFMAN & al. 1993) in his zone of M. with the exception of the basal waltersdorfensis fiegei + M. dresdensis + M. lusatiae, and higher, Event (see Text-figs 2 and 4). The base of the are probably some of the morphotypes found dobrogensis Zone coincides either with the base among representatives of the M. striatoconcentri of the European hannovrensis or is an equivalent cus group. of a slightly higher horizon corresponding to the The correlation presented here explains many erectus III Event. of the apparent inconsistencies existing between the European inoceramid zonation, as presented by WOOD & al. (1984; and presented recently by CORRELATION WITH KAUFFMAN'S WALASZCZYK & WOOD 1998), and the North ZONAL SCHEME American record at the T/C boundary as published recently by KAUFFMAN & al. (1993). These incon Direct correlation with KAUFFMAN'S zonal sistencies concern mostly the presence in the scheme for the topmost Turonian and Lower KAUFFMAN'S scheme of many of the Late Coniacian is very difficult or actually impossible Turonian mytiloids in the lowermost Coniacian, or because many of his zonal inoceramid indexes were a lack of rapid evolutionary turnovers in the inoce referred to in open nomenclature and have never ramid faunas of the latest Turonian such as in the been characterized by him subsequently. The fol European record. lowing remarks are based thus on the comparison of our scheme and the inoceramid zonation he applied to the Pueblo section (KAUFFMAN & PRATT 1985), AMMONITE/INOCERAMID ZONAL shown in our Text-fig. 2. This comparison reveals a SCHEME marked difference between our concept of the Turonian/Coniacian boundary and that of During the Second Internation Symposium on KAUFFMAN. Moreover, it indicates considerable dif Cretaceous Stage Boundaries (Brussels, 1995), the ferences in the taxonomic concept of many inoce Coniacian Working Group of the Cretaceous ramid species characterizing the boundary (see Subcommission on Stratigraphy recommended WALASZCZYK & COBBAN, in press). KAUFFMAN (in that the First Occurrence (FO) of Cremnoceramus KAUFFMAN & PRATT 1985 and in KAUFFMAN & al. rotundatus (sensu TRaGER non FIEGE) [= C. 1993) placed the T/C boundary at the base of the deformis erectus (MEEK)] be the criterion for "barren interval", which lies very low within the recognition of the Turonian-Coniacian boudnary. scupini Zone (see Text-figs 2, 4). In terms of the The boundary was said to lie between the LO of Salzgitter-Salder section, it is the level correspond the ammonite Prionocyclus germari and the FO ing approximately to the Micraster Ecoevent, about of the ammonite Forresteria ("thus preserving 50 m below the actual Turonian/Coniacian bound some of the original ammonite definition of the ary (see WOOD & al. 1984). The base of the erectus boundary"). In Europe, however, the lowermost Zone, as here recognized, corresponds to the base Coniacian ammonite record is very poor, and first of KAUFFMAN'S Cremnoceramus defonnis Forresteria is not known until the base of the deformis Zone (Text-fig. 2). His zone of inconstans Zone (in WALASZCZYK & WOOD'S Inoceramus (Cremnoceramus?) deformis n. subsp. 1998 scheme) located relatively high in the Lower (small early form) + I. erectus n. subsp. (late form) Coniacian succession (see KAPLAN 1986, KAPLAN corresponds approximately to the interval spanning & KENNEDY 1995, 1996). According to the much lhe Didymotis I Event through the waltersdOifensis better record in the Western Interior, first Event. Forresteria co-occurs with definitely Turonian It is very difficult to find out which forms were inoceramids [Mytiloides herbichi (ATABEKIAN) the basis of KAUFFMAN'S concept of forms like and M. scupini (HEINZ)] from the upper scupini Inoceramus rotundatus or I. erectus. Inoceramids Zone (Text-fig. 2), distinctly below the FAD of occurring in the interval included by him in these Cremnoceramus deformis erectus (MEEK) (see zones are extremely rare, and we have not seen also KENNEDY & COBBAN 1991). In the Pueblo TOURONIAN - CONIACIAN BOUNDARY IN THE UNITED STATES 505 section the oldest specimen of F. brancoi is FIEGE)]' and consequently, the original intention known from bed 23 (Prionocycloceras? of SCOTT of the boundary definition to preserve its & COBBAN 1964) well below the Didymotis I ammonite character is rejected. Event (see also Text-fig. 2). KAUFFMAN'S statement (in KAUFFMAN & al. 3. Two North American sections, i.e. Pueblo, 1993, 1996) on the coincidence of the FAD of C. Colorado, and Wagon Mound, New Mexico, were rotundatus (=C. erectus) and the boundary proposed during the Brussels Symposium on the between the Turonian Prionocyclus and Cretaceous Stage Boundaries as potential bound Coniacian Forresteria (F. peruana and F. bran ary stratotypes. None of them, however, appears to co i) was based on his different concept of inoce be better than the selected German section of ramid fauna in that interval and, consequently, Salzgiter-Salder, in spite of the fact that recent much lower position of the Turonian/Coniacian studies revealed a hiatus and/or condensation boundary (see former chapter). close to the boundary in its succession. The Pueblo Concluding, inoceramid fauna at the level rec section is much more condensed in comparison ommended during the Brussels Meeting as the with the Salzgitter-Salder section, however, its Turonian/Coniacian boundary, allows for a very excellent exposure, accessibility, preservation in a refined zonal subdivision of that interval and its protected area, and relatively complete record very precise correlation in a frame of Euramerican make this section a very convenient reference sec biogeographical region. Moreover, the published tion for the Turonian/Coniacian boundary in the data suggest its usefulness for the correlation also Western Interior. The Wagon Mound section (the with other regions (North Pacific Province, New only other section formally proposed in Brussels Zealand) although further studies are needed besides Salzgitter-Salder) was mis-interpreted in (NODA 1984, 1996; CRAMPTON 1995, 1996). respect of its biostratigraphical position. The However, in contrast to the original intention of entire succession of this section is of Late the Coniacian Working Group, the use of Turonian age, and distinctly below the Forresteria as defining the base of the Coniacian Turonian/Coniacian boundary as defined in should be rejected. Brussels. CONCLUSIONS Acknowledgements 1. The topmost Turonian and lowermost 1. WALASZCZYK acknowledges, with warmest thanks, Coniacian succession of the Western Interior con the Research Fellowship of the Fulbright Foundation. tains the same inoceramid faunas as recognized in The U.S. Geological Survey provided the specimens Europe, and allows application of the same inoce used in this study. ramid biozonation in the boundary interval. Moreover, the recognition of the boundary events (Didymotis I, waltersdOljensis, erectus I, erectus REFERENCES II, and ?erectus III ), described originally from Europe, allows almost bed-by-bed correlation of CECH, S.1989. 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