Upper Cretaceous

40ArI39Ar age and correlation of the nonmarine Two Medicine Formation (Upper Cretaceous), northwestern Montana, U.S.A. RAYMOND R. ROGERS Department of Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, U.S.A.

CARLC. SWISHER I11 Geochronology Center, Institute of Human Origins, 2453 Ridge Road, Berkeley, CA 94709, U.S.A. AND

JOHNR. HORNER

Can. J. Earth Sci. Downloaded from www.nrcresearchpress.com by Dr. Raymond Rogers on 08/16/11 For personal use only.

Museum of the Rockies, Montana State University, Bozeman, MT 59427, U.S.A. Received June 2, 1992 Revision accepted February 12, 1993 The age of the nonmarine Two Medicine Formation of northwestern Montana is currently based upon correlations with K-Ar-dated Western Interior ammonite zones. 40Ar/39Ardating of biotite and plagioclase separated from four bentonites and one crystal-rich tuff permits for the first time direct determination of the age of Two Medicine strata. Biotite and plagioclase from a bentonite 10 m below the top of the Two Medicine Formation yield concordant 40Ar/39Arages of 74 Ma, while biotite and plagioclase from two bentonites and a crystal-rich tuff from approximately 100 m above the base of the formation cluster in age around 80 Ma. The total duration of Two Medicine deposition is estimated using these new radioisotopic ages via extrapolation of an average rock accumulation rate. The new 40Ar/39Arages facilitate regional correlation of the dinosaur-dominated paleofauna recovered from the Two Medicine Formation, and help constrain the timing of the Claggett and Bearpaw transgressions. The ages support correlation of the richly fossiliferous upper lithofacies suite of the Two Medicine Formation with exposures of the Judith River Formation in Dinosaur Provincial Park, Alberta, Canada. Radioisotopically dated exposures of the Judith River Formation within Montana that include important Judithian "age" mammal localities correlate app~oximatelywith middle and lower parts of the middle lithofacies suite of the Two Medicine Formation. The new 40Ar/39Arages further indicate that the transgressions of the Claggett and Bearpaw seas culminated within northwestern Montana at ca. 79.6 and 74.0 Ma, respectively. L'Lge de la Formation de Two Medicine continentale, du nord-ouest du Montana, est fond6 actuellement sur les corrklations avec les zones d'ammonites de I'Ouest Inttrieur dattes au K-Ar. Les datations 40Ar/39Arsur biotite et plagioclase extraits de quatre bentonites et d'un tuf riche en cristaux permettent pour la premibre fois la dktermination directe de l'lge des strates de Two Medicine. La biotite et le plagioclase de la bentonite qui apparait i 10 m sous le sommet de la Formation de Two Medicine fournissent des Lges 40Ar/39Arconcordants de 74 Ma, tandis que la biotite et le plagioclase extraits de deux bentonites et d'un tuf riche en cristaux, localists ii approximativement 100 m au-dessus de la base de la formation, donnent des Lges regroupts autour de 80 Ma. La dur& totale du temps de skdimentation de Two Medicine est estimte I? l'aide de ces nouveaux Lges radioisotopiques partant de l'e~tra~olation du taux moyen d'accumulation des roches. La dttermination de ces nouveaux Lges 40Ar139Arfacilitent la mise en corrtlation rtgionale de la paltofaune domin6e par les dinosaures qu'a livrks la Formation de Two Medicine, et ils contribuent i prkciser la date des kvbnements des transgressions de Claggett et Bearpaw. Ces lges ktayent la mise en corrtlation de la suite, abondamment fossilifbre, du faciBs supkrieur de la Formation de Two Medicine avec la Formation de Judith River dans le Parc provincial des dinosaures, Alberta, Canada. Les affleurements datks par radioisotopes de la Formation de Judith River dans le Montana, qui incluent les importantes localitts de mammifkres "d'Lge" Judithien, sont corrklb approximativement avec les horizons mkdian et infkrieure du milieu de la suite lithofaciologique de la Formation de Two Medicine. Les nouveaux Lges 40Ar/39Arindiquent, en outre, que les extensions maximales des transgressions marines de Claggett et de Bearpaw, i l'intkrieur du nord-ouest du Montana, datent d'il y a environ 79,6 et 74,O Ma, respectivement. [Traduit par la rkdaction] Can. I. Earth Sci. 30, 1066-1075 (1993)

Introduction The nonmarine Two Medicine Formation of northwestern Montana has traditionally been assigned to the Late Cretaceous Carnpanian Stage, based upon correlations with ammonite zones dated by K-Ar methods (Gill and Cobban 1966, 1973; Gill et al. 1972; Obradovich and Cobban 1975). This Campanian age designation is tentative owing to uncertainties in correlating marine and nonmarine strata. Correlation is further complicated by the diachronous boundaries of the formation (Lorenz 1981; Lorenz and Gavin 1984). At its westernmost exposure, the lower contact of the Two Medicine Formation may be of Santonian age (Hirsch and Quinn 1988) and the upper contact may extend up into the Maastrichtian (Lorenz 1981). We report here the first radioisotopic ages derived from Pnnted in Canada / Imprime au Canada

Two Medicine deposits. The ages were determined by laserfusion 40Ar/39Ardating of individual plagioclase and biotite phenocrysts extracted from four bentonites and one crystal-rich tuff occurring within the formation. These new ages partially constrain the duration of Two Medicine deposition, facilitate correlation of the dinosaur-dominated Two Medicine paleofauna with vertebrate assemblages in equivalent strata, and suggest the timing of incursion of the Claggett and Bearpaw seas within northwestern Montana.

Two Medicine Formation The Two Medicine Formation is exposed in stream drainages and occasional road-cuts from the town of Wolf Creek, Montana, to the United States - Canada border (Fig. 1). The


ROGERS ET AL.

-600 m thick formation consists of volcaniclastic sediment derived from the Cordilleran highlands and the Elkhorn Mountains Volcanics (see below). Fine-grained lenticular sand-bodies and variegated mudstones and siltstones dominate the sedimentary sequence. Two Medicine deposits comprise the proximal, nonmarine facies of two eastward-thinning clastic tongues. Nonmarine nearshore marine equivalents within central Montana include upper horizons of the Eagle Formation and the Judith River Formation (Fig. 2). The Claggett Formation and the lower part of the Bearpaw Formation are westward-thinning marine correlates (Stebinger 1914; Gill and Cobban 1973). Correlative strata in southern Alberta include the Pakowki, Belly River, and Judith River formations, and upper and lower horizons, respectively, of the Milk River and Bearpaw formations (Russell 1970; McLean 1971) (Fig. 2). Deposition of the Two Medicine Formation commenced with the Telegraph Creek - Eagle regression and continued until maximum transgression of the Bearpaw Sea (Gill and Cobban 1973). Correlation with K -Ar-dated marine ammonite zones suggests that the formation in the vicinity of the type area (Fig. 1) spans approximately 10 Ma, from the Scaphites hippocrepis I1 Zone (ca. 83.4 Ma, all ages are corrected using International Union of Geological Sciences constants following Dalrymple 1979) to the Baculites compressus Zone (ca. 73.3 Ma). Near Augusta, Montana (southwest of the type area, Fig. I), ammonite zonation suggests that the Two Medicine Formation spans approximately 12 Ma, from the Scaphites hippocrepis I Zone (ca. 83.9 Ma) to the Baculites cuneatus Zone (ca. 71.7 Ma) (Gill et al. 1972; Gill and Cobban 1973; Obradovich and Cobban 1975; Lorenz 1981) (Fig. 2).

Elkhorn Mountains Volcanics and bentonites in the Two Medicine Formation The Elkhorn Mountains Volcanics (Klepper et al. 1957) comprise a thick sequence of calc-alkaline intrusive and extrusive rocks in west-central Montana (Fig. 1). Exposures of this volcanic pile cover roughly 7770 km2, with estimates of the original areal extent ranging as high as 25 900 km2 (Smedes 1966). Smedes (1966) divided the -3500 m thick volcanic sequence into three stacked units. The lower unit consists primarily of lava flows and volcanic breccias; the middle unit comprises thick sheets of welded rhyolitic tuff with intercalated lava flows and breccias; and the upper unit consists primarily of volcaniclastic mudstones, siltstones, sandstones, and conglomerates with minor tuff interbeds. The Elkhorn Mountains Volcanics are considered to have erupted primarily during the Carnpanian (Viele and Harris 1965; Smedes 1966; Robinson et al. 1968; Tilling et al. 1968; Gill and Cobban 1973). Viele and Harris (1965) and Schmidt (1966) attributed volcanic-rich alluvial facies in southern exposures of the Two Medicine Formation to the Elkhorn Mountains Volcanics. Viele and Harris (1965) also documented complex interfingering of lava flows and tuffs with volcaniclastic alluvial facies of the southern Two Medicine Formation, and designated the Big Skunk Formation to include interstratified volcanic - fluviatile deposits between the Elkhorn Mountains Volcanics and typical Two Medicine strata to the north. At least 13 discrete altered ash beds are intercalated within the Two Medicine Formation in the type area. In light of the interdigitating contact between the Elkhorn Mountains Volcanics and Two Medicine Formation, these altered ash beds are best

1067

490

FIG. 1. Map showing locations of sampled bentonites and tuff, and regional outcrop area of the T~~ Medicine Formation (Tm) and the Elkhorn Mountains Volcanics (Ev) (based on Ross et al. 1955 and Viele and Harris 1965). The type area of the Two Medicine Formation is exposed along the valley of the Two Medicine River.

CAN. J . EARTH SCI. VOL. 30. 1993

'WESTERN INTERIOR AMMONITE ZONES

Ma

-

Rrrruliter c l k i

Baculdesjenreni

Baculites reesidei

Bearpaw Formation

Bearpaw Formation


BmJila t1~yafu.r

Baculiles gregoryensis

Baculiles perplerus

Bmdiles tp. ((smrrolh)

Baculiles arperiformis

Baculites obtrcsus

(weakflanking ribs) Baculites sp. (smoolh)

FIG.2. Correlation chart of Campanian strata of northwestern and north-central Montana and southern Alberta. Western Interior ammonite zones from Hancock (1991); zonation of formations based on Gill et al. (1972), Gill and Cobban (1973), Lerbekrno (1989), and Eberth et al. (1990). Corrected K-Ar ages (in parentheses) are based on Obradovich and Cobban (1975); 4 0 ~ r / 3 9 Aages r are based on Eberth and Deino (1992) (indicated by *) and Eberth et al. (1990) (indicated by **). The Santonian-Campanian and Campanian-Maastrichtian boundaries follow Obradovich and Cobban (1975) and Berggr~net al. (1985), respectively. Nonmarine rocks are stippled.

1069

ROGERS ET AL.

TABLE 1. Locations, stratigraphic positions, and sedimentologic characteristics of dated horizons within the Two Medicine Formation Sample

Location

WOFSIU5

Type area (Fig. l), sec. 33, T. 32N, R. 6W, Flag Butte Quadrangle South of Choteau (Fig. I), sec. 28, T. 23N, R. 5W, Sevenmile Hill Quadrangle Type area (Fig. I), sec. 35, T. 32N, R. 8W, Rocky Ridge Quadrangle Blacktail Creek (Fig. I), sec. 8, T. 29N, R. 8W, Four Horns Lake Quadrangle South of Choteau (Fig. I), sec. 28, T. 23N, R. 5W, Sevenmile Hill Quadrangle

TM-3


TM-4 TM-6 RTITM-7

Stratigraphy

- 108 m above base

of Two Medicine Formation 105 m above base of Two Medicine Formation 480 m above base of Two Medicine Formation 10 m below top of Two Medicine Formation 105 m above base of Two Medicine Formation

-

considered distal expressions of explosive volcanic activity within the Elkhorn Mountains. Thomas et al. (1990) also implicated the Elkhorn Mountains Volcanics as a probable source of devitrified ash beds in the Judith River Formation of southern Alberta, Canada. Altered ash beds of the Two Medicine Formation satisfy the geologic and mineralogic definitions of a "bentonite" as outlined by Grim and Giiven (1978, pp. 155- 158). X-ray diffraction analyses of claystone comprising Two Medicine bentonites show an almost complete domination of the clay fraction by smectite (Rogers 1990). "Popcorn" weathering is characteristic, and color ranges from olive-grey to tan-yellow (when intercalated with carbonaceous strata). Basal contacts are invariably sharp, while upper contacts vary from sharp to gradational. Phenocrysts of euhedral biotite and plagioclase, when present, are localized near basal contacts. The bentonites range inthickness from a few centimetres to over one metre, and significant lateral variation in bed thickness is not uncommon. Bentonites occur interstratified with caliche-rich paleosol sequences and carbonaceous lacustrine-paludal deposits, suggesting deposition and preservation in both subaerial and subaqueous settings.

Campanian Stage boundaries Considerable contention surrounds the placement of the Santonian -Campanian and Campanian -Maastrichtian boundaries in the Western Interior of North America (Jeletzky 1968; Obradovich and Cobban 1975; Bergstresser and Frerichs 1982; Berggren et al. 1985; Lillegraven and McKenna 1986; Eaton 1987; Hancock 1991; Lillegraven 1991). Difficulties in correlating local sections characterized by endemic species with European stratotypes, and significant dissension between foraminiferal and molluscan zonal schemes fuel the current debate. Lillegraven (1991) correlated the Campanian stratotype in the Aquitaine Basin of France with strata in eastern Texas based on the mutual occurrence of advanced forms of the cosmopolitan ammonite Scaphites hippocrepis. This biostratigraphic correlation recommends relocation of the Santonian - Campanian boundary to a lower stratigraphic position in the Texas Gulf Coast section, and thereby corroborates the "traditional" (e.g., Obradovich and Cobban 1975) placement of the boundary in the Western Interior. We accept Lillegraven's (1991) argu-

Sedimentologic characteristics

-

Tan-yellow bentonite, 10 cm thick, clay fraction dominated by smectite, sharp upper and lower contacts, interbedded with carbonaceous shales Olive-grey bentonite, -30 cm thick, lower contact sharp, clay fraction dominated by smectite, upper contact varies laterally from sharp to gradational, overlies RTITM-7 Olive-grey bentonite, 50 cm thick, clay fraction dominated by smectite, sharp upper and lower contacts

-

Olive-grey bentonite, 15 cm thick, clay fraction dominated by smectite, sharp upper and lower contacts Medium- to coarse-grained red crystal tuff composed of euhedral to subhedral crystals of plagioclase, K-feldspar, biotite, and quartz, and grey-green mud rip-up inclusions derived from underlying bed, varies in thickness laterally, averaging 5 cm, sharp upper and lower contacts

ment, and adopt a Santonian -Campanian boundary below the zone of Scaphites hippocrepis I (ca. 83.5 Ma) as proposed by Obradovich and Cobban (1975). Currently proposed positions of the Campanian - Maastrichtian boundary in the Western Interior span 18 ammonite zones (Jeletzky 1968; Pessagno 1969; Obradovich and Cobban 1975; Bergstresser and Frerichs 1982; Berggren et al. 1985; Lerbekrno and Coulter 1985; Lillegraven and McKenna 1986; Eaton 1987; Goodwin and Deino 1989), a time interval of roughly 10 Ma. In a critical assessment of the Campanian-Maastrichtian boundary, Berggren et al. (1985) advocated a boundary calibrated at ca. 74.5 Ma based upon combined magnetostratigraphic, biostratigraphic, and radioisotopic data. Because of the current debate, we opt to evaluate our results using the "traditional" Santonian -Campanian boundary (83.5 Ma) of Obradovich and Cobban (1975) and the "magnetobiochronologic" CampanianMaastrichtian boundary (74.5 Ma) of Berggren et al. (1985).

40Ar/39Ardating Sample localities Four bentonite beds (WOFS-US, TM-3, TM-4, TM-6) and one crystal tuff (RTITM-7) are dated in this study. Approximate locations of dated beds are illustrated in Fig. 1 and details of locations are given in Table 1. The uppermost bentonite analyzed, TM-6, was sampled 10 m beneath the top of the Two Medicine Formation in the vicinity of Blacktail Creek. The crystal tuff RTITM-7 and the overlying bentonite TM-3 were sampled 105 m above the base of the formation (Lorenz 1981) near Choteau, Montana. Bentonites WOFS-U5 and TM-4 were sampled at 108 and 480 m respectively above the base of the formation in the type area (Fig. 1).

-

-

Methods Biotite and plagioclase for 40Ar/39Ardating were separated from available hand samples of the four bentonites (TM-3, TM-4, TM-6, and WOFS-US) and the crystal-rich tuff (RTITM-7). In two cases, TM-4 and TM-6 from the upper part of the Two Medicine Formation, a minor component of potassium-rich feldspar (sanidine) was discovered in the hand samples, albeit in insufficient quantity for detailed dating. Consequently, the resultant 40Ar/39Arages are considered here in

CAN. J. EARTH SCI. VOL. 30, 1993

TABLE2. Analytical data for 40Ar/39Ar analyses of four bentonites and one crystal-rich tuff from the Two Medicine Formation Sample and lab No.

Mineral dated


Biotite Biotite Biotite Biotite Biotite Plag Plag Plag Plag Anorth? Plag Sanidine Plag Plag Plag Plag Plag Plag Plag Plag Plag Plag Plag Biotite Biotite Biotite Biotite Biotite Biotite Biotite Biotite Biotite Biotite Biotite Biotite Plag Plag Plag Plag Plag Plag

40Ar* 40Ar/39Ar 37Ar139Ar 36Ar/39Ar 40Ar*/39Ar (%)

Age Ma

+la

+SE

ROGERS ET AL.

TABLE2 (concluded)


Sample and lab No.

Mineral dated

40Ar/39Ar

Biotite Biotite Biotite Biotite Biotite Biotite

2.40677 2.37528 2.38879 2.40990 2.39665 2.38064

Plag Plag Plag Plag Plag Plag

2.71110 2.47436 2.49786 2.40720 2.42963 2.38267

*denotes radiogenic argon. Abbreviations: anorth, anorthoclase; plag, plagioclase. NOTES: 'Altered or detrital grains? b~lteredgrain?

part preliminary, requiring the collection of additional larger samples of the upper bentonites to obtain more reliable sanidine ages. In the laboratory, the bentonite samples were disaggregated first using H202 and then warm tap water. Following disaggregation, the samples were passed through 20, 40, 60, and 100 mesh screens, leaving primarily pristine biotite and feldspar. Following 10 min of cleaning with distilled water in an ultrasonic cleaner, the minerals were dried and the biotites and feldspars were separated using a Frantz isodynamic separator. Pristine euhedral crystals of biotite and feldspar were then handpicked under a binocular microscope. The feldspars were treated for an additional 5 min in dilute 0.7% hydrofluoric acid in an ultrasonic cleaner to remove any remaining altered clay, followed by 10 min in distilled water. The biotite was treated for an additional 10 min in distilled water only. The minerals were irradiated with two centrally located monitor minerals using the hydraulic rabbit core facility of the Omega West research reactor at Los Alamos National Laboratory for 28 h. Following irradiation, individual crystals were loaded into separate wells of a copper sample disk, placed within the sample chamber, bolted onto the extraction system, and baked out at 200°C for 8 h. Total fusion of the samples was accomplished with a 6W Coherent Ar ion laser. The released gases were purified by two Zr-Fe-V getters operated at approximately 150°C. Argon was measured in an on-line Mass Analyzer Product 215 noble-gas spectrometer, operated in the static mode, using automated data collection techniques. Sample fusion, gas purification, and mass spectrometry were completely automated, following computer-programmed schedules. Ages were calculated using a J value calculated from six replicate analyses of individual grains of the coirradiated monitor mineral Fish Canyon Tuff sanidine with a reference age of 27.84 Ma (modified from Cebula et al. 1986) intercalibrated in-house with Minnesota hornblende MMhb-I with a published age of 520.4 Ma (Samson and Alexander 1987). Ca and K

corrections used during this study were determined from laboratory salts: 36Ca/37Ca= 2.557 x 4.6 x lop6, 39Ca/37Ca= 6.608 x lop4 f 2.53 X lop5, and 40K/39K= 2.4 x f 7.0 x Mass discrimination during this study, as determined by replicate air aliquots delivered from an on-line pipette system, was 1.005 f 0.0002. Decay constants are those recommended by Steiger and Jager (1977) and Dalrymple (1979).

+

Results Analytical data for the 40Ar/39Aranalyses are presented in Table 2. Two samples from low in the Two Medicine section, RTITM-7 and TM-3, yielded concordant 40Ar/39Arages on both plagioclase and biotite. The stratigraphically lowest of the two, RTITM-7, a crystal-rich tuff, yielded mean ages of 80.044 f 0.078 Ma (SE) and 80.002 f 0.1 14 Ma (SE) calculated from six replicate analyses of biotite and plagioclase, respectively. The overlying bentonite, TM-3, yielded slightly younger, but also concordant, mean ages of 79.715 0.031 Ma (SE) and 79.771 f 0.112 Ma (SE) calculated from six replicate analyses of biotite and plagioclase, respectively. A third bentonite, WOFS-US, also from the lower part of the Two Medicine Formation, but from the type area farther to the north, yielded a mean age on five analyses of plagioclase of 79.603 f 0.096 Ma (SE). The biotite, however, yielded an apparent reproducible age of 78.266 f 0.073 Ma (SE) based on four analyses. A fifth analysis yielded an age of 79.862 f 0.302 Ma (1a), an age concordant with the plagioclase analyses. The older ages are considered here as best reflecting the age of the WOFS-U5 bentonite, with the younger ages possibly reflecting slight alteration. Two samples from near the top of the Two Medicine Formation, TM-4 and TM-6, yielded concordant 40Ar/39Armean ages on plagioclase (four analyses each) of 74.270 f 0.079 Ma (SE) and 74.076 f 0.048 Ma (SE), respectively. A single analysis of sanidine from TM-4 yielded an age of 74.066 f

+

CAN. 1. EARTH

1072

0.722 (la), while a single anorthoclase analysis yielded a statistically older age of 75.376 0.384 Ma (la), possibly representing a detrital grain. The age determinations for biotite from TM-6 varied widely, from 70 to 76 Ma, indicating varying degrees of alteration and possible detrital contamination. An overall mean of these five analyses of 73.57 1.1 Ma (SE) is concordant with the feldspar age, given the large accompanied uncertainty.

+

SCI. VOL.

30, 1993

West

East

SWEETGRASSARCH

IEFODED)


+

Discussion Age of the Two Medicine Formation The total duration of Two Medicine deposition may be estimated by calculating an average rock accumulation rate for strata bracketed by samples WOFS-U5 and TM-4 (both of these samples are located in the type area) and applying this rate to the complete Two Medicine section. Approximately 372 m of strata is bounded by these samples (see Table I). Dividing this stratal thickness by 5.30 Ma (WOFS-U5 (plagioclase) minus TM-4 (plagioclase)) yields a rock accumulation rate of approximately 7.0 cmlka. Applying this rate, the 600 m thick Two Medicine Formation would have accumulated for approximately 8.6 Ma, from roughly 82.6 to 74.0 Ma. This estimate suggests that basal sediments of the Two Medicine Formation are early Campanian in age, while uppermost Two Medicine strata are earliest Maastrichtian (Santonian- Campanian boundary sensu Obradovich and Cobban 1975; Campanian - Maastrichtian boundary sensu Berggren et al. 1985). Unfortunately, direct extrapolation of this rock accumulation rate to the lower 105 m of the Two Medicine section may be inappropriate owing to the presence of a disconformity (unconformity?) 100 m above the base of the formation. This areally extensive erosional surface is marked by moderate relief (1 -2 m) and a persistent caliche - clay pebble lag, and may coincide with a proposed 80 Ma eustatic fall in sea level (Haq et al. 1988; Olsson 1991) that ostensibly led to subaerial exposure and erosion elsewhere within the Western Interior foreland basin (DeGraw 1975; Weimer 1988; Schurr and Reskind 1984; Van Wagoner et al. 1990). Therefore, our age estimate based upon an extrapolated rock accumulation rate should be viewed only as a lower limit. A maximum duration of Two Medicine deposition is estimated by comparing the new Two Medicine radioisotopic ages with K-Ar-dated ammonite zones. The uppermost horizon dated within the formation (TM-6, 10 m below top of formation) yielded an age of ca. 74. l Ma (plagioclase). The Scaphites hippocrepis I1 ammonite zone, dated at ca. 83.4 Ma (K- Ar), correlates approximately with the base of the formation in the type area (Gill et al. 1972; Gill and Cobban 1973). These ages suggest a duration on the order of 9.3 Ma, from roughly the Santonian -Campmian boundary to the earliest Maastrichtian. Discrepancies between our age estimates and those founded solely upon ammonite zonation (Gill et al. 1972; Gill and Cobban 1973, see Fig. 2) are probably attributable to inexact correlation between marine and nonmarine strata, inaccurate K- Ar age determinations, and the presumption of equal zonal durations. Because our estimates are partially constrained by intraformational radioisotopic ages, they should be more accurate.

-

-

-

Timing of Claggett and Bearpaw transgressions 40Ar/39Arages reported here also constrain the timing of incursion of the Claggett and Bearpaw seas into northwestern

FIG.3. Schematic cross section showing stratigraphy of bentonite beds used to date incursions of the Claggett and Bearpaw seas into 0.096 Ma, northwestern Montana. Sample WOFSIUS (79.603 plagioclase) occurs below a shell bed of shallow marine - brackish bivalves that formed as a lag during transgression of the Claggett Sea. Sample TM-6 (74.076 0.095 Ma, plagioclase) occurs immediately beneath the Horsethief transition zone (Cobban 1955), which accumulated during regression of the Bearpaw Sea. Transgressive deposits of the Bearpaw Sea overlie Two Medicine deposits to the east of sample TM-6.

+

+

-

Montana (Fig. 3). WOFSlU5 occurs 12 m below an areally extensive shell bed composed of the brackish-water bivalves Gyrostrea, Corbula, and Corbicula. This shell bed developed as a transgressive lag during westward incursion of the Claggett Sea (Lorenz 1981), and remains the only unequivocal expression of the Claggett transgression within Two Medicine deposits. Based upon the stratigraphic proximity of this transgressive lag and sample WOFSIUS (79.6 f 0.096 Ma, plagioclase), maximum transgression of the Claggett Sea into northwestern Montana can be tentatively dated at ca. 79.6 Ma. Transgression of the Bearpaw Sea into northwestern Montana can be tentatively dated using sample TM-6, which yielded an 0.048 Ma (plagioclase). This bentonite was age of 74.1 sampled 10 m below the top of the Two Medicine Formation in the vicinity of Blacktail Creek. At this locality, the Two Medicine Formation underlies interbedded sandstones and shales of the Bearpaw-Horsethief transition zone (Cobban 1955), which accumulated in nearshore marine -beach environments during regression of the Bearpaw Sea (Gill and Cobban 1973). Transgressive deposits of the Bearpaw Sea sharply (disconformably) overlie the Two Medicine Formation a short distance to the east of the TM-6 locality. Our results place a lower limit of ca. 74.0 Ma on transgression of the Bearpaw Sea into northwestern Montana.

-

+

Regional correlation Limited exposure and post-cretaceous erosion over the Sweetgrass Arch have inhibited direct correlation between Two Medicine deposits in northwestern Montana and correlative Judith River deposits of central Montana and Alberta. The 40Ar/39Arages reported here provide a means of correlation with Judith River strata, and thus permit comparison of the Two Medicine and Judith River paleofaunas within a lowresolution temporal framework (Fig. 4). Judith River strata of Dinosaur Provincial Park, Alberta, Canada, are world renowned for their diverse dinosaurian paleo-

ROGERS ET AL.

I

1073

I II..., ml

JRF

TMF

(KC)

600

JRF (DPP)

TM-6

74.270 +I- 0.157 Ma (plag) 74.066 +I- 0.722 Ma (sanidine)

c

:

m

cn cn

I

c


=4 m

..........

WOFSU5 78.266 +I- 0.146 Ma (biotite)

:"* I

TM-3 79.771 +I- 0.1 12 (plag) 79.715 +I- 0.031 (bbvds) RTITM-7 80.002 +I- 0.1 14 (plag) 80.044 +I- 0.078 (biotite)

r

E FIG. 4. Tentative correlations between composite Two Medicine section (TMF) and Judith River Formation (JRF) of Montana (Kennedy Coulee, KC) and Alberta (Dinosaur Provincial Park, DPP) based on radioisotopic ages and regional stratigraphic relations. Weight of vertical line indicates confidence in correlation. Radioisotopic data for Judith River Formation from Goodwin and Deino (1989) (indicated by *) and Eberth and Deino (1992) (indicated by **). Two Medicine lithofacies suites from Lorenz (1981).

fauna, and are often cited as recording the zenith of dinosaurian diversity prior to their demise at the end of the Cretaceous (Sloan 1976; Dodson 1983; Clemens 1986; Dodson and Tatarinov 1990). A thorough analysis of Judith River chronostratigraphy within Dinosaur Provincial Park is underway (Thomas et al. 1990; Eberth and Deino 1992; Eberth et al. 1992); results suggest that exposures of the Judith River Formation in the park range in age from ca. 76.5 to 74.5 Ma. Our data indicate that the abundant vertebrate assemblage (see Horner 1989) recovered from the upper lithofacies suite (approx. upper 280 m; Lorenz 1981) of the Two Medicine Formation is roughly contemporaneous with the Judith River paleofauna of Dinosaur Provincial Park. Judith River exposures in northern Montana (Kennedy Coulee, Hill County) have yielded radioisotopic dates ranging from ca. 78.0 to 79.5 Ma (Goodwin and Deino 1989). Dated horizons bracket two Judithian "age" mammal localities (Montellano 1986; Goodwin and Deino 1989). This stratigraphic interval ( 38 m, Goodwin and Deino 1989) correlates approximately with middle to lower reaches of the middle lithofacies suite (Lorenz 1981) of the Two Medicine Formation (see Horner 1989 for faunal overview) and the Taber Coal Zone of southern Alberta (Eberth et al. 1990; Eberth and Hamblin 1993). Correlations of greater resolution must await further radioisotopic dating and (or) interformational geochemical fingerprinting of bentonites (e.g., Huff 1983; Huff and Kolata 1990; Thomas et al. 1990).

-

Conclusions This report presents initial findings of an ongoing study of Two Medicine and Judith River geochronology within Montana. The new 4°Ar/39Ar ages reported here place the Two Medicine Formation within a geochronologic framework consistent with recent radioisotopic studies of Goodwin and Deino (1989), Thomas et al. (1990), Eberth and Deino (1992), and Eberth et al. (1992). Future dating (and further refinement of preliminary ages reported herein) will promote reconstruction of the geologic history of the region within a chronostratigraphic framework, and foster correlation of its unique and abundant paleontologic resources. Acknowledgments We thank D. Eberth, M. Patzkowsky, and an anonymous referee for editing this manuscript. We are also grateful to S.M. Kidwell for suggestions and comments. Support for 40Ar/39Aranalyses was provided by the Institute of Human Origins. Fieldwork was funded by grants from Sigma-Xi, the Geological Society of America, and the American Museum of Natural History (Theodore Roosevelt Fund) awarded to R. Rogers and by National Science Foundation grant NSF EAR855241 1-PYI to S. Kidwell. Berggren, W.A., Kent, D.V., and Flynn, J.J. 1985. Jurassic to Paleogene: Part 2. Paleogene geochronology and chronostratigraphy. In The chronology of the geological record. Edited by N.J. Snelling. Blackwell Scientific Publications, Oxford, pp. 141- 195. Bergstresser, T.J., and Frerichs, W.E. 1982. Planktonic foraminifera from the Upper Cretaceous Pierre Shale at Red Bird, Wyoming. Journal of Foraminifera1 Research, 12: 353 -361. Cebula, G.T., Kunk, M.J., Menhert, H.H., Naeser, C.W., Obradovich, J.O., and Sutter, J.F. 1986. The Fish Canyon Tuff, a potential standard for the 40Ar/39Arand Fission-Track methods. Terra Cognita, 6: 139 - 140. Clemens, W.A. 1986. Evolution of the terrestrial vertebrate fauna during the Cretaceous -Tertiary transition. In Dynamics of extinction. Edited by D.K. Elliott. John Wiley & Sons, New York, pp. 63-85. Cobban, W.A. 1955. Cretaceous rocks of northwestern Montana. Billings Geological Society, 6th Annual Field Conference, Guidebook, pp. 107-119. Dalrymple, G.B. 1979. Critical table for the conversion of K- Ar ages from old to new constants. Geology, 7: 558-560. DeGraw, H.M. 1975. The Pierre -Niobrara unconformity in western Nebraska. In The Cretaceous System in the Western Interior of North America. Edited by W.G.E. Caldwell. Geological Association of Canada, Special Paper 13, pp. 589 -606. Dodson, P.J. 1983. A faunal review of the Judith River (Oldman) Formation, Dinosaur Provincial Park, Alberta. Mosasaur, 1: 89- 118. Dodson, P., and Tatarinov, L.P. 1990. Dinosaur extinction. In The dinosauria. Edited by D.B. Weishampel, P. Dodson, and H. Osmolska. University of California Press, Berkeley, pp. 55 -62. Eaton, J.G. 1987. The Campanian -Maastrichtian boundary in the western interior of North America. Newsletters on Stratigraphy, 18: 31 -39. Eberth, D.A., and Deino, A.A. 1992. Geochronology of the nonmarine Judith River Formation of southern Alberta. Society of Economic Paleontologists and Mineralogists, 1992 Theme Meeting, Mesozoic of the Western Interior. Eberth, D.A., and Hamblin, A.P. 1993. Tectonic, stratigraphic, and sedimentologic significance of a regional discontinuity in the upper Judith River Group (Belly River wedge) of southern Alberta, Saskatchewan, and northern Montana. Canadian Journal of Earth Sciences, 30: 174-200.


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