Magnetic Polarity Stratigraphy and Biostratigraphy of Middle-Late Paleocene Continental Deposits of South-Central Montana Robert F

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9-1987 Magnetic Polarity Stratigraphy and Biostratigraphy of Middle-Late Paleocene Continental Deposits of South-Central Montana Robert F. Butler University of Portland, [email protected]

David W. Krause

Philip D. Gingerich

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Citation: Pilot Scholars Version (Modified MLA Style) Butler, Robert F.; Krause, David W.; and Gingerich, Philip D., "Magnetic Polarity Stratigraphy and Biostratigraphy of Middle-Late Paleocene Continental Deposits of South-Central Montana" (1987). Environmental Studies Faculty Publications and Presentations. 4. http://pilotscholars.up.edu/env_facpubs/4

This Journal Article is brought to you for free and open access by the Environmental Studies at Pilot Scholars. It has been accepted for inclusion in Environmental Studies Faculty Publications and Presentations by an authorized administrator of Pilot Scholars. For more information, please contact [email protected]. MAGNETIC POLARITY STRATIGRAPHY AND BIOSTRATIGRAPHY OF MIDDLE-LATE PALEOCENE CONTINENTAL DEPOSITS OF SOUTH-CENTRAL MONTANA1

ROBERT F. BUTLER, DAVID W. KRAUSE, AND PHILIP D. GINGERICH Department of Geosciences, The University of Arizona, Tucson, Arizona 85721 Department of Anatomical Sciences, State University of New York at Stony Brook, Stony Brook, New York 11794 Department of Geological Sciences and Museum of Paleontology, The University of Michigan, Ann Arbor, Michigan 48109

ABSTRACT

Exposures of the Fort Union Formation on the divide between Hunt Creek and Cub Creek in the northern Clark's Fork Basin, Carbon County, Montana, were selected for magnetostratigraphic study of the transition between the Torrejonian and Tiffanian Land-Mammal Ages. Paleomagnetic samples were collected from 25 sites within a 160 m-thick section of the Fort Union Formation at that location. Rock-magnetic analyses indicate that alternating-field demagnetization to peak fields in the 10 to 40 mT interval success- fully removed secondary components of natural remanent magnetism (NRM). Characteristic NRM directions define three polarity zones, a 50 m-thick normal polarity zone bracketed by two reversed polarity zones. The Cub Creek local faunule CC-2 (To3 or Til) occurs within the upper portion of the normal polarity zone. Cub Creek local faunules CC-1, CC-3, and Eagle Quarry (all Tii) occur in the upper reversed polarity zone. These data, along with faunal and magnetostratigraphic data from the San Juan Basin, New Mexico, and the southern Clark's Fork Basin, Wyoming, allow the transition between the Torrejonian and Tiffanian Land-Mammal Ages to be correlated with the later portion of chron 27. Paleomagnetic and paleontologic data from isolated quarries in the southern Clark's Fork Basin allow Mantua Quarry (Pu1) to be correlated with chron 29r, while Rock Bench Quarry correlates with the later portion of chron 27r. Data from the Crazy Mountain Basin in Montana indicate that Silberling Quarry (To3) correlates with chron 27r, while Douglass Quarry (Til), Scarritt Quarry (Ti2), and Locality 13 (Ti3) correlate with chron 26r.

INTRODUCTION zones into a broader context. The objective The transition between the Torrejonian here, therefore, is to report the results of (To) and Tiffanian (Ti) Land-Mammal Ages magnetic polarity stratigraphic sampling in (middle and late Paleocene, respectively) ap- the eastern Crazy Mountain and northern pears to represent a major period of faunal Clark's Fork basins, and to relate that infor- change (Sloan 1969; Gingerich 1976), but it is mation to earlier studies in the southern still one of the most poorly known intervals in Clark's Fork and San Juan basins (Butler et mammalian history. Correlations between al. 1981; Butler and Lindsay 1985). late Torrejonian and early Tiffanian mammal Archibald et al. (in press) have recently re- localities from separate depositional basins viewed the North American Land-Mammal have therefore proven difficult. Particular Ages for the Paleocene epoch. They have difficulty in correlating land-mammal age revised definitions of zones within these subdivisions between areas in the northern land-mammal ages, discussed correlations part of the Western Interior (e.g., Crazy between various faunal localities, and doc- Mountain Basin of south-central Montana, umented magnetostratigraphic correlations Clark's Fork Basin of northwestern Wyo- where such data are available. A specific ming and south-central Montana) and the problem discussed by Archibald et al. (in south (e.g., San Juan Basin of northern New press) is the transition between the Torre- Mexico and southern Colorado) makes it im- jonian and Tiffanian Land-Mammal Ages. portant to place the temporal position of The Torrejonian has been divided into three biochronologic zones (Tol to To3) and the Tiffanian into six (Til and Ti6), but inter- 1 Manuscriptreceived January9, 1987;accepted pretations vary regarding the number of April 20, 1987. zones that can be adequately defined within each of these land-mammal ages. Also, no [JOURNALOF GEOLOGY,1987, vol. 95, p. 647-657] © 1987 by The University of Chicago. All rights magnetostratigraphic data have yet been pub- reserved. lished from a stratigraphic section containing 0022-1376/87/9505-006$1.00 the Torrejonian/Tiffanian transition. When 647

This content downloaded from 64.251.254.77 on Mon, 28 Oct 2013 18:26:12 PM All use subject to JSTOR Terms and Conditions 648 R. BUTLER, D. KRAUSE, AND P. GINGERICH publishingthe magnetic polarity stratigraphy terminating in slickensided surfaces, etc.) of the Black's Peak Formation (Big Bend, that would probably make construction of a Texas), Rapp et al. (1983) interpreted it to complete magnetostratigraphic section diffi- contain the Torrejonian/Tiffanianboundary, cult or impossible even if exposure was more but Archibald et al. (in press) consider the continuous. Accordingly, magnetostratigraphic lowest faunal level in the Big Bend section to data from the Crazy Mountain Basin are lim- be Tiffanianrather than Torrejonian. ited to determinations of paleomagnetic Magnetostratigraphicstudy of the southern polarity of several of the most important Clark's Fork Basin, Wyoming, by Butler Torrejonian and Tiffanian fossil localities. et al. (1981) determinedthe positions of the These data are presented here. late Tiffanian,Clarkforkian, and Wasatchian A single stratigraphic section containing Land-Mammal Ages (and zones therein) both late Torrejonian (To3 zone) and early withinthe magneticpolarity time scale. How- Tiffanian (Til zone) fossil localities does exist ever, the earliest zone of the Tiffanianrepre- in the northern Clark's Fork Basin near the sented in that section is Ti2. This zone is con- Wyoming-Montana state line on the divide tainedwithin the basal reversed polarityzone between Hunt Creek and Cub Creek (fig. 1). (Powell A-), which was correlated with Magnetostratigraphic data from that section chron 26r of the magneticpolarity time scale. are reported in this paper. Also included is a From magnetostratigraphicstudy of early summary diagram of geochronologic calibra- and middle Paleocene continentaldeposits of tion of North American Land-Mammal Ages the San Juan Basin, New Mexico, Butler and for the Paleocene and early Eocene epochs Lindsay (1985)have shown that the youngest provided by magnetostratigraphy. Torrejonian zone (To3) represented in those deposits correlateswith the magneticpolarity LOCALITIES AND PALEONTOLOGY time scale in the interval between the later The mammaliantaxa used by Archibaldet part of chron 27r and into chron 27. How- al. (in press) to delimit To3, namely Pan- ever, no section in the San Juan Basin con- tolambda and Pronothodectes, appear to tains Tiffanian superposed on Torrejonian. have either been rare taxa (Pantolambda)or Therefore the Torrejonian/Tiffaniantransi- to have had limited geographic distribution tion is surmised to occur somewhere in the (Pronothodectes). The first appearance of intervalbetween the later portionof chron 27 Pantolambda was retained as the lower and the early portionof the followingpolarity boundaryof zone To3 by Archibaldet al. (in chron 26r. Obviously it would be desirableto press) primarily for historical reasons and obtain magnetostratigraphicdata from a con- also because a better substitutewas unavail- tinuous section that contains both late Torre- able. Pronothodectes is locally much more jonian and early Tiffanianfossils in order to abundant than Pantolambda and therefore confirmand perhaps refine the placement of could be proposed but, unfortunately, it ap- the Torrejonian/Tiffaniantransition within pears to have been geographicallyrestricted the magnetic polarity time scale. to the northernpart of North America (i.e., Our first attempt to accomplish this objec- from Wyoming northward). Thus, correlative was to collect a magnetostratigraphic tions between the classic and type Torrejo- section from the eastern Crazy Mountain nian in the San Juan Basin and areas in the Basin of south-central Montana. This re- north have yet to be fully resolved. gion contains the best known and most The first appearance of Plesiadapis well-sampled sequence of middle and late praecursor marks the beginningof the Tiffa- Paleocene localities for fossil mammals.It is nian Land-MammalAge and the Plesiadapis the only known basin in which the last Torre- praecursorLineage Zone (Ti1).The next two jonian (To3)and the firstthree Tiffanian(Til- Tiffanian zones are the Plesiadapis anceps Ti3)zones are sampled.However, the quality and the Plesiadapis rex Lineage Zones. The and continuity of outcrop (more accurately type localities of Plesiadapis praecursor, P. the lack thereof) do not permit a continu- anceps, and P. rex are Douglass Quarry, ous magnetostratigraphicsection to be con- ScarrittQuarry, and Locality 13, all of which structedin that region. In addition,we found occur in the eastern Crazy MountainBasin of evidence of structural complications (beds south-centralMontana. This basin, therefore,

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paleomagneticBasinline. and state

Mountain

stratigraphicCrazy the the (1981). of and Montana/Wyomingal. et sitespart the

fossilnear Butler of in Basin northeastern locationtheFork discussed thein is Clark's area showingthe townshipsin Bench troughthree of township partsone structural of Coulee-Polecat Basinoccupypart Sand

rocks sites the occupyof Basinsites Basin-Bighorn sedimentarymountains Creek stratigraphy Mountain Mountain volcanic Mesozoic Crazy and and CrazyCreek/Hunt here. the Cubpaleomagnetic of

CenozoicPaleozoic Crystalline Eocene discussed 1.-MapMontana. early FIG. sectionscentraland

This content downloaded from 64.251.254.77 on Mon, 28 Oct 2013 18:26:12 PM All use subject to JSTOR Terms and Conditions 650 R. BUTLER, D. KRAUSE, AND P. GINGERICH is of fundamental importance for biochro- quarriesare now consideredto be late Torre- nologic zonation of the early and middle Tif- jonian (To3) in age and Scarritt Quarry is fanian. thoughtto be of early, but not earliest, Tiffa- The Crazy Mountain Basin is a broad nian age (Ti2 of Gingerich 1975, 1976; Ar- structuraldepression that is the most north- chibald et al. in press). Mammalianfaunas westerly part of a NW-SE trending trough from Gidley and Scarritt quarries were re- that also includes, from northwest to south- viewed by Rose (1981). In 1940, a fourth ma- east, the Stillwater, Clark's Fork, and jor locality, Douglass Quarry,was developed Bighorn basins (Gingerich 1983; fig. 1). The by Silberling and a small field crew from CrazyMountain Basin is dividedinto western PrincetonUniversity (Bell 1941).A few addi- and eastern portionsby the Crazy Mountains tional specimens were discovered at Doug- and is boundedto the south by the Reed Point lass Quarryby University of Michigan field Arch. The Stillwater Basin is in turn sepa- parties during brief visits to the area in the rated from the Clark's Fork Basin by the summersof 1978and 1980.Princeton Univer- Nye-Bowler Lineament. sity and University of Michigan collections Paleocene mammalswere first discovered from Douglass Quarry, accorded an earliest in the eastern Crazy MountainBasin in 1901 Tiffanianage (Til), were describedby Krause by E. Douglass. These were, in fact, the first and Gingerich (1983). Large collections of fossil mammalsever found in Fort Union de- fossil mammals from Scarritt and Douglass posits. Douglass (1902a, 1902b)described the quarries, and additionalspecimens from Sil- specimens and drew tentative correlations berling Quarry, Locality 13, and several with Torrejonianbeds in the San Juan Basin. other localities have since been obtained by Douglass was accompaniedby a resident of State University of New York at Stony nearby Harlowton, Albert Silberling, who Brook field crews during the summers of later found two of the richest localities in the 1982-1986. Crazy MountainBasin known to date: Gidley Much of the Crazy MountainBasin is cov- and Silberling quarries. Douglass (1908) de- ered by vegetation, and the known quarries scribed the early collection from these two and surface localities yielding Torrejonian quarriesin 1908. In 1909 and 1911, however, and Tiffanianfossils can only be placed into Silberling,along with J. W. Gidley, obtained relative stratigraphic position by leveling a much larger collection from the two quar- techniques. Gidley and Douglass quarries, ries for the U.S. National Museum. These which bound the Torrejonian-Tiffaniantran- specimens comprisedthe subject materialfor sition, are separatedby a covered intervales- a series of papers on the mammalianfauna of timated to be approximately 500 m thick the Crazy Mountain Basin by Gidley (1909, (Simpson 1937a). Searching for continuous, 1915, 1919, 1923). well-exposed stratigraphicsections spanning The mammalianfaunas from Gidley and the Torrejonian-Tiffanianboundary, we in- Silberlingquarries were treatedas a whole by vestigated a series of previously known Simpson (1935, 1937a),who also listed or de- Princeton University fossil vertebrates lo- scribed mammalianspecimens from 55 other calities in badlands on the divide between localities in the basin (discovered primarily Cub Creek and Hunt Creek east of the town by Silberling).In 1935, a third majorquarry, of Belfry in the northernClark's Fork Basin, Scarritt Quarry, was developed by Simpson Carbon County, Montana, and prospected and Silberling,and the fauna from it was de- for new localities in the area. Tertiarystrata scribed by Simpson (1936, 1937b). In addi- in this area consist of dark gray to olive gray tion, one importantsurface locality, Locality carbonaceousshale and mudstone, with sub- 13, produced mammals of middle Tiffanian ordinateinterbedded sandstone, lignite, dark age (Ti3) (Simpson 1937a). All of these lo- ferruginousconcretions, and orangefreshwa- calities are in Sweetgrass and Wheatland ter limestones, all mapped and correlatedas counties, Montana. the Lebo Member of the Fort Union For- Simpson's classic studies document a se- mation by Stow (1938; see also Rice 1976; quence of middleand late Paleocene mamma- Hickey 1980;Gingerich 1983). lian faunas from localities of estimated super- The Lebo Member(at least its upper part) positionalrelationships. Gidley and Silberling is well dated as Torrejonian in the Crazy

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TABLE 1

LATE TORREJONIAN (TO3) AND EARLY TIFFANIAN (TI1) FOSSIL VERTEBRATES FROM PRINCETON UNIVERSITY (PU) AND UNIVERSITYOF MICHIGAN(UM) LOCALITIESON CUB CREEK/HUNT CREEKDIVIDE EAST OF TOWN OF BELFRY, CARBON COUNTY, MONTANA

Locality Age Taxon Specimen

PU Section 28 To3 or Til Pronothodectes jepi or Nan- PU 17930 (right dentary) nodectes intermedius PU 17931 (left dentary) (Mammalia,Primates) PU Section29 Tit Aphronorusorieli PU 17308(left P4, right P4) (Mammalia,Insectivora) Stelocyon arctylos PU 17929(left and right (Mammalia,Condylarthra) dentaries-holotype) Pantodontaindet. PU uncatalogued(incisor (Mammalia) crownand molar frags.) PU Sections28 and29 To3or Til Pantodontaindet. PU uncatalogued(tooth and (Mammalia) bonefrags.) UM CubCreek 1 (NW4, Til Dinosaur?(Reptilia) UM79667 (bone, reworked) SW4, Section28) Champsosaurussp. UM 79668(bone) (Reptilia,Eosuchia) Chriacuspelvidens UM 79666 (rightdentary) (Mammalia, Condylarthra) UM CubCreek 2 (/4 corner To3or Til Trionyxsp. UM 80162(carapace frags.) betweenSections 28 and (Reptilia,Chelonia) 29) Allognathosuchussp. UM 80162 (teeth) (Reptilia,Crocodilia) Mammaliaindet. UM 80161 (tooth frags.) UM 80710(tooth frags.) UMCub Creek 3 (center Til Aphronorusorieli UM 80163(left maxilla) NE/4 of Section29) (Mammalia,Insectivora) Chriacus pelvidens UM 80164 (teeth) (Mammalia,Condylarthra) UM 83001(teeth) UM EagleQuarry (center Til Plesiadapispraecursor UM 80166(right dentary) SE/4of Section28) (Mammalia,Primates) Phenacodus bisonensis UM 80167 (left dentary) (Mammalia,Condylarthra)

NOTE.-All localities are in sections 28 or 29, T 8 S, R 23 E (USGS Long Draw 7- ' quadrangle).

MountainBasin (type area; Simpson 1937a) available and, consequently, it is difficultto and on Polecat Bench in the southernClark's relate any of the specimens to the Hunt Fork Basin (Stow 1938;Jepsen 1940). Hence Creek/CubCreek stratigraphicsection). The it is naturalthat fossil mammalsfound in the specimens include two plesiadapid primate Lebo Memberon the Cub Creek/HuntCreek dentariesfrom Section 28 identifiedon labels divide were all initially assumed to be Torre- as Pronothodectes and a very fragmentary jonian in age. In recent years, however, ear- pantodontfrom Sections 28 and 29 identified liest Tiffanianmammals have been found at on labels and maps as Pantolambda caviric- the top of the Hunt Creek/CubCreek section tis. Two additionalspecimens were collected as well. by Simons' Princetonfield partyfrom Section The first fossil mammals from the Hunt 29 but not initially identified to genus and Creek/CubCreek area includefive specimens species. found in 1955by Elwyn Simons, then a grad- The first specimens from this area to be uate studentat PrincetonUniversity. Each of publishedwere the two plesiadapiddentaries, these specimens is accompanied by locality identifiedby Gingerich (1976, p. 19) as Pro- informationplacing them in Section 28 and/or nothodectesjepi (with a caveat that "the ma- 29 of T 8 S, R 23 E, CarbonCounty, Montana terial is too fragmentaryfor an unequivocal (table 1), but more precise locality informa- determination").They may, in fact, be re- tion is not available(topographic maps of the ferable to Nannodectes intermedius,an ear- Cub Creek area have only recently become liest Tiffanian species. The Cub Creek pan-

This content downloaded from 64.251.254.77 on Mon, 28 Oct 2013 18:26:12 PM All use subject to JSTOR Terms and Conditions 652 R. BUTLER, D. KRAUSE, AND P. GINGERICH todont was omitted from Simons' review of (clays and fine silts) were preferentially col- Paleocene Pantodonta(Simons 1960;accord- lected. At five sites distributed throughout ing to J. H. Ostrom it is very fragmentary, the stratigraphic section, seven oriented certainly not identifiableto species or even block samples and a bulk sample (2 kg) for genus-it was never assigned a catalogue magnetic separation were collected, while number in the Princeton collection). The three oriented samples were collected from fourth Princeton specimen, consisting of left each of the remaining 15 sites. Samples from and right dentaries, was later assigned to a sites containing seven oriented samples were new genus and species of arctocyonidCondy- subjected to detailed progressive demagneti- larthra,Stelocyon arctylos Gingerich(1978), zation studies to investigate stability of natu- a taxon providing no informationabout age ral remanent magnetism (NRM). All NRM because it is as yet unknownelsewhere. The measurements were done with a cryogenic fifth Princeton specimen is here identifiedas magnetometer (ScT C-102), alternating-field Aphronorusorieli, a species elsewhere con- (AF) demagnetization with a Schonstedt fined to the earliest Tiffanian (Til). Judging GSD-5 tumbling-specimen demagnetizer, from available evidence, PU specimens from thermal demagnetization with a mumetal Section 28 could be either latest Torrejonian shielded furnace with magnetic field <10 nan- or earliest Tiffanianin age, while those from noTesla in the specimen region, and strong- Section 29 are earliest Tiffanian. field thermomagnetic analyses with a com- University of Michigan collections come puterized Cahn 2000 microbalance (Galbrun from four new localities on the Hunt Creek/ and Butler 1986). Cub Creek divide. The faunafrom each local- Curie temperatures revealed by strong- ity is listed in table 1. Locality CC-1 is from field thermomagnetic analysis of magnetic the same stratigraphicinterval as CC-3 with separates from five stratigraphic levels within Aphronorus orieli, and both therefore are the Hunt Creek section were between 150 likely to be of earliest Tiffanianage. CC-2 is and 2500C. As shown by Butler and Lindsay at a level stratigraphicallybelow CC-1 and (1985) for Late Cretaceous and Paleocene CC-3, and it may be either latest Torrejonian sediments of the San Juan Basin, the domi- (To3) or earliest Tiffanian(Til) in age. Eagle nant ferrimagnetic mineral in the Fort Union Quarryis at the base of the highest sandstone Formation of northwestern Wyoming is an in- exposed in Section 28, and it is hence at termediate composition (x = 0.55) titano- the top of the Cub Creek/Hunt Creek hematite. Acquisition of isothermal remanent stratigraphicsection. The fauna from Eagle magnetism (IRM) in progressively higher Quarry, while small, includes well pre- magnetizing fields indicates that IRM is effec- served dentaries of Plesiadapis praecursor tively saturated in magnetizing fields of be- and Phenacodus bisonensis. Plesiadapis and tween 200 and 300 mT. Thus, we do not see Phenacodus make their first appearance in any evidence in the bulk magnetic properties the Tiffanian Land-MammalAge and Plesi- for high coercivity phases such as hematite in adapispraecursor is the principalindex fossil addition to the detrital titanohematite. of earliest Tiffanianzone Til. Results of analysis of the NRM were simi- The following discussion of paleomagnetic lar to those obtained by Butler et al. (1981) on stratigraphyutilizes a 160m stratigraphicsec- sediments of the nearby southern Clark's tion measuredon the Hunt Creek side of the Fork Basin north of Powell, Wyoming. NRM Hunt Creek/Cub Creek divide. All of the intensities from the Hunt Creek section av- Princeton and University of Michiganfossil eraged approximately 4 x 10-4 A/m prior to localities discussed here occur in the upper demagnetization. Examples of progressive 50 m of this section, and those of certain AF demagnetization experiments are illus- earliest Tiffanian age are likely to be in the trated in figure 2. Above demagnetizing fields upper 30 m. of 40 mT, directions of NRM became very weak (<1 x 10-4 A/m), and components of PALEOMAGNETISM viscous remanent magnetism (VRM) ac- At Hunt Creek, paleomagnetic samples quired on short time scales in the laboratory were collected from 20 levels (sites) within a complicated the measurement of remaining 160 m thick section. Finer-grainedlithologies NRM at higher demagnetizing fields. The

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EB 013 EB 021

UP, N

HORIZ,E

DOWN,S

- i x 10-4A/m - 1.0 x 10-4A/m 1.0 O INCLINATIONA DECLINATION 0 INCLINATIONA DECLINATION FIG.2.-Example vector demagnetizationdiagrams of progressive AF demagnetizationbehavior for representativesites in the Hunt Creek section. Bar indicates scale for each diagram.Numbers adjacent to data points indicate peak AF in mT. progressiveAF demagnetizationresults indi- VGPLATITUDE POLARITY cate that secondary components of NRM Eagle subparallelto the present geomagneticfield at Quarry the sampling locality are generally removed by AF demagnetization to peak fields of about 10 mT. Demagnetizationin the 10-40 mT interval reveals a trend of the remaining NRM toward the origin on vector demagnetization diagrams suggesting isolation of a characteristicNRM vector. Thermal demagnetization revealed dominant unblocking temperatures<3000C, con- FIG.3.-Paleomagnetic polaritystratigraphy for Hunt Creek section. Lithologic column, site aver- sistent with the Curie temperature results. age VGP latitudefollowing magneticcleaning, and No high blockingtemperature components of interpreted polarity column are plotted against NRM such as those found by Butler and stratigraphicthickness. Solid data points in VGP Lindsay (1985) in magnetically overprinted plot indicate sites with within-sitegrouping of di- sediments of the San Basin were ob- rections significant from random at the 95% Juan confidencelevel using the statisticaltest of Watson served. The studies of NRM stabilitythus in- (1956). Black (white) intervals in polarity column dicate that secondary components are suc- indicate normal(reversed) polarity. cessfully removed by AF demagnetizationat 10 mT and that the polarityof the characteristic component is revealed by AF demagneti- analysis of the site by site stability of NRM zation in the 10-40 mT interval. The charac- and allowed confidence in extracting polarity teristic componentof NRM is interpretedas a determinations from these weakly mag- depositionalremanent magnetism carried by netized sediments. Data from one site were detritaltitanohematite. rejected because NRM intensity fell below 5 In order to determine the polarity stratig- x 10-5 A/m during the demagnetizing pro- raphy of the Hunt Creek section, all paleo- cedures. magnetic samples were progressively de- A stratigraphic plot of the resulting data is magnetizedat several AF levels in the range shown in figure 3. Polarity zones were 5-40 mT. Site mean NRM directions were defined through site by site analysis from determined at every AF level, and vector which virtual geomagnetic pole (VGP) lat- demagnetizationdiagrams were constructed itude is simply a convenient parameter for a for each site. This techniqueproved useful in summary stratigraphic plot. Two reversed

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PALEOMAGNETIC RESULTS FROM FOSSIL MAMMAL QUARRIES

Site A.F. J I D Locality Zone No. mT A/m o o N R MantuaQ. Pu1 PB028 30 1.8 x 10-4 -47.9 153.5 3 1.62 Rock Bench Q. To3 PB029 20 6.5 x 10-4 -28.2 177.5 3 2.89 PB030 20 2.9 x 10-4 -51.6 165.7 4 3.71 Douglass Q. Til CM010 20 2.5 x 10-3 -58.2 213.2 4 3.98 CM011 20 2.1 x 10-3 -44.5 147.8 4 3.93 ScarrittQ. Ti2 CM022 60 9.2 x 10-4 -32.0 143.1 3 2.70 Locality 13 Ti3 CM021 40 1.2 x 10-4 -47.8 104.8 8 3.90 SilberlingQ. To3 CM012 20 5.1 x 10-4 -79.4 178.1 4 3.66

NOTE.-A.F. is demagnetizing field, J is intensity of NRM, I and D are mean inclination and declination of cleaned NRM, N is number of samples, R is vector sum of the N unit NRM vectors. Where two sites were collected at a single quarry, the paleomagnetic sampling sites bracket the quarry. polarity zones (A- and C-) with an inter- calities in the Crazy Mountain Basin. Al- vening normal polarity zone (B +) are deter- though the magnetostratigraphic results from mined and are given the prefix "Hunt the lower portion of the Polecat Bench sec- Creek." The normal polarity zone (Hunt tion in the southern Clark's Fork Basin (But- Creek B +), defined by data from five strati- ler et al. 1981) did not allow confident desig- graphically superposed sites, has a thickness nation of polarity zones, collections made in of about 50 m. This is thinner than any polarity 1983 at the Mantua and Rock Bench quarries zone in the Clark's Fork Basin, where the have now allowed polarities of these quarries narrowest zone was at least 100 m thick (But- to be determined. Procedures and results ler et al. 1981). Accordingly, we must address were similar to those described above, with the issue of whether Hunt Creek B + could the exception that the dominant ferrimagnetic be an overprinted zone like that found in mineral in the continental deposits of the similar Laramide continental sediments of Crazy Mountain Basin is magnetite rather the San Juan Basin (Butler and Lindsay than titanohematite. Characteristic directions 1985). The procedures used by Butler and of NRM (summarized in table 2) clearly indi- Lindsay (1985) to detect the overprinted zone cate that all of these localities are within sedi- in the San Juan Basin were virtually identical ments of reversed paleomagnetic polarity. to those used here. But no evidence, such as The determination of reversed polarity for high coercivities detected in IRM acquisition Rock Bench Quarry contradicts the state- or high coercivities or high blocking tempera- ment by McKinney and Schoch (1983, p. 803) tures of NRM, was observed for sites within that "the typical late Torrejonian fauna of Hunt Creek B+. In addition, the average Rock Bench Quarry lies in what may be a declination observed (Do = 342°) for the five normally magnetized magnetozone ..." sites within this normal polarity zone is close to the expected declination (Dx = 3480) pre- DISCUSSION AND CONCLUSIONS dicted by the 60 Ma reference paleomagnetic Magnetostratigraphic studies in the San pole of Diehl et al. (1983) and distinct from Juan Basin (Butler and Lindsay 1985) indi- the declination of the present geomagnetic cate that zone To3 correlates with the mag- field (D = 160) at the sampling locality. Thus netic polarity time scale from approximately several lines of evidence indicate that Hunt the midpoint of chron 27r to the later portion Creek B + is a reliable recording of a normal of chron 27. Because the Cub Creek faunule polarity geomagnetic interval during deposi- CC-2 of To3 or Til zone is contained within tion of the Hunt Creek stratigraphic section. the upper half of polarity zone Hunt Creek Polarities of NRM were also determined B +, this normal polarity zone must correlate for one Torrejonian (Silberling Quarry) and with chron 27. The overlying reversed polar- three Tiffanian (Douglass Quarry, Scarritt ity zone (Hunt Creek C-) containing the Quarry, Locality 13) vertebrate fossil lo- Eagle Quarry and faunules CC-1 and CC-3

This content downloaded from 64.251.254.77 on Mon, 28 Oct 2013 18:26:12 PM All use subject to JSTOR Terms and Conditions MIDDLE-LATE PALEOCENE CONTINENTAL DEPOSITS 655 (all of Ti1 zone) must therefore correlate with is to be expected, as zone Ti2 in the southern chron 26r. Given that the Hunt Creek section Clark's Fork Basin is entirely within sedi- does not extend to a normal polarity zone ments of reversed polarity correlated with correlative with chron 26, it is not possible on chron 26r. The Locality 13 fauna (Ti3) from the basis of this section to determine an accu- the Crazy Mountain Basin is within sedi- rate position of the Tit zone within chron 26r. ments of reversed polarity. Zone Ti3 from the However, in the southern Clark's Fork Basin southern Clark's Fork Basin correlates with magnetostratigraphic section (Butler et al. the magnetic polarity time scale in the inter- 1981), Ti2 and the earlier portion of zone Ti3 val between the later portion of chron 26r and also occur within a reversed polarity zone into chron 26. Thus, the Locality 13 fauna correlative with chron 26r. It thus becomes must be correlative with the older Ti3 lo- evident that the Ti zone must occur early calities in the Clark's Fork Basin. Silberling within chron 26r (and perhaps late within Quarry contains a fauna of zone To3 and is chron 27). Accordingly, the transition be- contained in reversed polarity sediments. tween the Torrejonian and Tiffanian North Given the known limits of To3 discussed American Land-Mammal Ages (= transition above, Silberling Quarry must correlate with between To3 and Til zones) occurs in the later the later portion of chron 27r. portion of chron 27. The results of magnetostratigraphic anal- As indicated in table 2, Rock Bench Quarry yses from the San Juan Basin (Butler and containing a fauna of zone To3 is contained within sediments of reversed paleomagnetic polarity. Given the above determination of the Torrejonian/Tiffanian transition and the magnetostratigraphic limits of zone To3 in the 01 22r San Juan Basin, Rock Bench Quarry corre- 52- 23r- lates with the later portion of chron 27r. 24 Table 2 also reports paleomagnetic results for 53- 54 Mantua Quarry, which contains zone Pul, the 24r initial Puercan zone. The polarity is reversed 55- and, given the correlation of San Juan Basin 56 - 25 Puercan localities (Pu2 and Pu3 zones) with 57 - 25r chron 29, the Mantua Quarry Put zone must 58- 26 correlate with the immediately preceding reversed polarity interval, chron 29r. The Hells 59 - 26r Hollow local fauna (Pu1) and stratigraphically 60 - lower Lancian faunas also occur within a re- - 61 27r versed polarity zone correlative with chron 62- 27r 29r (Archibald et al. 1982 and in press). Thus 63- 28 the Puercan Land-Mammal Age is initiated 64 - during the later portion of chron 29r. 29 Determinations of the paleomagnetic 65 - 29r polarities of fossil localities in the Crazy 66 - 30

Mountain Basin (table 2) allow some refine- 67 - 31- ments of correlations between those faunas 68- and faunas from the Clark's Fork Basin and 69 - other locations. Douglass Quarry contains a zone fauna. Sediments bracketing Doug- Til FIG.4.-Correlation of North American Land- lass Quarry are of reversed polarity as are MammalAges (NALMA) and zones therein with those containing Eagle Quarry in the Hunt the magnetic polarity time scale of Harlandet al. Creek area. From the above arguments, both (1982).Intervals of vertebratefossil zones are indi- by adjacent column. of these Til localities correlate with the early cated vertical bars to polarity Land-mammalage boundaries that are relatively part of chron 26r. poorly defined are shown by slanted lines to indi- Scarritt Quarry contains faunas of zone TiZ cate the range within which those boundariesmay and is of reversed polarity. This observation occur.

This content downloaded from 64.251.254.77 on Mon, 28 Oct 2013 18:26:12 PM All use subject to JSTOR Terms and Conditions 656 R. BUTLER, D. KRAUSE, AND P. GINGERICH Lindsay 1985), southern Clark's Fork Basin defined, the transition between the Puercan (Butler et al. 1981), and this paper are sum- and Torrejonian Land-Mammal Ages is less marized in figure 4. Ranges of the Paleocene well determined. However, even this rela- and early Eocene vertebrate fossil zones and tively poorly determined transition in the correlations to the magnetic polarity time early Paleocene is now known to a precision scale are primarily determined from the San of about 0.5 m.y. (accepting the precision of Juan Basin and southern Clark's Fork Basin relative age calibration of the magnetic polar- results. Additional data from the North Horn ity time scale of Harland et al. 1982). Mag- Formation (Tomida and Butler 1980) and Big netostratigraphy has provided a major ad- Bend, Texas region (Rapp et al. 1983) are also vancement in geochronologic calibration of used. Definitions of vertebrate zonation fol- North American Land-Mammal Ages. low Archibald et al. (in press). The current status of knowledge regarding correlation of ACKNOWLEDGMENTS.-Contributions to North American Land-Mammal Ages of the field and laboratory work were made by Paleocene and earliest Eocene with the mag- Kathy Flanagan, E. H. Lindsay, Steve May, netic polarity time scale is summarized in this and Bob McCord. We thank J. H. Ostrom, figure. Yale University, for information regarding Note that we have constructed figure 4 the Cub Creek pantodont in the Princeton with the view that vertebrate zones will be University collection, D. Baird for loan of the considered to be of roughly equal duration Princeton University specimens (now housed unless clear evidence to the contrary exists. at Yale University), and Karen Klitz for Because of gaps in the fossil record, dura- drafting figure 1. Research was supported by tions of vertebrate zonations (e.g., Ti2, Tol, NSF Grants EAR 82-06262 and EAR 84- etc.) shown in figure 4 cannot be taken lit- 05360 to R. F. B., EARTHWATCH Grants erally. While transitions between Torrejo- (1983-1986) and NSF Grant BSR 84-067097 nian, Tiffanian, Clarkforkian, and Wasatch- to D. W. K., and NSF Grant EAR 84-08647 ian Land-Mammal Ages are quite accurately to P. D. G.

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