Correlation Chart of Frontier Formation from Greenhorn Range, Southwestern Montana, to Mount Everts in Yellowstone National Park, Wyoming

DEPARTMENT OF THE INTERIOR MISCELLANEOUS FIELD STUDIES U.S. GEOLOGICAL SURVEY MAPMF-2116 PAMPHLET

CORRELATION CHART OF FRONTIER FORMATION FROM GREENHORN RANGE, SOUTHWESTERN MONTANA, TO MOUNT EVERTS IN YELLOWSTONE NATIONAL PARK, WYOMING

By R.G. Tysdal, T.S. Dyman, D.J. Nichols, and W.A. Cobban

INTRODUCTION figure 2, and thickness relationships-for the measured sections are illustrated in figure 3. In discussing The Upper Cretaceous Frontier Formation in stratigraphic relationships, we proceed from eas* to southwestern Montana was deposited in the foreland west, from shelf to foreland basin. All megafossils basin of the Cordillera in the west and shelf facies of reported here, including those in cited references, were the foreland in the east. Lateral and vertical changes identified by co-author W.A. Cobban and are shown in the lithology of the formation reflect both in tables 1 and 2. All palynomorphs, including those depositional environments and regional tectonism. in cited references, were identified by co-author D.J. The measured sections presented here form a 120-km Nichols and are shown in tables 3 and 4. In the (75-mi) transect that is about normal to the axial citation of previous studies, we have attempted to trend of the mid-Cretaceous seaway in which the include all relevant M.S. and Ph. D. theses, but marine rocks of the formation were deposited, and others, of which we are unaware, may exist. about normal to the orogenic trend in the hinterland to the west (fig. 1). During the Late Cretaceous to MEASURED SECTIONS early Tertiary Laramide orogeny, Cretaceous strata in southwestern Montana were deformed and General locations of the measured sections are foreshortened (telescoped) by eastward transport of shown in figure 1, and detailed locations are giver on strata above major thrust faults. Frontier rocks of the the chart. The four sections were measured originally Madison Range composite section and the Mount by Tysdal; the Ruby River section was partly Everts section are not known to be separated by any remeasured with the help of Dyman. None of the major Laramide-age thrust faults that would have sections have been published previously. The moved the sections closer together than they were Frontier Formation in the Madison Range is shown when deposited, and hence, the facies present are in a composite of two measured sections, about 21 broadly similar. Frontier strata of the Ruby River km (13 mi) apart: the section at Lincoln Mountain section, in the Greenhorn Range, however, lie within represents the lower part of the formation, the section a thrust plate that is bordered on the east by the at Big Sky represents the upper part (fig. 1). The Hilgard thrust fault (fig. 1). The plate moved four sections were measured where exposures are the eastward during the Laramide orogeny and the facies best that we know of in their general areas. All of the Frontier present within it differ significantly sections were measured with a Jacob's staff equipped from those exposed farther east, both in depositional with a hand level. environments and thicknesses. The measured sections are shown here in a The purpose of this report is to review computer-generated graphic format produced by the stratigraphic relationships and nomenclature for the SRG (Stratigraphic Report Graphic), a stratigrarhic Frontier Formation in the region of the measured applications program developed by the U.S. sections, and to discuss contact relationships with Geological Survey and Petroleum Information overlying and underlying formations. These goals are Corporation of Denver, Colo. The SRG accepts accomplished through the presentation of new data, sedimentologic, paleontologic, lithologic, including the faunal and palynofloral identifications paleoecologic, and nomenclatural data for outcrop and the four measured sections displayed graphically sections and cores, and displays these data in a scale- in this report, and the reinterpretation of existing data. variant format (Petroleum Information Corporation, Regional stratigraphic relationships are shown in 1984; Dyman and others, 1985). The SRG is proprietary to Petroleum Information Corporation, STRATIGRAPHY but is available by contract to the U.S. Geological Survey for a variety of research applications. Frontier strata of the Madison Range composite section and the Mount Everts section in PREVIOUS WORK the eastern part of the region considered here record deposition in a relatively stable, slowly subsiding Early workers in the areas of the four measured shelf setting--a continuation of conditions that existed sections commonly conducted reconnaissance studies during deposition of Lower Cretaceous rocks. In and used map units that encompassed thick sequences contrast, Upper Cretaceous rocks of the much thicker of strata. These thick stratigraphic units were Ruby River section in the Greenhorn Range record subdivided by later workers into several formations, deposition in a rapidly subsiding foredeep of the including the Frontier Formation. Only the early foreland basin, reflecting a change in stability from studies that used the thick sequences of strata are that which existed during deposition of the Lower mentioned in this section. Studies that concerned the Cretaceous rocks. The nomenclature, however, does Frontier directly are cited later for each measured not change, although a wide variety of lithologies and section where appropriate in the discussions of thicknesses commonly are assigned to the Frontier contacts, paleontology, and correlation. In Formation in Wyoming and southwestern Montana. Yellowstone National Park, early studies began in the Lower and Upper Cretaceous formations that late 1800's, whereas in the Greenhorn Range the underlie the Frontier in the area of the measured earliest studies did not begin until about the middle of sections also record deposition in a fairly stable shelf the 1900's. setting; lithologies of individual formations are fairly In the northern part of Yellowstone National uniform, and the nomenclature also is uniform. In Park, strata of the Frontier Formation were included ascending order, the Lower and Upper Cretaceous in the Colorado Formation by Iddings and Weed formations are the Thermopolis Shale and Muddy (1894,1899) and Hague (1896). Directly north of the Sandstone (Lower Cretaceous), and Mowry Shale park, at Cinnabar Mountain (locality CM , fig. 1 [the (Upper Cretaceous) (fig. 2). West and southwest of same locality was denoted by DS, for Devil's Slide, the Greenhorn Range, the lithology of these in Tysdal and others, 1989b]), rocks of the Frontier formations gradually changes, and the nomenclature were mapped within the Colorado Group by Iddings reflects this change. An arbitrarily chosen geographic and Weed (1894) and Calvert (1912). line of demarcation (not shown in fig. 1) for the In the Madison Range, early studies, as well as change in nomenclature lies just south of the Ruby some published in recent years, generally involved River section, in the middle of the valley separating mapping in which the workers did not distinguish the Greenhorn and Gravelly Ranges from the Frontier strata from the overlying and underlying Snowcrest Range (Tysdal and others, 1989a, b). marine Cretaceous rocks. Frontier strata are part of a Lithic correlatives of the Thermopolis, Muddy, and unit mapped as "Colorado and Montana formations" Mowry lie within the Blackleaf Formation in the (Peale, 1896), Colorado Group (Becraft and others, Snowcrest Range and areas farther west 1970; Garihan and others, 1983), and an unnamed Strata directly above the Frontier in the Mount unit (Swanson, 1950). Along the western flank of Everts and Madison Range areas are assigned to the the central part of the Madison Range, Hadley (1969a, Cody Shale. No Cody strata are known to occur in 1980) used the name Frontier Formation for a thick the Greenhorn Range or the nearby Gravelly and succession of strata that included the Frontier Snowcrest Ranges (fig. 2). Formation as used here, but also included several formations that underlie and overlie the Frontier as MOUNT EVERTS SECTION now defined. In the Greenhorn Range and the contiguous Frontier strata of the Mount Everts section northern part of the Gravelly and Snowcrest Ranges, (fig. 1, locality ME) in the northern part of reconnaissance work was conducted by Klepper Yellowstone National Park are composed chiefly of (1950), who assigned strata of the Frontier to the medium- to light-gray, fine- to medium-grained Colorado Group. Mann (1954, 1960) assigned the sandstone and lesser amounts of siltstone. The beds same strata to the Colorado Formation. McBride contain abundant grains of quartz, feldspar, and chert; (1988) mapped Frontier strata in the central part of glauconite is present locally. Light-colored bentonite the Snowcrest Range as part of his map unit of and porcellanite are common interbeds in the upper Blackleaf and Frontier Formations undivided, whereas part of the section. The rocks were deposited in Sheedlo (1984) did not apply a name to equivalent shoreface environments and locally in upper offshore strata in the northern part of the range. (prodelta) environments. In general, they record a regressive, shallowing-upward succession. Trough Mowry and Frontier at Cinnabar Mountain, F-aser crossbeds and Ophiomorpha burrows oriented at a and others (1969) measured a total thickness of 250.2 high angle to bedding attest to the shoreface setting of m (821 ft). Wilson (1934) also measured the parts of the formation. Cretaceous rocks at Cinnabar Mountain, assigning 91 The Mowry-Frontier contact is conformable m (300 ft) to the Frontier and 64 m (210 ft) to the and gradational. Different definitions of the strata Mowry; his Thermopolis Shale unit (beneath the assigned to the Frontier at Mount Everts concern Mowry) contains an additional 94 m (310 ft) of STndy placement of the Mowry-Frontier contact. In this shale above a sandstone unit that most likely is the report, the base of the Frontier is placed at the base of Muddy Sandstone. The combined thickness of the the first ledge-forming sandstone above slope- Frontier, Mowry, and sandy shale that we believe is forming, dark-gray muddy siltstone and silty part of the Mowry is 250 m (820 ft). mudstone, which we assigned to the upper part of the Ruppel (1972, p. 28) reported a thickness of Mo wry Shale. As thus defined, we measured a only 12-18 m (40-60 ft) of Frontier strata at Mount thickness of 56.5 m (185 ft) for the Frontier. Everts, but he presented no measured section with Merewether and Cobban (1986) reported about which we could compare our data. Ruppel (1972, p. 80 m (270 ft) [their calculation of metric equivalence 28) stated that the sandstone mapped in the northern of feet] of Frontier strata at Mount Everts. Rocks part of Yellowstone National Park is correlative with that we placed in the Mowry Shale contain a the lower sandstone member of the Frontier ir the sandstone unit about 11.5-15 m (38-49 ft) beneath the Gardiner area as mapped by Fraser and others (1969). base of the Mowry-Frontier contact as we chose it; if We disagree with this interpretation, and with the sandstone were considered the base of the Frontier, Ruppel's statement (p. 29) that the middle shale and the Frontier thickness would be 71.5 m (234 ft), upper sandstone of Fraser and others (1969) are really closer to the thickness measured by Merewether and part of the Cody Shale. We measured the Cinnabar Cobban (1986). We did not include this sandstone in Mountain section and conclude that the Frontier-Cody the Frontier because it crops out only locally and does contact was placed accurately by Fraser and others not provide a mappable base for the formation. (1969). The difference in thickness reported fcr the Fraser and others (1969) measured 151 m (497 Frontier at Cinnabar Mountain, as with that reported ft) of Frontier at Cinnabar Mountain (fig. 1, locality at Mount Everts, is in placement of the Mowry- CM) directly north of Yellowstone National Park. Frontier contact. They described the formation as consisting of upper At Mount Everts, we measured 201 m (660 ft) and lower sandstone members, each about 17 m (55 of Mowry and Frontier combined. Brown (1957) ft) thick, separated by a medial shale member. A measured 152 m (498 ft) of Mowry and Frontier tripartite nature also characterizes the Frontier where combined at Mount Everts, but it is clear from his measured near McLeod 80 km (50 mi) northeast of text, his illustrated line of section measured, and from Gardiner (fig. 1, locality ML); here, the lower the absence of bentonitic strata known to be present sandstone member is more than 30 m (100 ft) thick in the lowermost part of the Mowry in the area of his and forms a cliff (Tysdal, unpub. data). The lower section, that his thickness for the Mowry did not sandstone near McLeod thins toward Cinnabar include the lowermost part of the Mowry. Mountain, where it contains much interbedded The Frontier-Cody contact is paraconformable siltstone and mudstone. at Mount Everts. Basal strata of the Cody Shale, are Strata of the medial shale unit of the Frontier middle Coniacian in age, whereas those of the at Cinnabar Mountain are similar to those in the uppermost Frontier are middle Turonian in age upper part of the Mowry Shale at Mount Everts and (Merewether and Cobban, 1986) (see paleontoTogy are included as part of the Mowry in some areas. discussed in following paragraphs). Pebbles are Where the lower sandstone of the Frontier is poorly present locally at the top of the Frontier in the exposed, where it contains a large amount of vicinity of Mount Everts, forming a zone 1-2 in. intertongued siltstone and mudstone, or where it is thick (exaggerated in the measured section). The absent, the two units must be treated as one and pebbles may represent a transgressive lag gravel mapped as the Mowry Shale. Fossils are needed to related to deposition of the overlying Cody Shah., or distinguish the Mowry from the fine-grained clastic they may be genetically related to Frontier strata; we rocks of the Frontier equivalents. interpret the gravel as part of the Frontier. Differences in reported thickness of the The age of the Frontier at Mount Evers is Frontier are related to placement of the Mowry- Cenomanian and Turonian. The uppermost sandstone Frontier contact. Different interpretations of the unit of the Frontier of the Mount Everts section thickness of the Frontier at Mount Everts and at yielded fauna of the middle Turonian Collignoniceras Cinnabar Mountain can be placed in perspective by woollgari zone (table 1, Mesozoic locality D11S21). comparing the composite thickness of the Frontier Marine palynomorphs of early to middle Cenomanian and the underlying Mowry Shale. For the combined age were collected by us from a carbonaceous shale lens within sandstone in the lower part of the Frontier faunal remains could be of species from the older C. at Mount Everts (table 3, paleobotany locality woollgari zone, although this is not the favored D6799-A). A significant time break must therefore interpretation. If the latter possibility is true, tHn lie within the sequence between the sample horizon of the two measured sections could contain some time- the shale lens (unit 14 of ME section) and that of the equivalent strata. However, the two sections do not C. woollgari horizon (unit 36 of ME section). share a key bed or like sequences, and fossils of the P. Palynomorphs from unit 30 (paleobotany locality hyatti zone were obtained from uppermost strata of D6799-B) of the measured section, between the the Big Sky section. previous palynomorph horizon and the horizon of C. The thickness of the composite section toHs woollgari, have a broad age-range of Cenomanian- 217 m (713 ft). At no place was the contact between Turonian, and thus do not provide an age constraint. the upper and lower parts of the composite section Merewether and Cobban (1986) considered the entire exposed. We have no estimate of thickness for strata sequence of marine sandstones to be middle Turanian "missing" from between those present at Lincoln in age, and that a middle Cenomanian-early Turonian Mountain and those at Big Sky. The preserved hiatus existed between the sandstone and underlying thickness of the Frontier is not uniform throughout "nonmarine" strata at the base of the Frontier. the study area. The Madison Range area was exposed The early to middle Cenomanian age of our to erosion following deposition of the Frontier and palynomorphs in the basal part of the Frontier at some strata of the upper part of the formation were Mount Everts indicates that both (1) the medial shale removed. member of the Frontier at Cinnabar Mountain and its Mount Everts lithic equivalent (which we placed in Lincoln Mountain section the Mowry) and (2) the lower sandstone member of the Frontier at Cinnabar Mountain and its equivalent- The lower part of the Frontier Formation is age strata at Mount Everts (which we also placed in 102 m (336 ft) thick at the Lincoln Mountain section the Mowry) may be as old as early Cenomanian. (fig. 1, locality LM) and is composed chiefly of More detailed biostratigraphic work needs to be done dark-gray, fine-grained sandstone and siltstone before a more detailed assessment can be made comprised of quartz, feldspar, chert, rock fragments, concerning the age of these strata. and locally glauconite. Sandstone in the upper part of the section is rhythmically interbedded with mudstone MADISON RANGE and is interpreted as a delta-front sandstone. COMPOSITE SECTION Sandstone in the lower part of the section contains abundant sedimentary structures indicative of a The Frontier Formation in the Madison Range shallowing-upward succession of lower to middle was first distinguished from other Cretaceous strata shoreface environments. The middle part of the by Tysdal and Simons (1985) during mapping in the section seems to be mostly nonmarine and contains a central and southern part of the range. The formation thick, prominent porcellanitic tuff. An unconformity generally is not well exposed and at no place in the probably exists at the top of a brackish-water mapped area was a continuous section found for the limestone-coquina that caps the middle part of the entire formation. Two measured sections are measured section. presented here and, on the basis of their megafaunal The Mo wry-Frontier contact is conformat Tc content, are interpreted as comprising a composite and was placed at the base of the first sandstone unit section of the Frontier. The lower part of the above slope-forming, dark-gray mudstone and silty formation is best exposed at Lincoln Mountain (fig. mudstone that was assigned to the Mowry Shrle 1) and the upper part is nearly completely exposed in (Tysdal and others, 1989a, b). Much of the previous a roadcut west of the village of Big Sky, about 21 km work in the east-central part of the Madison Ran?e (13 mi) to the north. constitutes fairly detailed theses studies. Hall (1961), Megafossils from the top of the Lincoln Lauer (1967), Ray (1967), Rose (1967), Bolm Mountain section are of the early middle Turonian (1969), Kehew (1971), and Walsh (1971) us^d Collignoniceras woollgari fauna! zone. Megafossils Cretaceous strata undivided, or undifferentiated, for obtained from near the middle of the Big Sky section rocks that include part or all of the Frontier are fragmented and not abundant, but are interpreted to Formation. Wilson (1970) and Vuke (1982, 198*) represent the late middle Turonian Prionocyclus hyatti identified Frontier in their studies but assigned some faunal zone. Co-author W.A. Cobban believes that of the lower Frontier strata at Lincoln Mountain to the weight of the evidence favors interpretation of the the Mowry Shale; Schwartz (1972) assigned the sane faunal remains from Big Sky as probable specimens strata to his Blackleaf Formation unit D, which he of Scaphites arcadiensis Moreman and Inoceramus equated with the Mowry; and Austin and Stoever flaccidus White, both species of the P. hyatti zone. (1950) assigned the same strata to the Aspen(?) Cobban further believes a possibility exists that the Formation, a name used in western Wyoming and eastern Idaho. The assignments were made, at least in part of the Frontier in the Big Sky section. Hie part, to include porcellanitic tuff (units 13 and 14 of Frontier is unconformable with the overlying Cody LM section) that lies about 33 m (108 ft) above the Shale. base of the Frontier as mapped by Tysdal and Simons Megafossils of the late middle Turanian (1985) and Tysdal (1989) and as defined in some detail Prioncyclus hyatti fauna! zone were collected from by Tysdal and others (1989a, b). Porcellanitic tuff in unit 18, near the middle of the Big Sky section (table the Frontier that is similar to pastel-colored 1, Mesozoic locality D11923), and from unit 54, the porcellanitic (and bentonitic) strata in the lower part uppermost stratum (table 1, Mesozoic locality of the Mowry Shale, 84 m (274 ft) beneath the base D12918). Nonmarine and brackish-water fossils that of the Frontier at Lincoln Mountain, probably led are not age diagnostic were found in delta-plain nr.ks some of the above authors to include the strata with beneath the P. hyatti fossils of the uppermost stratum this additional volcanic material in the Mowry. The (table 1, Mesozoic localities D12908, D12919, pastel strata were assigned to the Vaughn Member of D12920, and D12921). No palynomorphs were the Mowry Shale by Tysdal and others (1989b). recovered from our samples of this section. At Lincoln Mountain, Tysdal and others (1989a) reported palynomorphs of late Albian to early RUBY RIVER SECTION Cenomanian age from the basal strata of the Frontier (paleobotany locality D6950, table 3; unit 6 of LM The entire section of the Frontier Formation section). The palynomorph assemblage is now measured near the Ruby River (fig. 1, locality PR) considered to be early Cenomanian in age. This consists of interbedded .medium-gray, fine- to reinterpretation is reflective of a new interpretation of medium-grained sandstone and interbedded olive-gran molluscan megafossil data for the Cretaceous Western to gray mudstone and siltstone, and locally coaly Interior (Cobban and Kennedy, 1989); age mudstone and coal. These rocks are composed assignments of palynomorph assemblages are keyed dominantly of quartz, chert, feldspar, and rock to the molluscan fossils. The basal sandstone of the fragments; many of the sandstone beds have a "salt- Frontier at Lincoln Mountain yielded the long- and-pepper" appearance. Strata of the lower part of ranging marine brachiopod Lingula subspatulata Hall the measured section represent estuarine and coaly and Meek along with several other megafossils, backswamp sediments of a delta plain. Most of the including an unidentifiable juvenile ammonite, that upper part contains a prominent assemblage of course are not age diagnostic (table 1, Mesozoic localities stream-channel deposits of sandstone that display D11689, D11690). Megafossils from Mesozoic erosive bases and trough crossbeds that decrease in locality D11927 (table 1; unit 19 of LM section), size upward in a deposit, and that commonly are about in the middle of the measured section, are capped by plane-bedded sandstone. Associated brackish-water forms that also are not age diagnostic. overbank strata are represented by the olive and gray The early middle Turanian ammonite Collignoniceras mudstone, silty mudstone, and muddy siltstone that woollgari (Mantell) (table 1, Mesozoic locality commonly contain carbonaceous plant debris. D12343) was found at the top of the Lincoln Brackish-water and coastal sediments are present Mountain section. locally in the uppermost part of the section. The Mowry-Frontier contact may be Big Sky section unconformable in the Greenhorn Range where lowermost sandstone of the Frontier overlies poorly The upper part of the Frontier Formation is exposed porcellanitic and bentonitic strata of the 115 m (377 ft) thick at the Big Sky measured section Vaughn Member of the Mowry Shale. In the (fig. 1, locality BS). Most of the upper part of the Madison Range and farther east, the Vaughn is formation consists of repeated deposits of light- to overlain by dark-gray mudstone or muddy siltstone medium-gray, fine-grained sandstone and siltstone that forms the upper part of the Mowry. No lithic composed of quartz, feldspar, chert, and rock equivalent of the upper part of the Mowry is present fragments. Hummocky crossbeds are common in above the Vaughn Member in the area of the Frontier some units of the sandstone and Ophiomorpha section in the Greenhorn Range; the strata may have burrows occur in some of these units. These strata been eroded, although it is possible that the up~>er are interpreted as offshore mudstone and siltstone and part never was deposited there or that equivalent lower to middle shoreface sandstone. The upper 25 m (contemporaneous) strata are of different facies. The (82 ft) consists of dark-gray mudstone and siltstone, upper part of the Mowry in the Madison Range, is capped by a sequence of fine-grained sandstone and Cenomanian in age; megafauna and palynomorphs siltstone-mudstone. These upper strata are interpreted (tables 1 and 3) from the base of the Frontier as nonmarine delta-plain deposits. Thin beds (less Formation in the Ruby River section are than 1 m [3 ft] thick) of tuff, porcellanitic tuff, and Cenomanian-Turonian in age, as discussed below; bentonite occur sparingly through the entire upper hence, the paleontological data are inconclusive as to the existence of an unconformity between the top of locality as our D12355. Hadley's (1980) collection the Vaughn Member and the base of the Frontier in locality D1766 is higher stratigraphically than D1768 the Ruby River section. but we could not determine its stratigraphic position. The top of the Frontier section measured near Some generic names listed by Klepper (1950) and the Ruby River is truncated by a segment of the Hadley (1980) have been changed in table 1 to reflect Snowcrest thrust fault. No Cody Shale is known to current usage. occur in the Greenhorn, Gravelly, or Snowcrest Assemblages of palynomorphs from the Ruby Ranges. In the central part of the Snowcrest Range, River section are generally indicative of Cenomanian about 24 km (15 mi) southwest of the Ruby River through Turonian age (table 3, paleobotany localit:

114' 113° 112' 111' 46 _ I BUTTE

LIVINGSTQN

Blacktail Mountains 45« ) s Tendoy \. Mtns -A A \ Snowcrest

MONTANA Area of figure

10 20 30 40 KILOMETERS 44.

Figure 1.-Index map of geographic features and locations of measured sections of the Frontier Formation in the Greenhorn and Madison Ranges, southwestern Montana, and at Mount Everts in Yellowstone National Park, Wyoming. Abbreviations for measured sections are as follows: 55, Big Sky; LM, Lincoln Mountain; ME, Mount Everts; RR, Ruby River; CM, Cinnabar Mountain; ML, McLeod. Only the first four sections are shown on chart. AGE THIS REPORT NORTHWESTERN WYOMING

Snowcrest Greenhorn Mount Everts, Range and and and Lima area Gravelly Gallatin and Ranges Had is on Ranges

C CO Beaverhead 0 4-> ?* CO J-l Group Cody Shale Cody Shale H (fl (part) C O- (part) (part)

O t=^KZi?^x-rc= u c o CO retace Turon: Frontier Frontier Frontier

Formation Formation Formation c CO 0) C " r^« 4-) -_î--î-^^4-J _ _ >-_j_--3-îr"iL^3L^LîL^: l_ J _l_---L_~ -*» J-v CO CO ^^ *o J *> Upper £ ^ Mowry §c 0) g Vaughn !-l r-l 7 part "

4J <> r-l Kootenai Kootenai Formation C C C8 crj crj Formation Formation bJ CUr-j

Figure 2.-Correlation chart of middle Cretaceous rocks in southwestern Montana and northwestern Wyoming. Asterisk (*) denotes section missing due to thrust fault. Placement of Lower-Upper Cretaceous boundary reflects new interpretation of molluscan megafaunal data for the Cretaceous Western Interior (Cobban and Kennedy, 1989).

10 RUB Y FEET/ DETERS RIV ER O-ir-0 Thrust foul t

-200

-- 1000- -

-400 MADISON RANGE COMPOSITE SECTION

'°v BIG SKY (BS) \ and \ LINCOLN -- \ MOUNTAIN (LM) 2000 600 \ \_

BS \ °*^ \ MOUNT -800 i- . . ^v.^ EVERTS bhach«i- I e Frontier Formation \ LM

3000 Mowry Shale

h 55 Km(35 mi ) t>\f 55 Km(35 mi ) «> I

Figure 3.-Diagram of thickness relationships of the Ruby River, Big Sky and Lincoln Mountain, and Mount Everts measured sections. (Ruby River section, shown in three columns on chart, here is shown in one column.)

11 Table 1. Distribution ofmollusks identified (by W.A. Cobban) in collections from measured sections presented here

[Within each measured section (Ruby River, Big Sky, Lincoln Mountain, Mount Everts), vertical order of collections reflects relative stratigraphic position. Vertical arrangement of measured sections represented in table, however, has no stratigraphic significance. Sampled horizons are indicated by arrows at left margin of measured sections on chart. Sampled horizons are uncertain for collections of Klepper (1950) and Hadley (1980), and thus are not indicated by arrows on measured sections and do not reflect relative stratigraphic position. Symbols are as follows: X, present; -, not present in collection]

r 30 c _ . Q o ao a o a 2 o o o o o o S I o o o o w /i fik^ ^^ "-* K) M K) KJ K) CC D1766 w w w w 1192O,1] EVERTSMT U* 0* O *0 O W P" unit4 6unit D1768 W «0 CM -0 33 00 *0 0?* M P* ^* y K) H» O fO K) M. -< iepper(l95O) adley(19BO) ui w en M w *0 O W **l NJ C/J C/J *0 CO O I** CD & w ui w < 3 s* m H a? Z

1 II Co 11ignoniceras woollgari (Mantel 1) 2 II Inocerarnus sp. II 3 II Ostrea sp. II 4 II Oxqtoma sp. If X 5 II P1acenticeras sp. II X 6 II Tur-ritella sp. 7 II shark tooth II e ii Lingula subspatulata Hall & Meek ii 9 II ftphrodina sp. II 10 II Camptonectes sp. 11 II Goniomqa sp. II 12 II juuenile ammonite, indet. II II x X X X X II 13 Cor-bula sp. II II 14 Lepfesthes sp. II II 1 5 II Uelor-itina sp. II II II 1 6 II Corbula, aff. C. subt r-i gona 1 i s Meek & Haqden II 17 II Melania sp. II 18 II Ueloritina? sp. II 19 II Carinorbis s p. II ?% ?s 20 II Inocer-amus flaccidus White II 21 II Pqcnodonte sp. II Scaphites arcadiensis II 23 II ft n o i n i a s p . ii ii I! 24 II Brachidontes sp. II II II 25 II PIesiel1iptio n. sp. II II II 26 II Protelliptio n. sp. II II II 27 II unionid bivalves II II II 28 II Anom i a ? sp. 29 II Baculi tes sp. II 30 II Inoceramus lamareki stumckei Heinz II 31 II Rhi_|nehostreon suborbiculatum (Lamarck) II 32 II Scaphites car-lilensis Morrow II 33 II "Barbatia" micronema Meek? X II 34 II ftporrhais? sp. II II II 35 II Crassostrea soleniscus (Meek> II II II 36 II Cqrena aequi1 atera1is Meek? II II ^^ II 37 II P 1 eur i oca rd i a pauper-culum Meek II II ^^ II 38 II Anomia, aff. ft. micronema Meek II X 39 II Brachidontes, aff. B. multilinigera Meek II X 40 II Corbula, cf. C. perundata Meek & Haqden II ^% 41 II Eu 1 imella? sp. II X 42 II Mesostoma? n. sp. II 43 II RhLjtopho rous ? cf. R. meek i White

f\ II 44 II Turbonilla, cf. T. coa1vi11ensis Meek

f\ II 45 II Ursiriwus? sp. <2 spp.?> II II II 46 II Co rbi cu1 a sp. II II x% II 47 II Co rbula n. sp. II II ^s II 48 II ftnomia, cf. A. micronema Meek II II x% x% II 49 II Pleuriocardia sp. II 50 II Carqocorbula? cf. C.? waria Stephenson II II II 51 II Co rbicula? sp. II II X X II 52 II Crassostrea sp. II II II 53 II Parmicorbula? sp. II 54 II Pu. r gul i f era ? sp. II II 55 II Senis dakotensis (Meek t: Haujden^ II II II 56 II Corbula, aff. C. engelmanni Meek II II II 57 II Corbula, aff. C. - pqr i f o rrnis Meek II II ii 53 !i r*qrgulifera sp. II II II 59 II PIi catula sp. Table 2.-Alphabetized list ofmollusk species. Index number corresponds to column number in table 1

Index Species No. 23 Anomia sp. 39 Ariomia, aff. A. micronema Meek 48 Anomia, cf. A. micronema Meek 28 Anomia? sp. 9 Aphrodina sp. 34 Aporrhais? sp. 29 Baculites sp. 33 "Barbatia" micronema Meek? 24 Brachidontes sp. 39 Brachidontes, aff. B. multi1inigera Meek 1O Camptonectes sp. 19 Carinorbis sp. 5O Caryocorbula? cf. C.? varia Stephenson 1 Collignoniceras wool1 gari (Mantel1) 46 Corbicula sp. 51 Corbicula? sp. 47 Corbula n. sp. 13 Corbula sp. 56 Corbula, aff. C. engelmanni Meek 57 Corbula, aff. C. pyriformis Meek 16 Corbula, aff. C. subtrigonalis Meek & Hayden 4O Corbula, cf. C. perundata Meek & Hayden 35 Crassostrea soleniscus (Meek) 52 Crassostrea sp. 36 Cyrena aequilateral is Meek? 41 Eulimella? sp. 11 Goniomya sp. 2O Inoceramus flaccidus White 3O Inoceramus lamarcki stumckei Heinz 2 Inoceramus sp. 14 Leptesthes sp. 8 Lingula subspatulata Hall & Meek 17 Melania sp. 42 Mesostoma? n. sp. 3 Dstrea sp. 4 Oxytoma sp. 53 Parmicorbula? sp. 5 Placenticeras sp. 25 PIesielliptic n. sp. 37 Pleuriocardia pauperculum Meek 49 Pleuriocardia sp. 59 Plicatula sp. 26 Protelliptio n. sp. 21 Pycnodonte sp. 5B Pyrgulifera sp. 54 Pyrgulifera? sp. 31 Rhynchostreon suborbiculatum (Lamarck) 43 Rhytophorous? cf. R. meeki White 22 Scaphites arcadiensis 32 Scaphites carlilensis Morrow 55 Senis dakotensis (Meek & Hayden) 44 Turbonilla, cf. T. coalvillensis Meek 6 Turritella sp. 45 Ursirivus? sp. (2 spp.?) 15 Veloritina sp. 18 Veloritina? sp. 12 Juvenile ammonite, indet. 7 shark tooth 27 unionid bivalves______

14 Table 3.--Distribution of palynomorphs identified (by DJ. Nichols) in collections from measured sections preserted here

[Within each measured section (Ruby River, Lincoln Mountain, Mount Everts), vertical order of collections reflects relative stratigraphic position. Vertical arrangement of measured sections represented in table, however, ha? no stratigraphic significance. Sampled horizons are indicated by arrows at left margin of measured sections on chart. Species 1-12 are marine taxa; all others are nonmarine. Symbols are as follows: X, present; -, not present in collection]

1 II RIVERRUBY ! ! D6799-A D6799-B EVERTSMOUNT D695O D7412 LINCOLNMTN. D68O1 D&773-A D68O4 D6773-B D68O5 D68O6 ! 1

1i !1 1 1 1 1 1 1 1 1 1 Baltisphaeridium mu 1t isp inosura Singh 1 .... x 1 2 Camarozonosporites insignis Morris 1 X X X X X X " ! 3 Cicatricosisporites spp. 1 ...... 1 4 Or ibroper idinium eduardsii (Cookson & Eisenaclc) Da wet) 1 X X x X X X X X X Cqathidites minor Couper 1 5 ll1 ...... 6 1! Cu,c 1 onephel ium uannophorum Oaveu, i Diconodinium pusillum Singh Eucomrni idites minor Groot & Pennq 1 B II1 X X X > X XXX 1 9 ( Gleicheniidites senonicus Ross \ X X X X XX* 1 10 Laevigatosporites sp. 1 ...... 1 11 II 01igosphaeridium sp. 1 1! 1 12 II Owoidinium werrucosum

Sir Triporoletes cenornani anus (Figasie) SriwasTawa 1 1 1 40 1 ftppendicisporites sp. 10 Cupuliferoipollenites sp. 1 Nqssapollenites albertensis Singh 1 1 i &f l Toroisporis sp.

15 Table 4.--Alphabetized list of palynomorph species. Index number corresponds to column number in table 3

Index Species No. 36 Append!cisporites auritus-Agasie 4O Append!cisporites sp. 1O 33 Araucariacites austral is Cookson 1 Baltisphaeridium multispinosum Singh 2 Camarozonosporites insignis Morris 24 Chichauoadinium vestiturn (Brideaux) Below 29 Cicatricosisporites hallel Delcourt & Sprumont 3 Cicatricosisporites spp. 37 Cicatricososporites sp. 18 Corollina sp. 19 Costatoperforosporites foveolatus Deak 4 Cribroperidinium edwardsii (Cookson & Eisenack) Davey 20 Cupuli-f eroidaepollenites sp. 41 Cupuliferoipollenites sp. 5 Cyathidites minor Couper 21 Cyathidites sp. "T" 6 Cyclonephelium vannophorum Davey 7 Diconodinium pusillum Singh 31 Echinatisporites varispinosus (Pocock) Srivastava 8 Eucommiidites minor Groot & Penny 30 Foraminisporis wonthaggiensis (Cookson & Dettmann) Dettmann 9 Gleicheniidites senonicus Ross 1O Laevigatosporites sp. 25 Luxadinium propatulum Brideaux & Mclntyre 34 Microreticulatisporites sp. 42 Nyssapollenites albertensis Singh 38 Nyssapollenites sp. 26 Odontochitina operculata (Wetzel) De-flandre 22 Oligosphaeridium pulcherrimum (De-flandre & Cookson) Davey & Williams 11 Oligosphaeridium sp. 12 Ovoidinium verrucosum (Cookson & Hughes) Davey 28 Palaeohystrichophora in-fusorioides De-flandre 13 Pityosporites spp. 14 Pterospermella sp. 32 Ouadripollis krempii Drugg 15 Retitrescolpites spp. 35 Retitricolporites spp. 23 Stereisporites spp. 16 Taxodiaceaepollenites hiatus (Potonie) Kremp 43 Toroisporis sp. 39 Triporoletes cenomanianus (Agasie) Srivastava 27 Vitreisporites pallidus (Reissinger) Nilsson 17 obligate tetrads, unidentified