The St. Eugene Formation and the Development of ...

The St. Eugene Formation and the Development of the Southern Rocky Mountain Trench

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JOHN J. CLAGUE' Deparrmenr of Geological Sciences, University ofBritish Columbia, Vancouver, British Columbia Received January 4,1974 Revision accepted for publication March 18, 1974

The Tertiary history of the southern Rocky Mountain Trench is inferred from a study of the distribution, stratigraphy, fabric, lithologic composition, structure, and palynology of the Miocene St. Eugene Formation in southeastern British Columbia. The St. Eugene Formation consists of flood-plain and fan facies and represents the upper part of up to about 1500 m of sediments which accumulated in the proto-Rocky Mountain Trench upon cessation of Laramicte deformation and after initiation of extension and block faulting in the eastern Cordilfern during Emene or early Oligocene time. Deep Tertiary basins in the southern Rocky Mountain Trench are bounded on the east and west by high-angle faults parallel to the Trench margins and on the north and south by faults transverse t o the trend of the Trench. Block faulting of a half-graben style wets probably contemporaneous with sediment deposition, but at least 600 m of displacement on the east boundary fault postdates deposition of the St. Eugene Formation. Although there i s no present seismic activity along the Rocky Mountain Trench north of latitude 49'N, Holocene fault scarps and earthquakes in a zone along the Rocky Mountains of the United States attest to the continuation of block faulting south of 49"N. The St. Eugene microflora includes at least 39 genera of f e r n , gymnosperms, and anthophytes. Phytogeographic reconstraction based upon the habitats of extant counterparts indicates floral elements growing on poorly dra~nedlowlands, adjacent slopes, and montane upIands; thus, there was moderate to high relief in southeastern British Columbia during St. Eugene time. The climate apparently was temperate, with warm summers, miId winters, and abundant, uniformty distributed precipitation. This contrasts with the present climate of the southern Rocky Mountain Trench which is semiarid with hot summers and cold winters. and suggests that the mountain barriers which presently restrict cml, moist, Pacific maritime nit masses to the coast were lower during the Miocene, or that the polar seas were relatively warm. L'histoire de la partie sud de la tranchie des Montapes Rocheuses durant le Tertiaire se dkduit de I'itude de la distribution, stratigraphic, texture, structure, composition lithologique, et paiynologie de la formation de St-Eugene du Mioctne du sud de la

Colom hie Britannique. La formation de St-Eughe est reprhentk par le facZs de plaine d'inondation et celui de cBne d'kboulis: elle reprisente Ea partie supkieure des sCdiments qui ont pu atteindre jusqu'k 1500 m. dY&paisseurqui .se sont acnunul& dans la tranchie initiale des Montagnes Rocheuses 21 la fin de la dgormation Laramienne et aprks le dkbnt dcs faitles de tension de I'esl des Cordilli.res durant 1'Eockte ou au dkbut de I'OligmPne. Les bassins profonds du Tertiaire dans la partie sud de la tranchie des Montagnes Rocheuses sont born& ii I'est et 5 I'ouest par des failles abmptes parallkles 2 la bordure de la tranchh, au nord et au sud par des failles tranaversales. Las blocs faill6s d'apparence "graben" sont probablement cantemporain & Ia saimentation, mais il y a au moins 600 m. de dtplacement sur la cat6 est qui est posttrieur k !a skdimentation de la formation de St-EugZne. Meme s'il n'y a plus d'activitC sCismique lc long de la tranchCe des Montagnes Rocheuses au nord de la latitude 49"N, Ies escarpernents de faille Holockne et Ees trernblements de terre dans une zone le long des Rocheuses arnrifoi~aieae Carplnus-ostrya carya castanea Clienopudiacede-Amaranthacehe COmposlLae

coiylus Krl~G'eae

Fraxlnus Crarnlneae Ilex Juglans Liquldambar Myrlca Nyssa

Pachysandra-sarcococca P1dCd""S PtCrDCaryd

Qucrcus SalIx

?'Ilia Ulmul-delkovs

*Location o f s a m p l e s i t e s :

(11 49-04'N,

( 3 ) 49'36'N.

palms and broadleaf evergreens are replaced by warm temperate, deciduous anthophytes and conifers. In the late Tertiary, cool temperate taxa and herbs are important floral elements. Although many late Tertiary megafloras from the Pacific Northwest have been described (references in Chaney 1959; Axelrod 1964), detailed palynological studies are few. Table 2 illustrates the similarity between the St. Eugene and some other microfloras of Miocene age in British Columbia and the northwestern United States which are located in Fig. 12. Because microfloras and megafloras from the same fossil locality often differ (in part because of relative differences in pollen, leaf, and fruit productivity and the relative

transportability and preservation of these materials), direct comparisons are not made with Miocene megafloras. Pie1 (1969, 1971) has described a succession of middle to late Tertiary pollen floras in central British Columbia. The Miocene assemblage, indicative of a warm temperate climate with abundant summer rainfall, contains essentially the same taxa as the St. Eugene Formation. The deciduous broadleaf element which characterizes Piel's Miocene unit is subordinate to the coniferous element in the succeeding Mio-Pliocene assemblage which Pie1 believes to have existed under a cool temperate climate. Also clearly different is the subtropical to warm temperate Oligocene flora which includes many genera not found in younger rocks of the

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FIG. 11. Selected plant microfossils from the St. Eugene Formation. A-Lycopodium; B-Osn~lolda; C-Pulypodiaceuc-Der~nstaedtiaceac~; D-Triplanosporites; E-Cedrus; FEphedra; G-Glyptostrobus; H-Picra; I-Tsuga; J-Aesculus; K and L-Alnus; M-Betula; N-Carya; 0-Compositae; P-flex; Q-Juglans; R-Liquid-ambar; S-Nyssa; T-Pterocctr ya; ~ P a c 1 1 y s a n d r . a - S u r c o c o c c a V-L ; 111, 1r.s-ZelXo~~o; V i and X-unidentified tricolpate pollen. Bar length represents 10 pm.

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C A N . J . EARTH SCI. VOL. 11, 1974

TABLE 2.

Comparison of the St. Eugene microflora and Miocene microfloras from British Columbia and the northwestern United States. The floras are located in Fig. 12.

Palynomorph taxa, SL Eugene Forrnat~on

(I1

(2)

3

lb)

(5)

(6)

(1)

(8)

(9)

(lo)*

(11)"

(12)*

(13)*

(14)

D l v l s l o n Lycopodophyfa

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Lycopodium Selaginella Dlvlslon Pcerophyca Deltoldo~pora Osmunda

Y

X

x

x

x

x

x

Y

x

K

t

X

X

x x

x

I

*+

**

tt

th

*f

I

X

Y

X

X

X

~

I

X

X

L

X

X

T

x

x

x

X

I

I

Polypod~a~eae-Dennstaadrraceee LaevlgafOSpOrlteS T r l p l a n o s p o r i ces

D l v l s l o n Camferophyta Ables Cedris

Y

Y

x x

*.

x

Cupressaceae-Taxaceae Ephedra Glypto~trobus Metdsequoia Plcra Pinus Podocarpus Pseudotsuga-Larlx

Y

1

Y

X

x

x 7.

x

x

x

tf

*f

X X

+* X X

X

X

Y

X

X

I

X

X

1

x

1

X

X

X

X

X

X

I:

*

* *

X

X

1

'

6

%

Seguala

Y

T a x o d l um Tsug.3

X

D l v l s l o n Anthaphyta Acer AOSCU~US dlnus Betula

X

X

t

X

l

X

Y

X

X

X

X

X

X

X

X

X

X

X

1

X

x X

x

x

Y

x

x

I

composltac

Y

x

Y

Y

X

x

x

X

X

x

X

x

Carylus hrliaceae rriix2nus

X

x

x x

Chenopndlaceae-Amaranthaccaa

x

X

X

x

Caprlfol~aceae Carplnus-ostrqa carya Castanea

Y

X

X

X

Y

X

X

X

X

X

X

X

Gramlneae Ilex Jugla"* Llquldambar Myrlca

X X

Y

x

X

X

*

X *

I

X

X

x x

Quercus SdIlX Tllia u1mus-Zelkova

Number o f genera o r famllles absent from S t . Eugene ~ m . * * ' --

-

X

X

I

:

X

X

r

X

I

*

X

Y

NyIsa

Pachysandra-"iircococca Platanus Ptecocarya

I

x x I

1

i

I

*

I

x x

x

x

x 1

x x

x

x

*

x x

X

X

x

X

X

X X

X

X

O

h

C

Y

L

I

L

X

x

X Y

x

a

X

X

X

L

X

X

X

X

X

X X X

h

U

B

I

~

?

x

x

1

1

x x

x x

X

X

-

(1) South-central British Columbia, Mio-Pliocene (Piel 1969). ( 2 ) South-central British Columb~a,Miocene (Piel 1969). ( 3 ) South-central Brltlsh Columbia, Miocene or early Pliocene (Mathews and Rouse 19631, (4) Queen Charlotte Islands, Britlsh Columbia. Mlo-Pllocene (Marrln and Rouse 19661, (5) Whatcum Basln, Brltish Columbia. Miocene (Hopkins 1966). ( 6 ) Sucker Creek, Oregon, Miocene (Graham 1965). ( 7 ) Mascall, Oregon. Miocene (Chaney 1959). ( 8 ) Stinking Waters. Oregon, Miocene (Chamy 1959). (9) Blue Mountains, Oregon, Miocene (Chaney 1959). (10) Western Oregon, composite Miocene microflora (Gray 1964). (11) Eastern Oregon, composite Miocene microflora (Gray 1964). (12) Washingfan, composite Miocene mlcroflora (Gray 1964). (13) Idaho, composite Miocene rnliroflora (Gray 1964). (14) Kilgore, Nebraska, Miocene (MacG~nltle 1962). *Microflora includes only woody a n g l o s p e r m s and gymnosperms. **Genera of Taxodiaceae not ident~fied. ***Gymnosperms and anglosperms.

area (e.g., Diervilla, Engelhardtia, Psilastephanocolpites, Sciadopitys, and Sigmopollis) . Martin and Rouse (1966) have discussed a late Miocene or early Pliocene microflora from the Queen Charlotte Islands off the coast of British Columbia. Generically, the flora is remarkably similar to that of the St. Eugene Formation (Table 2), although gymnosperms

(specifically Cedrus, Picea, and Pinus) are relatively more abundant in the St. Eugene flora. Fern spores, especially the Polypodiaceae-Dennstaedtiaceae complex, are abundant in both floras. The microfossil assemblage from the Queen Charlotte Islands apparently grew on a coastal lowland under a relatively humid, mild temperate climate.

5

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CLAGUE: SOUTHERN ROC:KY MOUNTAIN TRENCH

FIG.12. Location of some Miocene palynomorph assemblages in British Columbia and the northwestern United States. (1) South-central British Columbia, Mio-Pliocene (Pie1 1969); (2) south-central British Columbia, Miocene (Piel 1969) ; (3) southcentral British Columbia (Mathews and Rouse 1963); ( 4 ) Queen Charlotte Islands (Martin and Rouse 1966); (5) Whatcom Basin (Hopkins 1966); (6) Sucker Creek (Graham 1965); ( 7 ) Mascall (Chaney 1959); ( 8 ) Stinking Waters (Chaney 1959); (9) Blue Mountains (Chaney 1959); (10) western Oregon (Gray 1964) ; (1 1 ) eastern Oregon (Gray 1964); (12) Washington (Gray 1964); ( 1 3 ) Idaho (Gray 1964); (14) Kilgore (MacGinitie 1962).

Eocene and Miocene microfossil assemblages from the Whatcom Basin, southwestern British Columbia and northwestern Washington have been described by Hopkins (1966, 1968). The younger assemblage includes the following dicotyledons and gymnosperms: Acer, Alnus, Carpinus, Carya, Castanea, Cedrus, Engelhardtia, Fagus, Glyptostrobus, Ilex, Juglans, Keteleeria, Liquidambar, Metasequoia, Momipites, Picea, Pinus, Pterocarya, Quercus, Salix, Tilia, Taxodium, and UlmusZelkova. Again the similarity with the St. Eugene Formation is marked. Absent are many of the characteristic Eocene palynomorphs including Anemia, Azolla, Cicatricosisporites, Pistillipollenites, and Platycarya. The Miocene flora grew on the Whatcom Basin lowland and upland basin margins under a temperate to warm temperate climate. Microfossil assemblages from five localities in south-central British Columbia are considered by Mathews and Rouse (1963) to be late Miocene or early Pliocene in age. Associated volcanic rocks at two localities yielded K-Ar dates of 10 2 2 and 12 and 13 m. y. These floras are somewhat less diverse than the St. Eugene flora, contain a higher percentage of total conifer pollen and a lower per-

931

centage of spores (especially the family Polypodiaceae), and lack Betula and Cedrus, two common constituents of the St. Eugene Formation. These floras, then, appear slightly younger than the St. Eugene microflora, although phytogeographic factors such as nearness to uplands may in part explain the differences. The St. Eugene flora is generally similar to Middle and Upper Miocene floras from Mascall, Blue Mountains, Stinking Waters, Sucker Creek, and Trout Creek of eastern Oregon (Chaney 1959; Graham 1965). Tuffs associated with the last two have been dated at 16.7 and 13.1 m.y. respectively. Differences among the floras are attributed in large part to phytogeographic variables and, to a lesser extent, age. Gray (1964, her Table 1 ) has compiled Tertiary microfloral lists for Washington, Oregon, Idaho, and California. The St. Eugene microflora is closely related to the Miocene assemblages from these states (Table 2) and shows less similarity to both Oligocene and Pliocene floras. Somewhat more distant from southeastern British Columbia is the Miocene Kilgore fossil locality in northern Nebraska (MacGinitie 1962). Nevertheless, the Kilgore and St. Eugene floras are similar, the major difference between the two being the comparative rarity of gymnosperm pollen from Nebraska. In summary, the palynomorph assemblage of the St. Eugene Formation can be considered Miocene in age on the basis of similarities with other microfloras from western North America assigned to this epoch by means of vertebrate and invertebrate fossils and radiometric age dates. The correctness of this age assignment is further indicated by the degree of modernity of the flora. Wolfe and Barghoorn (1960) have shown a close relation between mega- and microfloral age and the proportion of fossil genera still living near their fossil localities in western North, America (Fig. 13). The progressive modernization of floras during the Tertiary presumably resulted from gradual cooling and topographic differentiation. The closeness of the relationship in Fig. 13 based on widely separated floras is somewhat surprising and suggests that local climatic and topographic considerations were of secondary

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C A N . J . E A R T H SC:I. VOL. 11, 1974

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st. tuecnc

C

brmt,

I LCM

I

I 01,~-

AGE

Moment

R,-

I

OF FLORA

FIG. 13. Relation between age of flora and percent of fossil genera still living near their fossil localities for mega- and microfloras from the western United States and British Columbia (from Wolfe and Barghoorn 1960, their Fig. 1). About 44% of the fossil genera of the St. Eugene Formation are still extant in the general area of the unit.

importance in comparison to age in characterizing the degree of floral modernity. The St. Eugene assemblage with about 44% native genera is determined to be of middle Miocene age and is grouped with such floras as the Sucker Creek (41 % ), Mascall (40% ), and Trout Creek (47% ) . Paleoecology Inferred from Ecological Requirements of Extant Counterparts The paleoecology of fossil plant assemblages traditionally has been assessed by determining the distribution and ecological requirements of the extant counterparts of fossil genera. This approach was first used in studies of fossil leaves by such workers as Brown (1934), Chaney (1936, 1938, 1959), Axelrod (1941), and Dorf (1959, 1963), and later was applied to palynological studies by, among others, Traverse ( 1 955), and Rouse et a/. (1970). In such studies it is generally assumed that fossil plants had environmental requirements similar to their extant counterparts. This assumption has certain limitations (Wolfe and Hopkins 1967; Wolfe 1971; Hopkins et al. 1972), but its application here is supported by the observation that the St. Eugene flora consists of associations of plants that today occur together in restricted climatic provinces. Also, although most modern genera have wide geographic and ecological ranges, plant associations are more restricted in their distribution and ecological requirements. The St. Eugene flora resembles the modern flora of the eastern and southeastern United

States, especially of those areas between the Appalachian Mountains and the Mississippi Valley where oak-hickory-walnut-elm-beech forests border upland areas dominated by mixed deciduous and coniferous elements. A similar resemblance between Miocene floras of the Columbia Plateau and the modern floras of the eastern and southeastern United States has been noted by Chaney (1959). The presence of Taxodium and other lowland genera indicates the presence of swamps or poorly drained lowlands in southeastern British Columbia. In contrast, the strong representation of Pinaceae suggests a nearby montane region. Mixed deciduous hardwood and conifer forests probably occupied the slopes between these two habitats. The St. Eugene flora is also similar to the extant flora of warm temperate parts of central and southeastern China. Asian genera no longer native to North America include Cedrus, Glyptostrobus, Metasequoia, and Pterocarya. Numerous other St. Eugene genera are associated with these Asiatic trees under climatic conditions close to those existing in southeastern North America. Thus, the St. Eugene plant community consists of plants which now occur as two separate floral elements subject to the same summerwct, temperate climate (Chaney 1948, 1959). If the climatic requirements of Glyptostrobus and Taxodium are about the same today as during the Miocene, the lowland habitats in southeastern British Columbia during the Miocene probably had the following climate: average annual precipitation, 100 to 150 cm, evenly distributed through the year; mild winters with temperatures seldom below freezing and warm summers (Fullard and Darby 1967). If upland slopes were climatically similar to the Appalachian-Mississippi Valley region, the Miocene mixed deciduous hardwood-conifer forest received similar amounts of precipitation, with the mean temperature between 10 and 18OC. In contrast, the present climate of the floor of the southern Rocky Mountain Trench according to the Koppen classification is Bsk (Continental Cold Semiarid) (Krajina 1965 ) . Average annual precipitation at Newgate' ( 1918-1954) and Cranbrook (1916-1954) is 35 and 37 cm respectively and is distributed approximately

933

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CLAGUE: SOUTHERN ROCKY MOUNTAIN TRENCH

evenly through the year. Mean temperatures at Newgate and Cranbrook are 6 and 5°C respectively. The climate is truly intemperate, with average summer temperatures above 16°C and average winter temperatures well below freezing (Kelley and Sprout 1956). The present climate is controlled in part by the southward movement of cold continental air from the Arctic and by the northward movement in the summer of warm, dry air from the interior of the United States. Low precipitation in part results from isolation of the area from the moderating influence of the Pacific by a series of north- to northwest-trending mountain ranges. Mathews and Rouse ( 1963) have suggested that the humid, warm climate necessary to sustain the Miocene floras of southcentral British Columbia might have resulted if the Coast Mountains were much lower during the Iate Tertiary than at present. Another factor responsible for the humid, temperate climate may have been the existence of a warm polar sea during the Tertiary (Piel 1971, p. 1895). Under such conditions and with atmospheric circulation similar to that of the present, warm, moist air would move south through British Columbia instead of the cold, dry air which is a factor controlling the present distribution of vegctation. High precipitation would result as these warm, moist air masses were driven against mountain ranges by westerly winds from the Pacific. The presence of late Tertiary mountain ranges in southeastern British Columbia is shown by regional tectonic evidence and by the composition of the St. Eugene flora. The Rocky Mountains originated during the Laramide Orogeny which probably began in the late Cretaceous and ended before the early Oligocene (Douglas et 01. 1968, p. 464). Epeirogenic uplift continued from the Oligocene until late Pliocene, with many areas of the Rockies uplifted 1500 m or more during this interval (Cook 1960). Diverse habitats for St. Eugene floral elements also indicate moderate to high scIief in southeastern British Columbia during the Neogene; for example, Glyptostrnhirs and Taxndirdnr were probably limited to lowlands near the depositional site, whereas the strong representation of conifers such as Abies and Picea suggests the presence of montane uplands.

Paleogeography Inferred from Physical Characteristics o f Sediments Moderate to high relief near the depositional site is also suggested by the physical characteristics of the St. Eugene Formation and the Eocene-Oligocene Kishenehn Formation (Price 1962; Johns 1970) in the Flathead Valley to the east. Gravel and fanglomerate of the St. Eugene Formation were deposited by rivers from high-gradient tributary valleys and by mudflows from adjacent mountain fronts (Fig. 14). Barnes (1963, p. 70) has concluded that the Kishenehn Formation accumulated in large part in flood-plain lakes, swamps, and river channels; deposition of coarse conglomerate marginal to the broad flood plain was by mudflows from bordering uplands.

Neogene Deformation in Southeastern British Columbia The St. Eugene Formation provides evidence for Tertiary tectonic activity in the eastern Cordillera. The restricted occurrence of the St. Eugene Formation in the Trench and the derivation of the strata from adjacent uplands indicate that the Trench was already a major physiographic form by the Miocene. Alden (1953) has suggested that the Purcell and adjacent trenches developed by erosion and faulting prior to the Miocene. This is deduced from the fact that lake beds were deposited in the southern Purcell Trench and later covered by Columbia River basalts. Alden interpreted some bedrock benches in major valleys and basins of western Montana and Idaho as remnants of Pliocene valley bottoms produced during regional uplift prior to glaciation. Most of the major British Columbia drainage routes were established by the Miocene as evidenced by occurrences of Miocene sediments along the

FLCTXtPLAlN {ACIES

FAN ' F ~ C 15

FIG. 14. Block diagram showing the proposed model of Tertiary sedimentation controlled by block faulting in the southern Rocky Mountain Trench, British Columbia (diagram modified from a lecture sketch by W. M. Davis in King 1959, his Fig. 86).

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C A N . J . EARTH SCI. VOL. 1 1 , 1974

Fraser River and its tributaries near Quesnel and Prince George (Pie1 1969). The Miocene age of the St. Eugene Formation, however, represents only a minimum age for the formation of the southern Rocky Mountain Trench as a major physiographic form and sedimentation trough. The occurrence of deep structural basins with Tertiary fills in the Trench indicates that unexposed sediments overlying Precambrian and Paleozoic bedrock in these basins may be older than Miocene. About 2000 m of late Eocene and early Oligocene sediments crop out in the Flathead Valley to the east in a similar stratigraphic and structural setting (Russell 1954; Barnes 1963; Price 1966). These sediments unconformably overlie strata deformed during the Laramide Orogeny and, therefore, establish an upper age for this event. Late Cretaceous and early Tertiary postorogenic sediments accumulated in intermontane troughs along the northern Rocky Mountain and Tintina Trenches (Eisbacher 1972). Sedimentation in the southern Rocky Mountain Trench was probably controlled by faultblock topography. The eastern margin of the Trench coincides with a major normal fault (Leech 1966). Coarse fanglomerate derived from the east and northeast occurs on the downthrown side of the fault. Similar occurrences of fanglomerate on the west side of the Trench may be associated with a series of steeply dipping, north- to northwest-striking faults north of 49"N latitude. Thompson (1962) has concluded that the deep (up to 1500 m) bedrock basins beneath the Trench were formed by normal faulting. Presumably these basins are bounded by faults transverse to the trend of the Trench. East-west faults striking beneath the unconsolidated sediments in the Trench have been mapped by Leech (1960). At least some faulting postdates deposition of the St. Eugene Formation. Strata along Gold Creek are offset by northeast-striking dip-slip faults. The clast composition of fanglomerate adjacent to the normal fault on the east margin of the Trench indicates a minimum of 600 m of post-St. Eugene displacement. Mafic igneous clasts are absent from this fanglomerate (elevation 760 to 910 m) despite the bedrock occurrence along the adjacent

mountain front of a mafic lava as high as 1500 m. Rather, the fanglomerate consists of lithologies present in bedrock units stratigraphically above the lava. This suggests that units below and including the lava were not being eroded in this area during St. Eugene time and therefore were lower than the fan apices. Subsequently, these units were elevated relative to the fanglomerate at least 600 m aIong the bounding normal fault. Strata originally dipping west to southwest off the Galton Range scarp were mildly deformed; they now dip south to southeast. Again there is a similarity between these conditions and those occurring somewhat earlier in the Flathead Valley, where conglomerate and breccia were deposited in a structural basin on the downthrown side of the Flathead fault (Price 1966). Displacement was simultaneous with deposition as the sequence of lithologies that occur as clasts is opposite that of the normal stratigraphic succession of bedrock lithologies in the hanging wall of the fault. Cenozoic normal faults are widespread across the Cordillera. Pardee (1950) and Alden ( 1953) have described block-faulted basins in northwestern Montana which are underlain by Cenozoic sediments similar to the St. Eugene Formation. Both fan and Aoodplain facies are present, and interbeds of ash are locally common. Much of the strata are faulted and tilted. Alden has concluded that during deposition there was considerable relief in northwestern Montana resulting from block faulting superimposed on epeirogenic uplift. Uplands and basins were in about the same positions as they are now, and there is no evidence of widespread peneplanation in the area following deposition of the basin sediments. Faulting has persisted to the present in the Rocky Mountains of the United States (Woollard 1958; Milne et al. 1970), but the zone of seismicity dies out near the International Boundary. In summary, deposition of St. Eugene and other basin sediments in the eastern Cordillera postdates the major compressional deformation of the Rocky Mountains during the Laramide Orogeny. This deformation, which culminated in the development of the Columbian Foreland Thrust and Fold Belt, may have resulted in part from subduction along the Pacific

I

I

I

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CLAGUE: SOUTHERN ROC:KY MOUNTAIN TRENCH

margin of the Cordillera (Wheeler et al. 1972). Except off southern British Columbia and Washington, under-thrusting ended by the Oligocene and was replaced by lateral shifting of plates along transform faults off the coast and by isostatic uplift elsewhere. Uplift of the Cordillera was accompanied by extension and block faulting which produced the major basins and trenches of the Rocky Mountains in southern Canada and the United States. Sediment deposition was controlled by faulting along the margins of grabens and half-grabens. In places, uplift, extension, and faulting were accompanied by the widespread and abundant extrusion of lavas and the intrusion of epizonal plutons (Wheeler et al. 1972). Faulting continued after deposition of the Miocene St. Eugene Formation in southeastern British Columbia and has continued to the present in the Rockies south of 49 ON. The Rocky Mountain Trench (south of 50°N), then, consists of tectonic depressions formed by Cenozoic half-graben block faulting superposed on late Cretaceous to early Tertiary allocthonous fold and thrust structures. These depressions have been modified by sediment i nfilling, glaciation, and postglacial effects (Leech 1966). The remarkable continuity of the Trench from 47ON to beyond 59"N and its trend across structures produced during the Lararnide Orogeny sugeest that it delineates a structural boundary of continental proportions. This is supported by abundant geophysical evidence which shows the Trench to be a major crustal boundary. Between 49' and 56"N the Trench is the western limit of broad, highamplitude aeromagnetic anomalies found over the Rockies and the Plains (Haines et al. 1971 ) . Geomagnetic depth-sounding results indicate that the Trench marks a transitional zone between a conductive, hydrated lower crust to the west and a resistive lower crust to the east (Caner 1970). Gravity data show the craton ending at the Trench north of 50°N, although to the south the craton extends west of the Trench farther into the Cordillera. Finally, seismic results show that the lithosphere may thin toward the Cordillera (Wickens and Pec 1968). Berry et al. (1971) have suggested from this geophysical evidence that the Rocky Mountain Trench between 50" and 56ON is the margin of the Precambrian

935

craton. They thus agree with Price and Mountjoy (1970) who consider the Trench to mark a Paleozoic hinge zone along the craton margin.

Conclusions (1) The St. Eugene Formation consists of flood-plain and fan facies deposited in a depression similar in morphology and position to the present southern Rocky Mountain Trench. The flood-plain facies includes both highenergy river gravels deposited off tributary valleys and shallow-lake or slack-water silt and sand. The fan facies consists of talus and fanglomerate deposited along the margins of the proto-Rocky Mountain Trench and derived from adjacent fault-bounded uplands. (2) Up to 1500 m of sediment are present in a series of structural basins underlying the Trench and bordered by high-angle faults transverse to the Trench trend. The Miocene St. Eugene Formation includes only the uppermost strata of these basin fills, and it is probable that deeper sediments are in part Oligocene and perhaps Eocene in age. Sediments of Upper Eocene and Oligocene age occur in the structurally comparable Flathead Valley to the east and provide an upper age limit for the Laramide Orogeny in the southern Canadian Rockies. Compressional deformation which produced the Rocky Mountain fold and thrust belt during the late Cretaceous and early Tertiary was followed by a period of uplift and extension accompanied by block faulting in the Cordillera. This change in tectonics may be due to the cessation of subduction along the northern Pacific margin and its replacement by lateral displacement of plates along transform faults. The southern Rocky Mountain Trench formed by block faulting of a half-graben style. Deposition of sediment in the Trench and erosion of the uplands were probably contemporaneous with faulting, although 600 m or more of displacement along the east boundary fault occurred after deposition of the St. Eugene Formation. ( 3 ) Palynological results substantiate conclusions based on the texture, structure, and lithologic composition of the St. Eugene Formation. The microflora includes elements which grew on poorly drained lowlands and

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montane uplands. The climate was humid with abundant summer precipitation and mild, moist winters; it was thus more temperate than the present climate of the area. Factors which may have controlled the Miocene climate include a relatively warm polar sea and lower mountain barriers between southeastern British Columbia and the Pacific Ocean.

Acknowledgments The project was supported by National Research Council Postgraduate Scholarships and Grant A-1 107. Thanks are extended to W. H. Mathews and G . E. Rouse for guidance, criticism, and encouragement during all phases of this study. Drafts of the manuscript were reviewed by W. C. Barnes, R. J. Fulton, R. E. Kucera, W. H. Mathews, G. E. Rouse, and 0. Slaymaker. A. C. Clague assisted in the field and typed the manuscript. ALDEN,W. C. 1953. Physiography and glacial geology of western Montana and adjacent areas. U.S. Geol. Surv., Prof. Pap. 231,200 p. ANDERSON, T. W. and STEPHENS, M. A. 1971. Tests for randomness of directions against equatorial and bimodal alternatives. Stanford Univ., Dept. Statistics, Tech. Rept. 5, 19p. ANDREWS, J. T. and SMITH, D. I. 1970. Statistical analysis of till fabric: methodology, local and regional variability (with particular reference to the north Yorkshire till cliffs). Quart. J. Geol. Soc. London 125, pp. 503-542. AXELROD, D. I. 1941. The concept of ecospecies in Tertiary paleobotany. Proc. Natl. Acad. Sci. U.S. 27, pp. 545-55 1. 1964. The Miocene Trapper Creek flora of southern Idaho. Univ. Calif. (Berkeley) Publ. Geol. Sci. 51, 181 p. BARNES, W. C. 1963. Geology of the northeast Whitefish Range, northwest Montana. Unpubl. Ph.D. thesis, Princeton Univ., Princeton, N.J., 163 p. BENNEMA, J. 1%3. The red and yellow soils of the tropical and subtropical uplands. Soil Sci. 95, pp. 250-257. BERRY, E. W. 1929. The age of the St. Eugene silt in the Kootenay Valley, British Columbia. Trans. R. Soc. Can., Sect. IV, Ser. 3,23, pp. 4748. BERRY,M. J., JACOBY,W. R., NIBLETT,E. R., and STACEY, R. A. 1971. A review of geophysical studies in the Canadian Cordillera. Can. J. Earth Sci. 8, pp. 788-801. BOULTON, G. S. 1971. Till genesis and fabric in Svalbard, Spitsbergen. In: Till: a symposium, R. P. Goldthwait (Ed.), Ohio State Univ. Press, Columbus, pp. 41-72. BROWN,C. A. 1960. Palynological techniques. C. A. Brown, Baton Rouge, Louisiana, 188 p. BROWN,R. W. 1934. The recognizable species of the Green River flora. U.S. Geol. Surv., Prof. Pap. 185-C, pp. 45-77. CANER,B. 1970. Electrical conductivity structure in

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Appendix Fabric no.

levat ti on*

Location

-

Fan-fabric results. Number of observations

(m) - -

WHEELER,J. O., AITKEN,J. D., BERRY,M. J., GABRIELSE,H., HUTCHISON,W. W., JACOBY,W. R., MONGER,J. W. H., NIBLETT,E. R., NORRIS,D. K., PRICE,R. A., and STACEY,R. A. 1972. The Cordilleran structural province. In: Variations in tectonic styles in Canada, R. A. Price and R. A. Stacey (Eds.), Geol. Ass. Can., Spec. Pap. 1 1, pp. 1-81. WICKENS,A. J. and PEC, K. 1968. A crust-mantle profile from Mould Bay, Canada to Tucson, Arizona. Bull. Seismol. Soc. Am. 58, pp. 3821-1831. WOLFE, J. A. 1971. Tertiary climatic fluctuations and methods of analysis of Tertiary floras. Palaeogeography, Palaeoclimatology, Palaeoecol. 9, pp. 27-57. E . S. 1960. Generic change WOLFE,J. A. and BARGHOORN, in Tertiary floras in relation to age. Am. J. Sci. 25&A, pp. 388-399. WOLFE,J. A. and HOPKINS,D. M. 1967. Climatic changes recorded by Tertiary land floras in northwestern North America. In: Tertiary correlations and climatic changes in the Pacific, K. Hatai (Ed.), Proc. 1 lth Pacific Sci. Congr., Tokyo, 25, pp. 67-76. WOOLLARD, G. P. 1958. Areas of tectonic activity in the United States as indicated by earthquake epicenters. Trans. Am. Geophys. Un. 39, pp. 1135-1 150. WRIGHT,H. E., JR. 1957. Stone orientation in Wadena drumlin field, Minnesota. Geogr. Ann. 39, pp. 19-3 1 .

--

Axis of minimum clustering**

Axis of maximum clustering*' el =I

Al Pi - -

-

A3 P3 -

e3

S3

71.2 71.5 70.5 69.3

0.104' 0.101' 0.111' 0.125+

--

Paraglacial alluvial fan. Fraser Canyon, British Columbia 1 2 3 4

50~46'00"~ 121°50'10"~ 50046'05"~' 121°50'20"W 50~46'10"~'12lo50'25"w 50~46'20"~:121°50' 1 5 " ~

420 400 400 400

60 60 60 60

94 2 33.5 0.696' 42 13 39.4 0.597' 74 1 41.5 0.561' 117 13 39.9 0.588'

0 208 336 247

62 77 83 71

St. Eugene fanglomerate, Rocky Mountain Trench. British Columbia

*Approximate. * * ~ x e s calculated by the "eigenvalue" method for the analysis of axial orientation data (Mark 1973).

The two

vectors are orthogonal. A = azimuth; P = plunge; e = standard scattering angle around the associated vector; S = Statistic against which the null hypothesis that the population is uniform may be tested (Anderson and

Stephens 1971). '~opulation is nonuniform, 99% confidence limits. +Population is nonuniform, 95% confidence limits. &~ompositefabric of two or mare individual fabrics taken at intervals of several meters to about 100 m laterally.

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