a structural interpretation across part of the northern rocky mountains, british columbia, canada
TRANSCRIPT
A structural interpretation across part of the northern Rocky Mountains, British Columbia, Canada
ROBER I- I . THOMPSON Geologic.ci1 Su~.z~l?: oJ'Canad(1, 3.303-33rd Street N. U'., Ctrlgciry, Alta. . C'ancrda 7'21: 2A7
Received November 10, 1978
Revision accepted February 6, 1979
The northern Canadian Kocky Mountains, as exemplified by the Halfway River map-area (940) in British Columbia, consists of a rugged and mountainous structurally complex Foothills sub- province of large amplitude box and chevron-style folds in rocks of late Paleozoic and Mesozoic age. anti a structurally diverse Rocky Mountain subprovince with open folds and apparently inconspicuous thrust faults in upper Precalnbrian to upper Paleozoic rocks; separating them is a narrow topographically subdued and heavily vegetated 'transition interval' comprising more penetratively folded and faulted shales and thin-bedded carbonate rocks of late Devonian and Mississippian age.
Flat thrust faults, with displacements in the order of 10 km, which occur under the eastern margin of the Kocky Mountain subprovince (mountain front) extend across the 'transition interval' and beneath the western margin of the Foothills subprovince. These faults terminate within a decollement along the Devonian and Mississippian Besa Kiver shale, as the displacement on them is transformed into disharmonic kink-type box and chevron folds in overlying units and into tectonic thickening within the Besa River shale. Because most of the major thrust faults along the Rocky Mountains are 'blind' and cannot be traced to surface exposures, one is left with the erroneous impression that very little lateral displacement (foreshortening) has occurred in the northern Canadian Rocky Mountains.
The basic change from a well organized thrust-fault terrane in the southern Rockies to a more diverse fold terrane with few large mappable thrusts in the north is consistent with changes in the stratigraphic character of the rock prism that was deformed: the proportion of thick incompetent shale units increases northward, and major lateral carbonate to shale facies transitions traverse the eastern margin of the Kocky Mountain subprovince.
Despite the differences in structural style from south to north, strain patterns within the northern Rocky Mountains are consistent with the lateral eastward movement of a detached prism of sedimentary rocks, and support the basic tenets of thin-skinned tectonics.
La partie nord des montagnes Rocheuses canadiennes, telle qu'on I'observe dans la region cartographique de Halfway River (Y4B) en Colombie-Britannique, comprend la sous-province des Avant-monts, accidentee, montagneuse et de structure complexe avec ses plis coffres et en chevrons de grande amplitude datant de la fin du Paleozoique et du Mesozoi'que, et la sous- province des montagnes Rocheuses de structure plus variee avec des plis ouverts et des failles de chevauchement peu apparentes a premiere vue dans les roches du Precambrien superieur et du Paleozoique superieur; entre ces deux zones, on rencontre un "intervalle de transition" etroit, d'expression topographique attenuie et masque par une vegetation tr-es dense, qui comprend une plus grande quantite de shales et de carbonates a !its minces failles et plisses penetrativement qui datent du Ilevonien et du Mississippien.
Des failles de chevauchement plates avec cies deplacements de I'ordre de 10 km qu'on ren- cuntre sous la bordure est de la sous-province des Rocheuses (front montagneux) s'etendent 2 travers 1' "intervalle de transition" et en dessous de la bordure ouest de la sous-province des Avant-monts. Ces failles se terminent avec un decollement le long dl1 shale de Besa River du Devonien et du Mississippien, alors que le deplacement sur les shales se transforme en plis dysharmoniques cofYres de type kink ou en chevrons dans les unites au-dessus et en epaississe- ment tectonique dans le shale de Besa River. Parce que la plupart des failles majeures de chevauchement dans les Rocheuses sont "aveugles" et qu'on peut r\! peine les retracer en affleurements, on reste avec la fausse impression qu'il y a eu tres peu de deplacement lateral (retrecissement a I'avant) dans la partie nord des Kocheuses canadiennes.
1-e changement principal a partir d'une region bien organisee avec des failles de chevauche- ment dans la partie sud des Rocheuses h une region plus diversifiee, plissee avec des failles de chevauchement majeures en nombre restreint au nord s'accorde bien avec les changements dans le caractere stratigraphique du prisrne rocheux qui a ete Jeforme: la proportion d'unites de shale epaisses non competentes augmente vers le nord et les transitions laterales majeures dans les facies des carbonates aux shales traversent la bordure est de la sous-province des Rocheuses.
En depit des differences de style structural du sud au nord, les patrons de deformation dans la partie nord des Rocheuses s'accordent avec le mouvement lateral vers l'est d'un prisme detach6 de roches sedimentaires et supporte les principes de base de la tectonique des couches minces.
Can. J. EarthSci., 16,1228-1241 (1979) [Traduit par le journal]
0008-4077/79/06 1228- 15$0 1 .OO/O 01979 National Research Council of Canada/Conseil national de recherches du Canada
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Intrgedugltisn The purpose of this paper is to describe the basic
elements of a structure section across part sf the northern Canadian Rocky Mountains, and to out- line the nature and significance of some sf the similarities and differences in structural style that exist between the northern and southern Canadian Rockies. The observations and interpretations that follow are based on a systematic mapping program within the Halfwfay River map-area (94B), British Columbia (Thompson 1978; Fig. 1).
The %in-skinned' regional structural style of the southern Canadian Rocky Mountains has been clearly portrayed in the structure cross-sections of Bally e f a/ . (1966) and Price and Mountjoy (1976), which are based on geometric and kinematic prin- ciples that have been outlined and reviewed by ~ b u g l a s (1950. 1958) and Dahlstrom (1969, 1970. 19-77] among others. These structure sections pro- vide a well documented geometric and kinematic model from which a gravitational spreading process model for the evolution of the southern Rocky Mountains and other similar belts has been de- veloped (Price and Mountjoy 1970; Price 1972; El- liot 1976). In contrast with the attention that has been focused on the structure of the southern Woc- kies. littie has been written concerning the nature s f its northern extension into British Columbia, and that creates the impression of a fundamentally dif- ferent structural style and process of structural evo!utioa (Taylor 1972; Stott and Taylor 8972; Taylor and Stote 1973).
The southern Canadian Rocky Mountains can be conveniently subdivided 1ifa:o four linear belts or subprovinces-the Foothills, Front Ranges, Main Ranges, and Western Ranges-which are each characterized by distinctive topographic strati- graphic and structural features, and are separated by major thrust faults and thrust systems that can be traced, locally with en echelon offsets, at least north to the 54th parallel (North and Henderson 1954). However farther north, near the Peace River, the disti~~ctions between adjacent subdivi- sions become less clear. The topography, which is an expression of the stratigraphic succession and stre~ctural style, changes abruptly, and the thrust fdealts and fault zones that form sharp boundaries between contrasting terranes to the south become less continuous or die out. North of the Peace River (56th parailel) only two subdivisions are evident: the FgaothiHHs subprovince on the east and the Rocky Mountain subprovince on the west; both contain high mountains and a diversity of structural styles, and there is a considerable overlap in the
FIG. I . Halfway River map-area in relation to the major tectonic subdivisions of the Canadian Ccjrdillera within the Provinces of British Columbia and Alberta. Line A-A' is Ioc:~tion of strati- graphic cross-(iection, Fig. 6 .
ages of rocks exposed within each stabprovince. The very large thrust faults that are characteristic farther south, are replaced by Barge-amplitude folds, and minor thrust faults. At first glance it is difficult to avoid the conclusion that there has been much less foreshortening than the 200 km plus es- timated for the southern Rocky Mountains (Bally eb CB&. 1966; Brice and Mountjoy 1970).
Do these changes from south to north signal a significant change in the basic mode of deformation within the Rocky Mountain system? I will present the view that the principles of thin-skinned tee- tonics applied so successfulSy to the southern Canadian Rocky Mountains are equally applicable to the northern Canadian Rockies, but with varia- tions in detail; and that the changes that occur along strike are analogous to changes that occur within the Appalachian fareland belt from the thrust- dominated southern part to the fold-dominated central part.
Regional Fsan~ewark The northern Rocky Mountains comprise two
basic parts. the Foothills subprovince in the east and the Rocky Mountain subprovincekn the west are separated by a topographically subdued and heavily vegetated 'transition interval9 4 to 8 km wide (Fig. 2).
'The terms Rocky Mountains and Rockies are used in a gen- era1 sense here and refer to the Rocky Mountain System; Rocky Mountain subprovince refers specifical~y to the mountain ter- lane west of the Foothills subprovince.
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1230 CAN. I. EARTH
FIG. 2. Map of Halfway River area showing major geologic subdivisions, axial traces of prominent folds. and traces of im- portant thrust fzibalts* Line W-E is location of structure cross- section, Fig. 8.
The Foothills subprovince consists of two physiographically (and to a lesser extent geologi- cally) distinct belts. The 'outer' part of the belt constitutes a physiographically subdued region tinderlain by low-amplitude long wavelength folds that gradually merge eastward with undeforrned strata of the Plains region; whereas the western or 'inner' part rises abruptly from this adjacent plateau region to form a topographically high faid terrane 30 to 50 km wide (Fig. 3). For the purpose s f this discussion the structure section to be described begins at the eastern margin of the topographically high, well exposed 'inner' part of the Foothills sub- province and extends westward to include the east- ern part of the Rocky Mountain subprovince (see section line W-E, Fig. 2).
The Rocky Mountain subprovince is a fold- thrust terrane made up of Proterozoic and Paleozoic rocks; but unlike its southern counterpart, the dis- tinction between Front, Main, and Western Ranges is not possible because structural styles change rapidly both along and across strike; in addition the major thrust Fiults that are exposed do not have the through-going linear continuity typical s f t hmst faults farther south. By comparison the northern Rocky Mountains subprovince is a relatively di- verse terrawe, both geologically and physiograpki- cally .
Unlike the southern Rockies, there is no single large thrust fault that separates the Foothills sub- province from the Rocky Mountains subprovince. Along the western margin s f the broad topsgraphi- cally subdued valley that marks the "transition
FIG, 3. The abrupt change from plateau to mountains at the outer-inner Foothills boundary, as viewed northward from the Halfway River.
Fro. 4. A view eastward from the eastern edge of the Rocky Mountain subprovince at latitude 56"35'N, showing the transi- tion interval and the abrupt rise of Foothills folds in the distance. Note that the east-dipping pans1 of carbonate rocks (lower right) ;at the mountain front is unfauIted.
FIG. 5. Complex recumbent folds within shales and thin lime- stones that underlie the transition interval. Such exposures are rare, owing to subdued topography and heavy vegetation cover.
intervat' the Rocky Mountain front is for the most part defined by the east-dipping limb of a large anticline made up of middle Paleozoic carbonate rocks. East of this valley, Foothills structures rise abruptly in an en-echelhn series of large box and chevron folds consisting of upper Paleozoic and Mesozoic rocks. Despite the subdued topography and the lack of obvious structural features in the "ransition interval' (Fig. 41, the complex folds and faub ts hidden beneath this heavily vegetated terrane (Fig. 5) are an Important part of the tectonic foreshortening and mark a detachment zone which
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accommodates the change in structural gegbmetry 9" 5.s between a deeper level represented by the Rocky 2 - c z d Mountains and a shallower level represented by the 4
Foothills.
Stratigraphy Here as elsewhere along the Rocky Mountain
fold and thrust belt the deformed rocks are part of an easterly tapering wedge that comprises an older miogeocline-shelf sequence of Late Precambrian through Jurassic age, and a younger clastic wedge sequence of late Jurassic to early Tertiary age (Fig* 6).
The miogeocline-shelf sequence comprises a westerly prograded succession of carbonate and terrigenous clastic rocks that transgsessively on- lapped crystalline basement rocks of the Canadian Shield. The basal part, sf late Precambrian and early Cambrian age, is dominated by medium- and fine-grained terrigenous clastic rocks. North of the Halfway River area this comprises up to 12 000 rn of strata of Helikian age (Taylor and Stott 19733, as well as more than 10 000 rn s f Hadrynian and lower Cambrian rocks. but in the Halfway River region the Helikiarr succession is not exposed, and the Hadrynian, although present, is sf an unspecified thickness; the lgbwes Cambrian succession is prob- ably less than 1088 m thick. This uncertainty about stratigraphic thicknesses of Proterozoic and lower Cambrian strata is the reason for not completing Fig. 6 beyond the upper Cambrian stratigraphic level. The remainder of the Paleozoic successisn is characterized by platform carbonate buildups that pass laterally (westward) into time-equivalent siltstone, shale, and thin-bedded limestones of deeper water facies. The result, in gross aspect, is a series of carbonate terraces that step up eastward across the region, and record, with each step, a - - - marine transgression of deeper water terrigenous clastic rocks over regressive shallow-water earbs- nate shelf deposits. Four- of these casbsnate ter- races (middle Ordovician, middle Silurian, lower Devonian, and Mississippian), which traverse part sf the Halfway region, have exerted an important influence on structural styles. Triassic shales, silt stones, and shallow-water carbonate rocks comprise the upper part of the miogec~cline- platform sequence, and outcrop exclusively within the Foothills belt.
The total thickness of the miogeocline-platform sequence increases westward beneath the Plains fmm 21Wm at the British Columbia-Alberta boundary to a minimum total thickness in the srder - - - of 10 000 m beneath the Rocky Mountain subprov- ince.
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1232 CAN. J. EARTH SCI. VOL. 16, 1979
The ci;astic-wedge sequence consists mainly of non-marine shales and sandstsnes, which repre- sent detritus that was shed from the Cordillera as it was being deformed, and was deposited on its east- ern flank as a series of of-lapping easterly tapering wedges. Aggregate thickness of the clastic wedge in the Halfway Region is approximately 1500 rn. Un- Iike the southern Rockies, which has a considerable thickness of both Iower Cretaceous and upper Cretaceous-Tertiary deposits, only lower Cre- taceous strata cxcur in the Halfway River region.
A Structure Section Across the Northern Resckies The FOCBBI~~IBS Se'qb?~c-'ng
The Foothills subprovince is a fold terrane con- sisting s f large-amplitude box- and chevron-style folds (Figs. 72, 7b) that involve rocks of Mississip- pian through Cretaceous age. Regionally the folds are not periodic and it is common for fold axes to merge and diverge along strike (Fig. 2: Fitzgerald 1968). Limb dips of 60" or greater are csmmon and are remarkably constant along trend. Although the synclines tend to be broad flat-bottomed struc- tures, the anticlines may cc~nsist of a complex array of chevron and box folds that make up an anti- cliinonii closure. Box and chevron foids are not unique to the northern Foothills, they persist far- ther north along the eastern margin of the Mac- kenzie Mountains; and they are also remarkabl y similar in style to folds described from the centraj Appalachians (Faill 1369) and the Jura (Eaubscker 1977).
The planar character and consequent uniform dip of fold limbs, and the narrow angular nature of hinge zones prc~mpted me to fol8ow the example of Faill (1969) and to treat these box and chevron folds as mega kink bands. This circumvents the space or balanced shortening problem inherent in trying to project the shape of a concentric fold to depth (e.g., Price 1965; De Sitter 1964). It can be seen in the cross-section of Fig. 8a that an incompetent shale succession of Pennsylvanian-to-Triassic age forms a detachment zone beneath the kink folds, which accommodates any adjustments necessary to maintain the kink forin and t s balance the amount of shortening from one stratigraphic levei to another.
Beneath the incompetent succession s f Pennsyl- vanian-to-Triassic shale is a thick competent car- bonate uni t s f Mississippian age (Prophet Forma- tion, Mp). Well intersections together with SUP- face exposure show that this unit is involved in Footkills folds. Consequently 1 show it shortened by the slime amount as the overlying beds. Where surface control is very good (as in the eastern part
FIG. 7. (a) Western part of large box fold of Triassic strata, viewed northward across the Halfway River at longitude 1%3"23'W. (b) -4 succession of chevron f d d s in Triassic I . C P C ~ S ,
viewed northward from the Peace River at longitude 12%"55'W.
of the section), the amount of shortening can be simply calculated by measuring around marker beds, because the preservation of primary struc- tures in these beds makes it clear that they have not undergone much penetrative strain, and therefore the present length is about the same as the original length. Where control is lacking an area method of calculation was adopted wherein an excess volume of rock is calculated and restored to an undeformed stratigraphic thickness to give a nseasure of the amount of shortening2. This method has the ad- vantage of being relatively insensitive to small folds and faults not mapped at sur-fidce.
Once the structure section (Fig. 8a) was cons- pleted down to the level of the Mississippian carbo- nates, it became important to determine the ulti- mate depth of deformation: did it persist down to a master detachment at the top of the crystalline basement or is there a major glide zone at a higher level separating deformed from undeformed strata?
2A4 description of this method is beyond the scope of this paper and the reader is referred to papers by Bkncher (19551, Gwinn (197O), and Elliott (1977) for further information.
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The most rational explanaticraa is that the bulk of Foothills defc~rmation involves rocks no $icier than the middle Devonian shales of the Besa River For- mation, and that the underlying middle Paleozoic carbonate units have remained an essentially pas- sive basement. This conclusion ic based on two lines s f reasoning: (1) stratigraphic markers within the Wat-bottomed box synclines are aligned with or are close to the segismsal slope s f these markers as defined by relations outside the deformed belt: this implies that the top of the Devonian carbonate suc- cession is also aligned with the regional slope; and (2) tws wells drilled into the cores of major anti- climses (Headstone Creek. d-87-1, near the eastern margin of the inner Fg~othBIls. and Shell-Hudson's Bay Schoo%er, b-47-B, within the central part of the inner Foothills) hiled to reach the Devonian carbo- nates despite having been terminated within 708 rn of the projected regional dip of eke carbonate suc- cession. This does not negate the possibility of structura% il~voIvement of the Devonian carbo- nates, but it does place a limit on the magnitude of invcalvernent in that the space constraints within the anticlinal cores are too restricted t s accommodate wholesale thrust repetitions as have occurred Farther west in the Rockies subprovince. Therefore I have approximated the top of the Devonian car- bonate succession as an undeformed passive sur- face above which the Besa River shale (D,,) is a zone of dkcollement ; given its lack of competency I have interpreted the Besa River shale to be a vis- cous cushion that can Wow into anticlinal cores and conform to any structural complexides within the overlying more rigid Mississippian carbonate units (blp). It should be pointed out that the approxima- tion s f the top of the Devonian as a planar surface does not necessarily apply t c~ all pasts of the inner Foothills belt, wells drilled to the north (e.g., Shell-H.B. Klingzurt b-82-F) have encountered De- vonian rocks above regional within the cores of major anticlines; however, the interpretation that the Besa River shale is a major detachment zone of regional extent remains valid.
Although this logic applies to much of the Foot- kills belt, it must break down somewhere along the western Foothills margin, because there strati- graphic levels within the synclines no longer remain at regionai and, a few kilometres farther west, the top of the Devonian carbonate succession is ex- posed at surface.
The ROC/CC? Mc~untains segment Before discussing the western margin of the
Foothills subpr~vince and the "transition interval9 we can consider the eastern margin of the Rocky
FIG. 9. View looking sasarkh aEong the eastern margin of the Rocky Mourrtwin stalspra~vince. &owing the unfaulted east- dipping iimb of the large frontal anticline composed of middle Paleozoic carbowate rocks.
Mountains subprovince, which is defined by the shaglow- to steep-dipping eastern limb of a large anticline consisting of lower and middle Paleozoic carbonate rocks (Fig. 9). The lack of frontal thrust may lead to the initial impression that nsost of the deforrssation has invsBved vertical displacements, with little or no lateral displacements. It is only where a deep, eroded east-west valley transect and a steepening of the plunge allow observation s f deeper levels within the anticline, that one can gain a true picture of what the frontal limb represents.
One such locality is Mount Burden (Figs. %$a, b). where a flat thrust fault can be fcjllowed across the strike for 80 km. Over most of this length the fault separates relatively flat-lying Upper Cambrian t s Lower Ordovician shales from underlying Devo- nian and Mississippian Besa River shales; but at its eastern end the complete lower and middle Paleozoic carbonate succession in the Ranging wall dips eastward and is cut off abruptly against the fault, which then has Besa River shales in both the hanging wall and foot wall. En other words, the large frontal antipline at this locality is act~lallgr an al- lockthonous plate of carbonate rocks, which has been displaced at least 10 km relative to unmderlying rocks. The reason that large thrust fmlts have not been mapped along the strike of the mountain front is that the Fdults have been deflected into and foHlow the incompetent Devonian and Mississippian shales that form the dkcollernent between the box and chevron folds and the Devonian carbonate SLHC- cession in the Foothills. At the Mount Burden loc- ality, some of these shales (and thin carbonates) are preserved within the frontal portion of the thrust plate; there (Fig. 10b) the upper surface of the car- bonate succession is essentially undeformed and gently east-dipping whereas the overlying incom- petent units are folded into a series of stacked re-
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I234 CAN. J . EA4RTH SCI. VOL. 16, 1979
FIG,. 10. (a) View tcpvvard the southwest (oblique to the northwest-southea~t structural grain) of the Mount Burden threast plate. The fault can kc braced from east (left) to west (right) caver 16) km, brat cannot be followed eastward beyond the hanging wag! cut-offor along the trend c~f the mountain front. Arrow points to area shown in Fig. 18b tviklain the Mua~nt Burden plate; the: black lines above the fault tract: approximate the stratigraphic boundary between rigid middle Bevcbtrian and older carbonate rocks, and overlying incompetent middle and upper Devonian limestone and shale (see Fig. 2 for location of Mount Burden). (k) Detailed view toward the south, of structural disharnmony within the toe region of the Mount Burden thrust plate. The black line adjacent to D<. represents the planar upper (stratigraphic) boundary of middle Devonian dolostones (Stone Formation); the black line adjacent to B,, outlines a succession of stacked recumbent folds within limestone and shale of the Devonian Pine Point Formation. The Pine Point Fe~rrnation has deformed and shortened independently of the underlying more rigid tiolostones (D,).
cumbent IssscIines. Conseqa~ently, there must be a zone ef detachment or decsupling within the al- lochthonous plate itself, which allows the overlying incesmpetent units to deform independently of the underlying rigid carbonates. If this is so, then the movement that c ~ ~ c u r r e d on the thrust at depth is balanced by complex folding (and probably a myriad of minor and unobserved f a ~ ~ l t s ) at higher structural levels. Carried t s its logical con- claasic~n this BHne of reasoning implies that where the amount of shortening due to folding is equal to that due to movement on the thrust fault, the f~arjt ceases to exist and therefore it need not project to surface. Structural disharmony of this type has been documented by Fitzgerald and Braun (1965)
from within the ir~competent carbonate-shake pack- age, and a simkar style of compensation between structural levels has been described by Matwe and Bos (11978) for the Sukunka area of British Colum- bia. A similar situation exists 60 krn north of Mount Burden, near Robb Lake. A thrust fault placing Ordovician and Silurian carbonate units onto Besa River shale with a minimum lateral displacement s f 5 krn is well displayed (Fig. I la), yet the fault can be mapped only 2 krn southward along strike of the mountain front (Fig. 1 Hb), before it abruptly and hyster isusly ' disappears.
Thus the large anticline in the Paleozoic carbo- nate rocks at the muunntain front (Fig, 9) is ail- lochthonous and the easterly dipping beds in its
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FIG. 11. (a) View toward the south of a thrust fiiu8t that places Silurian and Ordovician carbonate rocks onto middle Devonian dolostones (Q) and the Devonian Besa River [br); minimum displacement is 5 km (located 12 km north of Rvbh Lake at 5;POB'N !at., 123°45'W long.). (b) View northward, showing the abrupt termination ofthe thrust shc~wn in Fig. 1 la, 2 km south of that location. In the foreground are fdds of Besa River shale (Qr) and middle Devonian dolostone (D,) apparently unaffected by the major thrust immediately to the north. In the distance the mountain front is defined over a brief interval by the thrust fault; in the hreground (left) it is defined once again by the eakt- dipping limb of a large 3nticline. Without the mcxe northerly up-plunge exposures there would be littie reason to suspect the allc~chthonous nature of the anticline.
eastern flank mark a ~ t m p or step along which the underlying thrust fault rises from a deeper dkcolle- ment zone in the lower Paleozoic succession (Kecli-aika Formation) to the higher dkcolfernent in the Devonian and Mississippian Besa River For- mation. The counterpart of this offset ramp B B ~ step occurs s n the footwall of the Pdult at least 18 km to the southwest.
Comp/eting $he Link bptwcepa F(20~&lil/,r and Rockies
Duplication of the Ordovieian-Devonian carbo- nate succession across a Rat thrust fault under the eastern margin of the Rocky Mountkiin subprovince does not entirely satisfy the correlation of struc- tures acrsss the "ransition interval' and western margin of the FocptKTls with the rest of the Foot- hills; because the top of the Devonian carbonate
surface as projected westward beneath the "transi- tion intervai' from the east is 2 km deeper than the dkcollement in the Besa Riven- Formatic~n which underlies the altochtkonous anticline at the mom- tain front. This discrepancy can be attributed t s st not her. duplication of the Lower and middle Paleozoic carb~mate successic~n on another thrust fault, which rises into the eastern Besa Riven- dkcoilernent beneath the western margin c~f the Foot hills (Fig. 8a).
This interpretation leads to a balanced section (Dahlstrom 1969) in which the amount of tectonic shortening expressed as folding and faulting of De- vonian and younger rocks in the Foothills and "ransition interval' is equivalent to the amount of tectonic shortening of the lower Paieozoic carbo- nate sequence owing to the tkv~ thrust fiiults that duplicate the lower Palecszanic section ~anmder. the western Footkills and the eastern Rocky bfoun- tains. On the othen- hand, if the same space beneath the 7rannsiti~sn interval9 and western Foothills were occupied by tectonically thickened Besa River shale the relative amount of shor-tensing in the Besa River Formation (as expressed for example by the relative thickness to cross-sectisr~ area) would be out of balance with the ;amount of tectonic short- ening of the sther stratigraphic units in the same section.
Direct evidence C B ~ this type of thrust Fnu%t dupli- cation of eke Iower Rileozoic crarbonate succession under the western Foothills margin occurs at Mount Bertki (Fig. 121, where a large thrust sheet of middle Paleozoic carbonate rocks is exposed within a large area csf upper Paleozoic shale and is sunounded by Foothills structures. This rather ex- ceptional example in which part of the thrust hukt has been exposed by erosion provides an actua%istie basis and a useful prototype fk~r the interpretation that the lower Paleozoic carbonate succession is duplicated by thrust faulting at depth beneath the 'transition interval' and western margin of the Foothills su bprovince.
A hlodel af Thrust Fault to Fold Link~ge, and CalcnBatian s f Net Snpracmstal Shortening
The field relationships described above and de- picted in csoss-section (Fig, 8a) are shown schematical%y in Fig. 13" There are two main dkcollement zones: the Cambro-Ordovician shales (Kechika Group) and the Devonian and Mississip- pian Besa River shales. Between them is a thick, competent beam of carbonate rocks, iBClX3SS which
There has been no attempt to draw Fig. 13 to scale; all strratigr.aghic thicknesses and fauit displacements are hypotheti- cal.
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1236 CAN. 3 . EARTH SCI. VOL. 16, 1978
Fre. 12. Northward view of Mount Bertha (59"1OpN !at; 123"45'W long.) showing Silurian and Devonian thrust onto Besa River shale. The t~4nsit ion interval is located west (Heft) of the photograph. The exposed thrust is interpreted as a splay from a 'blind' thrust beneath the western margin of the Foothills.
the thrusts cut very abruptly. As a thrust plate moved eastward, the ramp along which the fault cut abruptly through the lower Paleozoic carbonate rocks into the Besa River shales moved over the flat dkcollernent in the Besa River shales, and the dis- harmonic box and chevron folding thi~t developed in the Mississippian and younger rocks probably extended back over the top of the carbonates within the overriding plate, as this plate moved farther eastward than its cover. A second detachment in the incompetent overl ying shales decoaapHed the upper part of the section from the carbonates, al- lowinng it to deform independently and with an en- tirel y different style. The net effect is that tectc~nic shortening, which is expressed as a duplication of section across a discrete fault surface at depth, is transformed across a dkcollernent zone in Mississippian-Devonian shales into an equivalent tectonic shortening expressed by n complex array of folds with smwll fdults. At the point in the section where the displacement on the rnsjor thrust fault is balanced by the shortening due to folding within and above the overlying incompetent shales, the fault ceases to exist-hence the 'pin' across the right side of Fig. 13. In most areas the erosion level is deep enough to expose only the top of the folded thrust plate and the adjacent deformed succession of younger shales, but not the thrust along which the lower Paleozoic carbonate rocks have been dis- placed eastward.
Figure 8b is a palinspastic reconstruction of structure section W-E (Fig. 8.w) made by unfolding and unfaulting the deformed units in a progressive manner, starting at the eastern limit s f the structure section. This procedure incorporates the assump- tions that the structure section is essentially cor- rect. and that the amount of penetrative strain within the more competent marker units is suf-
ficiently small such that cross-section bed thick- nesses remained essentially constant through- out deformation. Applying these constraints, folded bed lengths equal initial bed lengths.
The eastern limit of the section shown in Fig. $a was used as a vertical datum, and the top of the Fernie Formation as a korzintal datum. The top of the Fermie Formation is a particujarly convenient choice, because it represents a sea-level reference for the miogeocline and therefore permits an as- sessment of its initial character immediately prior to the advent of foiding and thrusting. Extrapola- tion of this datum well beyond the existing surface exposures of Mesozoic rocks, combined with the lack c~f data concerning the nature and thickness of upper. Paleozoic and Mesozoic sedimentation, make the western part of the reconstruction less reliable.
Bed lengths were measured along competent marker units, around folds, and between faults, and were plotted in Fig. 86 relative to the vertical and horizontal datums; fault trajectories were plotted from the me;tsurements made on the stmcture sec- tion to show the relative positions of ramps or steps prior to eastward translation; and the upper limit of preserved ruck was also inccx-psrated.
Compariscm of Figs. $a and 8b reveals 12 krn of shortening across the Foothills caused primarily by folding. Since Foothills structures are interpreted to be linked with an equal amount of shortening, owing to thrust faulting within lower and middle Paleozoic rocks beneath the western Foothills and "transition interval', the first major ramp or step plotted across the lower and middle Paleozoic suc- cession in Fig. $b Is located 12 krn west of the estimated position of the hanging-wall step shown beneath the western Foothills in Fig. 8a. Estima- tion of shortening across the "transition interval' is hampered by poor exposure and penetrative de- formaticjn within the Besa River shales; however, the interpretation that shortening within this structural Ievel is linked to displacement on the thrust that underlies the large fl-oental anticline per- mits a rough approximation. In the structure sec- tion, 10 km of relative eastward displacement of strata cc~mprisiglg the mountain-front anticline is estimated; therefore, an equal amount of shsrten- ing was absorbed by the Besa River shales and is represented by the substantially thickened succes- sion within the 'transition interval' in Fig. 8a. The second (more westward) major step across the lower and middle Paleozoic succession in Fig. 8b represents the ramp over which the frontal anticline making rap the eastern part of the Rocky Mountains was formed.
Using the above figures, a rough estimate of the
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THOMPSON 1237
ZONE OF DECOUPLING rnnr MERGES WITH
I N HANGING MAIN FAULT
SHORTENING DUE 7C
I THRUST FAL~L~ A7 L X ~ J x X '
FIG. 13. A schematic representation of the compensation mechanism that permits major thrust fdults to remain buried. Detachment within the hanging wall plate allows differential shortening of the incompetent units relative to the rigid units. This has the effect of reducing displacement on the major thrust east of the hanging wall cut-off. When total shortening due to folding above the d6collernent zone equals displacement on the thrust fault. the f;iult ceases to exist. The "pin' within the hang- ing wall plate was located arbitrarily.
total net shortening across the Foothills, 'transitioii interval' and including the 'blind' thrust beneath the mountain front is 22 km.
The geometry of the preserved rock surface (ero- sion surface) in the palinspastic restoration (Fig. 8b) shows that erosion is deepest along the axis of major anticlines and that substantial section is re- moved across the western part of the restoration, where the traces of the 'blind' thrusts are plotted. Had blind thrusts involving gross repetition of lower and middle Paleozoic strata occurred be- neath the eastern part of the inner Foothills, deeper erosion over broader regions would be expected.
The net tectonic shortening of 22 km is consid- erably less than the 70 km estimated for the interval across the Foothills into the first Front Range (McConnel Thrust Sheet) in the southern Rockies4, but this decrease from south to north over a strike length of 880 km is not surprising considering the relative narrowness of the mountain belt in the north as compared with the south.
The basic structure of the eastern Rocky Moun- tains in northeastern British Columbia (Fig. 8a) is no different from that shown in cross-sections of the southern Rockies (Bally ct ~ i l . 1966; Price and Mountjoy 1970; Keating 1966). The faults cut up- ward through the stratigraphic sticcession from west to east and carry older rocks over younger rocks; they :ire listric in form, with substantial hed- ding gIide or detachment zones in incompetent units and abrupt steps or ramps in competent units; and displacements on the scale of kilometres have occurred on them. Furthermore, the crystalline basement surface must be an undeformed passive element at least as far as the west end of the section in Fig. 8a. Thus, the fundamental geometric attri- butes, which form the cornerstones for interpreta- tions of the structures of the southern Canadian
4This figure was calculated from the palinspastic restorations of Price and Mountjoy (1970, Figs. 2-3).
Rockies (Dahlstrom 19701, are equally applicable to the northern Canadian Rocky Mountains.
The Clastic Wedge and Timing Co~lstraints The pattern of deposition and erosion within the
clastic-wedge succession (Fig. 6) suggests that the deformation spread progressively across the Rockies and Foothills subprovinces during a time span that began in the 1,ate Surl~ssic and ended in the early Tertiary.
Commencing with the Late Jurassic - Early Cre- taceous Minnes Group, detritus derived from sedimentary rocks of Proterozoic and Paleozoic age farther west (and which may include material as young ;is Triassic age; Stott 1967a. 1968, 1973) began to accumulate as a foreland clastic-wedge succession. The depoaxes for each successive component of the clastic-wedge sequence migrated progressively eastward. This is consistent with the general pdttern and timing of deformation and foredeep deposition within the southern Rockies (Bally et l a ! . 1966; Price and Mountjoy 1970) and the Wyoming-Idaho-Utah thrust belt (Armstrong and Oriel 1965; Koyse et a!. 1975).
'Three unconformity-bounded clastic sequences make up the clastic wedge of northeastern British Columbia: ( I ) the Fernie Formation and Minnes Group of Jurassic through Valanginian age; (2) the Bullhead Group of ?late Neocoilaian to early Cenomanian age; and (3) the Smoky Group and Wapiti Formation of Campanian through Maas- trichtian age (Stott 1975). Because all three se- quences are involved within Foot hills folds, defor- mation must have persisted beyond Maastrichtian time. An upper time limit cannot be precisely defined, but the lack of younger clastic-wedge de- posits suggests that deformation probably ended by early Paleocene time. The lack of evidence of structural diachroniety within the clastic-wedge sequences suggests that Foothills deformation began after most 01- all of the wedge was deposited-probably after Campanian time. The link between blind thrusts at the Rocky Mountain front and Foothills structures further suggests that deformation along the eastern margin of the Rocky Mountain subprovinces was also restricted to late Cretaceous-early Tertiary time.
It is not possible to date precisely the succession of events that occurred west of the Rocky Moun- tain front. Presumably rocks west of and near the present day Rocky Mountain 'Trench were de- formed and uplifted starting in Jurassic time. De- formation probably was interrupted by a hiatus in late Valanginian time, after which tectonic activity resumed until Cenomanian time. During this sec- ond oropenic pulse, thrust faulting and folding
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CAN. J. EARTH SCI. VOL. 16, 1979
FIG. 14. A comparison of palinspitstic reconstrractions of stratigraphic sections across the miogeocline for the 1-Ialfway River area (above) and southern Rockies (below; adapted frona Price and Mountjoy 1970, Figs. 2 - 3 ) . The vertical line pattern is drawn across incompetent units, more rigid carbonates and quartzites are left open. Thc limits of Foothills and Rocky Mountains structures are indicated by solid vertical lines. 'The hinge line shown represents the approximate locus of accelerated thickening for- the Iowet- and middle Paleozoic carbonate successions in each section.
progressed eastward to include the western part of the Rocky Mountain subprovince. A long hiatus ensued, e%lhich spanned most of Turonian through Santonian time before the final Iate Cre- taceous-early Tertiary pulse was initiated.
A Comparison with the Southern Canadian Rockies; Similarities with the Appalachian Foreland Belt The differences in structtlral style betbleen the
northern and southern Rocky Mountains are viewed here as a consequence of variations with- in the basic framework of thin-skinned dkcolle- ment tectonics. The fact tkat there is a greater diver- sity of structural styles in the northern Rockies can be attributed to the different stratigraphic character-between north and south-ad" the rocks that were deformed. A rough comparison of the positions of Foothills-Rockies subprovince bound- aries plotted on palinspastic reconstructions of the deformed belt emphasizes this (Fig. 14): Those parts of the ~niogeocline-platform succession that form the Foothills and Rocky Mountains in the Halfway River section, rind the Bow-Athabasca section s f Price and hlountjoy (1970) can be com- pared with respect to a conmmon reference frame.
defined by the hinge zone between the platform armd naiogeocline5.
In the Halfway Kiver section deformation is re- stricted, primarily, to the thicker moi-c distal part of the prism wcst of the hinge zone. The eastern mar- gin of the Rocky Mountains subprovince occurs near mrijol- lower-middle Paleozoic carbonatc-to- shale facies transitions. Thus it is quite normal that structures are more varied in character. They rep- resent a response to the greater lateral a s well as vertical variation in stratigraphy along the eastern margin of the subprovince.
By comparison, the rocks making up the south- ern Rockies are pi-edorninar~tly homogeneous rigid carbonates, which lie east of major lateral carbonate-to-shale facies transitions. Structural homogeneity in the form of discrete thrust faults that 'break' to stirface is a natural consequence of the greater stratigraphic homogeneity.
It is idso true (Fig. 14) that the Rockies armd Foothills subprovinces decrease in width north- - -
51t is acknowledged that position of the hinge lines shifted during time; in this case thc hinge lines are chosen to coincide approximately with the locus of thickening within the Iower and middle Paleozoic stratigraphic succession.
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THOMPSON 1239
ward, with a concomitant decrease in the amount of supibacrustal shortening. If the easter-n edge of the Foothills subprovince is thought of as a "ti-ain front', then strain in the south extended across the entire width of the miogeoclinal prism and eastward to include a considerable width of the thin cratonic succession. In the Halfwriy River section, the 'strain front' stopped in its eastward progression while still within the micageoclinal prism. From this one can make the rather tentative conclusion that a broader width of sedimentary veneer was deformed in the south owing to a longer lasting and (or) more intense orogenic pulse accompanied by greater total foreshcal-tening.
The stratigraphic and stl-uctural changes that occur along the strike of the Canadian Rocky iVlountains are remarkably similar to those that occur along the strike oft he Appalachian Foreland belt. The concept of thin-skimned tectonics was applied to the thrust belt of 'Tennessee. Kentucky, and West Virginia by Rich in 1934, but arguments persisted concerning the applicability of this model no!-thward, along strike in Pennsylvania, where folds are the dominant surface structural feature (see Fig. 2 in Root 1973 for a comparison of struc- tural styles between the central and southern ,4p- p:ilachians). Some argued that the folds reflected movement at depth within the crystalline basement along high-angle faults (e.g., Coopel- 196 l ) , while others contended that the folds were detached along ddcollement zone(s) within the sedimentary cover and that the crystalline basement remained uninvolved (e.g., Rogers 1964).
The combination of seismic reflection surveys and deep drilling during the 1960's has shown that the thin-skinned model does apply to the Pennsyl- vania segment of the foreland belt; furthermore, interpretive cross-sections by Gwlnn (1964), Root (1973). and Harris and klilici (1977) show that thrust-fitult repetitions at depth may be compen- sated for by folds at su~T~ice-the same basic re- lationship that is shown in the Halfway River sec- tion. The northward change from a thrust- dominated to a fold-dominated terrane coincides with northward increase in the proportion of in- competent units within the deformed sedimentary succession.
beds of Triassic and younger age (e.g.. Grizzly and Su kunka fields) that are imbricated within the cores of large open anticlines. 1 suggest that further po- tential exists within older stratigraphic intervals along the western margin of the inner Foothills belt and eastern margin of the Rocky Mountains sub- province. If the basic tenets used to construct the structure section of Fig. 8a are correct, then the large 'blind' subsurfr~ce imbrication(s) of lower and middle Paleozoic rocks that form part of the geometric link between Rocky Mountain and Fcdsthills structures are ideally situated for the en- trapment of hydrocarbons; furthermore, they should be within easy I-each of conventional drilling techniques and equipment. A single unbroken im- bricate slice, as shown in the section, is undoub- tedly a gros5 oversimplification; it is more i-ealistic to expect scvei-a1 thrust slices, some probably tightly folded. However, this basic model can serve as a first approximation only, upon which more detailecl investigations can be planned.
Conclusions The basic structure of the northern Rocky
klountains, as exemplified by the Halfway River map-area, is dominated by easterly overthrusting within the northeastern tapering wedge of miogeocline-platform and clastic-wedge deposits and resembles that of the southern Canadian Rockies (e.g., Price and Mountjoy 1970; Bally et (11 . 1966). C'onspicuous contrasts in details of stnictural style reflect the influence of major lateral carbonate-to-shale filcies transitions, as well as the presence caf thick incompetent shale units between Inmore competent carbonate units. Thus, the lack of major thrust faults is more apparent than real; most remain buried disharmonically within the incom- petent Devonian and Mississippian Besa River Formation, beneath the conspicuous box- and chevron-style folds of the younger units.
The northern Rocky Mountains are interpreted as less structurally mature than the southward ex- tension of the belt. Had deformation continued and strain proceeded eastward to include a broader width of the cratonic rock succession, stl-uctural styles, width of the thrust belt, and amount of foreshortening would have more closely resembled the present-day southern Canadian Rocky Moun-
Implications with Regard to Exploration for tains. Hydrocarbons
The structural interpretation presented here is of Acknowledgments potential use in the search for hydrocarbon re- This papel- draws heavily from the important re- sources. Significant discoveries have been made gional studies of my predecessors, especially E. J. within the outer Foothills subprovince, mainly in W. Irish, and from the extensive knowledge and
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1240 CAN. J . EARTH SCI. VOI,. 16, 1979
experience of my colleagues G. C. Taylor and D. F. Stott. Their advice and support during the initial stages of field work within the Halfway River map- area is sincerely appreciated. In addition I wish to express my gratitude to J . D. Aitken, H. Bielens- tein, P. Gordy, and R. A. Price for reviewing an car8ier version of this papel- and making many im- portant and helpful suggestions. Finally 1 thank both my Survey colleagues and those within the oil industry for- taking the time to discuss Rocky Mountain geology with me. Although this con- tribution reilects the influence of those mentioned above, it does not necessarily support their views, and final responsibility rests with me.
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