MIDDLE CAMBRIAN LITHOSTRATIGRAPHY OF SOUTHEASTERN ...

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MIDDLE CAMBRIAN LITHOSTRATIGRAPHY OF SOUTHEASTERN. CANADIAN CORDILLERA 1. D. K. NoRRIS2 and R. A. PRICE. Ottawa, Ontari o. ABSTRACT.



BULLETIN OF CANADIAN PETROLEUM GEOLOGY VOL. 14, NO . 4 (DECEMBER, 1966), P . 385-40 4

38 5

MIDDLE CAMBRIAN LITHOSTRATIGRAPHY OF SOUTHEASTERN CANADIAN CORDILLERA 1 D . K.

NoRRIS2

and R . A .

PRICE

Ottawa, Ontari o ABSTRACT The thin Cambrian sequence in the Flathead-Crowsnest Pass area of th e Canadian Cordillera is distinctly different in character from the classical Cambrian succession of the Canadian Rockies to the north . It is unconformable o n Purcell (Precambrian) rocks and consists of a basal diachronic sandstone fro m a few feet to 150 feet thick, a sequence of shales up to 290 feet thick, a lowe r carbonate unit up to 510 feet thick, and an upper carbonate unit that wa s initially more than 225 feet thick . These units comprise the Flathead, Gordon , Elko and Windsor Mountain (proposed) Formations respectively . This succession is most closely related to the Cambrian of the southern Alberta Plains an d the northern Rocky Mountains of Montana, and is most probably entirely of . Middle Cambrian age . It has been bevelled toward the northwest by pre-lat e l Middle Devonian erosion which has locally cut deeply into the underlying Purcel rocks . Several channels in the upper part of the Windsor Mountain Formatio n filled with fossil plant-bearing siltstone provide a record of earlier Devonia n fluvial erosion and deposition . The contrast between the Cambrian succession in and south of Crowsnest Pas s and that in adjacent areas can be ascribed to the influence of the tectonicall y positive area Montania, which, like a gigantic trapdoor structure, appears t o have been bounded on its northwestern and western margins by faults or ver y steep flexures . Montania was apparently a feature of great antiquity . Coars e detritus accumulated along its northwestern and western flanks in both Winder mere (Late Precambrian) and Early Cambrian time . The patterns of thicknes s and facies variations among the remnants of the Cambrian succession in th e Flathead-Crowsnest Pass area that have escaped pre-Devonian and later erosion , indicate that this succession was initially much more widespread and thicker , and that all of Montania was inundated during the Middle Cambrian . Th e renewed uplift which left its distinctive imprint on the pattern of pre-late Middl e Devonian erosion, appears to have assumed the same character as that which occurred in the Precambrian and Early Cambrian . The local absence of Cambrian rocks over the northwestern part of Montania is a result of this late r erosion rather than of non-deposition .

INTRODUCTIO N

The Cambrian succession in the Flathead-Crowsnest Pass area of th e southeastern Canadian Cordillera contrasts markedly with the classical Cambrian sequence of the Canadian Rockies immediately to the north . The Flathead-Crowsnest strata comprise a basal, diachronic sandston e and shale, overlain by carbonate rocks, and appear to be most closel y related to the Cambrian sequence of the southern Alberta Plains and the northern Rocky Mountains of Montana . With recent increased knowledge of the character, areal distribution , contact relations, and age of this succession, it can be correlated to th e classic Cambrian sequence of the Canadian Rockies, to the subsurfac e 1 Read before the Edmonton Geological Society, Edmonton, May 26, 1966 ; manuscript received June 3, 1966. 2 Geological Survey of Canada .



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D . K, NORRIS and R . A . PRIOR

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MIDDLE CAMBRIAN OF S .E . CORDILLERA

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sequence beneath the Alberta Plains and to the Cambrian sequence o f the northern Rockies of Montana (see Figures 1 and 2) . On this basi s it can be analyzed in terms of its implications for Cambrian paleogeography in the Cordillera . Previous Work The present state of our knowledge of Cambrian stratigraphy in th e area under consideration is based on the work of numerous individual s and it is our intent to mention contributions only insofar as they affec t historical precedence in the recognition, dating and correlation of th e rock units . The presence of Middle Cambrian strata in the southeastern Cordiller a of Canada was first recognized by Schofield (1914, p . 82) and Burlin g (1914, p . 125) . Together they examined the section on the hillslope s above the Burton Mine (abandoned), about 2 miles northwest of th e town of Elko . Schofield introduced the names Burton and Elko for a clastic and a carbonate unit, respectively, lying unconformably on th e Precambrian Roosville Formation and overlain unconformably by Devonian rocks . Burling reported an Albertella fauna from the shales of th e Burton Formation and referred it with some degree of certainty to th e early Middle Cambrian (Burling, 1914, p . 128) . He recognized a lowe r sandstone and an upper shale member in the Burton Formation . Adams and Dick (1915, p . 13), while searching for phosphate deposit s along the east flank of the Flathead Range, discovered the well expose d Cambrian section between the north and south branches of Lost Cree k and reported the presence of Albertella in yellowish orange weatherin g limestone and shale . They assigned the fossil beds to the early Middl e Cambrian, the sandstone unit below to the uppermost part of the Precambrian Kintla Formation, and the carbonate rocks immediately abov e to the Devonian System . In northern Clark Range, Hage (1943) reported Middle Cambrian strat a unconformably overlying Late Precambrian formations . He divide d the Paleozoic succession on Windsor Mountain into a lower sandston e unit, a middle green shale unit with sandstone and limestone interbeds , and an upper dominantly carbonate succession . The lower two units were reported to be Middle Cambrian and the upper, Devonian an d possibly older . Price (1958, p . 45-49) described the Cambrian succession in Flathea d Range as comprising three distinctive units, a light grey quartzite at th e base, a green and greyish green shale, and a massive grey dolomite a t the top . The quartzite was equated with the basal unit on Windso r Mountain and correlated with the diachronic basal Cambrian quartzit e (North, 1953) represented by the Flathead Sandstone of Montana an d the Cranbrook Formation of southeastern British Columbia. The shal e was correlated with the Gordon Shale of Montana and with the Ross Lak e Shale Member of the Cathedral Formation of the classic sequence of th e Canadian Rockies . The dolomite was shown to be unconformably over lain by the Devonian Fairholme Group and was correlated with the dolomites of the Cathedral Formation . As a result of later work in Mac Donald and Galton Ranges (Price, 1962, p . 13-14), the dolomite unit was referred to the Elko Formation, and on the basis of the dating of the



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Elko Formation (Leech, 1958, p . 11) was assumed to be Middle and/o r Upper Cambrian . By 1962 three main Middle Cambrian rock units were recognized in th e southeastern Cordillera of Canada . The basal, diachronic sandstone ha d been referred casually to the Flathead Formation by North (1953, p . 110 ) and this usage is now generally accepted (Rasetti, 1956, p . 746 ; Price , 1962, p . 12 ; Fritz and Norris, 1966, p . 107) . The overlying shale with sandstone and limestone interbeds in Flathead Range was referred t o the Gordon Formation by Price (1958, p . 48 ; 1962, p . 13) . This nam e was extended to Windsor Mountain and Elko by Fritz and Norris (1966 ) and accordingly it was recommended that the name Burton be suppressed . The name Elko is appropriate for all of the overlying Cambrian carbonat e sequence at the type area northwest of Elko and in Flathead Range . In Clark Range however, younger Cambrian rocks occur above the Elk o Formation . The first comprehensive examination of fossil collections from th e Middle Cambrian successions northwest of Elko, in Flathead and Clar k Ranges, and from northern Montana has resulted (Fritz and Norris , 1966) in a firm correlation of rock units among these localities, an d between them and the classic units in Bow Valley . Moreover, it ha s shown that the Cambrian rocks of the Flathead-Crowsnest Pass regio n of the Canadian Rockies are most probably entirely of Middle Cambria n age . Faunal control of Cambrian formations on Windsor Mountain, Flat head Range and at Burton Mine, is shown in Figure 3 . Acknowledgments

We are indebted to G . B . Leech for his assistance, suggestions an d criticisms during our investigations and to R . A . Spillers for his assistance, so cheerfully rendered, in the field . The identification and interpretation of fossil collections made in the course of our investigations is th e work of W . H . Fritz, D . C . McGregor, B . S . Norford and A . W . Norris . DESCRIPTION OF FORMATION S

The sandstone, shale and carbonates of the lower Middle Cambrian i n this area comprise a conformable succession that rests with regional unconformity on Precambrian rocks of Purcell age and is in turn overlai n with regional unconformity by Devonian rocks . The gradational nature of the contact between the Flathead and Gordon Formations on the on e hand, and between the Gordon and Elko Formations on the other, an d the petrographic nature of these units would suggest that they provide a record of the progressive change from a littoral environment, through a deeper water, offshore neritic environment, to a carbonate bank or plat form environment during the eastward transgression of the Cambria n sea . Flathead Formatio n

The Flathead Formation (Weed, 1900 ; Deiss, 1936) consists of inter bedded quartz sandstones, quartzites and mudstones that rest unconformably on Precambrian Purcell (Beltian) rocks and grade up into the green



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D . K . NORRIS and R . A . PRICE

shales of the Gordon Formation . The sandstones and quartzites are commonly limonitic, yellowish grey, fine- to coarse-grained, thin- to medium bedded, medium to coarsely crossbedded, weathering yellowish orang e with the cross laminations standing out in colour relief as various shade s of purple . On the fresh surface the limonite occurs as scattered flecks , presumably derived from the decomposition of pyrite . The formation i s conglomeratic locally, at various stratigraphic levels, but most commonly at its base and top where there are subangular to subrounded pebble s up to 1 cm long . Light grey, fine-grained quartzite and grey and re d sandstone pebbles predominate in the lowest few inches of the formatio n and were probably derived from the underlying Purcell (Belt) System . In thin section the sandstone and quartzite consists of a mosaic o f quartz and minor chert grains, about 1 mm long, set in a matrix of quart z grains generally less than one-third the size, and with varying amount s of silica cement. The quartz is of both strained and unstrained varieties . Limonite occurs as patchy interstitial fillings . Quartz overgrowths are rare . Interbeds of mudstone generally less than 3 inches thick are common . Those in the lower part of the succession are pale red and those toward s the top grey . The former are comparable in colour with the dominan t rocks of the Precambrian terrain from which the detritus was derived , and the latter with the colour of the first few tens of feet of mudston e at the base of the overlying Gordon Formation . The thickness of the Flathead Formation varies irregularly along th e same outcrop and from place to place throughout the region . Fo r example, it varies from 54 feet (C . O . Hage, unpublished data) to 94 fee t within a quarter mile along the east flank of Windsor Mountain, and fro m less than 25 feet on Mount Broadwood to approximately 150 feet in Flathead Range (Price, 1962, p . 12) . Above the Burton Mine near Elko th e basal sandstone and conglomerate member of the Burton Formatio n (herein referred to the Flathead Formation) is only 8 feet thick (Leech , 1958, p . 11) . No fossils are known from the Flathead Formation and it has bee n assigned to the early Middle Cambrian on the basis of its stratigraphi c position and contact relations to the fossiliferous Gordon beds above . The presence of a Plagiura-Poliella faunule in the lower part of the Gordo n near Elko (Leech, 1958 ; Fritz and Norris, 1966) and of an Albertell a faunule 60 feet above the base of the Gordon at Windsor Mountain indicate that the basal Gordon beds are of early Middle Cambrian age . Th e fact that the Flathead is gradational into the Gordon suggests that it i s probably also of early Middle Cambrian age (Fritz and Norris, 1966, Fig . 2) Gordon Formation

In northwestern Montana the Gordon Formation (Walcott, 1917, p . 7 ) consists of green and grey shales that occur between the Flathead (? ) Sandstone and Meagher Limestone . These shales carry an Albertell a fauna . In the southeastern Cordillera of Canada the Gordon Formation consists of fissile, greyish green, micaceous shale, variegated with brown or



MIDDLE CAMBRIAN OF S .F . CORDILLERA

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pale red toward the base, but commonly medium grey immediately abov e the Flathead Formation . Interbeds of resistant, yellowish brown, biogenic and glauconitic sandstones from 1 inch to 6 inches thick are common in the lower two-thirds of the formation, and mottled brown an d grey, argillaceous limestone becomes progressively more abundant upward . Within the upper one-third of the shale succession some of the limeston e interbeds are made up almost completely of fossil debris, with trilobit e fragments dominant. Scattered pellets of glauconite occur and quartz clasts are conspicuously lacking . In thin section the sandstone interbeds consist of sub-rounded quart z grains, commonly 0 .5 to 1 .0 mm long, with interstitial secondary silica . Limonite occurs as scattered clots throughout the rock . In some bed s the sand grains are frosted ; and glauconite pellets, commonly up to 1 m m long, are abundant in these . Within the upper 50 feet of the Gordon Formation on Windsor Mountain and in Flathead Range there are zones of limestone up to 18 fee t thick . The limestones are characteristically very fine-crystalline, pal e olive grey mottled with greyish orange, and occur in nodular beds 1 t o 3 inches thick . In thin section the mottling is represented by limonit e encrustations on grains and fine-crystalline aggregates of dolomite an d sparry calcite . Oolites stained with limonite are common, and fossi l debris occurs in abundance in some layers . The thickness of the Gordon Formation ranges from 147 feet above th e Burton Mine, to approximately 150 feet in MacDonald Range and o n Mount Broadwood, to 200 feet on the east flank of Flathead Rang e between the north and south branches of Lost Creek, to 288 feet on the north ridge from Windsor Mountain . It is 274 feet thick in Scapegoat Basin, Lewis and Clark Range, Montana (Deiss, 1938, p . 1090), and it s equivalent on Gable Peaks in Flathead National Forest, Montana, terme d the Wolsey Formation, is 165 feet thick (Ross, 1959, p . 61) . The contact between the Gordon and Flathead Formations is gradational and therefore apparently conformable . The uppermost beds o f the Flathead are highly argillaceous and the lowermost shales of th e Gordon contain sandstone interbeds . Regionally, the contact between these two formations represents a change from a littoral to a neriti c environment during the Middle Cambrian transgression . In the southeastern Canadian Cordillera the Gordon Formation is earl y Middle Cambrian in age . It contains Plagiura-Poliella, Albertella an d Glossopleura faunules (Fritz and Norris, 1966) . A Glossopleura faunule is reported 13 feet below the top of the Gordon at Windsor Mountai n (Fritz and Norris, 1966) and a Plagiura-Poliella faunule within 20 fee t of the base at Elko (Leech, 1958, p . 11 ; Fritz and Norris, 1966) . Th e fact that these faunules have not been recognized in other sections ma y be due to lack of exposure, as in the upper part of the succession at Elko , or to barren intervals, as in the lower part of the formation on Windso r Mountain . Elko Formatio n The Elko Formation (Schofield,, 1914, p . 83) comprises a unit up t o 510 feet thick of cliff-forming grey dolomite with a zone of dolomite-



D . K . NORRIS and R. A . PRICE

392

Flathead Range LOCALITIES

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'Assume Gordon Fm . is 210' thick at North Kootenay Pas s . . Inclucles collections of G . B . Leech t 1958 )

FIG . 3 .-Fauna of Cambrian formations of southeastern Canadian Cordillera .

mottled, dark grey limestone at the base . It is gradational downward into the Gordon Formation and is overlain, apparently conformably, b y the Windsor Mountain Formation or unconformably by the Devonia n Fairholme Group . Its thickness varies as a result of pre-late Middl e Devonian erosion, and locally it has been removed completely (Leech , 1958, p . 17) . The Elko dolomites are mainly medium to light grey and fine- to medium-crystalline, and commonly show a faint textural mottling whic h becomes etched into relief on weathered surfaces . Bedding is not always apparent, and may be defined in terms of parallel zones of differing texture . Locally these are zones with discontinuous vuggy porosity, o r with intercrystalline porosity, or with different patterns of textural mottl -



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ing . Towards the base of the sequence the dolomite grades through a zone of medium to dark grey, texture-mottled, medium-crystalline dolomite, to a basal zone of dolomite-mottled dark grey limestone . The basa l limestone zone varies in thickness from about 30 to 120 feet, presumably due to lateral gradations from limestone to dolomite in the upper beds . The limestone is typically dark grey, fine- to very fine-crystalline an d mottled with fine- to medium-crystalline light brown and pale yellowis h orange dolomite . Thin interbeds of olive grey, strongly calcareous mud stone occur locally toward the base . The limestones are similar to, if no t identical with, those toward the top of the Gordon Formation . The dolomites consist of a mosaic of anhedral to subhedral grains an d are devoid of any obvious `primary' sedimentary fabric on a microscopi c scale . The mottling can be related to almost imperceptible patchy variations in crystal size and amount of intercrystalline porosity . On a microscopic scale the limestones consist of a mosaic of fine- an d very fine-crystalline calcite with rare to abundant disseminated very fine to medium-grained clastic calcite . The clastic fraction consists predominantly of a variety of organic skeletal fragments including triaxial spicules, fragments of trilobite carapaces, brachiopod shell (?) fragments , and echinoderm fragments . Calcispheres averaging 0 .1 mm in diamete r are also present . Fine- to medium-crystalline dolomite occurs as a mosai c of euhedral to subhedral grains in irregular patches with diffuse boundaries . The dolomite grains are commonly encrusted with limonite .

The contact between the Elko and Gordon Formations is gradational , and the base of the Elko Formation is drawn at the top of the highest , thick shale bed . The upper contact on the other hand is abrupt, bu t apparently conformable . On Windsor and Citadel Mountains mediu m dark grey dolomite forming the top of the Elko Formation is overlain b y recessive weathering, greyish orange silty dolomite of the Windsor Mountain Formation . No fossils have been reported from the Elko Formation but its Middl e Cambrian age can be inferred from its stratigraphic position relative t o fossiliferous formations above and below . On Windsor Mountain a Glossopleura faunule occurs 13 feet below the base of the Elko Formation and a faunule belonging either to the Glossopleura or Eathyui . iscus Elrathina faunizones occurs in the overlying Windsor Mountain Formation , Windsor Mountain Formatio n

The Windsor Mountain Formation is proposed as the dolomite an d limestone succession lying between the Elko Formation and the Devonian System on Windsor and Citadel Mountains in southwestern Alberta . At its type section on the west flank of Windsor Mountain, the formation is 222 feet thick . To the west and northwest it is absent becaus e of pre-late Middle Devonian erosion . The basal unit of the formation is slightly calcareous, silty dolomit e that is seldom well exposed and forms a prominent physiographic brea k toward the top of the Cambrian carbonate succession in the area (Pl . 1 , 2) . It is overlain by a mottled limestone and dolomite succession simila r to that toward the base of the Elko Formation .



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PLATE 1 .-Paleozoic succession on the west face of Windsor Mountain . Bas e of Windsor Mountain Formation (Wb) is marked by a conspicuous ledge an d the base of the Devonian sequence (Db) by channeling along the unconformity .

The Middle Cambrian age of the Windsor Mountain Formation is established by the recovery of trilobites approximately 50 feet above the bas e of the formation (Fritz and Norris, 1966) . The faunule may belong t o either the Glossopleura or Bathyuriscns-Elrathina faunizones . The lower contact of the Windsor Mountain Formation is apparentl y conformable . It is remarkably planar (Pl . 1, 2) and there is no indication of an hiatus between the faunules of the Windsor Mountain an d Gordon Formations . The upper contact is, however, obviously disconformable and deep channelling is apparent (Pl . 1) . It marks an important stratigraphic hiatus that spans the interval from late Middle Cambria n to Early or Middle Devonian . SUB-CAMBRIAN UNCONFORIvIITY

A regional unconformity at the base of the Flathead Formation separates the Cambrian sequence from rocks of the Purcell System, a succession of Precambrian sedimentary rocks older than the Late Precambrian Windermere System . On a local scale the pre-Flathead erosion surface shows little or n o apparent relief . It is essentially planar . Moreover, the stratification i n the Flathead Formation generally appears to be concordant with that i n the underlying Purcell rocks . However, in at least a few localities, in



MIDDLE CA11BRIAN OF S .E . CORDILLERA

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the general vicinity of Windsor Mountain, there is a distinct angular discordance of up to several degrees between the Flathead and underlyin g rocks (P1 . 3), and locally, as for example at North Kootenay Pass, th e top of the Purcell succession shows evidence of pre-Flathead weatherin g (Price, 1958, p . 49) . On a regional scale, some 5,000 feet of Purcell strata, comprising th e Roosville and Phillips Formations, and at least part of the Gateway Formation, are truncated against the pre-Flathead erosion surface . Th e pattern of truncation is shown schematically in the Cambrian subcrop map of Figure 4 . The depth of pre-Flathead erosion is greatest in the north ern and northeastern parts of the area where the Flathead Formatio n overlies the Gateway Formation and probably some of the older Purcel l formations as well . The rate of truncation of the Purcell rocks is greatest in a northwesterly direction, perpendicular to the northeasterly trending zone of the Moyie and Dibble Creek faults (Leech, 1960) . The Lower Paleozoic stratigraphy on either side of the Moyie and Dibbl e Creek transverse faults is very different . These faults are believed t o lie near the site of an important pre-late Middle Devonian structure o n which the south side stood high and the north side stood low (Leech , 1958) . North of these faults Purcell, and locally, Windermere strata ar e overlain by a considerable thickness of Lower Cambrian rocks belongin g to the Cranbrook and Eager Formations . This is in marked contrast with the thin Middle Cambrian succession, comprising the Flathead, Gordon , Elko and Windsor Mountain Formations, that overlies the Purcell on the south side . The rapid northwesterly truncation of Purcell strata along

PLATE 2,-Cambrian carbonate rocks on the southeast face of Citadel Mountain . Base of Windsor Mountain Formation (Wb) is marked by a ledge abov e cliffs of Elko dolomites and base Devonian sequence (Db) marks top of cliff .



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PLATE 3 .-Angular discordance between Flathead Formation and Purcel l strata, east face of Windsor Mountain (photo by C . O . Hage) .

the sub-Cambrian unconformity indicates that the transverse structur e was active in pre-Flathead time . It marks the northwestern edge of th e positive area, Montania (Deiss, 1941), that played so fundamental a rol e in controlling the patterns of Early Paleozoic sedimentation in this par t of the Cordillera. Montania apparently consisted of a tilted block that had been up lifted, like a gigantic trapdoor structure, along faults or very steep flexures which outlined its northwestern and western boundaries . It appears to be a tectonic element of great antiquity . It is devoid of Windermer e rocks and presumably emerged as a positive element when the coars e clastic sediments at the base of the Windermere succession began t o accumulate in regions to the northwest and west. Accordingly, muc h of the erosion that preceded deposition of the Flathead Formation ma y have occurred during Windermere time, rather than during the Earl y Cambrian . However, coarse conglomerates in the Lower Cambrian Cranbrook Formation on the northwest flank of Montania (Rice, 1941) indicate that at least some significant erosion of Montania occurred during the Early Cambrian . Furthermore, the deposition of several thousand feet of Lower Cambrian clastic sediments, comprising the Cranbrook Formatio n and at least the lower part of the Eager Formation (Rice, 1937 ; 1941), of f the northwest flank of Montania suggests that there was substantia l relief across its margin immediately prior to the Middle Cambrian FIathead transgression that apparently inundated all of Montania .



MIDDLE CAMBRIAN OF S .E . CORDILLERA

PRE-DEVONIAN

39 7

EROSION

The Cambrian sequence of the southeastern Canadian Cordillera an d adjacent parts of northwestern Montana was bevelled toward the north west by pre-late Middle Devonian erosion which occurred in conjunctio n with renewed uplift along the northwestern margin of Montania . Th e effects of this uplift are most conspicuous in the western Canadia n Rockies near Lat . 49° 35' where the Devonian lies unconformably o n the Silurian to the north of the Dibble Creek fault and on the Purcell an d Cambrian rocks of the Montania succession to the south (Leech, 1958 , p . 17 ) Southeast of the immediate vicinity of the Dibble Creek fault, th e Devonian Hollebeke Formation unconformably overlies the Cambrian Elko and Windsor Mountain Formations, generally with a sharp an d apparently concordant contact that provides little or no indication of the presence of a major stratigraphic hiatus . However, locally there is considerable relief along the contact and it is marked by a fossil regolith or channel-fill deposits (Price, 1958, p . 49 ; 1964, p . 430) . At Citadel Mountain, 6 miles south of Windsor Mountain, a channe l has been cut to a depth of' at least 15 feet into the Windsor Mountai n Formation . Light yellow mudstone occurring in the deeper parts of th e channel may represent relicts of a paleosol, but the remainder of th e channel is filled with coarse-grained, dolomitic, quartz sandstone an d quartz granule conglomerate . The sandstone unit extends beyond th e limits of the channel and grades upward into argillaceous dolomites o f the lower member of the Devonian Hollebeke Formation . The channe l was filled by the initial deposits of the transgressing Devonian sea . A Iocal relief of about 60 feet is clearly evident along the top of th e Windsor Mountain Formation on the face of the west shoulder of Windsor Mountain (Pl . 1) . On the south side of this shoulder a channel cu t into the Windsor Mountain Formation is filled with about 55 feet o f brownish grey, massive, flaky to platy weathering siltstone . The siltstone completely fills the irregularities along the top of the Windso r Mountain Formation and is overlain by a sequence of dolomites, conglomeratic, coarse-grained sandstones, and mudstones like that tha t elsewhere mark the base of the Devonian Hollebeke Formation, and li e in direct contact with the Windsor Mountain or Elko Formations . Th e brownish grey siltstone is a local occurrence of a channel-fill deposi t older than the Hollebeke Formation . Hage (1943) reported fossil plan t stems of Hostimella appearance from this unit . Additional plant fossils , collected from 15 feet above the base of the unit, were identified by D . C . McGregor as impressions of vascular plant axes of probable Devonia n age . McGregor also recovered a few broken and extremely corroded spores from a specimen of the siltstone, but the general form of th e few that were reasonably intact offered no clue as to their age . It would appear that the Devonian marine transgression that inundated al l of Montania was preceded, during earlier Devonian time, by fluvial erosion and deposition of channel-fill deposits . On a regional scale, thickness variations indicate that the northwesterly truncation of the Cambrian sequence along the pre-Devonian unconformity is relatively gradual, except in the area northwest from

D. K. N'O~I~IS a~td t~. A. P R I C E

398

Pre -Devonian Erosional Edge o{ Cambrian Cambrian Exposures ROOSVILLE FORMATION I s o p a c h s of B.oo sville Formation ( i n t e r v a l - i000 feet)

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