UPPER DEVONIAN STRATIGRAPHY, NORTHEAS'T'ERN BANKS ...

BULLETIN OF CANADIAN PETROLEUM GEOLOG Y VOL . 19, NO . 4 (DECEMBER, 1971) P . 705-729

UPPER DEVONIAN STRATIGRAPHY, NORTHEAS'T'ERN BANKS ISLAND, N .W .T . ' J . E . KLOVAN 2 and A . F . EMBRY III '

ABSTRAC T Devonian strata of northeastern Banks Island consist of 3700 feet of sand stones, siltstones, and shales, with minor but spectacular carbonate reef buildups . These strata range in age from lowermost Frasnian to mid-Famennian and ar e time equivalents of the Melville Island Group of Melville Island . The sequenc e of environments represented by these rocks may be related to the developmen t of a northerly derived clastic wedge prograding southward through time . Th e area underwent Ellesmerian deformation, which produced gentle folds and normal faults which were re-activated during Tertiary deformation . INTRODUCTIO N

Banks Island is the most southwestern of the Canadian Arctic Island s (Fig . 1) . It comprises an area of 27,000 sq mi . Over most of this area the land is of low relief and presents a flat, dismal scene of muskeg , lakes, and a few broad rivers . The northeastern portion of the island , in contrast, is a deeply dissected plateau affording good exposures of Lat e Devonian strata . During the summer of 1969, we spent six weeks in this area (Fig . 2 ) with a twofold purpose : first, to describe in detail the Devonian strat a on northeastern Banks Island and second, to study the magnificentl y exposed Devonian carbonate build-ups . The present paper is concerned primarily with the first objective .

'Revised Manuscript received June 30, 1971 . 2 Department of Geology, University of Calgary, Calgary, Alberta . : ; Mobil Oil Canada Ltd ., Calgary, Alberta . The authors express their thanks to Alminex Ltd ., Amoco Canada Ltd ., Banff Oil Ltd ., Canadian Superior Oil Ltd ., Chevron Standard Ltd ., and King Resource s Co . for the financial support necessary to undertake this work . Additional financial assistance came through National Research Council grants to both authors . Inuvik Research Laboratory and Polar Continental Shelf Project provide d accommodation and some transport to us while in the arctic . Charles Ellswort h of Reindeer Air Services ably piloted our aircraft in the field . Bernard Plauchut, Elf Oil Canada, gave invaluable advice on both logistical and geological problems . P . Wolters did all our drafting ; G . Marney was our geological field assistant . To all of these we express our sincere appreciation . Finally, we thank the staff of the Department of Geology, University of Calgary , for their aid . 705

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Devonion outcrop pattern and study area, Banks Island, N .W .T .

UPPER DEVONIAN STRATIGRAPHY

707

PREVIOUS WORK

The work of Thorsteinsson and Tozer (1962) marks the first systemati c geological survey of Banks Island . Early fossil collections by Armstrong (1857) led to the widespread belief that much of the northern coast wa s underlain by Permian strata . Porsild (1950) first established the fac t that these rocks are of Devonian age . Descriptions of the physiography and wildlife of the area, with brie f notes and photos of geological features, are to be found in the account s by Manning (1953, 1956) of his circumnavigation of Banks Island b y canoe . The work of Thorsteinsson and Tozer (19623, however, is th e first and only published comprehensive geological study of Banks Island . It covers the physiography, regional geology, and logistics of workin g in the area in admirable detail, considering the reconnaissance nature o f their work . Gallup (1962) studied the Devonian strata of northeast Banks Island but his report adds little to that of Thorsteinsson and Tozer . PRESENT WOR K

Access to the area from Inuvik, via Sachs Harbour, was by means of a single-engine Otter and Beachcraft 18 aircraft . A base camp was established on the southwest shore of a large lake five miles west of Caste l Bay (74°00'N, 118°55'W), where there is a natural airstrip suitable fo r aircraft of the DC-3 type . Commercial charters from Inuvik can b e arranged for supply trips . Within the study area, transportation was solely by means of a Pipe r Super Cub aircraft equipped with oversize "Tundra" tires . This aircraf t is admirably suited to work in this terrain and, in the hands of a skilfu l pilot, affords access to most localities of interest . Communication with civilization was by means of a Marconi CH5 radi o powered by a small generator . With this set-up we were able to establish contact with Inuvik, Sachs Harbour or Resolute Bay almost every day , which parties in nearby camps using smaller, battery-operated radio s were not able to do . Field work began on June 20, 1969 and ended July 27 . Poor weather that year meant that only one out of three days was suitable for work . Personal injuries and mechanical difficulties with the aircraft furthe r hampered the study . Because northeastern Banks Island is included in a bird sanctuary , Federal permission is required before surveys can be conducted in th e area . REGIONAL SETTIN G

Thorsteinsson and Tozer (1962) have described the regional geolog y of Banks Island and the islands in the immediate vicinity , Structurally, northeastern Banks Island consists of a gently westwar d dipping homocline, perhaps the westward flank of the Minto Arch . A broad, gentle syncline is present in the central portion of the study are a and gently eastward dipping strata occur in the west . This rather simpl e structural setting is complicated by normal faults and subtle flexures .

708

J . E . KLOVAN and A . F . EMBRY III

Some 50 miles to the west of the study area, at Cape Crozier, is a n anticline which brings Late Devonian strata to the surface . This structur e has been interpreted as part of the Prince Patrick Uplift by Thorsteinsso n and Tozer (1962) . Late Devonian beds are some 3700 ft thick and, except for a 200-ftthick limestone near the middle of the sequence, consist of terrigenou s clastics . Drab, green-grey silty sandstone, siltstone and shale are th e predominant rock types . Clean, quartzose sandstone, coal, and ironston e are relatively minor components . STRATIGRAPH Y

Type sections for the Middle and Upper Devonian of the western Arcti c Islands have been established on eastern Melville Island by Tozer an d Thorsteinsson (1964) . The units of interest comprise the Melville Islan d Group, which is subdivided into the Weatherall, Hecla Bay and Gripe r Bay formations . The units range from Givetian to Famennian in age . Devonian strata exposed on northeastern Banks Island have bee n equated with the Late Devonian portion of the Melville Island Grou p (Thorsteinsson and Tozer, 1962) and are considered to be "more or les s equivalent to the Griper Bay Formation" (Thorsteinsson, 1970, p . 566) . As we will attempt to demonstrate, the Banks Island section correlate s with much of the Melville Island Group . Rocks lithologically equivalen t to the Weatherall Formation comprise the lower half of the section , the Hecla Bay Formation is present near the middle of the sequenc e and the upper portion of the section is lithologically and perhaps tim e equivalent to the lower portion of the Griper Bay Formation . Outcrops along M'Clure Strait extend for a distance of sixty mile s between Parker Point and Cape Vesey Hamilton . In general, the strata dip to the northwest at angles on the order of 2° . Twelve overlapping sections, K-69-6 to K-69-17 (Fig . 3) inclusive, wer e measured by pogo-stick . The individual sections have been compiled into a 3660-ft-thick composite section for the area (Fig . 4) . This was aide d by tracing individual beds on a panoramic photograph of the entire coast . In some cases, correlation between individual sections is rather tenuou s due to the lack of definite marker beds and poor exposure . Further, ove r a distance of 60 mi, individuaI beds are almost certain to undergo facie s change . The composite section, therefore, should not be construed a s representing a sequence of rocks at a given point . The lithologic descriptions on the composite section are taken largely from field notes . Faunal and floral identifications were made by Pedder , Harrington, and Hills and Smith . Detailed fossil descriptions are give n in subsequent articles of this Bulletin .

Weatherall Formatio n The lowest exposure of the Weatherall Formation occurs just north west of Parker Point at section K-69-6 ; the top is placed at the base of a distinctive white sandstone unit . The formation is 2610 ft thick but th e base is not exposed .

UPPER DEVONIAN STRATIGRAPHY

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A typical exposure is shown on Plate 1, Figure 1 . Beds consist o f alternating shales, siltstones and sandstones . A distinctive limestone uni t occurs near the top of the formation . Shales are generally grey to greenish grey, largely silty and vary i n fissility' from dense mudstone to highly fissile shale . In many instance s the fissility has undoubtedly been destroyed by burrowing organisms . A s many of the shaly intervals are covered, only sporadic outcrop of in-plac e shale occurs . Siltstones are similar in appearance to the shales ; however, they tend to stand out as slightly resistant ribs . Bedding varies from very thin t o massive . Again, organic burrows are a common sedimentary structur e and the burrowing has undoubtedly modified the bedding characteristics . Sandstones in the Weatherall Formation are predominantly immatur e (Plate 3, Fig . 1) . Compositionally, they are quartzose greywackes . Man y are micaceous . The predominant modal grain size is very fine to fine grained . Except for a few beds, they are poorly sorted with a large percentage of silt and clay . Only one or two intervals consist of potential reservoir rock . Like the shales and siltstones, many of the sandstone beds show well-developed organic burrows . Other common sedimentar y structures are cross-beds and ripple marks . Where present, cross-bed s tend to be of the planar type, ranging from cross-laminations to bed set s 2 to 3 ft thick . Not surprisingly, burrows and cross-beds tend to be mutually exclusive . Another conspicuous rock type is ironstone . It occurs in almost equa l proportions as nodules and irregular layers in all three lithologies . Th e association of ironstone with the sandstones is particularly interesting . Commonly, sandstone intervals are capped by a red-weathering, ironston e layer which contains abundant marine fossils . The sandstone itself i s generally barren. In fact, almost all fossil occurrences in the formatio n come from such sediments . Wood fragments and carbonaceous material are also quite commo n constituents . Some wood fragments are up to 6 in long . Not uncommonly , they are found associated with marine invertebrates in the ironstone layers . More commonly, however, wood fragments and abundant carbonaceou s particulate matter are found in the cross-bedded sandstones . One coa l seam occurs at 1890' and a highly carbonaceous shale interval, some 11 5 ft thick, is present between 1905 ' and 2020' on the composite section . The only distinctive beds present in the otherwise monotonous sequenc e of elastics are the carbonates and associated sediments that occur betwee n 2385' and 2512' on the composite section . These beds comprise the Merc y Bay Member of the Weatherall Formation (Embry and Klovan, 1971, p . 739), and are perhaps the most striking geological feature of northeaster n Banks Island . Thorsteinsson and Tozer (1962) have called attention to th e reefoid nature of this unit . Gallup (1962) also briefly commented on th e nature of the carbonates . Embry and Klovan (1971) gave a detaile d account of the unit, and only the general features are discussed below . The Mercy Bay Member is about 200 ft thick over the entire area o f study . Although extremely variable in character, the unit is best describe d as a biostrome ; that is, a thin, relatively widespread limestone made up , for the most part, of the skeletons of carbonate-secreting marine organ-

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sedimentation over a large area . This again suggests that an event o f major proportions occurred at the Frasnian-Famennian boundary . In any situation where regional facies changes of the type propose d here are taking place, stratigraphic nomenclature is often detrimental t o the understanding of the true stratigraphic relationships . An example o f a semantic tail wagging a stratigraphic hound is well shown by the clasti c Catskill delta of the Appalachians . Not until the work of Chadwick an d Cooper were the myriad of formational names melded into a meaningfu l stratigraphic framework . We therefore feel that the terminology w e propose, which is similar to that presently used for the Catskill roc k units (Pocono, Catskill, Chemung, Portage depositional phases of Rickard , 1964), will lead to a better understanding of the Middle and Upper Devonian strata of the Arctic Islands . STRUCTURAL GEOLOG Y

Except for two distinctive stratigraphic units, the entire sequence o f strata is remarkably consistent in over-all appearance ; thin- to medium bedded, drab grey and greenish grey shales, siltstones and fine-grained , silty sandstones . The Mercy Bay Member, near the middle of the sequence, is a distinctive and mappable marker on northeastern Bank s Island . For the purpose of mapping, therefore, the section was informall y subdivided into three large units : a lower, pre-carbonate unit, a carbonat e and carbonate-equivalent unit, and a post-carbonate unit . Paleontologi c and palynologic work subsequent to field mapping has shown that thes e map units do not correspond exactly with the formal stratigraphic unit s proposed above . That is, the pre-carbonate, carbonate and lowermost 10 0 ft of the post-carbonate map unit comprise the Weatherall Formation, th e next 140 ft of the post-carbonate map unit is the Hecla Bay Formation , and the rest of the section comprises the Griper Bay Formation . Figure 10 is a geological map based on the use of the Mercy Ba y Member as a mappable marker . The dominant structural feature is a broad syncline whose axis inter sects M'Clure Strait just west of Rodd Head . East of this axis, beds di p northwestward at angles of about 2° . Near Parker Point, the strata ar e almost flat-lying . The only structural disturbances noted on the easter n limb of the syncline were one normal fault, downthrown to the west wit h an apparent displacement of 50 ft (Plate 1, Fig . 1), and a minor anticlina l flexure near Rodd Head . The western limb of the syncline is slightly more complex . It i s characterized by numerous anticline-syncline couples and normal faults . Folds are very gentle with structural dips averaging 5' and neve r exceeding 10° . Fold axes, trending north-south, are spaced 3 to 5 m i apart . Normal faults, with stratigraphic throws up to 300 ft, paralle l the folds ; in all cases the western block is downthrown . Unrecognize d repetition of the carbonate unit by faulting led Thorsteinsson and Toze r (1962) to assume that three distinct carbonate units were present whereas , m fact, there is only one (Plate 4, Fig . 1) . The foregoing observations indicate that intensity of deformation o n northeastern Banks Island increases to the west . The structural complexity displayed at Cape Crozier, farther west, supports this view .

UPPER DEVONIAN ,STRATIGRAPHY

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Geological map of northeastern Banks Island, N .W.T .

The age of these structures is not known with certainty . Similar trending structures are present in overlying Cretaceous and Tertiary strata to the west of the study area . The youthful topographic expressio n of the faults, and the fact that outliers of Isachsen Formation occur i n the core of a syncline within the study area, all suggest a Tertiary ag e of deformation . Two phases of deformation are suggested by the fact that the Isachse n Formation unconformably overlies older Devonian strata at Mercy Bay than it does 25 mi to the east at Pim Ravine . This is the opposite of wha t would be expected if Devonian strata maintained the gentle westward di p of the Prince Albert Homocline (Thorsteinsson and Tozer, 1962) . The folded and faulted Devonian strata of northeastern Banks Islan d were possibly deformed during the Mississippian Ellesmerian orogeny , and the structures may have been reactivated during the Tertiary . Thorsteinsson and Tozer (1961) have documented reactivation of Paleozoic structures by Tertiary deformation in other areas of the Arcti c Islands . Slumping occurs on both a large and a small scale, and recognition of this phenomenon affects stratigraphic interpretation of the carbonat e unit . Plate 4, Figure 2 shows an incipient slump developing . A larg e block of limestone has broken off the main mass and is creeping down slope . In Figure 3 of the same plate, two huge masses of limestone are

722

J . E . KLOVAN and A . F . EMBRY III

now situated on the valley floor, but have been derived from the mai n carbonate strata some 400 ft above . These strongly resemble complet e in situ bioherms . Blocks of this nature and size have been observe d halfway down valley walls, and attest to the recent age of the movement . ECONOMIC GEOLOGY

The occurrence of limestone bioherms and biostromes and thick sand stone sequences in the study area suggests that equivalent strata in th e subsurface of the surrounding regions may serve as reservoir rocks . As has been mentioned previously, the dominant lithologies in th e clastic sequence are shales, siltstones, and immature sandstones . Nevertheless, some beds of fine- to medium-grained sandstone do possess porosities effective enough to serve as reservoirs . Further, many of thes e beds are seen to wedge out into impermeable rocks (Plate 1, Fig . 1, fo r example), and may well prove to form traps in the subsurface . The Hecla Bay Formation consists of poorly indurated rock with extremely high porosity . Whether this is entirely a manifestation of surface weathering is uncertain, yet the well-sorted nature of the sand s would indicate that it is a favorable reservoir rock and would be a prim e target in the subsurface . The nature of the preservation of the spores indicates that the shales and siltstones could serve as excellent source rocks of petroleum . Although the carbonates of the Mercy Bay Member have very lo w porosity at outcrop, in the subsurface they may serve as potential reservoirs and traps . Individual bioherms have dimensions similar to thos e at the Zama Lake field in northern Alberta . Certainly, the folds an d faults present in the western portion of the study area should provid e ample trapping mechanisms in the subsurface . Although no specific conclusions can be reached at this time, the are a appears to offer attractive potential for hydrocarbon accumulation . CONCLUSIONS

Devonian strata of the Melville Island Group exposed on northeaster n Banks Island range in age from lowermost Frasnian to mid-Famennian . They consist of a sequence of compositionally immature clastic rocks . A spectrum of depositional environments ranging from marine shelf, with

PLATE 1 Fig . 1 . Sea cliffs 600 ft high along M ' Clure Strait near K-69-13 exposing the Weatheral l Formation . Note the gentle westward dip . A bed of nearshore facies sandstone pinches ou t to the right . A normal fault can be seen at the extreme edge of the photo . View lookin g southwest . Fig . 2 . Sea cliffs along M'Clure Strait from M'Clure River to K-69-16 . The distinctiv e white-weathering sandstone dipping gently to the right is the Hecla Boy Formation . Vie w looking northwest . Fig . 3 . Outcrop of Hecla Bay Formation near head of M'Clure River . Cross-beddin g and the friable nature of the rock are evident . Hoodoos shown are typical of this formation .

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occasional development of carbonate reefs, through nearshore and coasta l plain is present . The vertical distribution of these facies may be interpreted in terms of three transgressive-regressive cycles related to regiona l tectonic adjustments . The section on Banks Island is equated with that exposed on Melvill e Island . The Weatherall Formation on Banks Island is the marine-shel f equivalent of a portion of the Frasnian Hecla Bay Formation of Melvill e Island . Only 140 ft of Hecla Bay Formation, in its typical nearshor e facies, is present on Banks Island . These regional relations may b e interpreted in terms of a clastic wedge development with a norther n source prograding southward through time . The Griper Bay Formatio n at both localities is dominantly coastal-plain facies and appears to be time equivalent . The strata and structures in the area appear to offer attractive potential for hydrocarbon accumulation .

Armstrong, A, 1857, A personal narrative of the discovery of the Northwes t Passage : London, Hurst and Blackett, 616 p . Embry, A . F. and Klovan, J . E ., 1971, A Late Devonian reef tract on northeastern Banks Island, N .W .T . ; Bull . Can . Petroleum Geology, v . 19, no . 4, p . 730-781 . Gallup, W . B ., 1962, The Devonian stratigraphy of Banks Island, N .W .T . : un published manuscript . Harrington, J . H ., 1971, Brachiopoda of the Melville Island Group (Upper Devonian) Banks Island, N .W.T . : Bull . Can . Petroleum Geology, v . 19, no . 4, p . 782-798 . Hills, L . V ., Smith, R . E . and Sweet, A . R ., 1971, Upper Devonian megaspores , northeastern Banks Island, N .W .T . : Bull . Can . Petroleum Geology, v . 19 , no . 4, p . 782 . 798 . Manning, T . H ., 1953, Narrative of an unsuccessful attempt to circumnavigat e Banks Island by canoe in 1952 : Arctic, v . 6, p . 170-197. Manning, T . H ., 1956, Narrative of a second Defence Research Board Expeditio n to Banks Island, with notes on the country and its history : Arctic, v . 9, p . 3-77 . McCave, I . N ., 11968, Shallow and marginal marine sediments associated with the Catskill Complex in the Middle Devonian of New York : Geol. Soc . America, Spec . Paper 106, p . 75-107 . Porsild, A . E ., 1950, A biological exploration of Banks and Victoria Islands : Arctic, v . 3, p . 45 . 54 . Rickard, L . V ., 1964, Correlation of the Devonian rocks in New York State : New York State Mus . and Sci . Service, Map and Chart Ser . No . 4 . Thorsteinsson, R ., 1970, Geology of the Arctic Archipelago, in Douglas, R . J . W . (ed .), Geology and Economic Minerals of Canada : Geol . Surv . Canada Econ . Geology Rept. No . 1. Thorsteinsson, R ., and Tozer, E . T ., 1961, Banks, Victoria and Stephansson Islands , District of Franklin, Northwest Territories : Geol . Surv . Canada, Mem . 330 , 85 . Tozer, E . T . and Thorsteinsson, R ., 1964, Western Queen Elizabeth Islands, Arcti c Archipelago : Geol . Surv . Canada, Mem . 332, 242 p .

PLATE 2 Fig . 1 . The distinctive 150 ft-thick sandstone is the Hecla Bay Formation exposed nea r K-69-14 . Where developed, the carbonate Mercy Bay Member rests on the top of th e prominent sandstone in the middle of the photo . Fig . 2 . Abundant wood fragments in the Heclo Bay Formation . Pole marked in inches .

UPPER DEVONIAN STRATIGRAPHY PLATE 2

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PLATE 3 Photo micrographs of sandstones of the Melville Island Group . Nicols partially crossed . 10ths of mm .)

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Fig . 1 . An example of the sandy marine-shelf facies of the Weotherall Formatio n consisting of moderately sorted very fine, subangular quartz grains in a clay matrix . K-69-13-360' . Fig . 2 . Hecla Bay Formation here, a moderately sorted, medium-grained, angular t o subangular quartz sandstone . The matrix consists almost entirely of carbonaceous matter . Note the high porosity . K-69-14-100' . Fig . 3 . Griper Bay Formation, lower coastal-plain facies . Grains are poorly sorted, fine A few plagioclase grains are not readily evident . to medium-grained, angular quartz . K-69-14-320' . Fig, 4 . Griper Boy Formation, upper coastal-plain fades . Quartz grains are moderatel y sorted, fine- to medium-grained, very angular to angular . K-69-14-360 ' .


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J . E. KLOVAN and A . F . EMBRY III

PLATE 4 Fig . 1 . A normal fault in Mercy River Volley . The Mercy Bay Member outcropping alon g the skyline is downthrown some 300 ft to the west, occurring again in the bottom of th e valley . Overlying quartz sandstones ore on the extreme left of the photo . Fig . 2 . Two Recent normal faults (slump planes) are apparent, with two huge blocks o f Mercy Boy Member beginning to slide down into the river valley . Cliffs are approximatel y 100 ft high . Fig . 3 . Two large slump blocks in the volley of an unnamed river flowing northwar d into Mercy Bay . The slump block in the foreground morphologically resembles an in sit u bioherm, but in fact is only a mass of limestone that has slid down from the cliffs above .


72 9