Fox

INTRODUCTIONIn the western part of the Arctic Archipelago of Canada two structural provinces are dominant.

They are the Ellesmerian Orogenic Province and the Sverdrup Basin. The Ellesmerian OrogenicProvince extends in a great arc from northeastern Ellesmere Island to eastern Prince Patrick Island.That part of it that lies on Bathurst and Melville islands, west of the Cornwallis Fold Belt, includesthe Parry Islands Fold Belt, which is typified by the structures of Bathurst and Central Melvilleislands.

The western sector of the Sverdrup basin lies north of the Parry Islands Fold Belt. Its axis strikessouthwest and its southern erosional margin is the northern limit of exposure of the Parry Islandsfolds. The illustrations that follow show typical Parry Islands fold structures, and a major diapiricstructure of the Sverdrup Basin.

PARRY ISLANDS FOLD BELTThe Parry Islands fold belt has a known length of about 565 to 600 km (350 to 375 mi). Its

width as seen on Bathurst Island is about 175 km (120 mi), and on central Melville Island is about150 km (90 mi). The full width is not known because its northern margin is hidden beneath therocks of the Sverdrup Basin; it probably does not extend far beneath the basin.

The known stratigraphic succession ranges in age from Early Ordovician to Late Devonian(Figure 2). It is believed that there are also Cambrian and Proterozoic sedimentary rocks, butnothing is known of their sedimentary facies or thicknesses. In southern Melville Island thePaleozoic and Proterozoic sedimentary section appears to be at least 10,000 m (33,000 ft) thick, asconcluded from seismic reflection data.

The Cambrian through Late Devonian beds are the southern basinal rocks of the FranklinianMiogeosyncline. These fall readily into two megasequences the Lower Ordovician to MiddleDevonian consists of units of limestone, dolomite, shale, halite, and anhydrite, and the late Middlethrough Upper Devonian sequence consists of a great thickness (as much as 4,500 m; 14,750 ft) ofshale, siltstone, and sandstone.

The response of these beds, primarily by folding, to southerly directed orogenic pressureproduced the Parry Islands Fold Belt. Two parts of the succession played a vital role in determiningthe style and intensity of folding from place to place. The Bay Fiord salt served as a decollementzone, a mobile substratum, above which folding proceeded. In areas where there is no Bay Fiordsalt the Ellesmerian Orogeny produced structures very different from the typical Parry Islands folds.An important role also was played by the Late Ordovician to Middle Devonian facies. In areaswhere that interval comprises limestone and dolomite of the Blue Fiord, Read Bay, and Allen Bayformations, the orogeny produced broad, relatively short, rather unsystematic folds. Where the

interval comprises the Eids and Cape Phillips shales, with the Bay Fiord salt at depth, the foldserected are the typical Parry Islands type.

Thus the Ellesmerian Orogeny produced two distinct styles of deformation. In areas such assouthwest Melville Island, where the section contains thick carbonate sequences, the folds mayhave wave lengths of 40 km (25 mi) or more and amplitudes as great as 5,000 m (16,000 ft). Inareas where the post-salt succession consists mainly of shale, the folds commonly are 100 km (60mi) or more long, with average wave length of 12 km (7 mi) and amplitudes of 3,000 m (10,000 ft)or less; such structures characterize central and eastern Melville Island and most of Bathurst Island.

Figure 1 shows the trace of a composite seismic reflection profile across central Melville Island.It is 235 km (146 mi) long. Figures 3 and 4 are uninterpreted and interpreted versions of thatsection, and Figure 5 is a conversion to geological terms of what the seismic sections show.

At the southwest end of the section, beneath Dundas Peninsula, the Lower and Middle DevonianEids shale retains its identity, but the underlying Cape Phillips shale passes southward, ratherabruptly, to Read Bay/Allen Bay limestone and dolomite. Moreover, the Bay Fiord evaporites gradeabruptly southward to carbonate rocks. Near the northeast end of the section, about 10 to 12 km (6to 8 mi) southwest of King Point anticline, the Eids shale passes laterally northward to Blue Fiordlimestone, and the Cape Phillips shale grades northward into Read Bay/Allen Bay limestone anddolomite.

The characteristics particularly well illustrated by this section are:1) the increasing intensity of defo rm ation from Dundas Peninsula northeast to King Point and beyo n d ;2) the lack of consistency in the dip azimuths of the thrust faults;3) the seeming failure of the thrust faults to penetrate upward through the Cape de Bray and

Weatherall fine grained clastic succession; (This failure may be more apparent than real. Dips in theaxial zone of such folds as Beverley Inlet and King Point anticlines are commonly in the 50 to 600range, and at Beverley Inlet drilling revealed high angle reverse faulting in the Weatherall beds.)

4) the presence of prodeltaic clinoform beds in the central sector of the section and their absencefrom the Dundas and King Point areas, which are Ordovician to Middle Devonian carbonatedomains; (Clinoform beds are known in the Weatherall and Cape de Bray formations, but they arepresent only in areas where the Ordovician to Middle Devonian interval is represented by shale.)

5) the detachment of the folds above the base of the Bay Fiord salt; and,6) the base of the saltrises beneath the anticlines. (Those apparent flexures probably are to some degree spurious, createdby velocity pull-up. The velocity of the sediments, at least up to the top of the Weatherallformation, probably is not much less than that of the salt. Therefore, on the geological section theapparent folding at the base of the salt has been subdued, but not eliminated.)Broad low amplitudefolds, striking generally west, can be mapped at the base of the salt throughout the Parry Islandsfold belt. The mild deformation of the pre-salt bed is disharmonic relative to the Parry Islands folds.

SVERDRUP BASINThe Sverdrup Basin lies north and west of the visible part of the Ellesmerian Orogenic Province.

It is about 1,300 km (800 mi) long, and in its central part is as much as 400 km (250 mi) wide. Theaxial strike of its northeastern half is about N45E, and of its southwestern half the strike is aboutN60E. In its depocenter, it is believed to contain about 12,000 m (39,000 ft) of Carboniferous,Permian, and Mesozoic sediments (Balkwill and Fox, in press), comprising 5,000 m (16,000 ft) ofCarboniferous and Permian shales and evaporites and 7,000 m (23,000 ft) of Mesozoic shales andsandstones.

Vesey Hamilton salt wall is in the southwestern sector of the basin (Figure 6). Upper Paleozoicand Mesozoic sediments in this area are probably about 10,000 m (32,000 ft) thick, and consist ofabout 5,000 m (16,000 ft) of Carboniferous and Permian siltstone, shale, and evaporites, and 5,000m (16,000 ft) of Mesozoic sandstone and shale. The Bjorne formation, at the base of the Mesozoicsuccession, comprises about 1,800 m (5,900 ft) of sandstone with minor shale. Figure 7 illustratesthe bulk of the stratigraphic succession above the Bjorne.

The salt wall is in an area of numerous diapirs, two of which are onshore on northern SabinePeninsula. All the diapirs shown on Figure 6 are piercement structures that reach the surfaceonshore, and the sea bottom offshore. There are other structures in the area that are probably theresult of stunted diapirism; halokinesis was sufficient to create these structures, but was notsustained or powerful enough to enable the salt to pierce the entire section and reach the surface.

The Vesey Hamilton salt wall is the most spectacular of the diapirs. It is illustrated in crosssection by a SE-NW directed seismic line that crosses the middle sector of the wall (Figures 8, 9,and 10). It strikes at N35E and is 50 km (30 mi) long with an average width of 8 km (5 mi). Thisenormous mass of salt rose from the early Pennsylvanian Otto Fiord formation. Structurally andstratigraphically the picture demonstrated by Figures 8, 9, and 10 is a simple one, except in theimmediate vicinity of the wall. The length of this section is about 38 km (24 mi) and the effects ofthe diapirism extend across about 18 to 20 km (11 to 12 mi). The shooting was done offshore andrecorded on the sea ice.

The effects of the diapirism are best observed on its southeast flank. There, pronounced thinningof the top Bjorne to top Pollux sandstone interval indicates Early through Middle Triassic uplift.overstepping near the diapir is clearly visible in the interval between the top of the second ScheiPoint limestone and the top of the Pollux sandstone. Moreover, the top Pollux to top Borden Islandsandstone interval is reduced, and also shows structural overstepping. All the intervals to the base ofthe Valanginian shale show distinctly reduced thicknesses as the salt wall is approached.

The near-diapir stratigraphic relationships at the base of the Valaginian shale are clearlyrevealed. The beds below that shale, near the diapir, can be seen to be truncated, indicating that alocal unconformity is present. Moreover, above that unconformity, younger beds can be seen to

STRUCTURE SECTIONS ACROSS PARRY ISLANDS FOLD BELT AND VESEY HAMILTONSALT WALL, ARCTIC ARCHIPELAGO, CANADA

F.G. FoxPanarctic Oils

Detached Sediments / Decollement Tectonics (A-Subduction) 3.4.1-54

Fox

overstep.Even in beds below the top of the Bjorne formation local thinning toward the salt wall is

apparent. It follows, then, that diapirism began in Early Triassic time, and continued in an episodicway. During some intervals it merely resulted in stratigraphic attenuation over the crest. Duringother times it was sufficient to expose the crest to erosion for brief intervals, resulting in truncationof beds over the crest. Such strong pulses of halokinesis were followed by intervals of quiescence,during which sediments were again deposited over the crest, overstepping the immediatelyunderlying beds. This process, begun during Early Triassic time, was repeated in Middle Triassic,Early and Middle Jurassic, and Early Cretaceous time.

The record of uplift cannot be perceived beyond late Early Cretaceous time, at least in this area.There are no detailed data relating to the post-Christopher (Cenomanian to Maestrichtian) rocks inthis part of the Sverdrup Basin.

A PARADOXA scrutiny of the record sections (Figures 3, 4, 8, and 9) might lead the observer to conclude

that they display two vastly different styles of structure, which is true, and that they have nothing incommon, which is not true. Different in style they surely are. Nevertheless they have a commonfactor that controlled the development of both kinds of structure. That factor is salt; Ordovician inthe Parry Islands Fold Belt, and Early Carboniferous (Pennsylvanian) in the Sverdrup Basin.Without salt the Parry Islands folds certainly would lack the conformation that they have, and theVesey Hamilton Salt Wall would not exist. The real paradox lies in the fact that both kinds ofstructure require compression, although not to an equal degree. The Parry Islands folds resultedfrom Ellesmerian compressive stress acting horizontally from north to south. The salt wall and itsneighboring diapirs resulted from vertical compression of the salt, aided by its natural buoyancy, thepressure being supplied, in a dilational stress field, by the sedimentary overburden bearing down onthe mobile salt.

ACKNOWLEDGMENTSThe author is grateful for suggestions from D.C. Waylett and G.R. Varney of Panarctic Oils, and

for permission from Panarctic and A.I.E.G. et al partners to publish the salt wall seismic profile.

REFERENCESB a l k we l l ,H . R . ,1 9 7 8 ,E volution of Sve rd rup Basin,A rctic Canada:AAPG Bull., v. 62, p. 1004-1028.Balkwell, H.R., \and F.G. Fox, in press, Incipient rift zone, western Sverdrup Basin, Arctic Canada,

in A.F. Embry, ed., Proceedings, Third International Symposium on Arctic Geology: CanadianSoc. Petroleum Geols., Mem. 8.

Davies, G.R., 1975, Hoodoo L-41; diapiric halite facies of the Otto Fiord Formation in the SverdrupBasin, Arctic Archipelago: Geol. Survey Canada, Paper 75-1, pt. C, p. 273-278.

Kerr, J.W., 1974, Geology of the Bathurst Island Group and Byam Martin Island, Arctic Canada:Geol. Survey Canada, Mem. 378.

Sweeney, J.F., 1977, Subsidence of the Sverdrup Basin, Canadian Arctic Islands: Geol. Soc.America Bull., v. 88, p. 41-48.

Tozer, E.T., and R. Thorsteinsson, 1964, Western Queen Elizabeth Islands, Arctic Archipelago:Geol. Survey Canada, Mem. 332.

Trettin, H.P. (coordinator), 1972, The innuitian province, in R.A. Price and R.J.W. Douglas, eds.,Variations in tectonic styles in Canada: Geol. Assoc. Canada, Spec. Paper No. 11, p. 83-179.

Detached Sediments / Decollement Tectonics (A-Subduction) 3.4.1-55

Index map.