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Geological and geophysical evidence in support of the hypothesis of anticlockwise rotation of northern Alaska
Marine Geology, 93 (1990) 317-329 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
317
Geological and Geophysical Evidence in Support of the Hypothesis of Anticlockwise Rotation of Northern Alaska ASHTON F. EMBRY Geological Survey of Canada, 3303-33rd St. NW, Calgary, Alta. T2L 2A7 (Canada) (Received J a n u a r y 10, 1989; revision accepted April 6, 1989)
Abstract Embry, A.F., 1990. Geological and geophysical evidence in support of the hypothesis of anticlockwise rotation of n o r t h e r n Alaska. In: J.R. Weber, D.A. Forsyth, A.F. Embry and S.M. Blasco (Editors), Arctic Geoscience, Mar. Geol., 93: 317-329. The most popular hypothesis for the origin of the Amerasia Basin of the Arctic Ocean is t h a t it formed by the anticlockwise rotation of n o r t h e r n Alaska and adjacent n o r t h e a s t e r n Siberia away from the Canadian Arctic Islands in Cretaceous time. Because there is no obvious p a t t e r n of magnetic stripes in the Amerasia Basin, and the geological evidence presented to support the anticlockwise rotation model has been r a t h e r general and unconvincing, the hypothesis has been repeatedly challenged and alternative hypotheses for the origin of the Amerasia Basin have been proposed. Recently released seismic reflection data from the Chukchi Sea and the Canadian Arctic Islands, in combination with surface and well data, have allowed regional structural and stratigraphic trends to be established both in the Canadian Arctic Islands and n o r t h e r n Alaska Chukchi Sea region. To test the validity of the anticlockwise rotation model, Upper Devonian structural and depositional trends, Carboniferous-Jurassic basin axes and Lower Triassic facies belts have been plotted on Harland et als.'* Arctic pre-Cretaceous plate reconstruction which used the anticlockwise rotation hypothesis as its basic premise. In all cases coherent and reasonable tectonic and depositional reconstructions were obtained. Furthermore, seismic lines and well logs from the n o r t h w e s t e r n A l a s k a - C h u k c h i Sea region and the southwestern Sverdrup Basin, areas which are juxtaposed in the reconstruction, were compared and found to be remarkably similar. Overall, the available geological and geophysical data support the anticlockwise rotation model.
Introduction
Two main basins, the Amerasia and Eurasia, comprise the present Arctic Ocean and they are separated by a narrow ridge of continental crust, the Lomonosov Ridge. The Eurasia Basin is the northern extension of the North Atlantic Ocean and it formed by seafloor spreading away from the axial Nansen Ridge in Tertiary time (Vogt and Avery, 1974). The spreading history of the basin is reasonably Geological Survey of Canada Contribution 56688. "1984. 0025-3227/90/$03.50
well known due to the recognition of clearly defined magnetic anomalies. The Amerasia Basin is a triangular-shaped oceanic basin which includes two prominent bathymetric highs, the Alpha Ridge complex and the Chukchi Cap. The Alpha Ridge complex appears to be oceanic in origin (Asudeh et al., 1987) but the nature of the Chukchi Cap is unknown. Heat flow and depth to oceanic basement studies indicate a Cretaceous age for the Amerasia Basin (Lawver and Baggeroer, 1983) and thus it formed earlier than the Eurasia Basin. The origin of the Amerasia Basin is not well
© 1990 Elsevier Science Publishers B.V.
understood and a number of hypotheses for its formation have been proposed over the last 30 years. The first and most popular hypothesis is that the basin originated by the anticlockwise rotation of northern Alaska and adjacent Siberia away from the Canadian Arctic Islands (Carey 1958; Rickwood, 1970; Tailleur, 1973). The pole of rotation is interpreted to have been located in the Mackenzie Delta region. In this model both the Canadian Arctic margin and Alaskan-Siberian margin are rift margins with the Lomonosov margin being a transform margin (Fig.l). Although this hypothesis has been repeatedly advocated, little convincing geological or geophysical evidence has been provided by its supporters. There is no obvious pattern of magnetic anomalies which would allow the spreading history to be interpreted and the plate movements to be reconstructed. Furthermore, in regard to geological evidence, most workers have noted only general geological similarities between Arctic Alaska and the Canadian Arctic Archipelago (e.g., Grantz et al., 1979). However, such generalities can be accounted for in other hypothetical plate tectonic models and are not convincing evidence for the anticlockwise rotation of Alaska. Reasonably persuasive data which support the anticlockwise rotation model are recently published palaeomagnetic data from northern
Alaska (Halgedahl and Jarrard, 19,n7). H o w ever, due to the problems associated with determining and interpreting palaeomagnetic results from northern Alaska (e.g. very steep magnetic inclinations and resetting of p a l a e o magnetic directions) these data ar~ not r e garded as conclusive. As a result of this lack of solid evidence to support the rotation of Alaska, other plate tectonic models to explain the origin of the Amerasia Basin have been proposed. Most of these (Herron et al., 1974; Churkin et al., 1980; Jones, 1982) are incompatible with existing geological and geophysical data and they have not been adopted by other workers. These hypotheses are reviewed by Lawver et al. (1985) and are not considered further here. A serious rival to the anticlockwise rotation of Alaska model is the southward translation of Alaska model (Bally, 1976; Christie, 1979; Kerr, 1981; Dutro, 1981). This model proposes that the Amerasia Basin opened by the rifting and southward translation of northern Alaska and adjacent northeastern Siberia away from the Lomonosov Ridge-Barents Sea region. In this model the Lomonosov margin and the Alaskan-Siberian margin are rift margins with the Canadian Arctic margin being a transform margin (Fig.2). Recently a number of workers have refined this model and its popularity
Fig.1. Anticlockwise rotation of northern Alaska and adjacent northeastern Siberia model for the opening of the Amerasia Basin (stippled).
Fig.2. Southward translation of northern Alaska model for the opening of the Amerasia Basin (stippled).
ANTICLOCKWISE ROTATION OF NORTHERN ALASKA
seems to be increasing (Crane, 1987; Smith, 1987; Hubbard et al., 1987). However, as is the case for the anticlockwise rotation of Alaska model, only broad generalities have been offered in support of this model. Furthermore the validity of this model depends on the existence of a major left-lateral strike-slip fault through the northern Yukon and northern Alaska (Kaltag Fault, Fig.2). Present geological and geophysical data available for the northern Yukon appear to deny the existence of such a fault (Bamber and Waterhouse, 1971; Poulton, 1982; Lane, 1988). However, the purpose of this paper is not to argue against the southward translation of Alaska model, a task much better accomplished by those working in the northern Yukon. Rather it is to provide specific geological and geophysical data which are supportive of the anticlockwise rotation of Alaska model. The data and interpretations presented in this paper are based on my studies of the geology of the Canadian Arctic Archipelago over the past twenty years and on published information for northern Alaska and adjacent offshore areas. It is only within the last six years that critical seismic reflection data have become publically available for the northern Alaska and Canadian Arctic areas. These data have provided the key to reconstructing palaeogeographic and structural trends in both areas and such trends are ideal for testing the anticlockwise rotation of Alaska model. Various structural and palaeogeographic trends have been plotted on present-day geography and also on the plate reconstruction of Harland et al. (1984) which restores northern Alaska and adjacent northeastern Siberia against the Canadian Arctic Islands. This reconstruction was chosen because it uses the anticlockwise rotation of Alaska as its basic premise and because it was published before the geological data used to test the validity of the model were compiled.
Geological trends Previous workers have noted t h a t the Phanerozoic column of northern Alaska is similar to
319
t h a t of the Canadian Arctic Islands and that the two areas have similar geological histories (Tailleur and Brosg6, 1970; Grantz et al., 1979; Smith, 1987). These authors indicated that these similarities were evidence for a closer positioning of the two areas before the Amerasia Basin had opened in Cretaceous time. However, as noted in the previous section, just how these two areas were formerly juxtaposed is unclear. Figure 3 illustrates a Devonian Cretaceous correlation chart for northern Alaska and the Canadian Arctic Islands, and, as shown, the stratigraphic similarities between the two areas are striking. Embry (1989) elaborates on these similarities and concludes t h a t the two areas were tectonically and depositionally linked throughout the Devonian to Cretaceous interval. The Devonian to Cretaceous succession of Arctic North America can be divided into three megasequences, each having been deposited under a distinctive tectonic setting: (1) Middle-Upper Devonian clastic strata deposited in foreland basins which flanked areas of orogenesis (Ellesmerian Orogeny). (2) Carboniferous-early Middle Jurassic clastics and carbonates deposited in basins which developed by rifting and thermal subsidence along the area previously occupied by the Devonian orogenic belt (e.g. Sverdrup Basin of Arctic Canada and H a n n a Trough of Arctic Alaska). (3) Early Middle Jurassic-Cretaceous clastic strata deposited in a foreland basin which flanked the Brooks Range Orogen (Colville Trough), in previously established extensional basins, and in new extensional basins related to rifting and seafloor spreading in the Amerasian Basin (e.g. Dinkum Graben, offshore northeastern Alaska, and Banks Basin, southwestern Arctic Islands). These three megasequences are approximately equivalent to the three tectonic sequences of Lerand (1973) (Franklinian, Ellesmerian and Brookian) which have been commonly recognized in the stratigraphic succession of northern Alaska (e.g. Bird and Molenaar, 1987).
As indicated above, the first two megasequences developed prior to the formation of the Amerasia Basin. Thus, geological trends recognized in D e v o n i a n - l o w e r Middle Jurassic strata within Arctic North America are well
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suited to test the validity of various plate tectonic restorations. The geological trends which are used herein to test the anticlockwise rotation of Alaska model are: (1) Upper Devo nian stratigraphic and structural trends; (2) Carboniferous-Middle Jurassic depocentre trends, and (3) Lower Triassic facies trends. Upper Devonian
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Both n o r t h e r n Alaska and the Canadian Arctic Islands were affected by Devonian orogenesis (Ellesmerian Orogeny). A clastic wedge derived from the orogenic highlands and deposited in an adjacent, tectonically linked foreland basin is present in both areas (Embry and Klovan, 1976; Nilsen and Moore, 1982). Figure 4 illustrates the general structural and depositional trends for the Devonian orogenic belts and their related clastic deposits. A large area of highly deformed Lower PalaeozoicProterozoic strata is present in the nort h e rn part of the Canadian Arctic Islands. A foreland basin with up to 5000m of Middle U p p e r Devonian clastic strata occurs to the south. P a l a e o c u r r e n t and facies trends indicate a general longitudinal (southwesterly) transport of sediment in the basin. Structural trends within the highly deformed strata and the clastic wedge form an overall curvilinear pat t ern with nort hw est erl y trends occurring in the west. These northwesterly trends are approximately perpendicular to the Amerasia Basin margin. On Banks Island to the south, M i d d l e - U p p e r Devonian' clastic strata and underlying carbonates are very gently deformed. The structures trend n o r t h - s o u t h and deformation increases to the west. However, seismic reflection data for the continental shelf Fig.3. Upper Palaeozoicand Mesozoiccorrelation chart for northern Alaska and Sverdrup Basin (Canadian Arctic Islands). Data for northern Alaska from Bird and Molenaar (1987), Hubbard et al. (1987) and Thurston and Theiss (1987). Data for Sverdrup Basin from Embry (in press). M.H. = Murray Harbour Fro.; R.P. = Roche Pt. Fm.; H.B. = Hoyle Bay Fro.; P.B. = Pat Bay Fm.; S.P. = Sandy Pt. Fro.; K . C . = K a r e n Creek Fm.
ANTICLOCKWISE ROTATION OF NORTHERN ALASKA
321
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adjacent to Banks Island indicate t h a t deformation of the Lower Palaeozoic and older strata remains mild upto the shelf edge, which is at the limit of seismic resolution (J. Dietrich, pers. commun., 1985). In northern Alaska and adjacent northern Yukon, Devonian and older strata have been severely deformed during Cretaceous and Tertiary. Consequently Devonian structural and stratigraphic trends are not very clear. A broad area of Lower Palaeozoic-Proterozoic strata which were deformed in Late Devonian time extends from the Chukchi Sea to the northern Yukon (Tailleur and Brosg~, 1970; Bell, 1974; Grantz et al., 1982). A thick Upper Devonian clastic wedge occurs south of the belt of highly deformed strata and palaeocurrent data indicate derivation from the deformed strata (Nilsen and Moore, 1982). Structural trends appear to be east-west and to wrap around an eastern terminus of the deformed belt in northern Yukon and adjacent Northwest Territories. In the Chukchi Sea area seismic reflection
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data have revealed the presence of an isolated wedge of mildly deformed rocks interpreted to be Devonian clastic strata and underlying carbonates (Grantz et al., 1982; Craig et al., 1985; Thurston and Theiss, 1987). This area of clastic strata is surrounded by highly deformed lower Palaeozoic strata. Structures in the clastics have a n o r t h - s o u t h trend (W. Masterson, pers. commun., 1988). As shown on Fig.4 there are a number of Devonian structural and stratigraphic trends which are truncated by the Alaskan and Canadian margins of the Amerasia Basin. These include the following: (1) Two separate areas of Ellesmerian orogenesis with the orogenic front being approximately perpendicular to the continental margin in various areas. (2) A north-south, gently deformed foreland basin on Banks Island and the adjacent continental shelf with no adjacent orogenic belt. (3) An apparently isolated, wedge-shaped area of mildly deformed Devonian clastic strata in the Chukchi Sea.
The Devonian trends of Fig.4 were plotted on the Harland et al. (1984) reconstruction resulting in a coherent structural and depositional pattern (Fig.5). The Ellesmerian Orogenic Belt appears as a continuous entity with a sinuous trend from northern Ellesmere Island to the northern Yukon. The mildly deformed, foreland basin on Banks Island becomes paired with the orogenic belt of northern Alaska. Also within this reconstruction the isolated wedge of Devonian clastic strata of the Chukchi Sea becomes part of the regional foreland basin. Furthermore it is apparent t h a t the Upper Devonian clastic wedge of northern Alaska was deposited on the opposite side of the orogenic belt compared with the Devonian
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clastic wedge of the Arctic Islands. This observation counters Nilsen's (1981) interpretation that the anticlockwise rotation of Alaska model is untenable because the Devonian clastic strata of Alaska have very different palaeocurrent directions compared with those of the clastic wedge in the Canadian Arctic Islands. As an additional check on the validity of the proposed reconstruction of Devonian trends a published seismic line from the Chukchi Sea which images the Devonian clastic strata and underlying carbonates (Craig et al., 1985, plate 1) was compared with an unpublished, but publically available, seismic line on southern Prince Patrick Island (Fig.6). The location of
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Fig.5. Distribution and structural trends of highly deformed lower Palaeozoic strata and Devonian clastic strata plotted on pre-Cretaceous Arctic reconstruction of Harland et al. (1984) (from Embry, 1988). A coherent depositional and structural picture results from using this reconstruction. 1 = seismic line from Chukchi Sea and 2 = seismic line from Prince Patrick Island (see Fig.6).
ANTICLOCKWISEROTATIONOF NORTHERNALASKA 35 km
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these lines are plotted on present-day geography (Fig.7) and on the H a r l a n d et al. (1984) r e c o n s t r u c t i o n (Figs.5 and 8). Figure 5 suggests th at the two areas crossed by the seismic lines were quite close in the Devonian and t hat the strike directions of the two lines would have
been approximately coincident. There is a remarkable similarity between the two seismic lines, as might be expected if the proposed r e c o n s t r u c t i o n is valid. On both lines a thin interval of post-Devonian strata unconformably overlies a thick interval (2-3 s two-way
Amerasia Basin
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ALASKA
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Fig.7. Basin axes for Sverdrup Basin (Canadian Arctic Islands), Hanna Trough (Chukchi Sea) and Arctic Alaska (northern Alaska). I---South Meade No. 1; 2= seismic line of Fig.6a; 3= seismic line of Fig.6b; 4= Sandy Point L-46.
Fig.8. Basin axes for Sverdrup Basin, Hanna Trough and Arctic Alaska plotted on pre-Cretaceous Arctic reconstruction of Harland et al. (1984). Note the juxtaposition of the Sverdrup Basin and Hanna Trough axes on this reconstruction suggesting that these basins were once part of the same through-going seaway, l f S o u t h Meade No. 1; 2=seismic line of Fig.6a; 3--seismic line of Fig.6b; 4--Sandy Point I~46.
ANTICLOCKW1SE ROTATION OF NORTHERN ALASKA
travel time or about 4000-6000 m of strata) of interpreted Devonian clastic strata. In both areas these clastic strata are characterized by interval velocities of 4-4.5 km s- 1 (Craig et al., 1985; T. Brent, pers. commun., 1988). The clastics overlie Lower Palaeozoic carbonates in which two reflectors about 1 s apart can be identified. It is worth noting that the Chukchi Sea seismic line compares more closely with the southern Prince Patrick seismic line than any other Arctic Canada seismic line outside the southern Prince Patrick area. In summary, Devonian structural and depositional trends in Arctic North America support the anticlockwise rotation of Alaska model.
Carboniferous-Middle Jurassic trends A new tectonic regime was initiated throughout Arctic North America in earliest Carboniferous (Embry, 1989). Extensional basins developed by rifting and thermal subsidence along the area previously occupied by the Devonian orogenic belt. Thick intervals of Carboniferous-Middle Jurassic strata were deposited in these basins. In the Canadian Arctic Archipelago the main depocentre was the Sverdrup Basin (Balkwill, 1978). The axis of the basin has been delineated using outcrop data and subsurface well and seismic data. The axis trends southwest in the eastern and central parts of the basin and then swings northwest in the western part (Fig.7) (Embry, in press). Land areas which supplied sediment to the basin were present to the east, south and northwest. The basin axis is apparently truncated at the margin of the Amerasia Basin. In northern Alaska and the adjacent Chukchi Sea, Carboniferous-Middle Jurassic strata were deposited in a basin with an axis which trends approximately east-west on land (Arctic Alaska Basin) and which then turns northward in the Chukchi Sea (Hanna Trough of Grantz et al., 1982; Central Chukchi Basin of Thurston and Theiss, 1987) (Fig.7). Land areas which provided sediment lay to the north and west. As was the situation in Canada, the basin
325
axis appears to be truncated at the margin of the Amerasia Basin. Figure 3 shows that the Carboniferous-Middle Jurassic successions of the basins of northern Alaska and the Canadian Arctic Islands are very similar and I suggest that the now-separated basins were at one time parts of a single entity. The two basin axes have been plotted on the Harland et al. (1984) reconstruction and, as shown on Fig.8, the axes are almost perfectly aligned. Thus it would indeed appear as if the H a n n a Trough is the westward continuation of Sverdrup Basin. To further test this conjecture the Triassic-lower Middle Jurassic succession of a well in northwestern Alaska (South Meade No. 1) was compared with an equivalent succession penetrated in a well in the southwestern Sverdrup Basin (Sandy Point L-46). The present-day and reconstructed locations of these wells are shown in Figs.7, 8 and 11. Note that on the reconstruction (Figs.8 and 11) the well locations are reasonably close together and in approximately the same tectonic position (basin margin). Figure 9 illustrates the correlation of the Triassic-lower Middle Jurassic successions of the two wells. The stratigraphy in the two wells is very similar and distinctive lithological units and log markers can be readily correlated. Overall it has been found that the Triassic-lower Middle Jurassic strata of the southwestern Sverdrup Basin are more similar to equivalent strata in northwestern Alaska than they are to equivalent strata in the eastern Sverdrup Basin (see Embry (in press) for stratigraphic comparisons between the eastern and western parts of the Sverdrup Basin). Another test of the validity of positioning northern A l a s k a - C h u k c h i Sea region against the Canadian Arctic Islands in pre-Cretaceous time is to plot facies trends for a given interval and ascertain whether a reasonable palaeogeographic pattern is obtained. Such a test was performed for Lower Triassic strata. In the Sverdrup Basin, thick Lower Triassic sandstones of fluvial to shallow-shelf origin (Bjorne Formation) occur along the eastern and
326
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southern basin flanks. Shale and sittstone of outer shelf to slope origin (Blind Fiord Formation) occur farther basinward (Fig.10). Shelf sandstones reappear in the Lower l'riassic succession along the northwestern basin flank and the northwestern edge of the basin is interpreted to be present on the continental shelf (Embry, in press). In the southwestern part of the basin the Lower Triassic facies and thickness trends are northwesterly, perpendicular to the Amerasia Basin margin (Embry, in press). In the northern Alaska-Chukchi Sea region, fluvial-nearshore sandstones (Ledge Sandstone, Ivishak Formation), which were derived from the north, occur along the North Slope (Hubbard et al. 1987, fig. 12). The sandstones become finer grained southwards and eventu-
PANARCTIC SANDY POINT L-46
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Fig.9. Correlation of Lower Triassic-Middle Jurassic subsurface sections from northern Alaska (South Meade No. 1) and southwestern Sverdrup Basin (Sandy Point L46). Well locations are in Fig.7. The wells are reasonably close together and are in the same tectonic-depositional position on the reconstruction of Harland et al. (1984) (Figs.8 and 11). G.l.=Grosvenor Island Fm.; M.S.= Maclean Strait Fro.; McC.I.=McConnell Island Fro.; for other abbreviations, see Fig.3.
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327
ANTICLOCKWISE ROTATION OF NORTHERN ALASKA
ally c h a n g e facies to s h a l e a n d s i l t s t o n e ( K a v i k M e m b e r , I v i s h a k F o r m a t i o n ) of offshore s h e l f to b a s i n a l o r i g i n (Fig.10). In t h e C h u k c h i Sea area, s e i s m i c - s t r a t i g r a p h i c r e l a t i o n s h i p s indicate that sands were derived from both the east and west with a central shale trough occurring a l o n g the axis of H a n n a T r o u g h ( G r a n t z et al., 1982; T h u r s t o n a n d Theiss, 1987) (Fig.10). T h e l a n d a r e a to the w e s t w a s called C h u k c h i P l a t f o r m by G r a n t z et al. (1982) a n d T h u r s t o n a n d T h e i s s (1987). T h e a b o v e - d e s c r i b e d L o w e r T r i a s s i c facies t r e n d s w e r e p l o t t e d on the H a r l a n d et al. (1984) r e c o n s t r u c t i o n and, as i l l u s t r a t e d on F i g . l l , a coherent palaeogeographic picture appeared. Two significant land areas which provided s e d i m e n t to the b a s i n s w e r e p r e s e n t . T h e C a n a d i a n Shield a n d f l a n k i n g A r c t i c P l a t f o r m were the dominant source areas and the t h i c k e s t , m o s t w i d e s p r e a d s a n d s t o n e s fringe this l a n d m a s s . A s e c o n d l a n d m a s s o c c u r s to t h e n o r t h w e s t of the A r c t i c I s l a n d s a n d i n c l u d e s
the Chukchi Platform. This land area, named C r o c k e r l a n d by E m b r y (1989), also p r o v i d e d c l a s t i c s to t h e S v e r d r u p B a s i n a n d H a n n a T r o u g h . A d e e p - w a t e r a r e a d o m i n a t e d by s h a l e a n d s i l t s t o n e follows t h e b a s i n axes.
Conclusion In r e c e n t years, sufficient d a t a h a v e b e c o m e a v a i l a b l e to a l l o w r e g i o n a l s t r u c t u r a l a n d d e p o s i t i o n a l t r e n d s to be e s t a b l i s h e d for t h e D e v o n i a n - M i d d l e J u r a s s i c s u c c e s s i o n s in b o t h the n o r t h e r n A l a s k a - C h u k c h i S e a r e g i o n a n d t h e C a n a d i a n A r c t i c Islands. T h e s e t r e n d s a r e older t h a n t h e f o r m a t i o n of t h e A m e r a s i a B a s i n ( C r e t a c e o u s ) a n d s o m e a r e a t a h i g h a n g l e to the present Amerasia Basin margins. Such geological trends allow a given plate tectonic model for t h e o r i g i n of t h e A m e r a s i a B a s i n to be e v a l u a t e d in r e g a r d to its success or f a i l u r e in p r o v i d i n g r e a s o n a b l e p a l a e o g e o g r a p h i c a n d tectonic reconstructions. It has been found
Fig.11. Early Triassic palaeogeography of Arctic North America on pre-Cretaceous Arctic reconstruction of Harland et al. (1984). The established facies belts neatly link up on this reconstruction, as does the northern land area (Crockerland) which has been identified in the western Chukchi Sea and north of the Canadian Arctic Islands. 1 = South Meade No. 1; 2= Sandy Point L-46.
328
that the plate tectonic model which restores northern Alaska and adjacent northeastern Siberia against the Canadian Arctic Islands provides a reasonable restoration of various geological trends in Arctic North America. The following trends become aligned to form a through-going pattern on the above restoration: (1) the tectonic front of the Ellesmerian Orogeny, (2) the Carboniferous--Middle Jurassic basin axes, and (3) Lower Triassic facies trends. In summary, the hypothesis that the Amerasis Basin opened by the antictockwise rotation of northern Alaska and adjacent northeastern Siberia away from the Canadian Arctic Islands is supported by geological and geophysical data from northern Alaska and the Arctic Archipelago.
Acknowledgements The Geological Survey of Canada supported this research through the Frontier Geoscience Program and allowed its publication. I would like to thank Polar Continental Shelf Project for substantial logistical support during field studies. Discussions with J. Dixon, M. Cecile, and A. Grantz on the origin of the Amerasia Basin have been enjoyable and enlightening. T. Brent and C. Harrison advised on the interpretation of the Prince Patrick seismic line. L. Lane, S. May, J. Sweeney and D. Forsyth critically read the manuscript and suggested worthwhile changes. The manuscript was expertly processed by Billie Chiang. Paul Wozniak kindly drafted the figures. References Asudeh, I.,Green, A.G. and Forsyth, D.A., 1988. Canadian expedition to study the Alpha Ridge complex: results of the seismic refraction survey. Geophys. J., 92: 283-301. Balkwill, H.R., 1978. Evolution of Sverdrup Basin, Arctic Canada. Am. Assoc. Pet. Geol. Bull., 62: 1004-1028. Bally, A.W., 1976. Canada's passive continental margin -a review. Mar. Geophys. Res., 2: 327-340. Bamber, E.W. and Waterhouse, J.B., 1971. Carboniferous and Permian stratigraphy and paleontology, northern Yukon Territory, Canada. Bull. Can. Pet. Geol., 19: 29-250. Bell, J.S., 1974. Late Paleozoic Orogeny in the northern
~ !,~i~Yukon. In: J.D. Aitken and D.J. Glass ~Edltors), Proceedings of the Symposium on the Geology of the Canadian Arctic. Can. Soc. of Pet. Geol., pp.25 38. Bird, K.J.and Molenaar, C.M., 1987. Stratigraphy. In: K.J. Bird and L.B. Magoon (Editors). Petroleum Geology of the Northern Part of the Arctic Wildlife Refuge. Northeastern Alaska. U.S. Geol. Surv. Bull., 1778:37-60. Carey, S.W., 1958. A tectonic approach to continental drift. In: S.W. Carey (Editor), Continental Drift, a Symposium. Univ. Tasmania, Hobart, pp.177 355. Christie, R.L., 1979. The Franklinian geosyncline in the Canadian Arctic and its relationship to Svalbard. Skr. Nor. Polarinst., 167:263 314. Churkin, M., Carter, C. and Trexler, d., 1980. Collisiondeformed Paleozoic continental margin of Alaska, a foundation for microplate accretion. Geol. Soc. Am. Bull., 91: 648-654. Craig, J.D., Sherwood, K.W. and Johnson, P.P., 1985. Geologic report for the Beaufort Sea Planning Area, Alaska. U.S. Miner. Manage. Serv., Outer Cont. Shelf Rep. 85-0111, 192 pp. Crane, R.C., 1987. Arctic reconstruction from an Alaskan Point of View. In: I. Tailleur and P. Weimer (Editors), Alaskan North Slope Geology. Pac. Sect. Soc. Econ. Paleontol. Mineral. and Alaskan Geol. Soc., Bakersfield, Calif., pp.769 783. Dutro, J.T., 1981. Geology of Alaska bordering the Arctic Ocean. In: A. Nairn, M. Churkin and F. Stehli (Editors), The Ocean Basins and Margins, Vol. 5, The Arctic Ocean. Plenum, New York, pp.21 36. Embry, A.F., 1988. Middle-Upper Devonian Sedimentation in the Canadian Arctic Islands and the Ellesmerian Orogeny. In: N.J. McMillan, A.F. Embry and D.J. Glass (Editors), Devonian of the World. Can. Soc. Pet. Geol. Mem., 14, (2): 15-28. Embry, A.F., 1989. Correlation of Upper Paleozoic and Mesozoic Sequences between Svalbard, Canadian Arctic Archipelago and northern Alaska. In: J.D. Collinson (Editor), Correlation in Hydrocarbon Exploration. Norw. Pet. Soc., Stavanger, pp.89-98. Embry, A.F., in press. Mesozoic History of the Arctic islands. In: H.P. Trettin (Editor), Innuitian Orogen and Arctic Platform: Canada and Greenland. (Geology of Canada, 3.) Geol. Surv. Can. Embry, A.F. and Klovan, J.E., 1976. The Middle-Upper Devonian clastic wedge of the Franklinian Geosyncline. Bull. Can. Pet. Geol., 24: 485-639. Grantz, A., Eittreim, S. and Dinter, D.A., 1979. Geology and tectonic development of the continental margin north of Alaska. Tectonophysics, 59: 263-291. Grantz, A., Dinter, D.A., Hill, E.R., Hunter, R.E., May, S.D., McMullin, R.H. and Phillips, R.L., 1982. Geologic framework, hydrocarbon potential, and environmental conditions for exploration and development of proposed oil and gas lease sale 85 in the central and northern Chukchi Sea. U.S. Geol. Surv. Open-File Rep. 82-1053, 84 pp. Halgedahl, S.L. and Jarrard, R.D., 1987. Paleomagnetism of the Kuparuk River Formation from oriented drill core: Evidence for rotation of the Arctic Alaska Plate. In: I. Tailleur and P. Weimer (Editors), Alaskan North
ANTICLOCKWISEROTATIONOF NORTHERNALASKA Slope Geology. Pac. Sect. Soc. Econ. Paleontol. Mineral. and Alaskan Geol. Soc., Bakersfield, Calif., pp.581 617. Harland, W.B., Gaskell, B.A., Heafford, A.P., Lind, E.K. and Perkins, P.J., 1984. Outline of Arctic post-Silurian continental displacements. In: A.M. Spencer (Editor), Petroleum Geology of the North European Margin. Norw. Pet. Soc. and Graham and Trotman, London, pp.137 148. Herron, E.M., Dewey, J. and Pitman, W., 1974. Plate tectonics model for the evolution of the Arctic. Geology, 2:377 380. Hubbard, R.J., Edrich, S.P. and Rattey, R.P., 1987. Geologic evolution and hydrocarbon habitat of the "Arctic Alaska Microplate". Mar. Pet. Geol., 4:2 35. Jones, P.B., 1982. Mesozoic rifting in the western Arctic Ocean Basin and its relationship to Pacific seafloor spreading. In: A.F. Embry and H.R. Balkwill (Editors), Arctic Geology and Geophysics. Can. Soc. Pet. Geol. Mem., 8: 83-99. Kerr, J.W., 1981. Evolution of the Canadian Arctic Islands; a transition between the Atlantic and Arctic Oceans. In: A. Nairn, M. Churkin and F. Stehli (Editors), The Ocean Basins and Margins. Vol. 5, The Arctic Ocean. Plenum, New York, pp.105-199. Lane, L.S., 1988. The Rapid Fault Array: a foldbelt in Arctic Yukon. In: Current Research, Part D. Geol. Surv. Can. Pap., 88-1D: 95 98. Lawver, L. and Baggeroer, A., 1983. A note on the age of the Canada Basin. J. Alaskan Geol. Soc., 2:57 66. Lawver, L., Grantz, A. and Meinke, L., 1985. The tectonics of the Arctic Ocean. In: I. Dyer and C. Chryssostomidis (Editors), Arctic Technology and Policy. Hemisphere Publ.. New York, pp.147 158. Lerand,M., 1973. Beaufort Sea. In: R.G. McCrossan (Editor), The future petroleum provinces of Canada: their geology and potential. Can. Soc. Pet. Geol. Mem., 1: 315 386. Nilsen, T.H., 1981. Upper Devonian and Lower Mississip-
329 pian Redbeds, Brooks Range, Alaska. In: A.D. Miall (Editor), Sedimentation and Tectonics in Alluvial Basins. Geol. Assoc. Can. Spec. Pap., 23: 187-219. Nilsen, T.H. and Moore, T.E., 1982. Fluvial facies model for the Upper Devonian and Lower Mississippian (?) Kanayut Conglomerate, Brooks Range, Alaska. In: A.F. Embry and H.R. Balkwill (Editors), Arctic Geology and Geophysics. Can. Soc. Pet. Geol. Mem., 8:1 12. Poulton, T.P., 1982. Paleogeographic and tectonic implications of the Lower and Middle Jurassic facies patterns in northern Yukon Territory and adjacent Northwest Territories. In: A.F. Embry and H.R. Balkwill (Editors), Arctic Geology and Geophysics. Can. Soc. Pet. Geol. Mem., 8:13 27. Rickwood, F.K., 1970. The Prudhoe Bay Field. In: W.L. Adkinson and M.M. Brosg~ (Editors), Proceedings of the Geological Seminar on the North Slope of Alaska. Pac. Sect. Am. Assoc. Pet. Geol., Los Angeles, Calif., pp.L1 Lll. Smith, D.G., 1987. Late Paleozoic to Cenozoic reconstructions of the Arctic. In: I. Tailleur and P. Weimer (Editors), Alaskan North Slope Geology. Pac. Sect. Soc. Econ. Paleontol. Mineral. and Alaskan Geol. Soc., Bakersfield, Calif., pp.785-794. Tailleur, I., 1973. Probable rift origin of Canadian Basin, Arctic Ocean. In: M. Pitcher (Editor), Arctic Geology. Am. Assoc. Pet. Geol. Mem., 19:526-535. Tailleur, I. and Brosg~, W.P., 1970. Tectonic history of northern Alaska. In: W.L. Adkinson and M.M. Brosg~ (Editors), Proceedings of the Geological Seminar on the North Slope of Alaska. Pac. Sect. Am. Assoc. Pet. Geol., Los Angeles, Calif., pp.E1-E19 Thurston, D.K. and Theiss, L.A., 1987. Geologic report for the Chukchi Sea Planning Area, Alaska. U.S. Miner. Manage. Serv., Outer Cont. Shelf Rep. 87-0046, 193 pp. Vogt, P.R. and Avery, O.E., 1974. Tectonic history of the Arctic basins: partial solutions and unsolved mysteries. In: Y. Herman (Editor), Marine Geology and Oceanography of the Arctic Seas. Springer, New York, pp.83 117.
317
Geological and Geophysical Evidence in Support of the Hypothesis of Anticlockwise Rotation of Northern Alaska ASHTON F. EMBRY Geological Survey of Canada, 3303-33rd St. NW, Calgary, Alta. T2L 2A7 (Canada) (Received J a n u a r y 10, 1989; revision accepted April 6, 1989)
Abstract Embry, A.F., 1990. Geological and geophysical evidence in support of the hypothesis of anticlockwise rotation of n o r t h e r n Alaska. In: J.R. Weber, D.A. Forsyth, A.F. Embry and S.M. Blasco (Editors), Arctic Geoscience, Mar. Geol., 93: 317-329. The most popular hypothesis for the origin of the Amerasia Basin of the Arctic Ocean is t h a t it formed by the anticlockwise rotation of n o r t h e r n Alaska and adjacent n o r t h e a s t e r n Siberia away from the Canadian Arctic Islands in Cretaceous time. Because there is no obvious p a t t e r n of magnetic stripes in the Amerasia Basin, and the geological evidence presented to support the anticlockwise rotation model has been r a t h e r general and unconvincing, the hypothesis has been repeatedly challenged and alternative hypotheses for the origin of the Amerasia Basin have been proposed. Recently released seismic reflection data from the Chukchi Sea and the Canadian Arctic Islands, in combination with surface and well data, have allowed regional structural and stratigraphic trends to be established both in the Canadian Arctic Islands and n o r t h e r n Alaska Chukchi Sea region. To test the validity of the anticlockwise rotation model, Upper Devonian structural and depositional trends, Carboniferous-Jurassic basin axes and Lower Triassic facies belts have been plotted on Harland et als.'* Arctic pre-Cretaceous plate reconstruction which used the anticlockwise rotation hypothesis as its basic premise. In all cases coherent and reasonable tectonic and depositional reconstructions were obtained. Furthermore, seismic lines and well logs from the n o r t h w e s t e r n A l a s k a - C h u k c h i Sea region and the southwestern Sverdrup Basin, areas which are juxtaposed in the reconstruction, were compared and found to be remarkably similar. Overall, the available geological and geophysical data support the anticlockwise rotation model.
Introduction
Two main basins, the Amerasia and Eurasia, comprise the present Arctic Ocean and they are separated by a narrow ridge of continental crust, the Lomonosov Ridge. The Eurasia Basin is the northern extension of the North Atlantic Ocean and it formed by seafloor spreading away from the axial Nansen Ridge in Tertiary time (Vogt and Avery, 1974). The spreading history of the basin is reasonably Geological Survey of Canada Contribution 56688. "1984. 0025-3227/90/$03.50
well known due to the recognition of clearly defined magnetic anomalies. The Amerasia Basin is a triangular-shaped oceanic basin which includes two prominent bathymetric highs, the Alpha Ridge complex and the Chukchi Cap. The Alpha Ridge complex appears to be oceanic in origin (Asudeh et al., 1987) but the nature of the Chukchi Cap is unknown. Heat flow and depth to oceanic basement studies indicate a Cretaceous age for the Amerasia Basin (Lawver and Baggeroer, 1983) and thus it formed earlier than the Eurasia Basin. The origin of the Amerasia Basin is not well
© 1990 Elsevier Science Publishers B.V.
understood and a number of hypotheses for its formation have been proposed over the last 30 years. The first and most popular hypothesis is that the basin originated by the anticlockwise rotation of northern Alaska and adjacent Siberia away from the Canadian Arctic Islands (Carey 1958; Rickwood, 1970; Tailleur, 1973). The pole of rotation is interpreted to have been located in the Mackenzie Delta region. In this model both the Canadian Arctic margin and Alaskan-Siberian margin are rift margins with the Lomonosov margin being a transform margin (Fig.l). Although this hypothesis has been repeatedly advocated, little convincing geological or geophysical evidence has been provided by its supporters. There is no obvious pattern of magnetic anomalies which would allow the spreading history to be interpreted and the plate movements to be reconstructed. Furthermore, in regard to geological evidence, most workers have noted only general geological similarities between Arctic Alaska and the Canadian Arctic Archipelago (e.g., Grantz et al., 1979). However, such generalities can be accounted for in other hypothetical plate tectonic models and are not convincing evidence for the anticlockwise rotation of Alaska. Reasonably persuasive data which support the anticlockwise rotation model are recently published palaeomagnetic data from northern
Alaska (Halgedahl and Jarrard, 19,n7). H o w ever, due to the problems associated with determining and interpreting palaeomagnetic results from northern Alaska (e.g. very steep magnetic inclinations and resetting of p a l a e o magnetic directions) these data ar~ not r e garded as conclusive. As a result of this lack of solid evidence to support the rotation of Alaska, other plate tectonic models to explain the origin of the Amerasia Basin have been proposed. Most of these (Herron et al., 1974; Churkin et al., 1980; Jones, 1982) are incompatible with existing geological and geophysical data and they have not been adopted by other workers. These hypotheses are reviewed by Lawver et al. (1985) and are not considered further here. A serious rival to the anticlockwise rotation of Alaska model is the southward translation of Alaska model (Bally, 1976; Christie, 1979; Kerr, 1981; Dutro, 1981). This model proposes that the Amerasia Basin opened by the rifting and southward translation of northern Alaska and adjacent northeastern Siberia away from the Lomonosov Ridge-Barents Sea region. In this model the Lomonosov margin and the Alaskan-Siberian margin are rift margins with the Canadian Arctic margin being a transform margin (Fig.2). Recently a number of workers have refined this model and its popularity
Fig.1. Anticlockwise rotation of northern Alaska and adjacent northeastern Siberia model for the opening of the Amerasia Basin (stippled).
Fig.2. Southward translation of northern Alaska model for the opening of the Amerasia Basin (stippled).
ANTICLOCKWISE ROTATION OF NORTHERN ALASKA
seems to be increasing (Crane, 1987; Smith, 1987; Hubbard et al., 1987). However, as is the case for the anticlockwise rotation of Alaska model, only broad generalities have been offered in support of this model. Furthermore the validity of this model depends on the existence of a major left-lateral strike-slip fault through the northern Yukon and northern Alaska (Kaltag Fault, Fig.2). Present geological and geophysical data available for the northern Yukon appear to deny the existence of such a fault (Bamber and Waterhouse, 1971; Poulton, 1982; Lane, 1988). However, the purpose of this paper is not to argue against the southward translation of Alaska model, a task much better accomplished by those working in the northern Yukon. Rather it is to provide specific geological and geophysical data which are supportive of the anticlockwise rotation of Alaska model. The data and interpretations presented in this paper are based on my studies of the geology of the Canadian Arctic Archipelago over the past twenty years and on published information for northern Alaska and adjacent offshore areas. It is only within the last six years that critical seismic reflection data have become publically available for the northern Alaska and Canadian Arctic areas. These data have provided the key to reconstructing palaeogeographic and structural trends in both areas and such trends are ideal for testing the anticlockwise rotation of Alaska model. Various structural and palaeogeographic trends have been plotted on present-day geography and also on the plate reconstruction of Harland et al. (1984) which restores northern Alaska and adjacent northeastern Siberia against the Canadian Arctic Islands. This reconstruction was chosen because it uses the anticlockwise rotation of Alaska as its basic premise and because it was published before the geological data used to test the validity of the model were compiled.
Geological trends Previous workers have noted t h a t the Phanerozoic column of northern Alaska is similar to
319
t h a t of the Canadian Arctic Islands and that the two areas have similar geological histories (Tailleur and Brosg6, 1970; Grantz et al., 1979; Smith, 1987). These authors indicated that these similarities were evidence for a closer positioning of the two areas before the Amerasia Basin had opened in Cretaceous time. However, as noted in the previous section, just how these two areas were formerly juxtaposed is unclear. Figure 3 illustrates a Devonian Cretaceous correlation chart for northern Alaska and the Canadian Arctic Islands, and, as shown, the stratigraphic similarities between the two areas are striking. Embry (1989) elaborates on these similarities and concludes t h a t the two areas were tectonically and depositionally linked throughout the Devonian to Cretaceous interval. The Devonian to Cretaceous succession of Arctic North America can be divided into three megasequences, each having been deposited under a distinctive tectonic setting: (1) Middle-Upper Devonian clastic strata deposited in foreland basins which flanked areas of orogenesis (Ellesmerian Orogeny). (2) Carboniferous-early Middle Jurassic clastics and carbonates deposited in basins which developed by rifting and thermal subsidence along the area previously occupied by the Devonian orogenic belt (e.g. Sverdrup Basin of Arctic Canada and H a n n a Trough of Arctic Alaska). (3) Early Middle Jurassic-Cretaceous clastic strata deposited in a foreland basin which flanked the Brooks Range Orogen (Colville Trough), in previously established extensional basins, and in new extensional basins related to rifting and seafloor spreading in the Amerasian Basin (e.g. Dinkum Graben, offshore northeastern Alaska, and Banks Basin, southwestern Arctic Islands). These three megasequences are approximately equivalent to the three tectonic sequences of Lerand (1973) (Franklinian, Ellesmerian and Brookian) which have been commonly recognized in the stratigraphic succession of northern Alaska (e.g. Bird and Molenaar, 1987).
As indicated above, the first two megasequences developed prior to the formation of the Amerasia Basin. Thus, geological trends recognized in D e v o n i a n - l o w e r Middle Jurassic strata within Arctic North America are well
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suited to test the validity of various plate tectonic restorations. The geological trends which are used herein to test the anticlockwise rotation of Alaska model are: (1) Upper Devo nian stratigraphic and structural trends; (2) Carboniferous-Middle Jurassic depocentre trends, and (3) Lower Triassic facies trends. Upper Devonian
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Both n o r t h e r n Alaska and the Canadian Arctic Islands were affected by Devonian orogenesis (Ellesmerian Orogeny). A clastic wedge derived from the orogenic highlands and deposited in an adjacent, tectonically linked foreland basin is present in both areas (Embry and Klovan, 1976; Nilsen and Moore, 1982). Figure 4 illustrates the general structural and depositional trends for the Devonian orogenic belts and their related clastic deposits. A large area of highly deformed Lower PalaeozoicProterozoic strata is present in the nort h e rn part of the Canadian Arctic Islands. A foreland basin with up to 5000m of Middle U p p e r Devonian clastic strata occurs to the south. P a l a e o c u r r e n t and facies trends indicate a general longitudinal (southwesterly) transport of sediment in the basin. Structural trends within the highly deformed strata and the clastic wedge form an overall curvilinear pat t ern with nort hw est erl y trends occurring in the west. These northwesterly trends are approximately perpendicular to the Amerasia Basin margin. On Banks Island to the south, M i d d l e - U p p e r Devonian' clastic strata and underlying carbonates are very gently deformed. The structures trend n o r t h - s o u t h and deformation increases to the west. However, seismic reflection data for the continental shelf Fig.3. Upper Palaeozoicand Mesozoiccorrelation chart for northern Alaska and Sverdrup Basin (Canadian Arctic Islands). Data for northern Alaska from Bird and Molenaar (1987), Hubbard et al. (1987) and Thurston and Theiss (1987). Data for Sverdrup Basin from Embry (in press). M.H. = Murray Harbour Fro.; R.P. = Roche Pt. Fm.; H.B. = Hoyle Bay Fro.; P.B. = Pat Bay Fm.; S.P. = Sandy Pt. Fro.; K . C . = K a r e n Creek Fm.
ANTICLOCKWISE ROTATION OF NORTHERN ALASKA
321
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adjacent to Banks Island indicate t h a t deformation of the Lower Palaeozoic and older strata remains mild upto the shelf edge, which is at the limit of seismic resolution (J. Dietrich, pers. commun., 1985). In northern Alaska and adjacent northern Yukon, Devonian and older strata have been severely deformed during Cretaceous and Tertiary. Consequently Devonian structural and stratigraphic trends are not very clear. A broad area of Lower Palaeozoic-Proterozoic strata which were deformed in Late Devonian time extends from the Chukchi Sea to the northern Yukon (Tailleur and Brosg~, 1970; Bell, 1974; Grantz et al., 1982). A thick Upper Devonian clastic wedge occurs south of the belt of highly deformed strata and palaeocurrent data indicate derivation from the deformed strata (Nilsen and Moore, 1982). Structural trends appear to be east-west and to wrap around an eastern terminus of the deformed belt in northern Yukon and adjacent Northwest Territories. In the Chukchi Sea area seismic reflection
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data have revealed the presence of an isolated wedge of mildly deformed rocks interpreted to be Devonian clastic strata and underlying carbonates (Grantz et al., 1982; Craig et al., 1985; Thurston and Theiss, 1987). This area of clastic strata is surrounded by highly deformed lower Palaeozoic strata. Structures in the clastics have a n o r t h - s o u t h trend (W. Masterson, pers. commun., 1988). As shown on Fig.4 there are a number of Devonian structural and stratigraphic trends which are truncated by the Alaskan and Canadian margins of the Amerasia Basin. These include the following: (1) Two separate areas of Ellesmerian orogenesis with the orogenic front being approximately perpendicular to the continental margin in various areas. (2) A north-south, gently deformed foreland basin on Banks Island and the adjacent continental shelf with no adjacent orogenic belt. (3) An apparently isolated, wedge-shaped area of mildly deformed Devonian clastic strata in the Chukchi Sea.
The Devonian trends of Fig.4 were plotted on the Harland et al. (1984) reconstruction resulting in a coherent structural and depositional pattern (Fig.5). The Ellesmerian Orogenic Belt appears as a continuous entity with a sinuous trend from northern Ellesmere Island to the northern Yukon. The mildly deformed, foreland basin on Banks Island becomes paired with the orogenic belt of northern Alaska. Also within this reconstruction the isolated wedge of Devonian clastic strata of the Chukchi Sea becomes part of the regional foreland basin. Furthermore it is apparent t h a t the Upper Devonian clastic wedge of northern Alaska was deposited on the opposite side of the orogenic belt compared with the Devonian
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clastic wedge of the Arctic Islands. This observation counters Nilsen's (1981) interpretation that the anticlockwise rotation of Alaska model is untenable because the Devonian clastic strata of Alaska have very different palaeocurrent directions compared with those of the clastic wedge in the Canadian Arctic Islands. As an additional check on the validity of the proposed reconstruction of Devonian trends a published seismic line from the Chukchi Sea which images the Devonian clastic strata and underlying carbonates (Craig et al., 1985, plate 1) was compared with an unpublished, but publically available, seismic line on southern Prince Patrick Island (Fig.6). The location of
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Fig.5. Distribution and structural trends of highly deformed lower Palaeozoic strata and Devonian clastic strata plotted on pre-Cretaceous Arctic reconstruction of Harland et al. (1984) (from Embry, 1988). A coherent depositional and structural picture results from using this reconstruction. 1 = seismic line from Chukchi Sea and 2 = seismic line from Prince Patrick Island (see Fig.6).
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these lines are plotted on present-day geography (Fig.7) and on the H a r l a n d et al. (1984) r e c o n s t r u c t i o n (Figs.5 and 8). Figure 5 suggests th at the two areas crossed by the seismic lines were quite close in the Devonian and t hat the strike directions of the two lines would have
been approximately coincident. There is a remarkable similarity between the two seismic lines, as might be expected if the proposed r e c o n s t r u c t i o n is valid. On both lines a thin interval of post-Devonian strata unconformably overlies a thick interval (2-3 s two-way
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Fig.7. Basin axes for Sverdrup Basin (Canadian Arctic Islands), Hanna Trough (Chukchi Sea) and Arctic Alaska (northern Alaska). I---South Meade No. 1; 2= seismic line of Fig.6a; 3= seismic line of Fig.6b; 4= Sandy Point L-46.
Fig.8. Basin axes for Sverdrup Basin, Hanna Trough and Arctic Alaska plotted on pre-Cretaceous Arctic reconstruction of Harland et al. (1984). Note the juxtaposition of the Sverdrup Basin and Hanna Trough axes on this reconstruction suggesting that these basins were once part of the same through-going seaway, l f S o u t h Meade No. 1; 2=seismic line of Fig.6a; 3--seismic line of Fig.6b; 4--Sandy Point I~46.
ANTICLOCKW1SE ROTATION OF NORTHERN ALASKA
travel time or about 4000-6000 m of strata) of interpreted Devonian clastic strata. In both areas these clastic strata are characterized by interval velocities of 4-4.5 km s- 1 (Craig et al., 1985; T. Brent, pers. commun., 1988). The clastics overlie Lower Palaeozoic carbonates in which two reflectors about 1 s apart can be identified. It is worth noting that the Chukchi Sea seismic line compares more closely with the southern Prince Patrick seismic line than any other Arctic Canada seismic line outside the southern Prince Patrick area. In summary, Devonian structural and depositional trends in Arctic North America support the anticlockwise rotation of Alaska model.
Carboniferous-Middle Jurassic trends A new tectonic regime was initiated throughout Arctic North America in earliest Carboniferous (Embry, 1989). Extensional basins developed by rifting and thermal subsidence along the area previously occupied by the Devonian orogenic belt. Thick intervals of Carboniferous-Middle Jurassic strata were deposited in these basins. In the Canadian Arctic Archipelago the main depocentre was the Sverdrup Basin (Balkwill, 1978). The axis of the basin has been delineated using outcrop data and subsurface well and seismic data. The axis trends southwest in the eastern and central parts of the basin and then swings northwest in the western part (Fig.7) (Embry, in press). Land areas which supplied sediment to the basin were present to the east, south and northwest. The basin axis is apparently truncated at the margin of the Amerasia Basin. In northern Alaska and the adjacent Chukchi Sea, Carboniferous-Middle Jurassic strata were deposited in a basin with an axis which trends approximately east-west on land (Arctic Alaska Basin) and which then turns northward in the Chukchi Sea (Hanna Trough of Grantz et al., 1982; Central Chukchi Basin of Thurston and Theiss, 1987) (Fig.7). Land areas which provided sediment lay to the north and west. As was the situation in Canada, the basin
325
axis appears to be truncated at the margin of the Amerasia Basin. Figure 3 shows that the Carboniferous-Middle Jurassic successions of the basins of northern Alaska and the Canadian Arctic Islands are very similar and I suggest that the now-separated basins were at one time parts of a single entity. The two basin axes have been plotted on the Harland et al. (1984) reconstruction and, as shown on Fig.8, the axes are almost perfectly aligned. Thus it would indeed appear as if the H a n n a Trough is the westward continuation of Sverdrup Basin. To further test this conjecture the Triassic-lower Middle Jurassic succession of a well in northwestern Alaska (South Meade No. 1) was compared with an equivalent succession penetrated in a well in the southwestern Sverdrup Basin (Sandy Point L-46). The present-day and reconstructed locations of these wells are shown in Figs.7, 8 and 11. Note that on the reconstruction (Figs.8 and 11) the well locations are reasonably close together and in approximately the same tectonic position (basin margin). Figure 9 illustrates the correlation of the Triassic-lower Middle Jurassic successions of the two wells. The stratigraphy in the two wells is very similar and distinctive lithological units and log markers can be readily correlated. Overall it has been found that the Triassic-lower Middle Jurassic strata of the southwestern Sverdrup Basin are more similar to equivalent strata in northwestern Alaska than they are to equivalent strata in the eastern Sverdrup Basin (see Embry (in press) for stratigraphic comparisons between the eastern and western parts of the Sverdrup Basin). Another test of the validity of positioning northern A l a s k a - C h u k c h i Sea region against the Canadian Arctic Islands in pre-Cretaceous time is to plot facies trends for a given interval and ascertain whether a reasonable palaeogeographic pattern is obtained. Such a test was performed for Lower Triassic strata. In the Sverdrup Basin, thick Lower Triassic sandstones of fluvial to shallow-shelf origin (Bjorne Formation) occur along the eastern and
326
~' r~ ~
southern basin flanks. Shale and sittstone of outer shelf to slope origin (Blind Fiord Formation) occur farther basinward (Fig.10). Shelf sandstones reappear in the Lower l'riassic succession along the northwestern basin flank and the northwestern edge of the basin is interpreted to be present on the continental shelf (Embry, in press). In the southwestern part of the basin the Lower Triassic facies and thickness trends are northwesterly, perpendicular to the Amerasia Basin margin (Embry, in press). In the northern Alaska-Chukchi Sea region, fluvial-nearshore sandstones (Ledge Sandstone, Ivishak Formation), which were derived from the north, occur along the North Slope (Hubbard et al. 1987, fig. 12). The sandstones become finer grained southwards and eventu-
PANARCTIC SANDY POINT L-46
USGS/NPRA SOUTH MEADE 1 S.P. 7864 ~-~-~
:
r~
FM.
i,: 2
SHUBLIK
:-
FM.
IVISHAK
Fig.9. Correlation of Lower Triassic-Middle Jurassic subsurface sections from northern Alaska (South Meade No. 1) and southwestern Sverdrup Basin (Sandy Point L46). Well locations are in Fig.7. The wells are reasonably close together and are in the same tectonic-depositional position on the reconstruction of Harland et al. (1984) (Figs.8 and 11). G.l.=Grosvenor Island Fm.; M.S.= Maclean Strait Fro.; McC.I.=McConnell Island Fro.; for other abbreviations, see Fig.3.
:: ; :::
FM.
~]
LIMESTONE
-----7
Amerasia Basin
ALA..c
/ o
'
km
3.._..oo
°o
CANADA
Fig.lO. Early Triassic palaeogeography of Arctic North America.
- nearshore shelf sandstone dominant
~
Flevlal
~
Offshore shelf - basin shale - slltstone dominant
,ana
327
ANTICLOCKWISE ROTATION OF NORTHERN ALASKA
ally c h a n g e facies to s h a l e a n d s i l t s t o n e ( K a v i k M e m b e r , I v i s h a k F o r m a t i o n ) of offshore s h e l f to b a s i n a l o r i g i n (Fig.10). In t h e C h u k c h i Sea area, s e i s m i c - s t r a t i g r a p h i c r e l a t i o n s h i p s indicate that sands were derived from both the east and west with a central shale trough occurring a l o n g the axis of H a n n a T r o u g h ( G r a n t z et al., 1982; T h u r s t o n a n d Theiss, 1987) (Fig.10). T h e l a n d a r e a to the w e s t w a s called C h u k c h i P l a t f o r m by G r a n t z et al. (1982) a n d T h u r s t o n a n d T h e i s s (1987). T h e a b o v e - d e s c r i b e d L o w e r T r i a s s i c facies t r e n d s w e r e p l o t t e d on the H a r l a n d et al. (1984) r e c o n s t r u c t i o n and, as i l l u s t r a t e d on F i g . l l , a coherent palaeogeographic picture appeared. Two significant land areas which provided s e d i m e n t to the b a s i n s w e r e p r e s e n t . T h e C a n a d i a n Shield a n d f l a n k i n g A r c t i c P l a t f o r m were the dominant source areas and the t h i c k e s t , m o s t w i d e s p r e a d s a n d s t o n e s fringe this l a n d m a s s . A s e c o n d l a n d m a s s o c c u r s to t h e n o r t h w e s t of the A r c t i c I s l a n d s a n d i n c l u d e s
the Chukchi Platform. This land area, named C r o c k e r l a n d by E m b r y (1989), also p r o v i d e d c l a s t i c s to t h e S v e r d r u p B a s i n a n d H a n n a T r o u g h . A d e e p - w a t e r a r e a d o m i n a t e d by s h a l e a n d s i l t s t o n e follows t h e b a s i n axes.
Conclusion In r e c e n t years, sufficient d a t a h a v e b e c o m e a v a i l a b l e to a l l o w r e g i o n a l s t r u c t u r a l a n d d e p o s i t i o n a l t r e n d s to be e s t a b l i s h e d for t h e D e v o n i a n - M i d d l e J u r a s s i c s u c c e s s i o n s in b o t h the n o r t h e r n A l a s k a - C h u k c h i S e a r e g i o n a n d t h e C a n a d i a n A r c t i c Islands. T h e s e t r e n d s a r e older t h a n t h e f o r m a t i o n of t h e A m e r a s i a B a s i n ( C r e t a c e o u s ) a n d s o m e a r e a t a h i g h a n g l e to the present Amerasia Basin margins. Such geological trends allow a given plate tectonic model for t h e o r i g i n of t h e A m e r a s i a B a s i n to be e v a l u a t e d in r e g a r d to its success or f a i l u r e in p r o v i d i n g r e a s o n a b l e p a l a e o g e o g r a p h i c a n d tectonic reconstructions. It has been found
Fig.11. Early Triassic palaeogeography of Arctic North America on pre-Cretaceous Arctic reconstruction of Harland et al. (1984). The established facies belts neatly link up on this reconstruction, as does the northern land area (Crockerland) which has been identified in the western Chukchi Sea and north of the Canadian Arctic Islands. 1 = South Meade No. 1; 2= Sandy Point L-46.
328
that the plate tectonic model which restores northern Alaska and adjacent northeastern Siberia against the Canadian Arctic Islands provides a reasonable restoration of various geological trends in Arctic North America. The following trends become aligned to form a through-going pattern on the above restoration: (1) the tectonic front of the Ellesmerian Orogeny, (2) the Carboniferous--Middle Jurassic basin axes, and (3) Lower Triassic facies trends. In summary, the hypothesis that the Amerasis Basin opened by the antictockwise rotation of northern Alaska and adjacent northeastern Siberia away from the Canadian Arctic Islands is supported by geological and geophysical data from northern Alaska and the Arctic Archipelago.
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