The breakup unconformity of the Amerasia Basin, : Evidence from Arctic Canada

I Geological Survey of Canada, 3303 33rd Si NW, Calgary, Alberta, Canada T2L 2A7 JAMJbo DIXON J

ABSTRACT Ridge and the Chukchi Borderland. The Alpha- tion of this methodology to the Amerasia Basin Mendeleyev Ridge, which divides the basin into (Lawver and Baggeroer, 1983) led to the inter- To delineate more closely the age and evo- the Makarov Basin and the Canada Basin, is pretation that the basin opened sometime in lution of the Amerasia Basin of the Arctic interpreted to consist of thickened Cretaceous time. A greater precision (for exam- Ocean, a breakup unconformity has been which developed as a hot-spot track (Forsyth ple, Early Cretaceous) which has been attached identified in sedimentary basins along the and others, 1986). The nature of the Chukchi to these data and interpretations by some au- Canadian margin of the basin on the basis Borderland is unknown. thors (for example, Sweeney, 1985) is unsup- of one or more of the following criteria. Like other ocean basins, the Amerasia Basin ported by the available data and methodology. (1) Strata underlying such an unconformity is interpreted to have formed by sea-floor The methodology which best represents our are cut by major normal faults which extend spreading. The most widely accepted hypothesis hope for dating the Amerasia Basin with current into the basement, whereas strata overlying is that the basin opened by the counterclockwise data is the identification of a breakup uncon- the unconformity are relatively unfaulted. rotation of northern Alaska and adjacent north- formity on the rifted margins of the basin. In (2) A major decrease in subsidence rate in the eastern Siberia, away from the Canadian Arctic theory, the age of the breakup unconformity is marginal basins coincides with the time of Archipelago about a pole located in the Mack- coincident with the initiation of sea-floor spread- breakup and the formation of the unconform- enzie Delta region (Carey, 1958; Rickwood, ing in the adjacent ocean basin (Falvey, 1974). ity. (3) Volcanic rocks occur beneath the 1970; Tailleur, 1973; Grantz and others, 1979 Application of this methodology has led to three unconformity. The widespread late Albian- and in press). Paleomagnetic data from northern different interpretations for the initiation of Cenomanian unconformity is interpreted to Alaska (Halgedahl and Jarrard, 1987) support spreading in the basin: Hauterivian (Grantz and be the breakup unconformity and thus this this hypothesis, and Embry (in press a) recently May, 1983), early Albian (Hubbard and others, time interval would coincide with the initia- demonstrated that a plate-tectonic reconstruc- 1987; Craig and others, 1985) and Cenomanian tion of sea-floor spreading in the Amerasia tion using this model results in coherent, (Embry and Osadetz, 1988; Dixon, in press). Basin. Sea-floor spreading and the opening of through-going Devonian to Jurassic structural These conflicting interpretations are not the re- the Amerasia Basin by the counterclockwise and stratigraphic trends in the Arctic region. In sult of different age interpretations of the same rotation of northern Alaska and adjacent this model, the Canadian Polar Margin and the unconformity. Rather, each set of authors se- northern Siberia away from the Canadian Alaskan Polar Margin are conjugate rift lected a different regional unconformity as the Arctic Islands are interpreted to have oc- margins. breakup unconformity. curred during Late Cretaceous time and to The timing of sea-floor spreading in the Because previous applications of this method- have ceased near the Cretaceous-Tertiary Amerasia Basin is not well established because ology have resulted in conflicting results, we boundary when the active plate margin the usual methods for dating ocean floors cannot have written this paper with the following aims: switched to the site of the present Eurasia be applied to the basin. The basin is covered by (1) to evaluate, on both theoretical and empiri- Basin. the shifting polar ice pack which prevents drill- cal grounds, the validity of the breakup uncon- ing, and only shallow cores have been obtained formity methodology in dating adjacent oceanic INTRODUCTION from the sea floor. The oldest sediment so far crust; (2) to determine relatively objective crite- recovered is latest Campanian or Maastrichtian ria for the recognition of a breakup unconform- The ocean floor over most of the globe is from a locality near the crest of ity; (3) to apply the above criteria to the ge- reasonably well dated due to numerous Deep (Mudie and Blasco, 1985). Furthermore, the ological and geophysical data bases of the Sea Drilling Project (DSDP) and Ocean Drill- magnetic anomalies over the basin are of very Canadian Polar Margin so as to identify the ing Program (ODP) wells which reach oceanic low amplitude and cannot be confidently corre- breakup unconformity; and (4) to interpret the crust, and the presence of magnetic anomalies lated with the world standard. A number of tectonic evolution of the Amerasia Basin. which can be correlated with the world standard workers have offered possible ages for these of dated anomalies. An exception to this is the magnetic anomalies, but the wide range of sug- BREAKUP UNCONFORMITY: oceanic basement of the Amerasia Basin of the gested ages (Jurassic-early Tertiary) for the DEFINITION AND IDENTIFICATION Arctic Ocean, the age of which is presently the anomalies underscores the futility of such an ex- CRITERIA subject of much speculation. The Amerasia ercise (Ostenso, 1972; Taylor and others, 1981; Basin is a small, triangle-shaped ocean basin Vogt and others, 1982). Falvey (1974) first proposed the concept that which is completely enclosed by continental Parsons and Sclater (1977) demonstrated that an unconformity, which is approximately time areas (Fig. 1). Two prominent bathymetric highs both heat flow and depth to oceanic basement equivalent to the onset of sea-floor spreading in occur within the basin, the Alpha-Mendeleyev correlate with the age of oceanic crust. Applica- a given ocean, is present in the stratigraphic rec-

Geological Society of America Bulletin, v. 102, p. 1526-1534, 8 figs., November 1990.

1526

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/11/1526/3380885/i0016-7606-102-11-1526.pdf by guest on 29 September 2021 Figure 1. A. Location map for Amerasia Basin and adjacent areas. (1) Atigi G-04 well; (2) Skybattle Bay C-15 well; (3) seismic reflection line of Figure 4; (4) seismic reflection line of Figure 8. QEI: Queen Elizabeth Islands. B. Detailed location map of wells and seismic line illustrated in Figure 4 in the Mackenzie Delta region. C. Detailed location map of wells and seismic line illustrated in Figure 8 in the northern Queen Elizabeth Islands area.

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ord of the adjacent continental margin. He named it the "breakup unconformity" and in- terpreted that it was "caused by erosion during the final uplift pulse associated with pre-breakup upwelling in the mantle" (Falvey, 1974, p. 102). Falvey further noted that the unconformity was usually localized and occurred mainly on the basin flanks and high blocks (see Falvey, 1974, Fig. 9). Subsequent to Falvey's pioneering work, so- phisticated thermal-mechanical models have been developed to explain the origin and evolu- tion of rifted continental margins (McKenzie, 1978; Beaumont and others, 1982, 1984; Issler and others, 1989). In these models, a margin undergoes two main phases of development: rift and drift. During the rift phase, the continental lithosphere is actively extended until it finally ruptures, initiating the drift phase. In the rift phase, high rates of subsidence and sedimenta- tion are localized in half grabens. Adjacent horsts undergo erosion or are covered by rela- tively thin sedimentary packages. Sediments can vary from continental clastics to restricted or fully marine deposits. Volcanic flows may be interbedded with the syn-rift deposits. During the subsequent drift phase, the active plate mar- gin is seaward of the rifted margin, and the mar- gin undergoes regional subsidence due to thermal contraction of the lithosphere and sedi- SHALE-SILTSTONE SUBAERIAL UNCONFORMITY ment loading. In these models, suggested mechanisms for Figure 2. Jurassic-early Tertiary correlation chart for Canadian and American areas border- the occurrence of a breakup unconformity are ing the Amerasia Basin. Mackenzie Delta data from Poulton (1982) and Dixon (1982 and in lateral conduction of heat to the basin flanks press). Banks Island data from Plauchut and Jutard (1976), Miall (1979), and Embry (in press (Cochran, 1983), depth-dependent extension b). Queen Elizabeth Islands (Sverdrup Basin) data from Embry (in press b). Northern Alaska (Royden and Keen, 1980), and simple shear ex- data from Detterman and others (1975), Grantz and May (1983), Craig and others (1985), and tension (Hegarty and others, 1988; Issler and Bird and Molenaar (1987). others, 1989). Another explanation for the generation of an unconformity at the transition from rift to drift is the significant change in the ian margin, Veevers (1986) on the southern it is possible to develop criteria for the recogni- horizontal stress field in the marginal continental Australian margin, and Boote and Kirk (1989) tion of a breakup unconformity. These criteria lithosphere which would occur at that time. As on the northwestern Australian margin. include the following. demonstrated by Cloetingh (1986), such a de- Given the likely presence of a breakup uncon- 1. The unconformity and its correlative con- crease in horizontal tensional stress would result formity, the primary problem is its identifica- formity separate strata which were deposited in uplift of the basin flanks. tion. Numerous unconformities occur on the mainly in actively developing half grabens, from The existence of a breakup unconformity is flanks of a continental margin, and their pres- strata which were deposited over a broad area. also supported by empirical data. The age of the ence forms the basis of sequence stratigraphy Thus the pre-breakup strata are cut by major initiation of sea-floor spreading on the margins (Vail and others, 1977). The breakup uncon- normal faults which extend into the basement, of the Atlantic Ocean and on the Australian formity and its correlative conformity compose whereas the post-rift strata are unfaulted or have margins is known due to the identification of just one of many sequence boundaries, and it is undergone substantially less faulting (due to well-dated magnetic anomalies. Studies in these sometimes difficult to distinguish it from other minor reactivation of rift faults and differential areas have demonstrated the existence of a re- sequence boundaries. For example, Falvey subsidence over fault scarps). Faults which do gional unconformity which is about the same (1974) misidentified the breakup unconformity not cut basement are excluded in this criteria. age as the oldest adjacent oceanic sea floor. Such in the example he cited when he proposed the 2. Subsidence rates in half grabens are com- studies include those of Klitgord and others concept. His rift-onset unconformity for the monly high during rifting. A notable decrease in (1988) on the eastern United States margin, southern Australian margin was subsequently subsidence rate occurs in graben areas at the Enachescu (1987) on the Newfoundland mar- shown to be the breakup unconformity by time of the development of the breakup uncon- gin, Balkwill (1987) on the Labrador margin, Veevers (1986). formity; following this, the subsidence rate on Mauffret and Montadert (1987) on the Spanish From both theoretical considerations and the margin declines due to thermal decay during margin, Chang and others (1988) on the Brazil- published examples of breakup unconformities, the drift phase. This change is readily observed

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as a significant inflection point on a computed Mackenzie Delta-Beaufort Sea zone" (Young and others, 1976; Cook and oth- tectonic subsidence curve (for example, Veevers, ers, 1987a, 1987b). The fault zone can be traced 1988, Fig. 4; Chang and others, 1988, Fig. 10; Oil and gas exploration in the Mackenzie northeastward into the Arctic Platform Hinge Hegarty and others, 1988, Fig. 9). Delta and adjacent Beaufort Sea has been active line (Figs. 1 and 4; Dixon and Dietrich, 1985, 3. Volcanic strata occur mainly in the syn-rift since 1965, and consequently there is a plethora Fig. 2) which underlies the Beaufort Shelf and deposits and thus underlie the breakup uncon- of publically available, reflection seismic data Amundsen Gulf. The seismic data have been formity (Falvey, 1974; Klitgord and others, and more than 200 wells. Furthermore, deep- correlated to exploratory wells, and the results 1988; Chang and others, 1988). reflection seismic has been recently shot by the show that faulted Lower Cretaceous and older Geological Survey of Canada (GSC). One of the strata are overlain unconformably by a prograd- THE AMERASIAN BREAKUP GSC lines was shot in a northwest direction on ing succession of Upper Cretaceous to Tertiary UNCONFORMITY: CANADIAN DATA the east side of the Mackenzie Delta (Cook and strata (Figs. 3 and 4). The youngest Lower Cre- others, 1987a, 1987b), and another was shot taceous strata below the unconformity are mid- In this section, the stratigraphy and structure across the Beaufort shelf at the south end of dle Albian Arctic Red Formation (Dixon and of three areas along the Amerasia Basin (Mack- Amundsen Gulf (Dietrich and others, 1989). others, 1989). Arctic Red strata are also present enzie Delta-Beaufort Sea, Banks Island, and These data, in combination with surface studies, under the unconformity in most areas on the Queen Elizabeth Islands) are reviewed with spe- have allowed a good understanding of the Juras- upthrown side of the Eskimo Lakes fault zone, cial emphasis on the various unconformities sic to Tertiary stratigraphy and structure of the but locally Upper Cretaceous beds rest on Pa- present in the Upper Jurassic-Cretaceous suc- area (Figs. 2, 3, and 4). leozoic and possible Proterozoic strata. cession. The most likely candidate for the Seismic data clearly show that the southeast- Strata immediately above the unconformity breakup unconformity in each area is identified ern margin of the Beaufort continental shelf is are, in most cases, the Santonian-Campanian on the basis of the criteria listed in the previous normally faulted; the main trend of maximum Smoking Hills Formation. At the extreme section. throw is known as the "Eskimo Lakes fault southwestern end of Tuktoyaktuk Peninsula,

Figure 3. Simplified geologic cross section along the seismic line illustrated in Figure 4 (see Figure IB for line location). BU: breakup unconformity.

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ATIGI G-04 PARSONS P-41 ONIGAT C-38 of the area. As illustrated, the subsidence rate generally increases up to the time of the late •0.0 Albian-early Late Cretaceous unconformity and then markedly decreases, thus supporting the choice of this unconformity as the breakup un-

CO conformity. Other regional unconformities have a z been identified in this and adjacent areas and are o o of late Aalenian, late Callovian-early Oxford- L1J (0 ian, late Berriasian-early Valanginian, late z Hauterivian, mid-Aptian, and late Maastrichtian ages (Fig. 2). The tectonic significance of these is discussed below.

Banks Island Area

—3.0 Banks Island lies to the northeast of the Mackenzie Delta-Beaufort Sea area and forms 1. Reindeer Fm. (Lower Tertiary) 5. Parsons Group (Berriasian- the southwestern corner of the Canadian Arctic Hauterivian) 2. Smoking hills and Boundary Archipelago (Fig. 1). An unsuccessful search for Creek Fms (Upper Cretaceous) 6. Husky Fm. (Oxfordian- hydrocarbons in the 1970s left a legacy of seis- Berriasian ) 3. Arctic Red Fm. (Albian) mic reflection data (offshore and onshore) and 7. Lower Paleozoic 4. Mount Goodenough Fm. wells (onshore). The Jurassic to Cretaceous suc- (Barremian) 8. Proterozoic cession is thin because the area is landward of BU breakup unconformity the major down-to-basin fault zone (Arctic Plat- form Hinge line) on the margin of the Amerasia Figure 4. Reflection seismic section across the Eskimo Lakes fault zone, southern Tuktoyak- Basin. Three regional unconformities have been tuk Peninsula. The interpreted breakup unconformity (BU) is placed at the base of unit 2 recognized in the Jurassic-Cretaceous succes- (Smoking Hills and Boundary Creek Formations) (see Fig. IB for line location). sion: late Callovian-early Oxfordian, mid- Aptian, and late Albian-early Santonian (Figs. 2, 6; Plauchut and Jutard, 1976; Miall, 1979). however, Cenomanian-Turonian strata of the The application of criterion 1 for the identifi- Upper Jurassic-lowermost Cretaceous strata Boundary Creek Formation occur above the un- cation of a breakup unconformity indicates that occur in a fault-bounded trough in the center of conformity (Brideaux and Myhr, 1976). These the late Albian-earliest Late Cretaceous uncon- Banks Island (Banks Basin). Aptian to Albian stratigraphic relationships have been described formity is the obvious choice. As a further strata extend over much wider areas and in turn and illustrated by previous workers, including check, a tectonic subsidence curve has been are overlapped by Upper Cretaceous strata Lerand (1973), Cote and others (1975, Fig. 5), computed for the Upper Jurassic-Cretaceous (Miall, 1979) (Fig. 6). Hawkings and Hatlelid (1975, Fig. 10), and stratigraphic column in the Parsons Lake area Unpublished seismic data show that the Young (1973). (Atigi G-04 well, Fig. 5), which is representative Upper Jurassic-Albian succession is cut by normal faults and that significant thickness changes occur across the faults (J. Dietrich, 0 - 1986, personal commun.). Some faults extend LU BREAK-UP into the Upper Cretaceous-lower Tertiary sec- O Z tion but with much reduced offset. The faults 111 commonly die out within these strata. 9 m Because the Banks Island area represents a CO _SKYBANIJ^AYCI15_ OQ basin flank outside of the area of significant ex- 3 tension, the identification of a breakup uncon- CO ATIGI G-04 formity is rather tenuous. The application of ^1000- X/ criterion 1 (2 and 3 are not applicable) suggests that the unconformity at the base of the Kanguk o OK T B V H B A A C TCS C M PAL EOCENE OLIG MIO PL oI- Formation which developed in late Albian- LU LATE EARLY LATE early Late Cretaceous time represents the TERTIARY JR CRETACEOUS breakup unconformity in this area. 160 140 120 100 80 60 40 20 0 Queen Elizabeth Islands Ma The Queen Elizabeth Islands form the north- Figure 5. Tectonic subsidence curves for Skybattle Bay C-15 (Sverdrup Basin) and Atigi ern portion of the Canadian Arctic Archipelago G-04 (Mackenzie Delta) wells (see Fig. 1 for well locations). These curves were computed (Fig. 1). Seismic reflection and well data are using standard backstripping methodology. The initiation of sea-floor spreading in the adjacent available throughout the islands, including the Amerasia Basin (breakup) is interpreted to coincide with the marked decrease in tectonic land areas which border the Amerasia Basin. subsidence which occurs at the end of the Albian in both wells. The northwestern edge of the islands is under-

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lain by the Sverdrup Rim, which is a broad arch W CONTINENTAL SHELF BANKS BASIN E that separates the Amerasia Basin from the Sverdrup Basin. Sverdrup Basin was a major depocenter from Carboniferous to early Tertiary time (Balkwill, 1978). Carboniferous to Lower Cretaceous strata are progressively truncated northwestward out of the Sverdrup Basin onto the crest of Sverdrup Rim (Fig. 7). These strata are commonly cut by normal faults and are unconformably overlain by Upper Cretaceous to Tertiary strata that thicken both northward into Amerasia Basin and southward into the Sverdrup Basin (Fig. 8) (Meneley and others, 1975). On Sverdrup Rim, the oldest strata above the unconformity are Santonian in age (Embry, in press b). Due to a lack of data, the unconformity can be traced seaward only a short distance, but when it is followed into the Sverdrup Basin, it lies at the base of the Kanguk Formation or within the underlying Hassel Formation (Fig. 2). The Figure 6. Schematic geologic cross section of Mesozoic strata across Banks Island and oldest strata above the unconformity within the adjacent continental shelf. The cross section is based on seismic and well data. BU: breakup Sverdrup Basin are Cenomanian or early unconformity. Turonian; the youngest strata beneath the un- conformity are late Albian (Wall, 1983). Three intervals of tholeiitic basalt flows (up to 700 m thick) occur in the Hauterivian to late present in the Middle Jurassic-Cretaceous suc- DISCUSSION Albian strata which underlie the unconformity cession of the Queen Elizabeth Islands are late in the northeastern Sverdrup Basin (Embry and Aalenian, late Callovian-early Oxfordian, late Identification of the Amerasian breakup un- Osadetz, 1988). Diabase dike and sill intrusions Berriasian-early Valanginian, late Hauterivian, conformity has been previously attempted using also occurred throughout much of Sverdrup mid-Aptian, and late Maastrichtian-earliest Pa- data from northern Alaska. Seismic reflection Basin in Early Cretaceous time (Balkwill, 1978). leocene (Embry, in press b) (Fig. 2). and well data are available for the Alaskan mar- An interval of volcanic strata of Campanian to earliest Tertiary age occurs in the extreme northeastern portion of the Sverdrup Basin. These younger volcanic rocks are chemically N EUREKA SOUND dissimilar from those of the Lower Cretaceous and were accompanied by minor, localized dike and sill intrusions (Embry and Osadetz, 1988). These younger volcanics are closely related in age and composition to volcanic rocks in north- ern Greenland which are interpreted to be asso- ciated with the rift phase of the Eurasia Basin (Soper and others, 1982). The above described stratigraphic and struc- tural relationships are only from the extreme margin of the Amerasia Basin and from the ad- jacent Sverdrup Basin. The data suggest, how- ever, that the late Albian-Cenomanian uncon- formity represents the breakup unconformity because (1) on Sverdrup Rim, the unconformity separates normally faulted strata below from un- faulted strata above; (2) a computed tectonic subsidence curve for the Middle Jurassic-Cre- VOLCANIC

taceous succession of the Queen Elizabeth Is- SANDSTONE lands area shows that tectonic subsidence generally increases up to the time of the uncon- tr:>-;-J SHALE, SILTSTONE formity and then abruptly decreases (Skybattle Bay C-15, Fig. 5); and (3) widespread basaltic volcanism occurred prior to the development of Figure 7. Schematic geologic cross section of Mesozoic strata of the Sverdrup Basin, Queen the unconformity. Elizabeth Islands. The faulted northern flank of the basin comprises Sverdrup Rim. BU: Other regional unconformities which are breakup unconformity.

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South North

ISACHSEN J-37

40

Figure 8. Reflection seismic line on northernmost Ellef Ringnes Island (see Fig. 1C for location). The interpreted breakup unconformity (BU) separates seaward-dipping Upper Cretaceous-Tertiary strata from underlying faulted Jurassic and older strata (from Meneley and others, 1975).

gin, and these data have been illustrated and which occurs directly above the unconformity, for the relationship between unconformities [se- interpreted by Grantz and May (1983) and consists of transgressive, condensed deposits and quence boundaries] and maximum flooding Craig and others (1985). The general geological ranges in age from Hauterivian to Aptian in the surfaces). history of the area is summarized by the above west and also includes Albian strata in the east On the basis of criterion 1, the Hauterivian authors as well as by Bird and Molenaar (1987). (Bird and Molenaar, 1987). A prominent max- unconformity (LCU) is the most reasonable Grantz and May (1983) interpreted the Hauteri- imum flooding surface (downlap surface), marks choice for the breakup unconformity on the vian unconformity, which occurs beneath the the top of the pebble shale unit. Lower Creta- Alaskan margin. Other regional unconformities pebble shale unit (Fig. 2), to be the breakup ceous (west) and Upper Cretaceous (east) slope which have been identified in the Jurassic- unconformity. This unconformity, commonly clastics downlap onto this surface and represent Cretaceous succession of northern Alaska called the "Lower Cretaceous Unconformity" or prograding foreland basin deposits of the Col- are late Aalenian, late Callovian, late Berria- LCU, truncates Carboniferous to lowermost ville Trough. The Colville Trough migrated sian, mid-Cenomanian, and late Maastrichtian Cretaceous strata northward out of the Arctic northward in front of the advancing Brooks (Fig- 2). Alaska Basin onto the crest of the Barrow Arch Range Orogen and is filled with Cretaceous- If one accepts the counterclockwise rotation (Fig. 1). This arch trends east-west along the Tertiary clastics derived from the orogenic belt. hypothesis for the origin of the Amerasia Basin, northern coast, and Lower Cretaceous strata The foreland basin eventually migrated over the then the Alaskan and Canadian polar margins commonly rest unconformably on lower Pa- Barrow Arch onto the margin of the Amerasia are conjugate rift margins which should contain leozoic strata on its crest. Seaward of the crest of Basin. the same breakup unconformity. On the Ca- Barrow Arch, on the continental shelf, half Craig and others (1985) and Hubbard and nadian margin, however, the breakup uncon- grabens interpreted to contain up to 3 km of others (1987) interpreted the seismic data in a formity is identified as the latest Albian- Middle(?)-Upper Jurassic to lowermost Cre- fashion similar to that of Grantz and May Cenomanian unconformity, whereas on the taceous strata are present (for example, Dinkum (1983) but identified the top of the pebble shale Alaskan margin, it appears to be the late Graben, Grantz and May, 1983, Figs. 11, 12). unit as the breakup unconformity. Such an in- Hauterivian unconformity. The normal faults which cut basement are terpretation seems unreasonable because this A possible explanation for this disparity may commonly truncated by the Hauterivian uncon- surface is not a regional unconformity but a be found in the complex tectonics of the Alas- formity or extend a short distance into the over- maximum flooding surface which represents the kan continental margin. As described above, the lying Cretaceous strata. The pebble shale unit, time of maximum transgression (see Vail, 1987, Alaskan margin is a composite of extensional rift

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tectonics and external-load, foreland-basin tec- and older structural and facies trends are per- to the change from rift subsidence to foreland- tonics. Significantly, the late Hauterivian uncon- pendicular to the Amerasian margin (Harrison basin subsidence on that margin, than to the formity in northern Alaska marks the change and others, 1988; Embry, in press b). This early later rift-drift transition. Regional unconformi- from rift to foreland-basin subsidence on the Middle Jurassic unconformity is also readily ties are also coincident with the onset of conti- Barrow Arch and adjacent continental margin. recognizable in northern Alaska and the Mack- nental rifting, various pulses within the syn-rift During this transition period, extensional fault- enzie Delta region (Fig. 2). stage, and the cessation of sea-floor spreading. ing was subdued. It is suggested that this change The lower Bajocian to upper Albian succes- These regional unconformities are interpreted to in subsidence pattern produced a similar struc- sion on the margins of the Amerasia Basin there- have developed in response to a significant tural and stratigraphic expression as a change fore represents the syn-rift deposits. Four re- change in horizontal lithospheric stresses which from rift subsidence to thermal subsidence (rift- gional unconformities (late Callovian-early was associated with each tectonic event. An drift). Consequently, the Hauterivian uncon- Oxfordian, late Berriasian-early Valanginian, early phase of continental rifting, which initiated formity resembles a breakup unconformity on late Hauterivian, and mid-Aptian) are recogniz- the Amerasia Basin, began in early Bajocian the basis of criterion 1; however, criterion 2, the able within the syn-rift succession on the basin time, and the main rift phase occurred from late abrupt decrease in tectonic subsidence following flanks (Fig. 2). These events subdivide the suc- Hauterivian to late Albian. Volcanism and high the breakup unconformity, is not satisfied. The cession into distinct rift pulses, similar to the rates of subsidence characterize the main phase subsidence rate increased, rather than decreased, pulses which characterize both modern and an- of rifting. Sea-floor spreading began in Ceno- following the time of the unconformity (Craig cient rifts (Evans, 1988; Steckler and others, manian and continued until late Maastrichtian. and others, 1985), which would be expected in a 1988; Rosendahl and others, 1987; Chang and During this time, the Arctic Alaska plate (north- foreland-basin regime rather than in a thermal- others, 1988). Embry and Osadetz (1988) used ern Alaska and adjacent northeastern Siberia) subsidence regime. In summary, it is interpreted the late Hauterivian event to divide the period of rotated counterclockwise away from the Cana- that the overprinting of foreland-basin tectonics rifting into an early rift phase (Bajocian- dian Arctic Archipelago about a pole of rotation on the marginal-rift tectonics of the Alaskan Hauterivian) and a main rift phase (Barremian- located in the Mackenzie Delta region. Follow- margin prevented a typical breakup unconform- Albian). Subsidence rates were markedly higher ing this, the active plate margin switched to the ity from developing. in the main rift phase (Fig. 5), and volcanism Eurasia Basin. Because the Canadian margin was not af- appears to have been restricted to this phase. fected by foreland basin tectonics, the recogni- The duration of sea-floor spreading in the ACKNOWLEDGMENTS tion of the breakup unconformity is relatively Amerasia Basin is not known with certainty. straightforward. As discussed above, all three Due to the likelihood of a major change in the The authors would like to thank the Geologi- criteria for the identification of a breakup un- horizontal lithospheric stress regime of the con- cal Survey of Canada for supporting this re- conformity are satisfied by the latest Albian- tinental margins occurring with the cessation of search through the Frontier Geoscience Project. Cenomanian unconformity. Thus it seems rea- drift, this event is interpreted to be marked by a Jim Dietrich contributed greatly to the inter- sonable that this unconformity is the breakup regional unconformity. Taking into account the pretation of seismic lines in the Beaufort Sea unconformity of the Amerasia Basin. It is worth general age of the basin based on heat flow and and Banks Island areas. Dale Issler computed noting that this breakup unconformity is ex- depth to basement data, as well as the occur- tectonic subsidence curves for the Atigi G-04 pressed in northern Alaska as a major sequence rence of upper Campanian-Maastrichtian sedi- and Skybattle Bay G-04 wells. D. Issler, boundary which is at or near the boundary be- ment on Alpha Ridge, the widespread late A. Grantz, and T. O'Brien critically read the tween the Nanushuk Group and the overlying Maastrichtian unconformity (Fig. 2) is inter- manuscript and offered numerous suggestions Colville Group (Fig. 2). preted to mark the termination of spreading. for improvement. This is slightly younger than previous inter- EVOLUTION OF THE AMERASIA pretations (Santonian: Grantz and May, 1983; BASIN Sweeney, 1985) but does not conflict with cur- rent data. Notably the Eurasia Basin began to REFERENCES CITED rift in latest Cretaceous (Rowley and Lottes, Balkwill, H. R., 1978, Evolution of Sverdrup Basin, Arctic Canada: American From the above analysis, it is interpreted that Association of Petroleum Geologists Bulletin, v. 62, p. 1004-1028. sea-floor spreading began in the Amerasia Basin 1988). Thus a major plate-tectonic reorganiza- 1987, Labrador Basin: Structural and stratigraphic style, in Beaumont, G, and Tankard, A. 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