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The Breakup Unconformity of the Amerasia Basin, Arctic Ocean: Evidence from Arctic Canada

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The Breakup Unconformity of the Amerasia Basin, Arctic Ocean: Evidence from Arctic Canada The breakup unconformity of the Amerasia Basin, Arctic Ocean: 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 oceanic crust 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 Alpha Ridge 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. 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 1528 EMBRY AND DIXON 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
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