David A. Dinter

Geology and Tectonic Development of the Continental Margin North of Alaska

Abstract The continental margin north of Alaska, as interpreted from seismic reflection profiles, is of the Atlantic type and consists of three sectors of contrasting structure and stratigraphy. The Chukchi sector, on the west, is characterized by the deep late Mesozoic and Tertiary North Chukchi basin and the Chukchi Continental Borderland. The Barrow sector of central northern Alaska is characterized by the Barrow arch and a moderately thick continental terrace build of Albian to Tertiary clastic sediment. The terrace sedimentary prism is underlain by lower Paleozoic metasedimentary rocks. The Barter Island sector of northeastern Alaska and Yukon Territory is inferred to contain a very thick prism of Jurassic, Cretaceous and Tertiary marine and nonmarine clastic sediment. Its structure is dominated by a local deep Tertiary depocenter and two regional structural arches. We postulate that the distinguishing characteristics of the three sectors are inherited from the configuration of the rift that separated arctic Alaska from the Canadian Arctic Archipelago relative to old pre-rift highlands, which were clastic sediment sources. Where the rift lay relatively close to northern Alaska, in the Chukchi and Barter Island sectors, and locally separated Alaska from the old source terranes, thick late Mesozoic and Tertiary sedimentary prisms extend farther south beneath the continental shelf than in the intervening Barrow sector. The boundary between the Chukchi and Barrow sectors is relatively well defined by geophysical data, but the boundary between the Barrow and Barter Island sectors can only be inferred from the distribution and thickness of Jurassic and Cretaceous sedimentary rocks. These boundaries may be extensions of oceanic fracture zones related to the rifting that is postulated to have opened the Canada Basin, probably beginning during the Early Jurassic.

Quaternary sedimentation of the Alaskan Beaufort shelf: Influence of regional tectonics, fluctuating sea levels, and glacial sediment sources

Abstract The offshore stratigraphy of the Quaternary Gubik Formation of Arctic Alaska has been studied on high-resolution seismic profiles with a maximum sub-seafloor penetration of about 100 m. In general, marine transgressive subunits of the Gubik Formation are wedge-shaped on the shelf, thickening slightly seaward to the shelf break, beyond which they are offset by landslides and slumps. Beneath the eastern third of the Alaskan Beaufort shelf, active folding has created two persistent structural depressions, the Eastern and Western Wedge Terranes, in which the wedge morphology is especially well developed. The youngest transgressive marine wedge, which was deposited in such a way as to fill these depressions, leaving a generally flat present-day shelf surface, is inferred to be late Wisconsin or younger in age because it overlies a prominent disconformity interpreted to have been formed during the late Wisconsin glacial sea-level minimum. The thickness of this youngest wedge, Unit A, locally exceeds 40 m on the outer shelf, yet apparently relict gravel deposits collected from its seabed surface indicate that the depositional rate is presently quite low on the middle and outer shelf. Lithologies of the gravels are exotic to Alaska, but similar to suites exposed in the Canadian Arctic Islands. These observations suggest a depositional scenario in which the retreating Laurentide Ice Sheet shed sediment-laden icebergs from the Canadian Arctic Islands into the Arctic Ocean following the late Wisconsin glacial maximum. These bergs were then rafted westward by the Beaufort Gyre and grounded on the Alaskan shelf by northeasterly prevailing winds. Especially large numbers of bergs accumulated in the wedge terrane embayments—created as sea level rose—and melted there, filling the embayments with their sedimentary cargo. As glacial retreat slowed, depositional rates on the shelf dwindled. This mode of deposition in the Alaskan Beaufort wedge terranes may be typical of early post-glacial transgressive phases throughout Quaternary time. It has resulted in the preservation of disconformities that apparently formed during glacioeustatic lowstands, and whose seaward termination depths, appropriately corrected, may yield estimates of lowstand magnitudes. Knowledge of global sea-level fluctuations back through the Sangamon Interglacial (oxygen isotope stage 5e) and possible correlations with dated onshore deposits have facilitated a tentative correlation of major disconformities in the Beaufort Sea record with major 18 O enrichment maxima in the oxygen isotope curve back through stage 8. In this tentative scheme, close similarities between the two data sets occur both in magnitudes and in numbers of fluctuations intervening between major correlation points. Further testing of the Quaternary depositional model suggested here and of the resulting sea level curve awaits the collection and dating of core samples from the Beaufort wedge terranes.

Late Quaternary Depositional History of Alaskan Beaufort Shelf: ABSTRACT

Diverse nonmarine and shallow marine deposits blanketing the coastal plain and continental shelf of northern Alaska are known collectively as the Gubik Formation. In the Beaufort coastal region between Barrow and Prudhoe Bay and along the Chukchi coastline southwest of Barrow, five distinct marine subunits have been recognized within the Gubik, ranging in age from middle Pliocene to late Pleistocene. A sixth pre-Holocene transgressive marine subunit, about a meter thick and bearing abundant ice-striated dropstones that originated in the Canadian Arctic Islands, is present along much of the Alaskan Beaufort coast. The aggregate thickness of the Gubik Formation on the coastal plain is no more than a few tens of meters. Offshore beneath the Beaufort shelf, however, the Gubik Formation is locally thicker than 100 m and includes not only deposits that probably correlate with those mapped onshore but also subunits of intermediate and younger ages. These have been studied mainly through the interpretation of a network of high-resolution seismic reflection profiles that covers most of the Alaskan Beaufort shelf at 18 to 35-km intervals seaward of the 25-m isobath. In general, the Gubik Formation offshore appears to be a stack of wedge-shaped transgressive marine units that thicken toward the shelf break, beyond which they are disrupted by active slumps and landslides. This idealized geometry is altered in the area east of Canning River, where active faulting and folding have created persistent local highs and depocenters, and in the area between Smith and eastern Harrison Bays, where a complicated Quaternary drainage history has resulted in extensive local erosion of the marine wedges and in the deposition of relatively large deltaic sequences. Accumulation of the marine wedges must have occurred during periods when depositional rates were considerably higher than at present, perhaps during deglaciations of the Canadian Arctic Islands, when great volumes of sediment-bearing ice are likely to have been debouched into the Arctic Ocean. End_of_Article - Last_Page 662------------

Implications of Geologic Structure and Regional Sedimentation Patterns for Rifting Geometry of Arctic Basin: ABSTRACT

The smooth, passive continental margin north of Alaska is geometrically more complex than its simple physiography suggests. Multichannel seismic reflection profiles across the continental shelf reveal three sectors of contrasting structure and stratigraphy. The Barrow sector of central northern Alaska is characterized by a prominent arch in lower Paleozoic metasedimentary basement rocks overlain by a southward-thickening wedge of Mississippian to Lower Cretaceous (Neocomian) shelf sediments and a northward-thickening prism of Lower Cretaceous (Albian) to Tertiary clastic sediments. The Chukchi and Barter Island sectors, lying respectively west and east of the Barrow sector, comprise Mesozoic and Tertiary basins so deep that acoustic basement was not reached. We suggest th t this geometry (in which lower Paleozoic basement rocks extend much farther north in the central sector than they do in the east and west) is inherited from the configuration of the rift that opened the Arctic basin, probably beginning in Early Jurassic time. In this scheme, old, pre-rift highlands, originally continuous with the basement rocks of the central Barrow sector, were rotated northward away from Alaska along two sectors of the rift that lay well south of the present-day shelf edge. In these, the Chukchi and Barter Island sectors, the rift created room for the deep shelf basins observed there now. In the intervening Barrow sector, the rift opened along the present-day shelf edge, leaving a broad tongue of lower Paleozoic rocks, the site of upper Paleozoic and lower Mesozoic hi hlands, attached to Alaska. Presumably the sector boundaries were ridge-ridge transform faults during nascent rifting. The Chukchi-Barrow sector boundary is well defined by the trend of the Northwind Escarpment and the abrupt termination of the Barrow arch against the North Chukchi basin. The Barrow-Barter Island boundary is more obscure and is inferred from the provenance and distribution of Mesozoic sediments in northeastern Alaska and Yukon Territory. End_of_Article - Last_Page 714------------