David H. McNeil

Phanerozoic stratigraphy of Northwind Ridge, magnetic anomalies in the Canada basin, and the geometry and timing of rifting in the Amerasia basin, Arctic Ocean

Cores from Northwind Ridge, a high-standing continental fragment in the Chukchi borderland of the oceanic Amerasia basin, Arctic Ocean, contain representatives of every Phanerozoic system except the Silurian and Devonian systems. Cambrian and Ordovician shallow-water marine carbonates in Northwind Ridge are similar to basement rocks beneath the Sverdrup basin of the Canadian Arctic Archipelago. Upper Mississippian(?) to Permian shelf carbonate and spicularite and Triassic turbidite and shelf lutite resemble coeval strata in the Sverdrup basin and the western Arctic Alaska basin (Hanna trough). These resemblances indicate that Triassic and older strata in southern Northwind Ridge were attached to both Arctic Canada and Arctic Alaska prior to the rifting that created the Amerasia basin. Late Jurassic marine lutite in Northwind Ridge was structurally isolated from coeval strata in the Sverdrup and Arctic Alaska basins by rift shoulders and grabens, and is interpreted to be a riftogenic deposit. This lutite may be the oldest deposit in the Canada basin. A cap of late Cenomanian or Turonian rhyodacite air-fall ash that lacks terrigenous material shows that Northwind Ridge was structurally isolated from the adjacent continental margins by earliest Late Cretaceous time. Closing Amerasia basin by conjoining sea-floor magnetic anomalies beneath the Canada basin or by uniting the pre-Jurassic strata of Northwind Ridge with kindred sections in the Sverdrup basin and Hanna trough yield similar tectonic reconstructions. Together with the orientation and age of rift-margin structures, these data suggest that (1) prior to opening of the Amerasia basin, both northern Alaska and the continental ridges of the Chukchi borderland were part of North America, (2) the extension that created the Amerasia basin formed rift-margin grabens beginning in Early Jurassic time and new oceanic crust probably beginning in Late Jurassic or early Neocomian time. Reconstruction of the Amerasia basin on the basis of the stratigraphy of Northwind Ridge and sea-floor magnetic anomalies in the Canada basin accounts in a general way for the major crustal elements of the Amerasia basin, including the highstanding ridges of the Chukchi borderland, and supports S. W. Carey9s hypothesis that the Amerasia basin is the product of anticlockwise rotational rifting of Arctic Alaska from North America.

Emplacement and reworking of Cretaceous, diamond-bearing, crater facies kimberlite of central Saskatchewan, Canada

In central Saskatchewan, Canada, kimberlites were emplaced into Cretaceous marine and nonmarine clastic sediments. Core recovered from one drill hole that intersects kimberlite (Smeaton FAC/UK core 169/8) was selected for an integrated study involving sedimentology, volcanology, mineralogy, geochemistry, palynology, micropaleontology, organic petrology, and radiometric age determination. Only crater facies kimberlite has been observed; there is no indication of the locations of feeder dikes. Four varieties of kimberlite occur, all originating from subaerial volcanism: (1) fluvial-reworked kimberlite; (2) diamondiferous kimberlite lapillistone air-fall deposits; (3) kimberlite olivine crystal-tuff air-fall deposits; and (4) diamondiferous marine wave-reworked kimberlite. Within the multiple primary eruptive phases of the kimberlite air-fall deposits, the volcanic style changed upward with time, from violent Strombolian to more explosive volcanism. The bulk of the volcanism formed conformable, air-fall deposits on terrestrial sediments of the Cantuar Formation, resulting in the development of positive-relief tephra cones. Subsequent marine transgression associated with the Westgate Formation partially beveled the top of the cone. The kimberlite air-fall deposits contain microdiamonds, 5 to 25 μm in diameter. The maximum temperature and vitrinite reflectance values of coaly matter in the kimberlites indicate that these deposits, although originally derived from magma at high temperatures, did not thermally affect entrained surficially derived clasts or the country rock during emplacement. The chemical content of intrakimberlite shale clasts is markedly different from the marine and nonmarine shales and indicates significant synemplacement and postemplacement fluid movement through the volcanic pile. At least two episodes of kimberlite volcanism occurred in the middle and late Albian (paleontologically assigned). A U-Pb perovskite radiometric age of 101.1 ± 2.2 Ma from a kimberlite lapillistone from the younger episode of volcanism is internally consistent with biostratigraphic studies that constrain the kimberlite volcanism as post–middle Albian and pre–late Albian to late Albian.

Biostratigraphic and paleoclimatic significance of a new Pliocene foraminiferal fauna from the central Arctic Ocean

Abstract A Pliocene benthic foraminiferal fauna containing a previously unknown species association was found in the basal section of a piston core collected from the crest of Northwind Ridge (NWR) in the central Arctic Ocean. The fauna is dominated by Epistominella exigua, Cassidulina reniforme, Eponides tumidulus, Cibicides scaldisiensis, Lagena spp., Cassidulina teretis, Eponides weddellensis, Bolivina arctica , and Patellina corrugata . The presence of Cibicides scaldisiensis in the assemblage and the occurrence of Cibicides grossus higher in the core are indicative of an early Pliocene age. The morphologically distinctive species Cibicidoides sp. 795 of McNeil (in press) which occurs in the NWR core sample was previously known only from Oligocene through Miocene deposits in the Beaufort-Mackenzie Basin of Arctic Canada. Ehrenbergina sp. A and Cibicidoides aff. C . sp. 795, also present in the core, are new and endemic to the Arctic late Miocene and early Pliocene. These species, and possibly others, are survivors of the late Miocene (Messinian) sea-level crisis, which caused a significant faunal turnover in the Arctic Ocean. The predominantly calcareous assemblage indicates deposition above the calcium carbonate compensation depth in an upper bathyal environment. Paleogeographic affinities for the bulk of the assemblage indicate probable connections between the Arctic and the North Atlantic Oceans, but the endemic species identify environmental differences or partial isolation of the western Arctic Ocean. The species association suggests a cold but milder paleoclimate than that which existed during Pleistocene glacial intervals.

High-latitude application of 87Sr/86Sr: Correlation of Nuwok beds on North Slope, Alaska, to standard Oligocene chronostratigraphy

Strontium isotope ratios ({sup 87}Sr/{sup 86}Sr) calculated from three samples of benthic forminifers and one mallusc indicate an age range from 23.8 to 27.0 Ma for the Nuwok Member of the Sagavanirktok Formation, eastern North Slope, Alaska. Previous correlations derived paleontologically (foraminifers, ostracodes, and molluscs) for the Nuwok beds have differed greatly (Oligocene to Pliocene), but strontium isotope ratios substantiate the late Oligocene age determined from benthic foraminifers. The results constitute an important test case, illustrating the effectiveness of the strontium isotope method in correlating high-latitude carbonate samples to the standard Oligocene chronostratigraphy. In addition, a sample from the Kugmallit sequence in the Beaufort Sea Edlok N-56 well provided a strontium isotope age estimate of 30.7 Ma. The Edlok sample was paleontologically correlated to just below the early/late Oligocene boundary (30.0 Ma).

Foraminiferal Biostratigraphy and Seismic Sequences: Examples from the Cenozoic of the Beaufort-Mackenzie Basin, Arctic Canada

Cenozoic strata of the Beaufort-Mackenzie Basin were deposited in a series of 9 sequences on the continental margin of Arctic Canada. Seismostratigraphic and biostratigraphic (foraminiferal) schemes for analyzing these strata have evolved concurrently. Integration of these two methods is illustrated from selected Eocene to Miocene sections referenced to a transgressive-regressive sequence model. Marginal marine foraminiferal assemblages from seismic topset facies are low in diversity, dominantly agglutinated (Portatrochammina, Jadammina, Labros pira, Textularia), and markedly different in successive sequences. Mid-shelf deltaic facies are also dominated by agglutinated genera (Bathysiphon, Haplophragmoides, Insculptarenulla, Recurvoides, Reticulophragmium). In contrast, foraminiferal assemblages from non-deltaic, outer shelf to upper slope facies (seismic topsets to foresets) are mostly calcareous benthic species. In continental-rise facies (seismic bottomset facies), agglutinated foraminifers are also abundant (Ammodiscus, Ammolagena, Bathysiphon, Cystammina, Glomospira, Haplophragmoides, Insculptarenulla, Reticulophragmium, Recurvoides, Reophax), but typically are long ranging. Lower slope to continental rise turbiditic deposits are mostly barren, except for reworked microfaunas. Faunal changes at sequence boundaries indicate the scope of tectono-oceanographic reorganization which generated each depositional episode. For example, major faunal changes encountered at sequence boundaries near the Eocene/Oligocene and Miocene/Pliocene boundaries reflect global-scale events.