PHANEROZOIC STRATIGRAPHY OF NORTHWIND RIDGE, MAGNETIC-ANOMALIES IN THE CANADA BASIN, AND THE GEOMETRY AND TIMING OF RIFTING IN THE AMERASIABASIN, ARCTIC-OCEAN

Cores from Northwind Ridge, a high-standing continental fragment in th e Chukchi borderland of the oceanic Amerasia basin, Arctic Ocean, cont ain representatives of every Phanerozoic system except the Silurian an d Devonian systems. Cambrian and Ordovician shallow-water marine carbo nates in Northwind Ridge are similar to basement rocks beneath the Sve rdrup 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 wes tern Arctic Alaska basin (Hanna trough). These resemblances indicate t hat Triassic and older strata in southern Northwind Ridge were attache d to both Arctic Canada and Arctic Alaska prior to the rifting that cr eated the Amerasia basin. Late Jurassic marine lutite in Northwind Rid ge was structurally isolated from coeval strata in the Sverdrup and Ar ctic Alaska basins by rift shoulders and grabens, and is interpreted t o be a riftogenic deposit. This lutite may be the oldest deposit in th e Canada basin. A cap of late Cenomanian or Turonian rhyodacite air-fa ll ash that lacks terrigenous material shows that Northwind Ridge was structurally isolated from the adjacent continental margins by earlies t Late Cretaceous time. Closing Amerasia basin by conjoining seafloor magnetic anomalies beneath the Canada basin or by uniting the pre-Jura ssic strata of Northwind Ridge with kindred sections in the Sverdrup b asin and Hanna trough yield similar tectonic reconstructions. Together with the orientation and age of rift-margin structures, these data su ggest that (1) prior to opening of the Amerasia basin, both northern A laska and the continental ridges of the Chukchi borderland were part o f North America, (2) the extension that created the Amerasia basin for med rift-margin grabens beginning the Early Jurassic time and new ocea nic 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 bas in accounts in a general way for the major crustal elements of the Ame rasia basin, including the highstanding ridges of the Chukchi borderla nd, and supports S.W. Carey's hypothesis that the Amerasia basin is th e product of anticlockwise rotational rifting of Arctic Alaska from No rth America.