DETRITAL ZIRCON DATA FROM NORTHERN ALASKA: A SUMMARY OF INITIAL RESULTS

Detrital zircon results have been obtained from 30 rocks from the major sequences of northern Alaska in both autochthonous and allochthonous positions relative to the Late Jurassic and Early Cretaceous Brooks Range orogenic belt. Samples are from pre-Mississippian basement (n=4), the northerly-derived, compositionally mature Mississippian to Triassic Ellesmerian Sequence (n=13) and Jurassic and Cretaceous Beaufortian Sequence (n=2), and from the southerly-derived compositionally immature foreland basin strata of the Cretaceous and Tertiary Brookian Sequence (n=11). About 100 grains each were analyzed by LA-ICPMS in 25 samples and about 50 grains each by SHRIMP techniques in 5 samples.

Deformed lower Paleozoic basement rocks in the autochthon of the western North Slope contain abundant 420-475 Ma and 600-650 Ma grains and a complex variety of grain ages that range from 750 Ma to 2.6 Ga. Autochthonous basal Ellesmerian populations are complex but generally similar to underlying basement, suggesting derivation from local source areas. Most allochthonous Ellesmerian sandstones in the Brooks Range also display populations like those in pre-Mississippian basement, although the arkosic Nuka Formation is dominated by an anomalous population of 2.04 Ga zircons with minor 1.3 and 1.6 Ga zircons. In contrast, Brookian strata contain largely Phanerozoic zircons with significant peaks at 150-160, 210, 260, 300, and 360 Ma with much smaller populations at 500-550 Ma and 1.8-1.9 Ga. Although not all Brookian samples display all peaks, there appears to be no consistent pattern among samples from the orogenic belt, wedgetop, or foredeep deposits. Coeval Beaufortian sandstones representing forebulge and back-bulge deposits on the autochthon are similar to sandstones from underlying pre-Mississippian and Ellesmerian sandstones.

Although there are exceptions, these results suggest that many Ellesmerian and Beaufortian sandstones are shelfal sandstones derived from local basement sources. Brookian sandstones, however, do not reflect local northern Alaska sources and instead point to missing and now presumably eroded allochthons of Carboniferous, Permian, Triassic, and Jurassic rocks. The missing allochthons may have analogs in terranes south of the South Anyui, Kaltag, and Tintina fault systems.