CLUES TO THE PRE-MISSISSIPPIAN ARCHITECTURAL FRAMEWORK OF NORTHERN ALASKA FROM DETRITAL ZIRCON U-PB DATING OF THE UPPER DEVONIAN AND LOWER MISSISSIPPIAN ENDICOTT GROUP

The Upper Devonian and Lower Mississippian Endicott Group of northern Alaska is a widespread compositionally mature coarse-grained clastic sequence that covers much of the 1000-km-long Arctic Alaska terrane. It is a northward transgressive post-rift succession shed toward the south and southwest. It forms a thin carapace on older deformed rocks in up-dip areas in the north, and a 4-km-thick regressive-transgressive sequence in the south. Large magnitude north-directed thrusting during the Late Jurassic and Early Cretaceous Brookian orogeny produced over 90 km of shortening by emplacement of the basinal succession onto the thin up-dip succession.

We use detrital zircon U-Pb grain ages from 11 samples to constrain along- and cross-strike variations in the provenance of the Endicott Group. Analyses of most samples display peaks at ~355-370 Ma and ~400-480 Ma, which match the ages of zircons from Devonian magmatic rocks and early Devonian orogenic deposits in the central and eastern North Slope. In addition, western samples display peaks at ~550-700 Ma and ~850-950 Ma. All samples have a variety of ages between 1 and 2 Ga but significant peaks at 1000-1200 Ma and 1800-1900 Ma are observed mainly in the north and east. A potential match for the 550-700 Ma grain ages is found in the Silurian Iviagik Formation in the Lisburne Peninsula in western Alaska, whereas a source for the 850-950 Ma grains is unknown, although magmatic rocks of this age are found in Baltica. The 1000-1200 and 1800-1900 Ma peaks are common in the Proterozoic and Cambrian rocks of North American origin exposed in the northeastern Brooks Range.

The zircon dating indicates that the predominant source lithologies for the Endicott Group vary E-W across the North Slope. These results suggest that North American basement underlies the eastern North Slope, whereas Timanian and Baltic basement rocks may be present in the Lisburne Peninsula and Chukchi platform under the Chukchi Sea to the west. The same general E-W distribution of ages is found in both the basinal and up-dip deposits suggesting that either (1) separate drainage systems extended long distances from north to south from localized common source areas or (2) an important N-S tectonic boundary exists under northern Alaska that separates regionally extensive eastern and western source areas.