Lithology of the Lower Cretaceous Pebble Shale Unit, Arctic National Wildlife Refuge, Northeastern Alaska: Shale or Slate?
Dolores van der Kolk
University of Alaska Fairbanks, Department of Geology & Geophysical Surveys Fairbanks, Alaska
A 40-m thick section of dark-gray to black heterolithic mudstone crops out along the Canning River; the northwestern boundary of the Arctic National Wildlife Refuge in northeastern Alaska. The east-west trending, nearly vertical section is informally known as the Lower Cretaceous (Barremian to Aptian) pebble shale unit (PSU). The Kemik Sandstone underlies the PSU while the High Radioactive Zone (HRZ) associated with the Hue Shale overlies the PSU along the Canning River.
Samples were collected for vitrinite reflectance (Ro) and analyzed with an electron microprobe, XRF, and XRD. At the Canning River locality, major minerals in the mudstone include albite, kaolinite, muscovite, and quartz while minor minerals include chlorite, pyrite, gypsum, jarosite as well as trace pyrophyllite and rutile. Analyses of concretionary layers suggest that the section is stratigraphically zoned. Sideroplesite (Fe-Mg carbonate) + calcite is restricted to the lower 20.8 m; ankerite + calcite is present at higher stratigraphic intervals. The presence of pyrophyllite within PSU mudstone indicates heating to mid-greenschist conditions (T>350ºC). Microprobe compositions of co-existing Mg-calcite and ankerite in concretions indicate average temperatures of 375 +/-15ºC using the calcite-ankerite geothermometer. Although the name and outcrop appearance suggest that the PSU is shale, independent results suggest that the shale along the Canning River may be a slate. Maximum Ro values (n = 4) range from 1.5 to 2.06% suggesting temperatures of ~169 to 198ºC based on standard burial models. However, Ro values near 2% are compatible with temperatures >300ºC if burial and exhumation are relatively rapid.
AAPG Search and Discovery Article #90070 © 2007 AAPG Foundation Grants in Aid