AN ALASKAN HELICOPRION FROM ATIGUN GORGE, EAST-CENTRAL BROOKS RANGE, ALASKA

The 1986 discovery of a Helicoprion whorl has recently come to the attention of researchers of this Permian fish. Here we describe the occurrence of the whorl (interpreted to be a flat coil of teeth), and its stratigraphic and structural setting.

Atigun Gorge is located at the northern flank of the east-central Brooks Range, Alaska, near Mile 271 on the Dalton Highway, which parallels the Trans-Alaska pipeline between Livengood and Prudhoe Bay, Alaska.

In the vicinity of Mile 271, east-west trending Devonian-Cretaceous rocks are thrust-faulted and folded against adjacent younger Cretaceous rocks to the north. The structural juxtaposition forms the boundary between the Brooks Range and North Slope of Alaska. The fault and fold geometry of the various stratigraphic units vary depending on their thickness and lithology, but the Pennsylvanian-Jurassic sequence at Atigun Gorge is predominantly revealed as completely inverted in north-facing exposures.

The Helicoprion whorl was discovered by the author in 1986 in the Permian Siksikpuk Formation, which disconformably overlies thick massive carbonate rocks of the Mississippian-L.Pennsylvanian Lisburne Group (The whorl was formally identified by J.T. Dutro Jr. of the USGS in late 1986-early 1987). The Siksikpuk Formation at Atigun Gorge is found in talus and scree slopes beneath (but stratigraphically overlying) the castle-rock weathering limestones of the Lisburne Group. The whorl was found in float in the basal 2-3 m of the inverted Siksikpuk Formation. The dinner-plate sized yet incomplete whorl was preserved in a resistant yellowish-brown weathering siltstone which apparently was interbedded with the fissile dark gray more friable shale that comprises most of the Siksikpuk Formation in this area.

Although the Upper Paleozoic and Mesozoic rocks of Alaska's Brooks Range and North Slope have yielded a variety and distribution of fossils, this appears to be the only known Helicoprion find in northern Alaska.