Silurian fossils from steeply dipping slate and graywacke underlying the Prudhoe Bay oil field, and Ordovician and Silurian fossils from similar rocks in the Barrow test wells indicate that rocks of about the same lithology and age as the Neruokpuk Formation in the Brooks Range-Barn Mountains area continue west in the subsurface and form the basement of the Barrow arch. This belt of highly deformed, weakly metamorphosed, predominantly siliceous strata, including volcanic rocks, is part of an early Paleozoic geosyncline that was deformed, intruded by Silurian and Devonian granite, and deeply eroded by the end of the Devonian. Similar rocks affected by the same orogeny form the Innuitian fold belt in the Canadian Arctic Islands, but the connection between the two fold belts is obscured by rocks of mainly younger age in the Mackenzie River delta region and by water along the west side of Banks Island. South of the Mackenzie River delta, the Richardson trough with Its graptolitic shale and chert section separates carbonate strata of the Yukon shelf of northern Yukon Territory and east-central Alaska from the Mockenzie carbonate platform lapping onto the Ginadian shield. Now that the Neruokpuk Formation in the Barn Mountains is known to include groptolitlc shales and cherts of the some age as, and of partly similar lithology to, those in the nearby Richardson trough, it seems likely that deep-water pelagic sedimentation continued from the Neruokpuk geosyncline into the Richardson trough.
The argillaceous rocks of the Richardson trough, unlike those of the Neruokpuk geosyncline or Cordilleran geosyncline, grade downward into argillaceous limestone that lies on a Cambrian platform section. In the Ogilvie Mountains at the south end of the trough, shales interfinger with limestones, indicating that the trough was constricted into narrow channels and was separated partly from the Cordilleran geosyncline farther south. These relations indicate that the Richardson trough was not a basin of oceanic type or proportions but may be an aulacogen that developed across the Yukon shelf from an Arctic sea-floor-spreading center.