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The first discovery in North America of a succession of graptolite faunas across the Ordovician–Silurian boundary has been made on Esquibel Island, in south-eastern Alaska, where five graptolite zones are represented in an 18 m interval of shale in the Descon Formation. Despite the thinness of the graptolite zones, no stratigraphical breaks can be inferred from more complete graptolite successions known elsewhere. The Alaskan faunas can be closely correlated with those of the standard British zones—the Dicellograptus anceps Zone (uppermost Ordovician), the Glyptograptus persculptus Zone (lowermost Silurian) and the immediately overlying Akidograptus acuminatus, Orthograptus vesiculosus and Monograptus cyphus Zones of the Lower Silurian. They thus give evidence of the cosmopolitan distribution of graptolite faunas during latest Ordovician and earliest Silurian time.

Silurian graptolites and Ordovician and Silurian chitinozoans have been found in well cores from the argillitic basement rocks of the Prudhoe Bay oil field and Naval Petroleum Reserve No. 4 on the North Slope of Alaska. Monograptus spiralis (Geinitz), a late Llandoverian graptolite, and Middle Ordovician to Silurian chitinozoans belonging to the genus Conochitina Eisenack were discovered at a depth of 12,937-12,938 ft (3,943.2-3,943.5 m) in the ARCO-Humble Sag River State 1 well at Prudhoe Bay. Silurian chitinozoans of the genus Bursachitina Taugourdeau were found at a depth of 2,869-2,879 ft (874.5-877.5 m) in South Barrow test well 3 (NPR 4). South Barrow test well 1 yielded Silurian chitinozoans of the genus Sphaerochitina Eisenack at a depth of 3,341-3,346 ft (1,018.3 1,019.9 m) and the species Conochitina micracantha Eisenack, Cyathochitina campanulaeformis (Eisenack), and Lagenochitina baltica Eisenack, indicative of a Caradocian through Ashgillian age, at a depth of 3,535-3,537 ft (1,077.5-1,078.1 m). These findings prove that the basement rocks of the Barrow arch are of approximately the same age, at least over the 200 mi (325 km) between Prudhoe Bay and Point Barrow.

Construction of a map of basement rocks of Alaska has revealed a complexly deformed, arcuate Paleozoic continental margin that is a northwestern extension of the western margin of North America. Three key areas in Alaska (Nation River, Livengood, and Terra Cotta Mountains) have similar sections of Ordovician to Devonian and locally younger Paleozoic siliceous shale and chert, interfingering with limestone and containing various turbidity-current and slope deposits. Microplates moving north from the Pacific basin have collided with this Paleozoic continental margin and deformed and collapsed the original shale-out facies. A similar south-facing collapsed continental margin of middle and late Paleozoic age can be traced across the western Brooks Range of northern Alaska. We believe that this continental margin in the Brooks Range, although it is mainly younger, may be an extension of the same margin in central Alaska. This continental margin has been offset to the Brooks Range along the Porcupine megashear.

Sections of graptolitic shale within the Cordilleran foldbelt are very condensed. An average accumulation rate of 4 ∓ 1 m/m.y. calculated for these shales falls within the range of 2 to 5 m/m.y. for modern deep-sea clay and siliceous ooze. The Deep Sea Drilling Project has shown that once an age is determined for a stratigraphic horizon within a pelagic section, it is possible to estimate the ages of other horizons by assuming constant rates of sediment accumulation. This method, used on graptolitic shale, suggests that the time span represented by a graptolite zone is proportional to its thickness. The thickest zone analyzed is that of Nemagraptus gracilis , which is calculated to have spanned about 8 m.y., whereas the thinnest zone, Glyptograptus persculptus , took less than 1 m.y. to accumulate. The readily recognized zones of Nemagraptus gracilis and Climacograptus bicomis , which together consistently form thick biostratigraphic intervals, are the zones most commonly found, whereas the G. persculptus zone is very rare. A new subdivision of the Ordovician and Early Silurian time scale has been made on the basis of the calculated time duration of graptolite zones.