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Abstract

Early Mesozoic rocks in Arctic Alaska reflect a continuation of deposition in the late Paleozoic Cordilleran geosyncline. Starting in Early Jurassic time, the broad Cordilleran geosyncline was warped into three small geosynclines: the Colville, Koyukuk, and Kandik, separated by the east-trending Brooks Range geanticline and the narrow southwest-trending Ruby geanticline. These structural highs and lows were areas of erosion and deposition throughout the remainder of the Mesozoic.

Orogeny was widespread in the Cretaceous. One major orogeny took place during the Aptian; as a result, all post-Aptian strata lie with angular discordance on earliest Cretaceous to Devonian beds. Orogeny continued in Cenomanian time, and, by Late Cretaceous, folding was largely completed in the Koyukuk and Kandik geosynclines. The Brooks Range was uplifted at the end of Cretaceous time, and the rocks of the Colville geosyncline were moderately to intensely deformed. Major thrust plates developed, and the strata were thrust northward, so that rocks of similar age but widely different facies were commonly juxtaposed.

Early Triassic beds are primarily confined to northeastern Alaska, where 500-1,000 ft (150-300 m) of strata show a distinct northward coarsening of clastic components, indicating a source in that direction. Middle and Late Triassic time are represented by widespread deposits of black phosphatic limestone, calcareous shale, and chert totaling several hundred feet in thickness. These shelf deposits are similar to, and largely concordant with, the underlying Paleozoic strata.

During the Jurassic the Colville and Kandik geosynclines received 2,000–10,000 ft (610–3,050 m) of monotonous dark pyritic shale, siltstone, and graywacke. At the same time, mafic igneous flows and tuffs were ac-cumulating in the Koyukuk area. These rocks are largely discordant on older strata and locally discordant between successive Jurassic units.

The depositional pattern established in the Jurassic continued into the Early Cretaceous, when 5,000–15,000 ft (1,525–4,570 m) of mainly flysch-type sediments accumulated in the geosynclines. By middle Albian time, conditions favoring deposition of subgraywacke prevailed. Shifting shorelines caused better sorting in the 3,000–10,000 ft (915–3,050 m) of interfingering marine and nonmarine clastic rocks deposited during the remainder of Cretaceous time.

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