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 3 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 rest of the Mesozoic.

Orogeny was widespread in the Cretaceous. One major orogeny took place during the Aptian, and 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 complete 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 strongly 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 of strata show a distinct northward coarsening of clastic components, indicating a source in that direction. Middle and Late Triassic times are represented by widespread deposits of black phosphatic limestone, calcareous shale, and chert several hundred feet thick. 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 of monotonous dark pyritic shale, siltstone, and graywacke. At the same time, mafic igneous flows and tuffs were accumulating 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 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 of interfingering marine and nonmarine clastic rocks deposited during the rest of Cretaceous time.

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