Plant-bearing coastal plain deposits of Cretaceous age are exposed in cutbanks of the northward-flowing Kuk River system in Arctic Alaska. The Cretaceous strata have been folded into a broad synclinorium, with an east-west axis that crosses the Kaolak tributary near the southern end of the study area. Attitudes north of the axis show a uniform southerly dip of about 1 degree. Dip reversals are common south of the axis, indicating a crumpled south limb. Seven lithologic units, totalling about 4250 feet in thickness, have been recognized in the exposed section. Plant collections were taken from 40 localities, and their relative stratigraphic positions were determined on the basis of superposition. The succession of florules shows a temporal change from gymnosperm-dominated vegetation in older beds to angiosperm-dominated vegetation in younger units. Five sequential floras, representing major vegetation types, are recognized. All plant taxa have limited stratigraphic ranges in the local section. Correlations with intertonguing marine and continental beds in the Chandler-Colville region to the east indicate that Kuk River deposits and floras range in age from Middle Albian to Coniacian. The gymnosperm to angiosperm transition was quite rapid, and appears to have taken place in late Cenomanian-early Turonian time. Comparisons of Kuk River taxa with living analogues suggest that climates of the Cretaceous coastal plain of Arctic Alaska changed from warmer to cooler temperate during this 30 million year span of late Mesozoic time.

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