Evolution of vegetation and coal-forming environments in the Late Cretaceous of the North Slope of Alaska
Robert A. Spicer, Judith Totman Parrish, Paul R. Grant, 1992. "Evolution of vegetation and coal-forming environments in the Late Cretaceous of the North Slope of Alaska", Controls on the Distribution and Quality of Cretaceous Coals, Peter J. McCabe, Judith Totman Parrish
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During the Late Cretaceous, the Colville basin (North Slope, Alaska) was situated between 75° and 85°N paleolatitude and accumulated sediments from the Corwin and Umiat paleodeltas. These strata contain an estimated 2.75 trillion short tons of predominantly low-sulfur, low-ash coal. Plant fossils were collected in facies and taphonomic context, and show that the regional vegetation was dominated by conifers with riparian and understorey angiosperms, ferns, sphenophytes, and cycadophytes. All plants were deciduous, entered dormancy, or died back to storage organs during the winter. Organic accumulation was mostly the result of high summer productivity coupled with negligible postmortem losses of abscissed material during the 3- to 4-mo-long dark, cold winters. The climate was cool and temperate and water supply abundant and apparently uniform.
Palynofacies analysis provides an approach to monitoring mire development, with, for example, changes in palynomorph assemblages indicating floral turnover. Discrete associations of one or more nitric acid macerals from specific lithotypes are used to define palynofacies. Although some coals are allochthonous and sapropelic, others are autochthonous and cuticle-rich. Many yield abundant woody debris and resin.
Coal is abundant in the early Late Cretaceous strata. Measured coal beds range from 8 cm to 5 m thick, with a mean thickness of 96 cm. Most of the thick (>2 m), bituminous coals are wood-rich, and the megafossils suggest that many of the climax mire communities were dominated by the extinct conifer, Podozamites. The vegetation was diverse; growth rings in the woods indicate that growing conditions were ideal throughout the summer and that growth ceased abruptly each year, probably as a result of the rapid transition from the summer to winter light regime.
By contrast, the late Late Cretaceous environment was much less favorable to peat formation. The vegetation was still dominated by conifers, but the trees were smaller and diversity was drastically lower. Podozamites was either absent or greatly reduced in abundance. Growth rings in the woods indicate cooler summers and frequent interruptions of growth during the growing season, possibly by frost. Measured coal beds range from 5 to 61 cm in thickness, with a mean thickness of about 23 cm. The coal beds became thinner upsection within the upper Upper Cretaceous, and few are bituminous; most are lower rank than those of the lower Upper Cretaceous and contain a much higher proportion of clastics.
Figures & Tables
- Colville Group
- depositional environment
- Nanushuk Group
- North Slope
- organic residues
- sedimentary petrology
- sedimentary rocks
- United States
- Upper Cretaceous
- Colville Basin