Compositional Zoning of Clay Fractions in the Mesozoic Shales from Melville Island, Canadian Arctic.

Abstract

The clay mineral fractions from shales, in the Panarctic North Sabine H-49 well on Melville Island in the Canadian Arctic Islands, which ranged from Late Cretaceous to Early Triassic were studied using a S. E. M. equipped with an energy-dispersed electron microanalyzer. The clay fractions in samples from 2400 feet depth have relatively high Fe_2O_3 contents ranging from 11 to 15 per cent. The 3〜8 μm step scanning quantitative analysis of clay minerals revealed that each clay fraction has not only a different chemistry but also that mixtures of clay minerals such as montmorillonite, mica mineral, kaolinite and aluminum hybroxide in various ratios are present. Moreover, clay fractions exhibit four types of compositional zoning: Normal, Inverse and Transitional zoning and an unzoned type. The normal zoning is characterized by increasing of Al_2O_3 from core to rim. The inverse zoning shows a compositional trend opposite to that of normal zoning. The transitional zoning exhibits irregular Al_2O_3-rich patches in the low Al_2O_3 matrix. Clay fractions with homogeneous composition are very rare. The present study revealed that diagenesis of clay minerals proceeds microdimensionally zone by zone depending on the reaction process between pore fluid and clay fractions.