Abstract: The refractory shales of the Purgatoire and Dakota formations in central Colorado were studied to determine the genesis of the kaolinite. The compositions of the shales were similar, with variation only in relative amounts and grain size of the component minerals. The shales consist primarily of kaolinite, subangular to subrounded quartz, and hydromuscovite, with minor amounts of feldspar, montmorillonite, mixed-layer clays, and occasional traces of gypsum and zeolites.
An inverse relation between the kaolinite and hydromuscovite contents suggests that hydromuscovite was an intermediate mineral in the formation of the kaolonite. Derivation of vermicular kaolinite from feldspar and mica was evident from examination of thin sections. The thinly banded interfingered lenses of montmorillonite suggest a detrital origin rather than alteration in situ of a volcanic ash to bentonite.
Genesis of the kaolinite probably is similar to that proposed recently by Kesler for the Cretaceous clays of Georgia and South Carolina whereby feldspathic sands deposited in a near-shore environment were subsequently weathered to produce residual kaolinite. Winnowing of some of these clays and redeposition in downstream lagoons formed the fine-grained, carbonaceous, “flint” clays, whereas the plastic clays were formed by introduction of detrital montmorillonite.