Clay fractions and colours of some Devonian red beds in the Catskill Mountains, U.S.A

The fine fractions of samples from a cyclothemic section in the Catskill facies, Appalachian region, have been analysed by X-ray diffraction and determinations of iron content made. All the samples contain both illite and chlorite, and some contain kaolinite. Two of the cyclothems are richer throughout in kaolinite. This is attributed to a change in provenance and demonstrates the relative unimportance of diagenetic alteration of the clays. The redness is due to the presence of fine-grained hematite, much of it in hexagonal crystalline form. The clay content does not vary with the red or non-red state of the rocks, and this colour differentiation did not therefore involve alteration of the clays. A hypothesis to explain the red-non-red differentiation within the cyclothems is put forward.