The evidence is reviewed for the various hypotheses put forward to explain the origin of the Chalk montmorillonite.
Recent X-ray investigation of the mineralogy of the acid-insoluble clay fractions (buffered 2 N acetic acid at pH 3; <2 µ e.s.d.) of the Lower Chalk of England sheds light on the origin of the montmorillonite. The relations between the qualitative and semi-quantitative mineralogy of the clay fractions and the facies and stratigraphy in this formation have been studied in detail. Montmorillonite, illite, kaolinite, chlorite, vermiculite, pyrophyllite, mixed-layer minerals, quartz, low-temperature cristobalite and apatite have been identified; their semi-quantitative distribution reveals that two main antipathetic assemblages are present, between which all gradations occur. The first is characterized by montmorillonite, illite, quartz and by montmorillonite/illite (M/I) values of 0·7 and above; and the second by illite, kaolinite, chlorite, vermiculite and by M/I values of below 0·2. The distribution of these assemblages or of any particular mineral does not show obvious relations to the facies or stratigraphy.
There is strong evidence that the second of these assemblages is of detrital origin, introduced into the Lower Chalk seas by currents flowing mainly from areas to the east and south-east of England. There is no evidence to suggest that the montmorillonite and the illite of the first assemblage are of detrital or volcanic origin, and their distribution in the Lower Chalk is best explained by their neoformation in the sediment on the sea floor by precipitation from the porewaters. By extrapolation it is thought that most of the Chalk montmorillonite of clay-grade is of neoformational origin. Some from the Campanian and younger Chalks may be detrital. Locally in N.W. Germany and possibly Poland minor amounts may have been derived from the decomposition of volcanic glass in the Chalk seas.