The origin of the Triassic clay assemblages of Europe with special reference to the Keuper Marl and Rhaetic of parts of England

Abstract
Hypotheses are reviewed on the origin of the magnesium-rich Triassic clays which characterize the Germanic facies of western Europe and north Africa. Relations between clay minerals, megafacies and stratigraphy are described from 28 localities in the Triassic Keuper Marl, Tea Green Marl and Rhaetic sediments of England. Two clay mineral assemblages are recognized: (1) a detrital assemblage of mica with minor chlorite which occurs throughout all the sediments investigated, and (2) a neoformed assemblage of magnesium-rich clay minerals with a limited occurrence related to certain megafacies cycles which resulted from the transgression and regression of the Alpine facies into the Germanic facies; this assemblage includes sepiolite, palygorskite, chlorite, smectite, corrensite and irregular mixed-layer smectite/mica and smectite/chlorite minerals. The clay mineral neoformations resulted from reactions between the water masses in which the Germanic and Alpine facies were deposited. Controlling the distribution and types of minerals neoformed were the general and local variations in the chemistries of the Alpine and Germanic water masses, as well as competition for available magnesium from other mineral-forming reactions.