A Reconnaissance Survey of the Relationship between Clay Mineralogy and Geological Environment in the Negev (Southern Israel)

Abstract

ABSTRACT The clay minerals of the Negev sediments from the early Cambrian to the late Tertiary are largely derived from one source area and were deposited under climatic conditions supposed to have been relatively constant; they are therefore particularly suited for a study of their relationship to geologic age and environment of deposition. An approximate correlation exists between clay mineralogy and lithology, but this is frequently obscured because clay mineral assemblages, not yet adapted to their environment are frequently fossilized. No relationship to depth of burial or to the tectonic position has been observed. There is a gradual transition with time from one clay mineral assemblage to the next which justifies the concept of evolution of clay mineral assemblages. A long time interval is needed to stabilize a clay mineral assemblage. Transitional periods between one equilibrium and the next are characterized by a frequency of mixed layer structures. Certain definite trends emerge: the vegetation-less early Palaeozoic desert is distinguished by the abundance of illite; a greater density of vegetation on the Mesozoic land surface leads to the formation of kandite at the expense of illite; both kandite and illite occur under terrestrial conditions of deposition, while increasing marinity leads to the dominance of smectites. Slow diagenetic adaptation is considered the most likely mechanism of this change. Palygorskite, which is relatively widespread, occurs under brackish-lagoonal conditions. The virtual absence of chlorite is explained by the scarcity of basic source ro ks.