Inverse Graded Bedding in Primary Gypsum of Chemical Deposition

Abstract

ABSTRACT The Gypsum formation of the Sulphur series of Sicily shows thick gypsum beds, mostly made up of basal zones of primary gypsum with rhythmic structure and of thicker upper zones of secondary gypsum. The latter is characterized by swelling structures due to transformation from anhydrite. The rhythmic primary gypsum is bedded in two to three mm thick laminae, which present a structure of inverse graded crystalline mosaic with the finer grain sizes at the base and the bigger ones at the top. Some slightly reworked types of primary gypsum show loss of the grading and uniform grain size. Some rhythmites show a grading from finer to bigger sizes, ending in the deposition of anhydrite crystals which have been transformed to gypsum during the deposition of the followng lamina, thus deforming it. The inverse size grading is referred to a seasonal concentration increase by evaporation, possibly together with an average temperature increase. The return to the initial sizes at the base of each lamina is attributed to a rainy, perhaps cold season, which interrupts the evaporation and consequently the chemical deposition.