ABSTRACT

Despite data on the stability of anhydrite at high earth surface temperatures, the observation of Recent gypsum and the evidence for formation and preservation of metastable gypsum suggest that this form may be the common if not universal original calcium sulfate mineral. Gypsum beds can be produced by two mechanisms: (1) Precipitation and sedimentation in a standing body of water subjected to evaporation. Such deposits are original sedimentary facies, contemporaneous with other sedimentary facies. (2). Growth of abundant crystals by displacement of unconsolidated sediment or weathered rocks, which results in beds of gypsum which later is replaced by nodular anhydrite. These nodular anhydrite beds represent a diagenetic facies and postdate the host material. The distinction between se imented and nodular is important in the interpretation of any given evaporite deposit.

These primary deposits act as a source of calcium sulfate for subsurface growth of replacement and void-filling anhydrite. The latter two secondary types of anhydrite can be distinguished in reflected light, since a dark color is imparted to replacement anhydrite by included material.

From observations in Recent sediments, older outcrops, and the subsurface, there appears to be a characteristic cycle in the diagenesis of gypsum-anhydrite minerals: surface or near-surface gypsum is replaced by anhydrite as a result of burial and is in turn replaced by gypsum if the anhydrite is thereafter brought close to the surface.