An integrated system for macro-scale anhydrite classification

Abstract

Most anhydrite classification systems to date have focused primarily on the naming of anhydrite bodies, masses, or crystals themselves rather than focusing on both the mineral morphology and links to the sedimentary succession in which it occurs. Much of the reasoning for the lack of development of an integrated classification system for anhydrite may come from the inherent instability of the mineral, and therefore the difficulty making a link between any particular morphology and a specific formative process or environment. This sets anhydrite classification apart from other sedimentary classification schemes, as most of them (e.g., Dunham, McBride, etc.) naturally break into groups that can be related to sorting, textural maturity, mode of deposition, or other genetic process. A classification system for anhydrite has been developed that allows for information about the gross anhydrite volume and morphology, as well as host sediment type to be transmitted using a single type-name. This new integrated anhydrite classification scheme was developed using input from both previously developed schemes and field studies with a view to link anhydrite morphology and volume to precursor depositional process. These relationships have been shown to hold true in certain circumstances, with both gross anhydrite volume and morphology many times being characteristics that are particular to former depositional setting (bedded, laminated salinic anhydrite versus nodular sabkha anhydrite, as an example). By adding a host rock descriptor to the scheme, quite a bit of empirical information about the anhydrite-bearing rock is made available in a single name, which can then be more easily linked to genetic process. Such a scheme may have wide application in industry, where careful description and classification of anhydrite is a key component to understanding the distribution of reservoir rock types in the subsurface.