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Welcome to the Education Corner: Gypsum - Anhydrite Classification
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Future Links: Deep Hypersaline Seas • Gypsum-Anhydrite Transformations: Blades • Rosettes • Subaqueous Selenite • Clastic Gypsum
Much disagreement persists about interpreted depositional environments for ancient evaporite deposits for several reasons. Firstly, evaporite accumulation is not just a matter of deposition but also diagenesis. Secondly, modern evaporitic depositional settings comparable to those in the rock record simply do not exist today (Kendall 1982, Warren, 1999). Thirdly, considerable overlap exists in depositional textures from widely different depositional environments. Fourthly, diagenesis compromises a depositional process product analytical approach because evaporite deposits are highly sucseptible to alterations that mask or completely obliterate primary textures. Fifthly, primary depositional textures (e.g. anhydrite nodule formation) and secondary diagenetic fabrics (i.e. gypsum transformed into anhydrite nodules) seemingly have identical appearances. Sixthly, preservation of diagenetic transformation pathways are not commonly preserved (Schreiber 1978) or well documented. Adding to these overall challenges is a classification and terminology problem for anhydrite, the most important evaporite in the oil industry.
Maiklem et al. (1969) first noted the communication confusion resulting from an inconsistent usage of a variety of descriptive anhydrite terms. To address this messaging problem, they proposed an anhydrite classification scheme that unfortunately did not gain widespread popular acceptance. Certainly, their scheme does not facilitate environmental interpretations in the same sense as does the Dunham (1962) textural classification for carbonate rocks. But then the environmental interpretation challenge presented by anhydrite bearing strata seems greater than that of carbonate deposits. Beyond environmental interpretations, their classification seems complex and lacks the characterization of the various anhydrite fabrics with a one-word summary. This shortcoming compromises practical application to some extent. But the inclusion of bedding and deformation attributes seems an unfortunate complication that adds significantly to the number of anhydrite classes. Bedding attributes perhaps are best handled outside the classification just like they are in a sandstone classification. The same can be said for the deformation attributes included in the Maiklem et al. (1969) classification. Excluding these two parameters goes far to simplify the characterization of anhydrite structures and shapes. Based on our experience in the oil industry, an anhydrite classification should provide classes that are descriptive and facilitate depositional environmental interpretation. This is a huge order given that the mineralization of gypsum has few diagnostic environmental characteristics and its transformation into anhydrite commonly obscures whatever diagnositic features may have existed. A most important lesson learned from a study of modern evaporite depositional systems is that precipitated gypsum and anhydrite growth habits and distributions only enable us to identify if the minerals formed within sediment or as subaqueous precipitates or cumulates. Even distinguishing evaporites with a subaqueous as opposed to subaerial origin is fraught with challenges and generally require integration of sedimentary sequence information before such an interpretation can be substantiated. Hence, the interpretation of evaporite and more specifically anhydrite depositional environments will always require additional observations (Dean et al. 1975). But an analysis of anhydrite fabrics is paramount to assessing the most likely depositional processes and environmental setting. Essentially there are three attributes of anhydrite that seem most important for distinguishing between evaporites of subaqueous and subaerial origin:
1. does the anhydrite pseudomorph or represent altered forms of isolated gypsum crystals or crystal masses that have a known origin in the interstitial porosity of sediment
2. does the anhydrite structure mimic gypsum crystals or crystal masses that grew subaqueously
3. does the anhydrite cumulate mimic gypsum cumulate patterns of tiny crystals or crystal fragments
The work of Maiklem et al. (1969) and its application by Loucks and Longman (1982) inspired the anhydrite classification presented tentatively below. Ignoring anhydrite cement and anhydrite replacement of carbonate with its typical rectangular re-entrants, this scheme proposes thirteen anhydrite classes. My first application of the classification suggests it may be advisable to further combine some classes. For further improvement,your regarding this classification would be greatly appreciated.
DEAN, W.E., DAVIES, G.R., and ANDERSON, R.Y., 1975, Sedimentological significance of nodular and laminated anhydrite: Geology, v. 3, p. 367-372.
DUNHAM, R.J., 1962, Classification of carbonate rocks according to depositional texture, in Ham, W.E., ed., Classification of Carbonate Rocks: Memoir, American Association of Petroleum Geologists, p. 108-121.
KENDALL, A.C., 1982, Evaporites, in Walker, R.G., ed., Facies Models: Geoscience Reprint Series 1: Newfoundland, Canada, Geological Association of Canada, p. 259-296.
LOUCKS, R.G., and LONGMAN, M.W., 1982, Lower Cretaceous Ferry lake Anhydrite, Fairway Field, East Texas: Product of shallow-subtidal deposition, in Handford, C.R., Loucks, R.G., and Davies, G.R., eds., Depositional and Diagenetic Spectra of Evaporites — A Core Workshop: SEPM Core Workshop No. 3: Calgary, Society of Economic Paleontologists and Mineralogists, p. 130-173.
MAIKLEM, W.R., BEBOUT, D.G., and GLAISTER, R.P., 1969, Classification of anhydrite — a practical approach: Bulletin of Canadian Petroleum Geology, v. 17, p. 194-233.
SCHREIBER, B.C., 1978, Environments of subaqueous gypsum deposition, in Dean, W.E., and Schreiber, B.C., eds., Marine Evaporites: SEPM Short Course No. 4 Oklahoma City 1978: Tulsa, Society of Economic Paleontologists and Mineralogists, p. 43-73.
WARREN, J.K., 1999, Evaporites: Their Evolution and Economics: Oxford, Blackwell Science, 438 p.