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The Mission Canyon evaporite wedge, the Frobisher-Alida interval, has commonly been interpreted as typical nodular or "chicken-wire" anhydrite from a sabkha depositional setting. Upon examination of over 190 cores from North Dakota and Montana, we identified a variety of evaporite textures and interpreted several distinct origins for them. The following evaporite textures were recognized: (A) precipitative or primary, (B) intraclastic, (C) evaporite cement, (D) replacement, and (E) dissolution-stage.
Depositional evaporites (A, B) form by direct precipitation in supersaturated solutions (primary texture) or by reworking of primary evaporite (intraclastic texture). Primary texture forms by direct precipitation from a supersaturated brine occurring in shallow lagoons or tidal ponds (subaqueous evaporite) or within the sediment (nodular anhydrite). Three types of subaqueous textures were identified: (1) isolated laths, (2) rosettes or clusters of laths, and (3) large "swallowtails." Intraclastic texture results from the reworking of previously precipitated evaporite. It is recognized by angularity of the clasts, size sorting, and association with carbonate intraclasts. Depositional environment of this texture is interpreted as evaporitic shallow-water lagoons, punctuated by occasiona storm events.
Diagenetic textures include cementation, replacement, and evaporite dissolution. Cementation by evaporite was found primarily in carbonate grainstones and is usually poikilotopic. Replacement textures may develop early or late in the diagenetic history of the rocks. Early replacement was found in primary restricted carbonate facies. Original texture (algal laminations, bioturbation, carbonate grains) were usually preserved after replacement. Late-stage replacement was observed in more marine facies, with the original texture not preserved. Isolated nodules of fibrous anhydrite result.
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