Petrology and diagenetic features of the Fort Dodge gypsum beds

The Jurassic Fort Dodge Beds of Webster County, Iowa, are a laminated gypsum deposit with features similar to many other ancient sulphate deposits in the world. The laminations are zones of relatively pure gypsum alternating with zones of gypsum and clay. The clay is predominantly illite and occurs in the form of stringers or clusters of marl (clay and carbonate). The beds differ from most other deposits in their purity (89-96 percent calcium sulphate). Trace element distribution is chiefly related to the accessory mineral assemblage;Megascopic and microscopic features in the gypsum indicate that the beds have undergone considerable diagenetic alteration. Three dominant crystal varieties--euhedral to subhedral gypsum laths, anhedral gypsum, and microcrystalline gypsum--provide evidence that primary gypsum has been recrystallized to form secondary gypsum. Common throughout the deposit are veins of satin spar along lamination partings parallel to bedding. Evidence of an anhydrite precursor of the existing gypsum is rare, but the presence of possible anhydrite relicts in gypsum nodules together with other features suggests that conversion probably occurred at least locally within the gypsum body;A statistical analysis of several quantitative variables in the Fort Dodge Beds was performed to support tentative relationships suggested from thin section analysis. A (chi)('2) test showed that lamination color is chiefly dependent on the presence of clay. Analysis of variance indicated that the presence of clay and lamination thickness are related to crystal habit and recrystallization;The Fort Dodge Beds were probably deposited in a marginal marine basin during an arid to subarid period. There is little evidence for sabkha deposition.