The lower part of the Goose Egg Formation (Permian) of southeastern Wyoming consists of carbonate members interstratified with thicker, red clastic members. The carbonates are interpreted as having been deposited in subtidal, intertidal, and supratidal environments during transgressions of the Phosphoria sea.
Evidence of at least 2 periods of dolomitization is present in the carbonates. The first stage is represented by fine-grained dolomite, 5-20 microns in diameter. Abundant in strata interpreted as peritidal, this fine-grained dolomite is uncommon in strata interpreted as subtidal. This stratigraphic distribution suggests that the fine-grained dolomite formed in the depositional environment rather than later, in a postdepositional site. This conclusion is further supported by comparisons of features common to recent, as well as to other ancient, fine-grained dolomite which has been interpreted as penecontemporaneous in origin. Dolomitized Foraminifera tests and peloids indicate that at least some of the penecontemporaneous dolomite is a replacement phenomenon; however, the possibility that some is “primary” cannot be eliminated. The penecontemporaneous dolomite in the Goose Egg carbonates may have been formed by capillary concentration of hypersaline brines.
A later, postdepositional period of dolomitization is represented by euhedral and subhedral dolomite rhombs, 50-200 microns in diameter. Unlike the fine-grained, penecontemporaneous dolomite, the coarse dolomite is more evenly distributed throughout the carbonate members, contains numerous inclusions, and cross cuts other grains and crystals. This second period of dolomitization occurred during a late stage of diagenesis and was probably caused by circulation of ground waters rich in magnesium.