REPLACEMENT OF EVAPORITES WITHIN THE PERMIAN PARK CITY FORMATION, BIGHORN BASIN, WYOMING, USA

The Permian Park City Formation consists of cyclically bedded subtidal to supratidal carbonates, cherts and siltstones. Early diagenesis of Park City Formation carbonates occurred under the influence of waters ranging from evaporative brines to dilute meteoric solutions and resul ted in evaporite emplacement (syndepositional nodules and cements), as well as dolomitization, silicification and leaching of carbonate gain s. Major differences are seen, however, in the diagenetic patterns of subsurface and surface sections of Park City Formation rocks. Subsurfa ce samples are characterized by extensively preserved evaporite crysta ls and nodules, and preserve evidence of significant silicification (c hert, chalcedony and megaquartz) and minor calcitization of evaporites . In outcrop sections, the evaporites are more poorly preserved, and h ave been replaced by silica and calcite and also leached. The resultan t mouldic porosity is filed with widespread, very coarse, blocky calci te spar. These replacements appear to be multistage phenomena. Field a nd petrographic evidence indicates that silicification involved direct replacement of evaporites and occurred during the early stages of bur ial prior to hydrocarbon migration. Siliceous sponge spicules provided a major source of silica, and the fluids involved in replacement were probably a mixture of marine and meteoric waters. A second period of replacement and minor calcitization is inferred to have occurred durin g deep burial (under the influence of thermochemical sulphate reductio n), although the presence of hydrocarbons probably retarded most other diagenetic reactions during this time interval. The major period of e vaporite diagenesis, however, occurred during late stage uplift. The l ate stage replacement: and pore-filling calcites have delta(13)C value s ranging from 0.5 to -25.3 parts per thousand and delta(18)O values o f -16.1 to -24.3 parts per thousand (PDB), reflecting extensive modifi cation by meteoric water. Vigorous groundwater flow, associated with m id-Tertiary Mock faulting, led to migration of meteoric fluids through the porous carbonates to depths of several kilometres. These waters r eacted with the in situ hydrocarbon-rich pore fluids and evaporite min erals, and precipitated calcite cements. The Tosi Chert appears to hav e been an even more open system to fluid migration during its burial a nd has undergone a much more complex diagenetic history, as evidenced by multiple episodes of silicification, calcitization (ferroan and non -ferroan), and hydrocarbon emplacement. The multistage replacement pro cesses described here do not appear to be restricted to the Permian of Wyoming. Similarly complex patterns of alteration have been noted in the Permian of west Texas, New Mexico, Greenland and other areas, as w ell as in strata of other ages. Thus, multistage evaporite dissolution and replacement may well be the norm rather than the exception in the geological record.