K/Ar Dating of Authigenic Illite Constrains the Time of Diagenesis and Brine Migration in the Weber Sandstone of the Uinta-Piceance Basin, Colorado and Utah

Roger L. Burtner and Lori Hathon

Authigenic illite from the Weber Sandstone in the Uinta-Piceance basin and along the southern flank of the Uinta uplift was dated by the K/Ar method. It yieIded dates (58-100 Ma) which predate the Laramide uplifts in the area. The dates become progressively older from west to east toward the Eagle basin. This trend coincides with a regional increase in carbonate cement. Stable isotope analyses of authigenic illite from both Weber outcrop and

subsurface localities support the conclusion that the illite and associated cements precipitated from formation water that is isotopically similar to the Na Ca-Cl brine that is currently present in the Weber Sandstone in the Uinta Piceance basin. Oxygen isotope data indicate that the illite precipitated from this brine at temperatures on the order of 80°C. The illite K/Ar dates strongly suggest that the Uinta-Piceance basin brine migrated at least 58-100 Ma fron the Eagle basin where it originated as evaporated seawater in the Pennsylvanian Eagle Valley evaporites.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California