THE INFLUENCE OF SPRINGS ON THE FORMATION OF SOME MICROBIALITE BIOHERMS IN THE WILKINS PEAK MEMBER OF THE EOCENE GREEN RIVER FORMATION: WYOMING

Localized clusters of microbialite bioherms, stromatolites, barrier bars and sand tufa spires (clusters up to 50 m wide), occur at the local base of the Wilkins Peak Member of the Green River Formation near the town of La Barge, Wyoming. In this area, Wilkins Peak oil shale represents a significant transgression of Eocene Lake Gosiute over the fluvial Cathedral Bluffs Member of the Wasatch Formation. In general, the localized microbialites and barrier bars are embedded within Wilkins Peak oil shale and are associated with the Wilkins Peak “layered tuff”.

Six outcrops in Steed and Spur Canyons illustrate the vertical succession of three localized facies (from bottom to top): 1) vertically oriented sand tufa structures up to 3 m thick, 2) overlain by a 2-3 m thick barrier bar sequence composed of crossbedded microbial carbonate fragments, 3) microbialite bioherms or spring mound deposits up to 1.5 m thick. The sand tufa is considered strong evidence of vertical groundwater flow directly below the carbonate spring mounds and stromatolites.

Two outcrops at Little Mesa illustrate a similar vertical succession of localized facies but lack the vertical sand tufa structures and are embedded in kerogen-poor wackestone (instead of oil shale). At Little Mesa, the carbonate barrier bars are associated with overlying stromatolites and caddisfly-dominated microbial carbonate bioherms. The Little Mesa outcrops are 12 km northwest of the Spur Canyon outcrops and represent a more nearshore paleoenvironment (evidenced by the occurrence of caddisfly fossils and the lack of oil shale at Little Mesa).

The Steed Canyon, Spur Canyon and Little Mesa outcrops record evidence of a 12 km long and 50 m wide discontinuous accumulation of sand tufa, linear barrier bars and microbialites that paralleled the margin of Lake Gosiute during deposition of late Wilkins Peak sediments.

Similar facies associations can be found in analogue lakes such as Searles Lake and Mono Lake. In these lakes the localized accumulation of sand tufa structures, barrier bars and microbialite bioherms is associated with nearshore calcium-rich freshwater springs. The Wilkins Peak microbialite bioherms are probably located at the sites of similar springs that discharged into the more saline/alkaline Lake Gosiute.