MICROBIAL CARBONATES CONTAINING CADDISFLY CASES FROM THE EOCENE BRIDGER FORMATION (BRIDGER A)

Abundant microbial carbonate encrusted logs and stumps occur about 10 km south of Opal, Wyoming in the Eocene Bridger Formation (Bridger A). The logs are horizontal, up to 7 m in length and have a preferred NE/SW orientation. The stumps are in vertical position and often have a horizontal root system that is encased in microbial carbonate (micro-laminated carbonate). The microbial carbonate contains abundant caddisfly pupal cases. The caddisfly cases are pupal cases because they all have the same diameter (4-5 mm) and length (1.5 cm). Most caddisflys have a univoltine life cycle, so the inclusion of caddisfly pupal cases may indicate yearly events. The cases are encased in the microbial carbonates, and are oriented with the long axis of the caddisfly cases aligned with the long axis of the vertically oriented stumps. The caddisfly pupal cases are composed of ostracods and ooids.

The caddisfly-containing microbial carbonate bed occurs in Bridger A, about 20 m below the Lyman Limestone Marker Bed (G marker bed). The encrusted logs and stumps are embedded in a 20 cm thick ostracodal oolitic sparite that can be used for regional correlation (E marker bed). At the study site, the E marker bed contains fish bones, turtle bones and stromatolites. Toward basin center, the oolitic limestone grades into oil shale.

The caddisfly-containing microbial carbonates formed in high-energy nearshore areas of a large lake that expanded into forested areas. The lake was long-lived as evidenced by the incorporation of multiple generations of caddisfly pupal cases in the microlaminated microbial carbonate.