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An Analysis of Caddisfly Larval Cases From Domal Bioherms in the Upper Green River Formation, White Hill Cuesta, Ephraim, Utah


The paleoenvironmental conditions and growth patterns of ancient caddisflies (Order Trichoptera) have been studied in the Jinju Formation, South Korea (Paik 2005) and the Green River Formation, WY (e.g., Leggitt et al. 2007). Caddisfly cases are within or on the top of stromatolite bioherms, indicating these insects were aquatic early in their life cycle. This study extends known Green River localities of caddisfly-dominated carbonate mounds southward from Leggitt et al.’s (2007) study in Lake Gosuite to Lake Flagstaff, an intermontane basin of central Utah. The Green River Formation (Eocene) in central Utah is interpreted as a lacustrine unit with minor fluvial input, but shows lithologic variability between its Lower and Upper Members. This research focused on exposures of the Upper Green River in several quarries on one of the large cuestas.

The first objective of this research was a comparative investigation of two distinct quarries located 455 m from one another. One quarry (Quarry 3) was dominated by lime mudstone and showed no caddisflies present. Another quarry (Quarry 1), dominated by ostracodal-ooidal packstones/grainstones with other interbedded carbonates, contained caddisfly-larval cases in discrete bioherms. The stromatolite morphologic differences and the lithologic variability played a role in caddisfly preferences and presence. The second objective was to characterize the caddisflies found in the stromatolite bioherms and the lithologies exposed at Quarry 1. Methods included: petrographic analysis (thin section and acetate peel) of the host rock and of the caddisfly larval cases (transverse and longitudinal views), XRD analysis, and portable field magnetic susceptibility measurements. Additional trace fossil and paleocurrent data were obtained.

Results were compared to other Green River caddisfly studies (e.g., Leggitt et al. 2007). In central Utah caddisfly cases typically are found in distinct layers in discrete domal stromatolites. The alteration of caddisfly-rich and -poor layers in these bioherms supports the work of Leggett et al. (2007). Observations show that these caddisfly-stromatolite mounds (ave height 25 cm) exposes a unique stratigraphy. These mounds formed on a bed of much smaller (8 cm) digitate stromatolites, below which is a 6 cm silicified ooidal grainstone. Caddisfly characterization, coupled with the symbiotic presence of stromatolites, enables paleolake levels to be estimated for this margin of Lake Flagstaff.