A NOVEL METHOD TO DETERMINE RELATIVE LAKE DEPTH USING VERTICAL AND LATERAL TRENDS IN FISH TAPHONOMY (GREEN RIVER FORMATION, FOSSIL BUTTE MEMBER, WYOMING)

Fossil fish were collected at four localities within the Lower Sandwich Bed (an ash-bounded chronostratigraphic layer with an average thickness of about 10 cm) within Fossil Lake, southwestern Wyoming. Two localities were in deeper water, and two were near the lake’s margin in shallower water. The taphonomy of each fish collected (n=243) was graded (on a scale of 1 to 5) for its overall articulation, number of scales present, rib articulation, skull condition and fin condition. Plots were made to discover lateral and vertical taphonomic trends within the Lower Sandwich Bed.

The lateral plots show that higher taphonomic grades (better articulation) occur in deeper water (near the lake center) and in some areas near the lake margin where the Lower Sandwich Bed is thicker, and therefore more rapid deposition was occurring. Areas near the lake margin with average depositional rates, have overall lower taphonomic grades. Vertical plots of overall articulation through the Lower Sandwich Bed show that articulation patterns change lake-wide. For example, at the bottom ash layer, articulation is relatively poor (at all sites). Upward, articulation improves (1 cm above), becomes worse (3 cm above), and then improves again (5 cm above). These trends appear at each site.

Results from experimental work indicate that lateral changes in taphonomy occur mainly because of changes in water depth. In deeper water, decay gases are compressed and are insufficient to float or explode (and disarticulate) fish. Vertical trends in taphonomy might also be explained because of changing water depths. Vertical trends in fish taphonomy might be helpful in determining whether lake depths are stable or changing over time. From the vertical trends found in this study, it appears lake levels were fluctuating during Lower Sandwich Bed time. Lake level fluctuations might help establish seasonal depositional patterns and long term trends in lake history.