BEDROCK LITHOLOGY OF SEDIMENTARY FORMATIONS AND THEIR IMPACT ON STREAM CHEMISTRY

The Coal Creek watershed provides drinking and irrigation water for much of the population of Iron County, Utah. Although the watershed is characterized by low anthropogenic impact, Coal Creek has a notably limited biodiversity when compared to other streams in the region. Here we present results of an ongoing interdisciplinary study that investigates the processes that limit the biodiversity in the Coal Creek watershed system. The focus of this presentation is to better constrain how sedimentary rock composition controls water chemistry in the watershed and, by extension, if lithogeochemical processes potentially affect or limit biodiversity.

Field observations show that Coal Creek and other streams in the watershed have a steep gradient, are fed primarily by winter snow melts and summer storms. The stream system experiences large annual temperature variation, resulting in highly fluctuating discharge and rapid rates of erosion. The watershed spans a number of sedimentary rock units, including, in descending stratigraphic order, the Claron, the Wahweap, the Straight Cliffs, and the Tropic Shale formations. The region is heavily dissected by normal faults that occasionally place these units adjacent to one another. At the top of the watershed, the Claron Formation, comprised of alternating beds of pale to reddish orange sandy micritic limestones, calcite-cemented sandstones, calcareous mudstones, and pebble conglomerate. The Wahweap Formation is a fine grained sandy-silty sandstone, while the thicker Straight Cliffs Formation is comprised of interbedded sandstone, mudstone, shales, and a number of thin coal beds. The Tropic Shale is poorly exposed but underlies the lower portions of the watershed in the study area and may act as a relatively impermeable boundary between the surface water and lower rock units.

A total of 11 samples were taken from these formations. Current studies are investigating the sample mineralogy and textural relationships and will be discussed during this presentation. These observations will be integrated with elemental mass balance calculations to constrain to what extent erosion and leaching of these rocks can affect the water composition, and if such mechanisms provide a plausible explanation for the limited biodiversity observed in the Coal Creek watershed.