Travertine deposits and seep wetlands, lower Meramec River Hills, Missouri.

Ashley, David

Several rapidly growing travertine (calcium-carbonate) deposits and groundwater fed wetlands have been documented along the Mississippian bedrock outcrops of I-270 (St. Louis County). These features are caused by groundwater seepage from a dolomitic claystone of the Salem Formation, which acts as an aquitard to groundwater flow from the overlying closed karst aquifer.

Knowing the age of the rock-cut (1964), presented an opportunity to estimate coldwater travertine growth rates. Seep A has maximum dimensions of ~2-m wide, ~4-m in length and ~0.6 m thick. Thus, the maximum growth rate is ~10 cm per year, with an average closer to ~6 cm/year. Seep B has a growth rate of ~5 cm/year. Sandy moss (Didymodon tophaceus), a typical travertine forming species, covers much of the deposit. The blue-green algae (Cyanobacterium), Gloeocapsa, and the pinnate diatoms Navicula were common. Bryophytes, algae and vascular plants provide surfaces for new growth and are significant contributors to the rapid growth rate. Seep B has "hair-like" structures, due to calcified filamentous algae. Eventually, the plants become encased or "fossilized" by the growing travertine.

Groundwater geochemical analysis indicates a mixed calcium-magnesium bicarbonate type, with significant chloride, sodium and sulfate levels. The site is within a band of high chloride-sulfate groundwater along the Lower Meramec River Hills, which may be caused by groundwater flow through Pennsylvanian cyclothems containing pyrite and gypsum. Travertine deposits have typically been related to bedrock faulting. In this case, it maybe due to mineralized groundwater flow through intensively fractured bedrock.

* George (1), S.E. and C. Bourne (2). (1) MACTEC and (2) University of Missouri-St. Louis.