Investigating Lithologic Controls on the Morphology and Evolution of Bedrock Streams, Ouachita Mountains, Central Arkansas.

Abstract:

The incision of bedrock streams largely controls the topographic evolution of mountainous areas, and patterns of incision into bedrock hold information critical to unraveling past climate and tectonic uplift patterns. A popular tool in studying patterns of incision in bedrock streams is the channel steepness index, or channel gradient normalized by drainage area. The three main factors that are thought to affect channel steepness index are uplift rate, climate, and lithology. The Ouachita Mountains of central Arkansas provide a study site with currently uniform uplift (essentially zero) and climate, allowing us to explore how changes in lithology affect local channel steepness values.

The Ouachita Mountains are an intensely folded and faulted highland region, structurally related to the Appalachian Mountains to the east. Folding and faulting of this region occurred during the Paleozoic, and is no longer active. The trellised morphology of the stream network is controlled by past folding, as stream channels in the region generally flow along fold hinges. Bedrock in the area consists of Arkansas Novaculite, a massive chert that is highly resistant to erosion, and less resistant shale and sandstone members of the Bigfork and Mississippi Mountain Formation. Sense of bedding of geologic units is generally steep, although local folding causes high variation in bedding orientation.

Where bedrock channels transition from novaculite to shale, knickpoints and high channel steepness index values are observed in some streams, while others seem unaffected by this lithologic boundary. We explore 5 bedrock streams that flow over the novaculite/shale boundary to determine what lithologic factors have the largest impact on incision of bedrock channels. Analysis consists of measurements of channel morphology, detailed local geologic mapping of bedding and fold orientation, and measurements of rock strength along stream channels. Understanding how lithologic differences affect local channel steepness can yield information critical to our understanding of the evolution of bedrock streams, and the controls lithology imposes on their longitudinal form.