PRE-FLUVIAL INCIPIENT BEDROCK CHANNEL DEVELOPMENT: PRELIMINARY INVESTIGATION OF THE ROLE OF PHREATIC FRACTURE FLOW AND VADOSE WEATHERING

Although the idea that many if not most bedrock stream channels, regardless of lithology, are fracture controlled is well established, only meager research has been conducted on how bedrock fractures are transformed into incipient channels prior to the onset of fluvial erosion. The purpose of our research is to investigate possible linkage between groundwater fracture flow and incipient bedrock channel development. Two sites have been selected for preliminary investigation: Adrspach-Teplice in the Czech Republic, and Arches National Park, Utah.

At Adrspach slot canyons form along closely spaced joints in cross-bedded marine sandstone. Vertical canyon walls ~20-60 m high and ~0.25 to 20 m apart are pockmarked suggesting chemical weathering from top to bottom. There is no evidence of mechanical or fluvial erosion. Our hypothesis for Adrspach is: 1) groundwater circulating through the joints (phreatic fracture flow) chemically dissolved the sandstone cement, and over time effectively widened many of the joints. Over time some fractures underwent more chemical erosion than others, leading to a hierarchy of first (truck) and lesser order (feeding) fractures. In effect, a system of Strahler Order groundwater fracture flow developed, and 2) uplift of the sandstone body resulted in erosion of the overlying clay, permitted the subsequent removal of the weathered material that was present in the fracture system. At Arches fins, arches, and slot canyons have developed along joints in the Jurassic age Entrada Fm. Our hypothesis is: 1) the Entrada Fm was exposed to near-surface processes by the erosion of overlying sediments. Vadose recharge water moving along the joints chemically dissolved the sandstone cement along and adjacent to the fractured bedrock walls. The vadose water is able to chemically dissolve the sandstone cement because it is charged with CO2 from the plant communities that thrive along the fractures in the bedrock surface, 2) subsequent removal of the weathered material in the fracture system, by a variety of surface processes, resulted in narrow vertical canyon walls which were then widened and sculptured by spring sapping, flash flooding, and other processes.

Preliminary results of geophysical investigations, fracture mapping, core plug and geochemical analysis will be presented.