Part 5 CHEMICAL AND MECHANICAL WEATHERING PROMOTED BY SEEPAGE AND CATION EXCHANGE

Atmospheric carbon cycle: CO 2 concentrations, weathering, and climate

Susceptibility to weathering is proportionate to chemical exchange with oxygenated groundwater

As oxygenated groundwater percolates through rock it leaches the ground mass of soluble ions and absorbs free cations. These chemical exchanges promote oxidation and loss of cementation, which causes degrades tensile strength, thereby promoting mechanical weathering as an “after product.”

Honeycomb solution cavities, or alveolar weathering (“Tafoni”), in cliff of Entrada Sandstone testify to differential weathering caused by percolating groundwater dissolving carbonate cement beneath a more resistant crust. Between Kanab and Mt. Carmel Junction, Utah.

Demarcation between sandstone strata leached of iron (above) and that below. Note large solution cavities in the lower (red) strata. Back country of Capitol Reef National Park, Utah.

Channel potholes are formed by the grinding action of stones or coarse sediment whirled around by eddies or the force of the current at a bend. Fiery Furnace area of Arches National Park, Utah

Series of channel potholes carved into the Navajo Sandstone in backcountry of Capitol Reef. Note master joint providing structural control of the cascading channel, which descends in a series of falls and pools. Standing/percolating water accelerates chemical weathering

Double Arch at Arches Park was likely initiated by pothole seepage through the Slickrock Member of the Entrada Formation. As carbonate cement was leached from the sandstone, its tensile strength was diminished, leading to blocks in tension falling away, leaving the arch ribs that were in compression. The tensile strength of this material is between 1/30 th and 1/70 th of the compression strength. Note people for scale (arrows)

Bulk density, diagenesis and susceptibility to weathering tend to be influenced by hydraulic conductivity (permeability with respect to water).

The erosional escarpment formed by limestone and marl spires of Brice Canyon NP. These were formed by preferential solutioning along systematic regional joints.

These spires at Brice Canyon are erosional remnants caused by preferential flow paths of groundwater seepage. This view is taken from near the base of the escarpment. For pioneer shepherd Ebenezer Brice it was a heck of a place to loose one of his sheep.

Looking back at crest of the erosional escarpment at Brice Canyon. Note the caverns and arching roofs developed by solutioning along preferential flow paths. The roofs eventually collapse, leaving linear rows of spires.

Sandstone fins formed by tension along the northern flank of the Salt Creek anticline in Arches National Park. The anticline was caused by a rising salt diapir which was subsequently dissolved by Salt Creek.

Ground view of a sandstone fin at Arches Park. These are formed in the Slickrock Member of the Jurassic age Entrada Formation The sandstone exhibits extremely low splitting tensile strength, due to the leaching of interstitial carbonate cement. This low tensile strength tends to favor the development of natural rock arches

The arches form in thin sandstone fins along brittle horizons with extremely low splitting tensile strength These are typified by profuse exfoliation, as shown at middle left The stronger roof unit spans the opening in compression, without exhibiting any marked secondary fracturing.