Backwasting rock sheets of the Navajo Sandstone, Utah, USA. Issue 10 (26th June 2022) - Loope, David B.
Abstract: On many Navajo Sandstone outcrops in southern Utah, steep, decimetre‐high scarps (downslope‐facing margins of rock sheets) interrupt smooth slopes covered by crustose lichens. Scarps form where sheeting joints and water emerge at the land surface. Weathering at scarp faces causes upslope backwasting of rock sheets and leaves behind smooth, unweathered rock surfaces that become stabilized by lichens. We hypothesize that subsurface, low‐angle, open fractures in porous and permeable rocks are barriers to downward flux of water through the vadose zone. Runoff and lateral flow through the uppermost rock sheets deliver water to scarps, inducing backwasting. Monitoring of temperature and water/ice content within two surficial rock sheets revealed that, in the winter of 2020–2021, snowmelt that entered lichen‐covered surfaces persisted for 64 days and underwent 46 freeze/thaw cycles. Water gained from summer rains remained for a maximum of 11 days. Flaking rock and dangling lichen crusts that overhang scarps indicate backwasting is now active. We conclude that freeze/thaw cycles (rather than salt precipitation) drive backwasting at our study sites, all of which are located more than 1400 m above sea level where the climate is relatively cool, and snowfalls are frequent. Backwasting would likely be more rapid and extensive under a cooler, wetter (periglacial) climate. Although broad, gently sloping, lichen‐covered rock sheets dominate the land surface, they are not downwasting; weathering is instead restricted to the near‐vertical, lichen‐free, backwasting scarps. Abstract : Schematic cross‐section of sheet‐jointed Navajo Sandstone in southern Utah. Intact lichens show the sandstone is not downwasting. Instead, each rock sheet (separated by low‐angle fractures) is slowly backwasting. Weathering is concentrated at the scarps where water emerges to the land surface where freezing and thawing then forces sand grains apart.