Thursday, November 24, 2011

In search of relic periglacial wedges

Yesterday was sunny, cold, quite windy and there was little snow-cover in the basin -- probably much like it was here during the late Pleistocene.  It was a good day for a field trip in search of periglacial relics.

Wyoming is composed mainly of high mountain ranges and broad basins.  The ranges were glaciated during the Pleistocene and mountain glaciers sometimes extended several miles from canyon  mouths, but the basins generally were ice-free.   These basins are fairly high, 5000 to 8000 feet elevation, and periglacial environments appear to have been the norm, with permanently frozen ground subject to intense freezing cycles and other permafrost processes.
Mountains and basins of Wyoming.  Courtesy Wyoming Geological Survey.
Relic permafrost features have been found at many locations in Wyoming basins.  The most common are polygons and wedges created by repeated cold thermal-cracking of permanently frozen ground.  Most have been discovered fortuitously during excavation and other kinds of dirt-work.

Surface polygons are exposed only rarely, for example when an area is bladed off; these exposures are especially short-lived.
Sketch of relic frost polygons exposed on road cut along Interstate 80,
Walcott Junction, WY.  Fill is pale fine sand derived from a mantle of
aeolian deposits that was bladed off during construction.
Sand wedge; courtesy LIC.

More commonly an area of thermal-cracking is sliced through, producing longitudinal sections through the cracks.  Thermal cracks narrow with depth and so longitudinal sections generally are wedge-shaped, hence the names “sand wedge” and “ice wedge”.  Fresh cuts in quarries, roads, ditches and dumps are the best places to find wedges; these exposures also are ephemeral, eventually obscured by erosion, slumping and vegetation.

Most of the relic wedges reported from Wyoming basins are sand wedges, indicating cold dry permafrost conditions.  “Sand wedge” is a misnomer because fill material is not restricted to sand -- it can be finer or coarser.  The name is used to differentiate these wedges from ice wedges, which form in wet environments.

Ice wedges grow as water fills narrow cracks next to the existing wedge.  With post-glacial warming, the ice gradually melts and the casts are filled with material that falls or washes in.  Often the fill is layered, and may include clasts too large to have fallen into the original thermal cracks.  In contrast, sand wedges form in dry environments where thermal cracks are immediately filled with whatever blows in.  The wedge grows with repeated cracking and filling, and the fill may have a vertical texture.  Larger clasts are absent.

It was cold and very windy as we drove the back roads of the basin looking at road cuts and irrigation ditches.  There was a lot of disappointment -- all the road cuts were slumped or vegetated and many of the ditches were filled with snow.  But the last stop was a good one.  At one of the plains lakes, wave action had cut a nice exposure of sand wedges.
Sand wedge (red arrow) in Niobrara shale.

The host material is the Niobrara Formation -- gray and orange especially-gooey limey shales.  [This is the Niobrara shale being advertised as the next big oil shale play, in the Denver Basin at depths of 3000 to 14,000 ft.]  The fill is sand, sometimes with slightly coarser fragments.  The Niobrara’s thin layers of alternating colors make it easy to see deformation of host material by the wedge.

Sketch showing wedge from photo above with sandy fill, longitudinal crack;
layers in host material have been warped upward by the wedge.
The sandy fill is more prone to cracking than the sticky Niobrara shale.  Many of the wedges were penetrated by a narrow crack, and in several places, chunks had spalled off along an old frost crack/sand wedge.
Periglaciologist Fred once again in his Eureka pose
(Fred is 3" tall when kneeling).

Periglaciologist Fred made his debut back at the end of October, having found some wedges in a ditch in the west part of town filled with unusual unsorted unknown material.  Here he is thrilled to have found something he can explain.

Sometimes the more resistant wedge stands in relief to the
softer Niobrara shale.  Fred kneels near base of  wedge.
Closeup showing sandy texture with occasional slightly coarser clasts.
Deformed Niobrara shale adjacent to sand wedge.
Here the host material is warped downward. 
Google Earth:   41.160436° -105.933083° (Sodergreen Lake)

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