Heavy rainfall on sand dunes can lead to large-scale slumping of the lee face, as
shown here in the Pinacate Region of northern Mexico. Note breciation of
the slump sheet near its toe. Photo by Nick Lancaster
Soft-sediment slumps are present within the Early Jurassic Navajo Sandstone on
the Colorado Plateau near the Arizona-Utah border.
Slumps in the Navajo Sandstone can involve more than a meter of strata,
are truncated along their upper surfaces, and are overlain by undisturbed strata.
Some Navajo Sandstone outcrops contain depositional cycles that Hunter and
Rubin (1983) interpreted as annual rhythms.
The depostional cycles are the result of shifts in wind direction. The yellow bars
each show annual increments to the dune deposits. In the diagram, the lighter-colored
avalanche deposits were emplaced by the dominant winds out of the northwest; the
darker ones are composed of wind-ripple deposits emplaced by opposing winds.
The dominant, northwesterly winds may represent the winter circulation in the northern
hemisphere, when the anticyclone off the west coast of Pangea was in its southern
position. The opposing winds may have come in spring, summer, and fall.
When the rain-induced slumps lie within annual depositional cycles, it is possible
to relate the rain events to the seasonal changes in wind direction. The thin red lines
mark boundaries between annual cycles. The thick bars show the parts of the cycles
containing slumped (red) and unslumped (yellow) strata.
This graph shows the slumps (red) within 36 contiguous annual depositional cycles
in the Navajo Sandstone. Only four of the slumps (in cycles 5,7,11, and 33) took
place during the winter. All the rest are interpreted as the products of summer monsoon
rains that took place when the moist air from the tropics reached this subtropical
Along the southern edge of the Sahara in West Africa, heavy rains reach the
desert's edge. The Navajo Desert was in the subtropics, and the study area lay
about 17 degrees north of the paleoequator. During the pluvial interval in the early
and middle Holocene, the West African monsoon rains reached about 500 km
further north than today. Similar pluvial episodes probably took place during the