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

desert.

 

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

Early Jurassic.