Recognizing Eolian (i.e., Wind Blown) Deposits on San Salvador Island & Beyond
Mario Caputo
Gerace Research Centre, San Salvador Island, Bahamas
23 June 2010
How
does wind interact with sediments? Sediments can be moved by wind, water,
and glaciers. Wind is a turbulent transport agent.
What
makes eolian deposits? Wind moves bedforms (dunes). Dunes make
eolian deposits.
Ex: erg
(sand sea), Namib Desert, southwestern Africa
Ex: Kelso
Dunes, eastern Mojave Desert, California - has linear dunes (aka
seif dunes; aka longitudinal dunes) - the wind runs parallel to crest of
dunes.
Eolianite - rock
formed by wind-transported sand of carbonate grains, not quartz &
feldspar sand grains.
Given
a steady supply of sand and a good, persistent wind, dunes will form.
Ex: Michigan -
reworked glacial sediments.
Ex: Great
Sand Dunes, Colorado - reworked Medano River sediments, derived from the
Sangre de Cristo Range.
Ex: Barchan dunes of
Utah - sediments are reworked from wind-blown sands of the Entrada Sandstone.
Sand
can withstand several cycles of formation, deposition, lithification, and
erosion. Except at White
Sands, New Mexico, which has gypsum sand - it is quite soft (H=2) - it
doesn’t survive like quartz, so it doesn’t travel far.
Basic eolian processes & resulting strata
Wind
ripple strata - “topset” strata and “foreset” strata or cross-bedding
(sloping/slanting units).
Wind
ripples, grainflow (sandflow), and grainfall make cross-bedding.
Bahamian
limestones consist of aragonite (CaCO3) sand grains.
Utah
sandstones have quartz (SiO2) sand grains. The Navajo Ss.,
Page Ss., and Entrada Ss. are all siliciclastic eolian units there.
How sand is transported
Wind
creates a shearing effect above a sand grain. Get lift (Bernoulli
Effect), like an airplane. Wind-blown (lifted or pushed along) sediment
populations consist of fine sand - 1/16 mm to 1/2 mm.
Sand
moves by: 1) sand slides; 2) rolling; 3) saltation (bouncing), triggered by the
impact/strike of another grain. Saltation trajectories can be high.
Ex: sand stinging your legs during windy times.
Wind
“impact” ripples: ripple length (spacing) = saltation length (trajectory
controlled by wind speed & bounce height).
Forming wind ripple strata
Ralph
Hunter, now a retired USGS geologist, liked to dig out beautiful trenches &
smooth the walls to see wind-blown sand deposits. (Need moist sand
to trench & carve with a machete.) Such trenches show that wind
ripples move up as they move laterally - climbing
ripples. This is the result of horizontal & vertical
vectors. These control the angle of climb. Ripples climb.
Sediment deposits look horizontal, but they do have angular discordance.
There wouldn’t be a deposit if the ripples didn’t climb.
In
the Page Sandstone (~140 Ma, Paria Wilderness, southwestern Utah), you can see
the angle of climb.
At
the front of each ripple is a zone of erosion. If bedforms are moving
horizontally, previous ripples get chewed up - don’t get a deposit. If
ripples climb, you get a deposit.
On
San Salvador Island in the Bahamas, wind ripple marks are rare, but they occur
at French Bay in the Grotto Beach Formation. Wind ripple marks are well
exposed in the Jurassic Page Sandstone of southwestern Utah’s Paria
Wilderness. In both units, the ripple marks match up.
There
is an index of width-to-height that only wind ripples have.
Pinstripe
bedding - white stripes are microledges - seen in the Rice Bay Formation at
North Point on San Salvador. Darker bands/gray bands are recesses.
So, see alternating light-dark
ledges-recesses.
Can
see the same thing in the Moab Sandstone Member of the Entrada Sandstone of
Utah.
In
thin section, can see that some layers are cemented and other layers are not
cemented. This is the result of differential packing. The more
closely packed layers end up being better cemented. The closely packed,
better cemented portions are in the basal part - they form white stripes -
little ledges.
The
upper portions are coarser-grained, not as well packed, are darker, end up
being not as well cemented, and form recesses.
The
overall result is small-scale reverse grading. This happens even though
wind-blown sand is well-sorted. Wind is the most efficient sorter of
sediments.
Grainflow Processes
Ex: Silver Lake
State Park, western coast of Michigan.
Grains
at the summit of ripples fall down at the brink point - get avalanching down
the lee side.
Grainflows
in the eolianite limestones at North Point Peninsula on San Salvador Island are
thin to nonexistent. So, the original North Point dunes were small.
Sandflows
in cross-bedding form recesses in San Salvador’s Rice Bay Formation and in the Jurassic Page
Sandstone at Lake Powell (Monkey
Wrench Gang country).
_________________________________
Recess
Sandflow
(not well cemented)
_________________________________
Ledge
_________________________________ well cemented
Grainfall Process
Sand
is conveyed up the windward side of a dune. Finer sand of wind ripples
gets swept beyond the brink point and forms grainfall deposits - smooth,
unrippled surfaces without evidence of flowage.
Grainfalls
are difficult to identify in the rock record. They are relatively easy to
see in modern settings.
Can
identify grainfalls encasing sandflow lenses in the Jurassic Page Sandstone
& in the Grotto Beach Formation of San Salvador.
Grainfall
deposits in the rock record are recognizable by looking for sandflow lenses
encased in thin ledges (finer-grained than sandflow lens).
San
Salvador eolianites - grains are marine in origin - marine framework grains -
ooids, bioclastic grains, algal fragments - get washed up on beaches by waves
and tidal currents. Then winds blow the sediments into dunes.
Sediments get reworked by wind & vegetation - vegedunes.
Whole
dune-form preservation occurs by plant stabilization and early
cementation. You don’t see that in quartz-rich dunes.
Topset
strata, brinkset strata, foreset strata are also seen on Eleuthera Island in
the Bahamas.
Reactivation
surfaces represent pauses in dune movement and sand movement.
Can
see updip pinchout in carbonate eolianites in the Bahamas - you never
see that in quartz dunes preserved in the rock record.
San
Salvador's North Point Peninsula outcrops have sandflow lenses (recesses) in
grainfall deposits (ledges).
Quartz
can be frosted in any environment. Frosted grains are no longer a
reliable indicator of wind-blown origin.
Water
ripples have higher amplitudes than wind-blown ripples.
Symmetrical
ripples - can watch them form in the shallow waters around San Salvador
Island. Sediments get swept back-n-forth in ripple trough,
resulting in the buildup of sediments at the edges of troughs - get symmetrical ripples.
Rolling
& sliding mechanism of grain movement - creep.
Black
basalt sand dunes occur in Hawaii. Basalt sand dunes occur on Mars.
Bedding
is only visible is there’s slight differences in grain sizes and/or
grain mineralogies.
Swash
laminations form in high flow regime conditions. A sheet of sand gets
deposited. Swash has a higher velocity than backwash velocity. Get
a two-fold lamination (swash-backwash).
Won’t
see ripple forms in swash deposits. Will see parting lineation.
Can get ripples migrating up the
lee side of dunes due to the presence of an air eddy in the trough between
dunes.
