Recognizing Eolian (i.e., Wind Blown) Deposits on San Salvador Island & Beyond
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.
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.
(not well cemented)
Ledge _________________________________ well cemented
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.