Geology Reference
In-Depth Information
(a)
4km
(b)
Fig. 7.
(a) Space shuttle photograph
showing location of The Quicksands
west of the Marquesas Keys. (b) Low-
altitude aerial photograph of Quicksands
showing well-developed tidal bar belts
of carbonate sand trending N-S,
and large (several metres) sand
waves with crests oriented in a
roughly E-W orientation. Image
courtesy of the Image Science and
Analysis Laboratory, NASA, Johnson
Space Centre. Aerial photograph
modifi ed from Shinn
et al
. (1990).
Grain-size analysis of these samples shows that
the individual
Halimeda
plates making up the sand
bars in this region average about 3-5 mm (longest
dimension), with maximum lengths of 6-8 mm
(Fig. 9), making them similar in both size and shape
to the Phylloid algal plates found in the Paradox
Basin mounds (average 4-5 mm, maximum approxi-
mately 8-10 mm). In both the Paradox Basin mounds
and the modern sand waves, platy algae make up
the predominant grain type (8-Foot Rapids: range
9-55%, average 34%; Quicksands: range 10-66%,
average 45%) with subordinate amounts of normal
marine biota contributing to the sediment.
PHYLLOID ALGAL MOUNDS
DEVELOPED IN TIDAL PASSES?
The question of whether the Paradox mounds
exposed at 8-Foot Rapids might have formed as a
result of tidal currents piling algal plates into large
sand waves is signifi cant for a number of reasons.
