WAY-UP STRUCTURES:
1.1.1 Crossbedding
Cross-bedding is a series of
laminations included within a larger sedimentary bed. A directional current
allows the laminations to build upon the leeward (downstream) side of a
migrating bedform called a ripple. (Bigger versions of ripples are called
dunes, and they create cross-beds as they migrate, too.) Ripples and dunes form
in a directional current of either water or air. They accumulate at the angle
of repose for that size of the sedimentary particle, and within that medium
(saltwater, freshwater, or air). In general, it is a gentle angle of around 30°
of dip.
In terms of shape, cross-bedding is
most useful when the cross-beds show a pronounced concavity, with the scoop shape curving upward, like a smiley face. This concave-up shape is a
reflection of the cross-beds curving into a gentler orientation, approaching
parallel to the base of the main bed. We
call this “tangential,” from the geometric
term describing a line
intersecting a circle at one point. At the top of the bed, in contrast, they
are often truncated (“stopped short” or “cut off”), because the original upper tangential portion of the leeward side of the ripple has evidently been “planed
off” by post-depositional erosion.
A sketch showing how cross-beds
approach parallel with the main bed's bottom but at the top of the bed,
erosion has removed the tangential portion, resulting in a truncated contact (Figure 8). Another way of putting this is that
the angle between the crossbred and the main bed is typically small at the bottom
(close to parallel) and larger (around 32 degrees or so in dry sand) at the
top.
Figure 8: A cartoon showing the different cross-bed/bed
relationships at the upper and lower portion of the bed.
This leads to important insights, as
shown in the case study of the Mixed-Up Quartzites of Cape Agulhas.
Example 0X:
Examine this panorama of a specimen of cross-bedded red sandstone (Figure 9). There are two halves to the sample shown. One is
right-side-up. The other is up-side-down. Explore the two samples to see if you
can figure out which one is which, applying the criteria explained above.
Figure
9:
Specimen of cross-bedded red sandstone.
Really large cross-beds form via
aeolian (wind) deposition in dune fields. The shape and geopetal implications
are the same, but the cross-beds are much larger. Here is an example
(right-way-up) from coastal exposures in the western Orkney Islands:
Figure 10: Cross bedding exposures in the
western Orkney Islands
MAGIC BOX รจ Exceptions to the Rule
Cross-bedding can be tricky. Sometimes
cross-bed laminae accumulate without the tangential, concave-up portion, and
sometimes they preserve the upper convex-up portion at the ripple crest. Even
crazier, sometimes they accumulate not on the leeward side of a dune, but the
upstream (stoss) side! Let us take a moment to see what these complications
look like…
1.1.2 CLIMBING
RIPPLES
Image (Figure 11) showing annotation of a photograph of climbing ripples
in sandstone. A Swiss Army knife serves as a sense of scale. The climbing
ripples' foresets (leeward side laminae) build up and to the left, implying
current flow from the right toward the left.
Figure
11:
Climbing ripples in cross-sectional view, Morrison Formation, Greybull,
Wyoming.
Climbing ripples form when downstream
migration of a ripple or dune is accompanied by rapid vertical aggradation of
sediment. This tends to occur when the sedimentary load is higher than the
capacity of the current that’s carrying it. Climbing ripples are distinct
structures indicating critical to supercritical flow, but they often look about the same right-side-up as
they do up-side-down. The concave-up portion of the laminae in the lee of the
ripple is matched by the convex-up crest of the ripple itself, buried as soon
as it forms. As a result, they can mislead the historical geologist.
Consider the example in Figure 12; if you flipped the photo up-side-down, it would be difficult to tell:
The image is shown in Figure
13 shows a cross-section through three sets of climbing
ripples from modern (unlithified) sand deposits in the Mississippi Delta region
of Louisiana. Note how the largest, oldest, deepest set of cross-beds shows an
undulating set of internal laminations, and these “climb” up and to the right,
with tangential bases. This is likely indicative of supercritical flow. The the second set has a trough-like bottom that cuts into the older set of cross-beds,
and is truncated by the most recent, uppermost, thinnest set of cross-beds.
Figure 12: Photograph showing two sets of climbing ripples between planar laminated beds. The field of view is about 30 cm by 20 cm, and a lens cap serves as a sense of scale. The climbing ripples' foresets (leeward side laminae) build-up and to the left.
Figure 13: Photograph of a cross-section of modern sand deposits. The vertical trench wall shows three sets of climbing ripples, with the cross-bed sets climbing up and to the right.
TO BE CONTINUE....
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