61.2: Sedimentary way-up structures

Cavity fills

Annotated photograph showing a snail (gastropod) shell in cross-sectional view, filled on the bottom with limy mud and at the top with sparry calcite. The mud was deposited at the same time as the empty shell, partially filling it on the bottom of the empty space. The calcite spar was precipitated later by groundwater, filling the remaining open space at the top of the cavity. — Figure \(\PageIndex{1}\): In cavity fills, mud fills a portion of the empty space at the bottom and later diagenetic deposition of calcite fills in the remaining empty space (at the top) with spar.

When an empty space exists in a sedimentary deposit, such as the protected little hollow under a shell, it can be partially filled by sediment. We call this little pocket of space a “void” or a “cavity.” If the cavity is entirely filled with sediment, it’s useless as a geopetal structure. It only has geopetal value if it is partially filled in with mud. In fact, this is geopetal in the strictest sense of the term: it refers to these cavity fillings. Here is how it works: the mud settles under the influence of gravity, lining the bottom of the space (but not the top of the space). Later, as groundwater moves through the sediment, it carries with it dissolved ions, which may bond together, filling the available space with crystals. In the example at right, the crystals formed are of the mineral calcite, which occurs in a coarse form geologists call “spar.”

Use this principle to examine the following image, showing a cross-section through a bed of limestone collected in West Virginia. We have intentionally placed it in a vertical position for you to examine. As you will see, the limestone includes many snail shells (gastropod fossils). A lot of them show this key pattern of infilling with two substrates: gray limy mud and white calcite spar. Note that because the shells are made of calcite also, both the original shell and the subsequent spar appear the same coarse crystalline white:

Which way is depositional “up” in this sample? You can use the rotation tool at lower right Screenshot of the GIGAmacro rotation tool, a three-quarter circle of arc tipped with an arrowhead turning clockwise. to test various orientations, until you get the mud at the (original) bottom and the spar above it in the (original) top of the available space.

Hopefully you determined that the side of the sample next to the pencil was originally “down.”

The sample principle applies with minerals other than calcite. For instance, consider this slab of “Turitella agate,” a rock made of whole snail shells filled in with the tiny, clear, bean-shaped shells of small arthropods called ostracodes.

Compare and contrast these two shells from the slab, with their different way-up implications:

Animated GIF showing an annotated view of a photograph of a snail shell in cross-section, about 5 mm in diameter, filled in the "lower" half with oval-shaped ostracode shells, and in the upper half with empty space, into which are projecting an array of quartz crystals. The implication is that "up" is "up." — Figure \(\PageIndex{2}\): A gastropod shell serves as a cavity, partly filled with a sedimentary deposit of ostracode shells, leaving an upper chamber open for quartz crystals to attempt to fill. This one is right-side-up.

Animated GIF showing an annotated view of a photograph of a snail shell in cross-section, about 5 mm in diameter, filled in the "upper" half with oval-shaped ostracode shells, and in the "lower" half mostly filled with agate, and a little bit of empty space in the middle, lined all around the edges with quartz crystals. The implication is that "down" is "up." — Figure \(\PageIndex{3}\): A gastropod shell serves as a cavity, partly filled with a sedimentary deposit of ostracode shells, leaving an upper chamber open for quartz crystals to attempt to fill. But this one is up-side-down!

Explore the slab on your own if you want to search for additional examples. Or if you are ready to test yourself on a similar sample, from Texas, take the quiz below:

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

Is this sample right-side-up or up-side-down? Explain.

a. Right-side-up, since the bottom is filled with mud, and the empty cavity at the top is partially filled with sparry calcite crystals.

b. Up-side-down, since the bottom is partially filled with sparry calcite crystals, and the top is filled with mud.

Answer: a. Right-side-up, since the bottom is filled with mud, and the empty cavity at the top is partially filled with sparry calcite crystals.

Exercise \(\PageIndex{2}\)

Examine this sample for cavity fills.

Which way is depositional "up?"

a. The top of the screen (where the scale is) is the original depositional "up" direction.

b. The left of the screen (where the biggest shells are) is the original depositional "up" direction.

c. The bottom of the screen (opposite the side where the scale is) is the original depositional "up" direction.

Answer: a. The top of the screen (where the scale is) is the original depositional "up" direction.

Crossbedding

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. Another way of putting this is that the angle between the crossbed and the mainbed is typically small at the bottom (close to parallel) and larger (around 32 degrees or so in dry sand) at the top. — Figure \(\PageIndex{4}\): A cartoon showing the different crossbed/bed relationships at the upper and lower portion of the bed.

Cross-bedding is a series of laminations included within a larger sedimentary bed. A directional current allows the laminations to build up on 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 sedimentary particle, and within that medium (saltwater, freshwater, or air). In general, it is a gentle angle of around 30\(^{\circ}\) 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.

This leads to important insights, as shown in the case study of the Mixed-Up Quartzites of Cape Agulhas.

Examine this gigapixel panorama of a specimen of cross-bedded red sandstone. 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.

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:

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 the accumuate 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...

CLIMBING RIPPLES

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 below; if you flipped the photo up-side-down, it would be difficult to tell:

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 \(\PageIndex{6}\): Climbing ripples. (Photo by Brian Romans; reproduced with permission.)

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. — Figure \(\PageIndex{7}\): Climbing ripples in modern delta sediments, Louisiana. (Photograph by Robert Mahon; reproduced with permission.)

The image at right (click to enlarge) 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 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.

Finally, here is a gigapixel panorama of a specimen showing climbing ripples in cross-section:

ANTIDUNES

Standing surface waves on a small stream draining a beach — Figure \(\PageIndex{8}\): Standing surface waves will have corresponding antidunes on the sediment bed, in this small stream draining a beach at low tide.

If water is flowing fast enough to be in the upper flow regime, antidunes can form. They are mostly found in shallow channels (e.g., fluvial and tidal channels). If you are lucky enough to see them forming, you can recognize the situation when you see standing surface waves – watch closely and you will see the waves migrate upstream over time, meaning that the are bedforms developing immediately below the standing waves. If high flow is maintained, the antidunes will also migrate upstream. They do this by adding new sediment on their upstream (stoss) side rather than the usual situation for migrating ripples, which deposit sediment on the downstream (lee) side. So antidune cross-beds dip in the opposite direction from normal ripple or dune cross-beds. The good news is that the preservation potential of antidunes is low, because as soon as flow slackens they tend to wash out and be eroded.