Sapelo Island: Bedforms of the ebb tidal delta

We spent a lot of time with at the beach studying the formation of "bedforms" like ripples and dunes formed by currents reworking the sand. Waves, swash and tidal currents are the primary agents moving the sediment around and the type of bedforms they generate are specific to the type of current forming them.

Here are some straight-crested (or 2-D)"megaripples" we found on the ebb tidal delta. The steep (lee) side of these bedforms is the "avalanche" slope where sediment tumbles down, forming cross-bedding. The orientation of these megaripples suggests that they formed from a current flowing from the upper right side of this photograph to the lower left (obliquely past the camera position).

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This set of 3-D megaripples is similar to the 2-D ones (wavelength of ~2m, amplitude = ~10-20cm), but the crests are broken up and some of the bedforms are solitary, isolated dunes with an erosional trough in front of them. These form under conditions of stronger flow strength (higher current velocity or shallower water).

Also, 3-D ripples form a different type of cross-bedding. 2-D megaripples will form straight crested, tabular cross-bedding where the avalanche faces fill in the straight-crested hollow in front of the advancing megaripple. 3-D megaripple migration forms "trough" cross-bedding which looks similar

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Here is another example of a 3-D megaripple with an erosional trough in front of it. The turbulence of the flow over the bedform creates erosion in the eddie in front of the avalanche face.

Here, Dena, Jennifer and Eric have just dug a small trench across the front of a 3-D megaripple to examine the internal bedding.

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Closer up, the trench now forms a "T" shape so that we can see cross-bedding in 2 directions -- both perpendicular and parallel to the current direction. In this case, the long axis of the trench revealed trough-shaped (concave-upward) laminations that reflect the avalache face filling in the trough morphology. The short axis of the trench (where the machete is cleaning off the surface for viewing) is composed of steeply dipping cross-bedding indicating the current direction and the bedform migration direction (left to right, toward the partially visible trough).

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Here is a close-up view of a small trench oriented parallel to the migration direction of another 3-D megaripple bedform. This is the same orientation as the picture above, but this close-up allows you to easily see the steeply dipping cross-bedding that we use to determine paleocurrent directions (in this case, left to right). The scale of this trench is about 30cm deep.

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As a special treat, (for the sedimentologists among us), we offer this photo of "adhesion ripples" with a whelk's egg case in the sand for scale. This site is just below high tide. The very small, wrinkly-looking ripples visible along the bottom and left of the photo are the adhesion ripples, a small-scale type of eolian (wind-blown) bedform that forms when sand is blown by the wind onto a damp surface. The ripples grow by sand sticking (adhering) to the upwind side of the ripple. This makes the ripple grow upwind, and the dip of the ripples is downwind. This is a very diagnostic type of bedding (crinkly-looking ripple cross-lamination) that is very useful for identifying eolian sedimentation in the rock record.

Which way was the wind blowing for these adhesion ripples?

(ans: right to left)

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