Page 65 - Sedimentology and Stratigraphy

Processes of Transport and Sedimentary Structures
52 Nichols/Sedimentology and Stratigraphy 9781405193795_4_004 Final Proof page 52 26.2.2009 8:16pm Compositor Name: ARaju

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Fig. 4.8 Flow over a bedform: imaginary
streamlines within the flow illustrate the
separation of the flow at the brink of the
bedform and the attachment point where
the streamline meets the bed surface,
where there is increased turbulence and
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erosion. A separation eddy may form in

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+' the lee of the bedform and produce a
minor counter-current (reverse) flow.

that all dipped in the same direction and lay in the
same plane: this is planar cross-lamination. Sinu-
ous and linguoid ripples have lee slope surfaces that
are curved, generating laminae that dip at an angle to
the flow as well as downstream. As linguoid ripples
migrate, curved cross-laminae are formed mainly in
the trough-shaped low areas between adjacent ripple
forms resulting in a pattern of trough cross-lamina-
tion (Fig. 4.9).

Creating and preserving cross-lamination
Current ripples migrate by the removal of sand from
Fig. 4.9 Current ripple cross-lamination in fine sandstone: the stoss (upstream) side of the ripple and deposition
the ripples migrated from right to left. The coin is 20 mm on the lee side (downstream). If there is a fixed
in diameter.
amount of sand available the ripple will migrate
over the surface as a simple ripple form, with erosion
in the troughs matching addition to the crests. These
relationship between the two forms appears to be starved ripple forms are preserved if blanketed by
related to both the duration of the flow and its velocity, mud. If the current is adding more sand particles
with straight ripples tending to evolve into linguoid than it is carrying away, the amount of sand depos-
forms through time and at higher velocities (Baas ited on the lee slope will be greater than that removed
1994). Straight and linguoid ripple crests create differ- from the stoss side. There will be a net addition of sand
ent patterns of cross-lamination in three dimensions. A to the ripple and it will grow as it migrates, but most
perfectly straight ripple would generate cross-laminae importantly, the depth of scour in the trough is