Geology Reference
In-Depth Information
Fig. 10.17
Physical surface structures frequently observed on
tidal flats. (
a
) Asymmetrical wave ripples; (
b
) Ladderback ripples;
(
c
) Small wave ripples in the troughs of larger ripples and water-
level marks; (
d
) Late-stage runoff with linguoid current ripples
dissecting a field of flat-crested wave ripples; (
e
) Shallow intertidal
creek with small sand ribbons over shell pavement; (
f
) Late-stage
runoff features; (
g
) Current ripples in mud; (
h
) Thin fluid mud
sheet with scour windows displaying ripples on the surface of
underlying sand. (
i
) Circular tool mark formed by the rotation of
a protruding polychaete tube. Note the bird tracks surrounding
the structure; (
j
) Rippled sand bed with patchy wash-outs formed
shortly before emergence; (
k
) Intertidal dunes; (
l
) Shell pavement
sandy sediment where a rippled surface is overlain
by slightly elevated, smooth-topped sand patches
(Fig.
10.17j
). These patches are the remnants of an
eroded sand sheet that was locally stabilized by dia-
toms. The lower surface was subsequently covered by
wave ripples, whereas the surfaces of the elevated
patches were smoothed by wind-induced washover
shortly before emergence.
Tool marks are less frequent than other surface
structures on tidal flats but may on occasion be encoun-
tered where driftwood or dislodged algae have scraped
or rolled across the sediment surface in shallow water.
In contrast to this, a large variety of scour, prod and
roll marks induced by drifting ice floes are ubiquitous
on tidal flats in cold regions (Dionne
1974
; Reineck
1976
; Dionne
1988
; Pejrup and Andersen
2000
). A rather
