Environmental Engineering Reference
In-Depth Information
FIGURE 7
Photos (in color) and X-radiographs (gray scale) of modern estuary deposits of Willapa
Bay, Washington, USA. (A)
Arenicolites
and
Diplocraterion
in a Pleistocene firmground. The
tracemaker
Polydora
is indicated as
Po
. (B)
Gastrochaenolites
-shaped trace in a Pleistocene firm-
ground. The tracemaker was
Petricola pholadiformis
. (C) Inner-estuary bioturbate texture showing
a low-diversity and diminutive trace assemblage dominated by
Palaeophycus
(
Pa
),
Arenicolites
(
Ar
), and
Skolithos
(
Sk
). (D) The middle estuary shows larger trace fossils such as
Skolithos
(
Sk
),
Thalassinoides
(
Th
), and
Cylindrichnus
(
Cy
). (E) In the channels of the outer estuary, cross-bedded
sands are the dominant lithofacies.
intensities increase upward from BI
¼
1-2 in the lower point bar to BI
¼
5-6 at
the bar top.
At Willapa Bay, the outer estuary salinities are essentially fully marine
(27-30 psu). Nevertheless, rapid sediment transport and bedform migration
resulting from high-wave and tidal-current energies tend to inhibit infaunal col-
onization, such that the density of biogenic structures in the outer estuary is sur-
prisingly low (generally BI
¼
¼
2;
Fig. 7
E). Sheltered tidal-flat
deposits situated near the bay mouth are similar to those of the middle estuary
(i.e., intensely bioturbated; BI
0-1, rarely BI
¼
5-6) but are invariably dominated by the
Thalassinoides
-making sand shrimp
Neotrypaea californiensis
. In contrast,
the surface sediments on tidal flats formed along the exposed spit platforms
are too unstable (rapidly shifting) for most infauna. As such, most units are
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