Environmental Engineering Reference
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onto the tidal flats promotes mud deposition. By contrast, in the outer estuary,
much of the coastline is transgressive, and salt-marsh and glaciomarine deposits
are commonly eroded and exposed on the sea floor, making them available for
subsequent colonization. The range of burrowing fauna is remarkably
consistent throughout the embayment, with a dominance of Corophium voluta-
tor (an amphipod) in intertidal mud beds and a range of sessile and motile sub-
surface deposit-feeding and surface deposit-feeding polychaetes (e.g., Nereis
sp., Clymenella torquata ) in all deposit types ( Dashtgard and Gingras,
2005a; Dashtgard et al., 2008; Hauck et al., 2008; Pearson and Gingras,
2006; Pearson et al., 2007 ). In the outer estuary, where suspended sediment con-
centrations are lower and substrates are typically sand- or gravel-dominated,
bivalves (e.g., Mya arenaria , Ensis directus ), sediment-swimming polychaetes
(e.g., Nephtys sp.), and sessile deposit-feeding polychaetes (terebellids) are
increasingly common ( Dashtgard et al., 2008 ).
3.7.2 Trace-Fossil Distributions by Subenvironment
There is no strong longitudinal (ichnological) gradation through the inner and
middle estuary of Chignecto Bay. This is most likely because the rivers are
small and deliver too little freshwater even during peak discharge. The inner
and middle estuary of Chignecto Bay is fringed by a heavily rooted salt marsh
that abruptly grades into well-developed, typically broad, muddy tidal flats. The
salt marsh comprises vegetated platforms, shallow pools, and tidal channels that
drain the marsh ( Fig. 9 A and B), of which the vegetated platforms are nearly
devoid of bioturbation ( Fig. 9 C). Salt-marsh pools are either unbioturbated in
landward positions or intensely bioturbated in locations where they commonly
receive influxes of oxygenated water ( Fig. 9 D). The tidal channels are biotur-
bated where sedimentation rates are moderate to low ( Dashtgard and Gingras,
2005a ). Tidal flats are generally unburrowed in the wave-winnowed and sand-
silt-dominated salt-marsh deposits with abundant root traces (Rt). Black scale bar is 5 cm.
(D) Intensely bioturbated mud deposited in a salt-marsh pool near the marsh-intertidal zone contact.
This pool is regularly inundated by marine water. Note the presence of the bivalve (bi) Mya arenaria
and polychaete-generated Palaeophycus ( Pa ). Scale bar is 5 cm. (E)-(G) Interbedded laminated and
current-rippled beds and burrowed beds of the middle estuary mud flats. Laminated deposits re-
present winter deposition when bioturbation is severely restricted, while intensely bioturbated units
represent summer deposits ( Pearson and Gingras, 2006 ). The range of traces in these deposits is
produced by a low-diversity suite of infauna, mainly consisting of Corophium and Nereis . Traces
observed in these photos include Arenicolites ( Ar ), Diplocraterion ( Di ), Palaeophycus , and crypto-
bioturbation (cr). The pen in (F) is 13.5 cm long. The orange and black bands on the meter stick in
(G) are 10 cm each. The white arrow in (G) marks the basal scour surface of a tidal channel cut into
horizontally bedded and intensely bioturbated tidal-flat deposits (images provided by I. Armitage).
(H) Small-diameter threadworm burrows (t) in gravelly sand of the outer estuary. Scale in centime-
ters. (I) Permanent dwelling of the trophic-generalist Nereis ( Ne ) in muddy gravel. This trace is best
described as Palaeophycus . Coin is 2.1 cm in diameter. (J) X-radiograph of a bivalve and its trace
( Siphonichnus , Si ) in gravelly sand. Scale bar is 5 cm.
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