Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 463

cool temperate. The lagoons and estuaries represent preglacial valleys drowned
during the Holocene transgression. Kouchibouguac Bay is microtidal, with
mean and maximum tidal ranges of 0.67 and 1.25 m, respectively ( Davidson-
Arnott and Greenwood, 1974; McCann and Bryant, 1972 ).

3.2.1 Valley Margins and Substrate-Controlled Suites
In Kouchibouguac Bay, animals inhabit exposed cohesive lagoonal and salt-
marsh deposits. A depauperate assemblage of burrowers is present therein, pro-
ducing burrow suites that are analogous to the Glossifungites Ichnofacies. The
two dominant burrowers in muddy firmgrounds are the bivalve Petricola pho-
ladiformis ( Fig. 6 A) and the very small polychaete Polydora ligni . The lobster
Homarus americanus has also been observed, producing large-diameter firm-
ground tunnels in channel flanks and bases. A bay-margin assemblage attribut-
able to the Glossifungites Ichnofacies is not observed.

3.2.2 Trace-Fossil Distributions by Subenvironment
Hauck et al. (2009) show that the microtidal wave-dominated estuary at Kou-
chibouguac Bay displays a predictable ichnological distribution along its
length. Within the inner estuary, salinities range between 1 and 10 psu, and
the ichnological character comprises a low-diversity assemblage of biogenic
structures, mainly Skolithos , Palaeophycus , and Arenicolites ( Fig. 6 E and F).
Moreover, endobenthos in the inner estuary dominantly reside in channel-bar
tops and in small intertidal flats developed along the banks of the brackish-water
reach of the river ( Fig. 6 E and F). Subtidal settings of the inner estuary are char-
acteristically unburrowed into the fluvial reaches.
Toward the middle part of the Kouchibouguac estuary, the ambient salinity
is approximately 25 psu and a moderately diverse suite of traces is present,
including Psilonichnus , Gyrolithes , Planolites , Palaeophycus , Thalassinoides ,
and Siphonichnus. Bioturbation in the middle estuary is sporadically distrib-
uted, ranging from BI

3-5 in the subtidal flats
( Fig. 6 C and D). Notably, subtidal flats represent the spatially dominant part of
the middle estuary, suggesting that bioturbated media of low to moderate trace
diversity constitute the main sedimentary facies therein.
In the outer estuary (the most marine-influenced part of the system), animal
distributions are very patchy (BI

¼ 1-3 in the tidal channels to BI ¼

1-3) ( Fig. 6 B). Ten “ichnogenera” are
observed, with Skolithos , Siphonichnus , Arenicolites ,and Polykladichnus being
generally dominant. Cryptobioturbation and equilibrichnia are also common, with
Psilonichnus , Planolites , Thalassinoides ,and Palaeophycus present more rarely.

¼

3.3 Mixed-Energy Estuaries
Mixed-energy estuaries can be both strongly wave- and tide-influenced with
variable fluvial influence. A reduced tide versus wave energy promotes effec-
tive estuary mouth-bar development and limits tidal exchange between the