Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 462

from tidal exchange at the mouth of the system. Such bodies of water tend to be
fairly small so that fair-weather waves and storm waves are less effective at
reworking the substrate. As such, central basins tend to be characterized by
muddier deposits, and sand interbeds—which are associated with the flood-tidal
delta, deposited during river-flood events, or generated during storms—are pre-
dominantly oscillation ripple laminated, with rare current ripples and thin beds
of low-angle undulatory parallel lamination (micro-HCS). Tidal ranges are typ-
ically low in such settings so that, although tidal flats may form along bay mar-
gins, they tend to be thin and their deposits difficult to recognize.
Some wave-dominated estuaries are essentially filled, such that their central
basin consists of salt marshes that are dissected by tidal channels. The salt-
marsh deposits are preserved in the geological record as illuviated, root-bearing
strata that cap burrowed subtidal facies. The salt marsh can be thick, because of
vertical aggradation due to accumulation during relative sea-level rise.
Estuary-mouth deposits are sand dominated. Wave-dominated estuaries
develop on strongly wave-swept coasts, where barrier complexes can be readily
established and maintained. The tidal energy is generally sufficient to breach
the barrier locally, forming tidal inlets. The tidal energy is high where it is con-
stricted through the inlet but dissipates rapidly as it enters the central basin. This
decelerating flow commonly builds a flood-tidal delta complex. Intermittent
storms may result in washover events over the barrier into the central basin.
The above tripartite zonation is the result of pronounced energy partitions
that yield discrete substrate characteristics and are considered diagnostic of
wave-dominated estuaries. The clear relationship between resultant sediment
calibers and sedimentary process is extensively discussed in the geological lit-
erature (e.g., Dalrymple et al., 1990, 1992; Roy et al., 1980; Visher and Howard,
1974 ), as is the distribution of trace fossils ( MacEachern and Gingras, 2007;
MacEachern and Pemberton, 1994; Pemberton et al., 1992 ). The relatively
low volumes of water exchanged in wave-dominated estuaries—due to a small
tidal prism—greatly influence the biological dynamics of microtidal bays
within the estuary. Microtidal-bay water may be prone to partial thermal and
salinity stratification; thus, bottom waters can become eutrophic during neap
tides. Freshets (seasonal high-discharge, fluvial events) can markedly freshen
the bay, as freshwaters are sequestered within the middle, lower-energy parts
of the bay. These factors combine to make conditions unattractive for long-term
colonization, and intervals of unburrowed sediments in these settings are there-
fore common ( Gingras et al., 1999; Hauck et al., 2009 ).
3.2 Wave-Dominated Estuary Case Study:
Kouchibouguac Bay, New Brunswick, Canada
Kouchibouguac Bay is situated on the Northumberland Strait in the southern
Gulf of St. Lawrence. Kouchibouguac Bay comprises 29 km of arcuate barrier
islands fronting several estuaries ( Fig. 1 B). The climate of New Brunswick is