Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 470

The middle parts of tide-dominated estuaries characteristically display a
variably muddy intertidal zone transitional with a sand-dominated outer inter-
tidal flat. The subtidal channels are likely to be sand dominated—large lon-
gitudinal tidal sand-bars are common within and between channels—but mud
can still be transported seaward from the inner estuary. The water is mesoha-
line. As with the inner estuary, the subtidal deposits are typically unburrowed.
Owing to the presence of waves and variable tidal currents, the sandy inter-
tidal flats and estuary-margin shorefaces are sporadically burrowed, although
bioturbation is pervasive in the often muddy bay-margin zones.
The outer estuary is broadly similar to themiddle estuary, but is characterized by
broad and deep tidal channels, wherein bioturbation has a low preservation poten-
tial. Under conditions of persistent exposure to waves, the intertidal flat is replaced
by estuary-margin shoreface deposits ( Dalrymple et al., 2011 ). Estuary-margin
shorefaces and intertidal flats are, nevertheless, sporadically bioturbated.
3.7 Tide-Dominated Example:
Chignecto Bay, Bay of Fundy, Atlantic Canada
Chignecto Bay is one of two subbasins at the head of the Bay of Fundy. It is a
tide-dominated estuary with a tidal range of 10-13 m ( Dashtgard et al., 2007;
Desplanque and Mossman, 2001 ) and occurs in a cool temperate climatic zone
( Fig. 1 A). Due to the tidal mixing in the channels, the salinity is generally
between 15 and 20 psu. The depositional history of Chignecto Bay records
the postglacial history of the bay over the past 13.5 ka. The maximum lowstand
was reached approximately 6.5 ka ago, at which point Chignecto Bay was a
wave-dominated estuary subjected to microtidal conditions ( Amos et al.,
1991; Dashtgard et al., 2007 ). Tidal amplification occurred over the past
7ka( Grant, 1970; Scott and Greenberg, 1983; Shaw et al., 2002 ), and strongly
macrotidal to megatidal conditions have only persisted for the past 4 ka ( Scott
and Greenberg, 1983 ).
3.7.1 Valley Margins and Substrate-Controlled Suites
Chignecto Bay is currently undergoing a transgression, mainly resulting from
tidal amplification in the inner Bay of Fundy. During maximum lowstand,
Chignecto Bay was barred and the system corresponded to a wave-dominated
estuary with a quiet central basin, wherein mud was deposited in predominantly
subtidal positions ( Dashtgard et al., 2007 ). When the relative sea level rose and
tidal amplification led to transgression of the gravel barrier at the mouth of the
bay, the system evolved from a wave-dominated estuary to a tide-dominated
estuary. Mud deposits were subjected to erosion and served to provide the main
source of fine-grained material to the tidal flats in the middle and inner estuary
( Amos, 1987; Amos and Asprey, 1979; Amos et al., 1991 ).
Progradation of the tidal flats and salt marshes is common in the inner and
middle estuary, as an abundant sediment supply coupled with flow deceleration