Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 461

with grain-size and sedimentation rates, overwhelmingly dictates the resulting
longitudinal distributions of tracemaking organisms and their biogenic struc-
tures (e.g., Gingras et al., 1999; Hauck et al., 2009; Hertweck, 1992; Hertweck
et al., 2007; Howard and Frey, 1975; Hubbard et al., 2004; Noffke et al., 2009 ;
Fig. 3 ). This is in marked contrast with the dominant controls on ichnology that
prevail in more wave-exposed settings (i.e., the shoreface), wherein infaunal
distributions are mainly influenced by the magnitude of wave energy and fre-
quency and magnitude of storm events, as well as sedimentation rate, grain-size
availability, and grain-size distributions ( Pemberton et al., 2012 ).
Below, we draw from modern and ancient examples of tidally influenced,
marginal-marine depositional environments, with increasing tidal energy rela-
tive to wave and fluvial energy. From the modern, we consider a wave-
dominated estuary (Kouchibouguac, Canada), mixed-energy estuaries (Willapa
Bay and the Ogeechee River Estuary, USA), and a tide-dominated estuary
(Chignecto Bay, Canada), where discrete ichnological distribution patterns
can be discerned. From the rock record, we compare the wave-dominated
Viking Formation (Albian) of Alberta, Canada; a mixed(?) estuary from the
Montney Formation (Triassic) of Alberta, Canada; and a tide-dominated(?)
example from the McMurray Formation (Aptian to Albian) of Alberta, Canada.
Included in the depositional environments are a range of tidally influenced
subenvironments, including tidal flats, tidal channels, shoals, ebb- and flood-
tidal deltas, and salt marshes. It is emphasized here that these examples show
only a small part of the broad range of estuary occurrences. The character of the
estuary is heavily influenced by wave energy, volume of the tidal prism, fluvial
flux, and the available sediment sources. As such, although the examples below
likely provide useful comparisons,

they fall short of defining an all-

encompassing “estuary ichnology”.

3.1 Wave-Dominated Estuaries
Wave-dominated estuaries constitute one end member in the continuum of estu-
ary types proposed by Dalrymple et al. (1992) . Wave-dominated estuaries typ-
ically display a well-demarcated tripartite facies zonation (inner, middle, and
outer estuary, or alternatively, bay-head delta, central basin, and estuary mouth)
recognized by earlier workers (e.g., Dalrymple et al., 1992; Reinson, 1980;
Reinson et al., 1988 ).
Bay-head delta deposits encompass sediments supplied to the estuary via flu-
vial influx. Generally, the lowwave energies and veryweak tidal flux in the central
basin result in the bay-head delta developing a river-dominated, digitate morphol-
ogy. Distributary channels are locally developed in associationwith the delta front.
Under conditions of low fluvial flux, the tidal flow may impinge landward of the
bay-head delta, but for most wave-dominated estuaries, this effect is minimal.
Central-basin deposits are laid down in relatively shallow, standing bodies
of water, which receive sediment both from the river at the landward end and