Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 465

of the estuary, and with increasing tidal influence, discrete sedimentary and bio-
logical boundaries become uncommon. As such, clear tripartite divisions are
not developed, and the transitions through the inner, middle, and outer estuary
are gradational. Nevertheless, these estuaries also display discernible physio-
graphic zones.
The inner estuary is fluvially influenced and receives most of its sediment
from the fluvial reaches. The tidally influenced channels may possess a mean-
der form, and they expand in width and depth basinward. Salinities within the
inner estuary are typically very low.
The middle estuary receives sediment from both the estuary mouth and the
fluvial end of the system. As a result of tidal/fluvial interactions, facies succes-
sions of the transition zone from the inner estuary to the middle estuary are
markedly heterolithic. The channels' meander form is attenuated in the middle
estuary, and their channel widths and depths continue to increase basinward.
Salinities tend to be variable, depending on the fluvial flux into the estuary.
Within the middle estuary, the tidal-current energies tend to be low to moderate
compared to the outer estuary. This permits the emplacement and preservation
of a range of biogenic sedimentary structures.
The outer estuary is characterized by wide and deep, sand-dominated tidal
channels. The channels are gently sinuous to straight, and the dominant mode of
sediment storage is as tidal dunes and longitudinal tidal bars. Most of the sedi-
ment is derived from the tidal inlet. Although salinities can be high, energetic
tidal currents inhibit sediment colonization and trace preservation.
3.4 Mixed-Energy Example: Willapa Bay, Washington, USA
Willapa Bay is located in the southwestern corner of Washington, USA. The
bay is separated from the Pacific Ocean by a 27-km long spit (North Beach
Peninsula, Fig. 1 D). Owing to sediment supplied from the Columbia River,
the spit is progradational and is constructed by high-energy waves of the Pacific
Ocean. Willapa Bay is a mesotidal estuary, with a tidal range of 2-3.4 m. The
local climate is temperate.
The estuary sits within a Pleistocene-aged incised valley, entrenched into
Eocene basalts and sandstones (summarized in Anima et al., 1989; Clifton,
1983; Clifton et al., 1976 ). A stratigraphic record of three or more stacked
incised valleys is preserved within the main valley. Substantive stratigraphic,
sedimentological, and ichnological studies have been conducted at Willapa
Bay, and the area is particularly well known.
3.4.1 Valley Margins and Substrate-Controlled Suites
A striking heterogeneity in omission suites attributable to the Glossifungites
Ichnofacies is documented from these modern firmgrounds ( Fig. 7 A and B).
This variability is related to intertidal zonation, sediment texture, the absolute