Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 466

firmness of the firmground (substrate consistency), and the presence or absence
of a soft-sediment veneer ( Gingras et al., 2001 ).
In the middle estuary, valley-margin expansion is ongoing; wave and tidal
erosion are continually exposing compacted Pleistocene strata. These locales
typically reside within the intertidal zone, and a zonation of firmground bur-
rowers is readily observed. Shallow subtidal and lower to middle intertidal firm-
grounds are preferentially colonized by crustaceans or pholadiid bivalves.
These surfaces characteristically possess firmground Thalassinoides -, Gastro-
chaenolites -, and rare Psilonichnus -like traces. The upper intertidal zone is
dominated by small polychaetes ( Polydora ), which produce diminutive Diplocra-
terion- and Arenicolites -like traces ( Fig. 7 A). Firm, sand-dominated substrates
contain Gastrochaenolites -like burrows ( Fig. 7 B). Finally, a Thalassinoides -
dominated suite is locally observed at the base of tidal channels (e.g.,
Fig. 3 B). In Pleistocene units associated with the bay, nearly every observed
TS/SU is demarcated by Thalassinoides , Psilonichnus , or, more rarely, Gastro-
chaenolites , attributable to the Glossifungites Ichnofacies ( Gingras et al., 1999 ).

3.4.2 Trace-Fossil Distributions by Subenvironment
Willapa Bay displays notably different trace-fossil distributions than are
observed in purely wave-dominated estuaries ( Fig. 3 ). The fluvial reaches
and bay-head delta are gravel- and sand-dominated and are typically unbur-
rowed. Owing to a low degree of fluvial influence, the inner estuary exhibits
salinities between 0 and 17 psu and is characterized by 1-3 m thick, mud-
dominated, subtidal point-bar deposits. The point bars contain thinly bedded,
mud-dominated (70-95% mud) IHS, which contain biogenic sedimentary
structures comparable to Palaeophycus , Polykladichnus , Arenicolites , Diplo-
craterion , and Skolithos . Burrows are generally small ( 2 mm in diameter),

and animal distributions are patchy, leading to spatially variable bioturbation
(BI

<

1-3, Fig. 7 C). Bioturbation is present down to the channel thalweg. Mod-
erately burrowed (BI

¼

3-4) intertidal-flat deposits overlie the point-bar
deposits of the inner estuary, with suites dominated by small (

¼

2 mm in dia-
meter) Skolithos , Arenicolites , and Diplocraterion . Bioturbation intensities
increase upward from BI

<

¼ ¼

3-4 at the bar top.
In the middle estuary, salinities are generally 20-25 psu and traces such as
Skolithos , Planolites , Palaeophycus , Thalassinoides , Siphonichnus , Arenico-
lites , Cylindrichnus , and Piscichnus are present ( Fig. 7 D). Traces are larger
(2-20 mm in diameter) than those of the inner estuary. The intertidal flats of
the middle estuary are typically intensely bioturbated (BI

1-2 in the lower point bar to BI

5-6). Due to physi-
cal reworking in the channels, the abundance of traces and animal diversities is
markedly lower than in the intertidal flats (BI

¼

1-2). The subtidal deposits are
dominated by Thalassinoides , Skolithos , Planolites , Teichichnus , and shallow
motility traces of bivalves and, potentially, cryptobioturbation. Bioturbation

¼