Estuaries - Trace Fossils as Indicators of Sedimentary Environments - page 460

subaerially exposed coastal deposits (i.e., salt-marsh, soil-forming, or paludal
environments) may be highly burrowed (e.g., Dashtgard and Gingras,
2005a ), their associated fluvial deposits are normally unburrowed.
Many marine trace fossils do not typically occur in brackish-water environ-
ments. The ability to identifycharacteristic “marine” trace fossils, that are not asso-
ciatedwith brackishwater, is critical. Such trace fossils include all graphoglyptids,
and the common marine ichnogenera Nereites , Zoophycos , Spirophyton ,
Chondrites , Rhizocorallium , Asterosoma , Phoebichnus ,robust Rosselia ,and
large-diameter Ophiomorpha . At least since the Mesozoic, brackish-water strata
commonly contain Planolites/Teichichnus -dominated assemblages, Cylindrich-
nus/Skolithos -dominated assemblages, horizons dominated by Gyrolithes ,or
zones containing abundant Arenicolites . Siphonichnus can be locally common.
In more distal locales (i.e., closer to the “marine” basin), brackish-water deposits
may contain comparably small examples of Rosselia , Thalassinoides ,or Ophio-
morpha . In proximal positions (i.e., within the inner estuary), the trace-fossil
assemblage tends to be progressively dominated by Planolites and/or small
Skolithos . Although subaqueous fluvial deposits are normally unburrowed, trace
fossils such as rare Planolites , Skolithos , Camborygma ,and Protovirgularia
may be observed. Additionally, Buatois et al. (1997) documented the presence
of insect tracks on the intertidal portion of a tidally influenced fluvial bar in
Carboniferous strata of the Tonganoxie Sandstone, Kansas, USA.
By recognizing the proximal and distal shifts between stratigraphic levels, a
framework for overall transgression or regression can be established. In transgres-
sive regimes, comparably distal trace-fossil suites are positioned immediately
above proximal assemblages. In marginal-marine settings, trace-fossil suites
reflect highly variable depositional conditions and heterogeneous distributions
of infauna (e.g., Dashtgard, 2011a,b ). Correspondingly, the sequence-stratigraphic
resolution resulting from this ichnological approach can be of high fidelity.
3. TRACE DISTRIBUTIONS WITHIN WAVE- AND
TIDE-DOMINATED ESTUARIES
The distribution of food resources and salinity within estuaries is passively
related with one another through their codependence on the dynamics of the
tidal or tidal-fluvial system. Research in modern estuaries suggests that, in
these settings, the distribution of food resources and brackish water, coupled
(McMurray Formation, Cretaceous, Alberta, Canada). (H) Current reversals on sand waves from a
tidal-inlet deposit (Pleistocene, Willapa Bay, Washington, USA). (I) Siphonichnus ( Si ) in a sand-
dominated tidal-bar deposit (Pleistocene, Willapa Bay, Washington, USA). (J) Cylindrichnus
( Cy ) in a sand-dominated tidal-bar deposit (McMurray Formation, Cretaceous, Alberta, Canada).
(K) Skolithos ( Sk ) and Planolites in a sand-mud IHS from an inner estuary tidal-bar deposit (McMur-
ray Formation, Cretaceous, Alberta, Canada). (L) Illuviated, pedogenically altered medium with
insect-generated trace fossils Naktodemasis (McMurray Formation, Cretaceous, Alberta, Canada).
(M) Cross-bedded fluvial sandstone (Centenario Formation, Cretaceous, Argentina).