Ichnology Research Group, Department of Earth and
Atmospheric Sciences, University of Alberta, Edmonton, Alberta,
Canada.
Abstract: Lateral
Variability of Ichnofacies in Point-bar Deposits
at Willapa Bay, Washington
One of the important hydrocarbon plays over the past several
years has been the estuarine-filled incised fluvial system (IFS).
IFS commonly contain reservoir-quality sedimentary accumulations
which are stratigraphically sealed by fine grained, brackish
deposits. Key to the recognition of these estuarine facies has been
the characterization of brackish trace fossil assemblages. As the
IFS model is further developed, it is becoming apparent that the
standard tripartite model, which is typically applied in the
subsurface, lacks the sufficient detail to explain facies
distribution in these systems. Enhanced resolution
of
lateral
facies variability is required to fully understand the
palaeogeography and potential reservoir compartmentalization in
fossil IFS. These issues can be addressed by better understanding
the controls on, and the distribution and manifestation of
burrowing in modern estuaries.
Willapa Bay, a mesotidal estuary on the southwest coast of
Washington, provides a modern laboratory in which a better
understanding of trace distribution may be evolved. Studies along
intertidal and subtidal point bars from the lower estuary into the
upper estuary (primarily along the Palix River) at Willapa Bay,
indicate the burrowing fauna are sensitive to two primary controls,
salinity stress and substrate texture. The affects of these
controlling parameters overprint each other and are locally
difficult to separate. Clarification of this relationship can,
however, be applied to the subsurface for better resolution
of IFS
deposits in the subsurface.
For instance, striking textural controls are evident when comparing trace assemblages between muddy and sandy substrates. Muddy substrates, exhibit a low-diversity assemblage of trace-makers including the Heteromastus, Corophium, Neries, diminutive Mya, and Hemigrapsus. The resulting ichnocoenose is strongly tiered and consists of shallow Arenicolites, moderate depth (appx, 10-20 cm) siphonate passages, and deep-penetrating (upto 28 cm) Skolithos and Gyrolithes. Sandy to muddy sand point bar deposits exhibit a more diverse assemblage of trace-makers, such as Callianassa, Neries, Nephtys, and various threadworms in moderate abundances. Tube-dwelling maldanids are present in subtidal zones, especially where a significant amount of interstitial mud is present. Locally, robust Mya and Macoma sp. are present. The consequent trace assemblage is variable and shows distinct zonation down to the channel thalweg. Traces include Ophiomorpha-, Skolithos-, Gyrolithes-, Monocraterion-, Siphonites-, Rosselia-, Planolites-, and Palaeophycus-like burrows. Some of the traces are relatively large (4-7 mm diameter), particularly Maldanid and Terebellid tubes, and Callianassa burrows.
Salinity stress is typically recognized by an overall reduction of size of the trace-maker. Certain trace-makers, such as the decapods Upogebia p. and Callianassa c., show only limited reduction in body size. These hardy crustaceans typically have sharply demarcated population boundaries, up-estuary; this implies an ethological or physiological barrier controls their distribution.
Combining the aforementioned observations provides a key to better-understanding subsurface IFS. Other stresses, such as seasonal fluctuations of oxygen, turbidity, and temperature, must be considered if a comprehensive model of estuarine ichnology is to develop. These stresses, like salinity and texture, must be considered in relative isolation if such research is undertaken.
AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas