Characterizing
Subtle Deltaic Influences in the Shallow Marine (Lower Cretaceous) Viking
Formation at Hamilton Lake, Alberta, Canada
Dafoe, Lynn T.1, S. George
Pemberton1, James MacEachern2 (1) University of Alberta, Edmonton, AB (2) Simon Fraser University, Burnaby, BC
Ancient deltaic deposits are commonly
misinterpreted as nondeltaic (strandplain)
shoreface successions that imply deposition under
normal marine (unstressed) conditions. These misinterpretations can occur where strata possess only subtle indications of
deltaic sedimentation; a situation encountered in the Albian
Viking Formation at Hamilton Lake. Sedimentological,
ichnological, and stratigraphic
features of these Viking deposits, when carefully analyzed, exhibit subtle
evidence of physico-chemical stresses imparted by riverine influence on otherwise wave-dominated, fully
marine deposits.
In the Hamilton Lake area, the Viking
Formation comprises coarsening-upward successions, dominated by wave-formed
structures (oscillation ripple lamination and hummocky cross-stratification)
similar to shoreface deposits. Subtle sedimentological evidence of riverine
input is superimposed, however, signified by local synaeresis
cracks, soft-sediment deformation features, carbonaceous mudstone drapes, and
coaly fragments. Ichnological suites reveal low
diversities and reduced abundances of trace fossils in prodelta
deposits, characterized by moderately to subtly stressed expressions of the
archetypal Cruziana Ichnofacies.
Delta-front deposits yield low-diversity, moderately stressed suites
attributable to proximal expressions of the Cruziana
Ichnofacies. With increased numbers of tempestites, suites attributable to stressed expressions of
the Skolithos Ichnofacies
become increasingly intercalated. In general, there is a subtle suppression of
suspension-feeding structures, reduced bioturbation
intensities, and reduced ichnofossil diversities,
which are attributed to environmental stresses such as salinity fluctuations,
water turbidity, and sediment influx. The mapped distribution of the lower sand
body demonstrates a morphology consistent with both
wave and riverine influence. Integrated facies characteristics reflect the dominance of
wave-induced processes over associated riverine
influx; hence, strata are best described as a wave-influenced deltaic complex.