Michael J. Ranger

Chapter 16 – Estuaries

Estuaries are characterized by the mixing of fluvial and marine waters and the presence of energy gradients associated with wave, tide, and river processes. This chapter explores the influence that the juxtaposition of unsteady through reduced salinities and variable energy distributions has on the distribution and composition of trace fossils in estuary settings. Due to the wide physiographic range of estuary occurrences, a simple summary model for the identification of estuaries is not presented. However, some common ichnological and sedimentological elements of estuaries are explored: (1) the presence of brackish-water bioturbation, (2) the observance of bioturbated intertidal flat deposits, (3) the presence of burrowed inclined heterolithic stratification, and (4) the identification of Glossifungites-demarcated omission surfaces, which may be associated with transgressive erosion in marginal-marine settings. More generally, the distribution of food resources and brackish water—a result of tidal mixing and energy distributions—dictates the longitudinal distributions of tracemaking organisms and their biogenic structures. Considering the above ichnological characteristics, the identification of estuaries from ichnological datasets very much depends on a large number of observations from a spatially significant dataset.

Seismic Modeling of Fluvial-Estuarine Deposits in the Athabasca Oil sands using ray-tracing techniques, Steepbank River area, northeastern Alberta: DISCUSSION of C. W. Langenberg, F. J. Hein, D. Lawton and J. Cunningham

Langenberg et al. (2002) purport to identify discrete ‘channel complexes’ in outcrop exposures along the Steepbank River of northeastern Alberta, and they claim to have extrapolated these channel complexes into the nearby subsurface. They have used the resulting stratigraphic correlations as input for a seismic model constructed using ray-tracing techniques. The seismic model demonstrates the viability and utility of high resolution seismic over the shallow deposits of the Athabasca Oil Sands. Two major issues in Langenberg et al. (2002) deserve comment, however. 1. The criteria that Langenberg and his colleagues use for differentiating ‘channel complexes’ is simplistic, ill-defined and of questionable validity. Their extrapolation of these ‘channel complexes’ into the subsurface is not supported. Furthermore, the interpretation of the relationship between ‘channel complexes’ from the outcrop face is blatantly incorrect. These factors account for many inconsistencies evident in the cross-sections presented by Langenberg et al. (2002) and discussed below. 2. Langenberg and his colleagues use a stratigraphic framework that is untenable and confusing. Their proposed changes to the informal stratigraphic subdivision of the McMurray Formation are uncalled for and ill-advised at this time. The seismic modeling techniques used by Langenberg et al. (2002) do appear to support their conclusion, from a purely technical standpoint, that high-resolution seismic has evolved to become a useful tool for examining detailed stratigraphy within the McMurray Formation. However, multiple errors and inconsistencies in interpretation of the stratigraphy and sedimentology throw doubt on their depositional model. Therefore, the geology expressed in their seismic model is questionable and should not be used as a baseline example or a control for interpreting real seismic data in the surrounding area. We first address the criteria used for defining and differentiating ‘channel complexes’. Langenberg et al. (2002, p. 188) use the criteria that “Channel complexes generally contain a basal scour surface, …

Multivariate analysis of ichnofossil associations in the subsurface Bluesky Formation (Albian, Alberta, Canada)

Abstract The quantitative analysis of ichnological associations is in an early stage of development. Several studies of spatial distributions of ichnofossils have provided insight into spatial ecological interactions in modern and ancient environments. This study examines another dimension, the development of paleoecological and paleoenvironmental conditions through time. Ichnological data from diamond drill cores of 22 wells that penetrated the Bluesky Formation of northwestern Alberta were digitised. Q-mode cluster analysis distinguished 8 ichnocoenoses based on associations of ichnofossils and a few diagnostic physical structures. R-mode cluster analysis of these ichnofossil associations distinguished 3 major groups. The control provided by an existing subjective classification of the original facies intervals into trace fossil suites allowed an examination of the efficacy of several different clustering functions. Wards method utilising Euclidian distance for the similarity coefficients and the error sum of squares for the fusion transformation function appeared to be the optimum technique for this study. Two methods of Markov analysis employed the 8 ichnocoenoses as transition states. These were the Gingerich-Read row-scaling method, using Harpers binomial probabilities as a test of significance, and the Goodman iterative proportional fitting method to expected transition frequencies, using the χ2 statistic and Turks standardised residuals as a test of significance. The iterative proportional fitting technique showed significant deviation from independence at the 95% confidence limit. Both methods distinguished similar significant biofacies transitions. The resultant ichnocoenosis model for the Bluesky Formation in the study area is that of a nearshore to marginal regressive sequence following a sudden transgression over a coastal plain or swamp environment.