Ichnofacies Distributions in Wave-Dominated, Mesotidal Estuaries: Variations in Ichnological Patterns Along a Depositional Continuum

Murray K. Gingras¹ and James A. MacEachern^2
1 University of Alberta, Edmonton, AB
² Simon Fraser University, Burnaby, BC

Ancient estuaries are normally classified as wave- or tide-dominated, based on facies characteristics and distributions. Brackish bays and estuaries, however, also occupy wave-dominated coasts that endure mesotidal conditions. Differentiating such systems from sand-rich, microtidal deposits can be exceedingly challenging.

A series of modern estuarine settings were analyzed with respect to their physico-chemical conditions and infaunal colonization patterns. Modern wave-dominated estuaries occupying a spectrum of upper microtidal through mesotidal coastal systems (e.g., Willapa Bay, Tillamook Bay, Ogeechee River-Ossabaw Sound Estuary) have been compared to thoroughly mixed macrotidal systems (e.g., Gulf of California, the Bay of Fundy). The distributions of trace-making infauna within inner, middle and outer estuary positions have been assessed, and their structures assigned to the relevant ichnogenera.

Integrated ichnological-sedimentological analyses of marginal-marine settings demonstrate that ichnofacies are excellent geographical indicators. Trace-fossil distributions appear largely constrained by the prevalent degree(s) of tidal mixing. Thus, general tidal ranges might be inferred from the overall ichnological expressions of a deposit. Ichnological characteristics of mesotidal estuarine settings include: (1) relatively homogeneous distributions of ichnogenera; (2) preponderance of traces constructed by comparatively mobile infauna (e.g., Skolithos, Planolites, Palaeophycus, Thalassinoides, and Siphonichnus); (3) broad distributions of opportunistic structures (e.g., Arenicolites and Skolithos); (4) intensely bioturbated middle tidal flats with less burrowed upper and lower flats; and (5) sporadically burrowed channel-bottoms.

These are discernible from characteristics of microtidal systems (i.e., higher bioturbation intensities, marked differences in trace-fossil distributions), and modern macrotidal settings (i.e., low diversity subtidal assemblages, low population densities, dramatic increases in fine-grained substrate bioturbation).

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005