Reassessing the Use of “Offshore Transition”
Kerrie L. Bann1 and James A. MacEachern2
1Ichnofacies Analysis Inc., Calgary, Alberta, AB, Canada
2Earth Sciences, Simon Fraser University, Burnaby, BC, Canada
The “offshore transition” is generally understood to define a discrete subenvironment occupying a position between the fairweather-influenced distal reaches of the lower shoreface and the offshore. This terminology is applied rather loosely, in order to characterize a range of bedding types, grain sizes, primary sedimentary structures, lithological variations, ichnological suites, and bioturbation intensities. “Offshore transition” deposits, as described in the literature, are expressed as: 1) weakly to moderately bioturbated, thinly interbedded sandstone, siltstone and mudstone facies with predominantly wave- and/or storm-generated structures; or 2) thoroughly bioturbated silty and sandy mudstone to muddy sandstone facies. Careful integration of ichnological and sedimentological features, however, permits reliable identification of all subenvironments along a continuum from shelf through nondeltaic strandplain shoreface (e.g., lower offshore, upper offshore and distal lower shoreface) settings.
As such, we contend that there is no discrete depositional subenvironment along the shoreface profile that corresponds to the putative “offshore transition”. The term, if used at all, should be reserved for heterolithic, storm-influenced successions of tempestites interstratified with burrowed, offshore, mud-prone facies, where the specific position along the depositional profile is uncertain. It is a taphonomic expression of storm-affected deposits within the offshore regime, which may extend to the shelf in strongly storm-dominated settings. The term should not be employed for storm-influenced prodeltaic or distal delta-front successions, where well-established deltaic terminology already exists. Nor should it be used to characterize thoroughly bioturbated fairweather deposits, wherein ichnological suites can be utilized to accurately delineate the specific subenvironment.
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