Arnott, R.W.C. Department of Geology, University of Ottawa and Ottawa-Carleton Geoscience Centre, Ottawa, Ontario, Canada, K1N 6N5, warnott@acadvm1.uottawa.ca

Combined flow generates a diverse array of unique bed forms and bed configurations, which in turn form a myriad of different stratification styles. In an exploratory investigation of combined-flow bed forms using a narrow range of flow and sediment conditions, the bed phases were: no movement; small 2D ripples; small 3D ripples; current ripples; large 3D ripples; and plane bed. The influence of a unidirectional component (Uu) when applied to a purely oscillatory flow (Uo) was to depress the Uo needed to initiate each new bed phase and to cause the bed forms to develop a downstream cross-sectional asymmetry. In addition, at low Uu a symmetrical planform was maintained. Interestingly, very weak Uu caused bed forms to migrate into the mean flow, although this trend was quickly reversed as Uu was increased.

In the geological literature combined-flow conditions are commonly discussed in association with modern and ancient wave-dominated environments, but only rarely in context with tidal environments. In tidal settings, however, waves and tidal currents commonly co-exist. In the area near Ottawa, Ontario strata of the Middle Ordovician Rockcliffe Formation consist commonly of thinly bedded, small-scale cross-stratified sandstone capped by thin mudstone laminae. Sandstone beds have wavy basal contacts. Cross-stratification is bidirectional in different sets but also within the same set (even if there has been little bed aggradation). Superficially these beds appear similar to small-scale wave-rippled sandstone. However the repetitively interlaminated mudstone and the general paucity of bioturbation (although abundant along discrete horizons) argues against a purely oscillatory origin in a shallow open-marine setting. Rather, these ripples are interpreted to represent deposition in a physically-restricted shallow-marine setting, where oscillatory energy was generally too low to initiate sediment movement but in the presence of a reversing, although low speed tidal current, sediment transport was initiated and sustained so long as Uu existed. (Rare interbeds of hummocky cross-stratified fine to medium sandstone, however, suggest that Uo was episodically much higher). In addition, the observation of opposing direction of cross-stratification within a single set suggests that Uu fluctuated and at no time was high. As a result, bed forms typically migrated in the direction of the tidal current but at times migrated in the opposite direction during the same tidal cycle -- most likely during the ~late waning part of the tidal cycle.