Abstract

Some Canadian Holocene deltas that formed during and after relative-sea-level falls of over 100 m are modern examples of forced regressions and lowstand deltas. The principal controls on deltaic morphology and stratigraphy are rate of sea-level change, sediment supply (timing, type), basin physiography, and sediment dispersal processes in the receiving basin. Our observations indicate important alongstrike variability in deposition and erosion that affect facies architecture and stratal geometry. During rapid relative-sea-level fall, fluvial channels incise rapidly and do not migrate. Lateral channel migration, which would produce a widespread fluvial unconformity, begins once relative-sea-level fall has slowed and continues through the early part of relative-sea-level rise. Parts of the emerging shoreline cut off from the fluvial sediment supply become sites of shoreface incision into prodelta deposits (regressive marine erosion), and wave-cut terraces are developed. Where sandy sediment is then supplied to the shoreface, by rivers or updrift sources, sharp-based littoral sandbodies directly overlie the shoreface erosion surface. Other parts of the deltas below wave base that become abandoned through lobe switching can also be eroded by tidal currents. Deltas that have prograded into deep water show evidence of submarine slope instability, such as submarine channel development, base-of-slope turbidites, and thick- and thin-skinned failures that disrupt clinoforms. Isopach trends reflect the existence of underlying structure, with Holocene sediments locally either thinning or pinching over bedrock highs.

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