Abstract Elizabeth Reef is one of the southernmost coral reefs in the world. Located at approximately 30° south latitude in the Tasman Sea, this atoll is situated near the southern limit of environmental tolerance of reef organisms. The atoll has a well developed outer reef flat, encrusted with calcareous red algae, and a lagoon consisting of patch reefs, sand flats, and a central mesh-reef complex. A relatively small number of coral species occur, and significant coral growth is restricted to the lagoon. Offshore, there is a well developed erosional spur and grove zone (0 to 8 m; 0 to 26 ft), a deeper buttress zone (9 to 30 m; 29 to 98 ft) and carbonate sand flats (30 to 40 m; 98 to 131 ft) before the steep seaward drop-off. Detailed evaluation of eleven rotary cores from the Elizabeth Reef flat indicates that a period of more active coral growth existed in the past. C age dates on scleractinian corals from the cores indicate a maximum age of about 7000 years BP in the upper 3 to 4 m (10 to 13 ft) of the reef flat, with average accumulation rates of approximately 90 cm per 1000 years. The atoll has reached equilibrium with sea level, and it is now influenced more by erosion than growth.

ABSTRACT Heads of submarine canyons may occur anywhere on continental margins, from river mouths to continental slopes, producing a distinctive interface between shallow- and deep-marine environments. Inception of most canyons is subaerial, fluvially cut during lowered sealevel. Submarine mass flow also commences canyon formation. Submarine erosion shapes all canyons, and is especially effective in the headward region. Sliding and slumping are volumetrically most important as erosive agents, but sand spillover, bioerosion, sand flow, sand creep, and debris flow all play a part. Fluctuating channelized currents and low-velocity turbidity currents also erode and transport sediments. Canyons alter shelfbreak circulation and sedimentation. They remove detritus from fluvial outflow, longshore transport, and cross-shelf drift, and may influence the position of rip currents. On narrow shelves, surface waves diverge over canyon heads, providing a transport corridor for the return of turbid water. Suspensates downwell along canyons as high-density nepheloid layers. Channelized currents winnow fines in upper canyon heads; focused internal tides and waves may actually break, producing more extensive erosion. Although research on modern canyon systems has rapidly increased, detailed studies of ancient canyons remain sparse. An Eocene example from Southern California contains a tripartite fill representing progressive detachment from a nearshore source during a eustatic sealevel rise. Suspensate fallout, tractional flow, and mass-flow processes formed a basal amalgamated pebbly sandstone overlain by planar- to convolute-laminated sandstone, topped by variegated cut-and-fill mudstone channels. This tributary system fed the main canyon, filled with fining- and thinning- upward complexes. The Pigeon Point and Carmelo Formations of coastal California and Tethyan submarine canyons of Czechoslovakia display similar fining-upward canyon fills. Contrasting Fill sequences include coarse-grained units that dominate French Maritime Alps and New Zealand canyon complexes, and shales that plug canyons in the Gulf Coast, Sacramento Valley, and Israel. Shelf size and gradient, rates of eustacy, tectonism, and subsidence, and sedimentary-source input and migration interact to create this diversity of fills in ancient submarine canyons. Quantified analyses of canyon formation, maintenance, and Fill, and application of sedimentary hydrodynamics to observed mass transport processes and their resultant ancient counterparts, are still needed.