Marine slopes are bona fide geological settings in themselves, but from a geotectonic perspective they are the region where continental crust is transitional to oceanic crust, and where sediment bypasses the shelf as it heads towards the deep ocean floor – typically as submarine fans. Slopes, as their name suggests, have significantly greater dip than an adjacent shelf; the break between the shelf and slope is defined by this break in sea floor gradient. Slopes frequently are cut by gullies and submarine canyons; the gullies tend to be localized across the shelf-slope break, whereas canyons extend across the shelf (sometimes coming within a few 100m of the shore), to the full depth of the slope. Gullies and canyons focus sediment transfer to the ocean deep. The Black’s Beach and Point Lobos canyons were visited on an AAPG trip with Tor Nilsen; the Bowser Basin examples I worked on in the late 1980s – early 90s.
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The images:
The iconic, Eocene Pt. Lobos submarine canyon, California, where canyon-fill conglomerate (brown hues) is in abrupt contact with Salinian granodiorite (white weathering) – an example of a steep canyon wall.
Looking south, along the Pt. Lobos canyon axis. Conglomerate at the base, overlain by turbidites.
Layered Pt Lobos canyon-fill conglomerate against the blocky weathering granodiorite bedrock. California. The canyon wall is indicated by arrows.
Discordant packages of conglomerate canyon-fill, Eocene Pt. Lobos submarine canyon, California.
Interbedded canyon-fill conglomerate and turbidites, Eocene Pt. Lobos submarine canyon, California. Some of the conglomerate beds have debris flow characteristics, others may be down-canyon traction current deposits.
Local slope facies between channelised, canyon-fill conglomerate, presenting delicately laminated siltstone-mudstone, starved ripples with mud-drapes, thin graded beds (looking more like distal turbidites),soft-sediment load structures, and a few sand-filled burrows. Eocene Pt. Lobos submarine canyon, California.
Slump discordant packages of interchannel, thin-graded fine-grained sandstone, Eocene Pt. Lobos submarine canyon, California
A muddy debris flow consisting almost entirely of slope facies mudstone rip-ups, plus a few pebbles, overlain by clast-supported, canyon-fill conglomerate. Eocene Pt. Lobos submarine canyon, California
Black’s Beach, iconic coastal cliffs that reveal sediment gravity flow deposits (mainly turbidites and debris flows), and the remnants of an Eocene submarine canyon. This view is north of Scripps Pier, California.
Pebble-lined canyon floor at Black’s Beach, cutting into estuarine and other paralic facies (root structures and burrows are common). Eocene, California
Basal conglomerate filling the canyon floor, Black’s Beach, California
Typical channel conglomerates eroding into thick (proximal) turbidites and thinner channel overbank facies, Black’s Beach submarine canyon. Signs at the beach entrance warn of rock falls, house collapses, and other exposures.
A paper on this topic: Ricketts, B.D. and Evenchick, C.A. 1999. Shelfbreak gullies; Products of sea-level lowstand and sediment failure: Examples from Bowser Basin, Northern British Columbia. Journal of Sedimentary Research, v. 69, p. 1232-1240.
Slope facies, here consisting of relatively undisturbed thin, graded, very-fine grained sandstone-mudstone (thin turbidites), and a few small starved ripples in the laminated mudstone-shale. Bowser Basin, British Columbia.
Thin graded sandstone beds, starved ripples, laminated sandy mudstone, small slump folds, syn-sedimentary pull-aparts or boudinage, and microfaults, all features that are typical of slope facies mudrocks. Bowser Basin, British Columbia.
Left: laminated mudstone-siltstone and a few thin graded sandstone beds. Slope facies, Bowser Basin, British Columbia. Right: stratigraphic discordances occur at all scales in the Bowser Basin slope deposits. Many are caused by slumping, but discordant mudrock packages also arose from flows spilling over the channel-gully margins.
Two views of the Todagin canyon base, and bedded conglomerate-fill, most of which was deposited by debris flows, sometimes separated by thin turbidites. Each view shows about 25m of section. Bowser Basin, British Columbia.
Contrasting debris flow textures. Left: mud-supported clasts in a more plastic debris flow. Right: clast-supported frameworks that probably formed in a more fluid, sheared debris flow. Both types are common in the Todagin canyon succession, and in the gullies. Bowser Basin, British Columbia.