Submarine fans and channels: Beyond the slope (continental slope or delta slope) is the deep ocean floor, at depths usually measured in 100s to 1000s of metres. Sediment that has bypassed the shelf is transported through submarine canyons and gullies by turbulent flows of mud and sand (turbidites), or debris flows that are capable of moving a much greater range of clast sizes, from pebbles to chunks of rock or dislodged sediment having dimensions in the 10s to 100s of metres. A lower sea floor gradient at the base of the slope, plus frictional forces along the sea floor and overlying water, causes these flows to decelerate. The sediment accumulates in submarine fans, that have dimensions measured in 10s to 100s of kilometres.
The earliest models of submarine fan construction and architecture in the late 60s early 70s (e.g. Walker, Normark, Mutti and Ricci Luchi), and the plethora of model variations since, are based primarily on reconstructions from the rock record, with a smattering of new, actualistic observations. All these models have certain commonalities – in terms of their stratigraphic and geomorphic architecture, they contain elements of proximal to distal components of fan lobes, submarine channels, channel levees and overbank, and dislocation of slope, fan or channel sediment packages by slumping and sliding. Sediment dispersal is generally attributed to turbulent flows (turbidity currents), debris flows (ranging from highly fluid to plastic), and grain flows (less common), against a background of normal oceanic traction currents and pelagic-hemipelagic sedimentation. I have tried to illustrate as many of these attributes as possible in the images that follow.
Ancient submarine fan deposits illustrated here include: the Lower Miocene Waitemata Basin near Auckland, New Zealand; the Paleocene of Point San Pedro, Upper Cretaceous Pigeon Point successions, all in California; and Paleoproterozoic examples from Belcher Islands (about 1800-1900 Ma).
Waitemata Basin submarine fans
Typical exposure of Waitemata Basin strata around Auckland coastal cliffs – mid- and distal-fan turbidites at Takapuna Beach.Typical exposure of Waitemata Basin strata around Auckland coastal cliffs – thick, proximal submarine fan-channel capped by thinning-upward overbank facies, north end of Goat Island Marine Reserve.Waitemata Basin turbidites near the base of the succession, folded by compaction over paleotopographic highs on Jurassic-Permian metagreywacke basement. Omana Beach, south AucklandThick, proximal to mid-fan turbidites and possible channel overbank, Waitemata Basin, Goat Island Marine Reserve.The thicker, upper unit is is a laminated Tb Bouma interval with mudstone rip-up clasts, and a partly eroded-disrupted Td interval at the top – traced laterally this unit becomes composite. The thick mudstone beds are probably a combination of Td,e. Takapuna Beach, Auckland.Two turbidite flow units (above pen), the lower with well developed Bouma Tb, and the upper unit Tb-d with oversteepened and convoluted ripple drift (Tc interval). There is a thin layer of rip-up mud clasts at top – an event bed following the cessation of turbulent flow. Lower Miocene Waitemata Basin, Cockle Bay, Auckland.Distribution grain-size grading in this turbidite (a complete flow unit) begins in the lowermost B interval and ends with the gradational transition from the D to E (hemipelagic) intervals. Both basal and top contacts are scoured. From the Lower Miocene Waitemata Basin, Auckland, Aotearoa New Zealand.A thin Bouma Tb layer (at the coin) is overlain by a thin, rippled Tc (just above the coin), that subsequently was eroded by a thin, but coarse-grained sandy flow that ripped up local mudstone slabs and wafers. The middle grey mudstone is mostly Te (hemipelagic) with small bottom-current ripples redistributing sand across a thin layer. Waitemata Basin, Cockle Bay, south Auckland.Convoluted siltstone-fine sandstone, truncated by the next flow unit, in which there is a thin, gritty Ta interval. Waitemata Basin, Cockle Bay, south Auckland.Parallel laminated sandstone in the B division in an Early Miocene turbidite. Individual laminae range from about 5 to 10 millimetres thick. North Auckland, New Zealand. Thick, coarse-grained laminated Tb interval, Musick Point, Auckland.Thick Bouma Ta-b composites; most of the intervening, skinny Td mudstone (center) has been eroded. Waitemata Basin, Cockle BayDewatering of this turbidite (during very early burial) is indicated the concave-up dish structures, and small synsedimentary faults that terminate just above the dish structures. Waitemata Basin, Musick Point,Coalified wood fragment (outlined), intensely bored by Miocene Toredo-like marine worms, Waitemata Basin, Goat Island Marine Reserve.Very think, composite debris flows containing abundant pebbles, cobbles and boulders of basalt, and subordinate sedimentary and mafic igneous clasts. Interpreted provenance of the clasts varies between two extremes: an active, early Miocene volcanic arc on the western margin of Waitemata Basin; and more recently as debris from oceanic islands (see Shane et al, 2010, Geochemistry, Geophysics, Geosystems, open access). Left: Motuihe Island, Auckland. An iconic outcrop at Waiwera, north Auckland. Very think, composite debris flows containing abundant pebbles, cobbles and boulders of basalt, and subordinate sedimentary and mafic igneous clasts. Interpreted provenance of the clasts varies between two extremes: an active, early Miocene volcanic arc on the western margin of Waitemata Basin; and more recently as debris from oceanic islands (see Shane et al, 2010, Geochemistry, Geophysics, Geosystems, open access).Lower flow units have large rafts of locally derived, deformed mudstone. The debris flow is overlain by thick, proximal fan turbidites.Mixed matrix-supported and some clast-supported textures in thick, composite, Waitemata Basin debris flows. Note the scoured base. Waiwera (same as the left image above)Mixed matrix-supported and some clast-supported textures in Waitemata Basin debris flows. Karekare, Auckland west coast.A massive raft of columnar-jointed basalt, a remnant of either a lava flow of dyke from an oceanic island somewhere west of the basin. The weight of the block and compaction have pushed it into the underlying turbidite beds. Waitemata Basin, Army Bay, Auckland.Probably the most photographed slump fold in Waitemata Basin, Army Bay. The recumbent structure is detached from strata below along a relatively undisturbed glide plane. The lower limb is also cut by small faults.Classic slump folded turbidites, confined to a specific interval; strata above and below are relatively undeformed. Fold sandstone limbs are partly detached or pulled apart, and some mudrocks have been fluidized, Waitemata Basin, Takapuna, Auckland.Broken soft-sediment fold, enclosing partially fluidized mudrock below the central detached limb. Rheological behaviour here is brittle, plastic (ductile) and fluid. Waitemata Basin, Little Manly Beach. Isoclinal folding in thin-bedded mudstone-sandstone (left center), and a sandy turbidite bed deformed by rotated boudins (upper right). All these structures formed while the sediment was at a transition from relatively soft to weakly indurated. Waitemata Basin, Army Bay, north Auckland.Soft sediment deformation in Waitemata Basin, includes small thrusts (fault plane indicated by arrows), with folded strata in the hanging wall, and small drag folds in the footwall. Waiwera, north Auckland.Intensely folded turbidites on a horizontal, undeformed glide plane, Waitemata Basin, Orewa Beach, Auckland.
Point San Pedro, California
Paleocene turbidites, Point San Pedro, California.Successive cycles of thinning upward and thin bedded, distal fan turbidites, Point San Pedro, California. Cyclic, thinning upward interchannel, distal lobe facies, Paleocene Point San Pedro, California.Small slump package in thinly bedded distal fan facies, Point San Pedro, California. Combinations of brittle, ductile and fluid behaviour during deformation. Backpack for scale bottom right.Submarine channel sandstone overlain by thin sandy turbdites and overbank mudstone. Point San Pedro, California.Thick submarine fan channel sandstone (left) overlying fine-grained overbank facies, indicates migration of the channel thalweg. Point San Pedro, California.
Pigeon Point
Classic outcrops of pebbly mudstone – matrix-supported debris flows, that probably accumulated in proximal fan channels. Upper Cretaceous Pigeon Point, California.Classic outcrops of pebbly mudstone – matrix-supported debris flows (several flow units), that probably accumulated in proximal fan channels. Upper Cretaceous Pigeon Point, California.A variation on the debris flow theme, with well stratified conglomerate and commonly clast-supported frameworks (multiple flow units), that are inferred to have formed from more fluid flows than their pebbly mudstone counterparts. Upper Cretaceous Pigeon Point, California.A broader view of stratified, possibly surging debris flows in proximal fan channels. Upper Cretaceous Pigeon Point, California.Slump folded, and partly fluidized turbidites in Upper Cretaceous Pigeon Point, California.
Various localities, California
Thin Bouma Tb-c flow units, Pebble Beach, California. the middle unit has developed some excellent flame structures. the lower unit contains sand-filled burrows, and detached load casts.Dish structures and pillars formed by dewatering (fluid expulsion) during early burial by the overlying sandy turbidites. Rosario Group, San Diego.
A paper on the Omarolluk Formation: Ricketts, B.D. 1981: A submarine fan – distal molasse sequence of Middle Precambrian age, Belcher Islands, Hudson Bay; Bulletin Canadian Petroleum Geology, v. 29, p. 561-582.
Bedding style in the Omarolluk Fm. turbidite succession, Proterozoic, Belcher Islands (about 1800-1900 Ma). Mid fan channel sandstone and overbankBedding style in the Omarolluk Fm. turbidite succession, Proterozoic, Belcher Islands (about 1800-1900 Ma). – more proximal, thick sandstone facies (mostly Ta,b deposits), separated by thin hemipelagic shale.Channel overbank facies containing thin graded sandstone, thin sandstone beds with ripples and starved ripples, and Bouma Td-e mudstones. Omarolluk Fm. Proterozoic, Belcher IslandsFour incomplete Bouma cycles, each Tb with thin Tc. The whitish patches are very early diagenetic concretions. Omarolluk Fm. Proterozoic, Belcher Islands.Thin Bouma Tc-d mid-fan cycles, with ripple drift, flame structures and load coasts, and a small scour. Omarolluk Fm. Proterozoic, Belcher Islands.A Bouma Tb-c cycle with well developed and oversteepened ripple drift, overlain by a thicker Tb cycle with only a thin Td cap. Omarolluk Fm. Proterozoic, Belcher IslandsLaminated Bouma Td intervals and convoluted laminae (Tc). Omarolluk Fm. Proterozoic, Belcher IslandsSole structures beneath sandy turbidites – flute casts are superposed on grooves, Omarolluk Fm. Proterozoic, Belcher Islands.Sole structures beneath sandy turbidites – large flute casts are slightly deformed (block is about a metre across). Omarolluk Fm. Proterozoic, Belcher Islands.Large flute cast at the base of a sandy turbidite, paleoflow to top right. Omarolluk Fm. Proterozoic, Belcher IslandsDewatering of turbidites soon after deposition produced thin fluid-escape pillars (cross-section view below), that exited on the sea floor, now seen as small sand-mud volcanoes on bedding planes. Omarolluk Fm. Proterozoic, Belcher IslandsBedding planes exposure of small sand-mud volcanoes, formed during expulsion of water during early compaction. Omarolluk Fm. Proterozoic, Belcher IslandsViews normal to bedding of thick Bouma Tb units, and sheets of dewatering pillars formed during very early burial and compaction. Segregation of sheets through the sandstones is a function of different permeabilities between successive flow layers. Omarolluk Fm. Proterozoic, Belcher IslandsOblique views of thick, sandy Bouma Tb units, and sheets of dewatering pillars formed during very early burial and compaction. Segregation of sheets through the sandstones is a function of different permeabilities between successive flow layers. Dark globular shapes on left image, and white patches in the middle image, are early diagenetic calcite concretions (see images below). Omarolluk Fm. Proterozoic, Belcher IslandsProximal submarine channel conglomerate consisting almost entirely of reworked calcite concretions. Channel fill is overlain by thick, proximal turbidites and underlain by probable overbank facies.Detail of the channel conglomerate clasts. Elongate clasts are calcite concretions that formed in laminated and rippled Tc intervals; the ovoid and spherical concretions derived from coarser grained, calcite cemented Ta or Tb sandstones. Omarolluk Fm. Proterozoic, Belcher Islands.
