Platform slope to basin margins

Setting and geometry

• Carbonate platform slopes tend to be steeper than terrigenous platform slopes, since the CaCO3 deposits frequently show signs of syndepositional lithification.
• Carbonate slope deposits are transitional from the rapidly produced shallow-water platform carbonates to the slow rain of fine-grained pelagic sediments in the basin.
• Two types of slopes - depositional slopes (gradual slopes which merge with the basin floor) and by-pass slopes (steep slopes) (Figure 1).
• Mass-movement processes under gravity are the dominant modes of mobilization, transport, and deposition in the slope environment (Figure 2).
• Sediment input on carbonate slopes is usually along a line source rather than the point source of a canyon typical of terrigenous slopes. The debris apron has an overall wedge shape as opposed to the cone, which results from the channel-fan system.

Sedimentary/biogenic structures

• Sedimentary structures are associated with the different types of mass transport deposits (Figure 2).
• Biogenic structures are rare in the coarse slope deposits, but during calm episodes, when fine-grained sediment accumulates, an infauna, dominantly of worms and crustaceans, can survive, producing burrows within the sediment and feeding traces on the sea-beds.
  

Biota

• Most of this zone lies beneath the photic zone, so living algae are absent.
• Bottom dwellers are only common in stable areas of slow sedimentation. These locations are the sites of possible seabed lithification, so sessile epilithic organisms such as sponges and deep-water corals are found in scattered communities.
• Thickets of corals can also provide habitat for crustaceans, crinoids, bivalves, brachiopods, echinoids, anemones, etc.
  

Origin of slope deposits

1) Platform edge accumulations

• Reefal sediments, ooids and muds, washed off the platform

2) Resedimented slope deposits

• Rockfall, slide, and gravity-flow deposits (Figure 3)
• Carbonate turbidites - in some carbonate turbidite sequences the D and E divisions are omitted (Figure 4).
  

3) Pelagic rain falling from suspension

• Peri-platform ooze - mixture of storm-generated shelf-derived mud and pelagic mud, mostly aragonite
• Pelagic ooze - contain planktonic foraminiferans, pteropods, juvenile planktonic bivalves, and coccoliths, mostly calcite.
• Increases in the abundance of mixed-composition peri-platform ooze deposits in an interbedded succession with normal pelagic oozes may represent cycles in the flooding of the platform.

Basins

Setting and geometry

• Basinal limestones are deposited in water depths of 200m to 4500m, well removed from terrigenous supply.
• Deposits are usually flat and laminated
• Accumulation rates are very slow - 10-30 mm per 1000 years
  

Two type of carbonate sediment

1) Oozes

• Calcareous oozes produce blanket deposits of well-bedding chalks (firmly consolidated oozes) and basinal limestones (well-lithified oozes).
• Lithification increases with age and depth of burial.
• Oozes form today in parts of the ocean where the plankton thrives in abundance and where the seabed lies above the carbonate compensation depth (CCD) (Figure 5). Other controls are nutrient supply, water temperature, light, and salinity. These controls are dependent on climate and ocean circulation.
  

2) Platform derived sediments

• Platform sediments can spread many kilometers on to the basin floor, but are most prolific in linear prisms at the foot of the platform slope or as cones and fans of sediment at canyon mouths.
• These sediments are often dark colored (from terrigenous muds) and contain fine laminations.

Sedimentary/biogenic structures

• Large-scale sedimentary structures are rare in pelagic deposits, though locally large channels, slumps and mounds are recorded.
• Small scale structures - rhythmic bedding marked by alternations of clay-rich and clay-poor sediment, or alternations between pelagic layers and carbonate turbidites.
• Deep-sea currents can also produce ripples and winnowing sea-floor sands.
• Poor circulation - fine laminations, minimal biogenic activity, deposits are dark and organic rich.
• Strong circulation - light in color, more biogenic structures, fewer preserved sedimentary structures.

Biota

• Benthic macro-organisms found in well-oxygenated deep-sea deposits include echinoderms, sponges, crustaceans, bryozoans, and deep-sea corals.
• Poorly oxygenated areas contain mainly trace fossils
• And of course, calcareous planktonic organisms

References:

• Broecker, W.S. 1974. Chemical Oceanography. Harcourt Brace Jovanovich, New York.
• Cook, H.E. and Mullins, H.T. 1983. Basin margin environment. In Scholle, P.A., Bebout, D.G. and Moore, C.H. (eds.) Carbonate Depositional Environments. Am. Ass. Petrol. Geol. Mem. 33, 539-617.
• Enos, P. and Moore, C.H. 1983. Fore-reef slope environment. In Scholle, P.A., Bebout, D.G. and Moore, C.H. (eds.) Carbonate depositional environments. Am. Ass. Petrol. Geol. Mem. 33, 507-537.
• McIlreath, I.A. and James, N.P. 1979. Carbonate Slopes. In Walker, R.G. (ed.), Facies Models, Geoscience Canada Reprint Series 1, 133-149.
• Scoffin, Terence P. 1987. An Introduction to Carbonate Sediments and Rocks. Chapman and Hall, New York.