Geology Reference
In-Depth Information
Box 2.6.
Glossary of terms used to describe deep-water carbonates
Abyssal plains:
Area of deep sea bottom below 2000 m and between 4000 and 6000 m, including the nearly flat deep-
sea plains covered by sediment.
Accretionary slope
: A depositional slope, less steep than by-pass slopes. Characterized by net accumulation of sediments.
Aragonite compensation depth (ACD)
:
Level in the oceans where aragonite is dissolved. The bathymetrical position
of the ACD and the CCD depends on the fertility of the surface waters and the degree of undersaturation of deep
waters.
Aseismic volcanic ridges
: Volcanic seamounts, ridges and plateau, rising several kilometers above the deep ocean floor.
Baseofslope
:
The lower or lowermost part of slopes, continental rise.
Bathyal:
Sea bottom between about 200 (continental shelf edge, shelf break) and 2000 (4000) m, encompassing the
continental slope and continental rise, down to the deep sea..
Bypass slope:
Slopes on which sediment is transported from shallow to deeper water without significant deposition on
the slope.
Calcite compensation depth (CCD)
: The level in the deep oceans where the rate of dissolution of calcium carbonate
(calcite) balances the rate of deposition and below which carbonate-free sediments accumulate. The level is
characterized by a transition from carbonate ooze to deep-marine clay or siliceous ooze. Today, the CCD has a mean
depth of about 4.5 km, but varies between ocean basins. Note that large shifts of up to several thousand of meters of
the CCD have occurred during the Meso- and Cenozoic.
Chalk
: Fine-grained porous carbonate rock, predominantly composed of pelagic nanno- and microfossils, and deposited
on deeper shelves and basins.
Continental rise
: Setting at the foot of the continental slope between the slope and the deep ocean basin, characterized
by a gently sloping surface created by coalescing submarine fans. Sedimentation takes place by downward sediment
transport and deposition at the slope base. The continental rise may be subdivided by submarine canyons and deep-
sea channels.
Contourite
: Sediment deposited on the continental rise by contour-following bottom currents.
Debris flow
: A sediment gravity flow. Sediment transport by a cohesive matrix. Corresponds to mudflow.
Erosional slope
:
A steep slope. Net sediment loss. Erosion due to rockfalls, sediment gravity flows and the effect of
contour currents.
Grain flow
: A sediment gravity flow. Grains are supported during the sediment transport by dispersive pressure arising
from grain collisions.
Gravity flows
:
Downslope flow of sediment. Comprises turbidites, grains flows, and debris flows.
Guyot
:
A type of seamount with a platform top.
Hadal
:
Deepest area of sea bottom, lying specifically within the deep-sea trenches below the general level of the abyssal
zone, between 6000 and > 10,000 m.
Hemipelagic sediments
: Fine-grained muddy sediment consisting of a mixture of calcareous biogenic material and
terrigenous clay. More than 25% of the fraction coarser than 5
m is of terrigenous or volcanic origin. The sediment
has undergone lateral transport. Deposited near continental margins, on slopes, abyssal plains, and ridges.
Lysocline
: The level or ocean depth at which the rate of dissolution of calcium carbonate just exceeds its combined rate
of deposition and precipitation. The lysocline is characterized by a noticeable decrease in the percentage of deep-
marine carbonate sediments and by an interval separating well-preserved from poorly preserved microfossil
assemblages. The calcite lysocline is defined by planktonic foraminifera and calcareous nannofossils, the aragonite
lysocline by pteropods.
Oceanic basin
:
Comprises various deep-sea settings including abyssal plains, volcanic hills, mid-oceanic ridges and
deep-sea trenches.
Oceanic plateau
:
Topographically high area of the ocean floor that rises to within 2-3 km of the sea floor surface above
the abyssal floor. Many Pacific plateaus are covered by carbonate ooze overlying volcanic rocks.
Pelagic carbonates
: Fine-grained calcareous sediments of the open sea, composed chiefly of the skeletal debris of
calcareous planktonic organisms, and deposited in a range of deep-water settings (slopes, ocean floor, seamounts,
mid-oceanic ridges).
Periplatform sediments
: Intermixed platform-derived sediments and pelagic carbonate muds (Schlager and James
1978).
Resedimented carbonates
: Carbonate rocks characterized by redeposition of shallow-water carbonate sediment into
deep water. Submarine mass transport and redeposition is related to a variety of processes, including rockfalls, slides,
slumps, and gravity flows. Resedimented carbonates may be additionally reworked by deep-marine bottom currents.
Rockfalls
: Single blocks or many clasts derived from steep slopes of the shelf margins and deposited in talus piles at the
base of slopes or as blocks on the slopes. Abundant along fault-bounded platform margins and along fore-reef slopes.
Seamount
: Elevation of the sea floor, 1000 m or higher, flat-topped (Guyot) or peaked.
Slope
: Surface which connects the shelf or the shelf break with the continental rise or the deep-sea floor.
Turbidite
:
A sediment laid down by a turbidity current. Characterized by graded bedding, moderate sorting and a
vertical sequence of distinct sedimentary structures. Turbidity currents are initiated by the disturbance of the sediment
near the shelf margin or on the slope; deposit originally shallow-water sediments on the slope, at the foot of the slope
and on abyssal plains. Limestone turbidites are called 'calciturbidites' or 'allodapic limestones'.
