Geoscience Reference
In-Depth Information
parts of plankton. Carbonate mud is three to four times
more abundant and is referred to as an
ooze
if over 30 per
cent is derived from diatoms and coccoliths (marine
micro-organisms). Silicate solutions are less depth- and
temperature-sensitive than carbonates, so precipitated
silicates are found in deeper and colder ocean areas.
Carbonate rocks are associated with tropical/warm-
temperate oceans today, with a contrasting silicate mud
belt in the southern (Antarctic) ocean (
Figure 12.18
).
Silurian, Carboniferous and Jurassic limestones and
Cretaceous chalk represent former Laurasian and Tethys
carbonate oceans in the geological record. Hydrothermal
circulation adds a suite of
hydrogenic
sea-bed sediments
which become buried by younger sediments as they rift
away from their mid-ocean ridge source. Manganese (Mn)
nodules are widespread and, additionally, of increasing
resource potential for Fe, Cu, Co and Ni.
This relatively simple arrangement becomes compli-
cated by intercalation or lateral intergrowth of different
sediment bodies through sea-level change,
Marine sedimentation
Marine sediments vary according to their composition,
source of materials and location in either the offshore
slope system or the abyssal plain. Terrigenous sources of
eroded and transported continental erosion products are
mainly minerogenic in nature, and their abundance and
calibre
(particle size) diminish seawards. They are mostly
more stable minerals like quartz, potassium feldspars and
biotite flushed with dissolved minerals by rivers on to the
continental shelf at global annual rates of 1·8
10
13
kg
solids and 4
10
12
kg of solutes, or derived from marine
erosion of the coast itself. Heavy minerals such as gold
(Au), copper (Cu) and tin (Sn) settle out as
placer
deposits
among the shelf sediments. Biogenic rain-out of dead
marine organisms and their chemical derivatives increases
in importance away from the shore. They are the
predominant source of abyssal sediments, flooring 62 per
cent of deep-ocean basins as
calcareous
(calcite/aragonite)
and
siliceous
(silicate) muds derived from the skeletal
episodic
LONGITUDE
60°
120°
180°
120°
60°
0°
80°
80°
60°
60°
40°
40°
20°
20°
0°
0°
20°
20°
40°
40°
60°
60°
60°
120°
180°
120°
60°
0°
Calcareous sediments
Glacial/glaciomarine sediments
Deep-sea clay
Siliceous sediments
Terrigenous sediments
Continental-margin sediments
Source: After Davies and Gorsline (1986)
