Geology Reference
In-Depth Information
Fig. 2.11.
Sources and pathways of
modern
deep-sea sediments,
deposited in tropical,
temperate and boreal (polar) zones. Note that
only the radiolarian oozes have a fossil record
starting in the Paleozoic. Coccoliths (planktonic
algae) and planktonic globigerinid foraminifera
have become essential constituents of deep-sea
sediments since the Cretaceous. The fossil
record of deep-sea carbonates formed by
pteropods (gastropods adapted to pelagic life)
is scanty and limited to the Tertiary and
Pleistocene (Buccheri et al. 1980). The earliest
records of diatoms (siliceous algae) date from
the Middle Cretaceous, but the group did not
become important as sediment builders before
the Tertiary. Modified from Hay (1974).
In contrast to modern oceanic basins, many basins
found in the geological record occur in cratonic posi-
tions and represent intra-platform or intra-shelf basins
(e.g. pelagic epeiric basins). Note that the term basin
as used by carbonate sedimentologists does not neces-
sarily imply that carbonate basins are analogous to deep
oceanic basins. Carbonate basins generally lie well
above abyssal depth, commonly less than a few hun-
dred meters below sea level.
ooze, sediments derived from shallow-marine sources
and deposited on slopes and basins, deep-sea clays and
volcanic material, and glacial-marine sediments pro-
duced by ice-rafting (Fig. 2.11).
The basic classification of deep-marine sediments
considers grain size and composition. Currently used
compositional classifications were developed by sci-
entists participating in the DSDP and ODP programs
(Berger 1974; Kennett 1982).
2.4.5.2 Sedimentation Processes
2.4.5.3 Pelagic Sedimentation
Sedimentation in the deep sea is controlled by four
major processes (Cook and Mullins 1983; Cook et al.
1983; Enos and Moore 1983; Pickering et al. 1986;
Stow 1985, 1986):
(1) settling of marine pelagic and windblown material
from the water column,
(2) bottom transport by gravity flows including turbid-
ity currents, debris flows, grain flows, and slump-
ing,
(3) redeposition of the sediment by geostrophic bot-
tom currents including contour currents, and
(4) chemical and biochemical precipitation on the sea
floor. The first process is responsible for the depo-
sition of pelagic sediments, the second one for the
formation of a broad spectrum of resediments.
Biogenic deep-sea sediments are formed by the remains
of pelagic organisms contributing to the formation of
carbonate or siliceous oozes and muds.
The term 'pelagic' means 'of the open sea' and re-
fers to marine nektonic or planktonic organisms whose
environment commonly is the open ocean. Large terrig-
enous sediment input and submarine volcanism cause
mixtures of biogenic material and terrigenous clays re-
sulting in the formation of fine-grained muddy, 'hemipe-
lagic sediments'.
The character of the biogenic sediment depends on
the supply of the biogenic material, the dissolution at
and near the sea-bottom and in the water column, the
dilution of the sediment by non-biogenic components,
and alterations during diagenesis: The supply depends
on the fertility of the pelagic organisms, which in turn
is related to the availability of nutrients. Both nutrients
and sunlight control the primary production. The dis-
Modern deep-sea deposits comprise biogenic sedi-
ments including pelagic siliceous oozes and carbonate
