9.1 Weathering and Suspended River Loads
Later in the Earth's history, as a result of successive cycles of tectonic uplift, mountain building,
weathering and renewed sedimentation, sedimentary
and metamorphic rocks became increasingly important at the land surface and thus as a source of younger sediments (cf. Chap. 11.7). The composition of
successive sediment cycles therefore tended to become richer in stable secondary minerals (increase in
mineralogical maturity) through geologic time at the
expense of primary minerals (cf. Chaps. 6.5 and
11. 7).
However, there are many exceptions to this general rule.
For example, first-cyc1e sedimentary rocks (mainly sands
and gravel, containing a large proportion of unaltered primary minerals) are known from al1 geological epochs, not
only from the Precambrian. Volcanism and magmatism in
conjunction with mountain building provided new igneous
source rocks and relief, i.e. weathering-limited conditions,
for the formation of first-cycle sediments. On the other
hand, even first-cyc1e sediments may have already gained a
considerable proportion of stable neo-minerals if humid
c1imate and low relief al10wed intense pedogenesis.
Sorting processes during sediment transport may
have led to fine-grained deposits which are difficult
to discriminate from sediments derived from older
sedimentary rocks. Silty claystones and mudstones
are therefore less useful than coarser-grained strata
for evaluating sediment recycling. It appears that
only high relief areas or arid zones can significantly
contribute with fresh rock material and minerals to
the composition of successive sediments. Although
gravel and coarse sand are partially worn down during transport, the resulting silt-size particles may still
indicate the types of source rock. Older sedimentary
and metamorphic rocks can be identified by relics of
their cements or specific minerals. In subsequent cycles of erosion and deposition these particular signals
will become less distinctive.
Generally, it is assumed that the sediments of successive cycles of erosion and deposition become increasingly mature both mineralogically and texturally
(i.e. in their grain-size distribution).
9.1.4 Detrital Clay Minerals in Lake Basins and
in the Sea
General Aspects
Clay mineral associations of lakes with relatively
small drainage areas and a limited number of different rock types usually reflect the rock types and climate in their source area. Here, the general roles discussed in the previous Section can be applied. Marine clay mineral associations, derived from major
rivers and affected by changing current systems, are
more difficult to interpret. In addition, the transition
423
zone from fresh to sea water and the flocculation of
clay minerals lead to some sorting of the incoming
suspended river load (cf. Chap. 3.4). In wide areas of
the oceans eolian dust also plays a significant role
(Chaps. 2.3 and 5.1).
Fine-grained marine sediments on prodelta slopes
and in nearby regions usually exhibit clay mineral
suites similar to those of the entering rivers.
The c1ay mineral eomposition of the sediments in the Gulf
of Mexieo is about the same as that of the Mississippi
River at its mouth (Potter et al. 1975; cf. Chap. 11.3). The
sediments off the Niger delta (cf. Chaps. 3.4.2 and 14.4)
are rieh in kaolinite delivered by the Niger River. However,
the kaolinite eontent in the sediment deereases seaward due
to the specifie transport and settling behavior of the various
elay minerals (Chap. 3.4.2). The eastern Mediterranean
aceumulates sediments relatively rieh in montmoril1onite,
whieh for the most part is supplied by the Nile River. A
large portion of the suspended load in the Amazon River
(predominantly montmoril1onite and kaolinite) is transported to the northwest and deposited in the eoastal region
as mudflats (Weaver 1989).
The clay mineral distribution in modem ocean sediments reflects to some extent the global climatic
zones. The largest amounts of kaolinite are found in
low latitudes, where rivers from humid tropical regions enter the sea. The kaolinite content tends to
decrease towards the poles and from coastal waters
to the deep sea. Detrital chlorite is most abundant in
the polar and subpolar regions, where chemical
weathering is limited. Illite and smectite (montmorillonite) are the most important clay minerals in modem ocean sediments. Illite-rich sediments are found
in temperate to cold climatic zones, whereas smectite
is abundant in regions of extensive volcanic activity
both below the sea and on land.
Paleoceanography and Paleoclimate from
Clay Minerals
U sing the results from modem sediments, several
authors have tried to determine the regional or global
climate of the past (paleoclimate). For example, the
clay mineral composition of early Eocene finegrained sediments in the Atlantic ocean significantly
differs from the present situation (Robert and
Chamley 1991). The smectite content was found to
be generally higher at all locations investigated and
kaolinite is more abundant in regions of high latitudes than at present. Both findings (among other
evidence) indicate that the climate during the early
Eocene was globally warmer and in part seasonally
wet and dry.
Clay mineral associations mayaiso indicate the
predominance of certain source areas and the current
systems distributing them in the ocean. During the
Later in the Earth's history, as a result of successive cycles of tectonic uplift, mountain building,
weathering and renewed sedimentation, sedimentary
and metamorphic rocks became increasingly important at the land surface and thus as a source of younger sediments (cf. Chap. 11.7). The composition of
successive sediment cycles therefore tended to become richer in stable secondary minerals (increase in
mineralogical maturity) through geologic time at the
expense of primary minerals (cf. Chaps. 6.5 and
11. 7).
However, there are many exceptions to this general rule.
For example, first-cyc1e sedimentary rocks (mainly sands
and gravel, containing a large proportion of unaltered primary minerals) are known from al1 geological epochs, not
only from the Precambrian. Volcanism and magmatism in
conjunction with mountain building provided new igneous
source rocks and relief, i.e. weathering-limited conditions,
for the formation of first-cycle sediments. On the other
hand, even first-cyc1e sediments may have already gained a
considerable proportion of stable neo-minerals if humid
c1imate and low relief al10wed intense pedogenesis.
Sorting processes during sediment transport may
have led to fine-grained deposits which are difficult
to discriminate from sediments derived from older
sedimentary rocks. Silty claystones and mudstones
are therefore less useful than coarser-grained strata
for evaluating sediment recycling. It appears that
only high relief areas or arid zones can significantly
contribute with fresh rock material and minerals to
the composition of successive sediments. Although
gravel and coarse sand are partially worn down during transport, the resulting silt-size particles may still
indicate the types of source rock. Older sedimentary
and metamorphic rocks can be identified by relics of
their cements or specific minerals. In subsequent cycles of erosion and deposition these particular signals
will become less distinctive.
Generally, it is assumed that the sediments of successive cycles of erosion and deposition become increasingly mature both mineralogically and texturally
(i.e. in their grain-size distribution).
9.1.4 Detrital Clay Minerals in Lake Basins and
in the Sea
General Aspects
Clay mineral associations of lakes with relatively
small drainage areas and a limited number of different rock types usually reflect the rock types and climate in their source area. Here, the general roles discussed in the previous Section can be applied. Marine clay mineral associations, derived from major
rivers and affected by changing current systems, are
more difficult to interpret. In addition, the transition
423
zone from fresh to sea water and the flocculation of
clay minerals lead to some sorting of the incoming
suspended river load (cf. Chap. 3.4). In wide areas of
the oceans eolian dust also plays a significant role
(Chaps. 2.3 and 5.1).
Fine-grained marine sediments on prodelta slopes
and in nearby regions usually exhibit clay mineral
suites similar to those of the entering rivers.
The c1ay mineral eomposition of the sediments in the Gulf
of Mexieo is about the same as that of the Mississippi
River at its mouth (Potter et al. 1975; cf. Chap. 11.3). The
sediments off the Niger delta (cf. Chaps. 3.4.2 and 14.4)
are rieh in kaolinite delivered by the Niger River. However,
the kaolinite eontent in the sediment deereases seaward due
to the specifie transport and settling behavior of the various
elay minerals (Chap. 3.4.2). The eastern Mediterranean
aceumulates sediments relatively rieh in montmoril1onite,
whieh for the most part is supplied by the Nile River. A
large portion of the suspended load in the Amazon River
(predominantly montmoril1onite and kaolinite) is transported to the northwest and deposited in the eoastal region
as mudflats (Weaver 1989).
The clay mineral distribution in modem ocean sediments reflects to some extent the global climatic
zones. The largest amounts of kaolinite are found in
low latitudes, where rivers from humid tropical regions enter the sea. The kaolinite content tends to
decrease towards the poles and from coastal waters
to the deep sea. Detrital chlorite is most abundant in
the polar and subpolar regions, where chemical
weathering is limited. Illite and smectite (montmorillonite) are the most important clay minerals in modem ocean sediments. Illite-rich sediments are found
in temperate to cold climatic zones, whereas smectite
is abundant in regions of extensive volcanic activity
both below the sea and on land.
Paleoceanography and Paleoclimate from
Clay Minerals
U sing the results from modem sediments, several
authors have tried to determine the regional or global
climate of the past (paleoclimate). For example, the
clay mineral composition of early Eocene finegrained sediments in the Atlantic ocean significantly
differs from the present situation (Robert and
Chamley 1991). The smectite content was found to
be generally higher at all locations investigated and
kaolinite is more abundant in regions of high latitudes than at present. Both findings (among other
evidence) indicate that the climate during the early
Eocene was globally warmer and in part seasonally
wet and dry.
Clay mineral associations mayaiso indicate the
predominance of certain source areas and the current
systems distributing them in the ocean. During the