62
can no longer remain in suspension. The deposition
of dust is accelerated by the presence of vegetation
which creates surface roughness and thus impedes air
flow and subsequent resuspension. Settling dust often accumulates on the downwind side of hills and
other topographie highs, or in depressions; moist surfaces trap the dust grains.
Measured present-day rates of dust deposition on
land range between 10 to about 200 g/m 2 per year,
corresponding to sedimentation rates of approximately 0.5 to 10 cmlka. Locally and discontinuously,
the sedimentation rate of dust must have been higher
in the past, for example for Pleistocene loess with
rates up to several tens of cmlka (see below). The
contribution of dust to oceanic sediments is less
known, although this topic has been studied recently
by several workers (cf. Chap. 5). Dust deposition
rates in the ocean reported in the literature vary between 0.1 and more than 10 g/m 2 per year. Taking
into account the very 10w sedimentation rates of
deep-sea sediments (in large areas ca. 1 cmlka, cf.
Sects. 5.3 and 10.2), one can draw the conclusion
that eolian dust should make up a significant proportion of marine sediments in regions, where offshore
winds from deserts reach the open ocean.
The carbonate-free fraction of early Cretaceous to late Miocene pelagic to hemipelagic sediments in the North Atlantic consists predominantly of eolian silt and c1ay (Lever
and McCave 1983). In the eastem Atlantic, northwestem
Pacific, and northem Indian ocean, eolian dust may comprise more than 50% of the total modem sediment. These
sediments mainly consist of fine-grained quartz and clay
minerals characteristic of their source areas. During the last
8000 years, the dust flux into the Arabian Sea reached
nearly the same volume as the suspended load of the Indus
river (Sirocko and Samthein 1989). It appears that dust
deposition in the oceans increased in the Neogene in conjunction with a long-term climatic change.
Pleistocene Loess
Unconsolidated silty dust sediments of Pleistocene or
somewhat older age, which are deposited on land, are
termed loess. Unweathered and non-redeposited
loess is homogeneous, mostly buff or yellowish in
color, non- or weakly stratified, and highly porous. It
consists predominantly of quartz, feldspar, mica, clay
minerals, and carbonate grains in varying proportions. The principle grain size of loess ranges from
20 to 40 Jlm, but loess can also contain minor proportions offine sand ( 63 Jlm) and clay ( 4 Jlm). Loess
covers wide areas on the present land surface, is agriculturally important, and frequently used for dating
Pleistocene processes and sediments.
At least two principally different source areas of
loess are distinguished. In Europe and North America, most loess is derived from glaciated areas and
accumulated under cold and relatively dry conditions
Chapter 2 Continental Sediments
(cold loess) with steppe vegetation not very far from
the Pleistocene glaciers. Most of the silt-sized material was blown out of widely distributed, partially
abandoned meltwater streams.
In contrast, the so-called desert loess is derived
from arid, mainly non-glaciated regions. This is, for
example, true of most of the Chinese loess which
covers a large area and attains 10cally more than 300
m in thickness.
The Chinese loess and other central Asian loess profiles
provide excellent records of the climatic evolution in central Asia during the Pleistocene (e.g. Heller and Liu
Tungsheng 1984; Pye 1987: Liu Tungsheng 1991; Frechen
and Dodonow 1998). According to paleomagnetic dating,
loess accumulation in China began 2.4 Ma B.P. and took
place discontinuously in relatively cool, dry periods. During warmer and wetter phases, dust transport from the
mountainous desert areas slowed down and soils formed on
top of the 10ess deposits. The average sedimentation rates
of loess in Asia and Europe mostly varied between 2 and
about 25 cmlka, but values ranging from 50 to 300 cmlka
have been reported from China and Tadjikistan for the last
glacial period. This shows that the sedimentation rates were
in fact much higher during the cold stages when loess was
actually deposited.
Loess is easily eroded by running water and fluvial
action. The Yellow River (Huang Ho) in China and
some other rivers of eastem Asia carry extremely
high loads of suspended material, mostly derived
from loess, into the sea (cf. Sects. 9.3 and 1l.2.2).
Here, they cause unusually high sedimentation rates
of siliciclastic material and the rapid shallowing of
coastal seas with prograding shorelines.
Pre-Quatemary loess deposits have rarely been
reported. Considering the widespread occurrence of
Quatemary loess, lithified loess (loessite) should be
preserved more frequently in the ancient record than
known so far.
Possibly the thickest loessite (about 500 m) was described
from Upper Carboniferous to Lower Permian mixed fluvial
and eolian red beds in northwestem Colorado (Johnson
1989). Part of this sequence is characterized by homogeneous, structureless sandy siltstones interpreted as loessite.
This view as supported by intercalations of indistinct
paleosols, the lateral gradation of these beds into fluvial
deposits, and the inferred pa1eogeographic setting of the
sedimentary basin. Other occurrences of loessite were described from some Precambrian to Neogene deposits in
North America and Norway.
2.3.8 Remarks to Ancient Eolian Sediments
In the geologie past, the general principles goveming
the formation of deserts were the same as today, but
the proportion and regional distribution of deserts on
the land surface were sometimes quite different from
the present situation; During the cold phases of the
can no longer remain in suspension. The deposition
of dust is accelerated by the presence of vegetation
which creates surface roughness and thus impedes air
flow and subsequent resuspension. Settling dust often accumulates on the downwind side of hills and
other topographie highs, or in depressions; moist surfaces trap the dust grains.
Measured present-day rates of dust deposition on
land range between 10 to about 200 g/m 2 per year,
corresponding to sedimentation rates of approximately 0.5 to 10 cmlka. Locally and discontinuously,
the sedimentation rate of dust must have been higher
in the past, for example for Pleistocene loess with
rates up to several tens of cmlka (see below). The
contribution of dust to oceanic sediments is less
known, although this topic has been studied recently
by several workers (cf. Chap. 5). Dust deposition
rates in the ocean reported in the literature vary between 0.1 and more than 10 g/m 2 per year. Taking
into account the very 10w sedimentation rates of
deep-sea sediments (in large areas ca. 1 cmlka, cf.
Sects. 5.3 and 10.2), one can draw the conclusion
that eolian dust should make up a significant proportion of marine sediments in regions, where offshore
winds from deserts reach the open ocean.
The carbonate-free fraction of early Cretaceous to late Miocene pelagic to hemipelagic sediments in the North Atlantic consists predominantly of eolian silt and c1ay (Lever
and McCave 1983). In the eastem Atlantic, northwestem
Pacific, and northem Indian ocean, eolian dust may comprise more than 50% of the total modem sediment. These
sediments mainly consist of fine-grained quartz and clay
minerals characteristic of their source areas. During the last
8000 years, the dust flux into the Arabian Sea reached
nearly the same volume as the suspended load of the Indus
river (Sirocko and Samthein 1989). It appears that dust
deposition in the oceans increased in the Neogene in conjunction with a long-term climatic change.
Pleistocene Loess
Unconsolidated silty dust sediments of Pleistocene or
somewhat older age, which are deposited on land, are
termed loess. Unweathered and non-redeposited
loess is homogeneous, mostly buff or yellowish in
color, non- or weakly stratified, and highly porous. It
consists predominantly of quartz, feldspar, mica, clay
minerals, and carbonate grains in varying proportions. The principle grain size of loess ranges from
20 to 40 Jlm, but loess can also contain minor proportions offine sand ( 63 Jlm) and clay ( 4 Jlm). Loess
covers wide areas on the present land surface, is agriculturally important, and frequently used for dating
Pleistocene processes and sediments.
At least two principally different source areas of
loess are distinguished. In Europe and North America, most loess is derived from glaciated areas and
accumulated under cold and relatively dry conditions
Chapter 2 Continental Sediments
(cold loess) with steppe vegetation not very far from
the Pleistocene glaciers. Most of the silt-sized material was blown out of widely distributed, partially
abandoned meltwater streams.
In contrast, the so-called desert loess is derived
from arid, mainly non-glaciated regions. This is, for
example, true of most of the Chinese loess which
covers a large area and attains 10cally more than 300
m in thickness.
The Chinese loess and other central Asian loess profiles
provide excellent records of the climatic evolution in central Asia during the Pleistocene (e.g. Heller and Liu
Tungsheng 1984; Pye 1987: Liu Tungsheng 1991; Frechen
and Dodonow 1998). According to paleomagnetic dating,
loess accumulation in China began 2.4 Ma B.P. and took
place discontinuously in relatively cool, dry periods. During warmer and wetter phases, dust transport from the
mountainous desert areas slowed down and soils formed on
top of the 10ess deposits. The average sedimentation rates
of loess in Asia and Europe mostly varied between 2 and
about 25 cmlka, but values ranging from 50 to 300 cmlka
have been reported from China and Tadjikistan for the last
glacial period. This shows that the sedimentation rates were
in fact much higher during the cold stages when loess was
actually deposited.
Loess is easily eroded by running water and fluvial
action. The Yellow River (Huang Ho) in China and
some other rivers of eastem Asia carry extremely
high loads of suspended material, mostly derived
from loess, into the sea (cf. Sects. 9.3 and 1l.2.2).
Here, they cause unusually high sedimentation rates
of siliciclastic material and the rapid shallowing of
coastal seas with prograding shorelines.
Pre-Quatemary loess deposits have rarely been
reported. Considering the widespread occurrence of
Quatemary loess, lithified loess (loessite) should be
preserved more frequently in the ancient record than
known so far.
Possibly the thickest loessite (about 500 m) was described
from Upper Carboniferous to Lower Permian mixed fluvial
and eolian red beds in northwestem Colorado (Johnson
1989). Part of this sequence is characterized by homogeneous, structureless sandy siltstones interpreted as loessite.
This view as supported by intercalations of indistinct
paleosols, the lateral gradation of these beds into fluvial
deposits, and the inferred pa1eogeographic setting of the
sedimentary basin. Other occurrences of loessite were described from some Precambrian to Neogene deposits in
North America and Norway.
2.3.8 Remarks to Ancient Eolian Sediments
In the geologie past, the general principles goveming
the formation of deserts were the same as today, but
the proportion and regional distribution of deserts on
the land surface were sometimes quite different from
the present situation; During the cold phases of the