Page 74 - Global Tectonics

CONTINENTAL DRIFT 61

paleoclimatology, the study of past climates (Frakes, 5 Phosphorites. At the present day phosphorites
1979), may be used to demonstrate that continents have form within 45° of the equator along the
drifted at least in a north–south sense. It must be real- western margins of continents where upwell-
ized, however, that the Earth is presently in an intergla- ings of cold, nutrient-rich, deep water occur, or
cial period, and so parallels between modern and in arid zones at low latitudes along east-west
ancient climates may not be completely justifi ed. The seaways.
important paleolatitude indicators are listed below.
6 Bauxite and laterite. These aluminum and iron
1 Carbonates and reef deposits. These deposits are oxides only form in a strongly oxidizing
restricted to warm water and occur within 30° environment. It is believed that they only
of the equator at the present day where originate under the conditions of tropical or
temperatures fall in the narrow range 25– subtropical weathering.
30°C. 7 Desert deposits. Care must be employed in using
2 Evaporites. Evaporites are formed under hot any of these deposits because desert conditions
can prevail in both warm and cold environments.
arid conditions in regions where evaporation

exceeds seawater influx and/or precipitation, However, the dune bedding of desert sandstones
can be used to infer the ancient direction of the
and are usually found in basins bordering a
sea with limited or intermittent connection prevailing winds. Comparison of these with the
direction of the modern wind systems found at
to the ocean proper (Section 13.2.4). At the
present day they do not form near the their present latitudes can indicate if the
continent has undergone any rotation.
equator, but rather in the arid subtropical
high pressure zones between about 10° and 8 Glacial deposits. Glaciers and icecaps, excluding
50° where the required conditions prevail, those of limited size formed in mountain
and it is believed that fossil evaporites ranges, are limited to regions within about 30°
formed in a similar latitudinal range of the poles at the present day.
(Windley, 1984).
The results of applying these paleoclimatic tech-
3 Red-beds. These include arkoses, sandstones, niques strongly indicate that continents have changed
shales, and conglomerates that contain their latitudinal position throughout geologic time. For
hematite. They form under oxidizing condi- example, during the Permian and Carboniferous the
tions where there is an adequate supply of Gondwana continents were experiencing an extensive
iron. A hot climate is required for the glaciation (Martin, 1981) and must have been situated
dehydration of limonite into hematite, and at near the south pole (Fig. 3.9). At the same time in
present they are restricted to latitudes of less Europe and the eastern USA, coal and extensive reef
than 30°. deposits were forming, which subsequently gave way to
hot deserts with evaporite deposits. The northern con-
4 Coal. Coal is formed by the accumulation and
tinents were thus experiencing a tropical climate in
degradation of vegetation where the rate of
equatorial latitudes (see also Fig. 1.3).
accumulation exceeds that of removal and
decay. This occurs either in tropical rain forests,
where growth rates are very high, or in
temperate forests where growth is slower but 3.5 PALEONTOLOGIC
decay is inhibited by cold winters. Thus, coals
may form in high or low latitudes, each type
having a distinctive flora. In Wegener’s compila- EVIDENCE FOR

tion of paleoclimatic data for the Carboniferous
and Permian (Fig. 1.3), the Carboniferous coals CONTINENTAL DRIFT
are predominantly of the low latitude type,
whereas the Permian coals of Gondwana are of
the high latitude type. Younger coals were Continental drift has affected the distribution of ancient
typically formed at high latitudes. animals and plants (Briggs, 1987) by creating barriers to