Geoscience Reference
In-Depth Information
9.2 Coal
The majority of coal preserved in sedimentary
strata of the continents was created during two
major pulses of accumulation. The i rst was the
Permo-Carboniferous interval in the late Paleo-
zoic (
c
. 360 to 250 million years ago; Geological
Society of America 2009). The second interval
began in the Cretaceous (late Mesozoic) and
continued well into the Cenozoic (
c
. 145 to 20
million years ago). These intervals were extremely
favorable for l ourishing land vegetation and the
preservation of peat under humid climatic condi-
tions. Peat and coal also accumulated in lesser
amounts at other times between and since these
two main coal-forming intervals.
Figure 9-1.
Stromatolites exposed at low tide in the
Hamelin Pool Marine Nature Reserve, Shark Bay World
Heritage Area, western Australia. Modii ed from original
photograph by P. Harrison; obtained from Wikimedia
Commons
<
http://commons.wikimedia.org/
>
.
9.2.1 Paleozoic coal
examples include the Eocene Florissant Forma-
tion, a former lake in Colorado with spectacular
insect remains (Veatch and Meyer 2008), and
the Upper Carboniferous Hamilton Quarry, an
estuary channel-i lling in eastern Kansas with all
types of invertebrate and vertebrate fauna and
l ora (Mapes and Mapes 1988). These and other
examples demonstrate that wetlands served as
harbors and refugia for biodiversity in the past
(Greb and DiMichele 2006).
Among the organic deposits of wetlands,
peat is of major economic importance, because
upon burial it may be transformed into coal.
Thus, ancient wetlands produced the coal that
generates about one-third of the world's elec-
tricity supply (Greb and DiMichele 2006). It
comes as no surprise, then, that a tremendous
amount of research has been conducted on the
origins, disposition, and energy potential of
coal around the world. Amber is ancient tree
resin found in association with low grades of
coal, and it provides unique evidence regarding
life and environmental conditions in past wet-
lands. Approaching recent times, peat contains
a record of environmental and climatic condi-
tions spanning the past few millennia. This
record is crucially important for understanding
the recent evolution of the Earth's climatic
system as well as human impacts on the
environment.
Massive accumulation of coal during the late
Paleozoic was preceded by a long developmen-
tal phase of terrestrial vegetation that began
in the late Ordovician and early Silurian, when
the i rst land plants appeared about 450 to 430
million years ago. These were mosses and
lichens that were obligate wetland dwellers, but
not peat-forming plants (Greb, DiMichele and
Gastaldo 2006). Land plants diversii ed during
the Devonian, and marsh and forest swamp
environments emerged in which plants were
adapted for low-oxygen and low-nutrient condi-
tions. This set the stage for peat marshes and
peat swamps by the end of the Devonian (
c
. 360
million years ago), and peatlands became wide-
spread during the following Carboniferous
Period. Ombrotrophic tropical mires arose in
the middle Carboniferous, and
Glossopteris
l ora
spread into temperate climates during the
Permian (Greb, DiMichele and Gastaldo 2006).
Permo-Carboniferous coal is found on all
modern continents in a seemingly haphazard
geographic distribution. This pattern becomes
clear when the continents are reassembled into
the late Paleozoic supercontinent, known as
Pangaea (Fig. 9-2), which was i rst proposed by
Alfred Wegener in 1912. He relied on paleocli-
matic evidence to support his reconstruction
of Pangaea (Schwarzbach 1986). Coal deposits
were found in two primary climatic zones,
