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Vegetational
Scenario and Climate of
The
Coal-Bearing Gondwana
By: Usha
Bajpai
One of the
most important periods in the history of the earth, or rather the Southern
Hemisphere, that is, the Gondwana Supercontinent, was the Permian Period
(280-230 million years before present) when the major coal deposits of the
region were laid down. In India, the most important coal deposits are
approximately 280 million years old and were laid in basins aligned along
important river valleys of peninsular India. Major coal-fields are in Son,
Damodar, Mahanadi, Satpura and Wardha and Godavari basins. During this period
India lay between southern latitudes 70° and 40°.
Coal is a
compression fossil having been formed through an accumulation of plant material
that became crushed and compacted due to sediment load. Lignite represents an
early stage in coal formation. Coal contains bits of wood, pieces of bark, leaf
cuticles, resin bodies, spores and pollen. In rare instances coal is formed
entirely of cuticular fragments and amorphous organic material. This type of
coal is known as paper coal. One is always interested to know the ecological
and climatic conditions, besides the depositional environment under which coal
formation takes place.
Climate may
be defined as the prevailing weather condition that results from the exchange
of moisture, and heat between the earth and the atmosphere. Plants live under
various ecological environments on the land, from aquatic continental to upland
condition in extreme dryness, etc., which are reflected in the anatomy and
morphology of the plants. Thus, periodic changes in the earth's climatic
conditions can be deciphered from records of vegetation in time and space. This
has helped the palaeobotanist to decipher and interpret the past climate.
The study of
the fossil plants found in the Gondwana rocks, the Glossopteris Flora, provides
convincing evidence for the ecological and climatic conditions. Before the rise
of the Glossopteris Flora, there was more or less cosmopolitan vegetation, the
Lepidodendropsid Flora, in the Carboniferous, this flora vanished from the
areas that came under the influence of the glaciers. The retreat of the
glaciers in the earliest part of the Permian saw almost sudden and enigmatic
arrival of the Glossopteris Flora. Its ancestry is presently not known and is
probably rooted in rapid mutations that resulted due to the glacial episode.
The temporal and spatial spread of the Glossopteris Flora was facilitated by
amelioration of the climate due to rapid deglaciation. The flora comprised
different members of the Bryophyta, Lycophyta, Arthrophyta, Filicophyta, and
Gymnospermophyta. The gymnosperms formed the most dominant group of plants and
included the divisions Cordaitopsida, Cycadopsida, Coniferopsida, Ginkgopsida
and Glossopteridopsida. The most important group was the glossopterid group of
plants with tongue-shaped leaves, several genera of which are on record, for
example, Gangamopteris, Glossopteris, Maheshwariphyllum, Palaeovittaria,
Rhabdotaenia and Rubidgea. The wood that has been recovered almost invariably
shows an araucaroid organisation with clearly defined growth rings. It
indicates the prevalence of a temperate climate. In the earliest Permian
though, the presence of meagre vegetation comprising small plants indicates a
cold xeric environment. Later part of the Early Permian saw warming of the
climate and transgression of the sea. The climate became more humid, vegetation
proliferated, and the first coals were laid. The study of the leaf epidermis of
different plants has indicated that the cold temperate climate of the Early
Permian gradually changed over to a warm temperate climate of the Late Permian.
In between, at the end of the Early Permian, an inexplicable paucity of
vegetation is noticed which is sometimes ascribed to hot and dry spells.
However, the recovery of rich palynological assemblages, with lycopod
megaspores in large numbers, from the sediments of this period shows the
presence of favourable ecological niches. The prevalence of hypostomatic leaves
shows increase in precipitation, while the appearance of petiolate leaves
possibly indicates a rise in wind velocities in the Late Permian. It was during
this period that the last workable Gondwana coal was deposited. The association
of fungi and bacteria with the coal indicates that probably it was not
deposited in very deep waters.
Dr.
Usha Bajpai is a scientist at Birbal Sahni
Institute
of Palaeobotany,
Lucknow,
India. |
