Geology Reference
In-Depth Information
Cretaceous/Tertiary Coals
Extensive deposits of Cretaceous and Tertiary coals are found in the western North America, Northeastern Russia
and Siberia. They are also found in Europe, Japan, Africa, China, New Zealand, Australia, and South America. In
the Western United States, a seaway extended periodically from the Artic Ocean to the Gulf of Mexico. A seaway
also connected the Barents Sea with the Tethys Ocean. In the late Cretaceous, the Tethys Seaway was also
connected to the South Atlantic through the Niger Trough. An interior sea existed in central Australia in the mid-
Cretaceous (McCabe and Parrish, 1992).
Cretaceous coals developed in areas where the annual precipitation exceeded evaporation. In tropical areas, rainfall
was high and the humid climate preserved sediments. In higher mid-latitudes, evaporation was constrained by the
cooler climate (McCabe and Parrish, 1992). In the Western United States, which has the largest volume of
Cretaceous coal, wetlands developed along the margin of the retreating sea. The rising Rocky Mountains created
intermontane valley swamps. A warm humid climate and isolation from the Cretaceous sea produced extremely
thick, low-sulfur coals (Smith et al., 1994; WSGS, 2001).
During the early Cretaceous, conifer forests with ferns, gingophytes, and Czekanowskiales were the prevalent in
the coal-forming swamps. In the middle Cretaceous, although forests remained dominated by conifers, angios-
perms diversified and became an important component of the vegetation. In the late Cretaceous, conifers continued
to dominate the coal-forming swamps, but angiosperm trees and shrubs were also important (Saward, 1992).
Coal Swamps
A
site in which carbonaceous sediments could accumulate was created by erosion and the retreat of shallow
seas. Broad level areas at or very near sea level, such as coastal plains, deltas, or a partially filled basins,
could readily develop the marshy conditions needed for the growth of a carbonaceous swamp or mire. The
consistent gradual rise in sea level or continuous slow land subsidence was required for between 1000 and
100 000 years in order to form a 10m peat deposit which would be converted to a 1.5m coal seam (Ashley,
1928). Eustatic rise in sea level, due to the melting of glaciers in the southern hemisphere, or epeirogenic
sinking of the land would facilitate continual deposition of plant residues (Bennett, 1963; Kay and Colbert,
1965; Wanless et al., 1969).
Marshy conditions fostered the formation of peat and the preservation of the organic sediment. Under aerobic
conditions, plants are rapidly decomposed to cell carbon, carbon dioxide (CO
2
), and water (H
2
O). In a marshy area,
the movement of fresh H
2
O is inhibited, and an anaerobic environment develops, slowing the rate of microbial
decay and allowing carbonaceous sediments to accumulate. Toxic products also accumulate in the slow moving
H
2
O, decreasing microbial activity and preserving the sediments from further alteration. A rapid rise in sea level or
increase in the rate of subsidence would flood the swamp, halting growth, and burying the peat under inorganic
sediment.
In contrast to the slow deposition of peat, the deposition of inorganic sediments was relatively rapid, turbulent,
and variable. The rocks associated with coal seams are usually fine grained clastics, particularly shales,
mudstones, and siltstones. Black shales overlying many coal seams represent a gradational change as more
sediment was carried into the peat swamp. Sandstones immediately above a coal seam may be related to
erosion and subsequent deposition within an existing seam. Channel sands can be seen as stream erosion of an
existing coal or peat deposit and deposition of sediments within the stream channel (McCullogh et al., 1975).
The rate of sediment deposition within or immediately above the peat would affect the concentration of
syngenetic minerals, while sandstones deposited above a coal seam could increase the concentration of
epigenetic minerals.
Cyclothems
A
cyclothem is a series of repeating sediments representing the transgression and regression of H
2
O or the
submergence and emergence of land. In coal-bearing strata, changes in depositional environment produced a
cyclic repetition of beds. The cyclothem is defined as a series of beds deposited during a single sedimentary cycle.
