Courtesy of the Kentucky Geological Survey
Coal in Colorado was formed in peat mires during the Cretaceous and Paleocene-Epoch of the Tertiary Period, between 100 and 55 million years ago. Fresh-water bogs formed along the coastal plains adjacent the the shorelines of the Western Interior Seaway that existed in the mid-continent at the time. The climate was hot and sea level was generally high, and the seaway shorelines fluctuated greatly through geologic time.
There are four critical stages of the coal system: Stage 1 is the accumulation stage, where peat, or parent material of coal, accumulates in fresh-water marshes, swamps, and rain forests. The accumulation of organic material must exceed the oxidation or biodegradation of the environment. The second stage is called the burial and preservation stage. Conditions of subsidence and fresh-water preservation and burial must be critical for coal to form. Stage 3 is the diagenesis and coalification stage where peat is transformed or metamorphosed to form coal. Heat and pressure geochemically alter the organic material into coal. Regional heat flow, such as in parts of Colorado, also alter the process in sedimentary basins to where the coal rank is upgraded, sometimes from bituminous coal to anthracite. This entire process is called "coalification."
During coalification, peat undergoes several changes as a result of bacterial decay, compaction, heat, and time. Peat deposits are quite varied and contain everything from pristine plant parts (roots, bark, spores, etc.) to decayed plants, decay products, and even charcoal if the peat caught fire during accumulation. Peat deposits typically form in a waterlogged environment where plant debris accumulated; peat bogs and peat swamps are examples. In such an environment, the accumulation of plant debris exceeds the rate of bacterial decay of the debris. The bacterial decay rate is reduced because the available oxygen in organic-rich water is completely used up by the decaying process. Anaerobic (without oxygen) decay is much slower.
For the peat to become coal, it must be buried by sediment. Burial compacts the peat and, consequently, much water is squeezed out during the first three stages of burial. Stage four is the continued burial and the addition of heat and time causing the complex hydrocarbon compounds in the peat to break down and alter in a variety of ways. The gaseous alteration products (methane is one) are typically expelled from the deposit, and the deposit becomes more and more carbon-rich as the other elements disperse. The stages of this trend proceed from plant debris through peat, lignite, sub-bituminous coal, bituminous coal, anthracite coal, to graphite (a pure carbon mineral).
Because of the amount of squeezing and water loss that accompanies the compaction of peat after burial, it is estimated that it took 10 to 15 vertical feet of original peat material to produce one vertical foot of bituminous coal in Colorado. The peat to coal ratio is variable and dependent on the original type of peat, the coal rank, length of time of burial, and the depth at which it was buried.