Alexander R. Papp, 1998. "Depositional Environments and Sedimentary Geology Lithologies: controls on mining and CBM Development", Atlas of Coal Geology, Alexander R. Papp, James C. Hower, Douglas C. Peters
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Coal-bearing strata comprise a vast assortment of rock types that vary in composition and physical properties. Strata associated with coal seams are predominantly fine- to coarse-grained clastic sedimentary rocks. Characterization of the seam and associated rocks is important when assessing the resource potential of the deposit.
The distribution and type of rocks found are the result of a dynamic depositional system (see Coal Depositional Systems, Volume 1). Rocks immediately overlying and underlying a coal seam are referred to as the roof and floor, respectively. An assessment of the roof and floor strata is critical for mining and CBM development. The floor strata can range from weak underclay to a strong sandstone. The roof can vary from alternating fluvial sandstones and mudstones of the delta plain to a more consistent lithology of a regressive marine sequence. For example, a mire could develop behind a shoreline in a wavedominated delta system (Picture 1). During regression (seaward movement of the shoreline), the mire continues to follow the shoreline, depositing peat over older shoreline sand units. In turn, sediments of the delta plain are deposited over the mire. In that scenario, the coal seam would consist of a strong, marine, sandstone floor and a variable strength, fluvial roof consisting of alternating rock types (Picture 2). Conversely, the mire could undergo transgression and an episode of non-deposition (Picture 3). Regression could follow and delta-front sediments would then blanket the peat. In that scenario, the roof would comprise of a marine sequence which is often more consistent in character (Pictures 4, 5, and 6).