Application of sequence stratigraphy to coal-bearing coastal plain successions: implications for the UK Coal Measures

Abstract

Advances in the understanding of coal depositional environments have led to the notion that thick, low ash deposits in close proximity to siliciclastic sediment inputs are most commonly restricted to the products of raised mires. These mires are initiated, sustained and preserved in conditions of slowly rising base level (relative sea-level). Hence it is possible to consider the stratigraphic significance of economic coal seams within the concept of unconformity bounded depositional sequences (sensu Vail et al.). Time-equivalent clastic deposits tend to be minor (commonly with a heterolith fill) channel deposits, with brackish to freshwater lake and crevasse splay sediments, excluded from the peat mire either via the topographically raised nature of the mire or by the landward dislocation of fluvial facies. In this paradigm, spatially related major fluvial channel deposits, major seam splits and even washout deposits may not be time-equivalent to the coal seam. Regional correlations suggest that thick coal seams are time-correlative to significant flooding surfaces at the coeval coastline. Thus coals may be correlated with initial flooding surfaces over incised valley systems and sometimes with parasequence and parasequence set boundaries. This factor may be important in coal exploration. Application of these ideas to the classic Westphalian 'Coal Measures' of England indicates that the 'marine bands' may be regarded as flooding surfaces or condensed sections, with several being good candidates for maximum flooding surfaces. Between these marine bands the stratigraphy is punctuated by thick, stacked, regionally extensive fluvial units, incised into previously deposited sediments. These are interpreted as incised valley complexes. Systems tract assignment, based on changing accommodation space, indicates that the main coal zones fall dominantly within transgressive systems tracts. Depositional modelling of coal-bearing strata was largely initiated by work in the Carboniferous coal fields of the Central Appalachian Basin (e.g. Home et al. 1978; Howell & Ferm 1980). These models assumed that the nature of coal deposits (thickness, lateral extent, ash content and quality) was controlled by the autocyclicity of clastic environments which surrounded the peat-forming environments. Horne et al. (1978)

What Happened to the Coal Forests During Pennsylvanian Glacial Phases?

“…One notable coal that seems to fit this description on first appearance is the 8.5-m-thick Warwickshire Thick Coal, UK (Fulton, 1987), which, if deposited continuously, might represent -100 kyr, i.e., spanning one or more glacial cycles. Flint et al (1995), however, argued that this coal comprises several amalgamated parasequences and records successive deglaciation events on a stable interfluve rather than a persistent peatland. While this interpretation may be questioned, we know of no coals where an unequivocal case can be made for continuous accumulation through multiple climate cycles.…”

“…Dominated at different times by lepidodendrids, cordaitaleans, and marrattialean tree ferns (e.g., Phillips et al, 1985), these communities are often referred to as coal forests. Sequence stratigraphic studies show that most coal forests were established during the glacialto-interglacial transition and, to a more limited extent, during interglacial phases (Tandon and Gibling, 1994;Flint et al, 1995). Continent-wide peat (now coal) formation was triggered by a combination of climate change from seasonally dry (semiarid to subhumid) to ever-wet (humid to perhumid) and a coeval rise in sea level, which elevated the regional water table.…”

“…In contrast, HFF's position is that coals dominantly comprise midto late-TST deposits (Flint et al, 1995) because brackish fauna and estuarine fades locally occur below coal seams within expanded valleyfill successions (e.g. Gibling and Wightman, 1994;Feldman et al, 1995), indicating that sea level had risen significantly prior to the onset of peat (coal) formation.…”

On the fundamental difference between coal rank and coal type

“…Generally, preclusion or great reduction of sediment from the peatland has been attributed to mechanisms such as displacement of sediment deposition inland of coastal mires by rising sea level (e.g., Diessel, 1992;Flint et al, 1995); doming of the peat body, thus raising the surface above the sedimentary system (e.g., Ferm and Cavaroc, 1968;Greb et al, 1999); or baffling of sediment by vegetation growing at the margin of the peatland (e.g., Eble and Hower, 1995;Kravits and Crelling, 1981;Moore, 1991). These models implicitly or explicitly assume that peat formation takes place syndepositionally with local siliciclastic deposition.…”

No Major Stratigraphic Gap Exists Near the Middle-Upper Pennsylvanian (Desmoinesian-Missourian) Boundary in North America

“…Glacial-interglacial cycles not only influenced sea level, but also caused major fluctuations in climate (Tandon and Gibling, 1994;Flint et al, 1995;Hampson et al, 1999;Montanez et al, 2007;Poulsen et al, 2007;Tabor and Poulsen, 2008), and consequently in the composition of Pennsylvanian tropical vegetation (Falcon-Lang, 2004;Driese and Ober, 2005;Feldman et al, 2005;Falcon-Lang et al, 2009;FalconLang and DiMichele, 2010). During interglacial phases (late transgressive to highstand systems tracts), climate was largely humid to subhumid, and tropical forests were dominated by lycopsids, pteridosperms, and tree ferns (DiMichele and Phillips, 1994;DiMichele et al, 2001DiMichele et al, , 2007.…”