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Abstract
Peat successions preserved as coal seams are increasingly recognised as proxy records for ancient, high-resolution accommodation change in terrestrial strata. This is because (i) peat accumulation is typically interpreted as continuous, compared to terrestrial clastic sediments that are deposited in beds during discrete events; (ii) the water-table in the original mires very specifically defines the base-level of deposition for peat; and (iii) the sensitivity of the organic material to subtle changes in base-level results in changes in the composition of the resulting peat. The characteristics of coals from the Upper Carboniferous Upper Breathitt Group of the central Appalachian Basin (U.S.A.) were investigated with the objective of critically testing the method of using changes in coal composition as a means of investigating records of high-resolution base-level change in terrestrial environments. In the Hyden and Four Corners Formations, major coal seams/zones occur in the transgressive systems tracts and maximum flooding zone of fourth-order (c.140-210 kaduration) depositional sequences comprising fluvio-deltaic to marine sediments. Thickness measurements and the detailed petrographic composition of 239 coal samples from ten coals from a single vertical section through the Four Corners Formation were compared with the stratal geometries and stacking-patterns of the fourth-order sequences within which they occur. Coal seam thickness and vitrinite/inertinite ratio of coal correlate readily with the accommodationsetting of fourth-order depositional sequences derived from analysis of the non-coal strata, indicating that relative sea-level change was the dominant control on accommodation in mires landward of the shoreline. These results provide a framework for predicting vertical changes in coal quality through third-order composite sequences in the Breathitt Group and elsewhere. Lateral changes in coal composition were investigated by petrographic analysis of 275 coal samples from eleven vertical sections through the Fire Clay coal (Hyden Formation), along a >100 km SW-NEtransect. Using a volcanic ash-horizon interbedded with the coal as an independent datum, compositional cyclicity in the Fire Clay coal defines six depositional units recognisable over more than 100 km. The cyclic nature and basin-scale of the correlations strongly suggest that accommodation changes were driven by regional changes in the mire water-table, in turn driven by sea-level and/or climate fluctuations. Coals of the Hyden and Four Corners Formations typically contain abrupt vertical petrographic discontinuity surfaces, underlain by a degraded maceral assemblage characterised by disseminated inertinite and resistant peat components suchasliptinites and detrital minerals. Similar surfaces described elsewhere from bituminous and browncoals, and late Quaternary radiocarbon-dated peat successions, indicate that they represent surfaces of hiatus or degradation of the peat surface, associated with base-levelstillstand or fall. The recognition of hiatal surfaces in coal seams has important implications for studies that assume coals to represent continuous and time-invariant records of peat accumulation, but does not invalidate the technique of using coal seams as proxy records of high-resolution accommodation changes in terrestrial environments. It is concluded that coal seams do provide one of the best opportunities for investigating high-resolution accommodation changes in terrestrial environments, and the allogenic processes driving these changes. It is emphasised, however, that difficulty can be encountered in establishing regional-scale correlations through coal seams, and that conceptual models should not be applied rigidly during the interpretation of coal petrographic data-sets.
