Abstract
AbstractFluvial systems in which peat formation occurs are typified by autogenic processes such as river meandering, crevasse splaying and channel avulsion. Nevertheless, autogenic processes cannot satisfactorily explain the repetitive nature and lateral continuity of many coal seams (compacted peats). The fluvial lower Palaeocene Tullock Member of the Fort Union Formation (Western Interior Williston Basin; Montana, USA) contains lignite rank coal seams that are traceable over distances of several kilometres. This sequence is used to test the hypothesis that peat formation in the fluvial system was controlled by orbitally forced climate change interacting with autogenic processes. Major successions are documented with an average thickness of 6·8 m consisting of ca 6 m thick intervals of channel and overbank deposits overlain by ca 1 m thick coal seam units. These major coal seams locally split and merge. Time‐stratigraphic correlation, using a Cretaceous–Palaeogene boundary event horizon, several distinctive volcanic ash‐fall layers, and the C29r/C29n magnetic polarity reversal, shows consistent lateral recurrence of seven successive major successions along a 10 km wide fence panel perpendicular to east/south‐east palaeo‐flow. The stratigraphic pattern, complemented by stratigraphic age control and cyclostratigraphic tests, suggests that the major peat‐forming phases, resulting in major coal seams, were driven by 100 kyr eccentricity‐related climate cycles. Two distinct conceptual models were developed, both based on the hypothesis that the major peat‐forming phases ended when enhanced seasonal contrast, at times of minimum precession during increasing eccentricity, intensified mire degradation and flooding. In model 1, orbitally forced climate change controls the timing of peat compaction, leading to enhancement of autogenic channel avulsions. In model 2, orbitally forced climate change controls upstream sediment supply and clastic influx determining the persistence of peat‐forming conditions. At the scale of the major successions, model 2 is supported because interfingering channel sandstones do not interrupt lateral continuity of major coal seams.
Highlights
The repetitive nature and lateral continuity of coal seams in fluvial stratigraphic architectures have long been recognized in outcrops and the subsurface (e.g. Wanless & Weller, 1932; Cecil, 1990; Fielding & Webb, 1996; Paproth et al, 1996; Michaelsen & Henderson, 2000)
A stable primary characteristic remanent magnetization (ChRM) component can be isolated in most reversed polarity samples upward from 180°C (TH) or 20 m Tesla (mT) (AF) (Fig. 4): 61Á3% of the total directions showed a clear ChRM component directed towards the origin and were calculated with an anchored-principal component analysis (PCA) fit
The filters of the decompacted grain size index (GSI) records show a more regular pattern (Figs 9 and S4) than the filters of the undecompacted records (Fig. 8). These results show that decompaction of coal results in higher significant power spectra and higher representative filters and implies more reliable cyclostratigraphic results when a decompaction can be reliably estimated. It will be considered whether the results support the hypothesis of orbital-scale climate control on peat formation in the fluvial system interacting with autogenic processes
Summary
The repetitive nature and lateral continuity of coal seams in fluvial stratigraphic architectures have long been recognized in outcrops and the subsurface (e.g. Wanless & Weller, 1932; Cecil, 1990; Fielding & Webb, 1996; Paproth et al, 1996; Michaelsen & Henderson, 2000). Wanless & Weller, 1932; Cecil, 1990; Fielding & Webb, 1996; Paproth et al, 1996; Michaelsen & Henderson, 2000) Such features of coal seams in fluvial successions may reflect recurrent phases of peat formation through time over wide areas, that may point to orbital-scale climate control on peat formation in the fluvial system. In fluvial settings that are not (or are less) influenced by sea-level changes on orbital timescales, but are primarily allogenically controlled by orbital-scale climate control on river discharge and sediment supply, peat formation may be synchronous over a wide area Fielding & Webb (1996) argued for precession-scale climate control on peat formation in a fluvial system, based on a sedimentological analysis of the late Permian Bainmedart coal measures in Antarctica. The 19 kyr cycle durations summed to an estimated time duration of 2Á1 to 2Á3 Myr for the Bainmedart coal measures, which is within the range of palynostratigraphic age estimations of
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