麻豆传媒

The Boss Point Formation, part of the Cumberland Group, consists mostly of sandy braidplain and muddy lacustrine facies. The braidplain facies mainly comprise stacked, trough cross-bedded channel deposits, which contain accumulations of woody material, logs, and in some cases, thin coal seams within some of the bedsets. They muddy lacustrine facies nonerosively overlie the braidplain facies, and are thought to provide evidence for channel abandonment and avulsion. One outcrop near the base of the formation was studied in detail due to its particularly good exposure of the wood debris, which is present in different sandstone bodies within a section of about 3 m in thickness. The stratigraphy and sedimentology were described, and the size of wood fragments was measured, along with the thickness of the log-rinds. High-resolution photographs were taken in order to calculate the proportion of wood to sediment. From this, the amount of compaction that occurred to the wood-bearing beds during burial can be estimated, and their original thickness after deposition but prior to burial can be found. The findings show that the original depth of the channel was about 8.2 m, and though the lateral extent of the channel could not exactly be determined, it is thought to be quite large. Stratigraphic analysis shows that the debris is associated with flooding events, and the presence of shales above the log accumulations strongly suggests that the accumulations contributed to the abandonment of the channels. Our information, combined with previous studies of plant fossils in the area, suggest that the woody accumulations constitute a log jam deposit in the Boss Point Formation. The deposits in this area can also be compared to other log jam deposits, in Nova Scotia, USA and Poland. Recent evidence from palynology shows that the age of the Boss Point Formation is Yeadonian to Langsettian, older than any other documented log jam deposit. As a result, the data obtained from the deposits in this area not only provide information on how ancient channel systems were affected by the jamming, but also show one of the first instances of plants having evolved to be in sufficient numbers and to be large enough to significantly affect the dynamics of fluvial systems.

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Pages: 57
Supervisor: Martin Gibling