麻豆传媒

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The Sydney Mines Formation (SMF) forms the upper unit of the Late Carboniferous, Westphalian D to Stephanian, Morien Group - correlative regionally with the Cumberland Group. The lower Sydney Mines Formation was studied, and is intermittently exposed in the Sydney area over a distance of 45 km along depositional strike. It forms part of the onshore component and southern limit of the Sydney Basin. The lower SMF comprises a repetitive succession of alternating, predominantly grey sandstones, red and grey mudstones, minor limestone and coal seams of economic significance. These repetitive successions, bounded by coal seams, are referred to as cyclothems.

The SMF commences with the Emery seam cyclothem interval where it rests on the underlying South Bar Formation (SBF). The SMF thickens and, therefore, is expanded to include the Gardiner and McRury seam cyclothem intervals, in the Glace Bay Syncline area, where the SMF rests on the Waddens Cove Formation (WCF). The highest interval studied is capped by the lower leaf of the Bouthillier seam.

The thick economic coal seams, in close association with limestones, represent maximum flooding surfaces. A highstand systems tract that follows, typically forms bayfill deposits. Relative sea-level then progressively dropped to a lowstand sequence boundary, marked by calcretes, suggesting arid climatic conditions. Coincidentally, deep channel incision occurred, cutting down to and rarely through the limestone and coal below. Subsequent relative sea-level rise promoted channel body aggradation, localized subaerial exposure, formation of red beds, then eventual drowning and more bayfill deposition and peat formation. A return to more humid climatic conditions are inferred. This sequence of events defines a typical cyclothem.

The typical cyclothem is not represented in outcrop until the Stony to Phalen interval, where thick coal seams are developed. This interval also marks a significant change in fluvial style. It transforms from a dominance of low sinuosity, braidplain and sheet sand (Group A and B respectively) channel systems, carried over from the older SBF and WCF, to a more sinuous but variable fluvial style, including Groups C, D and E. This pattern continues upward, reflecting the influence of base-level fluctuation and the relative position of the exposed sections to a paleocoastline.

Some channel bodies are amalgamated to form thick, multi-story units, on the order of 18 m in composite thickness. An understanding of channel body geometry and fluvial style has important implications for underground coal mining operations and for hydrocarbon exploration potential.

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Pages: 162
Supervisor: Martin Gibling