Petrologic and stable isotopic study of the Walloon Coal Measures, Surat Basin, Queensland: peat accumulation under changing climate and base level

• Published in: International journal of coal geology Vol. 160-161; pp. 11 - 27
• Main Authors: Hentschel, A., Esterle, J.S., Golding, S.D., Pacey, D.V.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.04.2016
• Subjects: Basins; Climate; Coal; Coal petrology; Fires; Macerals; Palaeoenvironment; Peat; Sandstone; Seams; Stable organic carbon isotopes; Surat Basin; Walloon Coal Measures; Surat Basin; Stable organic carbon isotopes; Palaeoenvironment; Coal petrology; Walloon Coal Measures
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2016.04.010

The Late Jurassic Walloon Subgroup (recently dated as Oxfordian) is a productive, subbituminous coal seam gas source in the Surat Basin and can be subdivided from bottom to top into the Taroom Coal Measures, the Tangalooma Sandstone, the Lower and Upper Juandah Coal Measures, which have different coal character. The lower Taroom coals are commonly thick, associated with sandstones and interpreted to form as base level is rising, creating sodden anoxic conditions for peat accumulation. The middle Tangalooma to Lower Juandah contains fewer and thinner coals, and transitions upwards from a sandstone to siltstone dominated sequence responding to inundation with the development of floodplain lakes. The strata then coarsen upward in both grain size and coal thickness in the Upper Juandah Coal Measures, which may be eroded by an overlying unit, the Springbok Sandstone. This unconformable surface is basin wide and depending on age, can be tied into global changes in climate and base level. Existing models for peat growth under changing base level and the variability in terms of the conditions of peat formation through time, as well as throughout the basin, are tested. Environment of peat deposition and changes therein, are investigated by petrographic analysis of the Walloon coals, coupled with high resolution lithotype logging of core and organic stable carbon isotope analysis. Fine microlayering and abundance of root suberinite, telo- and detrovitrinite indicate precursory peat formation in a mostly herbaceous marsh to fen environment, in which bigger trees are either infrequent or absent, except for the lower seams of the Taroom Coal Measures and the upper seams of the Lower Juandah Coal Measures, where bright bands are thicker (≥10mm) and more frequent. No extended periods of dehydration-oxidation (<1vol.% mmf inertinite group macerals) are indicated until the deposition of the Upper Juandah Coal Measures that contain greater amounts (5 to 15vol.% mmf with rare 68vol.%) of inertinite group macerals. Suberinite is interpreted to reflect dense root mats that are resistant to decay by microbial activity. They leave behind their suberinised exoderms, which originally helped wetland plants to protect themselves from deleterious solutes or in case of a change to drier conditions provided protection from desiccation. The most common inertinite maceral found in the Upper Juandah Coal Measures is inertodetrinite, associated with detrovitrinite. After bush or swamp fires, pieces of charcoal on dried out peat surfaces are easily blown away by the wind and accumulate with sediment in standing water. Fusinites and semifusinites are mainly associated with telovitrinites and are likely to be the result of desiccation and (fungal) mouldering in addition to fire. Stable carbon isotopes of coal show a distinct positive shift in the Lower Juandah Coal Measures that sets in well before the increased inertinite content in the Upper Juandah Coal Measures. The enrichment in 13C could be linked to a change in climate during the high stand depositional cycle, marking the onset of late stage falling, where base level begins to drop, later creating exposures and water stress. A shift to a less humid climate in the Upper Juandah Coal Measures could have favoured the conditions for desiccation, mouldering and bush fires, which is reflected in the coal's maceral composition. The Surat Basin δ13C isotope trend follows the global trend found in marine carbonate samples from the same age interval that corroborates increasing enrichment towards the top of the coal measures (approximately middle Oxfordian), followed by a shift to more negative compositions, which corresponds to the onset of the Springbok Sandstone deposition on an unconformable surface. •The Surat Basin's Walloon coals (Oxfordian) record peat accumulation under changing base level.•The coals show a positive excursion in δ13C towards the top of the coal measures.•This follows the global δ13C trend as recorded in marine carbonates from the same age interval.•The positive excursion in δ13C corroborates a global climatic shift in the Mid-Oxfordian.