The use of stable carbon isotope trends as a correlation tool: an example from the Surat Basin, Australia

Abstract

The Surat Basin’s Middle Jurassic Walloon Subgroup is a productive coal seam gas source in Queensland, Australia. The Walloon Subgroup can be subdivided into the Upper and Lower Juandah coal measures, the Tangalooma Sandstone, the Taroom Coal Measures, and the Eurombah/Durabilla Formation, from top to bottom. Correlation across the basin is challenging due to high lateral variability and lack of extensive stratigraphic markers. The Walloon Subgroup is also, in places, incised by the overlying Springbok Sandstone, sometimes interpreted as far down as the Tangalooma Sandstone. New age dates suggest that the Walloon Coal Measures are Oxfordian in age and mark a period of high rates of Corg production and burial, and an intermittent decrease of atmospheric pCO2. The un- or dis-conformable base of the Springbok Sandstone coincides with a turning point of this supposedly global phenomenon. This study uses organic stable carbon isotope trends as a correlation tool within the Surat Basin’s Walloon Subgroup and its overlying Springbok Sandstone. Analysis of a stratigraphic suite of coal samples from several wells across the Surat Basin shows a gradual enrichment in 13C up section from the Taroom to the Lower Juandah Coal Measures, with the most positive δ13C values within the Upper Juandah Coal Measures. Thereafter there is a rapid reversal to more negative δ13C values for coal samples of the Springbok Sandstone. The upward enrichment occurs well before the shift in maceral composition to increased inertinite content in the coals, suggesting more global allogenic processes are controlling the carbon isotopic trend. The consistency of these trends lends a more confident correlation for sub-units within the Walloon Subgroup, and assists in determining the level of incision disconformity of the Springbok Sandstone.

Integrating coal petrology, geochemistry and geochronology to understand spatial variation in the Walloon Subgroup

“…Maximum depositional ages were determined by applying the 'youngest 1σ grain cluster' method of Dickinson & Gehrels (2009), as this style of age determination provided the best match to the CA-TIMS ages of the tuffs of the Walloon Subgroup (Table 5- Chinchilla 4 was adjusted based on stable carbon isotopic trends in coal (as outlined in Hentschel et al (2016b). BSS=basal Springbok Sandstone, UJCM=Upper Juandah Coal Measures, LJCM=Lower Juandah Coal Measures, TSS=Tangalooma Sandstone, UTCM=upper Taroom Coal Measures, LTCM=lower Taroom Coal Measures. with the Walloon Subgroup (Adams et al, 2007).…”

“…Previous work also showed that the coal measures of the Walloon Subgroup and their clastic interburdens show a high degree of lateral variation in their lithology and continuity. Regional isotope excursion detected in the coals as a correlation tool (Hentschel et al, 2016b). Depositional ages interpreted from U-Pb detrital zircon geochronology show that the 'upper coal measures' on the Roma Shelf are significantly younger than their eastern equivalents, and that these are better correlated with the coals of the overlying Springbok Sandstone (Hentschel et al, 2017).…”

“…The Springbok Sandstone does contain thin coals, with similar maceral composition to the Upper Juandah Coal Measures, making the units commonly difficult to distinguish. A test of regional correlations of the Walloon Subgroup and Springbok Sandstone is difficult due to a lack of conventional stratigraphic markers Hentschel et al (2016a;2016b). conducted a detailed petrographic and isotopic study of the Walloon coals and detected an excursion in carbon isotopic composition to less negative values within the Juandah Coal Measures, which follow a global trend recorded in Oxfordian sediment.…”

“…Wainman et al (2017) proposed that deposition of the Walloon Subgroup was diachronus across the Surat Basin due to increased subsidence east of the Burunga-Leichardt, Wambo, Goondiwindi-Moonie fault system (Figure 5-1b). This could be the case, but an alternative interpretation is that the Walloon Subgroup pinched out progressively westward, and the strata on the western side of the basin are correlative with the Springbok Sandstone Hentschel et al (2016b). tested the stratigraphic framework for the Walloon Subgroup, as proposed by OGIA(2016), by conducting a carbon isotope study of the Walloon coals.…”

“…tested the stratigraphic framework for the Walloon Subgroup, as proposed by OGIA(2016), by conducting a carbon isotope study of the Walloon coals. A significant positive excursion in isotopic composition, with a turning point coinciding with the Springbok Unconformity, was used as an indicator to test and adjust estimates of erosional extent of the Walloon Subgroup in Indy 3(Hentschel et al, 2016b) and Pleasant Hills 25 (Figure 5-2). Thus, the coals within the Springbok Sandstone were possibly mistaken for the coals of the Upper Juandah Coal Measures in these wells.…”