Syn‐rift sequence development in a fault‐controlled embayment (Early Permian Irwin River Coal Measures, Northern Perth Basin, Western Australia)

Abstract

Stratigraphic patterns and sequence development in tectonically active extensional basins remain poorly documented in comparison with passivemargin settings. Rift basin fills are generally characterized by coarseningupward trends in response to the rapid creation of accommodation by extensional faulting, and the progressive filling of graben during more quiescent periods. The Early Permian Irwin River Coal Measures in the Northern Perth Basin (Western Australia) record a complex stratigraphic arrangement of conglomerate, sandstone, mudstone and coal, and have been attributed to delta plain depositional environments that developed in a cool-temperate climatic setting during syn-rift activity. Sedimentary analysis of outcrop and core data from the fault-bounded Irwin Terrace is used to distinguish nine facies associations reflecting deposition in braided rivers, fixed-anastomosed channel belts, tide-influenced coastal environments and storm-affected distal bays. The broader depositional system is interpreted as a morphologically asymmetrical tide-dominated embayment with a fluvial and wave influence. The stratigraphic architecture of the Irwin River Coal Measures was strongly influenced by the evolving rift basin margin. Fault reactivation of the major basin-bounding Darling Fault in the early syn-rift phase caused footwall uplift and the inception of transverse palaeo-valleys occupied by braided fluvial systems. Fault block subsidence during the subsequent balanced, backstepping and drowning phases resulted in a dominantly retrogradational stacking pattern indicating progressive flooding of marginal-marine areas and culminating in deposition of distal marine elements. In the active rift basin, it is proposed that preservation of a shallow-marine syn-rift sequence was promoted by the geomorphological confinement of the embayed system increasing tidal current acceleration and hampering transgressive ravinement. The proposed sequence model demonstrates that transgressive successions can develop in the early syn-rift phase in response to footwall uplift and tectonic subsidence. The syn-rift sequence recording the filling of an embayment on a rift basin margin may be applied in similar tectonic and/or depositional contexts worldwide.

Neogene history of fluvial to shallow marine successions in the Kendari Basin, SE Sulawesi, Indonesia

“…Interpretation: The stacked fining upwards conglomerate and sandstone units separated by closely spaced erosion surfaces typically reflect multi‐storey channel fills (Allen, 1983; Bridge & Lunt, 2006; Dillinger & George, 2019; Miall, 1996). FAP1 includes a combination of major channels with multi‐storey sand bodies of intercalated stacked bars.…”

“…FAP1 includes a combination of major channels with multi‐storey sand bodies of intercalated stacked bars. Ripple cross‐laminated facies are indicative of near emergent conditions at channel margins and typically represent accretionary bar tops (Dillinger & George, 2019; Matoshko et al, 2016; Sambrook Smith et al, 2006). Convolute bedding reflects rapid sediment loading causing over steepening or dewatering (Plink‐Bjölrklund & Steel, 2004).…”

“…The cross‐stratified facies that commonly grade laterally and vertically into relatively massive or horizontal units represent deposition in longitudinal and transverse bars within a low sinuosity multi‐channel river (Miall, 1977; Ori, 1982; Rust, 1977). The lack of lateral accretion surfaces suggests predominant longitudinal, laterally or downstream accreting mid‐channel bars in braided channels (Di Celma et al, 2016; Dillinger & George, 2019; Martinsen et al, 1999; Miall, 2014). Stacked fining upward conglomerate and sandstone units separated by closely spaced, pebble‐rich erosion surfaces typically reflect multistorey channel fills (Allen, 1983; Dillinger & George, 2019; Miall, 1985, 1996).…”

“…Fining upward successions in such facies may also result from migration/avulsion of the nearby channel, whereas the coarsening upward trend might indicate a progradation of splays. Plant debris and root traces in the heterolithic facies indicate a low energy, non‐marine environment with episodic sub‐aerial exposure (Dillinger & George, 2019). This is consistent with the presence of red sediment layers that indicate an oxidising terrestrial environment.…”

“…The black layers suggest organic‐rich, probably plant material, accumulation. Non‐channelised sandstone beds (facies Sr and Sh) with unidirectional currents and high sedimentation rates may reflect crevasse‐splay deposits from overbank spills deposited on low‐relief floodplains (Dillinger & George, 2019; Ielpi, 2012; Michaelsen & Henderson, 2000).…”