Late Proterozoic Submarine Canyons of Adelaide Geosyncline, South Australia

Abstract

A well-exposed series of major basin-slope submarine canyons of late Proterozoic age occur within the mixed carbonate and siliclastic Wonoka Formation of the Adelaide Supergroup (Adelaide geosyncline), Flinders Ranges, South Australia. They were cut to depths of about 1.5 km into an underlying deltaic and basin-slope progradational sequence which includes the Brachina Formation and the ABC Range Quartzite. The canyons occur in close association with local synclinal structures. An unfolded section across one canyon (Patsy Springs Canyon) shows steep, almost vertical canyon walls, two axial channels, and a tributary canyon. The canyon-filling sequence includes canyon-wall and axial debris flow material, axial channel conglomerates, ripple cross-laminated sands and silts, hin- and thick-bedded turbidite sands, and a series of spectacular sand-filled megachannels which markedly truncate underlying units. Lenticular olistostromes of micritic carbonate clast breccias of carbonate-platform derivation occur throughout canyon-fill and immediately overlying strata. The sedimentary sequence preserved in the canyons demonstrates an episodic process of canyon backfilling by onlap of an aggrading submarine-fan sequence. Backfilled canyons were finally buried by progradation of a complex unit comprising interdeltaic carbonate platform, delta, and basin and slope environments. Evidence from the Wonoka Formation canyons suggests that initial canyon incision was triggered by a significant fall in relative sea level, which allowed fluvial systems to deliver their bed loa s directly to the contemporary basin slope. A subsequent relative rise in sea level was required during latter stages of canyon filling to account for delta and basin-slope progradation of the Wonoka Formation.