Abstract Salt-sediment interaction in the Adelaide ‘Geosyncline’ of South Australia has had a pronounced influence on deposition of the Late Precambrian to Early Cambrian succession that is well exposed in the northern Flinders Ranges. In this area, mini-basins are effectively windows into the salt withdrawal-related sedimentary section. Depositional systems of minibasins and associated syn-depositional synclines are characterized by interpreted shallow and deep-water facies. In this paper, shallow-water facies/sediments are defined as those deposited above storm wave base, while deep-water facies/deposits are classified as those deposited below storm wave base. The mini-basins are interpreted to have formed by either a combination of salt withdrawal and salt dissolution (Breaden Hill Mini-basin), or salt withdrawal (Beltana Mini-basin). Salt dissolution troughs are characterized by marine sediments interpreted to have been deposited in shallow water on the floor of the minibasin. In contrast, interpreted lowstand, basin floor and slope fans occupy the basal fill of salt withdrawal troughs. The Beltana mini-basin, the model discussed in this paper, is a classic example of an interpreted salt withdrawal mini-basin. An angular unconformity overlies slumped, lowstand facies of the mini-basin and marks the transition to regressive sedimentation. Regressive sedimentation of the highstand systems tract in this model is followed by pronounced subsidence and abundant sediment supply, resulting in a localized 2- to 5-fold increase in the depositional thickness of tidal marine facies. Stacking patterns suggest these facies can be assigned to the aggradational wedge systems tract that onlaps the rising flanks of the minibasin. A major angular unconformity caps this sequence, marking incision of sea floor valleys that are subsequently infilled with shoreface facies. Continued subsidence results in the formation of a broader syn-depositional syncline above the mini-basin. A submarine canyon directly overlies the axis of the main minibasin fill. It can be traced updip to a location where a deep-water dolostone overlies a surface of terrigenous sediment starvation. Salt withdrawal and sediment loading combined to drop the floor of the mini-basin, and as sediments of the mini-basin flanks were uplifted and rotated, the relative fall in base level resulted in canyon incision. As subsidence continued, over-steepening of the basin margin resulted in slump and debris flow facies derived from the wall of the mini-basin or the submarine canyon. These deposits were contemporaneous with, or closely followed by, deposition of a channelized, clast-supported breccia. Submarine canyons in mini-basins and syn-depositional synclines had no shallow-water, updip equivalent. Instead, the deep marine sediments, subsequently deposited, were assigned to the forced transgressive systems tract. Sedimentation was generally transgressive, but was punctuated by unconformity-based debris flows and channel-fill breccias. The Beltana mini-basin model predicts a number of stratigraphic, structural and combination hydrocarbon trap types that can be tied to the sequential formation of major unconformities within the minibasin.

Abstract This paper details the evolution of allochthonous salt systems in the Adelaide Geosyncline of South Australia, a divergent continental margin that developed after rifting of the Neoproterozoic supercontinent. It differs from previous studies in that it suggests salt-sediment interaction had a great influence on stratigraphic architecture of the late Precambrian to Early Cambrian succession. This stratigraphic architecture reflects the high subsidence and sedimentation rates in halotectonic mini-basins during divergent margin development of the Adelaide Geosyncline. Sedimentation can be tied to specific stages in the evolution of the various allochthonous salt systems. Lacustrine, fluvial, and evaporite facies of the rift phase, assigned to the late Precambrian Callanna and Burra groups, were deposited in salt-canopy mini-basins that were floored by shallow-water sediments. Debris flows indicated a stage in the development of the divergent margin where gravity flow deposits were associated with marine sedimentation. The transition phase heralded a series of marine incursions into mini-basins, climaxing in the extrusion of a very thick salt canopy that served as a basal detachment surface for the breakup unconformity. Overlying marine sediments of the late Precambrian Umberatana Group were deposited adjacent to reactive diapirs during the passive margin phase. As diapirs passed into the active stage, they separated mini-basins having counter-regional geometries. Progradation and aggradation of the passive margin resulted in salt-canopy mini-basins floored by deep-marine sediments of the late Precambrian Lower Wilpena Group that evolved into counter-regional salt systems. Christmas tree diapirs were characteristic of this final counter-regional phase.

Abstract Christmas tree diapirs in the Adelaide “Geosyncline” of South Australia are characterized by lateral tongues of allochthonous breccia derived from an autochthonous level that contains evaporites and a caprock assemblage of blocks (or ‘rafts’) of non-evaporitic lithologies. These breccias may be attached to a diapiric trunk (consisting of evaporites and breccias) that may be connected to the autochthonous level. This paper describes two prime examples that outcrop at Pinda Springs and Wirrealpa Springs in the central Flinders Ranges of South Australia. The Christmas tree diapirs evolved from an initially reactive triangular, or anvil-shaped, diapir and separate two mini-basins of different accommodation potential and often completely different sedimentary facies. An interpreted submarine salt glacier at Pinda diapir was extruded as an allochthonous salt tongue nearly 7 km long and 500 m thick. It overlies a major disconformity close to the diapir and the step-like base suggests aggradation and progradation within the transgressive systems tract. At Wirrealpa diapir, multiple tongues are contained within some 50 high-frequency sequences of the main mini-basin where deposition was influenced by the high subsidence rate, coupled with high sediment supply. The distinction is made between halokinetic sequences bounded by ravinement surfaces and the high-frequency sequences bounded by regressive surfaces of erosion. Both sequence types can be arranged into progradational, aggradational, and retrogradational stacking patterns. The model for Christmas tree diapirs, based on outcrop mapping, done as part of this study, suggests that a number of potential hydrocarbon traps can be related to different stages in the evolution of these halokinetic structures.

Abstract Salt-sediment interaction influence the stratigraphic architecture of depositional sequences during the passive margin stage of development of the Adelaide Geosyncline in South Australia. This late Precambrian to Early Cambrian succession outcrops in the Flinders ranges and is punctuated by submarine unconformities. The unconformities are interpreted as deep-water sequence boundaries that have no shallow water equivalent. However, they can be traced laterally into condensed sections or major flooding surfaces. These sequence boundaries are typically overlain by deep-water facies that reflect the high rate of subsidence coupled with high sedimentation rates typically found in halotectonic mini-basins. Large-scale slumping and canyon incision often mark the deep-water sequence boundaries. Canyon incision is associated with salt withdrawal or downbuilding in mini-basins. The architecture of submarine canyons is defined by the nature and correlation of bounding surfaces, sandstone interconnectivity and lithofacies elements. These features are often beyond the limits of seismic resolution in basins where passive margin development was influenced by salt tectonics.

Abstract Parts of two third-order Neoproterozoic (Marinoan) depositional sequences are documented in the Wilpena Group (Wonoka Formation and Bonney Sandstone) at Patawarta diapir, located in the central Flinders Ranges, South Australia. These sequences represent an overall regressive succession transitioning upwards from outer to middle wave-dominated shelf deposits to a tidally dominated barrier bar to coastal plain. The lower, middle, upper limestone and green mudstone informal members of the Wonoka Formation comprise the Highstand Systems Tract of the lower sequence. The Sequence Boundary is at the top of the Wonoka green mudstone member and is overlain by the Lowstand Systems Tract of the upper sequence, which includes the lower dolomite, sandstone and upper dolomite beds of the Patsy Hill Member of the Bonney Sandstone. The upper sequence Transgressive Systems Tract comprises the Bonney Sandstone. These units comprise one complete tapered composite halokinetic sequence (CHS). The lower halokinetic-sequence boundary is associated with the Maximum Flooding Surface of the lower depositional sequence and the upper halokinetic-sequence boundary is interpreted as the Transgressive Surface of the overlying depositional sequence where an angular truncation of up to 90° is documented.

Abstract An interval several hundred metres thick in the lower part of the late Ediacaran Billy Springs Formation (Fm.) of the NE Flinders Ranges, South Australia, includes both diamictic levels, ‘dropstones’ and isolated ‘stone-clusters’ in a thin-bedded, silty or fine sandy matrix, which is commonly laminated. Slumping at a variety of scales is prevalent, but extensive panels of ‘right-way-up’, moderately dipping beds host dropstones, which disturbed the laminae. It is difficult to explain laminated sediments peppered with dropstones other than by ice-rafting, and the stone-clusters comply with sinking pieces of ice loaded with debris. Although common reworking occurred in channels, larger erratics in this material are out of hydrodynamic equilibrium. Deposition occurred offshore on an unstable slope. Isotopic δ 13 C carb measurements show a strong negative excursion through the preceding Wonoka Fm., and several further similar negative excursions in the Billy Springs Fm. This record shows similarities to that of Ediacaran carbonates on the Yangtze platform, south China. Compilation based on a survey of palaeomagnetic data for Gondwana continents, Baltica and Laurasia permits a possible palaeogeography indicating a relatively high palaeolatitude at the time of deposition. Indications of age are imprecise but may be comparable with, or younger(?) than the c. 580 Ma Gaskiers Fm. of Newfoundland.