Basin Evolution and Tectonic Environment.

Deposition of the Curdimurka Subgroup in the Euchre Pack Domain occurred during four

major transgressions (Figure 3.21, Table 3.2). The first began with the deposition of the

Dome Sandstone in a continental, braided stream environment changing to sub-wavebase with the deposition of the Rook Tuff. There was then a rapid regression leading to evaporite deposition at the base of the Dunns Mine Limestone, followed by a second transgression and deposition of the Interbedded Carbonate - Siltstone Facies in the Dunns Mine Limestone.

The second regression occurred at the base of the Recovery Formation, reaching a maximum with the deposition of the Planar Laminated Sandstone Facies in RSds2.

The third transgression occurred with a gradual deepening through interbedded siltstone- sandstone facies to the Black Shale Facies in RSst4. There was then a third regression which lead to deposition of the basal Hogan Dolomite to the CSlt4 at or about sea/lake

level, with a series of short-term transgressions and regression until the final major

transgression leading to the deposition of the Black Shale Facies in the Boorloo Shale. The

final regression resulted in a shallowing upward package through the Boorloo Dolomite

resulting in deposition of the Carbonate - Chert Facies in a sabkha or playa environment.

There are two points to note about the basin evolution as outlined above. The first is that except for the first regression (Sub-cycle 2, Table 3.2), the changes are gradual, and the

second follows on from this; there are extended periods of stability with only minor changes in water level, particularly during deposition of the Recovery and Cooranna Formations.

The rapid change of the first regression may indicate that other processes were operating

and it is possible that the regression marks a period of basin isolation and evaporation of a standing body of water rather than a rapid change in subsidence or sediment supply. The following regression (Sub-cycle 4) was also rapid with a return to deep water conditions,

although with the addition of carbonate-dominated density flows. Decreasing grainsize and

bed thickness from northwest to southeast suggests that the source of the carbonate may have been emergent conditions to the northwest. There was no increase in clastic sediments at this time, suggesting either limited topography and sediment was being trapped before entering the deeper part of the basin.

Comparisons with depositional models developed for rift basins (e.g., Leeder and Gawthorpe, 1987; Prosser et al., 1993; Landon, 1994; Lambiase and Bosworth, 1996; Gawthorpe and Leeder, 2000) and the examples outlined does not provide unambiguous support for deposition of the Curdimurka Subgroup in a rift environment (von der Borch 1980; Rowlands et al., 1980; Preiss, 1987, 1993, 2000).

Initial rift sedimentation is dominated by coarse-grained clastic sediments (conglomerates and sandstone) deposited in alluvial fans on the immediate footwall to the bounding fault (Schultz, 1994; Landon, 1994; Smith, 1995; Gawthorpe and Leeder, 2000; Changsong et al., 2001; McLeod, 2002; Mack, 2005). Basinward, the coarse-grained units grade into

finer-grained clastic units and evaporitic units as in the Gulf of Suez (Schultz, 1994),

Table 3.4. Evidence for the depositional environment of the Curdimurka Group be-

ing either marine or lacustrine.

marine lacustrine

evaporite mineralogy (not conclusive) lithofacies associations thick black shale facies (not conclusive) dry and wet mud flat facies

aeolian sand flats absent absence of herring bone cross-bedding

lack of cyclicity absence of high energy sandy units

Reconcavo Basin (Figueiredo et al., 1994), Erlian Basin (Changsong, 2001) or the Mangas Basin (Mack, 2005). The only conglomerates in the Curdimurka Subgroup occur near the base of the Dome Sandstone, and these are matrix-supported pebble conglomerates less than one metre thick, and so alluvial fans are absent. Because the margins of the basin are not exposed, it is possible that the basin margin facies are present but do not crop out. In the Reconcavo Basin, the conglomeratic alluvial fan complex occupies a 10 km wide strip adjacent the basin bounding normal faults (Figueiredo et al., 1994), which could be hidden at depth adjacent to the Mulroona Gravity Ridge, northeast of the Euchre Pack Domain. Then, the observed portion of the Curdimurka Subgroup corresponds to the basin centre

fine-grained clastic and evaporitic units.

An alternative model is that of Lambiase and Bosworth (1995) who developed a four

stage rift evolution model; initial faulting, half-graben development, filling of half graben,

and regional subsidence. They concluded that early rift sedimentation, in the initial

faulting stage, is dominated by fluvial sands, followed by paludral and shallow lacustrine

sediments in the half-graben development stage (Lambiase and Bosworth, 1995). Harris (2000) suggested that initial sediments in the Congo Basin follow this pattern. The basal Curdimurka Subgroup has similarities with the model of Lambiase and Bosworth (1995) to the level of the top of the Dunns Mine Limestone (allowing for either lacustrine or marine conditions). In the Lambiase and Bosworth (1995) model, the lacustrine sediments

are succeeded by fluvial sediments in the third stage of rift development whereas in the

Gradual degradation of the source area leads to fining upwards of the last in-filling sediments unless other factors become involved.

Consequent drainage basins now become established and can expand due to cessation of differential subsidence. Sediment input rates increase as does grainsize. Transverse systems prgrade across the basin, Small- scale CU and FU cycles may be associated with fan progradation and lobe switching, and eustatic sea level changes.

Increase in displacement and subsidence rates lead to the drowning of the basin axis, then hanging wall settings and may be finally

footwall drainage basins. Longitudinal

systems dominate if feeder channels are still subaerial. Vertical stacking of basinal deposits occurs. Low rates of sedimentation lead to the creation of topography.

Initial response to rifting is by pre-existing systems with large established drainage basins and continual flow. There are generally positioned in the basin axis and the deposits are often seen as early continental basins. Sedimentation rate is greater than or equal to subsidence.

Debris flows, seismites etc triggered by seismic events

Constant repeat time of seismic

displacement events but

recorded in the stratigraphy at variable thickness intervals due

to chanes in sediment accumulation rates. Late Post-Rift Immediate Post-Rift Rift Climax Rift Initiation

Gradual coarsening upwards as drainage basins are established and infilling of deeper water environments occurs

Figure 3.28. An idealized section of the vertical stratigraphy through the basin centre for sedimentation within a half-graben (from Prosser, 1993).

Curdimurka Subgroup, the deeper water sediments are succeeded by shallow water to emergent conditions.

At the scale of the basin, the one-dimensional half-graben model of Prosser et al. (1993) emphasises the relationship between sedimentation and movement on the bounding faults.

Periodic uplift of the footwall results in the influx of conglomerates and sedimentary

breccias into the hanging wall basin (Figure 3.28). There is no evidence for the deposition of conglomerates but there are several sandstone bodies which may be distal to conglomerates; at the top of RSst1, RSds2 and in the Boorloo Shale. These may be periods of fault movement

on the basin margins but otherwise accommodation space is created smoothly and filled

smoothly, with no evidence of there being a series of rapid fault movements.

Rift basin sedimentation models and examples also show evidence of significant topographic

relief that eventually becomes subdued in the post-rift stage. Harris (2000) interpreted the shallow water lacustrine Toca carbonates as being deposited on the the crests of fault

blocks that are surrounded by grabens in which marl is deposited. A similar configuration

is noted on the Devonian Lennard Shelf, in a marine environment (Playfair, 1984). In sub- aerial settings, crests of fault-blocks are eroded but basal units typically vary in thickness along strike, perhaps by hundreds of metres (Schultz, 1994). During deposition of the Dome Sandstone, particularly in the Stony Range and South Hill Domains where there is extensive along-strike outcrop with little or no variation of thickness, there is no evidence

of significant basement topography in two dimensions. At the scale of the Euchre Pack

Domain, the only evidence for topography is inferred from some clastic units thinning to the southeast, and from the Dunns Mine Limestone discussed above.

3.7.3. Relationship between the Curdimurka Subgroup and the Burra Group.