Discusión y conclusiones del estudio

Cycle 4 begins with a rapid transition from the Black Shale Facies at the top of Cycle 3 to emergent in the Lower Hogan Dolomite (Figure 3.21). The lower 30 metres of the Hogan Dolomite comprises from its base, the Microbial Carbonate Facies, Carbonate-Chert Facies and the Black Shale Facies, and then back to the Carbonate-Chert Facies before the poorly outcropping siltstone above (Figure 3.21). Both to the north and south, it is affected by deformation but there is little change along the length of its outcrop until is it faulted out entirely. Most of the Upper Hogan Dolomite does not crop out except for narrow beds of dolomite protruding a few centimetres above the surface.

The upper few metres of the Hogan Dolomite comprises the Siltstone-dominant Siltstone- Sandstone Sub-facies with abundant well formed halite casts in siltstone. Above this is a two metre thick unit of dolomitic Carbonate-Chert Facies, with quartz and chert nodules

forming up to 20% of the rock mass. It also contains thin beds and lenses of fine-grained

sandstone up to 2 cm thick within the dolomitic unit.

The Cooranna Formation represents a series of short sub-cycles. It was divided by Murrell (1977) into four members; a basal sandstone-siltstone unit (CSlt1), two middle members that are mainly siltstone with minor sandstone (CSlt2 and CSlt3) and an upper siltstone

and sandstone unit (CSlt4: Figure 3.21). Carbonate beds occur in all four of these units,

although less so in the upper unit. Deformation has altered the thickness of the Cooranna Formation along its outcrop length, with repetitions due to thrusting in the north and middle of the area and removal of stratigraphy at the southern end due to low-angle normal faults (Figure 3.1). Because the Cooranna Formation is mainly siltstone and the faults are low- angle, juxtaposing one thinly-bedded siltstone block against another thinly-bedded siltstone

block, it is difficult to analyse any thickness changes due to depositional factors.

In CSlt1 the main lithologies are siltstone and sandstone with a few thin carbonate beds. Halite casts are moderately common. The facies present are the Sandstone Dominant Sandstone-Siltstone Facies and Siltstone-dominant Siltstone-Sandstone Sub-Facies (Figure 3.21, Table 3.2).

The basal section of the CSlt2 comprises repeating units of black shale, carbonates and siltstone. The carbonate beds are up to one metre thick and commonly contain rod-like pseudomorphs of K-feldspar. They are most common in the bottom third of the unit, decreasing in abundance up section. Three or four black shale beds are present, the thickest being about 20 m thick. The upper part of the unit comprises thinly interbedded siltstone and sandstone. The facies present are the Siltstone-dominant Siltstone-Sandstone Sub- Facies, the Black Shale Facies and the Carbonate-Chert Facies (Figure 3.21, Table 3.2).

CSlt3 comprises laminated siltstone with minor sandstone beds, thin carbonate beds and a one metre thick black shale bed. Carbonate beds contain chert after anhydrite. The Siltstone- dominant Siltstone-Sandstone Sub-Facies is dominant, with thin units of the Black Shale Facies and the Carbonate-Chert Facies (Figure 3.21, Table 3.2).

CSlt4 comprises interbedded siltstone and sandstone. It is more sandy toward the base and more silty toward the top. Sedimentary structures include ripple cross-beds, wavy bedding and lenses of sandstone. Desiccation cracks are common. There is a three metre thick black shale near the middle of the member. Hence, the facies present are Sandstone Dominant Sandstone-Siltstone Facies, Siltstone-dominant Siltstone-Sandstone Sub-Facies and the Black Shale Facies (Figure 3.21, Table 3.2).

As a whole, the Cooranna Formation can be interpreted as a time of fairly stable deposition at or about the same water level (Figure 3.21). In detail, it comprises a series of transgressive

and regressive cycles, finishing with the beginning of a transgressive cycle.

The Boorloo Siltstone represents a time of maximum transgression followed by a final

regression. It comprises two units, a black shale and siltstone dominated lower unit, here termed the Boorloo shale (BShl) and an upper unit comprising interbedded siltstone and dolomite here termed the Boorloo dolomite (BDol). The Boorloo shale is about 250 m thick and the Boorloo dolomite about 120 m thick in the centre and north of the Euchre Pack Domain (Figure 3.21). However, south of 6702500 mN (Figure 3.1), the BShl is about 150 m thick and the BDol is about 20 m thick. The thinning of the BDol is partly due to thinner clastic units between the carbonate beds and partly due to a fault having removed the top of the BDol. About 20 m above the base of BShl is a thin carbonate unit, up to 10 m wide and there is an eight metre thick sandstone unit about 50 m below the base of the BDol, which south of about 6702500 mN (Figure 3.1) has thinned to less than one metre.

BShl comprises mainly the Black Shale Facies (Figure 3.21, Table 3.2). The carbonate unit near the base is the Interbedded Carbonate-Clastic Facies but is too thin to show in Figure 3.21). From the middle to northern end of the area, it is intensely deformed, with folding and shearing destroying primary structures. South of 6702500 mN it is less deformed and contains rod-like pseudomorphs. The sandstone unit belongs to the Parallel Laminated

Sandstone Facies, being fine to medium grained and laminated with one bed of planar

cross-beds (Figure 3.21). Approaching the BDol, three thin carbonate units (less than 30

cm) occur, over about 30 m true thickness. They are black and very fine-grained and may

be due to transport of carbonate down-slope but are too thin to show in Figure 3.21. The BDol comprises up to 12 carbonate units, separated by clastic siltstone/sandstone intervals. From the base to the top, there is a gradual change in the dominant facies, from Interbedded Carbonate-Clastic Facies, to the Microbial Carbonate Facies with the Carbonate and Chert Facies at the top (Figure 3.21. Table 3.2). The carbonate is mainly dolomite although limestone occurs in one unit likely due to alteration of dolomite and in the upper-most carbonate unit. The clastic units are the Siltstone-dominated Siltstone- Sandstone Facies.

The base of the Boorloo Siltstone is a continuation of a transgressive phase, that commenced at the top of the Cooranna Formation (Figure 3.21). There is a brief regression leading to the deposition of the thin dolomitic unit close to the base, followed by transgression. Again, there is a short regression, with deposition of the sandstone unit, with another transgression,

until the final transgression of the BDol (Figure 3.21, Table 3.2).