Burial History of Jurassic Gondwana Surface West and Southwest of Lake Eyre, Central Australia

Cadernos Lab. Xeolóxico de Laxe Coruña. 2010. Vol. 35, pp. 221 - 242 ISSN: 0213-4497

Burial history of Jurassic Gondwana Surface west and southwest of Lake Eyre, central Australia

WOPFNER, H.1

(1) Institute of Geology, University of Cologne, Germany

Abstract The Gondwana Surface evolved during the second expansion phase of the super-continent in Late Triassic and Jurassic times. From an initial time of crustal stability it led up to the formation of expanding rift systems which initiated the final fragmentation of Gondwana in the Early Cretaceous. Along the western margin of the Eromanga Basin the stable phase is reflected by a thick regolith of kaolinised bedrock and the formation of the Gondwana Surface. During the Jurassic the surface was covered gradually by the quartzose and kaolinitic Algebuckina Sandstone, deposited by large, high volume river systems, dominated by low pH, “black water” conditions. Palaeobotanical evidence indicates a generally humid, subtropical climate. Expansion of the rift system along the southern margin of Australia coincided with the marine transgression from the east at the beginning of the Cretaceous. Various near-shore facies of the Cadna-owie Formation, including large individual boulders, attest to labile conditions during transgression. The conglomeratic Mt Anna Sandstone Member of the Cadna-owie Formation, containing well rounded pebbles of Gawler Ranges porphyry, demonstrates the formation of a new rift shoulder simultaneous with the marine transgression. Exhumation of the Gondwana Surface commenced in the Early Tertiary and continued episodically until the present time.

Key words: Gawler Range Massif; palaeosurface; transgression; fluvial sandstone; silicification. 222 ��Wopfner � CAD. LAB. XEOL. LAXE 35 (2010)

INTRODUCTION (KOGBE, 1976), the Jurassic part of the “Continental intercalaire” in North African During the Jurassic large parts of Gond- basins (GUIRAUD et. al., 2005), the base wana experienced widespread exposure of the Sunday River Beds in eastern South and morphological stability. Weathering Africa and pre-Cretaceous, current bedded, and planation produced a senile landscape. kaolinitic sandstones of the Luwegu Basin This surface is generally referred to as the in southern Tanzania, all witness compara- Gondwana Surface. In syneclises or regional ble morpho-tectonic histories (WOPFNER, depressions vast fluvial systems developed 1983). A similar story is revealed at the base and the sand bodies deposited within them of the “Upper Gondwanas” in Peninsular gradually covered parts of the Gondwana India (CASSHYAP, 1979; CASSHYAP and Surface. In Australia these sandstones (Hut- TIWARI, 1987). ton, Mooga, Algebuckina and equivalents) The burial of the Gondwana Surface became important artesian aquifers of the thus represents a global event connected “Great Artesian Basin”, now termed the with the second distensional phase of Gond- Great Australian Basin of which the Ero- wana (VEEVERS, 2000) and the subsequent manga Basin is part. Well documented ex- dispersion of Gondwana fragments. The posures of the Gondwana Surface exist at present paper deals with the development the northern margin of the Eromanga Ba- of the western and southwestern Eromanga sin near Mt Isa (TWIDALE and CAMP- Basin in Australia, where environmental and BELL, 1988) and along the southern basin tectonic forcing mechanisms are especially margin near Tibooburra and Milparinka, well demonstrated. where large fields of silicified tree trunks, some in growth position, attest to prolific EXPOSURES OF GONDWANA plant growth. (WOPFNER, 1983). The sur- SURFACE face is known also from rift structures and pericratonic basins along the southern mar- Positive identifications of the Gondwa- gin of Australia, like the Polda Basin and na Surface has been achieved either in the the neighbouring Bight Basin (HARRIS subsurface, where the contact with overly- and FOSTER, 1974; GATEHOUSE, 1995) ing deposits has been identified by drilling and the early graben developments within or along basin margins and the periphery of the Gambier-Otway Basin (WOPFNER inliers, where the surface has been exhumed and DOUGLAS, 1971; MORTON et al., from beneath covering sediments. In the 1995). The base of the Barbwire Sandstone subsurface the Gondwana Surface has been in the Canning Basin of Western Australia identified in a number of drill holes which also corresponds to the Gondwana Surface. penetrated Jurassic and older strata, espe- Here the surface leads from the cratonic cially near the margin of the Eromanga Ba- platform down into marine environments of sin, as for instance north and west of Ood- the distensional realm of the West Austral- nadatta (HESS, 1957; DEMAISON, 1969; ian Trough. WOPFNER, 1970), northwest of Marree In Africa the Gundumi Formation of (KRIEG et al., 1991) or in Fortville No the Iullemeden Basin in northern Nigeria 1-well close to the southern margin of the CAD. LAB. XEOL. LAXE 35 (2010) Burial history of Jurassic Gondwana Surface 223

Basin (WOPFNER and CORNISH, 1967), lents of the Algebuckina Sandstone are re- just to mention a few of the numerous inter- ferred to as Da Souza Sandstone, the edge sections of the surface in drill holes within of the emergent surface can be traced along the Eromanga Basin. a northeast-striking lineament to Finke Sta- Surface exposures are widespread along tion on the now defunct Central Australian the western margin of the Eromanga Basin. Railway Line, where it is developed on Per- Near the border between the Northern Ter- mian sedimentary rocks (Fig. 1). From there ritory and South Australia the surface has the edge is offset to the north and contin- planed adamellites and granodiorites of the ues along the northeast-striking Rumbalara Palaeoproterozoic Kulgera plutonic suite. Hills, whence it disappears underneath the The surface emerges from beneath a cover sand dunes of the Simpson Desert (see of Algebuckina Sandstone west of about WELLS et al., 1970). 134o 06’ E longitude (Fig. 1) and extends South of the Northern Territory/South westward, where TWIDALE and CAMP- Australian border the edge of the Gond- BELL (1995) recognised “a Jurassic granite wana Surface, still expressed on rocks of the inselberg landscape” in the Kulgera region. Kulgera plutonic suite, extends for about 15 In the Northern Territory, where equiva- kilometres towards Tieyon Station. About

Fig. 1. Generalised structure contour map of base of Meso- zoic of southwestern Eromanga Basin; also showing localities mentioned in text. As Triassic strata are absent west of the Dalhousie-Muloorina Hinge, the structure contour lines approxi- mate present subsurface position of Gondwana Surface in that region (modified and expanded after KRIEG et al., 1995). 224 ��Wopfner � CAD. LAB. XEOL. LAXE 35 (2010)

30 km southwest of Tieyon it is manifested NER et al., 1974). In both areas, Tieyon and on Early to Middle Proterozoic gneisses of Granite Downs, the emerging Gondwana the Musgrave metamorphic complex, before Surface is cut at an oblique angle by the Early being covered by aeolian sands and other Tertiary Cordillo Surface and its associated Quaternary materials. It re-emerges again silcrete (Fig. 2). As the weathering processes near Granite Downs, but here it has devel- on both surfaces involved the alteration of oped on folded sedimentary rocks of Neo- feldspars and other silicate minerals, includ- proterozoic age. In all the localities men- ing clays, to kaolinite, the thickness of the tioned so far the rocks below the Gondwana chemical alteration zone observed today Surface are intensely altered, all constituents may represent the cumulative effects of both except quartz having been transformed to weathering episodes (WOPFNER, 1964; kaolinite: this zone of chemical alteration WOPFNER and WALTHER, 1999). may in places exceed 30 metres in thickness. Very little is known about the Gondwana At sites like the Rumbalara Hills deposits of Surface to the west and northwest of the ba- red ochre are associated with the Surface, sin margin. Bevels which are well above the the formation of ochre resulting from the level of Tertiary terraces are fairly common alteration of pyrite after exhumation. in the Musgrave Ranges in northwestern The antiquity of the pre-Jurassic land- South Australia. Undoubtedly these bevels scape in that region was recognised by JACK relate to an ancient land surface. This con- (1915), as pointed out by TWIDALE and forms to the suggestion of TWIDALE and CAMPBELL (1993). However, the ledges CAMPBELL (1995) that the lower slopes of and mesas covered with siliceous duricrust the Musgrave and Everard Ranges have been ascribed by JACK (1915) to pre-Jurassic exhumed, “whereas the peaks rose as insel- erosion are now recognised as remnants of bergs” above the Jurassic fluviatile plains. an Early Tertiary (Eocene) surface (WOPF- The fact that Early Tertiary silcretes at the

Fig. 2. Schematic cross-section through western margin of Eromanga Basin at Lester’s Well, east of Granite Downs, showing present-day relationship between Jurassic Gondwana Surface and Early Tertiary Cordillo Surface (modified from WOPFNER, 1964). CAD. LAB. XEOL. LAXE 35 (2010) Burial history of Jurassic Gondwana Surface 225

southern margin of the Everard Ranges of the Leigh Creek coal basin, Algebuckina are only slightly above plain level indicates Sandstone forms a very prominent mesa. that the levels of corestones, situated well Here the Gondwana Surface is expressed below the summit surface, resulted from by a low angle unconformity between the deep chemical weathering of the Gondwana Rhäthian/Liassic Leigh Creek Coal Meas- Surface. Obviously there is ample scope for ures and the Algebuckina Sandstone. Silici- rewarding research in the future. fied stem fragments, up to 30 cm in length South of the latitude of Granite Downs are commonly embedded within the current exposures of the Gondwana Surface are bedded sandstone, indicating episodic input rare. The surface becomes prominent again of plant debris. Near Lyndhurst, about 30 at the southwestern margin of the Eroman- km north of Leigh Creek, a regolith consist- ga Basin, where the basin borders the Meso- ing of intensely kaolinised, Neoproterozoic proterozoic acid volcanics of the Gawler strata and large blobs of red ochre, meas- Ranges. Erosion of the “basal Jurassic sand- uring 30 to 80 cm, is ascribed also to the stone” (JACK, 1931), now recognised as Al- Gondwana Surface. gebuckina Sandstone and of the Mt Anna At the northeastern-most point of the Sandstone Member, both of which onlapped Flinders Ranges, the north-dipping Gond- the Gawler Ranges, has exposed large tracts wana Surface is identified by the base of the of the Gondwana Surface. Within the Rang- plant-bearing Algebuckina Sandstone at Mt es themselves the Gondwana Surface is re- Babbage (GLAESSNER and RAO, 1955). garded as the precursor of the Nott Surface, In earlier literature the sandstone at Mt viz. the ancient etch surface represented by Babbage was referred to the stratigraphic the bornhardt massif (CAMPBELL and unit ‘Blythesdale Sandstone‘ (WOODARD, TWIDALE, 1991; TWIDALE and CAMP- 1955) which induced TWIDALE (2007) to BELL, 1993). relate the pre-sandstone surface and asso- On the southern margin the Gondwana ciated summit surfaces to the Early Creta- Surface extends on to the Neoproterozoic ceous marine transgression rather than to and Cambrian strata of the Willouran the Jurassic fluvial event. Ranges south of Marree and the Flinders In addition to the exposures along the Ranges further to the southeast. On the basin margin, the Gondwana Surface is well Curdimurka 1:250,000 geological map documented on and around the periphery KRIEG et al. (1991) noted the widespread of the Precambrian inliers of the Peake and subsurface occurrence of deeply weathered Denison Ranges (Fig. 1). This up-faulted pre-Jurassic strata consisting of “greyish basement block is the type area of the Al- white clay passing down to mottled reddish gebuckina Sandstone and the succeeding, brown clay…”. KRIEG et al. (1991) named Early Cretaceous formations of the Neales this zone of deep chemical alteration Bop- River Group (WOPFNER et al., 1970). At eechee Regolith and placed it within the in- Algebuckina Hill, the actual type locality terval post-Permian and pre-Jurassic. The of the Algebuckina Sandstone, the forma- regolith is thus an equivalent of the weath- tion rests unconformably on basement of ering front associated with the Gondwana intensely folded Palaeoproterozoic Peake Surface. Near Copley at the southern end Metamorphics. The basement-rocks which 226 ��Wopfner � CAD. LAB. XEOL. LAXE 35 (2010)

comprise gneiss, migmatite and mica schist the sandstone has altered all constituent have been deeply weathered and kaolinised silicate minerals to kaolinite, leaving only (Fig. 3). The process which took place prior quartz and some other resistant minerals and during the early stage of deposition of unchanged. The altered rocks still preserve

Fig. 3. Unconformity between kaolinised, Palaeoproterozoic migmatite of Peake Metamor- phics (Pl) and Algebuckina Sandstone represents the Gondwana Surface. Bold exposure above the unconformity consists of medium-grained, ill-sorted, kaolinitic sandstone. Locality is about 4.8 km west of Algebuckina Hill.

Fig. 4. Completely kaolinised pegmatite, cutting across equally kaolinised migmatite of the Peake Metamorphics is recognizable only by the coarse fabric of remnant quartz. Pebbles of quartz and rip-up clasts of kaolinised basement are recognizable at the very base of the overlying Algebuckina Sandstone. CAD. LAB. XEOL. LAXE 35 (2010) Burial history of Jurassic Gondwana Surface 227

Fig. 5. Mesas of Algebuckina Sandstone and Cadna-owie Formation at Mt Anna rest on Gondwana Surface. The surface cuts across folded sedimentary rocks of the Neoproterozoic Burra Group, visible in the middle ground of the picture. Mt Anna, recognizable by its white cap of bleached Bulldog Shale (Aptian), is in central background of picture. Silcrete and fossil plant locality is at low mesa furthest to the right. the fabric of the original rock, which is well On palaeo-highs, like the Mt Margaret demonstrated in the case of a completely ridge, Algebuckina Sandstone was not de- kaolinised pegmatite (Fig. 4). posited and the Gondwana Surface did not Elsewhere, the Gondwana Surface has become covered until the Early Cretaceous been developed on folded and truncated transgression (WOPFNER, 1968; ROG- Neoproterozoic sedimentary rocks of the ERS and FREEMAN, 1996). No kaolinisa- Adelaide System as at Mt Anna further to tion is discernible at this location, indicating the south (Fig. 5; WOPFNER and HEATH, that such “bald heads” had either not been 1963; WOPFNER et al., 1970), on Artins- affected by kaolinisation due to denudation kian coal measures of the Mt Toondina or the old regolith had been eroded prior to Beds west of Algebuckina Hill (FREYTAG, the transgression. 1965; WOPFNER, 1970) or on Early Permi- Until the onset of the Early Cretaceous an glacial deposits west of Mt Dutton, the transgression the Gondwana Surface ap- northernmost inlier of the Peake and Deni- pears as a stable, senile landscape with oc- son Ranges (HEATH, 1965). At all these casional inselbergs and low swells. localities the Gondwana Surface is covered unconformably by Algebuckina Sandstone (ROGERS and FREEMAN, 1996). 228 ��Wopfner � CAD. LAB. XEOL. LAXE 35 (2010)

BURIAL HISTORY pebble layer thick, as demonstrated in Figure 6. The direction of the foreset beds is almost Algebuckina Sandstone invariably within the north to northeast sec- tor. However the proportion of pebble- to The first sediments to cover the - Gond cobble-sized clasts varies from locality to wana Surface were the fluvial deposits of the locality. At Mt Anna for instance it is con- Algebuckina Sandstone, described by WOP- siderably higher than at the type section. FNER et al. (1970). Within the type area The upper part of the Algebuckina around the Peake and Denison inliers the Sandstone, with the exception of the up- formation consists of fine to coarse grained, permost unit, consists of coarse-grained to quartzose, current bedded sandstones. It granule-sized, slightly kaolinitic and gener- comprises a lower succession characterised ally bimodal, quartz arenites. Kaolinite in by a pore-filling kaolinitic matrix and an upper, generally well sorted and clean sequence. As mentioned above, the Sandstone rests unconformably on older, strongly kaolinised rocks, ranging in age from Palaeo- proterozoic to Permian. Commonly a thin, basal conglomerate, consisting predomi- nantly of well rounded quartz pebbles and some rip up clasts occurs at the base of the sandstone. Occasionally, isolated pebbles of kaolinised volcanics or other igneous rocks are observed. West of Algebuckina Hill (Figs 3 and 4), however, the base of the section is formed by 20 cm to 50 cm of massive and resistant, kaolonitic sandstone. On clos- er scrutiny some relic current structures are discernible but most structures were obliter- ated by the invasion of pore-filling kaolinite, indicating that the process of kaolinisation and bleaching was not only synchronous with deposition but also continued after the latter had finished. Kaolinite extends into the lower parts of the formation, which consist of fine to Fig. 6. Typical lithofacies of lower Algebuckina medium grained, kaolinitic sandstones. The Sandstone, near Mt Anna, consists of planar current dominant sedimentary structures are uni- beds bounded by level erosional surfaces outlined by directional, angular current beds, bounded lag gravels. The lag gravels consist of quartz and kaolinitic shale pebbles. Climbing ripple marks are rec- on bottom and top by quasi-horizontal ero- ognizable at bottom of picture. The dark sandstone sional surfaces, commonly accompanied by at the top of picture belongs to the coarse-grained, lag gravels. The latter are usually only one bimodal facies of the upper part of the formation. CAD. LAB. XEOL. LAXE 35 (2010) Burial history of Jurassic Gondwana Surface 229

the matrix rarely exceeds 5 percent of the sandstones of the lower section by its excel- pore volume. Aggregates of 8 mm to 10 mm lent sorting and by the complete absence of spheres consisting of quartz grains cement- kaolinite or other matrix material. Further ed by carbonates and recrystallised kaolinite characteristics are sigmoidal current beds, are common (Fig. 7). The beds of this inter- dewatering structures as (Fig. 8) and incipi- val are invariably current bedded, often with ent silicifications. At Mt Anna the top 70 cm concave foresets and cut and fill structures. to 150 cm of this unit have been altered to The top unit of the formation is formed silcrete, containing well preserved casts of by fine to medium grained, clean, quartz plant fossils (Fig. 9; see WOPFNER and sandstone. At the Mt Anna section this HEATH, 1963; WOPFNER et al., 1970). unit is separated from the preceding units The silcrete is a typical early diagenetic sil- by an erosional surface and a basal gran- crete formed by quartz overgrowth in opti- ule conglomerate. The unit differs from the cal continuity with the original clastic quartz

Fig. 7. Aggregates of sandstone spheres, consisting of carbonate and kaolinite cemented sand are typical features of the coarse grained sands in the upper third of Algebuckina type section. Coin diameter is 2 cm. 230 ��Wopfner � CAD. LAB. XEOL. LAXE 35 (2010)

grains (homoaxial overgrowth). Thus, the quartz crystals of the silcrete form a dense and interlocking fabric. They are well oriented along the C-axes (Fig. 10). Homoaxial quartz overgrowth follows the position of the C-axis of the host grain. The orientation therefore reflects the grain orientation of the original sand fabric. This, together with the excellent sorting indicates a steady, quasi laminar flow regime just above the thresh- Fig. 8. Contorted folds, symptomatic of the collapse and dewatering of quicksand, are common structures old of bed load movement. The existence in the uppermost portion of well sorted Algebuckina of quicksand is evidenced by the dewater- Sandstone at Mt Anna locality. ing structures. Quicksand requires a fabric of random grain orientation and relates to movements at the low energy end of vortex systems. A more detailed discussion of the depositional environment is given below. The macro-flora of the Algebucki- na Sandstone comprises Cladophlebis cf.australis, Hausmannia cf. buchii, (see Fig. 9), several species of Microphyllopteris, Arau- cariaceae and Cycydites (HARRIS, in WOP- FNER et al., 1970). A review of macro- and Fig. 9. Surface of silcrete with well preserved casts of micro-floras by ROGERS and FREEMAN Cladophlebis cf.australis (upper left) and Hausmannia (1996) confirmed a Late Jurassic to Early cf. buchii (bottom right). Specimen is from silcreted Al- gebuckina Sandstone at extreme right in Fig. 5. Length Cretaceous age of the Algebuckina Sand- of Cladophlebis is 34 mm. stone, as suggested previously by WOPF- NER et al. (1970). In outcrop the Algebuckina Sandstone rarely exceeds 20 m in thickness, reaching a maximum of nearly 40 m in the region of the Finke erstwhile railway settlement. In subsurface the Dalhousie-Muloorina Hinge separates a western area where the formation generally measures less than 70m, from the region to the east, where the sandstone swells to a thickness of up to 750 m (KRIEG Fig. 10. Scanning electron micrograph of silcrete from the Mt. Anna locality displays a fabric of oriented, et al., 1995). interlocking quartz crystals. Surface was obtained by Generally the Algebuckina Sandstone chipping the specimen parallel to the bedding plane. extends to the very margin of the Eromanga Clear crystal faces are visible at top left of picture. Basin. However, in the area west of Mt Furn- Observations under the optical microscope show that crystal growth took place in optical continuity with well er and Coober Pedy (Fig. 1) the Algebuckina rounded, clastic quartz grains. CAD. LAB. XEOL. LAXE 35 (2010) Burial history of Jurassic Gondwana Surface 231

Sandstone is overstepped by the Cadna-owie The composition of the Cadna-owie Formation. In a drill hole near Mt Furner, 30 Formation is quite heterogeneous, com- metres of Algebuckina Sandstone were in- prising dark siltstones, carbonaceous clay- tersected, but in drill holes situated 130 km stones, lenses of granule conglomerates and WNW and 80 km WSW of Coober Pedy re- medium to fine grained, feldspathic and mi- spectively, the Neocomian Cadna-owie For- caceous sandstones. Many of the latter are mation rests directly on Early Permian coal tightly cemented by carbonate, to form hard, measures of the Mt Toondina Formation calcareous sandstones or sandy limestones. (WOPFNER, 1970). This indicates a mor- The calcite cement consists of discrete crys- phological high which may have separated tals of up to 1cm diameter, poikilitically two distinct fluvial systems. enclosing the clastic grains. The calcareous Deposition of the Algebuckina Sand- nature makes the Cadna-owie Formation an stone was a diachronous process. In the excellent seismic reflector which can be rec- rapidly subsiding parts of the basin east of ognised as “C”-horizon across the whole of the Dalhousie-Muloorina Hinge, palyno- the Eromanga Basin. Calcareous oolites are logical assemblages demonstrate that depo- commonly associated with the calcareous sition had begun already in Early Jurassic units, especially at the periphery of “base- times, whereas the macro flora at the Mt ment” inliers. Pyrite is ubiquitous through- Anna section is of latest Jurassic to Early out the Formation, either as botryoidal con- Cretaceous age. As some of the main sedi- cretions of up to 10 cm diameter, as fillings ment input into the basin was sourced from of cell lumina of fossil wood or as dissemi- the cratonic regions exposed to the south- nated small aggregates. west, west and northwest of the basin, riv- A special feature of the Cadna-owie ers must have flowed across the marginal Formation is the presence of isolated, well area for a long time without leaving much rounded cobbles and boulders within cal- sediment. The compressed sections of the careous sandstones in the lower third of the Algebuckina Sandstone along the basin formation. The diameters of the clasts range margins thus represent a long time of depo- from about 20 cm to 120 cm, but even larger sition and reworking. examples, exceeding 2 metres in maximum length have been reported. They consist Cadna-owie Formation largely of locally derived rocks, in the case of the Peake and Denison inliers, clasts of The second phase of inundation was in- yellow quartzite dominate. troduced by the marine transgression at the Within the upper half of the Cadna- onset of the Cretaceous. This event is evi- owie Formation, interbeds of medium to denced in the Cadna-owie Formation. The coarse grained, feldspathic sandstones with formation rests with an uneven, erosional pebbles of purple porphyritic rhyolites are surface on the Algebuckina Sandstone. A present. At the Mt Anna section the whole few lenticular pebble beds are present at the upper half of the Cadna-owie Formation base of the Formation, but generally, a typi- (11 m) is made up of such feldspathic sand- cal rudaceous transgressive facies is absent. stones. Thus the sandstone facies has been identified as Mt Anna Sandstone Member 232 ��Wopfner � CAD. LAB. XEOL. LAXE 35 (2010)

of the Cadna-owie Formation (WOPFNER participation on the total Cadna-owie sec- et al., 1970). The unit consists of well sorted, tion is 100 percent (Fig. 12). medium to coarse grained, feldspathic sand- The Cadna-owie Formation represents stones, characterised by concave current the transgressive event of the Lower Cre- bedding with individually graded foreset taceous Sea. Palynological and palaeonto- beds. Conglomerate beds consisting of well logical data indicate a Neocomian to earli- rounded pebbles of purple to orange col- est Aptian age (WOPFNER et al., 1970; oured porhyritic rhyolite occur as frequent KRIEG et al., 1995). interbeds (Fig. 11). Petrographic studies have demonstrated that the pebbles of por- Deposition and climate phyritic rhyolites derived from the volcanic complex of the Gawler Ranges, situated at The most significant features of the suc- the southwestern margin of the Eromanga cession of the Algebuckina Sandstone are Basin (WOPFNER et al., 1970; CAMP- the general absence of silicate minerals ex- BELL and TWIDALE, 1991; TWIDALE, cept kaolinite and the decreasing participa- 2007). This provenance is demonstrated tion of kaolinite from the bottom to the top further by increased prominence of the of the sequence. The kaolinisation of the Mt Anna Sandstone Member towards the underlying rocks preserving original rock source area of the Gawler Ranges, where its textures and the pore-filling kaolinite of the

Fig. 11. Exposure of current bedded Mt. Anna Sandstone Member at type section. The conglomerate lens incor- porated in the sandstone comprises an abundance of well rounded, orange coloured pebbles of Gawler Ranges porphyry. CAD. LAB. XEOL. LAXE 35 (2010) Burial history of Jurassic Gondwana Surface 233

Fig 12. Isopach map of Cadna-owie Formation in feet (1 ft = 30.48 cm) of area between the Peake and Denison inliers and Gawler Ranges, southwestern Eromanga Basin. Thickness of Mt. Anna Sandstone Member, expressed as percentage of total Cadna- owie thickness, demonstrates increased dominance of Mt Anna Sandstone proximal to the Gawler Range Massif (modified after WOPFNER et al., 1970). 234 ��Wopfner � CAD. LAB. XEOL. LAXE 35 (2010)

lower units of the Algebuckina Sandstone Conditions of high volume flow regimes are symptomatic for an aggressive, low-pH with moderate flow velocity and dominance pore water system. These pore waters acted of bed load transport correspond well with within the newly deposited sands as well as the planar current beds, bounded by hori- within the fluid saturated rocks below and zontal lag gravels observed in the lower, adjacent to the depositional system. KAN- kaolinitic units of the Algebuckina Sand- TOROWICZ (1984) for instance has dem- stone (Fig. 6). High volume flow regimes are onstrated, that pore-clogging kaolinite in maintained throughout Algebuckina sedi- Jurassic sandstones of the North Sea is not mentation but an increase of gradient with a depositional component, but was formed locally higher flow energies is indicated in during earliest, quasi syndepositional, the middle part of deposition, reverting to a diagenetic processes in freshwater environ- steady, high volume regime near the top. ments. Neoformation of kaolinite is known A further characteristic feature of the to occur in tropical soils (MURPHY et al., Algebuckina Sandstone is the decrease in 1995) and highly acid conditions within cer- kaolin content from the bottom to the top, tain rhizospheres are sufficient to transform leading to a steady increase of chemical ma- certain clay minerals into kaolinite (KHAD- turity of the sediment until pure and well AMI and AROCENA, 2008). Kaolinisation sorted quartz sandstone, consisting of more is not only effective in oxic conditions but than 95 percent SiO2, tops the sequence. This even more so in reducing conditions. This upward improvement of chemical maturity is evidenced by the abundance of pyrite suggests a continuous lowering of the pH to within the basal Algebuckina Sandstone a value where even kaolinite was removed. and the underlying kaolinised basement as When Al2O3 solubility exceeded that of for instance in the bore hole Fortville No. SiO2, excess silica crystallised in local con- 1 (WOPFNER and CORNISH, 1967), and centrations to form the hydraulic silcretes in Early Permian deglaciation sequences like the plant bearing quartzite at the top (DIEKMANN and WOPFNER, 1996; of the formation. This process takes place WOPFNER, 1999). at the level of true solutions (WOPFNER Conditions as described above exist in so and WALTHER, 1999). Similarly, the sil- called “black water” rivers. The Rio Negro icification of wood, preserving the most of the Amazon-Orinoco system in Brazil or intricate details on cell structures, can only the Gordon River in Tasmania may be cited be achieved by material exchange on the as analogue examples. The waters of the molecular level, requiring the existence of Rio Negro for instance have a pH-value as proper solutions for ion exchange (WOPF- low as 3.8, caused by high concentrations of NER, 1983). organic acids, saponins and other products To maintain such conditions a humid to of plant decay (SIOLI, 1965; GRABERT, pluvial climate is required. HARRIS (cited 1991). Black water rivers are characterised in WOPFNER et al., 1970) from his stud- by high flow volumes. They are further rec- ies of the palaeoflora deduced a “moist, ognised for their paucity of suspended sedi- subtropical climate” which corroborates ment load, the sediment freight consisting the sedimentological evidence. KRIEG et primarily of bed load or saltation material. al. (1995) and ROGERS and FREEMAN CAD. LAB. XEOL. LAXE 35 (2010) Burial history of Jurassic Gondwana Surface 235

(1996) reviewed the literature on the fos- position, i.e. with the flat side down, whereas sil flora and deduced a moist and warm to labile positions, as observed when emplaced cool-temperate climate. as drop stones from melting ice, are absent. As mentioned above, the Cadna-owie All these features, together with indicators Formation represents the onset of the Early for moderately warm surface waters like cal- Cretaceous transgression. Hence it incorpo- careous oolites have prompted these authors rates a great variety of litho-facies expres- to refute a glacial origin. They suggested sions, depending on the individual local instead that tectonic instability during the environment and coastal morphology (see transgression combined with near-shore re- WOPFNER et al., 1970). lated sedimentary processes like sediment One feature which has been subject to creep, were responsible for the emplacement discussions ever since their occurrence had of the rudaceous elements. Tectonism no been ascribed to glacial transport by JACK doubt was accompanied by earth quakes. (1931), are the pebbles and boulders incor- Tsunamis activated by earthquakes have to porated in the lower parts of the formation. be considered as an alternative mode of em- JACK’s (1931) observations referred pri- placement. The omnipresence of pyrite at- marily to the exotic boulders and cobbles tests to a negative Eh within the sedimentary which form a cover on all plains between the column, indicating high organic productiv- Gawler Ranges and the Peake and Denison ity and input of plant material as evidenced inliers. PARKIN (1956) has pointed out that by the abundance of fossil wood. many of these clasts displaying evidence The presence of the boulders within of glacial transport like soled and striated the Cadna-owie Formation and investiga- surfaces were derived from Early Permian tions of tree rings of fossil wood have lead tillites of the Arckaringa Basin (see WOP- FRAKES and FRANCIS (1988) to resur- FNER, 1970). Similarly, loose cobbles and rect the glacial model. The discovery of boulders of rhyolites have been weathered glendonite pseudomorphs in the overlying out from conglomerates of the Mt Anna marine Bulldog Shale was taken as further Sandstone. This explains the origin of most evidence for a cool to cold climate during of the exotics strewn on the surface, but the Early Cretaceous deposition in the Eroman- large clasts actually embedded in the lower ga Basin (DE LURIO and FRAKES, 1999). parts of the Cadna-owie Formation demand These authors deduced a seasonally freezing a specific interpretation. climate whereby the boulders were rafted off WOPFNER et al. (1970) demonstrated shore, locked into river ice. In the same paper that the cobbles and boulders occur at the DE LURIO and FRAKES (1999) support- periphery of basement highs and within spe- ed their argument by recalculating oxygen cific levels of the Cadna-owie Formation, isotope data published by DORMAN and generally in the lower half of the succession. GILL (1959). Using the assumption that the The clasts consist mainly of local basement present isotopic composition of glendonite material, they are water worn but do not pore waters are representative of the Early show any typical features of glacial trans- Cretaceous sea water composition and as- port and, they diminish in size away from the suming further mixing of high latitude me- high. They are always embedded in a stable teoric waters with seawater, DE LURIO and 236 ��Wopfner � CAD. LAB. XEOL. LAXE 35 (2010)

FRAKES (1999) derived at temperatures Whatever the temperature may have between -1o and 5o C, whereas DORMAN been in Cadna-owie time, climate was hu- and GILL (1959) had obtained temperature mid, providing considerable transport vol- values ranging from 13.8o to 16.5o C. Micro- umes by pH-neutral waters. The upward floral changes, especially an increase in coni- fining of individual foreset beds indicates fer pollen serves as an additional argument regularly changing flow volumes. These for a cold climate (KRIEG et al., 1995). could have been seasonal, but more likely The present author regards the occurrence episodic changes in precipitation, whereas of glendonite pseudomorphs primarily con- the conglomeratic interbeds can be related trolled by sulphate availability and negative to storm events. Eh rather than temperature. In this view the question of the palaeoclimate in the Early EXHUMATION Cretaceous remains open. The Mt Anna Sandstone Member is the The first evidence for the exhumation of product of a competent, high energy fluvial the Gondwana Surface is provided by the system which fltowed from an upland in the ubiquitous presence of well rounded pebbles region of the Gawler Ranges northeast to of silicified wood and amber coloured jas- enter the transgressing sea in large estuaries pers in the basal conglomerates of the Pale- and deltas. There its sediment freight was ocene/Eocene Eyre Formation in the eastern intercalated with marginal marine deposits. Lake Eyre Basin. The provenance of the The sudden appearance of the sandstone at silicified wood and the amber-coloured jas- about the middle of the Cadna-owie For- pers were unquestionable the exposures of mation requires a substantial increase in Jurassic sandstone and the “fossil forests” in gradient ascribed to tectonic uplift of the the Tibooburra region in northwestern New region of the Gawler Range Volcanics and South Wales (WOPFNER et al., 1974). neighbouring basement complexes, termed Sandstones of comparable age, interca- the Gawler Range Massif by WOPFNER lated with multiple silcretes at Mt Harvey, 2 et al. (1970). The ensuing erosion not only km northeast of Algebuckina Hill indicate a stripped the regolith from the Gondwana similar onset on the western margin of the Surface but also cut into the fresh rock, Eromanga Basin. This is corroborated by laying the foundation for the present mor- the development of silcretes of the Cordillo phology (CAMPBELL and TWIDALE, Surface at the present northwestern bounda- 1991; TWIDALE and CAMPBELL, 1993). ry of the Eromanga Basin. As demonstrated Both, WOPFNER et al. (1970) and CAMP- by WOPFNER and WALTHER (1999) and BELL and TWIDALE (1991) thought that indicated in Figure 2, these silcretes extend the Massif had been uplifted along north- over from the Cretaceous on to basement. west trending fault structures. Considering The youngest stratigraphic unit on which regional geodynamics however, vertical typical Cordillo Silcrete is developed, are the displacements along latitudinal trending terminal deposits of the Eyre Formation, structures may have imposed a controlling hence the first exhumation along the western influence. margin probably commenced also in Early Tertiary times. CAD. LAB. XEOL. LAXE 35 (2010) Burial history of Jurassic Gondwana Surface 237

The post Eocene – pre Miocene fold the Warrina Surface and was not exposed movements not only created the large anti- until post-Warrina fault movements uplift- clinal structures of the basin but also created ed the Ranges to their present elevation in new depot centres in the Lake Eyre region. Plio-Pleistocene times (WOPFNER, 1968). The following period of sedimentation and Figure 13 shows the present exposure of the the formation of ferricretes did not enhance modified Gondwana Surface at Mt Marga- exhumation and the succeeding depositional ret and the juvenile erosion on the up-fault- phase of the Warrina Surface covered large ed (eastern) side of the fault block. areas of already exposed Gondwana Sur- Along the southwestern margin of the face or modified its appearance. The Mt Gawler Ranges, the surface may have never Margaret Surface of ROGERS and FREE- been covered to any great extent. The level of MAN (1996) represents the modified or the Nott Surface was apparently lower in Ear- re-emerged Gondwana Surface in the Mt ly Tertiary times, as indicated by remnants of Margaret Range of the Peake and Denison Cordillo-equivalent silcretes between born- inliers. This is evidenced by remnants of Mt hardts (CAMPBELL and TWIDALE, 1991; Anna Sandstone with well rounded peb- TWIDALE and CAMPBELL, 1993). Late bles exposed on the Mt Margaret Plateau Tertiary uplift which elevated the southern (WOPFNER, 1968). However the surface rim of the Australian continent by some 200 had been covered by gypsiferous soils of m was responsible for that final adjustment.

Fig. 13. Exhumed Gondwana Surface cutting across deformed sedimentary rocks of the Neoproterozoic Burra Group now forms the surface of the up-faulted Mt. Margaret Plateau in the Peake and Denison inliers. Direction of view is northwest. 238 ��Wopfner � CAD. LAB. XEOL. LAXE 35 (2010)

CONCLUSIONS as Mt Anna Sandstone Member with the strata of the Cadna-owie Formation. The Gondwana Surface and its thick The uplift of the Gawler Range Mas- weathering mantle of kaolinised basement sif was associated with the widening of reflects the crustal stability experienced in the rift system between Antarctica and Gondwana at the time which followed the Australia. BOURNE et al. (1974) suggest- termination of the Pangaea depositional ed a northeast tilt of the Gawler Range phase in the mid-Triassic and the com- Volcanics along the northwest-trending mencement of dispersion of Gondwana Corrobinnie Depression which they inter- fragments in Late Jurassic to Early Creta- preted as the surface expression of a frac- ceous times. The time interval roughly co- ture zone. They observed a number of mi- incides with the “Gondwana II Extension” nor faults at the edge of the volcanics, but of VEEVERS (2000). there was no unequivocal evidence for a During the Jurassic the Gondwana major dislocation zone concomitant with Surface was dominated by large, high the surface expression of the depression. volume river systems, leading to gradual On the regional gravity map the complex burial of the subsiding surface by sand- of the Gawler Range Volcanics and the dominated fluvial and fluvio-lacustrine associated granitoids of the Hiltaba Suite deposits. Within the study area inflow was occur as an east-west trending, positive from the west and northwest and from gravity anomaly. A pronounced gravity the southwest to south. The Algebuckina slope bounds the positive anomaly in the Sandstone represents this initial phase of southwest, leading to a substantial grav- burial. Depositional mode, water and soil ity low, indicative of a different rock prov- chemistry and environment were analo- ince. This slope combined with a corre- gous to the present drainage system of the sponding change of the characteristics of Amazon River. magnetic anomalies support the existence Northeast tilt of Australia, i.e. away of a northwest striking dislocation zone from the rift shoulders, created by the below the Corrobinnie Depression. From Antarctic–Australia rift system, combined the southern margin of the positive grav- with a rise in sea level, opened the path for ity anomaly associated with the Gawler the marine transgression in Neocomian Range-Hiltaba volcanic-igneous complex, times and the deposition of the marginally a slope leads down to latitudinal-trending marine Cadna-owie Formation. Tecton- gravity depressions and magnetic anoma- ism which became active after the onset lies along about 32o 30’ S latitude. This is of the transgression was responsible for interpreted as an indication for the exist- the input of local rudaceous material. The ence of a further dislocation zone, paral- formation of a large basement block in the leling older rifts like the Polda Basin and southwest of the region, the Gawler Range similar structures in the Bight Basin fur- Massif led to the creation of a high energy ther west. Extensional movements along river system which transported large vol- these two dislocation zones uplifted the umes of sand and rudaceous material into Gawler Range Massif, thus creating a the advancing sea where it interfingered north- to northeast-dipping rift shoul- CAD. LAB. XEOL. LAXE 35 (2010) Burial history of Jurassic Gondwana Surface 239

der which became the source for the Mt distension. This event terminated the his- Anna Sandstone. The northwest-trending tory of Gondwana and the Australian Corrobinnie dislocation zone would have continent acquired its own, independent acted as an accommodation fault. These character. Exhumation of the Gondwana movements apparently were the final rift- Surface was initiated by epeirogenic move- ing event prior to the onset of drift and ments in the Early Tertiary.

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