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Geology...
The Sturt Gorge Recreation park is recognised as an area of great geological significance. The sturt river has carved a deep, narrow gorge to the Adelaide plains through the resistant glacial deposit knoewn as 'Sturt tillite'. The rock strata in Sturt Gorge were identified as having glacial origins in 1901. The Sturt Tillite formation holds the distinction of being the first area to provide definite evidence of glaciation (at such an early stage) in the geologocal history of the world. Sir Douglas Mawson (geologist and antartic explorer), writing to the then owner of the gorge said that:
Sir Douglas Mawson ©
2002 Australian Antarctic Division
Approximately 800 million years ago a large area of SA was covered by ocean. Immense masses of floating ice invaded this area up to several hundred kilometeres inland from the coastline as we know it today. Sturt tillite is believd to have formed from glacial material dropped from the floating ice. It consists of stones of all sizes, boulders and mudstones. The Tapley Hill formation overlies the Sturt Tillite and contains slates which were deposited as sediments in deep lakes that once covered the area about 700 million years ago. Siltstones and quartzites are the oldest rocks in the park and occur in small outcrops in the south east corner. Silver, lead, zinc and copper were mined in Sturt Gorge. Sturt Gorge Recreation Park occupies a foothill location on top of the Eden Fault Block which forms an undulating plateau at an elevation of approximately 200 metres. Slopes in the majority of the park are steep, with extensive cliffs formed where the Sturt River has carved a deep narrow gorge through resistant tillite strata. Maximum relief in the park is 135 metres, with elevations ranging from 50 metres where the Sturt River crosses the northern boundary of the park, to a maximum of 180—185 metres at several boundary locations and in the Craigburn addition. The little flat land which exists in the park is confined to small riverflats adjacent to the river and ridgetop areas in the Craigburn addition. The Sturt River is the main drainage line in the park. This river is one of the major catchments in the western Mount Lofty Ranges, draining a large area extending eastwards as far as Belair, Crafers, Heathfield and Cherry Gardens. This large catchment area has implications in the management of the park as water quality, river-bed weed infestations and their control, are all largely influenced by events upstream outside the park boundaries. Several minor tributaries of the Sturt River drain areas of the park. The largest of these is known locally as Spring Creek and drains the major valley, known as Spring Gully, in the western section of the park. The recent increases of housing in this vicinity and the provision of stormwater drains has greatly increased and concentrated runoff. In several areas this has caused or accentuated erosion problems. Notable erosion problems exist from stormwater outlets off The Boulevard and Broadmeadow Drive. A major and long-standing erosion gully exists in the cleared portion of the Craigburn addition. This gully was well-developed and readily apparent in 1949 aerial photographs, but probably dates from much earlier than this. This gully does not appear to be extending further up-slope, probably due to some lateral extension, but is undoubtedly deepening. Top of PageSturt Gorge is an area of considerable geological significance. The area first became of interest in 1901 when Howchin gave definite evidence of the glacial origin of rock strata exposed in the gorge (Howchin 1901, 1906, 1920). This formation, which became known as the Sturt Tillite, holds the distinction of being the first to provide definite evidence of glaciation at such an early stage in the geological history of the world (Howchin 1920).
Sturt Tillite outcrop The Sturt Tillite has not been dated directly, but the overlying Tapley Hill Formation has been dated at about 750 million years ago. This has led to an age of at least 750 million years being assigned to the Sturtian glaciation. Sturtian tillites can be traced within South Australia for about 1100 kilometres in a north—south direction and 320 kilometres in an east—west direction and at one tine they probably formed a continuous sheet of sediments over this entire area. Equivalent rocks of corresponding age are known in neighbouring states — New South Wales, Northern Territory and Western Australia — and Africa, indicating that these rocks correspond to a major period of glaciation, possibly of world—wide extent.
The occurrence of this formation in its type locality and in such magnificent exposure so close to Adelaide, heightens the significance of the gorge, particularly for educational purposes. Prior to the area coming under the control of the National Parks and Wildlife Service, the Geological Society of Australia (1967) recommended that:
Sir Douglas Mawson, in a letter dated 1946, and which is referred to bY the Geological Society of Australia (1967), perhaps best sunned up the significance of Sturt Gorge when he wrote:
The Sturt Tillite is believed to have been formed from glacial material dropped from floating ice (Howchin 1920; Sprigg 1946). It consists of stones of all sizes, and boulders and grits incorporated in bluish or brownish mudstones. The erratic boulders, some of which are more than a metre in diameter, have been transported by ice moving eastwards off crystalline basement rooks which at that time were exposed in highlands to the west (Daily et al. 1976). This deduction is based on the following grounds (Howohin 1920):
Today we know that there is a regional unconformity at the base of the Sturt Tillite; a few erratics probably do come from underlying beds; and the geosyncline is of Precambrian age extending into Cambrian times. Sprigg (1942) described the Sturt Tillite in this, the type locality, as having a:"well-cleaved slaty or phyllitic base in which, dispersed irregularly are a typically unassorted collection of sedimentary, igneous and metamorphic erratics typically unassorted collection of sedimentary, igneous and metamorphic erratics". The strata are dipping at variable, but usually shallow, angles to the west, although the bedding is not obvious and is frequently marked by strong cleavage along steep to nearly vertical planes. This cleavage results from, and reflects a history of, extensive folding and faulting. The intensity and duration of this period of glaciation can be gauged by the lmmense thicknesses which the beds attain; up to a maximum of 5500 metres in the Mount Painter region of the Flinders Ranges (Parkin 1969). In Sturt Gorge, the tillite beds are generally thought to attain a thickness of up to 200 metres. For a period which must have lasted tens of millions of years, immense masses of floating ice invaded the geosynclinal basin up to several hundred kilometres from the coast. This process is occurring at present in the Ross Sea bordering Antarctica. The waning or the intense glacial conditions is marked by a very sharp contact from Start Tillite to a remarkably wefl—lninated Series, the Tapley Hill Formation, which is also well presented in the park . This series is believed to have been deposited in quiet, relatively deep, lake waters (Sprigg 1942). The lower levels of the Tapley Hill Formation exhibit perfect lamination in which the variation in alternate layers is principally one or chemical composition and consequently colouration rather than grain size. Higher levels in the series become more calcareous and progressively lose the Line laminations which become coarse bands. These upper layers of the Tapley Hill Formation are believed to have been laid down in progressively shallower water and show crossbedding and other features indicative of shallow water sedimentation. This trend continues, and the series eventually passes upwards into a well marked limestone (the Brighton Limestone) which was probably formed in shallow water under warm conditions (Daily et aj.. 1976). The Brighton Limestone is not represented in Sturt Gorge Recreation Park, but crops out to the southwest, nearer the coast. In the higher levels of the park, areas of lateritic sandstone and gravels lie unconformably on the older rocks. These sediments of Tertiary age (about 20—50 million years) are the most recent in the park and were deposited by freshwater streams. They represent the remnants of the dissected high—level erosional surface of the Eden Fault Block, an old weathering surface at least several million years old, and possibly much older (Twidale 1975). The park also features a small area of siltstones and quartzites belonging to the Belair Subgroup (Daily et al. 1976; Parkin 1969). These sediments which disconformably underlie the Sturt Tillite (Coats 1967), are the oldest rooks exposed in the park and are confined to limited outcrops on the Craigburn addition.
With only minor exceptions, the steep slopes of the original park area exhibit limited soil development. The skeletal soils, largely without profile, are developed from the local parent material. In areas associated with Tapley Hill calcareous slates and dolomitic parent materials, there is a tendency towards the development of shallow, stony rendzina soils. Carbonate removed from the surface of these soils is concentrated to some extent as a layer of calcrete or calcareous silt overlying weathered parent material (Department of Mines 1970). Talus has developed below steep slopes where there has been an accumulation of soil and rock debris eroded from the slope. These areas, and the alluvial soils associated with the riverflats along Sturt River and Spring Creek, provide the only exceptions to the skeletal soils in the original park area. In the Craigburn addition; an area featuring generally lower relief and gentler slopes; soil development is more advanced. Much of this land has a surface of sandy sediments deposited in Quaternary and Tertiary times. In these areas, soils tend to comprise grey to brown-grey sands over bright yellow to reddish brown sandy clay, which in turn rests on weatbered slate or sands and soft clayey sandstone (Johnson 1961a). It is through soils of this type that the major erosion gully in the cleared area of the Craigburn scrub has developed. It would appear that this particular soil type is very susceptible to erosion if the vegetation covet is disturbed. Small areas of black reactive clay soils and gray-brown sands overlying white silty sandstone with brown ferruginous matted patches also occur. The sandstone is exposed on the surface in the area adjacent to Black Road. These soils, which are largely confined to the higher areas of the park, probably represent remnants or the old Tertiary land surface of the Eden Fault Block. |
