Category Archives: Permian glacigenes

Boulder lags in Rosetta Bay at Victor Harbor, South Australia

Overview

The significance of strewnfields of large granite erratics throughout the Inman Valley was discussed in an earlier note (Milnes, 2019).  They essentially pinpoint outcrop or subcrop of in-situ glacigene diamictite from which they have been exhumed, or within which they still remain partly encased.  The diamictite is generally plastered over smoothed and striated Cambrian Kanmantoo Group bedrock.

Good examples of these boulder lags occur on the beach at Rosetta Bay and were recognized early on as glacial erratics eroded from glacial till (for example, Howchin (1910; Fig. 1).  Milnes (2019) located and photographed remnants of the source diamictite underlying these boulder lags (Figs 2, 3).  However, recent drone photographs taken in the area by Isaac Forman (Serio.com.au) have shown that significant boulder lags occur on the landward side of Wright Island (Fig. 4) and, based on the forms visible beneath the sea surface, may also exist on the seabed in the Bay between the shore and the Island.

One implication of this observation is that the bedrock (Petrel Cove Formation, Cambrian Kanmantoo Group) is at a relatively shallow depth and that the glacial till from which the boulders were eroded (or within which they may be partly embedded) remains close to the sea floor.  Another is that the early held view that the granite landforms (particularly Wright Island and Rosetta Head) were ice-moulded during the Permian glaciation about 300 million years ago is probably correct.  And, finally, it may still be possible to find direct evidence of glacial pavements on parts of these granite landforms, like that discovered fortuitously many years ago at Port Elliot (Milnes & Bourman, 1972).

Fig. 1 (L) – Map from Howchin (1910) showing location of erratics in Rosetta Bay. Fig. 2 (C) -Boulder lag of mainly granite erratics on the beach near the boat-ramp, Rosetta Bay.  Fig. 3 (R) – Boulder lag of granite and other rock-type erratics on the beach at Petrel Cove.

SERIO-0796-Wright-Island

Fig. 4 – Isaac Forman (Serio.com.au) drone photo of Wright Island and the seabed on its landward side showing the boulder lag of granite erratics on the landward side of the Island and similar forms on the seabed (centre left).

References

Howchin, W., 1910, The glacial (Permo-Carboniferous) moraines of Rosetta Head and King’s PointTransactions and Proceedings and Report of the Royal Society of South Australia 34, pages1-12; plates 1-17.

Milnes, A.R., 2019, What’s the significance of the large granite erratics scattered through the Inman Valley in South Australia?  https://earthnotesblog.wordpress.com/2019/07/25/

Milnes, A.R.,  BOURMAN, R.P., 1972), A Late Palaeozoic glaciated granite surface at Port Elliot, South AustraliaTrans. R. Soc. S. Aust. 96, 149-155.

 

Dr Tony Milnes

New report

Professor Bob Bourman and I have just submitted a report to the Inman River Catchment Landcare Group (southern Fleurieu Peninsula, South Australia) entitled ‘The geology and landforms of the Inman River Catchment‘.  Some funds in support of the project came from the Regional Landcare Facilitator Programme, an initiative of the Australian Government’s National Landcare Programme.  Our time in researching the subject and writing the report was a voluntary effort.  front-page

The general aim of the project was to prepare an overview of the geology and geomorphology of the Inman Catchment.  This was to provide a basis for improving local knowledge and awareness of how landscape and landforms have changed (and continue to change) according to landuse and land management practices.  We enlisted the help of landowners and gained new insights via their responses to a wide-ranging questionnaire.

The report can be downloaded via the following link:  http://www.victor.sa.gov.au/page.aspx?u=856

Dr Tony Milnes, Earth Sciences, University of Adelaide

A Lesson Learned

Field observations

As part of ongoing studies of the nature and distribution of Permian glacigene sediments on Fleurieu Peninsula, we were shown to a location just below the plateau surface near Spring Mount, west of ‘Minnawarra’ Homestead (Fig. 1).  Here, on a north-facing spur high in the landscape, adjacent to scattered outcrops of extensively weathered and ferruginised bedrock, there was a surface scatter of rounded cobbles and pebbles cascading down towards a small dam in the valley. We would normally be looking for just this type of geological occurrence as evidence for Permian glacigene sediments from which the cobbles and pebbles (erratics, outwash gravels) would have been eroded. This site was somewhat removed from the usual locations of Permian sediments that are common nearby at lower elevations in the Inman Valley to the south, and in the Hindmarsh Tiers and Myponga valleys to the north. Nevertheless, the scatter of rounded clasts was distinctive.

Figure 1:  Location map.  Site studied marked by red star.

Figure 1: Location map. Site studied marked by red star.

On closer examination, the clasts were commonly of quartzite and fine-grained gneissic rocks but we could not identify any granite cobbles: boulders and cobbles of Encounter Bay Granites are very common in the Permian deposits along the northern margins of the Inman valley, for example. In addition, many of the cobbles and pebbles here were somewhat oblate in shape.

We traced the scatter of clasts downhill towards a small dam. On the eastern side of the dam a shallow cutting had been made in the hillslope to provide vehicular access to the dam.

Figure 2:  Pebbles and cobbles eroding from a bleached and weathered sand-silt material exposed at the base of a Xanthorrhea.

Figure 2: Pebbles and cobbles eroding from a bleached and weathered sand-silt material exposed at the base of a Xanthorrhea.

Figure 3:  Scatter of pebbles and cobbles eroding from a bleached, weathered and somewhat ferruginised sand-silt material.  Note the oblate character of many of the cobbles.

Figure 3: Scatter of pebbles and cobbles eroding from a bleached, weathered and somewhat ferruginised sand-silt material. Note the oblate character of many of the cobbles.

Various exposures en route to the cutting, for example on the downslope side of a Xanthorrhea (grass-tree), revealed cobbles and pebbles weathering from a bleached, weathered and generally poorly consolidated sand-silt matrix (Fig. 2), which is very like the situation we have observed in Permian glacigene deposits.  First observations on approaching the cutting by the dam confirmed these relationships (Fig. 3).  At the cutting, however, the actual field relationships of the clasts and the matrix are more clearly seen and the clasts tend to have a well defined ‘imbricate’ orientation in the host matrix (Fig. 4).  Immediately to the east in the cutting the relationships become clear:  the clasts are actually contained within steeply dipping weathered bedrock.  Their oblate shape and orientation is a function  of deformation of the bedrock and they are aligned parallel to the steeply dipping layering represented by schistosity roughly parallel to bedding (Fig. 5).  The weathering of the bedrock, which is related to its occurrence in close proximity to the deeply weathered pre-Tertiary summit surface of Fleurieu Peninsula, and which is particularly characteristic of the area around Spring Mount, has extensively altered the rock matrix but apparently little affected most of the contained clasts.

Some outcrops in the small tributary immediately to the west of the dam are of weathered and ferruginised schist and gneiss, but there are no rock clasts evident.

Figure 4:  Pebbles and cobbles with a clearly defined ‘imbricate’ habit within the bleached, weathered and somewhat ferruginised sand-silt matrix.  Again note the oblate character of many of the cobbles.

Figure 4: Pebbles and cobbles with a clearly defined ‘imbricate’ habit within the bleached, weathered and somewhat ferruginised sand-silt matrix.

Figure 5:  Pebbles and cobbles, now seen as significantly deformed parallel to the cleavage in steeply dipping bedrock which has been strongly altered by weathering.

Figure 5: Pebbles and cobbles, now seen as significantly deformed parallel to the cleavage in steeply dipping bedrock which has been strongly altered by weathering.

Clearly, the geology is not reflective of Permian glacigene deposits but of weathered and altered bedrock most likely to be the basal Proterozoic conglomerate that unconformably overlies the Barossa Complex basement inlier in this region. Reference to the geological map (Fig. 6) confirms this possibility. The essentially unweathered conglomerate, which is also spectacularly deformed, is well-exposed in the Inman Valley at Grey Spur, just south of the Spring Mount locality. The same formation (assigned to the Aldgate Sandstone; SARIG mapping) is exposed on the coastline at Lady Bay, south of Normanville, but the deformation here has been very intense and the contained cobbles and pebbles are significantly deformed.

Conclusion

The ‘lesson learned’ is that all is not as it initially may seem in field geology, and jumping to conclusions is not recommended. A close examination of field relationships in any locality, together with questioning of initial conclusions and gathering of all available evidence, might actually uncover an interesting story that would otherwise be missed.

Figure 6:  Geological map showing site location (red star) and distribution of basal Proterozoic conglomerate (Nol Aldgate Sandstone). Geology as follows:  Orange-brown (Lb) = basement Barossa Complex; dark brown (NoI, Nds, Nl etc) = Proterozoic; pale brown (Eec, Eeb etc) = Cambrian Kanmantoo Group; blue (CP-j) = Permian glacigene sediments; orange (T) = undifferentiated Tertiary weathered  zone materials; yellow (Q) = undifferentiated Quaternary alluvials.

Figure 6: Geological map showing site location (red star) and distribution of basal Proterozoic conglomerate (Nol Aldgate Sandstone). Geology as follows: Orange-brown (Lb) = basement Barossa Complex; dark brown (NoI, Nds, Nl etc) = Proterozoic; pale brown (Eec, Eeb etc) = Cambrian Kanmantoo Group; blue (CP-j) = Permian glacigene sediments; orange (T) = undifferentiated Tertiary weathered zone materials; yellow (Q) = undifferentiated Quaternary alluvials.

A R Milnes

R P Bourman