Into the Parrots Nest

At least 3 lava flows are evident from the different 'steps'

I had the opportunity a few weeks back to visit a quarry near the locality called Canaiba situated mid way between Casino and Lismore. The quarry is an operating variable quality rock quarry probably excavating Miocene aged basalt lavas from the geological unit known as the Lismore Basalt or possibly the earlier Eocene aged Alstonville Basalt. It was a site I'd wanted to visit for quite a while because the quarry is located at the lower side of a long ridge with an old abandoned quarry located at the top of the ridge on the way to a locality called Parrots Nest. In my mind having two quarries could give an interesting perspective on the any variations in lava flows.  But even before I visited the old quarry, while I was driving along the road to visit the operating one I noticed an interesting feature in the shape of a spur from the main ridge. Visible were several 'steps' in the spur. These steps create what is referred to, unsurprisingly, a stepped topography.

The steps are caused by the erosion of different lava flows. The flows are up to 20 metres of so thick which according to Duggan and Mason (1978) is a bit uncharacteristic for the Lismore Basalt (thin 2-3 thick flows). Looking back along the ridge it is pretty evident that the flows are of consistent thickness through the whole area.  They are probably from the Lismore Basalt that are related to the formation of the Tweed Volcano which was centered around present day Mount Warning. I wonder if there were closer vents that could be the source of the lavas but there is little evidence of any in the immediate vicinity. Indeed authors such as Cotter (1998) feel that the pre-existing topography was such that the area through Blakebrook Quarry (another site north of the quarry I was visiting) through to places like Parrots Nest may have been a valley. The swift flowing basaltic lavas flowed down these valleys filling them and creating thick sequences of rock.

The red layer overlain by another basalt lava flow
indicates the presence of a fossil soil horizon

The operating quarry cuts several of the lava flows that make up the ridge, the boundaries of the lava flows were very easy to make out because of the weathered zones especially the presence of palaeosols, that is, fossil soil horizons. The palaeosol gives an idea of the nature of the eruptions of lava too. Obviously enough time needs to have passed for the formation of a soil profile to occur on the earlier lava flow before the next lava flows over the top of it. Depending on the climatic conditions this could be many decades between flows or even thousands of years.

Anyway, a good trip even if it was just for the palaeosol or the stepped topography alone. But I'd like to do another blog on some of the macro scale igneous textures that are present in the lava including dykes, vesicles, voids and veins and I've still not got to the top abandoned quarry but when time allows I'll get there. I took some samples at the operating quarry to examine under the microscope to see if there were any microscopic textures that are of interest too, but once again, time does not seem to be on my side... though I will get to these tasks sometime!

References/bibliography:

*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.

The New England tablelands seem to be upside down

The geomorphology of the Northern Rivers and New England region can be quite complex. There are many features around the region that have developed as a direct result of the underlying geology. Whether it be the great escarpment, the Ebor Volcano, the backward Clarence River or various other situations, there is always a geological reason for the landscape we see today. In a previous post on the Maybole Volcano near Guyra I quickly mentioned that there is an “inverted topography” which has been created following the deposition of the lava from this volcanic area. Maybole is not isolated in this situation, indeed according to Coenraads & Ollier (1992) much of the basalts in the New England region from Armidale, Walcha, Llangothlin and even places on the other side of the watershed and great dividing range of the Northern Rivers such as Nundle or Inverell show what is technically referred to as relief inversion.

The area around Armidale is actually a good example of the relief inversion, as most hills actually demonstrate the situation nicely. Take, for example, the hill that the University of New England is situated on. The Hill is capped with Cenozoic (Miocene) aged calc-alkaline olivine basalt (part of the Central Volcanic Province) just to the east of the hill (in the paddock below the university carparks) below the level of the lowest basalt flow is a fossil soil horizon, known as a palaeosol. This palaeosol has been affected by lava being deposited on it and has been turned into a material known as silcrete (soil which has been cemented with silica). The old soil was developed on rocks of the Carboniferous aged Sandon Beds. The Sandon Beds outcrop on the lower slopes and in the valleys in and around Armidale but once they were the hills themselves.

The basalts were erupted to the surface the chemical composition of the lava meant that they were quite low in viscosity, that is it was very liquid and consequently the lavas flowed down the valleys that existed at the time. The valleys tended to fill up to varying degrees, leaving only a thin layer of volcanic rock on the existing hill crests of the Sandon Beds or none at all. In the following millions of years the process of erosion would be more effective on the non-volcanic rock and the hills would eventually become incised, turning into gullies and eventually larger valleys. The basalt in the old valleys would remain relatively un-eroded and be become the modern hills.

Evidence of this process can be seen from historic mining of some of the gold around Armidale. The ‘old timers’ would dig under the basalt along ‘deep leads’ which were originally gravel and sand deposits associated with old creeks and rivers. These deep leads had been alluvial gold deposits preserved by the basalt flows. Many of these were mined in the 1800’s and early 1900’s in many areas of the New England district including one quite recently in the Tilbuster area (Ashley & Cook 1988). The silcrete deposits mentioned previously are also examples of the process.

References/bibliography:

*Ashley, P.M. & Cook, N.D.J. 1988. Geology of the Whybatong gold prospect and associated Tertiary deep lead, Puddledock, Armidale District. New England Orogen - Tectonics and Metallogenesis. Conference Papers presented at the University of New England.
*Coenraads, R.R. & Ollier, C.D. 1992. Tectonics and Landforms of the New England Region. 1992 Field Conference - New England District. Geological Society of Australia Queensland Division.

Ground water in the Alstonville Plateau

A palaeosol in the Alstonville Basalt

Ground water is a valuable source of water for stock watering, domestic uses, irrigation and town water supply in the area of the Alstonville Plateau. For example both Ballina Shire Council and Rous Water operate ground water bores as sources of water for municipal use. The reason for the popular use of the ground water from this source is its yield and also freshness. The quality of the water in the aquifers is excellent and the quantity good. In fact the popularity of ground water from the Alstonville Plateau is such that it threatens to be over used with many aquifers being badly drawn down and for this reason the NSW state government has put in place a water sharing plan that prohibits new water extraction licenses from some areas of the plateau and all ground water bores in the area require a license.

So what is the Alstonville Plateau ground water source anyway. Where does the water come from? Well, in short, the Alstonville Plateau ground water source is a series of aquifers that occur in the Cenozoic basalt that defines the area of the Alstonville Plateau. The plateau extends from beyond? Lismore in the west almost to the coast at Lennox Head, past the little village of Newrybar in the north (almost to Bangalow) and south almost to the Richmond River at Broadwater. According to Brodie and Green (2003) there are several aquifers with the upper most being an unconfined source of water within the upper weathered and/or fractured zones of the basalt. Below this is at least one confined aquifer which flows through permeable layers such as paleosols (old soil horizons) or through fractures in the basalt. An example of a paleosol from the Alstonville Basalt is shown above (not acting as an aquifer in this case).

The unconfined aquifer is usually able to be intercepted within several metres of the surface but this depth can vary wildly depending on the depth of soil weathering zones and local topography. This shallow source is usually easy to find but yields are usually low and are often subject to drying out during periods of drought due to the local surface water influence on these aquifers. In general when it rains the streams tend to recharge the aquifers and when the weather dries out the aquifers tend to return base flow to the streams (until the aquifers run out of water).

The deeper aquifers are confined between layers of basalt. The layers that the water is found in is either made from substantially fractured rock or paleosols that were developed on lava flows and were subsequently covered up by new lava flows (i.e. are directly related to the eruptive conditions during the formation of the basalt). Interestingly, the dip direction of the aquifers is generally from east to west which is somewhat inconsistent with the idea that these rocks were sourced from the Tweed Volcano which is the established theory since Duggan and Mason published their paper on the volcanic rocks of the area in 1978.

The interesting thing about the importance of this ground water source is that despite the area being mapped as Lismore Basalt  most other areas of the Lismore basalt away from the Alstonville Plateau are not in as high demand for ground water as the Alstonville Plateau. Why is this? It is possible that there are peculiar features of the plateau such as extensive paleosols but it is possible that it is related to the plateau being derived from an older basalt unit that was identified by Cotter (1998) but has not been followed up in detail by any other authors since. See my older posts on this subject here and here.

References/Bibliography:


*Brodie, R.S. & Green, R. 2002. A Hydrogeological Assessment of the Fractured Basalt Aquifers on the Alstonville Plateau, NSW. Australian Bureau of Rural Sciences, Australia
*Duggan, P.B., Mason, D.R. 1978. Stratigraphy of the Lamington Volcanics in Far Northeastern New South Wales. Australian Journal of Earth Sciences V25.
*Cotter, S. 1998. A Geochemical, Palaeomagnetic and Geomorphological Investigation of the Tertiary Volcanic Sequence of North Eastern New South Wales. Masters Thesis, Southern Cross University.