1. Geology & Groundwater Origins
TAMBORINE MOUNTAIN
Geology
&
Groundwater Origins

2. Tamborine Plateau Formed on horizontal basalt lavas
Erupted from the Tweed Volcano over Mt Warning in NSW ~23 million years ago
One of several volcanoes active at that time

4. Erosion
Plateau is remnant of the final surface of the northern flank of volcano
Isolated by erosion from the radial streams of Canungra Creek and Coomera River
Same surface still visible to south on Beechmont

7. The basalts cap four groups of older rocks-
Meta-sediments’ of Neranleigh-Fernvale beds on east
Rhyolite lavas of Chillingham Volcanics in centre
Thin band of soft shales of Ipswich Coal Measures
Sandstones of Woogaroo Subgroup on west

8. The basalts
Lowermost flows are of the Albert Basalt from Focal Peak Volcano –limited in extent
Remainder are of the Beechmont Basalt from Tweed Volcano
Several sub-units recognised

10. Beechmont Basalt Unit A – 10 thin flows with inter-bedded sediments
Unit B, Cameron Falls Member – Single massive flow 20m thick
Unit C – 4 or 5 thin flows
Unit D, Eagle Heights Member – Single massive flow 25m thick forming upper scarp of mountain
Unit E – Weathered flows on plateau surface
(Lower units absent on eastern side)

11. Beechmont Basalt (contd.)
Thin flows of Units A and C are strongly fractured and contain zones of gas bubbles
These less resistant to weathering and form benches on sides of mountain

13. Water movement Rainfall seeps into ground to saturate rock
Level of saturation is the water table
Just because rock is saturated does not mean water can be extracted – often held only in tiny pores and fractures
Water can only be pumped where there are permeable zones which allow it to flow and be replenished relatively quickly

14. Controls on groundwater at Tamborine Mountain
Horizontal basalt flows
Fractures and gas bubbles in Units A & C which create permeable layers that feed groundwater laterally on to the sides of the mountain to form springs
Springs wax and wane depending on pressure head of water table, which is seasonally variable
Other permeable layers at base of basalt

16. Likely water sources
Best flows are likely from bores in Units A & C, and in deeper permeable layers at base of basalt sequence
Units B and D unfavourable, but small flows may be possible from localised fracture zones
Older rocks beneath basalt not of significance