TAMBORINE MOUNTAIN

Geology

&

Groundwater Origins

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

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

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

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

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)

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

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

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

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