What Is Sandstone?
Sandstone is a sedimentary rock, which means it was deposited in a water environment, much like you see the mud flats in Moreton Bay, near Brisbane, being deposited today. In geological terms, other types of rock are Igneous (eg. granites), Volcanics (eg. Basalts) and Metamorphics (eg. slates). Sedimentary rocks are formed in low temperature, low-pressure environments on the surface of the earth. Igneous rocks are molten rocks that came up from depth but didn’t reach the surface.
Volcanic rocks did reach the surface and usually flowed. Metamorphic rocks are any rocks that have been subjected to deforming temperatures and pressures. The main constituent of all sandstone is grains of quartz. The quartz is stuck together by a natural cement of clays and traces of other minerals. So if you took a ‘thin section’ of sandstone and looked at it under a microscope, you would see grains of sand with a cement matrix. Helidon Sandstone is mainly ‘fine’ to ‘medium’ grained, which means the grains of sand are relatively fine and hard to see with the naked eye. Coarse-grained sandstone can be found in Helidon but has a rougher texture to the hand.
When And How Was Helidon Sandstone First Deposited?
Helidon Sandstone takes its name after the geological formation named after the town of Helidon in South East Queensland, Australia. A formation is a uniform rock type that was deposited at the same time and is consistent enough to recognise in different locations. Helidon Sandstone is part of the Bundumba Group, which was deposited in the Mesozoic Era. The Mesozic Era is made of three separate geological time periods called Triassic, Jurassic and Cretacious. These periods range from 240 million years ago to 65 million years ago.
Helidon Sandstone was deposited in the Upper Triassic to Lower Jurassic, which makes it about 200 million years old (in a range from 210 to 190 million years). The sandstone was deposited in a fluviatile environment, which means rivers and estuaries. This was a freash water enviroment. A landmark of a fluviatile deposit is ‘cross bedding’ and you can see examples of this in road cuttings as you travel between the towns of Helidon and Gatton and of course in the quarries at Helidon. Bedding defines how the rock was originally deposited. Because rivers often flood, you often find bands of boulder or shade within the beds. This reflects turbulent times or very quiet times in the river deposits.
What Makes Helidon Sandstone Unique?
Helidon Sandstone is fine grained and relatively strong as a building material, it also has an unusual banding that often forms concentric rings and unusual colourings. Remember that sandstone was originally deposited in the Mesozoic Era at a time when dinosaurs walked the earth (about 200 million years ago). If you drive up the range from Helidon to the city of Toowoomba you know you are driving up to a high plateau. This plateau is made up of basalt. Around 22 million years ago, this part of the world was the site of great volcanic activity. In the Toowoomba area a major north south fissure opened up and great amounts of basalt were extruded out on to the surface. Any sediments along this fissure were totally destroyed by the volcanic activity. However, sediments near the main fissures, say within 10 to 15 km, were subject to a lot of heating by volcanic fluids and gases. Put simply, Helidon Sandstone was flushed with hot waters and gases and this altered the make-up of sandstone. The stone was close enough to be altered, but not so close as to be wiped out. It made the stone harder and recrystallised the quartz grains in the rock. In ‘thin section’ the quartz grains are tightly interlocking and this adds to the compressive strength. Most sandstones show some banding, which is usually bedding planes and also most sandstone is somewhat bland and featureless. The hot water and gas flushing of the sandstone at Helidon created different bands that make the stone unique. A classic feature of the banding is the concentric nature with bands forming circular patterns or following the edge of fracture plains.