Browns Field Walk, Wahroonga

Geologist John Martyn has worked in and published on volcanic environments both ancient and modern, originally having completed his PhD on the Great Rift Valley in Kenya. He will lead a walk at Browns Field, Wahroonga explaining how it sits over a diatreme of which there are eight known in the Ku-ring-gai GeoRegion.

Diatremes are massive, cylindrical or dyke-like bodies of fragmental rocks that have been blasted and bored up from depths of kilometres by extreme steam pressure created by igneous intrusion into deep rock aquifers. Most or all in the Sydney Basin are believed to be of Lower to Lower Middle Jurassic age. At the present surface we see their deeply weathered erosional cross sections which lie up to a kilometre below what were their ancient Jurassic eruption surface vents, long eroded away. We won’t clearly see actual volcanic rocks at Browns Field as they are limited to weathered stream bed exposures, but we will see the diatreme’s contact zone with Hawkesbury Sandstone.