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Thursday, 26 September 2013


Aplite dyke intruding granite (very weathered)
This is the first purely geological entry I’ve written for this Blog. Most readers will be familiar with the basic geology of the western Blue Mountains. Essentially, there is a series of sedimentary rocks resting on an eroded surface of older rocks, both igneous and sedimentary. The rocks that we see from lookouts such as Echo Point or Govett’s Leap consist of Triassic sandstones and siltstones (the Narrabeen Group, which forms the cliffs) and the Permian Illawarra Coal Measures (nonmarine) and Shoalhaven Group (marine) which form the slopes and more level areas below.

Once Victoria Pass is descended into the Hartley Valley, there comes a point near Hartley where the Permian Shoalhaven Group sedimentary rocks give way to the underlying granite, which occupies the lower ground. The sedimentary rocks and the granite are of quite different ages, Permian and Lower Carboniferous respectively, and the surface between them is called an unconformity (a time gap).
Shoalhaven Group conglomerate overlying weathered granite
At the very least, the time gap represented by this unconformity covers the time involved in forming the granite (as an intrusion into even older rocks), uplifting the earth’s crust and eroding away the overlying rocks and deeply weathering the now exposed granite. 

At this point in time the Sydney Basin began to form as the land subsided. The first sediments deposited in this vast subsiding area were the marine sandstones, conglomerates and siltstones of the Shoalhaven Group. These contain lots of pebbles (sometimes boulders) of the more resistant older local rocks. Most of these consist of hard quartzite, evidently derived from the nearby Upper Devonian Rydal Syncline. 
Detail of the unconformity

It’s not often you can put your finger on an unconformity like this, representing as it does perhaps 100 million years of time. The geological map is a small portion of one published in 1875 (see below). The excavation where the photographs were taken is in Portion 241. You can read the notes (21 and 22) which accompany the map. I have yet to find any pebbles containing Upper Devonian marine fossils nor any Permian marine fossils in the Shoalhaven Group sediments, but these are well known from other nearby localities. The cross section from the Sydney 1:250 000 geological map goes right through this spot and clearly shows the relationship between the various rocks.
A large quartzite cobble from the conglomerate
 References: “Geological Map of the Districts of Hartley, Bowenfells, Wallerawang and Rydal” (Department of Mines, New South Wales, 1875). The DIGS reference is Heritage Map H0252. Geological Map of Sydney 3rd edition 1966. The DIGS reference simply says “Map”, so try using the Report Identification Number R00019218 instead.

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