Do you trust a geological map?

The NSW Geological Survey have produced the maps that we use today. They have recently placed all of them online for people to view which is... well... excellent! They can be found here. Some areas have excellent, up-to-date 1:100 000 scale maps which are exceptional. However, it is worth mentioning that the best scale you will find for most of the state is 1:250 000 older 1970's maps. These maps are good but they were mostly done through looking at aerial photographs with limited field checking and since nearly 40 years have passed our understanding of the rocks has changed and this means that geological maps can be misleading if you are not careful.

Geology according to the current published maps
(after Brunker et al 1972)- scale approximate

I recently had the opportunity to be involved in a project looking for clay deposits in The Channon / Dunoon area. During the project it became obvious that the geology was not what was mapped (to the left is the geology that was mapped in 1972). The investigation that I was lucky enought to be involved with was pretty simple it just involved an excavator digging a few holes in the ground (testpits). Importantly I had an experienced Engineering Geologist to show me what was going on.

Rock weathers to form soils but there is rarely a distinct boundary between soil and rock, a transition occurs. This transition zone is called the regolith which lies below the soil proper at the surface, it is the  transition into saprolite (weathered rock) and then to unweathered rock. If the weathered rock was derived from shales, mudstones and other fine grained sediments then often these layers will become clay. It was this clay that was looked for.

A better interpretation following the testpit investigation
scale approximate

What was done was to dig into the regolith and depending on the characteristics of the saprolite it would be possible to tell what the original rock would have been. From the mapping it was assumed that what we would find would be related to the Lismore Basalt or Kangaroo Creek Sandstone. What was found was neithers. Instead layers of clayey and silty material and bands of weathered coal were visible as well as lithic sandstone. Coal would certainly not occur in abundance in volcanic rocks like the Lismore Basalt and nor does it occur in the Kangaroo Creek Sandstone. Lithic sandstone is also absent from these units. What must have found was lots more of the Walloon Coal Measures.

From my understanding of the area around Dunoon and on the basis of what was found during the hole digging exercise I put together a  rough new map of the area (the second map above). As you can see there is actually a fair amount of difference. So, don't take it for granted that when you look at a geological map it is exactly right. It should be used as a guide and your knowledge should be applied to check it. The amount of coal we found was so abundant that a discussion about this is probably worth another post in the future.

References/bibliography:

Brunker R.L., Cameron R.G., Tweedale G. and Reiser R., 1972, Tweed Heads 1:250 000 Geological Sheet SH/56-03, 1st edition, Geological Survey of New South Wales, Sydney