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Narellan Sports Hub

You will be standing in the carpark area next to the netball feilds, where there are several large sandstone rocks placed as a retaining wall.  The rocks infront of you have several large rocks with some lovely features including Liesegang rings, some minor inclusions and if you pay particular attention towards the base of the wall are some large concretions.  There are two oil shale inclusions please pick on and answer the following questions. 

What Are Concretions?

Concretions are compact, often rounded masses that form inside sedimentary rocks, distinct from the surrounding rock due to their color, composition, and resistance to erosion. Unlike pebbles or inclusions, concretions form in place as minerals precipitate from groundwater, binding surrounding grains together.

This ironstone concretion likely formed during the Triassic period, when water rich in dissolved iron moved through the porous sandstone. As the groundwater's chemistry changed — perhaps through contact with organic material or oxygen — iron began to precipitate and accumulate in this distinct lens-like shape.

How Do Concretions Form?

1. Nucleation – A core forms, often a fossil, shell, or piece of organic matter.
2. Mineral-Rich Groundwater – Water flows through the sediment, carrying dissolved minerals.
3. Precipitation – Minerals begin to precipitate from the solution due to changes in pH or redox conditions.
4. Growth – Layers of minerals (like iron oxides) build outward, often forming spherical or elliptical shapes.

This process can take thousands to millions of years. Ironstone concretions, like the one here, often form in sandy environments with periodic wet-dry cycles — ideal conditions for iron mobilization and precipitation.

Common Types of Concretions

Why Are They Important?

Concretions reveal:

• The direction and chemistry of ancient groundwater systems.
• The timing of cementation, helping interpret sedimentary rock formation (diagenesis).
• Preservation of delicate fossils due to rapid mineralization.
• Clues about past climates and environments.

This ironstone concretion is more resistant to weathering than the surrounding sandstone and protrudes as a dark, reddish-brown feature with flaky textures.

Geological Context: Hawkesbury Sandstone

If this site is within the Sydney Basin, the host rock is likely part of the Hawkesbury Sandstone — a Triassic sedimentary unit deposited in braided river systems about 200–250 million years ago. Iron concretions like this one indicate that iron-rich groundwater once flowed through these sand layers, cementing and preserving features we now see at the surface.

 Logging Requirements

To log this EarthCache as a find, please answer the following questions and message the answers to the cache owner:

1. Describe the concretion:
   • What is its approximate size, shape and how is the texture compared to the rest of the sandstone?
2. Colour Observation:
   • What colours do you see in the concretion and the host rock?
3. Erosion Resistance:
   • Looking at the concretion is it more or less weathered than the sandstone around it and what does this suggest about its composition?
4. Photo Requirement (To be included in your online log):
   Please include picture of yourself, your GPS or caching name at the site, but please don’t show close-up photos that reveal answers.