Welcome to Midge Point. First up, some detailed regional geology to set the scene.
The Northern New England Fold Belt extends along the eastern side of Queensland from Brisbane to Townsville. Mafic (dark coloured rocks like you see in Hawaii) to silicic (light coloured rocks, think of Mt St Helens) magmatism was widespread across the New England Fold Belt during the Late Devonian and Early Carboniferous (380 to 350 million years ago). Can you imagine having Hawaii and Mt St Helens mixed together in the same area!! The area around Midge Point forms part of a large igneous province. The figure below details the extent of the New England Fold Belt and zooms in on the Campwyn Volcanics and Edgecombe Beds around Midge Point.
Local Geology
Locally, the geology is dominated by green chloritic mafic and andesitic breccias and syn-volcanic intrusions or dykes. Today's Earthcache lesson is on igneous rocks (formed from molten magma). There are two types of igneous rocks. Extrusive rocks include lavas where molten rock flows over the Earth's surface like a river. The other are intrusive rocks. Imagine a chamber of molten rock deep in the Earth's crust that cools slowly leaving large typically resistant bodies of rock that can be kilometres to 10's of kilometres wide which geologists call plutons and batholiths (Yosemite in the US and Stanthorpe in Queensland are examples). Above these large bodies of molten rock, any cracks or fractures above them can be filled with molten rock to form a sill or a dyke.
What is a dyke?
At Midge Point, a dyke is not a man made feature to hold back water as seen in Holland. A dyke is a sheet of rock that is formed in a fracture of a pre-existing rock body. Magmatic dykes form when magma (molten rock) flows into a crack then solidifies as a sheet intrusion either cutting across layers of rock or through a contiguous mass of rock.
The Midge Point dykes are similar in chemistry to the surrounding host rocks, formed within 1000m of the Earth's surface (often called sub-volcanic) and formed during repeated extension of the fold belt. That is, the rocks here were pulled apart repeatedly which allow for molten rock to intrude along the fractures and cool preserving the rock formation history.
In the picture below is an example of dolerite dyke cross cutting horizontal limestone beds in Arizona. The Midge Point dyke is not as obvious but is different to the surrounding rocks.
Mafic versus Felsic
Mafic is a term to describe rocks that are lower in silica and higher in iron and magnesium and the minerals are darker in colour. The dolerite dyke in the image above is mafic. They can be grey, green and black and many shades of these colours.
Felsic is used for light coloured rocks, high in silica and low in iron. Examples of a felsic rocks are granite, a very common rock type. Pink, beige, white and pale grey are typical felsic rock colours.
Orientation
As dykes are tabular or sheet like then the orientation can be determined. The strike of a planar feature is the way it runs along the ground. The best way to measure this orientation is to stand on the feature and point your arms in the direction the rock is going. e.g. nort-south, east-west.
Dip is the angle of the plane off the horizontal. For example, steep dips are close to 60 to 75 degrees off the horizontal. The image above could be classified as steeply dipping to almost vertical
Earthcache description and questions
Look at the rock platform at GZ, all of the rocks are varying shades of green. You are looking for a feature that cross cuts the surrounding rocks, is slightly lighter in colour. This is the Midge Point Mafic Intrusion or dyke.
Q1. What is the thickness of the dyke?
Q2. Describe the two dominant colours observed within dyke?
Q3. How is it different to the surrounding rock?
Q4. What is the strike (the direction of the feature) and dip (I.e. horizontal, dipping or vertical of the dyke?
Q5. Please post a photo of the you, your GPS or a special object overlooking the boat ramp here. (Please no spoiler photos).
You are welcome to log your find straight away though logs without answers will be discreetly deleted within 14 days.
I hope you enjoyed Midge Point!
Map and information sourced from S. E. BRYAN,C. R. FIELDING, R. J. HOLCOMBE, A. COOK AND C. A. MOFFITT (2003). Stratigraphy, facies architecture and tectonic implications of the Upper Devonian to Lower Carboniferous Campwyn Volcanics of the northern New England Fold Belt* Australian Journal of Earth Sciences 50, 377–401