This is an expanded and rearranged version of my letter “T” contribution to the current weekly Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “T is for Timaru Today”.
T is for a Type of Tiny Tectonic Breccia?
I found this stone on Gemstone Beach, Southland, in April 2023. What fascinates me are the tiny fragments in the vein that runs nearly the full width of the stone. To me, it looks like a brecciation, a kind of type of breccia. The fragmentation has occurred under a kind of tectonic stress, the forces of weight and pressure, as large masses of rocks move and grind. The stress results in tiny cracks and openings and fragmentation. Every now and then I come across similar stones, sometimes the fragments being within a vein, sometimes within only part or even all of the stone.
Usually a breccia stone is defined as one that is made up of angular clasts (fragments) cemented together in a fine-grained matrix. A “monomictic” or “monolithic” breccia is one where the clasts are of a single rock type in contrast to a “polymictic” or “heterolithic” breccia where the clasts represent more than one rock type. Definitions of breccia usually refer to large-scale masses of rock, and they do not cover this form of brecciation WITHIN a small stone. Having looked through some of my finds, it seems to me there is a continuum from an “earthquake stone” (where there is an offset but no vein between the parts) to a vein (where the crack has been widened and filled) to a kind of tiny tectonic breccia (where fragments of the stone are in a vein, as with the stone above) to a brecciated stone (where a larger proportion, if not all, of a stone has been fragmented and the veins between have been infilled).
To illustrate this continuum: “Earthquake stones” show how tectonic stress can cause small fractures without brecciation – maybe more movement is required to create fragmentation. Two Gemstone Beach examples:
Another two Gemstone Beach finds, probably argillite, these having tiny breccia in veins:
Two stones with tiny breccia from tectonic stress, the first from Seadown Beach, Kakanui, the second being a piece of Slope Point petrified wood:
A stone with widely dispersed tiny clasts and one with small brecciation in one quadrant, both from Gemstone Beach:
Next are two Gemstone Beach stones in which the brecciation has involved most or all of the stone. In the second, the whole stone has been brecciated. It appears to me not to be a cementation of fragments from other stones but is rather a stone that itself has been brecciated in situ:
All of these breccia (above) would be classified as monomictic or monolithic, the breccia being from one rock type, usually in fact from one mother rock. They can be contrasted to the following four polymictic or heterolithic breccia:
The next Post in this Series is “U is for U-Shaped Trace Fossil Found Near Ure (Waima) River Mouth”. An Index for this Alphabetical Series is here.
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