This is a slight expansion of my letter “M” contribution to the current Alphabetical Series in the Facebook Group “New Zealand Lapidary, Rocks, Minerals, Fossils”. In the 2021 version of this Series I posted “M is for Muscovite Mica from Joyce Bay”.
M is for Mudstone from McCracken’s Rest
I found this stone on the beach at McCracken’s Rest in February 2021 (see Day 11 in this Post). It is an argillite mudstone with trace fossils. McCracken’s Rest is a roadside lookout just under eight kilometres west of Gemstone Beach, Western Southland. Perched on a hill on the edge of a cliff, the lookout provides a stunning view of Te Waewae Bay and the mountains of Eastern Fiordland.
The beach between Gemstone Beach and McCracken’s Rest (and beyond) lies below cliffs all the way along. Access even at this lookout is not easy. You have to hop over the fence and carefully make your way down a steep slope to the beach below.
A sandstone has grains that are visible to the eye while the tiny grains making up a mudstone like this one are not visible without magnification. Argillite is a hardened slightly recrystallised mudstone, sometimes called “metasedimentary”. Along the Te Waewae Bay coast, argillite stones come in grey, green and red, with some banded argillites being brown as well. Examples of these argillite stones can be found here.
The argillite of the Te Waewae Bay coast is a Brook Street Terrane sedimentary rock in which trace fossils are found. A “terrane” is a fragment of the Earth’s crust consisting of a distinct series of geological formations, and it has been transported by plate tectonic processes to its current position. New Zealand consists of a number of such terranes – see pages 34-51 of Peter Ballance’s “New Zealand Geology: An Illustrated Guide” (2017). The Brook Street Terrane runs from Nelson to the south coast of the South Island and has been torn in two by the alpine fault. When lying beneath the ocean surface, sediments of mud were inhabited by small animals which left traces, mostly of burrows and trails, that were preserved in the mudstone. All of this happened more than 250 million years ago, in the Permian Era, offshore from the ancient continent of Gondwana. For more on argillite mudstone and trace fossils along the south coast, see “The Fossilised Worm Cast Stones of Gemstone Beach and Riverton – Part Five: ‘Burrowing Worms’ of the Permian in Brook Street Terrane Rocks East of Tihaka Beach”.
The next Post in this Series is “N is for North Otago Neutral-Hued Quartzite”. An Index for this Alphabetical Series is here.
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