Friday, March 15, 2024

Drunk on Petrology - Greywacke Sauvignon Blanc

The next Drunk on Geology is for Greywacke Sauvignon Blanc from the Greywacke Winery in Marlborough, New Zealand.

The term "greywacke" (pronounced "grey wacky") is a rock name like sandstone or granite. However, the term is typically used as a "garbage" or "trash can" term, meaning that it is often used when better names aren't known. Greywacke is a grey sedimentary rocks composed of various cemented rock and mineral fragments like a sandstone or an arkose. While the term "greywacke" is often used without a specific meaning, there is a specific meaning to the term according to the Dictionary of Geological Terms 3rd Ed.:
Graywacke: (Note: this the the American spelling of the term "gray" for "grey") - An old term, now generally applied to a dark gray firmly indurated coarse-grained sandstone that consists of poorly sorted angular to subangular grains of quartz and feldspar, with a variety of dark rock and mineral fragments, embedded in a compact clayey matrix having the general composition of slate and containing an abundance of very fine-grained illite, sericite, and chlorite minerals. Graywacke commonly exhibits graded bedding and is believed to have been deposited by submarine turbidity currents. 
A piece of New Zealand greywacke. Image courtesy of The University of Auckland

Greywacke is a variety of argillaceous sandstone that is highly indurated and poorly sorted. It comprises a large percentage of the basement rock of New Zealand, and so is an important rock type throughout the country. Because it has been subjected to significant amounts of tectonic movement over a long period of time (some New Zealand greywacke is over 300 million years old), greywacke is commonly extremely deformed, fractured, and veined. Although greywacke can look similar to basalt, it differs in that it is commonly veined (with quartz being the vein mineral), and lacks vesicles.

Greywacke from New Zealand's Southern Alps. Image courtesy of

Here are New Zealand's Southern Alps, which are almost entirely comprised of the Torlesse greywacke. Some more information from the Greywacke website:
New Zealand does not have a designated national rock, but if one was ever chosen, it would have to be greywacke. This drab grey stone is found everywhere in New Zealand – on the mountains, in the rivers, on the beaches. It consists of layers of hard, muddy grey sandstone alternating with thinner layers of darker mudstone (argillite). Technically, the term greywacke refers to the sandstone (wacke is a German name for a type of sandstone), but it is also used as a general term for the entire rock. 
The label on the back of the bottle has a bit more geological context:
"Named after New Zealand's prolific bedrock, Greywacke (pron Greywacky) is the label of Kevin Judd, one of Marlborough's pioneer winemakers. Grown in prime vineyard sites in the central Wairau and Southern Valleys, this is a deliciously aromatic, finely balanced wine." 

More from the Greywacke website:
Greywacke (Grauwacke) was first used in the 18th century to describe rocks in the Harz Mountains of Germany. Ernest Dieffenbach, a German scientist who travelled widely in New Zealand between 1839 and 1841, was the first person to use it for local rocks. English geologists regarded greywacke as an uncouth foreign term, but it was adopted in Scotland. Archibald Geikie’s Text-book of Geology, published in 1903, gave descriptions of greywacke, and helped persuade New Zealanders that it was an appropriate term for their most widespread rock. In the 1960s, some geologists argued that the term greywacke was vague and imprecise. A subcommittee of the Geological Society recommended that it be dropped, but this was widely ignored. The term is possibly used more widely in New Zealand than anywhere else in the world.
Sets of turbidite deposits along the coast in Zumaia, Spain

As noted above, the greywacke "is believed to have been deposited by submarine turbidity currents". These submarine turbidity currents are also known as turbidites. Generally, these are underwater landslides. The landslides are denser than the surrounding water with the sediment tumbling down from higher to lower elevations. During the tumbling, sediment is kicked up into the surround water creating a "turbid" cloud. Once reaching a stable platform the cloud will settle out with the larger sediment fragments settling out first, followed by smaller and smaller sediments until the top layer is made up of a fine clay.  

As seen in the image above, when you have areas which are prone to turbidite deposits, these will often result in packets of rocks that cycle through that course to fine cycle. The above picture is from Zumaia, Spain where the rocks, once horizontal, have since been rotated almost vertically. 

Geologic Map of New Zealand. Image courtesy of

Greywacke is a primary component of the Torlesse Composite Terrain of New Zealand. Looking at the above geologic map of New Zealand, we can see how much of the islands, especially the southern island, are covered with the Torlesse. And besides just these areas where it is exposed, the Torlesse is found below most of the younger rocks across the islands. The composition of the Torlesse Composite Terrain includes not only greywacke but also argillite, as well as metamorphosed rocks that were altered from the original greywacke and argillite.

The Torlesse Composite was deposited by submarine turbidites in deep marine water which ranges in age from Middle Permian (~260 million years old) to the Early Cretaceous (~100 million years old) with the primary deposition taking place in the Late Triassic, or approximately 216 million years old. 

No comments:

Post a Comment

Due to spam, comments are turned off. Please contact me on Twitter @Jazinator for any comments.

Note: Only a member of this blog may post a comment.