I don’t know the culinary director of Araxi Restaurant in Whistler, BC personally, but apparently we like the same countertops. Black Marinace is billed as “granite” at your local countertop warehouse, and those skilled in the art of wordsmithing make it sound otherworldly: “Strikingly original and with hints of gold, Black Marinace is the perfect granite for spaces where glamour is a given…. Its appearance is reminiscent of a rich black velvet jewel box bearing precious gemstones. Because of this, it will look gorgeous in settings that seek to stand out above the norm.” - Levantina
I love this rock, and made it a center piece of my kitchen, but I definitely wasn’t thinking about a “rich black velvet jewelbox” at the time! Countertops are fascinating to me, and in this regard, Black Marinace does not disappoint. It’s geological history tells a tale of rivers on a lifeless planet and mountain building events to rival the Himalaya.
Black Marinace is not granite as the term is used in geology, but is a 1.7 Billion year old metaconglomerate from Brazil (1,2). Metaconglomerates start out as an accumulation of gravel with sand and/or mud between the gravel clasts. These sediments lithify (transform into stone) to form a rock called conglomerate. The conglomerate is then put under immense heat and pressure, which alters the minerals in the rock (a process known as metamorphism) to produce metaconglomerate.
The core of the South American continent is a region of really old rock that is similar to the Canadian Shield. These regions are called cratons and they are typically made of rocks older than 1 billion years. These cores are the regions around which the modern-day continents formed. Brazil contains most of the core of South America, and 1.7 billion years ago, braided rivers flowed across the craton, and alluvial fans formed on the margins of valleys. This was also a time when there were no plants on land. The cobbles and pebbles that accumulated in those braided rivers and alluvial fans were well rounded and were encased in sand and mud. These sediments cemented together to form conglomerate.
About 1.2 Billion years ago, Africa and South America collided through plate tectonic processes, causing the two landmasses to become one (2). That continent-to-continent collision would have formed a mountain range similar to the Alps or Himalaya, and rocks on both continents would have experienced immense pressure and heat which drove metamorphism. This resulted in the (1.7-billion-year-old) braided river and alluvial fan conglomerate deforming and some of the minerals in the rock changing. The mud and sand (matrix of the conglomerate) fused together to form an impermeable rock and some of the minerals changed to mica giving the rock it’s shiny appearance. The gravel clasts were forced together resulting in "embayed margins" between clasts and stretching of pebbles as they squeezed and deformed around each other (see top photo). An embayed margin refers to two rocks where one "fits" into the other, and contrast this with taking two rounded rocks from your back yard and trying to force one into the other. Hopefully that gives you some idea of the pressure involved in the formation of metaconglomerate!
So, the next time you walk past or eat at Araxi, visit the Royal Alberta Museum in Edmonton, or see Black Marinace in a kitchen, feel free to regale your friends with stories of rivers, mountains, and Earth's evolution. It is always crowd pleaser!
If you enjoyed learning about the geological history of granite countertops, subscribe and be the first to learn of new posts, geology in the news, and answers to subscriber questions.
(1) Souza, E. G. d., Scherer, C. M. d. S., Chemale, F., Bállico, M. B., dos Reis, A. D., and Rosseti, L. M. M., 2019, Paleoenvironment and age constraints of Paleoproterozoic alluvial fans in the Sao Francisco Craton, Brazil: Journal of South American Earth Sciences, v. 91, p. 173-187.
2) Trompette, R., Uhlein, A., Da Silva, M. E., and Karmann, I., 1992, The Brasiliano Sao Francisco craton revisited (central Brazil): Journal of South American Earth Sciences, v. 6, p. 49-57.