Cape Breton Highlands, Nova Scotia

Day 14

Old, old rocks
The Cape Breton Highlands are in northeastern Nova Scotia. This part of Nova Scotia is underlain by Grenvillian age basement rocks (~1.8 billion year old rock). The rocks outcropping at White Point (picture on right) in the Cape Breton Highlands are predominantly pink potassium feldspar, quartz, and plagioclase (granitic) gneiss. Black, biotite schist also outcrops at White Point.

The gneiss contains xenoliths or inclusions of biotite schist, indicating that the schist was emplaced prior to the gneiss. Gneiss is often metamophosed granite, which suggests that 1.8 billion years ago, granitic magma intruded the biotite schist. The magma ripped pieces of rock off the schist unit. When the magma cooled and solidified, the pieces of schist (xenoliths) were still trapped in the rock matrix. Both rocks were later deformed and metamorphosed.

In the picture on the right, the outcrop looks like it is made of layers of gneiss and schist. The layered appearance of the rocks suggests that the granitic magma preferentially intruded along foliations (mineral layers) in the schist.

An outcrop of Grenvillian age gneiss and schist in the Cape Bretton Highlands.

Mesozoic aged gypsum from an evaporite deposit.

Breaking up with a gypsum quarry
Gypsum is a hydrous calcium sulfate mineral (CaSO4-H2O). It has industrial uses, both in the paper milling process and as wallboard in homes.

Gypsum is an evaporite, a chemical sedimentary rock. Gypsum, like most other sedimentary rocks, is deposited in horizontal layers. As a chemical sedimentary rock, gypsum is deposited in shallow, warm, marine environments as an evaporite from sea water. When sea water is accumulated and evaporated in a shallow area, minerals such as gypsum, halite (NaCl), anhydrite (CaSO4), and sylvite (KCl) precipitate out. Halite, sylvite, and anhydrite are not associated with the Cape Breton Highlands gypsum deposit. These minerals either did not precipitate out from the sea water or they have already been weathered out of the evaporite deposit. Most likely, these minerals did not precipitate out of the sea water. These minerals are highly soluble in water and require high evaporation rates to precipitate out. Consequently, deposition of evaporite minerals such as sylvite and anhydrite is much less common than gypsum.

The gypsum was deposited during the Mesozoic Era. Shallow marine depositional environments are often associated with rifting. When rifting takes place, the crust becomes thinner as it gets pulled apart, forming a rift valley. If the rift valley extends to the ocean, ocean water can flood the rift valley and form a shallow sea. The deposition of this gypsum deposit is associated with the break-up of Pangea and subsequent continental rifting during the Mesozoic. In North America, evaporite deposits related to the break-up of Pangea occur in many places along the eastern seaboard from Connecticut up through Nova Scotia.

Furthermore, since gypsum is only deposited in warm environments, this gypsum deposit is evidence that the climate in Nova Scotia during the Mesozoic was much warmer than it is today. During the Mesozoic, the Nova Scotian landmass was likely at a similar latitude as the modern landmass, however, the overall climate throughout much of the Mesozoic was much warmer than the Earth's climate today. A modern analogy for the Mesozoic shallow sea in Nova Scotia is the Red Sea, which is a shallow, warm sea in a modern rift zone.