Regional Geology of the Maritime Provinces

The fusion of the three terranes that underlie the southern Maritimes was part of a bigger process that brought together most of the Earth's continental crust into a single supercontinent, Pangea, which dominated the planet from 350 to 200 million years ago (Fig. 5).

Figure 5: Global paleogeography of the Late Carboniferous, about 305 million years ago. From Atlantic Geoscience Society (2001).

Figure 6: Local paleogeography of the Early Carboniferous, about 335 million years ago. From Atlantic Geoscience Society (2001).

It was during this interval that the sedimentary rocks that we see today at places like Blue Beach, Hopewell Cape, Joggins and Sydney formed. But how, you may ask, could rocks and fossils be preserved from the Pangean heartland at a time when continental collisions and mountain building were, one might think, dominant themes? The answer is that, in active parts of the Earth's crust, complex jostling and adjusting of different crustal fragments and internal tectonic forces caused some areas to be uplifted while others sagged. And areas that sagged were ideal for accumulating and preserving sediment. Geologists call these depressed areas sedimentary basins - or simply "basins".

The southern Maritimes in the Carboniferous Period was one such area of uplands alternating with basins. Then, as now, areas such as the Caledonia Highlands in southern New Brunswick, the Cobequids, the Antigonish Highlands and Cape Breton Highlands of Nova Scotia were uplands. In between were a series of basins: for example the Sydney Basin, the Stellarton Basin, the Moncton Basin, and the Cumberland Basin. It is no coincidence that many of these basins contain famous coalfields, as we shall see. All the basins were interconnected and geologists somewhat confusingly call this ensemble of interconnected depressions in the crust as well as the intervening uplands, the Maritimes Basin. (It would have been easier perhaps if geologists had used the term "Maritimes basin and range province", but unfortunately we have to stick to the conventional terms.)

The geological history of the Maritimes Basin can be traced in the rocks from the Devonian to the Permian periods (Fig. 2), in a succession of rock "groups" (not the musical variety) that are termed:

the Pictou Group . 280-305 Ma
the Cumberland Group . 305-320 Ma . see Fig. 8
the Mabou Group . 320-330 Ma
the Windsor Group . 330-342 Ma . see Fig. 7
the Horton Group . 342-365 Ma . see Fig. 6

This list is ordered with the oldest at the bottom, a geological convention since this is the order that you would expect to see in a cliff face.

Rocks of the Horton Group were deposited in the latest Devonian to early Carboniferous (342-365 million years ago). They were formed mainly in extensive river systems, lakes and possibly deep "bays" (sometimes tenuously connected to the sea) within many of the individual basins of the Maritimes Basin (Fig. 6). TThis was atime when ancient amphibians left their footprints in the mud, for example at Blue Beach, near Hantsport. As noted above, there may have been minor incursions of the sea during Horton times, presaging the substantial inundation that is represented by rocks of Windsor times.

About 340 million years ago, coinciding with a generally much drier climatic episode, the Windsor Sea spread over all but the most mountainous areas of the Maritimes Basin (Fig. 7). Sea level rose and fell periodically during the 15 million years that the Windsor Sea existed. The record of this cyclicity is reflected in the repeating deposits of carbonates (mostly limestone), evaporites (gypsum, rock salt and potash) and red clastic sedimentary rocks. (Such red rocks are

Figure 7: Local paleogeography of the Early Carboniferous, about 335 million years ago, at the time of the Windsor Sea. From Atlantic Geoscience Society (2001).


known to geologists as "red beds". "Clastics" are rocks made of "clasts" like sand or silt grains or clay flakes, in contrast to "chemical" sedimentary rocks such as limestone and gypsum.) Windsor Group rocks are the source of most of the salt, gypsum and potash that is mined in the Maritimes.

With the coming of the late Carboniferous 325 million years ago, the climate grew much wetter, as reflected Nova Scotia's position straddling the equator. The Windsor Sea withdrew, probably to the northeast, and river and floodplain sediments dominate the Mabou and Cumberland groups

Figure 8: Local paleogeography of the Late Carboniferous, about 310 million years ago. From Atlantic Geoscience Society (2001).

Figure 9: Paleogeography of the North Atlantic region during Late Carboniferous time.
Adapted from Calder (1998).

During the latest Carboniferous and into the Permian, the generally red Pictou rocks evidence a change of climate to greater seasonality, including intense dry seasons: paleoclimatologists have called this climate "megamonsoonal". Drought-tolerant trees, such as Walchia (the world's only known fossil forest of which was recently found at Brule, NS), replaced the luxuriant growth of the coal deposits. Rocks of the Pictou Goup are found on the north shore of Nova Scotia, and form the characteristic red rocks of Prince Edward Island, most of which are Permian in age.