No, we haven't really traveled south of the Equator. But 400 million years ago, the Maritime Provinces (Newfoundland, Nova Scotia and New Brunswick) as well as Quebec had moved south thanks to Continental Drift and existed just south of the Equator. Why is this important? Because the "paleoclimate" was suggestive of equatorial environments, creating fossil evidence of life in benign temperatures. Paleoclimatology is a relatively new area of study in geology and factors into the story of the Maritimes. So does paleomagnetism, which analyzes the magnetic orientation of rocks when they were formed. Together with the actual geology of the rocks and their structural evidence, a far more complete picture of how the Maritimes were formed emerges and allows us to postulate that the Maritimes were in fact located south of the Equator.
We don't have a lot of photographs showing the formation of these rocks, so please put up with descriptions together with diagrams. We have taken liberally from the book mentioned in one of the first posts in the blog, Four Billion Years and Counting, Canada's Geological Heritage.
Geologists would be lost without names and dates. We'll throw a lot of names at you and will try to explain them. The dates are best seen with the timeline shown below. The vast expanse of time is particularly hard to fathom. I had an undergraduate geology professor who asked that we think of the Empire State Building, from the sub-basement to the top of the radio antenna. That height represented Earth's time. (Today, Earth is estimated to be 4.5 billion years old. In the early 1970s, the age was estimated to be 3.6 billion years old.) Now, place a dime on top of the antenna. That represents the period huminoids have been on Earth. Now place the thickness of a tissue on top of the flat dime. That represents recorded history. We're tiny and infinitesimal.
Up to about 780 to 750 million years ago, there was a supercontinent called Rodinia. Some time around 750 million years ago, Rodinia began to split up. By around 500 million years ago, its "offspring" became evident. Laurentia, which includes most of present-day North America plus Greenland; Amazonia, which underlies South America and especially Brazil; Baltica, which includes much of Scandanavia and western Russia; Siberia; and China.
Life forms developed as all of this was unfolding. In Historic Geology, we were always taught that 600 million years ago was an important dividing line as it separated the Pre-Cambrian Period from the Cambrian. Today, the Ediacaran Period (542 to 488 million years ago) indeed shows evidence of life forms and variety of life flourishing. The Mistaken Point fossils, deposited on ancient sea floors in southern Newfoundland, provide such diverse evidence of this that it has become a United Nations sanctioned locality.
By 520 million years ago, Laurentia had broken free of its southern neighbors. That's where our focus will now remain. And now this becomes a story of accretion once Laurentia is "free." With each "island arcs" floating free across the Iapetus Sea from Baltica and Protogondwanaland, there are successive collisions along the coast of Laurentia. And with each collision, there are successive mountain-building orogenies. The Taconic Orogeny was first and was the result of the Dashwoods collision. Then the Ganderia, Avalonia and Meguma collisions followed. Each contributed to building the continental margin and creating the Appalachians and additional mountains to the present-day east. Nova Scotia and Newfoundland benefit from this activity.
This brings us to about 440 million years ago.
Ganderia underlie parts of New England, Atlantic Canada and western Europe. Avalonia, named for the Avalon Peninsula of Newfoundland, underlies parts of Boston, Massachusetts, the Gulf of Maine, the Maritime Provinces and eastern Newfoundland. They underlie Cape Breton Island, around Louisburg (the fort on Cape Breton) and New Bruswick and the Bay of Fundy. Meguma underlies Nova Scotia but extends as far as under Cape Cod to the Grand Banks.
The Earth is dynamic and always changing. So here is how Laurentia splits up once Iapetus closes at the end of the Ordivician, 444 million years ago, and into the Silurian at around 390 million years ago. This is about the time of the Acadian Orogeny, a mountain-building event to the east of the Appalachians. We will see evidence of this activity with the Rose Blanche Pluton on the southwestern Newfoundland coast. The black lines are the suture lines delineating North America from Africa and Europe. These splits are not as clean as suggested here. Parts of North America went with Africa and parts of Africa remained with North America. But the larger picture is valid and shows how the Maritime geology carries over to England.
Crustal melting during the period from 380 to 360 million years ago created large granitic plutons that underlie large portions of Nova Scotia. These intrusive emplacements can be seen at the surface. Remember those granite boulders the kids launched from to go swimming in Rogue's Roost? They were gifts of the plutons.
The continents did split apart and, in our case, the Atlantic Ocean opened. All of the deformation of the surface (and the crust) created a basin and range environment. Many of the ranges exist today as upland on Cape Breton. Between the highlands were extensive depositional environments that were flooded by seas periodically. The seas deposited evaporites such as halites (which later formed Salt Mountain) and economically significant deposits of gypsum and potash. Over time, the dissolution of these evaporites allowed today's Bras d'Or Lakes to be flooded with sea water.
There have been fascinating episodes of glaciation and deglaciation, especially in the St. Lawrence Seaway and the Champlain Sea. But we'll save that for another time. As a teaser, it explains how whales got to Vermont!
Cheers,
Brio