The Queen Charlotte Islands are at the western edge of the continental shelf and form part of Wrangellia, an exotic terrane of former island arcs, which also includes Vancouver Island, parts of western mainland British Columbia and southern Alaska. While we’ll see that there are two competing schools of thought on Wrangellia’s more recent history, both sides agree that many of the rocks, and the fossils they contain, were laid down somewhere near the equator.
They had a long, arduous journey, first being pushed by advancing plates, then being uplifted, intruded, folded, and finally thrust up again. It’s reminiscent of how pastry is balled up, kneaded over and over, finally rolled out, then the process is repeated again. This violent lifestory applies to most of the rock that makes up the Insular Belt, the outermost edge of the Cordillera. Once in their present location, the rocks that make up the mountains and valleys of this island group were glaciated and eroded to their present form. Despite this tumultuous past, the islands have arguably the best-preserved and most fossil-rich rocks in the Canadian Cordillera, dating from very recent to more than 200 million years old.
On these details, there is a pretty broad consensus. On much else, including exactly where the Wrangellia terrane was born and how fast it moved to its present position, there is lively debate. The two rival theories each rely on a different type of evidence: biogeographic or paleomagnetic. Volumes have been written on this complex argument. We’ll follow a single thread of the debate to get a sense of the science being marshalled on both sides.
Biogeographic evidence compares the types of fossils found in rock with the historic ranges of the living creatures they were formed from, and sometimes with the ranges of comparable contemporary life forms. Paleomagnetic dating uses the history of the Earth’s magnetic fields (see side bar). It can give a rough estimate of how far north or south of the equator rock was formed. Palomagnetic data from Triassic rock of Wrangellia suggest that it was laid down somewhere in a band about two thousand miles wide, centered on the equator. The biogeographic work of geologist Dr. Tim Tozer on the fossil faunas of the Queen Charlottes concurs with this. The fossils found in the Triassic rock of Wrangellia are equatorial or low latitude life forms quite different from those found today on the Continent at the latitude of the Queen Charlottes. This suggests those rocks were in the equatorial region during the Late Triassic, just over 200 million years ago. Further supporting this, Dr. Jim Haggart, a geologist at the Geologic Survey of Canada, who has worked extensively with the fossils of the Queen Charlotte Islands, notes that Lower Jurassic ammonite faunas found on the Queen Charlottes are very similar to those found in the Eastern Pacific around South America and in the Mediterranean. The presence of these ammonites seems to indicate that the rock was still at or very near the equator during the Lower Jurassic epoch, which ended roughly 175 million years ago.
To this point in prehistory, the paleomagnetic and biogeographic evidence is in reasonable harmony (at least about how far south Wrangellia was created. There’s much debate about just where along the equator is was, but we’ll ignore that for simplicity’s sake.).
It is as we move up through the rock column to younger layers that the two lines of evidence start to conflict. Around the beginning of the Middle Jurassic, the fossils found on the Queen Charlotte Islands, Vancouver Island and in southern Alaska start to be dominated by forms found in high latitudes. This suggests that by the time of the Middle Jurassic, about 165 million years ago, Wrangellia had already completed much of its journey northward. Similar biogeographic evidence from Cretaceous rocks supports this idea as the Cretaceous ammonite, bivalve and radiolarian faunas all appear to be high latitude as well.
In contrast, the paleomagnetic data from Wrangellian Cretaceous rocks on Vancouver, Hornby and Texada Islands initially suggested that they were formed while Wrangellia was still at low latitudes, roughly around present-day southern Baja Mexico. More recently, however, paleomagnetists have conceded that Wrangellia may have gotten as far north as central California by the Early Cretaceous. The paleomagnetists now maintain that the Queen Charlotte Islands arrived at their present-day position by the Lake Cretaceous or even earlier, around 60 million years ago. This revised dating still means these two groups do not see eye to eye on just when the Charlottes arrived at their current position.
However, the paleomagnetists face some major stumbling blocks: They can’t locate many of the fault lines necessary to buttress their case. For example, rocks on either side of the valley at Churn Creek north of Lytton originated 2000 miles apart according to paleomagnetic calculations, but no fault line has yet been found running between the supposedly separately created sides. The paleomagnetists assert that it’s just a matter of time before the required faults are unearthed — mere formalities they are happy to leave to the geologists. Needless to say, many geologists are unimpressed with having these burdens thrust upon them by “paleomagicians” unable to back-up their case. Both sides are able to find many faults in the opposing theory, if not in the Wrangellian terrane.
So, are scientists discouraged by such vast gaps in their knowledge of the early Earth? On the contrary, they’re excited. Paleogeology would be a very dull field if all that was left to do was to cross a few trivial Ts and dot a few insignificant Is. As it is, there are vast opportunities to have your name go down in history by making major contributions to our knowledge of Earth’s prehistory.
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