No abstract

The magnitude 6.3 Timiskaming earthquake of 1935 resulted in discoloration in small lakes in the epicentral region. Sonar profiles for Lac Tee have shown that organic-rich gyttja has been removed from the sides and redeposited in the deep parts of the lake. Lake sediment cores from deep basins contain a 2–3 cm uppermost layer, which is interpreted as the normal accumulation of sediment since 1935. This is followed by a 20 cm chaotic zone of black gyttja mixed with partly tabular fragments of a previously formed silt layer, underlain by a dense 1–2 cm thick silt horizon. The latter is interpreted as having formed by rapid settling of the denser silicate portion of the resuspended sediment. This 20 cm zone is insufficient to account for the abnormal thickness of gyttja in the basins, so events of this kind would have had to have occurred repeatedly since deglaciation. The only major prehistoric event revealed by these ~100 cm cores is at a depth equivalent to about 1500 years ago, based on the amount of sediment deposited since 1935. A minor silting event occurred about 400 years ago. On this basis, the recurrence interval of magnitude 6 or greater earthquakes is longer at Témiscaming than at Charlevoix, Quebec (75 years historically), consistent with the present relative frequency of small earthquakes in these two regions.

Eastern Canada has experienced at least five earthquakes of estimated Richter magnitude 6 or greater during the last 350 years. The epicentres are usually under the St. Lawrence River, some 100 km east of Quebec City. Historical records of the earthquake of 1663, possibly the largest of these events, describe high levels of silting in streams for up to several months. A silt horizon in normally organic-rich lake sediments has been found in lakes with separate drainage systems and is interpreted as the 1663 event on the basis of rough sedimentation rates obtained from observation of 137Cs fallout of the 1950's and the effects of a dam on one of the lakes. Two other silt layers correspond reasonably well to the next largest earthquakes of 1791 and 1860 + 1870 (combined). Two deep layers were found that by extrapolation of the assumed 1663 layer yield dates of about A.D. 1060 and 600 and are believed to represent prehistoric earthquakes, though possibly not as large as that in 1663. Chemical analyses of the cores show that organic material, Ti, and especially K are very useful for identifying these layers and others that are not visible in the cores.

No abstract

Lower Paleozoic carbonatites associated with the St. Lawrence graben system are correlated with carbonatites of similar age (565 m.y.) in Greenland and Scandinavia. The correlation is well established in Canada where the intrusions all occur along a continuous rift system. The reconstruction of the whole province is made possible by restoring the continents to the positions they occupied prior to their separation.The combination of unique petrologic character, age, and structural setting shows that these rocks belong to a single alkaline rock province, defined by a rift system extending at least from central Canada to eastern Sweden that was active throughout its length about 565 m.y. ago.Periodic intrusion of alkaline rocks has occurred later, on both continents, associated with the components of the fracture system. In eastern Canada, alkaline rocks occur locally with ages of 520, 450, 300, and 110 m.y. in addition to the widespread 565 m.y. event.