| Popis: |
Paleomagnetism is used in Quaternary stratigraphic studies as a tool for correlation and relative age dating of equivalent strata or for the absolute dating of deposits. The method is based on the detection of changes in the earth's magnetic field and especially changes of polarity that are recorded by ferromagnetic sediments at the time of deposition. Dating by paleomagnetic characterization and geomagnetic polarity history is a relatively new technique. The large-scale features of the earth's magnetic field character have been well worked out for the past 5 million years or so. The detailed small-scale features for this period are still being discovered and defined through analysis of terrestrial sediments. Because of the much greater sedimentation rate on land, these are more likely to show short-lived events and record the excursions which ultimately will become useful correlative tools. Fine-grained sediments, lava flows, and baked pottery are the media most frequently used. Because reversals have occurred repeatedly in the past their identification within incomplete sedimentary records is only possible through comparison with other stratigraphic or radiometric data collected for similar or related sedimentary sequences. Continuously deposited marine or terrestrial sediments which show a high sedimentation rate provide isochrons which can be used for worldwide correlation. The recent flourishing of research activity into the secular variation of the earth's non-dipole field promises to greatly refine and embellish the geomagnetic timetable for the Quaternary. The only practical way of demonstrating the validity of interpreted magneto-stratigraphy is to show that results are reproducible in widely separated sections with different lithology and sedimentation rates. In Canada where Pleistocene deposits are largely glacial in origin, and were thus episodic, one must be aware that these deposits may only have recorded the earth's magnetic field in short time intervals. Possible subsequent alteration of these sediments by the processes outlined in this paper, must be borne in mind. Great Lake sediments and other glacial lake sediments provide excellent opportunity for magnetostratigraphic correlation and dating. |
