PALEOMAGNETIC STUDY OF THE MIDDLE DEVONIAN MCARRAS BROOK FORMATION, AVALONIA AND THE PALEOPOSITION FOR THE LAURENTIAN MARGIN

The McArras Brook Formation of the Avalon terrane in Nova scotia lies uncomformably on sedimentary rocks of late Silurian and early Devonian age, and consists of a coastal and riverbed exposure of six basaltic flows represented by ten paleomagnetic sites, seven red bed and crevasse infill mudstone paleomagnetic sites and three conglomerate field test sites. The McArras Brook Formation is, in turn, uncomformably overlain by late Visean sedimentary rocks. The basaltic flows and conglomeratic redbeds were emplaced in a pull apart basin during trancurrent margin-parallel motion postdating the Acadian orogeny in which Avalonia was first accreted to Laurentia. Magnetic directions obtained by laboratory stepwise demagnetization of magnetite and hematite from these mid Devonian lava flows, sedimentary red beds and conglomerate samples have yielded preliminary results that are consistent with the few previously published mid Devonian paleo positions of cratonic Laurentia. Preliminary results from conglomerate tests indicate the presence of a likely Late Paleozoic overprint direction, but also randomly distributed characteristic remanent magnetization directions implying the presence of primary, i.e. mid Devonian remanence. Hence, the sample sites that are intraformational with these conglomerate beds may bear primary characteristic magnetic remanence. Flows which appear to carry a primary, mid Devonian remanence have a southwest and up remanence direction, implying that the McArras Brook Formation was emplaced at low to mid latitude, in agreement with paleogeographic models which place this Laurentian margin at 30 degrees south in the mid Devonian.