Paleomagnetism of the Triassic red beds of the lower Fundy Group and Mesozoic tectonism of the Nova Scotia platform, Canada

Abstract

The lower Fundy Group red beds of the Wolfville and lower Blomidon formations in Nova Scotia and of Wolfville-equivalent units in New Brunswick are exposed around the Bay of Fundy. These red beds were sampled at twenty-six shoreline sites for paleomagnetic analysis, primarily by thermal step demagnetization methods. Samples from four sites proved too fragile to core; however, coating many of the rest of these poorly-indurated samples and cores with waterglass enabled them to withstand the rigours of coring, measurement and thermal cleaning. A stable hematite remanence was found in all twenty-one coherent sites with normal, reversed and mixed polarities. Conglomerate and fold tests indicate the remanence is either primary or very early diagenetic. Its mean direction is D = 196.5°, I = −3.8° ( α 95 = 7.2°) which converts to a pole position of 45.3°N, 97.1°E ( dp = 3.6°, dm = 7.2°). This pole indicates that the lower Fundy Group was likely deposited in the Early Triassic, rather than in the Middle and Late Triassic, as currently thought. It also indicates that the Nova Scotia platform was rotated about 8° clockwise after deposition of the lower Fundy Group and before extrusion of the North Mountain basalts. The consequences are briefly discussed for two alternative tectonic models of Mesozoic motion on the Minas Geofracture that led to formation of the Bay of Fundy rift as the Atlantic Ocean basin started to open in Early rather than Middle Triassic time. The Fundy Group pole is similar to those from granitic plutons in Maine, thereby increasing the probability that there is a significant error in the mid-Triassic portion of the apparent polar wander path for North America. Finally, magnetostratigraphic methods appear to be a promising way to subdivide and correlate between these red beds.