CORRELATION OF EXTRUSIVE UNITS OF NORTH MOUNTAIN BASALT AND CENTRAL HIGH ATLAS CAMP LAVAS USING GEOMAGNETIC PALEOSECULAR VARIATION

In the Newark basin [1], the level marking the end-Triassic extinction event occurs one Van Houten cycle (~20 kyr) before the first CAMP basalt and is preceded within less than another Van Houten cycle by reverse polarity Chron E23r, one of the shortest (~25 kyr) polarity intervals recognized in the Newark astronomically-tuned geomagnetic polarity time scale. This same tight chronostratigraphic sequence of events is recorded in the Fundy basin of Nova Scotia [2], where the initial CAMP extrusive unit (North Mountain Basalt) has a precise U-Pb zircon date of 201.27± 0.03 Ma [3]. In the Central High Atlas Mountains of Morocco, two magnetic excursions were found within the Intermediate Basalt (39Ar/40Ar date of 199.9± 0.5 Ma) and somehow might correlate to E23r [4, 5]; this would imply that the underlying Lower Basalt was erupted prior to the end-Triassic extinction event but the reliability of the excursion data seems doubtful [6]. Paleomagnetic study of the Moroccan basalts also revealed systematic groupings of magnetic directions, which were interpreted as a record of secular variation [5]. We sampled most if not all of the lava flows in the Fundy basin that comprise the ~300 m-thick North Mountain Basalt in outcrop (30 sites) as well as in several industry cores drilled near Margaretsville, Nova Scotia (e.g., GAV-77-3). We find only two directional groupings for the entire North Mountain Basalt, a finding that basically confirms the results of Carmichael and Palmer [7] and suggests very rapid extrusion. The progression of VGP clusters corresponding to the directional groups for the North Mountain Basalt resemble the VGP clusters corresponding to directional groupings reported [5] for the upper Lower and lower Intermediate lavas from Morocco. We speculate that the episodic volcanicity associated with initial phases of CAMP can be correlated over (predrift) distances of ~1000 km on submillennial time-scales of paleosecular variation.

References: 1, Olsen, P.E. et al., 2002, GSA Spec. Paper 356:505–522; 2, Deenen, M.H.L. et al., 2011, CJES 48:1282-1291; 3, Schoene, B. et al., 2006, GCA 70:426–445; 4, Marzoli, A. et al., 2004, Geology 32:973–976; 5, Knight, A.B. et al., 2004, EPSL 228:143–160; 6, Font, E.N. et al., 2011, EPSL 309:302-317; 7, Carmichael, C.M. & Palmer, H. C., 1968, JGR 73:2811-2822.