Publication Type:Journal Article
Source:LithosLithos, Volume 101, Number 1-2, p.74-101 (2008)
Keywords:CAMP, North Mountain Basalt, Nova Scotia, Pahoehoe flows, Tholeiites
The North Mountain Basalt (NMB) is a sequence of continental tholeiitc basalts that are part of the larger, 200 Ma Central Atlantic Magmatic Province (CAMP).The NMB occur within the Fundy Basin, one of several Mesozoic extensional basins that formed during the incipient rupture of Pangea. In addition to the NMB, the Fundy Basin was also filled by fluvial, alluvial, playa, lacustrine and aeolian sediments under arid to semi-arid conditions. Mapping of a 200 km long segment of the NMB along the southern coast of the Bay of Fundy indicates for the first time the lateral continuity of the three recognized lava units, the East Ferry, Margaretsville and Brier Island members (EFM, MM, BIM, respectively), that comprise the NMB. The EFM (< 180 m) and BIM (> 150 m) are massive and characterized by pervasive development of columnar jointing, both colonnade and entablature. In addition, whereas the EFM contains abundant layers of mafic pegmatite, the BIM is characterized by abundant mesostasis, possibly reflecting widespread quenching of this flow, and locally abundant segregation pipes due to mobilization of a late-stage felsic liquid. These two units represent expansive eruption of low viscoity flows onto a low slope surface, which encouraged uniform thickness and their massive nature, but the role of faulted basins cannot be ruled out. In contrast, the MM (170 m) consists of numerous (> 16) inflated pahoehoe sheet flow lobes. Evidence that these flows formed as pahoehoe flows is represented by exceptional development of the vesicle zonation (i.e., upper crust, vesicle sheets, vesicle cylinders, pipe vesicles) as observed in modern Hawaiian flows, historical Icelandic flows and ancient flows of the Columbia River Basalt Group. The thickness of upper crust zones indicates that the individual flows formed over months to years, whereas inclined basal pipe vesicles indicate the MM and BIM flowed to the south. The similar age and nature of the NMB to other basalt sequences in CAMP suggest that similar processes operated throughout this LIP, thereby having implications for such phenomena as atmospheric contamination and extinction events caused by eruption of these voluminous basaltic lava flows.