Basalt on the Bay of Fundy

northwest of Digby, Nova Scotia, Canada

to west [241 kb] down NE [243 kb] to east [256 kb]

Above left, centre and right: views to west, northeast and east showing landscape detail on a very small portion of the extensive North Mountain Basalt of Nova Scotia.

2 PTS (polished thin sections) of basalt [139 kb]

Above: a transmitted-light greyscale image showing two polished thin sections of basalt samples (FUN-1 at left, with FUN-3). Each of the glass backing slides is 45x26 mm in area, and all but the very edges of the slides are shown here. Although there are some opaque grains (iron-titanium oxides), most of the rock is translucent when reduced to the hair's-breadth thickness of 0.03 mm. Most of the dark material in FUN-1 is actually an iron-rich glass which coexists with and lies interstitial to the crystals, and which did not have sufficient time to crystallize before the magma cooled. The clear material is mostly feldspar and minor olivine, while mid-grey tones are prismatic silicates of the pyroxene family.

"Rock of the Month # 130, posted April 2012" ---

These views, and the associated samples, illustrate a segment of the southern shore of the Bay of Fundy known as the Digby Neck, a series of promontories and associated islands extending from the sheltered harbour of the Annapolis Basin southwest to Long Island and Briar Island. These samples are from a small private property southwest from the Prim Point lighthouse, which stands upon the western side of the entrance to Digby harbour, safeguarding the many vessels that pass, including the ferry that plies the Bay from Digby north to St. John, New Brunswick. The lighthouse, like most of the shore here, is underlain by basalt flows that vary in their grain size, internal textures, mineralogy and degree of post-crystallization alteration.

FUN-1 [252 kb] FUN 1+3 [243 kb] FUN-3 [252 kb]

Above centre, left and right: a photograph of sawn pieces of samples FUN-1 and FUN-3 flanked by photomicrographs of each basalt in thin section. The hand specimens are accompanied by offcut slices from which the thin sections (see above) were prepared. Left: FUN-1, basalt composed largely of grey laths of twinned plagioclase feldspar, colourful prisms of clinopyroxene, and partially devitrified glass. Glass is almost opaque, and so black here, except where partially recrystallized ("devitrified") to produce the concentric zoned patch at the lower right. Nominal magnification 50X, long-axis field of view 1.7 mm, in crossed-polarized transmitted light. Right: basalt FUN-3, same magnification, in plane-polarized light. Here we see a mass of small feldspar and pyroxene crystals in glass, and a patch of alteration to pale green chlorite.

Three basalt samples from the study area, although texturally distinct, share a common mineralogy. The mineralogy of the basalt can be summarized as follows, while noting that mineral proportions do vary from one sample to the next. In decreasing order of abundance, the basalt contains four main magmatic minerals, glass, and alteration minerals formed during or after the cooling of the magma, as follows:

  • plagioclase feldspar,
  • pale clinopyroxene, such as augite,
  • partially devitrified brown (and thus iron-rich?) volcanic glass,
  • olivine, in part oxidized and altered along partings,
  • magnetite (iron oxide),
  • chlorite (a later sheet-silicate alteration), and
  • other minor and accessory minerals.

The "spine" of the Digby Neck is composed of a prominent rock formation known as the North Mountain Basalt. The basalt outpourings represent an extensive volcanic episode that occurred 200 million years ago. This "LIP" (large igneous province) formed during rifting in a precursor event to the opening of the North Atlantic Ocean, which later saw similar basaltic magmatism in northwest Scotland, northern Ireland and coastal Greenland approximately 61 to 52 million years ago, and most recently the birth of Iceland, which has grown appreciably in the past 7 million years, with some 35 volcanoes active in the past 10,000 years alone.

Some relevant features of the North Mountain Basalt include:

  • Dated at about 200 million years old, which is
  • The time of the Triassic-Jurassic boundary,
  • Extends for 200 km along the south shore of the Bay of Fundy, from Brier Island to the Minas Basin,
  • The Digby Neck is roughly the western third of this expanse,
  • Part of the Central Atlantic Magmatic Province,
  • Comparable to early Mesozoic, diabase-dominant magmatism in the basins of northern Virginia, Maryland and Connecticut,
  • Composed of three suites of lavas, of which
  • The upper, Brier Island member forms the rugged coast of the Digby Neck.

Selected Recent References

Kontak,DJ, De Young,MYDW and Dostal,J (2002) Late-stage crystallization history of the Jurassic North Mountain basalt, Nova Scotia, Canada. I. Textural and chemical evidence for pervasive development of silicate-liquid immiscibility. Can.Mineral. 40, 1287-1311.

Webster,TL, Murphy,JB and Gosse,JC (2006) Mapping subtle structures with light detection and ranging (LIDAR): flow units and phreatomagmatic rootless cones in the North Mountain basalt, Nova Scotia. Can.J.Earth Sci. 43, 157-176.

Kontak,DJ (2008) On the edge of CAMP: geology and volcanology of the Jurassic North Mountain basalt, Nova Scotia. Lithos 101, 74-101.

Kontak,DJ and Dostal,J (2010) The late-stage crystallization history of the Jurassic North Mountain basalt, Nova Scotia, Canada. II. Nature and origin of segregation pipes. Can.Mineral. 48, 1533-1568.

Graham Wilson, 03-08 April 2012

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