Economy Mountain cliff face in Five Islands Provincial Park, Nova Scotia (east of Old Wife Point) shows a bleached zone, several meters thick, of Blomidon formation (Triassic redbeds) beneath the North Mtn. basalt. The Tr-J boundary and mass extinction horizon is a meter or two (?) below the basalt. I assume that the bleaching is a thermal effect of the lava. Paul Olsen says the boundary layer contains charred organic material.
Stevens "dike" at Old Wife Point in Five Islands Provincial Park, northern side of the Minas subbasin of the Fundy basin, Nova Scotia. The pile of fractured basalt (on the left) has sub-horizontal cooling columns, leading to the suggestion that it is a feeder dike to the North Mtn. basalt in this region (G. R. Stevens, 1987). However, the columns bend toward vertical to the right (northeast) where they are truncated by a fault. The basalt is greatly brecciated. Most likely, this is a tectonic block of basalt that has been deformed by the fault activity so evident here.
"Neptunian" dikes fill joints around cooling columns in the North Mtn. basalt (left side of image), as described by Schlische and Ackerman (1995) a few dozen meters from Wasson bluff near Glooscap Park, Nova Scotia. The authors describe these as silicified mud fillings that sifted down from the overhead basin sediment, possibly during a tectonic event. To me, they look like devitrified basaltic glass, although they are the same color as much larger and coarser clastic sedimentary dikes nearby in the basalt face of the bluff. The photos are from a ledge in the beach (a quarter for scale), and I tried to get a closeup of some thin white minerals that crystallized on the fracture surface of one of these infill dikes.(right side). I believe I recognized small rounded grains of pyroxenes in these infill dikes, and so perhaps these are actually a late-stage magma surrounding the columns. Similar infillings are found at other places. Gotta get a t-section.
Swallowtail Head Light dike on Grand Manan Island, New Brunswick is one of the few large dikes not yet studied. The exposure is spectacular, and is visible from the ferry boat as it approaches North Head. The dike intrudes Precambrian metabasalts, and it is structurally below the North Mountain basalt in fault contact to the west on this island. Our view of this NE-SW dike is toward the west, and the interior of the dike is exposed by erosion where it leans against the background cliff. I have been told that this dike is probably Paleozoic in age, although it is physically very similar to the large Mesozoic tholeiites. If our colleagues from Canada do not work on it, it is next on my list!
Swallowtail Head Light dike from the north. The great tabular body leans against the cliff, in an exposure unlike any other.
Seven Day's Work is the name given to this cliff of North Mountain basalt at the northeastern end of Grand Manan Island. I am told that the locals decided each layer took one day to form! The layers here probably represent individual flows in the middle or upper sections of the basalt. They are rather vesicular (including large vertical gas tubes) and zeolite collecting is good.
Seal Cove basalt on Grand Manan Island (southeastern shoreline) shows an interesting erosional pattern of cooling columns. I thought at first these were weathered pillows, but some are polygonal. I don't understand why the edges resist erosion, but I need to look at this again to see if there are any features like the "neptunian dikes" that surround columns at the Minas basin to the north.
Red Point Fault, on the shore of southeastern Grand Manan, is an exposure of the fault contact between the North Mountain basalt and a Carboniferous (?) siltstone. The basalt has slipped downward to the SW relative to the siltstone in the footwall. You can see some of the basalt columns tilted near the fault (yours truly for scale). In the footwall near the fault, the siltstone contains many quartz pods and veins, some visible in this photo. The basalt is brecciated for several meters above the fault.
Red Point breccia in the basalt above the fault. Looks just like the tectonic breccia around Steven's "dike" in the photo described above, except here it is rather weathered and somewhat greenish in color.
Southwest Head Cliff on Grand Manan is a great exposure of North Mountain basalt. Maine is off in the distance to the west, where a likely source dike is exposed along the shoreline farther southwest. This is the lower section of the basalt, which is a single great columnar flow. Or, it has been proposed that these columnar sections of the basalt are actually intrusive sheets and sills. The upper section exposed in the northern end of the island shows a series of distinct, thin flows (Seven Day's Work, see above).
North Mountain basalt thin section, sample from South Head. Left side is plain light, right side is crossed polarizers.
Caraquet dike near Bancroft, Maine, on the NW side of the Mattawamkeag River about 20 km west of New Brunswick. It is very fine grained and hard, and it appears to be offset a few meters by a right lateral fault. This locality yielded a 201 Ma Ar/Ar date (West et al. 1997). In its petrology, the Caraquet dike is very similar to the Buttress-Ware dike of Ct. and Mass., and perhaps we should look for a physical link as well.
Swan's Island dike is the easternmost known exposure of the Christmas Cove dike (location studied by Dave Bailey). Here the dike has turned toward the NNE along the western shore of Swan's Island, Maine and then it heads toward southern Mt. Desert Island. It is still quite large here. The dike probably continues NE or ENE but it may be confused with the numerous Paleozoic mafic dikes in this area (as at Schoodic Point). My guess is that it steps inland toward the NE to connect with the Minister's Island dike.
Christmas Cove dike where it crosses the peninsula at Christmas Cove in South Bristol, Maine. This photo looks NE, showing the interior of the dike as it strikes ENE into the bay. The northern contact is exposed around the far end, and cooling columns are also evident. The large broken blocks and rusty to gray surface are common field characteristics. The Christmas Cove dike is probably connected with the Higganum-Holden-Onway dike of southern New England, which together form a single dike system about 510 km in length.
Spruce Point dike, which is the Christmas Cove dike south of Boothbay Harbor. A "whaleback" outcrop of glacially-rounded ledges in a beach, and the smooth surface shows large pods of coarser dolerite (inset). This might be earlier dike material broken up and rounded by a later dike magma pulse. We also see two magma batches in the Higganum dike (Philpotts and Asher, 1994).
Mountain Road dike (part of the Christmas Cove dike) in the NE part of Harpswell Neck, Maine. Big cooling columns are evident, tilted at an angle. Possibly such tilted sections indicate a jump to another vertical joint, and this section can be eroded away to make a gap in the dike map. This part of the Christmas Cove dike does appear to be missing for a few km to the ENE. Jelle de Boer and his student are preparing to take a 1-inch core for magnetic analysis.
Mountain Road dike thin section in plain light. The clumps of green clinopyroxene have Ti-rich zones that are pink. Phencrysts of orthopyroxene (bronzite; upper right) several tenths mm in size are characteristic of this (dike) magma type.
Cumberland Foreside dike (part of the Christmas Cove dike), closeup of south contact in a glacially-polished surface, looking north. Notice the evenly spaced cross (cooling) fractures, and the thin selvedge or glassy contact zone. Also note the interesting left-lateral steps in the contact, which are consistent with the en-echelon pattern and steps of this dike on a regional scale, from Connecticut to New Brunswick (including the Minister Island dike, Maps & Figures section).
Onway dike near Onway Lake in Raymond, NH. Studied by the late Dan Sundeen, this exposure is in an abandoned railroad cut. Its chemistry and petrography are identical to the Higganum and Christmas Cove dikes, and it is certainly part of this dike system.
Hampden basalt Rte. 372 roadcut between Rtes. 9 and 5/15, East Berlin. Here you can see the contact of East Berlin siltstone with the overlying Hampden basalt, the youngest of the three lava flows of the Hartford basin. It dips eastward. It is thought that the silt was wet when the lava flowed over it, causing steam-filled "pipes" to form at its base. Some pipes are bent in the initial lava flow direction. The pipes later filled with calcite and zeolites. A quarter for scale.
Higby Mountain in Meriden, Connecticut is a typical "hanging hill" of the Hartford basin. This view is toward the northeast. The Holyoke basalt (the middle of the three Hartford basin basalts) forms west-facing steep cliffs of 80-m thick lava, dipping gently to the east. Just off to the right (south) side of the photo is a dike of much younger alkalic basalt, a rare series in Connecticut that remains poorly known.
Higganum dike at Ponset, in Haddam, CT (Rte 9 exit 9), looking NW. The dike here is tilted to a shallow angle (making a wide cut), but you can see the columnar fractures in the cliff. This cut was described by Philpotts and Asher (1994).
Holyoke basalt at the Tilcon quarry in North Branford, CT (looking east). This quarry face is about 40 m high, or about half of the basalt. There is a deformed zone of columnar basalt along the base of the overlying "entablature," in sharp contact with the lower sub-vertical "colonnade." The basal entablature layer wraps around ridges and valleys of several meters relief along the contact. In the inflation model of Self at al. (1997), the entablature is gradually solidified while the flow continued beneath it, and inflation pressure of the liquid deformed the overlying cap. Only after the flow stopped did the lower columnar section cool against this uneven contact. The great thickness of the entablature is evidence that the basalt flowed continuously through this area for many months, and was not merely a lava lake ponded against the basin border fault not far to the east. Also see Cashman et al. (1998). Another model is that the downward bulges of the entablature reflect zones of surface fractures that allowed rainwater to penetrate the upper basalt, which led to uneven cooling of the liquid interior.
Brevard Zone dike cutting saprolite (deeply weathered gneiss) in North Carolina, at the base of the famous Grandfather Mountain Window. This is one of the few southern Appalachian dikes that penetrates into the highlands from the Piedmont. It appears to be the far distal northern end of a very large dike that runs through the Carolinas called the Pageland dike. It shows spheroidal weathering (rounded blocks) but the rock remains surprisingly fresh inside.
Icelandic Fissure Eruption courtesy
of a Time-Life book on volcanoes -- I think this is part of the "Krafla
Fires." This is essentially a dike erupting in en-echelon segments
(notice the smoke in the distance from other segments). The gaseous
plume has plenty of water and carbon dioxide, but also poisonous chlorine,
fluorine, and sulfur dioxide (the Icelandic government sometimes issues
a warning to farmers to keep their livestock indoors to prevent fluoride
poisoning). I like to imagine this view was common at 200 Ma, but
only a brave pterosaur would see it from above! There was tremendous
damage to habitats and the atmosphere from such eruptions throughout Earth
history. Note to some colleagues: the magma in a fissure eruption
has to flow UP to get from its source to the surface, although when the
dike was forming, its contents must have followed a fracture that was propagating
both vertically and horizontally (or obliquely), of course.