Geology of the Roque Island Archipelago. Little Spruce Island Halifax Island Hickey Island . Trip 3 Islands. E – E’ Cross Section. Hickey. Halifax. Little Spruce. Trip 3 Geology Overview. Late Silurian, Early Devonian Double subduction zone Piroclastic flows Tuffs and ash deposits
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Little Spruce Island
E – E’ Cross Section
Where we’ll be…
Deb and Des formations with Machias fault on the northern end of island
Des: Shale, Siltstone, tuffs, and fossils
Shale: Fine grained sedimentary rocks. Particles less than .004mm in size. Starts as clay minerals and
muds. Easily splits into parallel layers. Needs still water for particles to settle. Can be black
red, green or black because of minerals.
Minerals: Quartz, feldspars, micas, pyrite
Siltstone: From silt compacted and bound together. Minerals grow in spaces between
silt grains binding them and forming rock. Recognizable layers. Grains can be felt but not seen
Minerals: Silica, calcite, iron oxides.
Tuffs: Fine grained volcanic ash. Grain size is less than 2mm in diameter. Ash deposits form sedimentary layers.
Basaltic Andesite is part of the Devonian Eastport basalt formation. It is an igneous, volcanic rock with texture ranging from aphanitic to porphyritic. Andesite is characteristic of converging plate margins.
Basaltic andesite consists primarily of plagioclase feldspar combined with pyroxene and hornblende. Common accessory minerals are magnetite, zircon, apatite, ilmenite, biotite and garnet.
Det (Set) – rhyolitic tuff-breccia and coarse bedded tuffs
Rhyolite is a volcanic rock of felsic composition at least 70% SiO2. It is the extrusive equivalent of granite and is a very viscous lava.
d – diabase and gabbro dikes, sills, small plutons from late Silurian and early Devonian
The intrusions probably accompanied basaltic volcanism throughout volcanic deposition in the area. Diabase contacts often have a narrow chill margin followed by a vesicle belt. Sills in the area are small and irregular in shape as the magma spread through bedded rock.
Deb also present