Transcript Geology of the Roque Island Archipelago

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
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Late Silurian, Early Devonian
Double subduction zone
Piroclastic flows
Tuffs and ash deposits
Sedimentary rock with Fossils
Where we’ll be…
Eastport Formation
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Part of Coastal Volcanic Belt
Stretches from Massachusetts to New
Brunswick Canada
Represents portion of Avalonia that accreted to
Laurentia during Acadian Orogeny
Originally dated as Early Devonian
Lisa Churchill-Dickson in 2004 published paper
dating it as Silurian from U-Pb Zircon dating
and different fossils.
Little Spruce Island
Siltstone
Shale
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.
Little Spruce Island Fossils
Pelecypods
Ostracodes
Gastropods
Geology of Little Spruce
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Deb – Basalt and basaltic andesite flows intruding
shallowly into muds and tuffs.
Part of the Eastport Formation, these flows overlie
lowermost flows and pyroclastic rocks, and some have
red weathered flow tops that may be overlain by
mudstone locally. Near Buckman Head this lava
intruded into soft mud to create spectacular bulbous
formations ten feet in diameter that are surrounded by
deformed shale bearing lava chips.
Geology of Halifax
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.
Minerology:
Basaltic andesite consists primarily of plagioclase feldspar combined
with pyroxene and hornblende. Common accessory minerals are
magnetite, zircon, apatite, ilmenite, biotite and garnet.
Geology of Hickey
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