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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 Sedimentary rock with Fossils Where we’ll be… Eastport Formation 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 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
