Morning (written by Brittany Pegg)
To kick off the field course, we explored outcrops in the Twillingate islands. These outcrops represent oceanic arc and back-arc igneous activity during the early Ordovician. Tectonically, they are part of the Notre Dame subzone of the Dunnage Zone, which is separated from the Exploits subzone by the Red Indian Line, a major structure related to the closing of the lower Paleozoic Iapetus Ocean.
The first stop visited was a hike up to the northernmost edge of Twillingate, at Sleepy Cove. Here, exposures of metamorphosed Cambrian mafic pillow lavas and bimodal volcanics from the Sleepy Cove group were seen along the edges of the cliffs. The majority of the rocks were metamorphosed to greenschist/amphibolite facies with mylonitic fabrics observed in some places. The pillows are amygdaloidal and infilled with calcite and chlorite. They are sheared and offset along abundant NE-trending faults. Rocks adjacent to the faults are infilled with carbonate, with minor pyrite mineralization.
The second stop visited was Jenkins Cove, located just southeast of Sleepy Cove. The Twillingate pluton that crops out here is a trondhjemite and tonalite intrusion. The pluton intrudes the pillows seen in Stop 1 and is the lithology that is the most exposed across the Twillingate islands.
The final stop visited in the morning was the Chanceport Fault, which is exposed just before the causeway leading to the Twillingate Islands. This fault is a 2 km-long thrust fault striking NE/SW. To the north of the fault is the Twillingate trondhjemite whereas the mudstone, graywacke, and pillow lavas with the red cherts of the overlying Chanceport Group are located south of the fault. The southernmost unit of the Chanceport Group is the only unit exposed at this locality; indicated by the presence of numerous pillow lavas as well as small exposures of jasper. Unlike the Twillingate pluton, the unit is well-preserved and unmetamorphosed, as well as being very structurally complex.
Afternoon (written by Jessie Kehew)
In a busy afternoon on Day 1, we saw various pre-, syn- and post-accretion lithologies related to the closure of the Iapetus Ocean. The purpose of these stops was to see the wide variety of lithologies that are involved in ocean closure. We first visited the pre-accretion Moretons Harbour island arc sequence. The Moreton’s Harbour Group (late Cambrian to mid Ordovician) is located north of the Chanceport Fault and is a sequence of volcanic and volcaniclastic rocks.
It hosts mineralized veins that were explored and mined for Arsenic in the late 19th – early 20th centuries. These are epithermal deposits, associated with felsic sub-volcanic intrusions. Our first stop was the Stewart’s Mine, part of the Little Harbour Formation, which hosts all the mineralization in the Moretons Harbour Group. We found the mine’s ore dump, which was rich in pyrite and arsenopyrite. The next stop was the Hayward’s Cove Formation, the youngest formation in the Moretons Harbour Group, where we saw the extrusive equivalent of the felsic dykes associated with the mineralization. These felsic extrusive rocks are predominantly massive quartz-feldspar crystal tuffs, but we also observed bedded tuffs that appeared to be rafted or slumped units.
We then drove south over the Chanceport Fault to view lithologies recording the syn-accretion history. The first stop was the Boyd’s Cove Dunnage Mélange, an early to middle Ordovician olistostrome that indicates tectonic instability interpreted to be associated with the Taconian Orogeny. This unit is a polymictic mélange, with clasts and blocks ranging in size from cobbles to 1 km-wide, encased in an argillite matrix. We viewed small clasts and rafted units on the ground to the northeast and southwest of the Reach Run Causeway. Next, we stopped at the Loon Bay Granite, the granitic portion of the Devonian Loon Bay Batholith. This unit is post-tectonic, which can help to provide a minimum age of deformation for the surrounding rocks. Next, we visited an outcrop of the Caradocian chert and shale, a late Ordovician unit associated with a period of quiescence following the cessation of island arc volcanism. At the roadside outcrop, we saw argillaceous cherts and intruding dykes, as well as stratiform pyrrhotite, pyrite and magnetite – exhalative sulfides that are present throughout the entire section. The final two stops record a flysch sequence of the late Ordovician to early Silurian, deposited during the Taconian Orogeny. The Samson Greywacke is at the base of the sequence, consisting of sandy turbidites, and up stratigraphy, interbeds of conglomerate increase in frequency, transitioning into the Goldson Conglomerate. The Goldson Conglomerate is a polymictic conglomerate with volcanic, plutonic and sedimentary clasts.