The Arches is located within the Humber Zone which is the most western of the 4 zones and is next to the Dunnage Zone.
Techtonostratigrapic Zones (East to West)
1. Avalon Zone
2. Gander Zone
3. Dunnage Zone
4. Humber Zone
All the rocks within the park are a part of the Humber Arm Supergroup which includes the Curling Group, Northern Head Group and the Cow Head Group. A southeasterly high angle fault is also located in The Arches Provincial Park.
The nearby Long Range Mountains are comprised of crystalline basement rock and are much older than the surrounding lowlands, the area is of Appalachian orogen. The mountains are considered a massif but are comprised of several inliers where older rocks are surrounded by younger ones and were reactivated during Appalachain deformation.
The rocks associated with the inlier are a part of the Humber Zone and are considered the foreland belt of Appalachian orogen. The highest point of the mountains is bounded in the south, west, north and locally east by proterozoic/paleozoic volcanic sedimentary cover rocks. The southern portion of the Long Range inlier is split into several different tectonic divisions:
. High-Grade Long Range Gneiss Complex
. Foliated Plutonic Rocks, Mostly Grenvillian in age
. Mafic Dikes (Long Range Dike Swarm)
. Thin remnants of Neoproterozoic to Early Paleozoic cover sequence
. Early Silurian gabbroic intrusions (430 Ma Taylor Brook gabbro)
. Minor felsic dikes, sills, and porphyries
. 541/250 million-year-old cover sequence quartzite-marble-dolomite
Overthrusting started in the Middle Ordovician and was possibly accentuated in the Devonian period, the thrusting came to an end during Appalachain deformation events. Compression and folding are greatest to the west of the overthrust and lies at the base of the Long Range Mountains but dies out further towards the western and eastern sides of the range.
The area is influenced by the drainage of rivers from the Long Range Mountains, streams pass through several deep glacial lakes at the base of the mountain front. Those Rivers, Portland creek and Parson's Pond reach the Gulf Of St. Lawrence and take eroded sediment to the bay.
The surrounding area around The Arches but not visible in the park is the Lower Head Formation, it is placed within the Middle Ordovician and is comprised of greywacke which is dark-coarse grained quartz and contains more than 15% clay interbedded with grey/green conglomerate and shale. The sandstone in the Lower Head Formation is medium-to thick-bedded with a tectonic contact above the formation.
The formation was rapidly deposited during syn-depositional deformation, the erosion of the underlying section of the Cow Head Group which created a small unconformity. The Lower Head was deposited by massive slope failures and considered turbidite deposition, this could have occurred as sediments on the continental shelf slide to the ocean floor.
SHALLOW BAY FORMATION
The rock formation that makes up The Arches is a part of the Cow Head Group and is a mix of three different formations, the younger Lower Head Formation, Shallow Bay Formation and the Green Point Formation. The lower portion of the Shallow Bay and Green Point Formations are located in the Furongain Period while the upper zone is placed within the Lower Ordovician.
The group is a succession of conglomerate, shale, limestone and sandstone. The limestone was deposited in shallow water while the intervening conglomerate layers were placed in deeper waters along with mudstone.
The Arches is comprised of conglomerate, limestone, mudstones and shales which have formed in the lower continental slope of Laurentia (The North American Craton which is a stable interior of a continent comprised of crystalline basement rock). The limestone platforms are formed from the death and decay of living organisms as their skeletal remains build up a reef.
Limestone broke from the carbonate platform at the rim of the continental margin and tumbled down the slope where it stayed which is why house-sized blocks of limestone are sometimes visible in the formation.
The Shallow Bay Formation is comprised of 4 different members:
. Downes Point Member:
Consists of a series of conglomerates 100 meters thick that range in age from Middle/Late Cambrian.
. Tuckers Cove Member:
60 Meters of interbedded quartzite calcarenite conglomerate and minor ribbon limestone (abrupt lateral changes in rock types), sandstone, siltstone and shale which are Middle/Late Cambrian in age.
. Stearing Island Member:
Series of mega-conglomerate 80 meters thick with minor interbedded limestone, quartzose calcarenite and shale which was deposited in the Upper Cambrian and Lower Ordovician or Upper Tremadoc.
. Factory Cove Member:
The member consists of a 100-meter thick sequence of ribbon/parted limestone with minor shale punctuated by beds or boulder conglomerate/mega-conglomerate and was deposited in the Late Arenig (Lower Ordovician Epoch 1.6 million years old).
The Arches formed from the exploitation of joints and fractures within the limestone of the Shallow Bay Formation, certain areas are eroded more than others which is where the arches form. Once the supports have been eroded away the arch than collapses on itself as the supports cant hold the weight of the above rock.
Once a window has opened within the rock formation it expands and eventually forms an arch, to be classed as an arch the opening must be three feet in one direction. Gravity forces layers of loose rock to fall from the ceiling and collapse, then wave action moves the sediment away along with weathering the base of the rocks.
COASTAL & GLACIAL EROSION
There was once an ancestral mountain range that was worn down by river, wave and wind action, the mountain range broke down further during the last 1-2 million years as several glacial episodes eroded the area more.
Those glaciers melted around 8,000 years ago and the sea level rose and covered the area, the wave action and seawater fractured the limestone/shale and exploited the rock. Sea Level then dropped to current and erosion by waves continue.
In the future, The Arches will be reduced to sea stacks or rock pillars this will take several thousands of years to happen but it is in the park's future.
The Arches, Newfoundland and Labrador, CAN
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Svend Stouge, Gabriella Bagnoli and Duncan McIlroy With contributions from J. Maletz, W. Douglas Boyce, Ian Knight and Sebastian Scorrer and D. McIlroy
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Arches National Park, Utah. National Park Service, USA
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