Carboniferous Lowlands
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PAG E 0 •• • •••••••• 0 • 0 •••• 0 • 0 • •• • • • •• • •• 0 •• • • 0 • • • 0 ••• • • ••• • ••••• • •••• 93 Carboniferous Lowlands D 51o I IIIlii 520 • 530 § 540 llllll 550 • 560 § 570 0 580 • 590 Figure 15: Region 500, Carboniferous Lowlands, and ils component Dislricls. Theme Regions: Natural History o f Nova Scotia, Volume II PAGE . 94 500 CARBONIFEROUS LOWLANDS ~ The Region has been divided into nine Districts on stones which contained abundant fish and plant re- the basis of topographic character and soil type: mains. Towards the end of this period in the Early 510 Till Plain Carboniferous a marine incursion took place; the ba 520 Coastal Plain sins became enlarged and interconnected, and an 530 Stony and Wet Plain inland sea formed with a shoreline on mainland 540 Clay Plain Nova Scotia which closely approximates the present 550 Coastal Fringe boundary of the Carboniferous deposits. Within this 560 Submerged Lowland sea was an archipelago of islands including the 570 Rolling Upland . Wittenburg Ridge, Mount Aspotogan, the Pictou 580 Hills and Valleys Antigonish Highlands, the elongated blocks of the 590 Dissected Plateau Avalon Zone in Cape Breton, and the Cape Breton highlands. REGIONAL CHARACTERISTICS In the marine basin a laminated limestone form 500 Carboniferous ed, followed by deposits of evaporites (mostly gyp Lowlands Lowland Nova Scotia is mainly Carboniferous sedi sum) and red and green shales. Near the islands, I mentary rocks on which deep soils have developed. shelly reefs developed, for example, at Gays River, Many of the rivers flow in valleys eroded from the Aspotogan, and East River (Mahone Bay). In the ini unresistant Wmdsor Group rocks, which include gyp tial incursion of seawater, the intermontane basins sum. Horton Group sandstones are more resistant and much of the surrounding area was flooded. The and form higher ground in District 510 in Cape limestone deposited at that time lies on the Horton Breton, and on the eastern mainland. Districts 580 deposits and, where formerly dry land was covered, and 590 abut the older upland blocks as foothills, directly on the underlying topography. As the basin while the less-resistant rocks of Districts 520, 550, enlarged, later deposits of Windsor rocks were laid and 560 form coastal plains, often with heavy soils. directly onto the older rocks of the basin margin, as Flat-lying sandstones and shales are poorly drained can be seen in Mahone Bay, St. Margarets Bay, and with numerous bogs (Districts 530 and 540), while on the eastern side of the Cape Breton highlands. the relatively infertile sandy soils derived from the Windsor deposits attain a thickness of 450 m in the resistant sandstones of District 570 cover a large area Shubenacadie area and up to 750 m in Cape Breton. of central Nova Scotia. At the end of the Windsor period, the land rose slightly and the sea withdrew to the east. Red and GEOLOGY· purple siltstones, shales, and sandstones (Canso Group) were deposited in streams and lakes. The The geological character of the area is best portrayed strata may once have been very extensive across through a description of the paleo-environment in Nova Scotia but are now found in only a few major which the sediments accumulated and the processes areas: around St. Georges Bay, the Strait of Canso, which affected them after deposition. Stellarton-Mount Thorn, and the Stewiacke Valley. The first deposits were coarse sands and grits They give red or purplish soils and, when exposed on (Horton Group) washed from the uplifted and folded the coast(e.g., Janvrin Island), bright red-coloured Meguma Zone and the highland blocks of the Avalon cliffs. This period was relatively quiet geologically; Zone (Pictou-Antigonish and Cape Breton) into ba there were no tectonic upheavals, and the topogra sins between mountains. In some areas these depos phy was subdued. The Early Carboniferous deposits its were preceded by extrusions of lava as crustal ad were by this time very thick and had buried the lower justments continued to take place. The first basins elevations of the terrain. Rivers developed wide val were in the Pictou and Mabou-Antigonish areas but, leys with broad floodplains and estuaries. Sediments as sediments accumulated, they became increasingly deposited in the environment had been compacted extensive. The early Horton deposits have no fossils to fine sandstones, black shales, and thin coal seams and now form resistant bands shouldering the up (Riversdale Group). These strata also may once have lands. Later came deposits of silts and fine sand- been very extensive, but deep erosion has confined Natural History of Nova Scotia, Volume II: Theme Regions PAGE 95 existing deposits to a few areas: a fringe around the • sistant Windsor Group deposits ·which have been Cobequid Hills, Pomquet, Port Hood, Broad Cove to deeply eroded in some places to well below present Margaree Harbour, and Port Hawkesbury to St. Peters. sea level. The rolling plains are underlain predomi About this time, conditions became suitable for nantly by Late Carboniferous sandstones of the the proliferation of fern-like swamp plants, which Cumberland, Pictou, and Morien groups. They cover flourished in extensive wetlands across the flat land northern Antigonish, Pictou, Colchester, and Cum scape. Great thicknesses of organic deposits accumu berland counties (Units 521 and 532) and the area lated and were compressed (Cumberland, Pictou, between Mira Bay and Great Bras d'Or Channel (Unit and Morien groups). These Late Carboniferous 531). The strata are fairly resistant and moderately to strata, which are predominantly thick sandstones strongly folded. In the Cumberland/Pictou area, and shales, cover almost the entire area north of the younger and more easily eroded strata are exposed in Cobequids and about half of Cape Breton County. the cores of the anticlines; consequently, the land The major coalfields of Springhill, New Glasgow scape has developed a ridge and valley topography Stellarton, and the Sydney area are all in this group. with moderate relief. Carboniferous strata are relatively unmetamor The upland areas are underlain by resistant phosed because, on a geological scale, they were Horton deposits. The rolling upland which lies be never deeply buried. However, they were affected tween the Chedabucto and St. Marys faults has been by compressive crustal movements during the affected by folding and faulting to quite a degree, yet Permian and Jurassic periods. The most intensely it remains a rather featureless elevated area across 500 affected area was a band across the northern part the central mainland (District 570). This suggests that Carboniferous of the province from Chignecto Bay, through the the topography reflects the more uniform resistance Lowlands Pictou coalfield, and up through the Aspy Valley in of the different rocks, and therefore a similar re I northern Cape Breton. Resistant strata such as the sponse to the forces of erosion. Late Carboniferous sandstones and the Horton The Cape Breton uplands are underlain by Horton grits were thrown into open folds, whereas the strata, interfolded with Windsor deposits. A large area softer strata in the zone, such as shales and gyp in western Cape Breton exhibits strong relief and is sum, were distorted and crushed. There was also varied and interesting. Windsor Group limestones movement along the Cobequid-Chedabucto and and gypsum have been eroded to form the deep val Aspy faults and related movement along associ leys occupied by the Bad deck, Middle, and Margaree ated parallel and crosscutting faults. rivers. Coastal fringe deposits skirt the Cape Breton Carboniferous deposits accumulated to a depth of highlands and form narrow coastal plains at the base many thousands of metres and engulfed most fea of steep cliffs (District 550). Resistant Horton sand tures of the old landscape. The Early Permian erosion stones form an upland plateau surface which these has removed hundreds of metres of Late and Middle valleys dissect. Carboniferous strata, exposed the underlying Wind The intervening hilly areas of the Carboniferous sor and Horton group blocks, and revealed ridges Lowlands have a topographic character transitional and valleys of the old erosion surface upon which between lowland, plain, and upland. Faults transect these beds were deposited. A wide range of different the basins and often set strata of different resistance rock types, with different responses to erosive forces, against each other. Downfaulted younger strata are is now exposed. The topography reflects both this set against older strata and are thereby preserved, for differential resistance, the structural character (folds example, the Pictou Valleys (Unit 582). and faults) of the area, and the relationship between the Carboniferous strata and the underlying rocks. Karst Topography Throughout the Carboniferous Lowlands, in areas LANDSCAPE DEVELOPMENT underlain by Windsor Group strata, pockets of gyp sum and anhydrite (dehydrated gypsum) occur at The geomorphology within the Carboniferous Low the surface and produce a special feature called karst lands is variable. There are lowland plains, rolling topography. uplands, and coastal fringe areas. The lowlands fall When anhydrite comes in contact with water, it into three main areas: the Windsor Lowlands (Unit expands and changes to gypsum. Gypsum is readily 511), from the Avon River to the Stewiacke Valley; the dissolved by rainwater and, when it lies above the Coastal Plain of the North Shore (District 520); and permanent water table, it crumbles and washes the Bras d'Or Lowlands (District 560) of central Cape away, leaving behind a small amount of reddish clay. Breton. These three areas are dominated by unre- Joints in the gypsum beds are enlarged and often Theme Regions: Natural History of Nova Scotia, Volume II PAGE ........................................................ 96.. subterranean channels are formed. Streams flow filtration rates are relatively high in the porous soils through these channels, sometimes enlarging them of this Region and, consequently, groundwater re into caverns.