NORTH AMERICA/Atlantic Margin 105 shifted sedimentary depocentres to deeper water on the rise and abyssal plain Many new canyons also developed Energy, Mineral, and Water Resources The knowledge base about energy resources on the North American continental margin varies by country: drilling off the US East Coast in the late 1970s and early 1980s produced disappointing shows for both oil and gas in the Baltimore Canyon trough and Georges Bank basin Neither the Carolina trough nor the Blake Plateau basin have been drilled or tested for petroleum maturation, although models suggest that maturation in the Carolina trough should have reached the oil window only in its deepest Early and Middle Jurassic rocks The petroleum potential in the rift basins buried beneath the margin remains unknown Currently the USA has a moratorium on leasing and drilling on its offshore Atlantic margin In contrast, Canada has had an active exploration and production programme on the Scotian Shelf and Grand Banks since the late 1970s The Mesozoic Jeanne d’Arc rift basin contains the most significant discoveries on the Grand Banks (Hibernia, Terra Nova, Hebron-Ben Nevis, and White Rose fields) Both oil and gas have also been produced from Upper Jurassic and Lower Cretaceous sandstones on the Scotian Shelf in the vicinity of Sable Island In the 25 years since exploration began, a robust offshore technology and infrastructure has grown to support the Canadian petroleum industry One of the best-known deposits of an unconventional hydrocarbon, gas hydrate, exists on the Blake Ridge off the coast of the south-eastern USA (Figure 13) Gas hydrate is an ice-like solid that holds high concentrations of methane in a crystalline water matrix, which forms under pressures, temperatures, and geochemical conditions that are often found in sediments near the seafloor deeper than about 500 m water depth (see Petroleum Geology: Gas Hydrates) The first dedicated gas-hydrate drilling leg of the Ocean Drilling Program occurred on the Blake Ridge in 1995 (Leg 164) Estimates of the total volume of methane in both hydrate and associated free-gas deposits on the Blake Ridge vary from 60 to 100 trillion cubic metres Occurrences of offshore minerals are known along the North American continental margin, but they have not been as extensively developed or exploited as their onshore equivalents, primarily because recovery and transportation are more expensive in an offshore environment Phosphates, used in agricultural fertilizers, are successfully mined in coastal-plain (i.e postrift) deposits in Florida and North Carolina, but phosphate deposits on the continental shelf between Georgia and North Carolina are largely unexploited Manganese nodules and ferromanganese crusts, potential sources of manganese, nickel, cobalt, copper, and platinum, have been sampled on the Newfoundland and New England Seamounts, but their distributions are largely unmapped More detailed studies of similar nodules and crusts have been carried out on samples from the Blake Plateau Placer deposits also exist offshore, the most famous being gold-bearing sands and alluvium along the Nova Scotian inner shelf, which are presumed to have a source in pre-Mesozoic basement rocks Mining for these intriguing finds has been largely uneconomical Placer deposits containing titanium and iron have been mapped off Florida and are inferred to exist on other parts of the margin The most plentiful mineral deposits are quartz sands, which are dredged mainly from the inner shelf for beach nourishment (for example, off the coasts of most of the states between New Jersey and Florida) Other resources that have been identified but not developed offshore include high-purity silica sand in the Gulf of St Lawrence and on the Grand Banks (which could be used for glass making), calcium carbonate sand (which is a key ingredient of Portland cement), clay deposits (which supply material for bricks, sewer pipes, and other construction materials), and coarse sand and gravel (which could be used in road building and maintenance) Quaternary glaciation (see Sedimentary Processes: Glaciers) and modern depositional processes have controlled much of the distribution of unconsolidated sands and gravels on the margin from New England to Newfoundland One offshore resource that has received little attention is fresh groundwater within sediments of the continental margin Freshwater springs are known along the Florida shelf One of the most spectacular is the Crescent Beach spring, about km off the coast of north-east Florida, in about 20 m water depth This spring produces a boil with a hydrogen sulphide odour on the sea surface during times of high discharge Within the fractured and porous limestone (see Sedimentary Rocks: Limestones) aquifer system of the south-eastern USA, this offshore fresh groundwater is derived from rainfall on land, and its pressure is considered to be in equilibrium with sea-level In contrast, shallow wells drilled on the continental shelf off New Jersey show a thin wedge of almost-fresh water that extends more than 100 km across the margin at relatively shallow depths (less than 100 m beneath the seafloor, Figure 14) This fossil freshwater accumulated during the Pleistocene falls in sea-level and has been protected from saltwater contamination