Non-Mineral Resources of the Inland Basin
Figure 7.2: Principal non-mineral resource-producing localities of the Inland Basin. Figure adapted from 1998 United States Geological Survey State Mineral Information.
The Inland Basin has an enormous wealth of non-mineral resources. The thick sequences of sedimentary rocks that dominate the basin are important for providing sandstone, carbonate rocks, shale and cement that are used in buildings and construction. Metamorphic gneiss of the Adirondacks, commercially called granite, is mined in Essex County, NY. The Inland Basin is also rich in fossil fuel resources, including coal, oil and natural gas.
Sedimentary Rock
The sandstone, siltstone and shale of the Inland Basin were formed by the Queenston and Catskill Deltas in the Ordovician and Devonian, and were composed of sediments eroding into the inland ocean from the successive Taconic and Acadian Mountains. As relative sea level rose and fell, different sediments were deposited in a given area. The shale represents deeper, quieter water; siltstone and sandstone represent shallower water and a more energetic environment. During periods when less sediment was being carried into the inland ocean, limestone and dolostone (carbonate rocks) formed.
Sandstone is quarried throughout the Inland Basin region as a dimension stone. The most famous dimension sandstone of the region is bluestone, found in northeastern Pennsylvania and the Catskills of New York. These well-laminated sandstones with distinct horizontal bedding are called "bluestone" because the mineral feldspar lends a bluish-tint to the rock, though a variety of colored sandstones are now commercially sold as bluestone. The bluestone industry dates back to the 1800’s, and quickly grew until bluestone became commonly used throughout the Northeast as flagstones, sidewalks, curbs, building stones and patios. The industry has gradually declined since its peak in the late 1800’s, and now cement has taken the lead economically.
Industrial sand, though once taken from surficial deposits left by the glaciers, is now produced from crushed sandstone or quartzite. Industrial sand, primarily composed of quartz (silica), is distinguished from glacial sand and gravel deposits, which are less uniform in composition. Industrial sand is important for sandblasting, filter sand, making bricks for furnaces and ovens, mixing with clay to make metal castings, and manufacturing glass. Limestone and dolostone, used in agriculture, the chemical industry, and construction, are also important components of cement and concrete. In some areas, limestone is also quarried as a dimension stone for buildings and facings.
Used in the steel and glass industries and for pottery and bricks, clay has also long been an important natural resource of the Inland Basin region. The extremely fine-grained, smooth nature of pure clay, which makes it ideal for these purposes, is a result of its environment of deposition. Clay-sized particles do not settle unless the water is a very low energy environment. Thus, the main sources of clay are glacial lake bottoms and the marine shales of the westward reaches of the Paleozoic inland ocean.
Metamorphic Rocks
Though sold commercially as "granite," Precambrian Grenville gneiss of the Adirondacks is quarried in Essex County, New York for use as dimension stone. The gneiss formed from the metamorphism of Grenville sedimentary rocks, deposited in the Iapetus Ocean.
Fossil Fuels
Fossil fuels include coal, oil and natural gas; the Inland Basin produces all three. These fossil fuels are clearly important to our economy and standard of living, providing the fuel we need for heating, cooling, cooking, driving and operating in everyday life.
The abundance of plant material in swamps, bogs and marshy areas makes these environments ideal for the formation of coal. As sediment is flushed into the swamp by water, plant material is buried. Bacterial decay of large quantities of plant material uses up available oxygen, causing aerobic decay rates to drop. In non-swampy conditions, running water replenishes oxygen to the bacterial community, and plant material rots away. As organic material gets buried more and more deeply, pressure on it builds from overlying sediments, squeezing and compressing the peat. Coal becomes successively more enriched in carbon as water and other components are squeezed out: peat becomes lignite, bituminous and eventually anthracite coal, which contains up to 95% carbon. Found in deformed rocks, anthracite is the cleanest burning of the four types of coal, because it has the highest amount of pure carbon. By the time a peat bed has been turned into a layer of anthracite, the layer is one-tenth its original thickness (Figure 7.3).
Figure 7.3: The formation of coal. Figures by J. Houghton.
In the Inland Basin, only Pennsylvania and Maryland have layers of coal. However, the Inland Basin coal is the northernmost extent of a long expanse of coal that stretches down the Allegheny Plateau the length of the Appalachians. The existence of coal in the region is a result of the inland ocean formed from the Acadian mountain-building event in the Devonian period. The inland sea became increasingly shallow as sediment from the Acadian Mountains filled in the ocean basin and worldwide sea level gradually dropped. Widespread coastal wetlands, river floodplains, and swampy areas were perfect for the accumulation of dead plant material, which was later compressed enough to become bituminous coal. Plants had only just arrived on the scene during the Silurian period. Diversification and evolution of plants was rapid, leading to a proliferation of swamp loving land plants during the Pennsylvanian, when the coals from the Inland Basin formed. During the Pennsylvanian, a tropical climate prevailed because the Northeast was at the equator. Globally, Pennsylvanian-age rocks produce more than 80% of the world’s coal.
Coal cyclically alternates with other sedimentary rocks during the Pennsylvanian. This cyclicity in sedimentation reflects cyclicity in sea level, repeatedly creating and submerging coastal environments appropriate for coal formation. Because the Inland Basin was not severely deformed and compressed, the coals of Maryland and Pennsylvania are bituminous, unlike the anthracite coal found further east in the intensely folded Appalachian/Piedmont region (Figure 7.4). Strip-mining is the primary means employed in the extraction of coal in the Inland Basin coal beds. The overlying layers of rock are stripped away and flat-lying coal layers are mined directly at the surface or outcropping.
Figure 7.4: Bituminous coal fields of the Inland Basin. Figure adapted from Isachsen, Y.W., et al, 2000, Geology of New York, A Simplified Account; Shultz, C., 1999, The Geology of Pennsylvania; and Schmidt, Martin F., Jr., 1993, Maryland's Geology.
Coal, oil and gas are all made of organic matter. The differences in the kinds of organic matter determine which type of fossil fuel is formed. Coal tends to be formed from land plants, accumulating in swampy areas. Oil, on the other hand, is made primarily of phytoplankton, bacteria and plant material from the ocean. Coal remains solid because of the nature of the land plant material, whereas the marine organic material transforms under high heat into oil and natural gas. Natural gas, primarily made of methane, forms either alone or in association with coal and oil, when high temperatures transform solid organic material to a gas.
Figure 7.5: Folds act as traps for oil and gas. Figure by J. Houghton.
Unlike coal, which forms and stays in one place, oil and gas form in one place and then migrate to another. Organic material from marine plants and animals becomes buried under increasing amounts of sediments that squeeze and heat up the organic material over time. The sediments containing the organic material eventually become sedimentary rock, commonly shale. The oil and gas generally do not stay in the rock that originally contained them because they tend to migrate upwards through cracks and permeable rocks to the surface where there is less pressure. If the oil and gas reach the surface, they evaporate into the atmosphere or are broken down chemically. However, if they are somehow trapped below the surface, the oil and gas pool within the rock. An impermeable layer, such as shale, is what halts the oil and gas migration to the surface. To pool the fossil fuels, in addition to an impermeable layer, a trapping mechanism is necessary. Folds or faults in rock layers are common trapping mechanisms (Figure 7.5).
Permeable vs. impermeable rocks
Rocks that are permeable allow fluids and gas (such as water, oil and natural gas) to move through the rock. Fractures within the rock and spaces between the grains of a rock are pathways for fluids and gas. Sandstone, limestone and fractured rocks generally are permeable rocks. Shale, on the other hand, is usually impermeable because the small, flat clay particles that make up the rock are tightly packed into a dense rock with very little space between particles. Poorly sorted sedimentary rocks may also be impermeable because the smaller grains fill in the spaces between the bigger grains, restricting the movement of fluids and gas (Figure 7.6).
Figure 7.6: Sorted and unsorted soil or rock affects porosity and permeability.
The Inland Basin has the combination of features necessary for the formation and trapping of oil and gas. The source of the oil and gas in the Inland Basin is the accumulation of dead plants, animals, phytoplankton and bacteria that were deposited on the floor of the inland ocean and buried by sediments. As the organic material was increasingly more deeply buried, it was squeezed and heated to become oil and gas and subsequently migrated upwards. The Devonian Oriskany Sandstone, a well-sorted sandstone that has excellent permeability and that is overlain by an impermeable layer, has provided a reservoir rock in which oil and gas pooled. The gentle folds of the region, formed during the Paleozoic mountain-building events, are excellent traps for oil and gas. The layers of salt found beneath Devonian rocks were instrumental in the folding of the overlying rock layers. Layers of salt beneath the surface are easily deformed by the weight of overlying rocks. Just as oil and gas try to migrate upward, so do layers of salt. As salt pushes upward, it warps and folds the overlying rocks. The folds provide traps for the migrating oil and gas.
The oil industry got its start in the Inland Basin. In 1859, Colonel Edwin Drake drilled the world’s first commercial oil well in Titusville, Pennsylvania. Though the amount of oil produced in Pennsylvania is small, it is high grade and thus relatively valuable. Though there are several oil fields in southwestern New York, very little oil is produced there (Figure 7.7).
Figure 7.7: Oil and gas fields of the Inland Basin. Figure adapted from Isachsen, Y.W., et al, 2000, Geology of New York, A Simplified Account; Shultz, C., 1999, The Geology of Pennsylvania; and Schmidt, Martin F., Jr., 1993, Maryland's Geology.
The natural gas industry in the United States also got its start in the Inland Basin. In 1821 in Fredonia, New York, William Hart drilled the first natural gas well. The potential of natural gas as a fossil fuel was not recognized, however, until the early 1900’s. In the past, natural gas was released into the air from coal mines and oil wells. Now that gas is a frequently used fossil fuel resource, it is no longer released to the atmosphere. Rock units that were sources of natural gas in the past are now used in many places throughout New York and Pennsylvania as underground gas storage. Gas is pumped into the rock in summer, and removed in winter for use as heating fuel.
