Cyclothems
The coal beds in the Illinois region that contain fossil peat lie in a
large structural depression in the earth's crust that geologists have named the Illinois Basin. The Illinois Basin is an oval area of the crust more than 400 miles long and 200 miles wide that sank very gradually for about a quarter of a billion years. The sinking basin formed a persistent depression in which layers of mud and sand, deposited by seas and rivers, accumulated. The basin area was not a very deep depression at any time during prehistoric eras, its continued sinking during such a long period allowed layer upon layer of water-carried sediment to collect in it.
The Midwest was mainly a low swampy area in which topical forests and fern trees grew. Sediment was carried into the region from deltas to the east. From time to time, the level of the sea fluctuated because of glacial conditions in the Southern Hemisphere. Swamps were flooded, and forests were destroyed. Slowly, layers rich in tree stumps, spores, branches, and leaves were deposited. Later, heat and pressure changed these layers into the coal beds that are so extensive in Illinois. To the west, marine limestones, sandstones, and shales accumulated in shallow seas. Some of the very large islands were formed by the buckling and uplifting of parts of the Earth's crust. Cyclothems are cyclic stratigraphic sequences during the Pennsylvanian period, that formed as a result of marine transgressions and regressions (rise and fall of sea level) related to the creating or melting of ice sheets at the South Pole.
Different sedimentary rocks are often stacked on top of one another in recognizable patterns that suggest an orderliness in formation. In particular, coal-bearing sequences not only show a repetition of coal seams, through sometimes hundreds of meters, but also other rocks in a more or less regular order.The main problem with regard to Pennsylvanian cyclothems is to explain sea transgressions over the delta plain. Marine incursions would take place after sudden sinkings of the plain or rise in sea level. Transgressions of the sea might be due to climatic changes. For example, a periodic melting of polar icecaps or delta out-building might be active during climatic periods of rapid debris supply to the basin, followed by marine transgressions during dry periods with little sediment supply.
large structural depression in the earth's crust that geologists have named the Illinois Basin. The Illinois Basin is an oval area of the crust more than 400 miles long and 200 miles wide that sank very gradually for about a quarter of a billion years. The sinking basin formed a persistent depression in which layers of mud and sand, deposited by seas and rivers, accumulated. The basin area was not a very deep depression at any time during prehistoric eras, its continued sinking during such a long period allowed layer upon layer of water-carried sediment to collect in it.
The Midwest was mainly a low swampy area in which topical forests and fern trees grew. Sediment was carried into the region from deltas to the east. From time to time, the level of the sea fluctuated because of glacial conditions in the Southern Hemisphere. Swamps were flooded, and forests were destroyed. Slowly, layers rich in tree stumps, spores, branches, and leaves were deposited. Later, heat and pressure changed these layers into the coal beds that are so extensive in Illinois. To the west, marine limestones, sandstones, and shales accumulated in shallow seas. Some of the very large islands were formed by the buckling and uplifting of parts of the Earth's crust. Cyclothems are cyclic stratigraphic sequences during the Pennsylvanian period, that formed as a result of marine transgressions and regressions (rise and fall of sea level) related to the creating or melting of ice sheets at the South Pole.
Different sedimentary rocks are often stacked on top of one another in recognizable patterns that suggest an orderliness in formation. In particular, coal-bearing sequences not only show a repetition of coal seams, through sometimes hundreds of meters, but also other rocks in a more or less regular order.The main problem with regard to Pennsylvanian cyclothems is to explain sea transgressions over the delta plain. Marine incursions would take place after sudden sinkings of the plain or rise in sea level. Transgressions of the sea might be due to climatic changes. For example, a periodic melting of polar icecaps or delta out-building might be active during climatic periods of rapid debris supply to the basin, followed by marine transgressions during dry periods with little sediment supply.