Explosive eruptions from volcanoes are recorded in the stratigraphic record throughout the Phanerozoic, but evidence of these eruptions in the form of preserved tephra layers appears to be concentrated at times of active plate collision and concomitant high stands of sea level. The products of volcanic eruptions are lavas, tephra, and gases. Basaltic magmas (low-silica content) are usually erupted in the form of lava flows, whereas rhyolitic magmas (high-silica content) are commonly explosively erupted as plinian and ultraplinian plumes, and associated pyroclastic flows. Fallout tephras are preserved in ancient sedimentary sequences as tonsteins, bentonites, and K-bentonites. Middle Ordovician K-bentonites represent some of the largest known fallout ash deposits in the Phanerozoic Era. They cover minimally 2.2 × 10 6 km 2 in eastern North America and 6.9 × 10 5 km 2 in central and northwestern Europe as a result of explosive volcanism, which affected both Laurentia and Baltica during the closure of the Iapetus Ocean. The three most widespread beds are the Deicke and Millbrig K-bentonites in North America and the Kinnekulle K-bentonite in northwestern Europe. Similar successions are well known in South America and China. Sedimentation rates of volcanic ejecta range from meters per year locally to ~1 mm/1000 yr in the deep sea. Volcanogenic sediments react with seawater to produce secondary phases such as zeolites and clay minerals. Studies of recent ashfall behavior suggest that the preservation potential in the stratigraphic record can be viewed as somewhat remarkable in that such sudden events are preserved at all, much less produce such a wealth of valuable geologic information.

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Abstract Interior Cratonic Basins, a product for the World Petroleum Basins series of the American Association of Petroleum Geologists (AAPG), was approved in 1984 and initiated in early 1985. 1 Contributors undertook to provide useful geologic information on the regional setting, stratigraphy, structure, tectonics and basin evolution, and oil and gas systems of seven cratonic basins. A detailed overview of the Illinois basin, selected by the AAPG ad hoc committee as a representative type (see Foreword ), is followed by less detailed reviews of six other selected interior cratonic basins: the Williston, Michigan, Baltic, Paris, Parana, and Carpentaria basins. The goal is to develop a better understanding of the basin-forming, basin-filling, and basin-modifying processes that control hydrocarbon plays and resultant oil and gas fields in this class of basins. The idea is to describe and document the variations, opportunities, and exploration problems that can be expected. We selected seven basins, productive and nonproductive, from four continents (Figure 1): five basins formed on Precambrian crust (Illinois, Michigan, Williston, Baltic, and Parana basins); one formed on accreted Paleozoic crust (Paris basin); and one formed on Paleozoic and Proterozoic volcanics and sediments and Proterozoic metamorphic rocks (Carpentaria basin). Some are rift related; some are not. Information from other interior cratonic basins balances the coverage. At the conclusion of the introduction, a selection of paleogeographic maps is presented for later reference throughout the volume on the time, place, and setting of the seven basins. We conclude the volume with a section on interiorcratonic basins and their place in

Abstract The Illinois basin is an oval depression that covers approximately 60,000 mi 2 (155,000 km 2 ) in parts of Illinois, Indiana, and Kentucky. The basin contains about 100,000 mi 3 (450,000 km 3 ) of Cambrian through Pennsylvanian sedimentary rocks. To include most petroleum-bearing strata, the boundary of the basin is demarcated by the —500 ft contour on top of the Ottawa Supergroup (Upper Ordovician). In northern and western Illinois, the boundary has been arbitrarily extended to coincide with the area covered by sediments of Pennsylvanian age (Figure 1-1). County names in the Illinois basin area are shown in Figure 1-2. Much of the bedrock throughout the basin is of Pennsylvanian age (Figure 1-3). Outcrops of Mississippian rocks surround the Pennsylvanian except in the northern part of the basin where they were completely removed by pre-Pennsylvanian erosion (Harris, 1979). The Illinois basin is separated from the Forest City basin to the west by the Mississippi River arch and the Ozark dome and from the Appalachian basin to the east by the Cincinnati arch (Figure 1-4). To the northeast, the basin is separated from the Michigan basin by the Kankakee arch and, to the south, from the Black Warrior and Arkoma basins by the Pascola arch. Older Paleozoic rocks are exposed along the axes of the surrounding positive structures. The New Madrid rift complex (Braile et al., 1982a, 1982b) underlies the southern part of the Illinois basin and, by our interpretation, consists of the Reelfoot rift (Ervin and McGinnis, 1975) and Rough Creek graben (Soderberg and Keller, 1981).

Abstract The shifting base level on the North American continent during the past 570 m.y. has resulted in the deposition of major rock stratigraphic units that are interregional in extent and separated by interregional unconformities. The major divisions of these stratigraphic units are the sequences of Sloss (1963), with further subdivisions (Sloss, 1982) derived from Vail's coastal onlap chart (Vail et al., 1977). Each sequence represents a major transgressive-regressive cycle of deposition that began at the cratonic margins and in the subsiding basins then gradually spread to the more stable cratonic interior. These major cycles are commonly complicated by minor interregional cycles and by a host of local effects (Sloss, 1963). The loss of sedimentary record because of erosion and/or nondeposition is greatest in the areas of uplift and least in the subsiding basins. Parts of the six primary sequences that span the entire Phanerozoic are represented in the Illinois basin. From oldest to youngest, these sequences include the Sauk, the Tippecanoe, the Kaskaskia, the Absaroka, the Zuni, and the Tejas (Figure 1-14). The active period of basin formation occurred during the 300 m.y. history of the Paleozoic Era. The Sauk, Tippecanoe, Kaskaskia, and Absaroka sequences were deposited during this time. By the end of Paleozoic time, subsidence within the basin had ceased. During Late Cretaceous and early Tertiary time, reactivation of the New Madrid rift complex, particularly the Reelfoot Rift, led to deposition of the Zuni and Tejas sequences at the southernmost part of the basin and at points farther