Abstract Salt (halite, NaCl) is the most soluble of common rocks; it is dissolved readily and forms a range of subsidence or collapse features as a result of human activities. Bedded or domal salt deposits are present in 25 of the 48 contiguous United States and underlie nearly 20% of the land area. These salts occur in 17 separate structural basins or geographic districts in the United States, and either local or extensive examples of natural or man-made salt karst are known in almost all of these basins or districts. Human activities have contributed to the development of salt karst. Boreholes or underground mines may enable (either intentionally or inadvertently) unsaturated water to flow through or against the salt deposits, thus allowing development of small to large dissolution cavities. If the dissolution cavity is large enough and shallow enough, successive roof failures can cause land subsidence or catastrophic collapse. Because salt dissolution proceeds rapidly, human-induced karst features often develop quickly and with dramatically adverse impacts. Industries associated with local salt-dissolution and collapse features include solution mining (e.g., Cargill sink, Kansas; and Grand Saline sink, Texas), petroleum activities (e.g., the Wink sinks, Texas; Panning sink, Kansas; and Gorham oil field, Kansas), and underground, dry mining of salt (e.g., Jefferson Island mine, Louisiana; and Retsof mine, New York).

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Abstract Roman Nose State Park comprises 540 acres (219 hectares) of public lands where layers of Permian gypsum, dolomite, and shale are well exposed and where fresh water flows from three natural springs. The site is in the central part of Blaine County, northwestern Oklahoma (Fig. 1), located about 6 mi (10 km) north of Watonga, in Secs. 23 and 24, T.17N., R.12W. The park allows free access at all times and provides picnic and camping facilities; the park also contains a resort lodge, meals, and cottages. For information and access to springs and other areas partially closed during winter months (October 1 till early April), contact the Park Superintendent, Roman Nose State Park, Route 1, Box 2-2, Watonga, OK 73772 (telephone: 405/623-4215). The area is on the Watonga Lake 7½-minute Quadrangle.

Abstract The Great Salt Plains comprise about 25 mi 2 (64 km 2 ) of flat, barren land, naturally encrusted with salt (halite), along the Salt Fork of the Arkansas River (Fig. 1). The salt plains, which also embrace the area where unique hourglass selenite crystals are currently growing, are almost entirely enclosed within the Great Salt Plains National Wildlife Refuge. Located in central Alfalfa County, Oklahoma, the land is about 4 mi (6.4 km) east of Cherokee and just northwest of Jet. Free access by car or bus is permitted at all times during the year to the salt flats in the vicinity of the observation tower on the west (site 1, Fig. 1) and to the dam and state park area in the east (including picnic and camping facilities and cabins). Also, free access by car or bus to the selenite crystal area (site 2, Fig. 1) is permitted through only one gate, which is unlocked from 8 A.M. until 5 P.M. only on Saturdays, Sundays, and holidays from April 1 to October 15 each year. The crystal area is about 1 mi (1.6 km) northeast of this gate and can be reached safely by driving along the specified road across the salt flats. Access to other parts of the Great Salt Plains is restricted by fences; quicksand, at scattered locations on the salt flats, makes trespass in nonauthorized areas hazardous. The area is covered by four 7½-minute quadrangle maps: Cherokee S, Cherokee N, Jet, and Manchester SW. For information concerning the

Abstract The Southern Midcontinent is a complex region characterized by great thicknesses of sediments preserved in a series of major depositional and structural basins separated by orogenic uplifts created mainly during Pennsylvanian time (Plate 5-A).Sedimentary rocks of every geologic system from Precambrian through the Quaternary are preserved within the region, and their diverse lithologies include limestones, dolomites, sandstones, shales, conglomerates, red beds, and evaporites. The strata are a mixture of marine and nonmarine deposits, and generally (except for the red bed-evaporite sequences), they are richly fossiliferous and are well suited to biostratigraphic correlation and interpretation of depositional environments. Dominant lithologies in most basins of the region are, in ascending order, as follows: a thin transgressive sandstone of Late Cambrian age that covered the basement-rock complex of intrusives, extrusives, and metasediments; overlain by a thick sequence of Late Cambrian through Late Mississippian (Meramecian) carbonates, with minor amounts of sandstone and shale; followed by a thick sequence of terrigenous clastics, with some carbonates, deposited from Late Mississippian (Chesterian) through Early Permian (Wolfcampian) time; then a thick series of red beds and evaporites were deposited during the remainder of the Permian; overlain, in the west only, by Triassic and Jurassic terrestrial red beds; then Cretaceous marine deposits in the south and west; and finally a mantle of Tertiary alluvial-fan, aeolian, and lacustrine sediments in the west. Owing to the great thickness of strata in most basins of the region, and the many surface and subsurface stratigraphie studies that have been conducted, a plethora of

The crustal structure and tectonic framework of the central Peru Margin, between lat 7° and 10°S were interpreted using mainly a 102 km long, multichannel seismic section and existing geologic and geophysical data in the region. Thrust faulting occurs in upper layer 2 basalts as the Nazca plate descends beneath the continental slope. This produces basaltic ridges (slabs) within the trench axis and at least 26 km landward beneath the overriding continental plate. Broken low-frequency reflectors within this diffracting subduction complex suggest that ophiolitic slivers of basalt are being incorporated into portions of it, forming a sediment-basalt melange. Three prominent forearc basins, Salaverry, Trujillo, and Yaquina Basins, occupy the central margin from east to west, respectively. Drill holes penetrated Tertiary sediments on the outer shelf and the nearby eastern flank of the Trujillo Basin and bottomed in a metamorphic arc massif. The massif is correlated with seismic refraction velocities greater than 5.7 km/sec and a density of 2.72 to 2.80 g/cm 3 which underlie the continental shelf. Our interpretation of the seaward limit of the massif is uncertain and depends upon the geophysical cirteria used. Each of three forearc models position the arc massif at about 26, 61, or 115 km landward of the trench, with the subduction complex occupying the region between the trench and the massif. The massif-subduction complex interface should be located through drilling to test the proposed models. The intramassif basins, Salaverry and Trujillo, have subsided during Tertiary time to allow the accumulation of 2 to 4 km of marine sediment. The Trujullo Basin apparently has not experienced much vertical movement since the late Miocene, based upon microfossil paleodepth indicators found in dolomicrites and glauconitic micrites dredged from the basin. However, abundant brecciated dolomicrites in the same dredges and disturbed strata in reflection records from both the Yaquina and Trujillo Basins suggest deformation of the basins during the Pliocene-Pleistocene by a compressional regime.