Search USGS⌕ Search

Geology topics

J. Wright Horton

Publications and source records attributed to J. Wright Horton.

51 records · Page 3Linked to original sources
USGS2001

Preliminary lithogeochemical map showing near-surface rock types in the Chesapeake Bay watershed, Virginia and Maryland

This preliminary experimental lithogeochemical map shows the distribution of rock types in the Virginia and Maryland parts of the Chesapeake Bay watershed. The map was produced digitally by classifying geologic-map units according to composition, mineralogy, and texture; rather than by age and stratigraphic relationships as shown on traditional geologic maps. This map differs from most lithologic maps in that the lithogeochemical unit classification distinguishes those rock units having key water-reactive minerals that may induce acid neutralization, or reduction, of hosted water at the weathering interface. The validity of these rock units, however, is independent of water chemistry, because the rock units are derived from geologic maps and rock descriptions. Areas of high soil carbon content, and sulfide metal deposits are also shown. Water-reactive minerals and their weathering reactions yield five lithogeochemical unit classes: 1) carbonate rock and calcareous rocks and sediments, the most acid-neutralizing; 2)carbonaceous-sulfidic rocks and sediments, oxygen-depleting and reducing; 3) quartzofeldspathic rocks and siliciclastic sediments, relatively weakly reactive with water; 4) mafic silicate rocks/sediments, oxygen consuming and high solute-load delivering; and, 5) the rarer calcareous-sulfidic (carbonaceous) rocks, neutralizing and reducing. Earlier studies in some parts of the map area have related solute loads in ground and stream waters to some aspects of bedrock lithology. More recent preliminary tests of relationships between four of the classes of mapped lithogeochemical units and ground water chemistry, in the Mid-Atlantic area using this map, have focused on and verified the nitrate-reducing and acid-neutralizing properties of some bedrock and unconsolidated aquifer rock types. Sulfide mineral deposits and their mine-tailings effects on waters are beginning to be studied by others. Additional testing of relationships among the lithogeochemical units and aspects of ground and surface water chemistry could help to refine the lithogeochemical classification, and this map. The testing could also improve the usefulness of the map for assessing aquifer reactivity and the transport properties of reactive contaminants such as acid rain, and nitrate from agricultural sources, in the Chesapeake Bay watershed.

Maryland, Virginia↗
USGS1999

Application of aerial gamma-ray spectrometric and magnetic surveys in geologic mapping: a case study in southern Virginia and northern North Carolina

Aerial gamma-ray spectrometric and magnetic survey maps are valuable aids for geologic mapping where rocks are poorly exposed in south-central Virginia and northernmost North Carolina. Broad low areas on the potassium and thorium gamma-ray survey maps distinguish the Carolina, Spring Hope, and Roanoke Rapids terranes from more highly radiogenic areas of the Raleigh and Triplet terranes, reflecting differences in the compositions of residual soils. Granitic rocks are delineated most clearly by potassium highs and less clearly by thorium highs. Nearly all the thorium highs other than those related to granites are associated with amphibolite-facies rocks of the Raleigh and Triplet terranes. Contrasting thorium lows within these terranes help to distinguish the individual rock units. In the Carolina and Roanoke Rapids terranes, high-gradient magnetic patterns delineate stratified metavolcanic and metasedimentary units that are not discernible from the gamma-ray surveys. Circular magnetic highs coincide with gabbro plutons, and numerous magnetic lineaments correspond to Jurassic diabase dikes. Magnetically uniform, low-gradient areas coincide with less mafic plutons. A magnetic lineament (high) coincides with the Nutbush Creek fault zone, and other faults are distinguished as boundaries between zones of contrasting geophysical properties. The gamma-ray spectrometric and magnetic survey maps most effectively indicate geologic features in the region if they are employed collectively, and if they are interpreted in concert with simultaneous geologic field investigations.

North Carolina;Virginia↗
USGS1997

Forum on Geologic mapping applications in the Washington-Baltimore urban area; proceedings; Reston, Virginia, April 23, 1997

The Forum on Geologic mapping applications in the Washington-Baltimore urban area was convened on April 23, 1997, at Reston, Virginia. The forum was cosponsored by the U.S. Geological Survey and the Maryland Geological Survey, with assistance from the Virginia Division of Mineral Resources. Spatial earth science information in the Washington-Baltimore area provides a scientific framework for environmental assessment, urban planning, and future resource and hazard investigations in this area of the Chesapeake Bay watershed, which has sustained three centuries of urban growth.

Circular↗
USGS1996

Middle Proterozoic age for the Montpelier Anorthosite, Goochland terrane, eastern Piedmont, Virginia

Uranium-lead dating of zircons from the Montpelier Anorthosite confirms previous interpretations, based on equivocal evidence, that the Goochland terrane in the eastern Piedmont of Virginia contains Grenvillian basement rocks of Middle Proterozoic age. A very few prismatic, elongate, euhedral zircons, which contain 12–29 ppm uranium, are interpreted to be igneous in origin. The vast majority of zircons are more equant, subangular to anhedral, contain 38–52 ppm uranium, and are interpreted to be metamorphic in origin. One fraction of elongate zircon, and four fragments of a very large zircon (occurring in a nelsonite segregation) yield an upper intercept age of 1045 ± 10 Ma, interpreted as the time of anorthosite crystallization. Irregularly shaped metamorphic zircons are dated at 1011 ± 2 Ma (weighted average of the 207 Pb/ 206 Pb ages). The U-Pb isotopic systematics of metamorphic titanite were reset during the Alleghanian orogeny at 297 ± 5 Ma. These data provide a minimum age for gneisses of the Goochland terrane that are intruded by the anorthosite. Middle Proterozoic basement rocks of the Goochland terrane may be correlative with those in the Shenandoah massif of the Blue Ridge tectonic province, as suggested by similarities between the Montpelier Anorthosite and the Roseland anorthosite. Although the areal extent of Middle Proterozoic basement and basement-cover relations in the eastern Piedmont remain unresolved, results of this investigation indicate that the Goochland terrane is an internal massif of Laurentian crust rather than an exotic accreted terrane.

Geological Society of America Bulletin↗
USGS1991

E-4 Central Kentucky to the Carolina Trough

E-4 is one of eight Geodynamics transects that cross the Atlantic margin of North America between Georgia and Newfoundland. Five of the transects are in the United States and three are in Canada. Transect E-4, which is 110 km wide and more than 1,100 km long, extends from the stable North American craton just west of the Grenville front near Lexington, Kentucky southeastward across Cape Fear, North Carolina, on the Atlantic coast to oceanic crust east of the Blake Spur magnetic anomaly. Like all of the other U.S. Atlantic coast transects, it crosses Cambrian and Jurassic continental margins of North America as well as the Appalachian orogen. The display, based upon published information, portrays the geology, tectonic style and geophysical expression of this segment of the eastern North American continental margin and interprets its Phanerozoic history. The Decade of North American Geology 1983 geologic time scale (Palmer, 1983) is used throughout the display and text.

Book↗