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J. Wright Horton,

Publications and source records attributed to J. Wright Horton,.

28 records · Page 2Linked to original sources
USGS2005

Crystalline-rock ejecta and shocked minerals of the Chesapeake Bay impact structure, USGS-NASA Langley core, Hampton, Virginia, with supplemental constraints on the age of impact

The USGS-NASA Langley corehole at Hampton , Va., was drilled 2000 as the first in a series of new coreholes drilled in the late Eocene Chesapeake Bay impact structure to gain a comprehensive understanding of its three-dimensional character. This understanding is important for assessing ground-water resources in the region, as well as for learning about marine impacts on Earth. We studied crystalline-rock ejecta and shock-metamorphosed minerals from the Langley core to determine what they reveal about the geology of crystalline rocks beneath the Atlantic Coastal Plain and how those rocks were affected by the impact . An unusual polymict diamicton, informally called the Exmore beds (upper Eocene), is 33.8 meters (m; 110.9 feet (ft)) thick and lies at a depth of 269.4 to 235.65 m (884.0 to 773.12 ft) in the core . This matrix-supported sedimentary deposit contains clasts of Tertiary and Cretaceous sediment (ranging up to boulder size) and sparse pebbles of crystalline rock . The matrix consists of muddy sand that contains abundant quartz grains and minor glauconite and potassium feldspar. Significantly, the sandy matrix of the Exmore beds contains sparse quartz grains (0.1 to 0.3 millimeter (0.004 to 0.012 inch) in diameter) that contain multiple sets of intersecting planar deformation features formerly referred to as shock lamellae. As many as five different sets have been observed in some quartz grains. Planar deformation features also occur in quartz grains in reworked crystalline-rock clasts in the Exmore beds. Such grains are clearly of shock-metamorphic origin. The presence of these features indicates that the quartz grains have experienced pressures greater than 6 gigapascals (GPa) and strain rates greater than 10 6 /second. Thus, the shock-metamorphosed quartz grains, although rare, provide clear and convincing evidence that the Exmore beds are of hybrid impact origin. Identification of shocked quartz grains in the Langley core adds to the number of sites in the structure where their presence is confirmed. Most of the clasts of crystalline rock that are in and just below the Exmore beds are rounded, detrital, and typical of coastal plain sediments. However, a few have angular shapes and consist of cataclastically deformed felsite having aphanitic-porphyritic to aphanitic texture and peraluminous rhyolite composition. Three of these clasts contain quartz grains that display two sets of planar deformation features of shock-metamorphic origin. Shock-metamorphosed quartz is an integral part of the cataclastic fabric in these three clasts, indicating that both the fabric and the shocked quartz were produced by the same high-energy impact event. Some felsite clasts have spherulitic textures that may be features either of an impact melt or of preimpact volcanic rocks. A weighted-mean total-fusion 40 Ar/ 39 Ar age of 35.3±0.1 Ma (±lσ) for 19 analyses of 4 North American tektites records the age of the late Eocene Chesapeake Bay impact event.

Chesapeake Bay↗
USGS2004

Deciphering multiple Mesoproterozoic and Paleozoic events recorded in zircon and titanite from the Baltimore Gneiss, Maryland: SEM imaging, SHRIMP U-Pb geochronology, and EMP analysis

The Baltimore Gneiss , exposed in antiforms in the eastern Maryland Piedmont, consists of a suite of felsic and mafic gneisses of Mesoproterozoic age. Zircons from the felsic gneisses are complexly zoned, as shown in cathodoluminescence imaging ; most zircon grains have multiple overgrowth zones, some of which are adjacent and parallel to elongate cores. Sensitive high-resolution ion microprobe ( SHRIMP ) analyses of oscillatory-zoned cores indicate that the volcanic protoliths of the felsic gneisses crystallized at ca. 1.25 Ga. These rocks were subsequently affected by at least three Mesoproterozoic growth events , at ca. 1.22, 1.16, and 1.02 Ga. Foliated biotite granite intruded the Baltimore Gneiss metavolcanic sequence at ca. 1075 Ma. The Slaughterhouse Granite (renamed herein) also is Mesoproterozoic , but extremely discordant U-Pb data from high-U, metamict zircons preclude calculating a precise age. The 1.25 Ga rocks of the Baltimore Gneiss are coeval with rocks emplaced in the Grenville Province during the Elzevirian orogeny, and the 1.22 Ga zircon overgrowths are coincident with a later stage of this event . Younger zircon overgrowths formed during the Ottawan phase of the Grenville orogeny. Backscattered electron imaging of titanites from felsic gneisses and foliated biotite granite reveals that many of the grains contain cores, intermediate mantles, and rims. Electron microprobe traverses across zoned grains show regular variations in composition. SHRIMP ages for titanite from the foliated biotite granite are 374 ± 8, 336 ± 8, and 301 ± 12 Ma. The ca. 374 Ma age suggests growth of titanite during a thermal event following the Acadian orogeny, whereas the late Paleozoic titanite growth ages may be due to greenschist-facies replacement reactions associated with Alleghanian metamorphism and deformation.

Maryland↗
USGS2002

SHRIMP and Conventional UPb ages of Ordovician granites and tonalites in the central Appalachian Piedmont: Implications for Paleozoic tectonic events

Discordant Cambrian U-Pb ages of questionable reliability for metamor-phosed plutonic rocks in the central Appalachian Piedmont of northern Virginia, Maryland, and the District of Columbia have led to different regional geologic interpretations and controversies. In this study, we use the sensitive high resolution ion microprobe (SHRIMP) for dating 25-m diameter areas on individual zircons from Piedmont granitoids in an effort to resolve previous controversial ages. Cathodoluminescence images of zoning within the zircons are used to select pristine areas of cores and overgrowths for dating and to help interpret the results. For comparison, hand-picked fractions of zircons from most of the samples were also dated by the conventional isotope dilution thermal ionization mass spectrometry (TIMS) U-Pb method. The combined SHRIMP and TIMS results of this study demonstrate that the Dale City Quartz Monzonite, Norbeck Intrusive Suite, Kensington Tonalite, Goldvein pluton, Lake Jackson pluton, Falls Church Intrusive Suite, Occoquan Granite (main batholith and satellite Bull Run Marina pluton), Georgetown Intrusive Suite, Dalecarlia Intrusive Suite, and related granitoids are parts of an extensive Early to Middle Ordovician magmatic arc. Emplacement of these plutons from about 485 to 450 Ma generally was contemporaneous with major regional deformation and metamorphism, called the Taconic orogeny in rocks of the northern Appalachians. The Ordovician ages supersede most of the discordant Cambrian(?) to Neoproterozoic dates that previously suggested a Potomac or Penobscot orogeny in Piedmont rocks of the Potomac Valley, raising questions about the nature and timing of that event. The Devonian Guilford Granite crosscuts all structures, providing a minimum age for these Paleozoic events.

Virginia, Maryland↗
USGS1989

Mélanges and olistostromes in the Appalachians of the United States and mainland Canada; An assessment

There is no completely accepted definition of a mélange, and the papers in this volume reflect this fact. In our regional assessment, the term mélange is principally used for a technically fragmented and mixed body of rock. A different term, olistostrome, is used for a chaotic and mixed rock body that formed by sedimentary processes such as slumping or gravity sliding. The term olistostromal mélange is used here if sedimentary processes and tectonic deformation were both involved in the fragmentation and mixing. In some cases there is evidence that these were effectively concurrent. Four main belts of Paleozoic mélanges and olistostromes have been recorded in the Appalachians of the northeastern United States and mainland Canada. These include: (1) olistostromes and olistostromal mélanges along Logan's line and the Taconic allochthons, which are related to thrusting during the Taconian orogeny; (2) mélanges associated with ophiolite fragments along the Baie Verte-Brompton line, which are thought to represent a Taconian suture; (3) mélanges containing ophiolite fragments along the Hurricane Mountain mélange belt (Boone and Boudette, this volume), which are thought to represent a Penobscottian terrane boundary; and (4) Acadian mélanges and olistostromes such as the Silurian Deadman Harbour mélange, an olistostromal mélange that probably formed at the front of an Acadian overthrust. In addition, Precambrian olistostromes have been recognized in southeastern New England and in the Green Head Formation of New Brunswick. These may have originated along normal faults on the rifted continental shelf of Gondwanaland, or they may relate to an earlier Pan-African cycle. These Precambrian olistostromes, therefore, accompany the extensional development of the Iapetus Ocean, whereas the Paleozoic olistostromes and mélanges mark its progressive closure. Five main sets of Paleozoic mélanges and olistostromes have been recorded in the central and southern Appalachians. These are: (1) a composite mélange-olistostrome belt in the Piedmont of Maryland and Virginia that includes olistostromes such as the Sykesville Formation (of unknown age) and mélanges such as those of the Morgan Run Formation (of debated age) and Mine Run Complex, some of which contain possible ophiolite fragments; (2) mélanges in the Blue Ridge Province from Virginia to Alabama, which contain possible ophiolite fragments; (3) Silurian or Early Devonian olistostromes of the Lay Dam Formation in the Talladega slate belt of Alabama; (4) mélanges, including the Falls Lake and Juliette mélanges in the Carolinas and Georgia, that border the Carolina terrane as defined by Secor and others (1983); and (5) broken formations and mélanges along major faults such as the Pulaski and Brevard, which are mostly related to Alleghanian thrusting. In addition, there is good evidence in the Ocoee Supergroup of the Blue Ridge Province for Late Proterozoic olistostromes related to the initiation of grabens prior to the opening of the Iapetus Ocean.

Alabama, Connecticut, Georgia, Kentucky, Maine, Ma↗
USGS1989

Preface

No abstract available.

Book chapter↗
USGS1989

Tectonostratigraphic terranes and their Paleozoic boundaries in the central and southern Appalachians

Parts of the central and southern Appalachian orogen appear to have evolved away from Proterozoic North America (Laurentia) and to have been accreted to it during the Paleozoic orogenies that collectively formed the orogen. Identifying each tectonostratigraphic terrane is a necessary step in understanding the evolution of the orogen. The terranes in the central and southern Appalachians are delineated, interpreted, and classified with varying degrees of confidence as: (1) Laurentian native terranes , (2) internal continental terranes of the Appalachian orogen, (3) disrupted terranes , (4) possible oceanic crustal remnants, (5) volcanic-arc terranes , (6) a continental terrane of Gondwanaland affinity, and (7) metamorphic complexes of undetermined affinity. The Laurentian native terranes consist of external massifs of Laurentian basement (Grenvillian and older), their rift- and shelf-facies cover rocks, and slope-rise prism deposits. External massifs are present in the Blue Ridge tectonic province, Reading Prong, and Honey Brook Upland. Rocks of the Talladega block are stratigraphically tied to Laurentia and, with the possible exception of the Hillabee greenstone, are also considered native. Offshore, deep-water, post-rift deposits of the Hamburg and Westminster terranes have no direct stratigraphic ties to Laurentia and are considered discrete native (not suspect) terranes . The internal continental terranes of the Appalachian orogen are isolated massifs of Middle Proterozoic (Grenvillian) continental basement and their cover sequences that occur within the metamorphic core of the orogen. These terranes , the Baltimore, Sauratown, and Pine Mountain terranes , could be either structurally isolated outliers of Laurentia or microcontinental fragments of Laurentian crust displaced by rifting or transcurrent faulting and later reassembled. Disrupted terranes in the central and southern Appalachians contain mélange complexes as well as more coherent terrane fragments (volcanic, ophiolitic, or continental) intermingled with the mélange complexes. Those identified include the Jefferson, Potomac, Smith River, Inner Piedmont, Falls Lake, Juliette, and Sussex terranes . The Bel Air-Rising Sun terrane (Baltimore Complex) in Maryland and Pennsylvania is the only terrane named separately as a possible oceanic crustal remnant. Similar mafic and ultramafic complexes are present in all of the disrupted terranes , but are too small to consider as separate terranes . Volcanic-arc terranes include the Chopawamsic, Carolina, Spring Hope, Roanoke Rapids, and Charleston terranes . The only terrane recognized as a continental terrane of Gondwanaland affinity is the Suwannee terrane , which contains rocks believed to correlate with those now exposed in west Africa. Metamorphic complexes of undetermined affinity are terranes that could not be clearly classified on the basis of available data. These include the Milton, Gaffney, Uchee, Crabtree, Goochland, Wilmington, and Hatteras terranes . The Penobscottian, Taconian, Acadian, and Alleghanian Paleozoic compressional events collectively assembled the various terranes into what is now the Appalachian orogen. Only the central and southern parts of the U.S. Appalachians are considered here. The Penobscottian orogeny, about 550 to 490 Ma, amalgamated the Potomac, the Chopawamsic, probably the Bel Air-Rising Sun, and possibly other exotic terranes at some unknown distance from Laurentia. This was followed by the Taconian orogeny, about 470 to 440 Ma, which accreted the previously amalgamated terranes and probably other terranes such as the Carolina terrane to Laurentia. The younger age limit for the Taconian event is partly constrained by Middle and Late Ordovician faunal assemblages in successor basin deposits of the Arvonia Slate and Quantico Formation. The significance of the Acadian orogeny, dated about 400 to 380 Ma in New England, is unclear in the central and southern Appalachians . In the Talladega block of Alabama and Georgia, an Early to Middle Devonian dynamothermal event is firmly bracketed between Early Devonian fossils and K-Ar ages that indicate a thermal peak no later than Middle Devonian time. A regional tectonothermal event and faulting of approximately this age are also suggested by isotopic studies in terranes to the east. The late Paleozoic (Alleghanian) continental collision between Laurentia and Gondwanaland, which formed the supercontinent Pangea, marks the final stage of accretionary history in the Appalachian -Caledonide orogen. Effects evident in the central and southern Appalachian region include: (1) the accretion of the Suwannee terrane and perhaps the Charleston terrane to what is now North America, (2) slicing and shifting of terranes along dextral strike-slip faults, particularly in the eastern Piedmont, (3) westward transport of native and previously accreted terranes in the western Piedmont and Blue Ridge as part of a composite crystalline thrust sheet, (4) deposition of clastic wedges in the Appalachian foreland, and (5) imbricate thrusting and folding of the resultant strata in the Valley and Ridge Province.

Appalachians↗
USGS1987

Alleghanian deformation, metamorphism, and granite emplacement in the central Piedmont of the southern Appalachians

Evidence of late Paleozoic (Alleghanian) penetrative deformation, amphilbolite-facies, metamorphism, and syntectonic granite emplacement has been found in an area of the central Piedmont in the southern Appalachians. The High Shoals Granite batholith in the Kings Mountain belt of south-central North Carolina consists of coarse-grained, megacrystic biotite granite with a strong, nearly vertical, gneissoid, foliation, defined by parallel feldspar megacrysts and biotite. U-Ph isotopic data on zircons from the granite yield a concordant age of 317 Ma (Pennsylvanian). This pervasively deformed late Paleozoic granite in the central Piedmont is far west of those known in the Kiokee and Raleigh belts. Field relationships indicate that the High Shoals Granite was emplaced during the late stages of regional F2 folding, close in time to the thermal peak of amphibolite-facies metamorphism. Mineral assemblages typical of Barrovian metamorphism are found in zones of decreasing grade away from the High Shoals contact. These zones, delimited by a kyanite(out)-sillimanite (in) isograd which completely surrounds the batholith and by part of a regional chloritoid (out)-staurolite (in) isograd west and southwest of it, conform to the shape of the batholith and cut across F2 folds. Furthermore, hornblendes from epidoteamphibolite facies to upper amphibolite facies metamorphic rocks in the Kings Mountain belt of North Carolina and South Carolina yield 40Ar/39Ar plateau ages of 318 to 323 Ma. These dates are analytically the same as the crystallization age of High Shoals Granite as established of zireon using the U-Ph method and demonstrate the synchroneity of intrusion of the High Shoals with the dominant regional metamorphism in this part of the Kings Mountain belt.

Southern Appalachians↗
USGS1984

Truncation of the Appalachian Piedmont beneath the Coastal Plain of Alabama: Evidence from new magnetic data

A new aeromagnetic survey of a part of southern Alabama reveals that magnetic signatures of the Appalachian Piedmont are truncated by a major magnetic lineament beneath the Gulf Coastal Plain. Mylonitic rocks have been recovered from a drillhole along this lineament, which is probably a fault zone of late Paleozoic and/or Triassic-Jurassic age. We suggest that this fault zone may initially have been the Alleghanian convergent suture between the North American craton and accreted terranes to the southeast. The zone may have been locally reactivated as part of an extensive buried Triassic-Jurassic graben system.

North Carolina, South Carolina, Georgia. Mississip↗
USGS1981

Shear zone between the Inner Piedmont and Kings Mountain belts in the Carolinas

The Kings Mountain shear zone, which marks the boundary between the Inner Piedmont and Kings Mountain belts near the NC-SC state line, is a northeast-striking, steeply to moderately dipping zone of ductile mylonitic deformation and late-stage semibrittle deformation. The zone is at least 60 km long and is no more than a few hundred metres wide. It truncates rock units of both belts. The juxtaposition of two lithologically different terranes suggests that displacement may be considerable, probably on the order of kilometres. Inconclusive evidence suggests that the northwest (Inner Piedmont) side is upthrown. The Kings Mountain zone is one of several in the southern Appalachian Piedmont that were active during a Middle to Late Devonian (Acadian?) deformational event, and it may be part of a regional fault system extending from AL to VA. The Kings Mountain, Lowndesville, and Towaliga zones may be a single zone more than 550 km long. © 1981 Geological Society of America.

North Carolina, South Carolina↗