Exposure age and erosional history of an upland planation surface in the US Atlantic Piedmont

The upland planation surface in the Piedmont of central New Jersey consists of summit flats, as much as 130 km² in area, that truncate bedding and structure in diabase, basalt, sandstone, mudstone and gneiss. These flats define a low‐relief regional surface that corresponds in elevation to residual hills in the adjacent Coastal Plain capped by a fluvial gravel of late Miocene age. A Pliocene fluvial sand is inset 50 m below the upland features. These associations suggest a late Miocene or early Pliocene age for the surface. To assess exposure age and erosional history, a 4·4 m core of clayey diabase saprolite on a 3 km² remnant of the surface was sampled at six depths for atmospherically produced cosmogenic ¹⁰Be. The measured inventory, assuming a deposition rate of 1·3 × 10⁶ atoms cm⁻² a⁻¹, yields a minimum exposure age of 227 000 years, or, assuming continuous surface erosion, a constant erosion rate of 10 m Ma⁻¹. Because the sample site lies about 60 m above the aggradation surface of the Pliocene fluvial deposit, and itself supports a pre‐Pliocene fluvial gravel lag, this erosion rate is too high. Rather, episodic surface erosion and runoff bypassing probably have produced an inventory deficit. Reasonable estimates of surface erosion (up to 10 m) and bypassing (up to 50 per cent of total precipitation) yield exposure ages of as much as 6·4 Ma. These results indicate that (1) the surface is probably of pre‐Pleistocene age and has been modified by Pleistocene erosion, and (2) exposure ages based on ¹⁰Be inventories are highly sensitive to surface erosion and runoff bypassing. Copyright © 2000 John Wiley & Sons, Ltd.     

Grenville age of basement rocks in Cape May NJ well: new evidence for Laurentian crust in u.s. Atlantic Coastal Plain basement Chesapeake terrane

The Chesapeake terrane of the U.S. mid-Atlantic Coastal Plain basement is bounded on the northwest by the Salisbury positive gravity and magnetic anomaly and extends to the southeast as far as the Atlantic coast. It underlies the Coastal Plain of Virginia, Maryland, Delaware and southern New Jersey. Rubidium/ Strontium dating of the Chesapeake terrane basement yields an age of 1.025±0.036 Ga. This age is typical of Grenville province rocks of the Middle to Late Proterozoic Laurentian continent. The basement lithologies are similar to some exposed Grenville-age rocks of the Appalachians. The TiO₂ and Zr/P₂O₅ composition of the metagabbro from the Chesapeake terrane basement is overlapped by those of the Proterozoic mafic dikes in the New Jersey Highlands. These new findings support the interpretation that Laurentian basement extends southeast as far as the continental shelf in the U.S. mid-Atlantic region. The subcrop of Laurentian crust under the mid-Atlantic Coastal Plain implies unroofing by erosion of the younger Carolina (Avalon) supracrustal terrane. Dextral-transpression fault duplexes may have caused excessive uplift in the Salisbury Embayment area during the Alleghanian orogeny. This extra uplift in the Salisbury area may have caused the subsequent greater subsidence of the Coastal Plain basement in the embayment. © 1999 Elsevier Science Ltd. All rights reserved.     

U–Pb systematics in heterogeneous zircon populations from the Precambrian basement of the Maryland Piedmont

The aim of the study was — besides the dating of metamorphic events — to evaluate the effects of multi-stage crystal growth, episodic and continuous Pb loss, and U gain on the discordant age patterns found for zircon populations of the polymetamorphic Baltimore Gneiss, the Precambrian basement in the Maryland Piedmont. Eight gneiss and migmatite samples were collected at two localities in the Phoenix and Towson dome, respectively. Their zircon populations were separated into twenty-three fractions of different size and optical appearance. A low-contamination method (T.E. Krogh, 1973) was used for the U-Pb analyses.Microscopy and electron-microprobe studies revealed internal heterogeneities of the zircon crystals: at least half of the grains of each population reflect more than one stage of crystal growth, with the last stage consisting of U-poor overgrowths (U: below 400 ppm, mostly below 200 ppm). Evidence for episodic U gain and overgrown material other than zircon has not been found. On a concordia diagram the “ages” obtained by upward extrapolations (1080 and 1180 m.y.) and downward extrapolations (421 and 455 m.y.) of the best-fit lines to the data points are in fair agreement with the geochronologic data found by other investigators and the probable times of metamorphic periods of Precambrian (Grenville) and early Paleozoic (Taconic) orogenies.Models of Pb loss by continuous diffusion cannot adequately explain the discordant age patterns: these are essentially the result of superposition of episodic Pb loss and zircon overgrowth during the Taconic (and Acadian?) metamorphisms. The zircon overgrowth appears to be present in all fractions, but its influence on the U-Pb systematics is generally not perceptible because it is overridden by the effect of episodic Pb loss. For the fractions showing the most discordant ages, the contribution of Pb loss to the discordancy was found to be at least 85 %.From the microscopic picture and the isotopic data, it appears that the bulk of the zircon substance crystallized during one or several high-grade metamorphisms accompanied by migmatization and granitization of the rocks in the course of the Grenville orogeny. Under consideration of zircon ages of Baltimore Gneiss rocks of Pennsylvania, the results point to a complex Grenville metamorphic history in the Maryland and Pennsylvania Piedmont, that lasted from at least 1200 m.y. until about 980 m.y. The granulite-facies metamorphism in the West Chester Prong, Pennsylvania, may be 50–200 m.y. younger than the metamorphic events in the gneiss domes of the Baltimore area. Although it seems that real differences exist with respect to the Precambrian ages of major zircon-forming events between the Phoenix and the Towson dome, the apparent difference of about 100 m.y. should be interpreted with caution, because it is impossible, so far, to evaluate quantitatively the influence of possibly much older inherited zircon components.     

Weathering,erosion and sediment composition in a high‐gradient river,Calabria, Italy

Source rock lithology and immediate modifying processes, such as chemical weathering and mechanical erosion, are primary controls on fluvial sediment supply. Sand composition and Chemical Index of Alteration (CIA) of parent rocks, soil and fluvial sand of the Savuto River watershed, Calabria (Italy), were used to evaluate the modifications of source rocks through different sections of the basin, characterized by different geomorphic processes, in a sub‐humid Mediterranean climate. The headwaters, with gentle topography, produce a coarse‐grained sediment load derived from deeply weathered gneiss, having sand of quartzofeldspathic composition, compositionally very different from in situ degraded bedrock. Maximum estimated CIA values suggest that source rock has been affected significantly by weathering, and it testifies to a climatic threshold on the destruction of the bedrock. The mid‐course has steeper slopes and a deeply incised valley; bedrock consists of mica‐schist and phyllite with a very thin regolith, which provides large cobble to very coarse sand sediments to the main channel. Slope instability, with an areal incidence of over 40 per cent, largely supplies detritus to the main channel. Sand‐sized detritus of soil and fluvial sand is lithic. Estimated CIA value testifies to a significant weathering of the bedrock too, even if in this part of the drainage basin steeper slopes allow erosion to exceed chemical weathering. The lower course has a braided pattern and sediment load is coarse to medium–fine grained. The river cuts across Palaeozoic crystalline rocks and Miocene siliciclastic deposits. Sand‐sized detritus, contributed from these rocks and homogenized by transport processes, has been found in the quartzolithic distal samples. Field and laboratory evidence indicates that landscape development was the result of extensive weathering during the last postglacial temperature maximum in the headwaters, and of mass‐failure and fluvial erosional processes in the mid‐ and low course. Copyright © 2000 John Wiley & Sons, Ltd.     

Delivery of upper-basin sediment to the lower neuse river,North Carolina,U.S.A.

Extensive storage of upper-basin Piedmont sediment and apparent low sediment supply to streams in lower-basin Coastal Plain areas generates questions as to the source of alluvium in lower reaches of rivers of the U.S. Atlantic drainage. This was investigated on the Neuse River, North Carolina, using a mineralogical indicator of sediment source areas. The utility of mica flakes for discriminating between Piedmont and non-Piedmont sources of sediment in the lower Coastal Plain reaches of the Neuse was established on the basis of an examination of the U.S. National Soils Database and of 26 soil surveys of the North Carolina Coastal Plain. From the Neuse River estuary to 48 km upstream there are no mica flakes in floodplain soils or in river bank and channel shelf sediments. Mica flakes become more common upstream. This suggests that a very small proportion of the sediment eroded in the Piedmont portion of the watershed is delivered to the river mouth. The small amounts which presumably do reach the lower Coastal Plain are so diluted by Coastal Plain-derived alluvium that no Piedmont origin can be discerned. This demonstrates a dominantly Coastal Plain source and underscores the importance of storage and discontinuous transport in fluvial sediment systems. More importantly, results suggest that upper- and lower-basin sediment dynamics are not only non-linearly related, but may be virtually decoupled.     

Mesozoic-Cenozoic tectonic transition in Kuqa Depression-Tianshan, Northwest China: Evidence from sandstone detrital and geochemical records

Succeeding to multiply collisions of different blocks in Late Paleozoic[1―5], complex intracontinental structural deformation occurred in the Tianshan area during Mesozoic-Cenozoic[6―16], which controlled coeval basin-range evolution and resulted in intensive modi- fication and adjustment of the Paleozoic oil-gas reser- voirs[17―19]. The Kuqa Depression is a secendary struc- tural unit of the Tarim basin, in which Mesozoic- Ce- nozoic deposits occur in thickness of 6000―7000 m. The Kuq…     

The significance of tectonic-related tertiary remagnetization along the margins of the Valencia trough

The paleomagnetic data from the margins of the Valencia Trough are derived from Mesozoic and Tertiary rocks from the Balearic Islands, Catalan Coastal Ranges and Eastern Iberian Chain. These rocks are affected by a complex structural evolution consisting of an initial compressive stage followed by one of extension. Cenozoic paleomagnetic data indicate that rotations occurred during the Paleogene compression and before the extension started (Lower Miocene) in the Catalan Coastal Ranges. In contrast, in the Balearic Islands the rotations are synchronous to both compressional (Late Oligocene-Middle Miocene) and extensional tectonics (post-Middle Miocene). In both areas the Mesozoic limestones are remagnetized. In the Catalan Coastal Ranges they display the same direction as the Paleogene syn-compressive deposits whereas in the Balearic Islands they conform with Lower Miocene (Burdigalian) syncompressive rocks. It is concluded that the processes of remagnetization that affected eastern Iberia are related to a compressive rather than an extensional tectonics regime     

Suspended‐sediment sources in an urban watershed,Northeast Branch Anacostia River,Maryland

Fine sediment sources were characterized by chemical composition in an urban watershed, the Northeast Branch Anacostia River, which drains to the Chesapeake Bay. Concentrations of 63 elements and two radionuclides were measured in possible land‐based sediment sources and suspended sediment collected from the water column at the watershed outlet during storm events. These tracer concentrations were used to determine the relative quantity of suspended sediment contributed by each source. Although this is an urbanized watershed, there was not a distinct urban signature that can be evaluated except for the contributions from road surfaces. We identified the sources of fine sediment by both physiographic province (Piedmont and Coastal Plain) and source locale (streambanks, upland and street residue) by using different sets of elemental tracers. The Piedmont contributed the majority of the fine sediment for seven of the eight measured storms. The streambanks contributed the greatest quantity of fine sediment when evaluated by source locale. Street residue contributed 13% of the total suspended sediment on average and was the source most concentrated in anthropogenically enriched elements. Combining results from the source locale and physiographic province analyses, most fine sediment in the Northeast Branch watershed is derived from streambanks that contain sediment eroded from the Piedmont physiographic province of the watershed. Sediment fingerprinting analyses are most useful when longer term evaluations of sediment erosion and storage are also available from streambank‐erosion measurements, sediment budget and other methods. Copyright © 2010 John Wiley & Sons, Ltd.     

Mesozoic basin evolution and tectonic mechanism in Yanshan, China

The Mesozoic basins in Yanshan, China underwent several important tectonic transformations, including changes from a pre-Late Triassic marginal cratonic basin to a Late Triassic-Late Jurassic flexural basin and then to a late Late Jurassic-Early Cretaceous rift basin. In response to two violent intraplate deformation at Late Triassic and Late Jurassic, coarse fluvial depositional systems in Xingshikou and Tuchengzi Formations were deposited in front of thrust belts. Controlled by transform and extension faulting, fan deltas and lacustrine systems were deposited in Early Cretaceous basins. The composition of clastic debris in Late Triassic and Late Jurassic flexural basins respectively represents unroofing processes from Proterozoic to Archean and from early deposited, overlying pyroclastic rocks to basement rocks in provenance areas. Restored protobasins were gradually migrated toward nearly NEE to EW-trending from Early Jurassic to early Late Jurassic. The Early Cretaceous basins with a NNE-trending crossed over early-formed basins. The Early-Late Jurassic and Early Cretaceous basins were respectively controlled by different tectonic mechanisms.     

The role of fault reactivation in the development of tropical montane lakes

This work details the role of fault reactivation in the development of tropical montane lakes by using basin morpho-structural analysis and seismostratigraphic studies. The upland lakes are severely faulted sinkholes, whose faults penetrate the Quaternary sedimentary units. Four main stages are related to the lake formation: (i) an Early Proterozoic tectonic deformation of the rocks along the southern border of the Carajás Structure, where the lake is placed; (ii) differential erosion by – and building of – the formation of the South Carajás Hill; (iii) Fe-rich crust formation by weathering and gravitational collapse faults following the E–W plateau border and the start of Violão Lake formation during the Pliocene–Pleistocene; and (iv) episodic fault-fracture reactivation by gravitational collapse causing pulses of subsidence in the lake and outlining its faulted borders. Dissolution of the lateritic crust and erosion by runoff drainage under wet climate conditions were coeval with fault activities, which allowed the deposition of relatively thick clastic deposits organized in three main seismostratigraphic units associated with major lake-level fluctuations. Initial fault reactivation under low-level water started lacustrine basin development with deposition of prograding fan deltas related to the main drainage. A second fault reactivation by gravitational collapse increased the lake accommodation space and resulted in the deposition of fine-grained sediments from dilute interflows or overflows until 36 000 cal year BP. At about 31 000 cal year BP, rapid decreases in the lake water level under redox conditions at the sediment/water interface allowed widespread siderite formation. A third gravitational collapse episode was responsible for the increase in the lake area and depth and the returning of clastic/organic deposition up to the present. This tropical montane lake can be seen as a representative example for understanding the formation of other upland lakes controlled by fault reactivation. © 2020 John Wiley & Sons, Ltd.     

The significance of rock structure,lithology and pre‐glacial deep weathering for the shape of intermediate‐scale glacial erosional landforms

The main landforms within the glacially scoured Precambrian rocks of the Swedish west coast are closely connected to the principal structural pattern and have lately been explained as mainly a result of etch processes, probably during the Mesozoic and with a possible second period of etching during the Neogene. To explore the effect of multiple glacial erosion on the rock surfaces, an island with two different lithologies and with striae from different directions was selected for a detailed study, focusing on the shape of roches moutonnées. Air‐photo interpretation of bedrock lineaments and roches moutonnées combined with detailed field mapping and striae measurements are used to interpret the structural and lithological control on the resulting shape. The study reveals a significant difference in shape between roches moutonnées in augen‐granite and orthogneiss. Low elongated and streamlined roches moutonnées occur in the gneiss area, striated by a Late Weichselian ice flow from the NE. This ice flow is subparallel with both the local dominant trend of topographically well‐expressed joints and the schistosity of the gneiss. Frequently, there are no signs of quarrying on the lee‐sides of the gneiss roches moutonnées and hence they resemble the shape of whalebacks, or ruwares, as typically associated with the exposed basal weathering surface found in tropical areas. The granite roches moutonnées were formed by an older ice flow from the ESE, which closely followed the etched WNW–ESE joint system of the granite. Late Weichselian ice flow from the NE caused only minor changes of the landforms. On the contrary, marks of the early ESE ice flow are poorly preserved in the gneiss area, where it probably never had any large effect as the flow was perpendicular to both schistosity and structures and, accordingly, also to the pre‐glacial relief. The study demonstrates that coincidence between ice flow direction and pre‐glacially etched structures is most likely to determine the effects of glacial erosion. Copyright © 2002 John Wiley & Sons, Ltd.     

Petrological influence on differential weathering and inselberg development in the Kora area of central Kenya

In the Kora area of central Kenya domed inselbergs are well developed on outcrops of granitoid migmatite, while positive relief features are rare on the surrounding gneiss. Block-strewn, vegetated hills occur on restricted areas of granoblastite, gabbro, and metagabbro. Schmidt Hammer measurements have shown that the apparent differences in resistance to weathering and erosion are not due to variations in rock hardness, since all the rock types have similar ‘R’ values. The results of geochemical analyses have shown that the migmatites are significantly more potassic than the surrounding gneiss. Samples of migmatite from the inselbergs were also found to be slightly richer in potassium than migmatite samples from the inter-inselberg areas. The variations in potassium content probably reflect differences in protolith composition, chemical fractionation during partial melting, and the effects of metasomatism. These findings support earlier suggestions that, other things being equal, potassium-rich granitoid rocks weather more slowly than less potassic rocks.     

Tortoises and hares: dissolution,erosion and isostasy in landscape evolution

Denudation mechanisms differ fundamentally between limestone and silicate rock types, which are subject to very different rate thresholds and enhancers/inhibitors. Silicates are removed largely by erosion, the mechanical entrainment and transport of particles. This is a relatively high energy, and highly episodic, process which occurs only when a minimum threshold ?ow velocity is exceeded; it is inhibited by vegetation cover and favoured by strongly seasonal runoff. Limestone is removed largely by chemical dissolution at a rate directly proportional to runoff. Dissolution is a relatively low energy process that can occur at any ?ow velocity or in static water; in general it is enhanced by vegetation cover and non‐seasonality of runoff. These contrasting factors in the denudation of silicates versus limestone can produce strikingly uneven rates of surface lowering across a landscape, sometimes akin to the well known ‘tortoise and hare race’, where the slow and steady denudation of limestones may in the long term exceed the sometimes rapid, but often localized and episodic, erosion of silicates. Prolonged exposure of limestone to a humid temperate climate in a tectonically stable environment produces low‐relief corrosion plains in which limestone uplands are anomalous and, in most instances, due to recent unroo?ng from beneath a siliciclastic cover. In a highly seasonal or semi‐arid climate almost the exact inverse may develop, with ‘?ashy’ runoff and sparse vegetation favouring erosion rather than dissolution. Even under a constant humid climate progressive unroo?ng of a thick limestone unit within folded siliciclastics may lead to a topographic inversion over time, with the limestone outcrop always forming a topographic low ?anked by siliciclastic uplands. Valleys will be initiated on anticlinal crests, where the limestone is ?rst unroofed, but progressive lowering of the limestone causes these valleys to migrate to their ?nal position in the synclinal troughs. In humid climates isostatic compensation in response to slow, but continuous, denudation of extensive limestone outcrops may be a signi?cant factor in the development of relief on adjacent, more slowly eroding, silicate outcrops. Copyright © 2004 John Wiley & Sons, Ltd.     

Geochronological evidence for pervasive Miocene weathering,Minas Gerais,Brazil

⁴⁰Ar/³⁹Ar laser incremental‐heating analyses of 22 individual grains of supergene cryptomelane from three weathering pro?les, up to 400 km apart, in the Rio Doce valley and Barbacena regions at Minas Gerais, Brazil, show that the formation of weathering pro?les in these regions is contemporaneous, suggesting a strong weathering event in the Middle to Late Miocene (10–8 Ma). The preservation of these Miocene samples at or near the present surface suggests that either erosion rates have been very low in the region since the Miocene or that a much thicker weathering mantle was present in the region originally. Assuming a constant thickness of weathering pro?les in the region throughout the Tertiary, we may calculate weathering front propagation rates of 4–8 m Myr^?1 during the past 10 Ma. Copyright © 2004 John Wiley & Sons, Ltd.     

Geophysical modeling of the northern Appalachian Brompton-Cameron, Central Maine, and Avalon terranes under the New Jersey Coastal Plain

A regional terrane map of the New Jersey Coastal Plain basement was constructed using seismic, drilling, gravity and magnetic data. The Brompton-Cameron and Central Maine terranes were coalesced as one volcanic island arc terrane before obducting onto Laurentian, Grenville age, continental crust in the Taconian orogeny [Rankin, D.W., 1994. Continental margin of the eastern United States: past and present. In: Speed, R.C., (Ed.), Phanerozoic Evolution of North American Continent-Ocean Transitions. DNAG Continent-Ocean Transect Volume. Geological Society of America, Boulder, Colorado, pp. 129–218]. Volcanic island-arc rocks of the Avalon terrane are in contact with Central Maine terrane rocks in southern Connecticut where the latter are overthrust onto the Brompton-Cameron terrane, which is thrust over Laurentian basement. Similarities of these allochthonous island arc terranes (Brompton-Cameron, Central Maine, Avalon) in lithology, fauna and age suggest that they are faulted segments of the margin of one major late Precambrian to early Paleozoic, high latitude peri-Gondwana island arc designated as “Avalonia”, which collided with Laurentia in the early to middle Paleozoic. The Brompton Cameron, Central Maine, and Avalon terranes are projected as the basement under the eastern New Jersey Coastal Plain based on drill core samples of metamorphic rocks of active margin/magmatic arc origin. A seismic reflection profile across the New York Bight traces the gentle dipping (approximately 20 degrees) Cameron's Line Taconian suture southeast beneath allochthonous Avalon and other terranes to a 4 sec TWTT depth (approximately 9 km) where the Avalonian rocks are over Laurentian crust. Gentle up-plunge (approximately 5 degrees) projections to the southwest bring the Laurentian Grenville age basement and the drift-stage early Paleozoic cover rocks to windows in Burlington Co. at approximately 1 km depth and Cape May Co. at approximately 2 km depths. The antiformal Shellburne Falls and Chester domes and Chain Lakes-Pelham dome-Bronson Hill structural trends, and the synformal Connecticut Valley-Gaspe structural trend can be traced southwest into the New Jersey Coastal Plain basement. A Mesozoic rift basin, the “Sandy Hook basin”, and associated eastern boundary fault is identified, based upon gravity modeling, in the vicinity of Sandy Hook, New Jersey. The thickness of the rift-basin sedimentary rocks contained within the “Sandy Hook basin” is approximately 4.7 km, with the basin extending offshore to the east of the New Jersey coast. Gravity modeling indicates a deep rift basin and the magnetic data indicates a shallow magnetic basement caused by magnetic diabase sills and/or basalt flows contained within the rift-basin sedimentary rocks. The igneous sills and/or flows may be the eastward continuation of the Watchung and Palisades bodies.     

The Nazca Group of south-central Peru: Age,source, and regional volcanic and tectonic significance

The Nazca Group, exposed east of Nazca, Peru, consists of a lower part composed of conglomerate and finer-grained clastic strata and an upper part made up of at least seven ash-flow sheets (cooling units or ignimbrites), collectively known as the Nazca Tuff. These tuffs were erupted between about 22 and 18 m.y. ago from a vent area in the vicinity of Pampa Galeras now marked by a collapse caldera. The early Miocene age of the Nazca Tuff provides additional evidence for a major pulse of largely pyroclastic felsic volcanism throughout the central Andes during the early Miocene. Recognition of the Pampa Galeras caldera supports the idea that many of these rocks were erupted from vent areas of the collapse-caldera type located near the eastern margin of the Coastal batholith.The Nazca Group overlies a major erosional surface cut on the Coastal batholith and its envelope that can be traced southward to the Chilean border. This surface is a continuation of the post-Incaic erosional surface to the north, which is overlain by conglomerate and radiometrically dated volcanic rock of late Eocene age. The post-Incaic surface therefore represents a major episode of regional uplift and pedimentation that followed early Tertiary orogeny. The absence of volcanic rocks of late Eocene/early Oligocene age overlying the Coastal batholith near Nazca and in southern Peru may reflect a general absence of post-Incaic volcanism in this portion of the Andes possibly related to differences in the angle of subduction and/or restriction of volcanic and volcaniclastic rocks of this age to depositional basins east of the batholith.     

Numerical modelling of combined erosion and weathering of slopes in weak rock

Due to various decay processes associated with weathering, the stability of artificial slopes in weak rocks may be affected well within their envisaged engineering lifetime. Conceptually, the decay following the initial stress release after excavation can be described as a process seeking equilibrium between weathering and erosion. The extent to which such an equilibrium is actually reached influences the outcome of the weathering‐erosion decay process as well as the effects that the decay has on the geotechnical properties of the exposed rock mass, and thus ultimately the stability of slopes affected by erosion and weathering. This paper combines two conceptual models for erosion and weathering, and derives a numerical model which predicts the resulting slope development. This can help to predict the development of a slope profile excavated in a weak rock in time, and can be extended with the addition of strength parameters to the weathering profile to enable prediction of slope stability as a function of time. Copyright © 2011 John Wiley & Sons, Ltd.     

Alleghanian orogenic-float on the Martic thrust during dextal transpression, central Appalachian Piedmont

During the Late Paleozoic Alleghanian orogeny, the mid-Atlantic Piedmont experienced transpressional deformation dominated by dextral strke-slip shear zones. The dextral displacement on these shear zones greatly influenced the geographic distribution of lithotectonic units. Transpressional deformation is evident in the Piedmont with the cogenetic development of domes and en-echelon antiforms between many of the shear zones. In the core of the Pennsylvania reentrant, major Alleghanian structures include the dextral Pleasant Grove shear zone and Tucquan-Mine Ridge antiform. Recent field mapping coupled with detailed metamorphic and deformation fabric studies have revealed that a major thrust, the Martic thrust, was also active during this time. Shear bands were identified during petrofabric analysis of the hanging wall rocks to the Martic thrust. The direction of displacement on these shear bands was parallel to the orogen, a direction contrary to earlier studies. Metamorphic mineral assemblages and ceased reaction textures, associated with ductile shear fabrics in the hangingwall rocks, are consistent with lower greenshist facies deformation. This low grade metamorphism, which is generally confined to sheared rocks, overprints the regional upper greenshist- to lower amphibolite-facies assemblages. Structural and magnetic modeling of the hangingwall block has revealed a complex geometry. A model of orogen parallel structural escape, or orogenic float, related to late Paleozoic dextral transpression is employed to explain the late reactivation on this important central Appalachian structure.     

A review of kentriodontine dolphins (Cetacea; Deiphinoidea; Kentriodontidae): Systematics and biogeography

Abstract Fossil dolphins belonging to the extinct family Kentriodontidae are small to medium-sized toothed cetaceans, which probably include the ancestors of some living species. Kentriodontids are known from rocks of Late Oligocene to Late Miocene age in various parts of the world. Among kentriodontids, species in the subfamily Kentriodontinae (e.g. species of Kentriodon Kellogg, 1927) are the most ubiquitous and generalized; these are now known from latest Oligocene to earliest Miocene strata in New Zealand and Patagonia, and Middle Miocene deposits in Maryland, Virginia, California and Japan. The diversity, morphologies and distributions of Miocene species of Kentriodontinae seem to parallel those of the living species of mostly pelagic delphinids in the subfamily Delphininae, and the fossil group may have been an ecological or behavioral/functional counterpart of the latter. Kentriodontines are inferred to have been wide-ranging neritic to pelagic animals that ate small fish and other nectonic organisms; they were probably active echolocators, and might have formed large schools. They are relatively common as fossils and, therefore, are potentially useful for intercontinental correlations of marine deposits.     

Radiometric ages for some rocks from Snares and Auckland Islands,Campbell Plateau

On the basis of K/Ar muscovite and biotite ages, and of Rb/Sr whole-rock, muscovite, and feldspar ages, the last cooling of granites from Snares and Auckland Islands on the Campbell Plateau took place in early Late Cretaceous times. The original emplacement is unlikely to have been older than Late Jurassic. These results from the basement rocks of the Campbell Plateau conform with data from Fiordland and southern Stewart Island in southern New Zealand and from Ford Range and other localities in West Antarctica. Two gabbros from Auckland Island yield Miocene K/Ar ages.