Burial and exhumation history of Pennsylvanian strata, central Appalachian basin: an integrated study

An inferred burial and exhumation history of Pennsylvanian strata in the central Appalachian foreland basin is constrained by integrating palaeothermometers, geochronometers and estimated palaeogeothermal gradients. Vitrinite reflectance data and fluid inclusion homogenization temperatures indicate that burial of Lower and Upper Pennsylvanian strata of the Appalachian Plateau in West Virginia exceeded ～4.4 km during the late Permian and occurred at a rate of ～100 m Myr^?1. Exhumation rates of ～10 m Myr^?1 from the late Permian to the early Cretaceous are constrained using maximum burial conditions and published apatite fission track (AFT) ages. AFT and radiogenic helium ages indicate exhumation rates of ～30–50 m Myr^?1 from the early to late Cretaceous. Radiogenic helium dates and present day sampling depths indicate that exhumation rates from the late Cretaceous to present were ～25 m Myr^?1. Exhumation rates for Upper and Lower Pennsylvanian strata within the Appalachian Plateau are remarkably similar. Early slow exhumation was possibly driven primarily by isostatic rebound associated with Triassic rifting. The later, more rapid exhumation can be attributed to thermal expansion followed by lithospheric flexure related to sediment loading along the passive margin.     

Denudation rates of a subequatorial orogenic belt based on estimates of sediment yields: evidence from the Paleozoic Appalachian Basin,USA

The Upper Mississippian (ca. 325 Ma) Pride Shale and Glady Fork Member in the Central Appalachian Basin comprise an upward‐coarsening, ca. 60‐m‐thick succession of prodeltaic‐delta front, interlaminated fine‐grained sandstones and mudstones gradational upwards into mouth‐bar and distributary‐channel sandstones. Analysis of laminae bundling in the Pride Shale reveals a hierarchy of tidal cycles (semi‐diurnal, fortnightly neap‐spring) and a distinct annual cyclicity resulting from seasonal fluvial discharge. These tidal rhythmites thus represent high‐resolution chronometers that can be used in basin analysis. Annual cycles average 10 cm in thickness, thus the bulk of the Pride Shale‐Glady Fork Member in any one vertical section is estimated to have accumulated in ca. 600 years. Progradational clinoforms are assumed to have had dips of 0.3–3° with a median dip of 1.7°; the latter infilled a NE‐SW oriented foreland trough up to 300 km long by 50 km wide in the relatively short time period of 90 kyr. The total volume of sediment in the Pride basin is ca. 900 km³ which, for an average sediment density of 2700 kg m^?3, equates to a total mass of ca. 2.4 × 10⁶ Mt. Thus, mass sediment load can be estimated as 27 Mt yr^?1. For a drainage basin area of 89 000 km², based on the scale of architectural channel elements and cross‐set thicknesses in the incised‐valley‐fill deposits of the underlying Princeton Formation, suspended sediment yields are estimated at ca. 310 t km^?2 yr^?1 equating to a mechanical denudation rate of ca. 0.116 mm yr^?1. Calculated sediment yields and inferred denudation rates are comparable to modern rivers such as the Po and Fly and are compatible with a provenance of significant relief and a climate characterized by seasonal, monsoonal discharge. Inferred denudation rates also are consistent with average denudation rates for the Inner Piedmont Terrane of the Appalachians based on flexural modelling. The integration of stratigraphic architectural analysis with a novel chronometric application highlights the utility of sedimentary archives as a record of Earth surface dynamics.     

Contourites associated with pelagic mudrocks and distal delta-fed turbidites in the Lower Proterozoic Timeball Hill Formation epeiric basin (Transvaal Supergroup), South Africa

Five genetic facies associations/architectural elements are recognised for the epeiric sea deposits preserved in the Early Proterozoic Timeball Hill Formation, South Africa. Basal carbonaceous mudrocks, interpreted as anoxic suspension deposits, grade up into sheet-like, laminated, graded mudrocks and succeeding sheets of laminated and cross-laminated siltstones and fine-grained sandstones. The latter two architectural elements are compatible with the Te, Td and Tc subdivisions of low-density turbidity current systems. Thin interbeds of stromatolitic carbonate within these first three facies associations support photic water depths up to about 100 m. Laterally extensive sheets of mature, cross-bedded sandstone disconformably overlie the turbidite deposits, and are ascribed to lower tidal flat processes. Interbedded lenticular, immature sandstones and mudrocks comprise the fifth architectural element, and are interpreted as medial to upper tidal flat sediments. Small lenses of coarse siltstone–very fine-grained sandstone, analogous to modern continental rise contourite deposits, occur within the suspension and distal turbidite sediments, and also form local wedges of inferred contourites at the transition from suspension to lowermost turbidite deposits. Blanketing and progressive shallowing of the floor of the Timeball Hill basin by basal suspension deposits greatly reduced wave action, thereby promoting preservation of low-density turbidity current deposits across the basin under stillstand or highstand conditions. A lowstand tidal flat facies tract laid down widespread sandy deposits of the medial Klapperkop Member within the formation. Salinity gradients and contemporaneous cold periglacial water masses were probably responsible for formation of the inferred contourites. The combination of the depositional systems interpreted for the Timeball Hill Formation may provide a provisional model for Early Proterozoic epeiric basin settings.     

An overview of the geology of the Transvaal Sequence and Bushveld Complex,South Africa

The Late Archaean-Early Proterozoic Transvaal Sequence is preserved within the Transvaal, Kanye and Griqualand West basins, with the 2050 Ma Bushveld Complex intrusive into the upper portion of the succession within the Transvaal basin. Both Transvaal and Bushveld rocks are extensively mineralized, the former containing large deposits of iron, manganese, asbestos, andalusite, gold, fluorine, lead, zinc and tin ores, and the latter some of the World's major occurrences of PGE, chromium and vanadium ores. Transvaal sedimentation began with thin, predominantly clastic sedimentary rocks (Black Reef-Vryburg Formations) which grade up into a thick package of carbonate rocks and BIF (Chuniespoort-Ghaap-Taupone Groups). These lithologies reflect a carbonate-BIF platform sequence which covered much of the Kaapvaal craton, in reaction to thermal subsidence above Ventersdorp-aged rift-related fault systems. An erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the Pretoria-Postmasburg-Segwagwa Groups within the three basins, under largely closed-basin conditions. An uppermost predominantly volcanic succession (Rooiberg Group-Loskop Formation) is restricted to the Transvaal basin. A common continental rift setting is thought to have controlled Pretoria Group sedimentation, Rooiberg volcanism and the intrusion of the mafic rocks of the Rustenburg Layered Suite of the Bushveld Complex. The dipping sheets of the Rustenburg magmas cut across the upper Pretoria Group stratigraphy and lifted up the Rooiberg lithologies to form the roof to the complex. Subsequent granitic rocks of the Lebowa and Rashoop Suites of the Bushveld Complex intruded both upper Rustenburg rocks and the Rooiberg felsites.     

Age of carbonatite and phoscorite magmatism of the Phalaborwa Complex (South Africa)

The Phalaborwa Complex (South Africa) has been dated by UPb analysis of uranothorianite and baddeleyite yielding an age of 2047 + 11/?8 Ma for caronatite and phoscorite magmatism. Baddeleyite yields an age comparable to the age determined on uranothorianite and provides a reliable age indicator. Ages of carbonatite and silicate magmatism cannot be resolved as different events. Confirmation of K-rich alkaline magmas produced in the mantle as early as 2047 Ma suggests that significant chemical heterogeneities existed prior to 2000 Ma.     

On the influence of kriging parameters on the cartographic output—A study in mapping subglacial topography

Geostatistics provides a suite of methods, summarized as kriging, to analyze a finite data set to describe a continuous property of the Earth. Kriging methods consist of moving window optimum estimation techniques, which are based on a least-squares principle and use a spatial structure function, usually the variogram. Applications of kriging techniques have become increasingly wide-spread, with ordinary kriging and universal kriging being the most popular ones. The dependence of the final map or model on the input, however, is not generally understood. Herein we demonstrate how changes in the kriging parameters and the neighborhood search affect the cartographic result. Principles are illustrated through a glaciological study. The objective is to map ice thickness and subglacial topography of Storglaciären, Kebnekaise Massif, northern Sweden, from several sets of radio-echo soundings and hot water drillings. New maps are presented.     

Tentative Detection of Circumstellar CO2 from the AGB Star R Crateris

We report on and discuss the detection of an emission feature at 14.98 μm from the oxygen-rich, semi-regularly pulsating Asymptotic Giant Branch star R Crateris, a feature which we suggest to be due to the ₀ ¹ Q-branch of circumstellar CO2. We also suggest a reasonable excitation mechanism, which could explain the height, the width and the asymmetry of the feature. This revised version was published online in August 2006 with corrections to the Cover Date.     

Petrography and geochemistry of the Middle Siwalik sandstones (tertiary) in understanding the provenance of sub-Himalayan sediments in the Lish River Valley,West Bengal,India

A petrography–geochemistry-based evaluation of the provenance of the sandstones of the Tertiary Middle Siwalik Subgroup in the Lish River Valley, West Bengal, is presented. The framework grains in the sandstones suggest mixing of sediments from spatially separated gneissic, quartzitic and phyllitic source rocks. Modal values of different framework minerals suggest that recycled sediments in an orogenic setting were deposited in the Middle Siwalik basin in the area. The major and trace element ratios suggest dominantly felsic input and mixing with subordinate basic material in an upper continental crustal setup. The major and trace element data also indicate that rocks of a passive margin setting acted as the source to the sediments. The present paper postulates that the Middle Siwalik sediments were derived from pre-Himalayan gneissic and metabasic rocks of an erstwhile passive margin setting and presently forming the Higher and Lesser Himalaya, respectively.     

Formation and significance of a middle Silurian ravinement surface on Gotland, Sweden

A laterally extensive and conspicuously smooth erosional surface is exposed near the Wenlock–Ludlow boundary on east-central Gotland, Sweden. It occurs in the reef complex area of a carbonate platform, has an undulating topography, and separates truncated bioherms and biostromes from overlying allochthonous high energy deposits. On a basin regional scale, the surface is associated to a shift from a prograding to a retrograding platform, and to a substantial hiatus in basin marginal areas (Estonia). The significance of the surface is further indicated by: (a) clear-cut truncation of the reef complex, including m-sized stromatoporoids, along a distance of at least 20 km, (b) an erosional relief exceeding 2.08 m, (c) a conspicuously smooth nature in both palaeolows and palaeohighs, (d) present, although scarce, subaerial diagenetic indications, e.g. shallow karst features at the unconformity surface and pendant/meniscus cement in the lowermost part of overlying strata, (e) a locally occurring basal conglomeratic lag in overlying strata, and (f) peritidal indications and, as evident from at least one quarry, onlapping geometry in overlying strata.

The unconformity has implications for the analysis of the middle Silurian Baltic basin evolution as well as for the interpretation of erosional surfaces on carbonate platforms in general. Based on the above characteristics, the formation of the unconformity is attributed to a relative sea-level fall, causing subaerial exposure, followed by transgressive abrasion in a rocky shore environment. The unconformity hence constitutes a ravinement surface which, based on the associated basin regional sedimentary changes, is interpreted as coinciding with a regional exposure surface (sequence boundary). It thus increases our understanding of the hitherto poorly understood palaeogeographic evolution of the middle Silurian Baltic basin. Further, the transgressive erosion was significant as well as recurrent, as indicated by the clear-cut truncation of large stromatoporoids at the unconformity surface and by truncated marine fossil fragments in upper edges of karst infills. The unconformity therefore constitutes a good example of the capability of transgressive erosion in creating stratigraphic incompleteness, and hence in removing subaerial indications, on previously exposed carbonate platforms.