Geological Society Special Publication
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Are gold deposits in the crust fractals? A study of gold mines in the Zimbabwe craton Abstract
The hypothesis that gold deposits in the earth are fractal dusts is tested using data on gold mine distribution in the Zimbabwe craton. Mine distribution reflects deposit distribution only indirectly because of undiscovered deposits and economic factors. Nevertheless, box counting shows an approximately linear relationship between the logarithm of the number of squares necessary to cover all mines in the craton and the logarithm of the square size between 10 and 60 km, indicating a fractal distribution. More detailed data from two areas within the craton have a similar relationship between square sizes of 2.5–20 km. Departure from the ideal fractal relationship is termed ‘roll-off’, and is very satisfactorily modelled by random sampling of a fractal dust. The results are consistent with the hypothesis that gold deposits are fractal dusts with fractal dimensions of
c.
1.0 and the distribution of mines can be regarded as a random sample of the distribution of deposits. Making this assumption, regions that have been incompletely but randomly explored can be identified and distinguished from regions containing large subareas that have been completely unexplored, and appropriate exploration strategies can be devised in either case.
Geological Society Special Publication - Tập 155 Số 1 - Trang 141-151 - 1999
Applications of ground-penetrating radar (GPR) to sedimentological, geomorphological and geoarchaeological studies in coastal environments Abstract
Acquisition of high resolution data regarding the stratigraphy and internal structure of coastal sedimentary sequences is becoming increasingly important in many sedimentological, geomorphological and geoarchaeological studies. Such information is usually obtained from the logging of field exposures, shallow trenches and cores. However, ground-penetrating radar (GPR), a relatively new and rapidly developing non-invasive geophysical technique, may also aid such investigations in certain coastal settings, providing additional, complimentary and often unique data sets. The technique is based on the transmission, reflection and reception of high frequency electromagnetic (radar) waves, with reflections occurring in the subsurface due to the varying electrical properties of the sediments. Such reflections can occur due the presence of primary sedimentary structures, lithological/material changes or water content changes. Examples presented from the UK indicate that the technique can accurately delineate the stratigraphy and internal sedimentary structure of coastal barriers, spits and strandplains, both above and below a fresh groundwater table. Sand and/or gravel-dominated beach-dune systems on moderate to high wave energy, macrotidal coasts appear to provide optimum settings for GPR deployment. In addition, the technique also has the potential to locate, map and provide stratigraphic context for a wide range of archaeological features that are commonly found in such coastal environments.
Geological Society Special Publication - Tập 175 Số 1 - Trang 139-171 - 2000
GPR stratigraphy used to infer transgressive deposition of spits and a barrier, Lake Bonneville, Stockton, Utah, USA Abstract
Ground penetrating radar (GPR) stratigraphic profiles of the classic cross-valley barrier and associated spits of Late Pleistocene Lake Bonneville, near Stockton, Utah, are used to infer transgressive depositional style and internal sedimentary structures. From onlapping patterns of radar reflections, which mimic subsurface stratigraphy, we reconstruct the following depositional sequence and style: (1) at the north end of the Rush Valley, the barrier formed by vertical accretion while keeping pace with hydro-isostatic-forced basin subsidence and/or slow lake-level rise; (2) a reorientation of the longshore transport pathway, induced by continued basin subsidence and/or a lake-level rise, produced a spit that prograded 2.5 km southwestward into Rush Valley. The NW-dipping radar reflections from the spit onlap SE-dipping reflections from the back-barrier, indicating that this spit was deposited after the barrier; (3) a final rise in lake level and/or basin subsidence again reoriented longshore transport and deposited the smaller upper spit. Radar reflections from the upper spit onlap the proximal eastern margin of the Stockton spit. This upper spit is the final landform deposited during the Bonneville highstand. The depositional sequence inferred from radar stratigraphy agrees with the transgressive hypothesis formulated in 1890 by G. K. Gilbert.
Geological Society Special Publication - Tập 211 Số 1 - Trang 79-86 - 2003
Late Permian anoxia in central East Greenland Abstract
The Upper Permian Foldvik Creek Group of the East Greenland Basin provides excellent possibilities for studying regional anoxia, because the basinal, anoxic facies of the Ravnefjeld Formation are well exposed throughout this 400 km long and more than 80 km wide basin.
The shales of the Ravnefjeld Formation are dominated by two units of laminated, organic-rich, calcareous mudshales. The absence of a burrowing or benthonic fauna, and the high content of organic matter and sulphur suggest these units were formed during a basin-wide anoxic episode. The character of the graded laminae indicates deposition from suspension. In contrast, thin bioturbated units appear to have been deposited during episodes of more oxygenated bottom waters. The TOC, HI and sulphur content are all lower in these bioturbated units.
The associated marine limestones of the Wegener Halvø Formation record a depositional history of transgressions and regressions that can be directly related to the laminated—bioturbated units of the Ravnefjeld Formation. The anoxic period can therefore be correlated with the maximum transgression of the basin, which induced rapid vertical growth of reefs. A stratified water column with anoxic bottom conditions and restricted circulation was apparently induced by the maximum flooding of the basin. The extensive vertical growth of carbonate platforms may have further restricted the circulation in the basin, producing a ‘silled basin’ effect.
Geological Society Special Publication - Tập 58 Số 1 - Trang 275-290 - 1991
Time scales of tectonic landscapes and their sediment routing systems Abstract
In regions undergoing active tectonics, the coupling between the tectonic displacement field, the overlying landscape and the redistribution of mass at the Earth's surface in the form of sediment routing systems, is particularly marked and variable. Coupling between deformation and surface processes takes place at a range of scales, from the whole orogen to individual extensional fault blocks or contractional anticlines. At the large scale, the attainment of a steady-state between the overlying topography and the prevailing tectonic conditions in active contractional orogens requires an efficient erosional system, with a time scale dependent on the vigour of the erosional system, generally in the range 10
6
–10
7
years. The catchment–fan systems associated with extensional fault blocks and basins of the western USA are valuable natural examples to study the coupling between tectonic deformation, landscape and sediment routing systems. Even relatively simple coupled systems such as an extensional fault block and its associated basin margin fans have a range of time scales in response to a tectonic perturbation. These response times originate from the development of uniform (steady-state) relief during the accumulation of displacement on a normal fault (
c
. 10
6
years), the upstream propagation of a bedrock knickpoint in transverse catchments following a change in tectonic uplift rate (
c
. 10
6
years), or the relaxation times of the integrated catchment–fan system in response to changes in climatic and tectonic boundary conditions (10
5
–10
6
years). The presence of extensive bedrock or alluvial piedmonts increases response times significantly. The sediment efflux of a mountain catchment is a boundary condition for far-field fluvial transport, but the fluvial system is much more than a simple transmitter of the sediment supply signal to a neighbouring depocentre. Fluvial systems appear to act as buffers to incoming sediment supply signals, with a diffusive time scale (
c
. 10
5
–10
6
years) dependent on the length of the system and the extent of its floodplains, stream channels and proximal gravel fans. The vocabulary for explaining landscapes would benefit from a greater recognition of the importance of the repeat time and magnitude of perturbations in relation to the response and relaxation times of the landscape and its sediment routing systems. Landscapes are best differentiated as ‘buffered’ or ‘reactive’ depending on the ratio of the response time to the repeat time of the perturbation. Furthermore, landscapes may be regarded as ‘steady’ or ‘transient’ depending on the ratio of the response time to the time elapsed since the most recent change in boundary conditions. The response of tectonically and climatically perturbed landscapes has profound implications for the interpretation of stratigraphic architecture.
Geological Society Special Publication - Tập 296 Số 1 - Trang 7-28 - 2008
Geochemical characteristics of basaltic volcanism within back-arc basins Summary
Back-arc basins are formed by extensional processes similar to those occurring at mid-ocean ridges. However, whereas the magmas erupted along the major ocean ridges are predominantly LIL element-, Ta- and Nb-depleted N-type MORB, many back-arc basins are floored by basalts transitional between N-type MORB and island arc or even calc-alkaline basalts (viz. enrichment of LIL elements (K, Rb, Ba, Th) relative to HFS elements (Nb, Ta, Zr, Hf, Ti)). On a broad scale, it is possible to relate basalt composition, tectonic setting of the basin, and maturity of the adjacent subduction zone. Thus, the Parece Vela Basin, formed during the earliest stages of the Mariana subduction system, is floored by basalts indistinguishable from N-type MORB, whereas the later Mariana Trough is erupting N-type MORB
and
basalts with calc-alkaline characteristics, commonly in close spatial proximity. The calc-alkaline component is best developed in narrow, ensialic basins such as Bransfield Strait, where the extension is adjacent to mature, continent-based magmatic arcs. This range of compositions, from N-type MORB to calc-alkaline basalt, can be satisfactorily explained only by invoking chemical variations in the composition of the mantle material supplying the back-arc basin crust. Two major processes may be suggested: (i) selective contamination of the mantle wedge by LIL-enriched hydrous fluids, perhaps together with sediments, derived from the descending, dehydrating oceanic lithosphere; and (ii) repeated melt (and incompatible element) extraction during basalt genesis. The former process will enrich the mantle source of back-arc basalts with LIL elements; the latter will deplete the source in all incompatible elements, but the net effect of both processes is to increase the LIL/HFS element ratio of the source regions. Consequently, as the subduction zone matures, the LIL/HFS element ratio of successive back-arc basalts will be expected to increase, from initial N-type MORB ‘background’ values, to ratios more typical of island-arc basalts. The model has implications for mantle dynamics in back-arc regions, because transfer of material from the subducted slab may destabilize the overlying mantle, potentially leading to diapiric uprise when tectonic conditions permit extension.
Geological Society Special Publication - Tập 16 Số 1 - Trang 59-76 - 1984
Stable isotope abundances in calcretes Summary
About 300 published and unpublished measurements of carbon and oxygen stable isotope ratios of calcretes are compared in an attempt to define general trends. The δ
13
C values of the carbonates range from − 12 to + 4‰ PDB. On a global scale the main factor responsible for the average
13
C content of calcrete in an area appears to be the
13
C content of the dominant plants in the region. No correlation could be found between
13
C contents and radiocarbon age. The carbonate δ
18
O values range from − 9 to +3‰ PDB. On a global scale, the factors discussed as possibly responsible for the
18
O content of a calcrete are the
18
O content of the local rainfall, temperature, and the extent to which the water underwent free surface evaporation before or during calcrete formation. Little or no correlation was found between
13
C or
18
O contents and rainfall, temperature, or evaporation rate on a world basis. Some relation between the
18
O content of calcrete and rain on a regional basis was confirmed. The range of
13
C and
18
O values suggest a diversity of origins and environments of formation.
Geological Society Special Publication - Tập 11 Số 1 - Trang 221-233 - 1983
The geochemistry and provenance of Ordovician turbidites in the Argentine Puna Abstract
Petrographical and major, trace and rare earth element data are combined in a provenance study of Early and Mid-Ordovician volcaniclastic turbidites in the Puna of NW Argentina. The tectonic setting of the depositional basin béhind a west-facing volcanic arc is well established from the literature, so that the new data may be used to test published geochemical discrimination schemes of tectonic setting of sedimentary rocks. However, there is a controversy whether volcanism took place in an island arc or a mature continentalmargin arc. The data show that the arc had a sediment source of average upper crustal composition dominated by silicic and intermediate magmatic rocks. Major elements document that subaerial weathering significantly influenced the detritus composition, whereas the HFS elements in particular retained the original source signal. The HFS and REE characteristics, as well as other parameters, compare favourably with those of Recent deep sea sands derived from continental arcs.
Geological Society Special Publication - Tập 142 Số 1 - Trang 127-142 - 1998
Geochemistry and provenance of Rhenohercynian synorogenic sandstones: implications for tectonic environment discrimination Abstract
The provenance of synorogenic greywackes from Devonian flysch successions occupying structurally similar positions at opposite ends of the Rhenohercynian zone in SW England and Germany is evaluated. Greywackes from both regions are petrographically and chemically similar, although minor differences are seen in the relative proportions of lithics; the Gramscatho group being richer in volcanic and metavolcanic clasts, whereas the Giessen group is richer in metasedimentary clasts. Absolute abundances of Ni-Cr-V and Zr-Hf-Y vary to a limited extent in each group and reflect variable mafic detritus and heavy mineral inputs respectively. Framework mode parameters and chemical data indicate the Rhenohercynian greywackes were derived mainly from a calc-alkali, acidic, dissected continental arc source, with minor MORB-like and argillaceous metasedimentary components. Upper continental crust-normalized multi-element patterns for the greywackes are characteristic of the continental arc/active margin tectonic environment. However, ‘Mid-Proterozoic’ model Nd ages for the Gramscatho greywackes suggest that an active Devonian arc source is unlikely. Instead the range of chemical and isotopic composition displayed mainly reflects mixing between acidic arc terranes of Proterozoic age and Devonian (Lizard-type) oceanic crust. Petrographic and geochemical discrimination diagrams alone cannot resolve the temporal decoupling between source and basin and may lead to an erroneous interpretation of tectonic setting.
Geological Society Special Publication - Tập 57 Số 1 - Trang 173-188 - 1991
Geochronology and metamorphic <i>P</i> - <i>T</i> - <i>X</i> evolution of the Eburnean granulite-facies metapelites of Tidjenouine (Central Hoggar, Algeria): witness of the LATEA metacratonic evolution Abstract
Central Hoggar, within the Tuareg shield to the east of the West African craton, is known for its complexity owing to the interplay of the Eburnean and Pan-African orogenies. The Tidjenouine area in the Laouni terrane belongs to the LATEA metacraton and displays spectacular examples of granulite-facies migmatitic metapelites. Here, we present a detailed petrological study coupled with
in situ
U–Pb zircon dating by laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) that allows us to constrain the relative role of the Eburnean and Pan-African orogenies and hence to constrain how the LATEA Eburnean microcontinent has been partly destabilized during the Pan-African orogeny; that is, its metacratonic evolution. These metapelites have recorded different metamorphic stages. A clockwise P–T evolution is demonstrated on the basis of textural relationships, modelling in KFMASH and FMASH systems and thermobarometry. The prograde evolution implies several melting reactions involving the breakdown of biotite and gedrite. Peak metamorphic
P–T
conditions of 860±50 °C and 7–8 kbar (M
1
) were followed by a decrease of pressure to 4.3±1 kbar and of temperature to around 700 °C, associated with the development of migmatites (M
2
). After cooling, a third thermal phase at
c
. 650 °C and 3–4 kbar (M
3
) occurred. U–Pb zircon laser ablation ICP-MS analysis allows us to date the protolith of the migmatites at 2151±8 Ma, the granulite-facies and migmatitic metamorphisms (M
1
–M
2
) at 2062±39 Ma and the medium-grade metamorphic assemblage (M
3
) at 614±11 Ma. This last event is coeval with the emplacement of large Pan-African granitic batholiths. These data show that the main metamorphic events are Eburnean in age. The Pan-African orogeny, in contrast, is associated mainly with medium-grade metamorphism but also mega-shear zones and granitic batholiths, characterized by a high temperature gradient. This can be considered as typical of a metacratonic evolution.
Geological Society Special Publication - Tập 297 Số 1 - Trang 111-146 - 2008
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