Large Outcrops Research Articles

Helicopter-based laser scanning: a method for quantitative analysis of large-scale sedimentary architecture

Abstract Studies of large-scale sedimentary architecture are mainly based on the interpretation of two-dimensional photomosaics. This method cannot account for the natural rugosity of outcrop exposures, introducing errors in the measurement of geobody sizes and orientations. In the past, three-dimensional outcrop studies have relied on time-intensive fieldwork, with irregular sampling and low geometric accuracy. More recently, terrestrial laser scanning, or LiDAR (Light Detection and Ranging), has been widely applied to small-scale outcrops, but range and accessibility preclude its usage on larger-scale outcrops. Oblique helicopter-based laser scanning, however, allows the collection of tens of kilometres of outcrop sections in a relatively short time frame. In this paper, a procedure for collecting and processing such virtual outcrop data is outlined, and the application of the technique for extracting dimensions of fluvial geobodies from two large and otherwise inaccessible outcrops from Utah is presented. The results are compared to interpretations from more conventional photomosaicking of the same outcrops. Results show that the use of helicopter-based laser scanning enables geoscientists to rapidly acquire georeferenced data that can then be used for sedimentological interpretation and analysis on reservoir scales. It is concluded that helicopter-based laser scanning promotes sedimentological research and is well suited to capturing quantitative geometrical data from large outcrops.

Evaluation of representative elementary volume for a vuggy carbonate rock—Part: Porosity, permeability, and dispersivity

Abstract This work highlights variation in petrophysical properties of a macroporous vugular carbonate rock. The macroporosity consists of vugs and moldic pores with sizes ranging from mm to cm scale. Carbonate rocks exhibit multi-scale heterogeneities that make the understanding of fluid flow and recovery a challenge. Properties such as porosity and absolute permeability often vary within a volume and sampling at the representative elementary volume (REV) is needed to determine the appropriate continuum properties for reservoir characterization and prediction. A classical approach to examine the variation with rock sample size and the REV is through analysis at different scales. Measuring petrophysical properties at various scales has therefore been a part of the REV investigation for the studied material. To study the variation with sample size and the effect of heterogeneities on fluid flow, laboratory tests were conducted at different scales, ranging from cm to m scale. Totally 37 sub-samples of a large outcrop block with different sizes and shapes have been included in this work. The main objective has been to determine porosity, effective permeability and dispersivity as function of rock sample size. Porosity–permeability correlations for heterogeneous material can be inadequate and strongly scale dependant. Thus, we also investigate the variation of the porosity–permeability ratio with sample size. The experimental results show that the variance and coefficient of variation decreases with increase in sample size for all investigated properties. Based on the reduction of variation with increased rock sample size a representative volume for measurement is proposed for the studied material. The results also indicate less variation for the porosity–permeability relationship with increased sample volume. It is shown that a single measurement at traditional laboratory scale can lead to erroneous interpretation of a measured property for the vuggy carbonate material. However, multiple measurements at laboratory scale can be used to determine the representative properties.

Species richness and diversity of a terrestrial insular environment: serpentine of the Barberton Greenstone Belt, South Africa

The serpentinite outcrops of the Barberton Greenstone Belt are considered to be amongst the oldest in the world and have been exposed for over 50 million years. Of the 30 large outcrops in eastern Mpumalanga, South Africa, seven were selected for detailed comparison of species richness and diversity with those of surrounding non-serpentine vegetation. Various non-parametric species richness estimators were calculated to predict the species richness of each site and these indicated that four serpentine outcrops have higher and three have lower species richness than the surrounding non-serpentine areas. The Shannon, Simpson’s and Fisher’s alpha indices of diversity were calculated for each site on and off serpentine and compared. The diversity values calculated show similar patterns to those of the species richness estimates. Significant differences in species composition between floras on and off serpentine were estimated using Sorenson’s Index of similarity. The serpentine outcrops of the Barberton Greenstone Belt show relatively high diversity when compared to some other serpentine outcrops around the world. Key words: Diversity indices, EstimateS, Modified-Whittaker plots, serpentine, species accumulation curves, species richness estimation.

Geological relationships of basalts, andesites and sand injectites at the base of the Paraná volcanic province, Torres, Brazil

The Cretaceous interaction of the heated, giant Guarani aquifer with lavas, dikes and sills of the Paraná volcanic province is here evaluated based on the description of the large exposures in Torres, southernmost Brazil. Chalcedony silicified sandstone dikes, sills and breccias containing volcanic clasts in a sandstone matrix are common in the Paraná volcanic province. Sandstone layers mark either the contact between lava flows or the base of the vertically-jointed, massive core. In Torres, one basaltic andesite and one andesite lava flow of the low-Ti Gramado type, and basalt and basaltic andesite sills and dikes, are correlated over 10km with rocks in the Graxaim quarry. Scintillometric emission rates vary systematically with contents of K, Th and U and are characteristic of each lava or intrusive rock. Major and trace elements are also characteristic of each lava. Three large outcrops of breccias display angular, amygdaloidal volcanic clasts immersed in a silicified sandstone matrix. No evidence was found of high temperature (1150°C) interaction of the lava with the loose, erg of the Botucatu Formation, such as fluidal volcanic clasts, quenched rims or thermal recrystallization of the sand; only a thin (5mm) layer of thermally modified sand is present on top of a paleodune in Torres.In Torres, the interaction of andesite with wet sand occurred after degassing of the lava, because the breccia clasts are amygdaloidal. All evidence points to hydrothermal processes for the formation of the breccias, such as lozenge fractures filled with silicified sandstone, preserved detrital characteristics of the sandstone grains and cold (below 130°C) contact between the volcanic rock and the sand. Most significant is the timing of sand intrusion, because thin (1mm) dikes of silicified sandstone cut fractures in the rock and the zeolite-filled amygdales. The sandstone dikes, sills and breccias are interpreted as a result of the ascent of overpressured heated water and its vapor from the huge, underlying Guarani aquifer, carrying fluidized sand and continuously silicifying the intruded sand. The three breccias are sills intruded into cold lava flows. The description and correlation of the magmatic bodies and the field relationships of the silicified sandstones establish the stratigraphy of the Paraná volcanic province in Torres and elucidate the processes responsible for the injection and effusion of sand in the Cretaceous.

Crater wall outcrop analysis for targeting subsurface sampling on Mars

Bedrock outcrops were analyzed inside craters on Mars at Meridiani Planum with correlating HiRISE based remote and the Opportunity rover based in-situ images. The aim was to get information on how outcrops could be identified from above in order to help targeting and interpretation of shallow subsurface sampling attempts on Mars by rovers like the planned ExoMars.During the analysis, outcrop indicators, such as albedo differences, rectangular blocks, protruding layers and steep walls were identified inside impact craters. Using shadow length measurements and analyzing density distribution of blocks, nearly vertical walls could also be identified even in images acquired under vertical viewing condition. There is a size dependent tendency in the occurrence of outcrops inside caters. Impact craters with diameters of 5–10m may indicate a subsurface lithology that differs from that visible on the surface, while craters with diameter above 50m may indicate subsurface layering. Many useful large outcrops are present inside craters above diameter of 100m and are possible targets for detailed in-situ analysis. The depth that can be reached by the drill of ExoMars is exhumed by outcrops inside craters above 50–80m diameter.The values presented here are valid only for Terra Meridiani since craters would excavate different subsurface layers in areas with different subsurface lithologies. Although future missions with the aim of subsurface sampling will probably land not at Terra Meridiani, but these terrains could be sedimentary plains too, so the experiences outlined in this work may still be useful there. The identified outcrops may help in the planning of in-situ work and also in the interpretation of results from drill activity.

Magnetic signature of the Sicily Channel volcanism

Widespread Late Miocene to Quaternary volcanic activity is know to have occurred in the Sicily Channel continuing up to historical time. New magnetic anomaly data acquired in the Pantelleria Graben, one of the three main tectonic depressions forming the WNW-trending Sicily Channel rift system, integrated with available profiles, are used to identify and map volcanic bodies in this sector of the northern African margin. Some of these manifestations, both outcropping at the sea-floor or buried beneath a variable thickness of Plio-Quaternary sedimentary cover, have been imaged by seismic reflection profiles. Three main positive magnetic anomalies have been found: to the S–E of the Pantelleria Island, the largest emerged caldera of the Sicily Channel, along the eastern margin of the Nameless Bank, and at the north–western termination of the Linosa Graben. Only the anomaly located off the south–eastern coast of the Pantelleria Island, associated with a large outcropping body gradually buried beneath a substantially undisturbed Upper Pliocene-Quaternary sediments, aligns with the trend of the tectonic depression. 2-D geophysical models produced along seismic transects perpendicularly crossing the Pantelleria Graben have allowed to derive its deep crustal structure, and detect the presence of buried magmatic bodies which generate the anomalies. Marginal faults seem to have played a major role in focussing magma emplacement in this sector of the Sicily Channel. The other anomalies represent off-axis volcanic episodes and generally do not show evident magmatic manifestations at the sea-floor. These magnetic maxima seem to follow a NNE-SSW-trending belt extending from Linosa Island to the Nameless Bank, where pre-existing crustal anisotropies may have conditioned magma emplacement both at deep and shallow crustal levels. In general, data analysis has shown that there is a structural control on magma emplacement, with the major magmatic features located in specific locations like boundary faults and transfer zones, in a manner similar to that found along several segments of the East African Rift system.

Evidence of Precambrian sedimentation/magmatism and Cambrian metamorphism in the Bitlis Massif, SE Turkey utilising whole-rock geochemistry and U–Pb LA-ICP-MS zircon dating

The Bitlis Massif is a regional-scale, south-vergent allochthon that was finally emplaced by collision of the Eurasian and Afro-Arabian plates during Miocene time. The Bitlis Massif includes a large outcrop of Precambrian continental crust, the closest counterpart to which is the Arabian–Nubian Shield ~1000km to the south. The Massif is sub-divided into two rock associations: a pre-Middle Devonian high-grade basement and a Middle Devonian-Triassic low-grade cover. The pre-Devonian basement comprises meta-granitic plutons emplaced into high-grade metamorphic rocks, including schist, paragneiss, amphibolite and eclogite. New laser-ablation zircon ages obtained for zircons separated from a meta-granite body and its host paragneiss provide constraints on magmatism, sedimentation and metamorphism. Whole-rock geochemical data indicate that the pluton crystallised from peraluminous I-type melts from an arc-type subduction influenced source. Sm–Nd isotope systematics suggest crustal contamination. Zircon dating yielded a 206Pb/238U age of 572±4.8Ma, interpreted as the time of crystallisation. Igneous zircons exhibit metamict metamorphic domains dated at 529Ma (Early Cambrian), interpreted as the time of latest Pan-African metamorphism. Nine detrital zircon grains from host paragneiss yielded Neoproterozoic ages (992–627Ma). Combined with the crystallisation age data, this suggests that the sedimentary protolith of the paragneiss was deposited from ~627 to ~572Ma (Ediacaran). The late Neoproterozoic ages suggest the Bitlis Massif is a peri-Gondwanan terrane with a likely origin in northeast Africa where similar early Neoproterozoic (0.9–1.0Ga) ages have been reported.

Reply to the discussion by Tanner on ‘The Highland Boundary Fault and the Highland Border Complex’ Bluck (2010) Scottish Journal of Geology , 46 , 113–124

B.J. Bluck replies : Many plates were shown in the paper by Bluck (2010, figs 2,3,4,5,7,) which illustrate quite clearly the stratigraphy and direction of younging of the rocks involved. These figures, not exhaustive, were taken from outcrops which span almost the entire length of the Highland Border Complex from Cowie, north of Stonehaven to Bute in the SW. As a sedimentologist I have disregarded ‘graded bedding’ in sheared quartzose sandstones of indeterminate origin and indifferently exposed, as a reliable way-up criterion (for example in the North Esk and elsewhere). The pattern of stratigraphical evidence is shown in photographs in Bluck (2010). 1. This is presented in Bluck (2010) fig. 4 where clasts of the rocks to the SE, which are made of tuffs, pillow lavas and cherts, occur in a conglomerate immediately overlying them to the NW. Clasts of chert and lava are also found in the basal parts of the quartz-arenite sequence immediately to the NW. 2. In the Prosen Water and Carity Burn clasts of serpentinite, many of which are rounded, occur immediately to the NW of a large outcrop of serpentinite to be followed by the Margie Series. 3. At Aberfoyle Jehu & Campbell (1917, …

Rock Outcrops Harbor Native Perennials in Type-Converted Coastal Scrub

Type-conversion of coastal scrub to exotic annual grassland has been extensive in certain parts of southern California, especially in drier inland locales. Field observations suggest that rock outcrops harbor native perennials associated with coastal scrub vegetation after conversion to exotic annual grassland has occurred. Surveys were conducted to test this observation. In addition, measures of fossorial mammal disturbance, soil depth, exotic annual plant abundance, and soil moisture were collected and used to infer potential mechanisms that may influence patterns of native and exotic vegetation. Results showed that larger outcrops are associated with more native perennial species than smaller outcrops, consistent with predictions based on species-area relationships. In addition, the edges of rock outcrops were shown to have greater native perennial species richness than the surrounding exotic annual grassland matrix. Higher soil moisture, lower soil depth, and less disturbance by Botta's pocket gopher (Thomomys bottae) were also found at the edge of rock outcrops compared to the surrounding grassland matrix. Furthermore, soil depth was positively correlated with pocket gopher disturbance and with exotic annual grass cover. Rock outcrops serve as refuge habitat for native coastal scrub perennials in areas where exotic annual grasses have become dominant. Rock outcrops may become increasingly important for conservation efforts if remaining coastal scrub vegetation continues to deteriorate.

Structure and kinematics of an outcrop-scale fold-cored triangle zone

Triangle zones are widespread structural elements that link fold and thrust belts with their foreland basins. We present a structural analysis of an outcrop-scale (~6 m [20 ft] wide) triangle zone that is exposed in the siliciclastic Carboniferous strata of the Harz Mountains in northern Germany. The geometry of the triangle zone is critically compared with larger outcrop and seismic-scale structures. The external form of the triangle zone is the same as that proposed for larger examples, with two bounding detachments that dip in opposing directions. However, the interior of the triangle zone is characterized by tight to isoclinal folds. We demonstrate how the triangle zone probably evolved from a fault-bend fold, which accreted further folds behind it. This is an alternative fold-based interpretation for a structure that is commonly modeled as a duplex stack. We present the resulting consequences for seismic interpretation and hydrocarbon reservoir evaluation.

Delta-front hyperpycnal bed geometry and implications for reservoir modeling: Cretaceous Panther Tongue delta, Book Cliffs, Utah

The Cretaceous Panther Tongue has an upward-coarsening and -thickening pattern and is well exposed in extensive large outcrops in the Book Cliffs area, west-central Utah. The deposits have been interpreted as having formed in a fluvial-dominated river delta environment that generated highly sediment-concentrated sustained (turbidity) flows during flooding, producing hyperpycnal-flow deposits on the delta front despite some resemblance to deep-water turbidites. The facies associations indicate terminal distributary channel, channel mouth, and proximal delta-front and distal delta-front depositional environments. The measured paleocurrents indicate a south-southwest transport of the sediments. The thickness of the hyperpycnal sandstone beds ranges from centimeters to meters. Sandstones are characteristically parallel laminated, sometimes structureless or rarely display inclined strata of cut-and-fill type. The sandstone hyperpycnal beds dominate the delta-front clinoforms and dip southward, consistent with the other paleocurrent indicators. Individual sandstone beds in the clinoforms have dips that range from 0.1 on the distal delta front (lower part of the outcrops) to 3 in the proximal parts (upper part of the outcrops). The hyperpycnal beds can be traced from a proximal mouth-bar environment to the distal delta front over a distance of hundreds of meters. As individual beds extend from mouth bar to distal delta-front environments, they become systematically finer grained and thinner. Over short distances (hundreds of meters), the beds thin with rates ranging between 0.0001 (i.e., dm/km) to 0.02 (i.e., tens of meters per kilometer). The sandstone beds thin to a greater degree in a dip direction than along strike, indicating a relatively strike-elongate (flow-normal) geometry of the hyperpycnal flows and of the delta lobes. The wider than longer geometry of the delta-front beds requires that reservoir development be more focused upon the downdip facies changes (heterogeneities) than the lateral (along strike) heterogeneities.

Using seafloor heat flow as a tracer to map subseafloor fluid flow in the ocean crust

Geofluids (2010) 10, 142–160AbstractWe describe how seafloor heat flow is determined, review current understanding of advective heat loss from oceanic lithosphere, and present results from three field areas to illustrate how heat flow measurements are used (along with complementary data) to resolve fluid flow rates and patterns. Conductive heat flow through much of the seafloor is lower than predicted by lithospheric cooling models as a result of hydrothermal circulation; this discrepancy is the basis for global estimates of the magnitude of advective cooling of oceanic lithosphere. Hydrothermal circulation also redistributes heat within the ocean crustal aquifer, leading to local variability. Heat flow studies in Middle Valley, a sedimented spreading center in the northeastern Pacific Ocean, indicate multiple scales of fluid circulation, delineate conditions at the top of a hydrothermal reservoir, and show the influence of primary and secondary convection. Heat flow studies on the eastern flank of the Juan de Fuca Ridge document the thermal influence of isolated basement outcrops surrounded by thick, low‐permeability sediments. Warm hydrothermal fluids seep from the crust through a small volcanic edifice, having flowed into the crust through a larger outcrop ∼50 km to the south. These fluids generate a local geothermal anomaly, but have little influence on regional heat loss from the plate. In contrast, heat flow surveys on part of the Cocos Plate, eastern Equatorial Pacific Ocean, indicate that regional conductive heat loss is just 10–40% of predictions from lithospheric cooling models. Basement outcrops in this area focus massive discharge of cool, hydrothermal fluid and associated heat (4–80 × 103 l s‐1 of fluid, 0.8–1.4 GW of heat). Seafloor heat flow studies will be increasingly important in coming years for understanding marine hydrogeologic regimes and the role of fluids in a variety of Earth processes and settings.

Wettability Impacts on Oil Displacement in Large Fractured Carbonate Blocks

Two-dimensional imaging of water and oil saturations during waterfloods in fractured carbonate rock models was obtained using nuclear tracer imaging and magnetic resonance imaging. Large outcrop chalk and limestone blocks were aged in crude oil to obtain wetting conditions from strongly water-wet to weakly oil-wet. The change in the oil recovery mechanism as the wettability shifted was investigated with and without the presence of fractures. Visualization of local, in situ fluid saturations during waterfloods improved the interpretation of the displacement process and oil recovery mechanisms. Experimental results demonstrate how fractures determine the displacement pattern differently depending on the matrix wettability conditions during waterfloods. At strongly water-wet conditions, the fractures had a minor impact on the ultimate recovery but significantly changed the progression of the water front compared to the unfractured case. At less water-wet or oil-wet conditions, capillary imbibition of water from the fracture to the matrix was reduced and fractures had a major impact on the ultimate recovery and water breakthrough time.

Magnetic susceptibility mapping of felsic magmatic lithounits in the central part of Bundelkhand Massif, central India

Late Archaean to Palaeoproterozoic felsic magmatic lithounits exposed in the central part of the Bundelkhand massif have been mapped and their redox series (magnetite vs ilmenite series) evaluated based on magnetic susceptibility (MS) data. The central part of Bundelkhand massif comprises of multiple felsic magmatic pulses (∼2600–2200 Ma), commonly represented by coarse grained granite (CGG-grey granite, CPG-pink granite), medium grained pink granite (MPG), fine grained pink granite (FPG), grey and pink rhyolites and granite porphyry (GP). However, the pink colour of these felsic rocks is the result of hydrothermal fluid-flushing leading to potassic alteration of grey granites. MS values of CGG vary from 0.058 to 14.75×10−3 SI with an average of 6.35×10−3 SI, which mostly represent oxidized type, magnetite series (73%) granites involving infracrustal (igneous) source materials. CPG (av. MS=3.95×10−3 SI) is indeed a pink variety of CGG, the original oxidizing nature of which must have been similar to the bulk of CGG, but has been moderately to strongly reduced because of distinctly more porphyritic nature together with partial assimilation of metapelitic (supracrustal) materials, surmicaceous enclaves, carbonaceous material included in the source materials, and to some extent, induced by hydrothermal and later deformational processes. MPG (av. MS= 1.15×10−3 SI) as lensoidal stock-like bodies intrudes the CPG and represent both magnetite series (18%) and ilmenite series (82%) granites, which are probably formed by heterogeneous (mixed) source rocks. GP (av. MS=6.26×10−3 SI) occur as dykes (mostly trending NE-SW) intrudes the MPG, CPG and migmatites and bears the nature similar to oxidized type, magnetite series granite. FPG (av. MS= 0.666×10−3 SI) trending NE-SW occur as lensoid bodies including a large outcrop, is intrusive into both CPG and MPG, and is moderately to very strongly reduced type, ilmenite series granites, which may be derived by the melting of metapelitic crustal sources. FPG hosting microgranular (mafic magmatic) enclaves commonly exhibit high MS values (7.31–10.22×10−3 SI), which appear induced by the mixing and mingling of interacting felsic and mafic magmas prevailed in an open system. Grey (av. MS=10.30×10−3 SI) and pink (av. MS=6.72×10−3 SI) rhyolites represent oxidized type, magnetite series granites, which may have been derived from infracrustal (magmatic) protoliths. Granite series evaluation of felsic magmatic rocks of central part of Bundelkhand massif strongly suggests their varied redox conditions (differential oxygen fugacity) mostly intrinsic to magma source regions and partially modified by hydrothermal and tectonic processes acting upon them.

The geology and mineralisation of the Carina Iron Deposit, central Yilgarn Craton, Western Australia

The Carina iron ore deposit is 105 km northeast of Southern Cross, WA. The area designated the Yilgarn Iron Ore Project (YIOP) comprises five prospects along an east–southeast trending greenstone belt that extends from the Helena-Aurora Ranges in the northwest to the Yendilberrin Hills in the southeast, over a strike length of approximately 40 km. The YIOP project encompasses the previously identified iron deposits of Bungalbin, Bungalbin East and Mt Walton. The Carina deposit was discovered in May 2007. The Carina deposit is located within and parallel to the structurally complex northwest trending Mount Dimer Shear Zone. The volcano-sedimentary Archaean greenstone sequence consists of ultramafic and mafic lavas and flows with interbedded sedimentary rocks dominated by chert and banded iron formation (BIF). Supergene enrichment processes formed a goethite ore body replacement of BIF sequences within a doubly plunging synclinal fold structure. Mineralised outcrops dominated by goethite form a low ridge 1500 m long, up to 90 m wide and drilled to over 200 m in depth. In the vicinity of the Carina deposit, iron occurrences have been identified over at least a 12 km strike length. The current exploration in the area began in 2006. Target generation in the area used existing state geological survey maps and regional magnetic survey data to focus on BIF occurrences associated with major structures. Early field investigations resulted in the identification of a large outcrop of predominantly goethite mineralisation at Carina. Detailed geological and structural mapping was completed, followed by close-spaced reverse circulation and diamond drilling. The mineralisation is predominantly goethite with specular and coarse platy hematite localised along structures that cut the goethite mineralisation. Mineralised zones are moderately vuggy and larger cavities occur in proximity to fault zones that transect the ore body. There is a capping by a thin veneer of canga or ferricrete up to 9 m thick. By July 2010, 184 reverse circulation and 11 diamond drill holes (seven surface holes, four tails) were completed. The current JORCcompliant resource estimate is based on assays from 170 RC and 11 diamond drill holes and using a 55% Fe cut-off is quoted as an Inferred Resource of 26·7 Mt at 58·9% Fe, 1·2% Al2 O3, 3·2% SiO2 , 0.11% P, 0·08% S, and 9·5 % LOI. Mining is planned to commence in early July 2011.

Comparison of three fracture sampling methods for layered rocks

Three methods of fracture data collection are tested against each other in layered dolomitic rocks to evaluate the effectiveness of each method in sampling fracture properties. The methods tested are the single scanline method (SSM), selection method (SM), and multiple scanline method (MSM). Finite element techniques were first used to build a base model with the exact locations, sizes and orientations of each fracture observed in the natural fracture network. Then, a second set of models were stochastically generated using statistics from each sampling technique. For each network, the overall fracture intensity was used to assess the effectiveness of each sampling technique in capturing the real fracture properties. Fracture network permeability was also calculated for each of two directions to evaluate the transmissive properties of the networks. Although all three methods produced good matches of relative intensity and permeability between natural and synthetic fractures, the results reveal that a well-placed scanline performed the best at recreating natural fractures. However, the results from one variation of the SSM were only slightly better than the results from both versions of the SM. In general, the SSM provides the best results but possibly at heavy costs in time and labor, whereas the SM gives comparable results with less expenditure of energy and time. Thus, the SM is an adequate technique and recommended for use at large outcrops or where time, access or budget constraints are a concern.

Geological position and isotopic age of intrusive charnockitic rocks in the Jennings Promontory (Eastern Wall of the Amery Glacier, East Antarctica)

Intrusive hypersthene granitoids (charnockitic rocks) are widespread in the mountainous system bordering the Lambert Glacier with the world’s largest outlet and Amery Ice Shelf continuing in East Antarctica. In the eastern margin of the Amery Ice Shelf, they constitute large bedrock outcrops in the Jennings Promontory (approximately 6 km 2 ) and Branstetter Rocks (approximately 0.7 km 2 ) and several outcrops approximately 9.5 km 2 in total in Gillock Island (Fig. 1). These outcrops of charnockitic rocks most likely characterize a relatively large massif, which extends in the northeasterly direction and occupies an area of at least 140 km 2 in size. This massif intrudes metamorphic granulitic rocks represented by the Reinbolt orthogneiss and Larsemann paragneiss groups (Fig. 1). According to [1], the protolith of the Reinbolt Group is 1174‐1019 Ma old, while the sedimentary Larsemann Group accumulated 932‐916 Ma ago. Granulitic metamorphism superimposed on igneous and sedimentary rocks of these groups occurred in the period of 533 to 529 Ma ago [2]. The charnockites of the Jennings Promontory are considered now as age analogues of similar rocks in the Loewe Massif (western margin of the Amery Ice Shelf), the crystallization age of which is estimated by the SHRIMP method to be 980 ± 21 Ma [2]. In addition, some researchers [3] correlate these charnockitic rocks with their metamorphosed porphyroblasrtic counterparts from the Reinbolt Hills dated by the U‐Pb method on zircons back to approximately 896 Ma [4]. These views are, however, inconsistent with the data on the age of granulitic metamorphism of the Reinbolt and

The impacts of past cultivation on the reptiles in a South African grassland

Habitat transfonnation is the primary anthropogenic threat to global biodiversity. Fragmentation of reptile populations following habitat transfonnation within a landscape can lead to the extirpation of species. We investigated the effects of land-use on the species richness and abundance of reptile assemblages in three habitat types (two natural and one modified) in the grasslands of Gauteng, South Africa. Using trap arrays, we surveyed reptiles in primary grassland with little or no rock cover, primary grassland with large quartzite outcrops and scattered rocks, and secondary grasslands that were historically ploughed and cropped. We measured vegetation height and vegetation cover at these same localities. We caught significantly fewer reptile species in the historically cultivated sites than in either of the two natural habitat types. Differences in the reptile assemblage of each habitat type were not explained by either the spatial location or the vegetation structure of our trap sites but were well explained by the sites' habitat type. Estimates of total species richness indicated that we were able to adequately sample the reptile assemblages in the three habitat types, further supporting our observation of reduced species richness in the secondary grasslands. We infer that habitat transfonnation associated with cultivation e.g., rock removal, has had a detectable, negative impact on the species richness and composition of the local reptile assemblages. We recommend that land-use planning in Gauteng emphasise the need for areas of inter-connected, untransfonned habitat in order to mitigate the negative impacts of habitat transfonnation on the local reptile diversity.

The Siegenian delta: land-sea transitions at the northern margin of the Rhenohercynian Basin

Abstract Analysis, interpretation and reconstruction of different depositional palaeoenvironments in the land–sea transition zone of the huge Lower Devonian siliciclastic delta system at the northern margin of the Rhenohercynian Basin (Central European Variscides, Germany) are the main subjects of this paper. Seven representative columnar sections have been taken at larger outcrops in the Mid-Rhine region. They furnished the main criteria for this analysis. The understanding and interpretation of the different depositional palaeoenvironments have been supported by regional geological lithological and biostratigraphical data. The sedimentary processes of distal meandering fluvial systems that gradually passed into fluvial-dominated deltaic systems of the elongate bird-foot type have been recognized. Lower delta slope to delta-foot plain conditions have been detected in a subtidal environment probably down to 100 m of former water depth. Thus, the Siegenian delta must have had a low inclined but wide slope down to the basin floor. Palaeoclimatic conditions at a latitude of about 10 to 30° south, in the basinal as well as in the source area of the Old Red Continent in the north, controlled the input of siliciclastic detritus as well as the depositional processes in the basin. Moreover, steady subsidence and an intermittent sea level played a prominent role during sedimentation. They held this system in balance for more than seven million years.

Full-Resolution 3D Radar Stratigraphy of Complex Oolitic Sedimentary Architecture: Miami Limestone, Florida, U.S.A.

Abstract At the field scale (m's to 100's m) it is often difficult to define the true three-dimensional form of depositional units or facies within sediments or sedimentary rocks, due to inherent spatial bias in traditional data sources such as outcrops and cores. Full-resolution 3D ground-penetrating radar (GPR) surveying has the potential to overcome this problem. Southwest of Miami, paleotopography on the upper Pleistocene Miami Limestone Formation indicates the former presence of a large oolitic barrier bar, backed by extensive ooid shoals. Data from three closely spaced cores and two large outcrops on the barrier bar allow initial subdivision of the Miami Limestone in to a lower mottled oolitic facies and an upper bedded oolitic facies. However, detailed correlation between the cores and outcrops is problematic due to spatial aliasing. A 250 MHz full-resolution 3D GPR data set was collected over an area of 45 m × 24 m that encompassed the location of the three cores. Depth of radar wave penetration was around 6 m, and both horizontal and vertical resolution was approximately 0.1 m for the 6500 m3 rock volume, such that the upper bedded oolitic facies was imaged without spatial aliasing. Radar surfaces and intervening radar facies define two main radar package types: low-angle sheet and sigmoidal. The low-angle sheet radar packages are interpreted as oolitic shoal deposits consisting of tidal bars covered by ebb-orientated, straight-crested sand waves. The overlying sigmoidal radar packages record barrier-bar formation through alongshore progradation. Comparable sedimentary successions from both Pleistocene and modern deposits in the Bahamas suggest that the three evolutionary stages recorded for the Miami Limestone are typical of many transgressive ooid shoal complexes.