Rock Varnish Research Articles

Pelagosit - neu für Israel: Seine prinzipielle Verschiedenheit vom echten Wüstenlack

Pelagosit - neu für Israel: Seine prinzipielle Verschiedenheit vom echten Wüstenlack

REMOTE SENSING: Geoscan airborne multi-spectral scanners as exploration tools for Western Australian diamond and gold deposits

The use of remote sensing in mineral exploration has evolved from basic photo-geology to interpretation of more sophisticated satellite and airborne multi-spectral data sets. Although the mineral mapping capabilities of Geoscan airborne multi-spectral scanners have been demonstrated for well-exposed and arid terrains, the question remains as to their effectiveness in deeply weathered regimes such as Western Australia. Geoscan airborne multi-spectral data have been collected for a number of Western Australian mineral deposits and prospects. The applicability of the data to exploration for similar deposits, based upon a series of simple image-processing treatments derived according to the known reflectance spectra of associated alteration minerals, has been assessed. Spectral characteristics of ultramafic rock such as kimberlite or lamproite are recognisable at Blue Well, but not over the Aries kimberlite pipe which has a spectral response dominated by the reworking and removal of desert varnish from detritus derived from its surrounding sandstone host. Shear-zone-hosted gold mineralisation in the deeply weathered Yilgarn Block is characterised by spectral signatures related to the presence of sericite, silica and iron oxides. The same minerals and spectral variations are also apparent in Carlin-style mineralisation at Kazput Pool. In each example studied, simple image-processing treatments are used to enhance the spectral characteristics of the deposit. These signatures are dominated by the present-day surface mineralogy of the deposit and are the result of the interaction between hydrothermal and supergene alteration. In an exploration mode, a method has been successfully developed and cost-effectively applied for first-pass testing for the basic spectral criteria of each geological model, followed by more detailed spectral discrimination of key mineral assemblages and early geochemical follow-up.

Three-Dimensional, High-Resolution Light Microscopy - A New Geological Tool

The three-dimensional microscope shows promise as a new tool in optical petrography. It has the same magnifying capability as the conventional compound optical microscope, but it has better resolution and increased depth of field. It also offers the ability to view and photograph objects in three dimensions. An object is obliquely illuminated by four light sources in such a way as to produce an enhanced stereoscopic effect. Fine-scale features, such as inclusions in feldspars, microfractures in quartz grains, and micro-scale layers in desert varnish, can be seen with much greater clarity than with the conventional petrographic microscope.

Fire, climate, and the persistence of desert varnish near Dampier, Western Australia

In the semi-arid region near Dampier, discontinuous black and dark red coatings (desert varnish) have developed mainly on outcrops of granophyre. Coatings which have persisted occur on the most stable surfaces of large, rather than small, boulders. Rock coatings are absent both from fire-affected outcrop margins and around sediment patches within boulder slopes, and from the spray zone on the coastal margin. Sea level rise between about 17,000 and 6000 yr B.P. would have produced a landward-migrating spray zone within which salt weathering probably destroyed varnish before inundation. The remaining rock coatings have variable chemistry, and include compound coatings with superimpositions of thin Fe-rich, thicker SiAlMnFe layers, and an upper coat of SiAlFe, indicating fluctuatinng environments. Within major outcrops, the best-developed coatings occur on large and stable boulders beyond the reach of fire and salt spray, and often in summit or mid-slope positions. Much varnish loss can be attributed to factors other than climatic change, with coatings presently forming at a slower rate than they are being destroyed.

Mapping the Piute Mountains, California, with thermal infrared multispectral scanner (TIMS) images

Thermal infrared multispectral scanner (TIMS) data were acquired in 1990 over the Piute Mountains, California, to evaluate their usefulness for lithologic mapping in an area of metamorphosed, structurally complex, igneous and sedimentary rocks. The data were calibrated and atmospherically corrected, and emissivity variations in the form of alpha residuals were extracted from which color composite images were made. There was an excellent visual correlation between the units revealed in the color composite image and the lithologic units mapped in the eastern side of the area. It was also possible to correct, improve, and extend the recent map. For example, several areas mapped as granodioritic gneiss had TIMS alpha residual spectra consistent with mafic rocks and were subsequently mapped as amphibolite. The presence of a swarm of mafic dikes, of which only a few had previously been identified, was also revealed. The images also showed color variations in granitoid plutons that correlated with compositional variations previously determined by extensive field and geochemical work. The western Piute Mountains is an area that has proven to be especially difficult to map with field and air photographic methods, due to heterogeneous nature of certain units and various levels of desert varnish. The images permitted the extremely heterogeneous Proterozoic schists, gneisses, and granites to be easily mapped because the small‐scale compositional variability was averaged to the TIMS pixel size (12 m × 12 m square). Areal measurements from the images show that the Proterozoic rocks in the Piute Mountains consist of granitoids (∼50%), biotite gneiss (∼15%), pelitic gneiss (∼15%), quartzite (∼10%), and amphibolite (∼10%). TIMS data can dramatically increase the efficiency and the quality of geologic mapping in well‐exposed heterogeneous areas with minimal vegetation cover where detailed mapping of lithologic contacts by traditional methods is unusually difficult.

Evidence Suggesting That Methods of Rock-Varnish Cation-Ratio Dating Are neither Comparable nor Consistently Reliable

Using samples from a prehistoric quarry site in the Mojave Desert, we tested and compared the two principal methods of rock-varnish cation-ratio dating, analysis of rock-varnish scrapings (R. I. Dorn, 1983, Quaternary Research 20, 49-73 and 1989, Physical Geography 13, 559-596) and analysis of rock varnish in situ (C. D. Harrington and J. W. Whitney, 1987, Geology 15, 967-970). Because we found no consistent relationship between the varnish cation ratio (K + Ca)/Ti, and the relative age of the varnish, neither method could be used to "cation-ratio date" the underlying chert artifacts. Moreover, in situ analyses yielded systematically higher cation ratios and lower Ti abundances than bulk analyses of scraped varnish or microprobe analyses of varnish in cross section. Our results, when considered along with other recently published evidence, indicate that these two methods of varnish chemical analysis and cation-ratio dating may not be comparable, nor consistently reliable, chronometers.

NEW PERSPECTIVES ON COLLUVIAL BOULDER DEPOSITS IN THE SOUTHWESTERN GREAT BASIN, USA

Colluvial boulder fields are ubiquitous on desert hillslopes in the Basin and Range Province. Five chronometric methods indicate that colluvial boulder fields in this region have been active in the late Pleistocene and Holocene: visual appearance, radiocarbon dating of wood, macrofossil analysis, varnish radiocarbon dating, and varnish cation ratio dating. A new method is presented to constrain the ages of colluvial boulder fields, using the presence of stable rock varnish and 36Cl; this method indicates that colluvial deposits at Buckhorn Mesa, southern Nevada, must be ≤ 310 ka. Rather than interpreting colluvial deposits from the traditional desert geomorphology framework of “relict” or “active,” we suggest that these features be studied from the perspective of ongoing processes using an independently established chronometic sequence.

Thermal infrared emission spectroscopy of natural surfaces: Application to desert varnish coatings on rocks

Thermal infrared Spectroscopy has become an increasingly important tool for remote compositional analysis and geologic mapping. Most published laboratory measurements have been obtained in bidirectional reflection or transmission, whereas remotely sensed thermal infrared data are obtained by measuring the emitted energy. Section 2 of this paper describes a laboratory technique for determining calibrated emissivities of natural surfaces. Equations are developed to account for the energy reflected from the environment and to determine directly the sample temperature from measurements of hot and cold blackbody targets. Two methods for determining emissivity are developed: one in which only a hot sample measurement is made and the reflected background energy is removed by modeling, and a second in which the sample is cooled and the reflected energy is measured directly. Relative emissivity can be obtained to approximately 1% and absolute emissivities can be obtained to 2–15%, depending on the validity of the assumption that the emissivity of the sample is unity at some wavelength. The emission data agree well with the hemispherically integrated reflection data but point out problems associated with bidirectional reflectance measurements. Section 3 applies emissivity measurements to the study of layered surfaces consisting of desert varnish coatings on granite and granodiorite rock suites. Two linear models are developed: the first assumes linear mixing of independent emission from the substrate and varnish (checkerboard model); the second models transmission through an absorbing/emitting medium. Regardless of whether the varnish is or is not relatively transparent and strongly absorptive, the spectral effect of varnish increases linearly with varnish thickness, indicating that thick patches of varnish dominate the spectral properties. As a result, median varnish thickness can be determined from spectral measurements. In addition, the composition of a substrate can be estimated through varnish layers up to 40–50 μm in median thickness, and the composition of the varnish material can be determined if the substrate material is known. The varnishes studied are composed primarily of clay materials, consistent with previous studies of varnish composition.

Manganese Accumulation in Rock Varnish on a Desert Piedmont, Mojave Desert, California, and Application to Evaluating Varnish Development

Rock varnish coatings tend to become thicker, darker, and more continuous over time, leading to the use of changes in overall varnish color and the percentage of clast surfaces covered by varnish as relative-age indicators. Manganese is the most characteristic element of subaerial rock varnishes, and the progressive development of varnish coats can be quantified by measuring the amount of Mn accumulated on a given area of rock surface. Manganese oxides were dissolved off varnished clasts collected from alluvial surfaces on the Soda Mountains piedmont in the Mojave Desert, California, and the amount of Mn was measured using inductively coupled plasma emission spectroscopy. On the distal piedmont, maximum varnish development increases from a mid- to late-Holocene surface, typically containing up to 0.15 mg/cm 2 of accumulated Mn, to an early- to mid-Holocene surface with up to 0.21 mg/cm 2. However, varnish is less developed on a nearby late Pleistocene surface, suggesting extensive abrasion of clasts on the Pleistocene desert pavements or disturbance of the clasts. Varnish is better developed on the proximal piedmont, typically containing up to 0.30 mg/cm 2 of Mn, although varnish from a Pleistocene surface is again no better developed than from a nearby early- to mid-Holocene surface. These data demonstrate that rock varnish can show significant spatial variation in degree of development on geomorphic surfaces of similar age, and imply that collecting varnish as old as a geomorphic surface may be difficult on surfaces as young as late Pleistocene.

Major-element chemistry and micromorphology of Mn-oxide coatings on stream alluvium

The Mn-oxide coatings on alluvium of six small streams in the eastern U.S.A. have been studied using a scanning electron microscope equipped with an energy dispersive X-ray analyzer. Low magnification surface analyses show that Mn is generally the most abundant followed by either Fe or Si. Several percent each of Ca and Al also occur. The Mn/Fe ratios range from 0.29 to 7.69 but average 2.37. Point analyses of coating cross sections show considerable variation in Mn/Fe ratios but no evidence of regular layering or microchemical laminations such as occurs in rock varnish. Two micromorphologies predominate: roughly circular structures which have a cellular appearance and spherical structures. The cellular structures are remarkably similar to porous Mn-oxide precipitated by bacteria in cultures prepared by Mustoe, suggesting that Mn-oxidizing bacteria are important in forming stream Mn-oxide precipitates.

Alkaline volcano of Paleocene age on the Southern Guinean Margin: Mapping, petrology, 40Ar- 39Ar laser probe dating, and implications for the evolution of the Eastern Equatorial Atlantic

A segment of the Southern Guinea Plateau Margin (S.G.P.M.) was surveyed during the 1988 Equamarge II cruise of the R/V Jean Charcot. Detailed Seabeam mapping reveals a huge 3000 m high volcano which was called the Nadir seamount, culminating at 840 m below sea level, and 6 adventive cones. Volcanics dredged between 2100 and 1300 m of water depth consist of hyaloclastite breccias and subaerial alkali basalts recovered as both massive blocks and eolian pebbles with a desert varnish. The data are consistent with the growth of a volcano up to emergence and, thereafter, its dismembering and subsidence. Volcanological, petrographic and geochemical features (REE) of the volcanics present similarities with those observed (1) in early basalts from Fuerteventura (Canary Islands) and Maio (Cape Verde Islands), (2) in lamprophyric volcanism from the Sierra Leone Rise and (3) in volcanics from the Gorringe bank. 40Ar- 39Ar laser probe dating was performed on two biotite phenocrysts from a massive basalt, using step heating and spot fusion procedures. Both experiments concordantly display a plateau-age of 58.6 ± 0.3 Ma and an integrated age of 58.6 ± 0.7 Ma, respectively. The Nadir seamount provides the third evidence of Paleocene alkaline volcanism in the Eastern Atlantic after the Gorringe bank and the northern Sierra Leone Rise (Krause seamount). These volcanoes are presumed to have been controlled by Fractures Zones. The ages and compositions of the Nadir and Krause seamounts, respectively, suggest a connection between the volcanic chains of both the Sierra Leone Rise and Guinea F.Z. which may have recorded the same hotspot track.

AMS Radiocarbon and Cation-Ratio Dating of Rock Art in the Bighorn Basin of Wyoming and Montana

Samples of organic matter and rock varnish from seven rock-art sites in the Bighorn Basin of Wyoming and Montana were collected for dating purposes. Petroglyphs sampled include Dinwoody-style figures, shield-bearing warriors, and other well-known Plains rock-art motifs. Accelerator mass spectrometry (AMS) dating of 10 petroglyphs yielded dates from the Early Archaic to the Protohistoric periods. A strong numerical relation between varnish leaching and time was found for petroglyphs older than 1,000 years, permitting the derivation of a cation-leaching curve (CLC) and calibrated cation-ratio (CR) ages for 15 different petroglyphs. No clear numerical relation between varnish leaching and time was found for petroglyphs less than 1,000 years old, possibly due to historical damage or past environmental conditions. As a result, calibrated CR ages could not be derived for six petroglyphs, and they are considered to be only younger than 1,000 years. Although further research is needed to establish whether one CLC can be used for all petroglyphs in the region, these studies constitute the first numerical chronology for rock art in the Bighorn area. Results indicate the occurrence of spatially discrete, but temporally concurrent styles in the Bighorn Basin during the last 800-900 years.

Quaternary Glaciations in the Rio Mendoza Valley, Argentine Andes

In the Rio Mendoza valley, five Pleistocene drifts and one Holocene drift are distinguished by multiple relative-age criteria, including surface-rock weathering, development of rock varnish, moraine morphology, soil-profile development, and stratigraphic relationships. Several absolute ages suggest a preliminary chronology. During the oldest (Uspallata) glaciation, a system of valley glaciers flowed 110 km from the Andean drainage divide and 80 km from Cerro Aconcagua to terminate at 1850 m. Drift of this ice advance is older than a widespread tephra dated by fission-track at 360,000 ± 36,000 yr. During the Punta de Vacas advance, ice terminated at 2350 m, while during the subsequent Penitentes advance, the glacier system ended at 2500 m. A travertine layer overlying Penitentes Drift has U-series age of 24,200 ± 2000 yr B.P. The distribution of Horcones Drift, which is inferred to represent the last glacial maximum, delimits an independent ice stream that flowed 22 km down Horcones valley to 2750 m. A later readvance (Almacenes) reached 3250 m. Confluencia Drift is considered to be Neoglacial in age and extends downvalley to 3300 m. The moraine sequence is compared with those studied by Caviedes (1972) along Rio Aconcagua on the Chilean flank of the Andes.

Data techniques for Quaternary lavas

Reliable age information on Quaternary volcanic events is difficult to obtain because 14C dating generally does not extend beyond 45,000 years, and recent applications of 40Ar/39Ar methods—proven techniques for older lavas—suggest that dating events younger than approximately 100,000 years will be problematic. Because of the gap left in coverage by these traditional techniques, attention has turned to alternative chronometers, such as thermoluminescence, surface exposure dating using the build‐up of cosmogenic nuclides, U‐series disequilibria, and desert varnish. This research has received impetus and funding because of the importance of young igneous processes in evaluating volcanic hazards, related questions of seismicity, and radioactive waste disposal.These new techniques need to be calibrated against each other using identical samples, which is being done in several volcanic fields throughout the southwestern United States [e.g., Anthony and Poths, 1992]. One such area, the Zuni‐Bandera volcanic field near Grants, N. Mex., was the site of a field conference [Laughlin et al., 1993] held from April 26 to 28, to report on recent determinations of flow chronology based on 14C, 3He, 40Ar/39Ar, and U‐series techniques. An informal group to continue research on correlated samples was also organized.

Biogeochemistry of oxalate in the antarctic cryptoendolithic lichen-dominated community

Cryptoendolithic (hidden in rock) lichen-dominated microbial communities from the Ross Desert of Antarctica were shown to produce oxalate (oxalic acid). Oxalate increased mineral dissolution, which provides nutrients, creates characteristic weathering patterns, and may ultimately influence the biological residence time of the community. Oxalate was the only organic acid detectable by HPLC, and its presence was verified by GC/MS. Community photosynthetic metabolism was involved in oxalate production since rates of (14)C-oxalate production from (14)C02 were higher in light than in dark incubations. Flaking of the sandstone at the level of the lichen-dominated zone a few millimeters beneath the rock surface can be explained by dissolution of the sandstone cement, which was enhanced by Si, Fe, and Al oxalate complex formation. Added oxalate was observed to increase the solubility of Si, Fe, Al, P, and K. Oxalate's ability to form soluble trivalent metal-oxalate complexes correlated with the observed order of metal oxide depletion from the lichen-dominated zone (Mn > Fe > Al). Thermodynamic calculations predict that Fe oxalate complex formation mobilizes amorphous Fe oxides (ferrihydrite) in the lichen-dominated zone, and where oxalate is depleted, ferrihydrite should precipitate. Hematite, a more crystalline Fe oxide, should remain solid at in situ oxalate concentrations. Oxalate was not a carbon source for the indigenous heterotrophs, but the microbiota were involved in oxalate mineralization to CO2, since oxalate mineralization was reduced in poisoned incubations. Photooxidation of oxalate to C02 coupled with photoreduction of Fe(Ill) may be responsible for oxalate removal in situ, since rates of (14)C-oxalate mineralization in dark incubations were at least 50% lower than those in the light. Removal of oxalate from Si, Fe, and Al complexes should allow free dissolved Si, Fe, and Al to precipitate as amorphous silicates and metal oxides. This may explain increased siliceous crust (rock varnish or desert varnish) formation near the surface of colonized rocks were light intensity is greatest.

New surface exposure ages for petroglyphs from the Olary Province, South Australia

AbstractExposed surfaces of rock engravings in the Olary Province of South Australia develop coatings of rock varnish, amorphous silica, and oxalate‐rich crusts. Instead of relying on any single approach, we use three experimental approaches to assign minimum ages to the petroglyphs. Three methods involve dating manganiferous rock varnish formed over the petroglyphs: 14C measurements of organic matter at the varnish/weathering rind interface provides numerical ages; calibrated ages are obtained by cation‐ratio dating; and sequences of layers within varnish are used to discriminate relative ages. A different approach is radiocarbon dating of organic remains within the weathering rind entombed by either rock varnish or amorphous silica. In addition to updating our 1988 results, we report two dozen new radiocarbon ages, eight new cation‐ratio ages, and studies of varnish stratigraphy. Although the techniques we use are experimental, our data suggest that petroglyphs have been made in the Olary Province for the last 40,000 radiocarbon years, making its rock art the oldest known in the world. Broader implications of our findings for the cultural history of the Olary Province and for different approaches to dating rock art are explored.

Distribution and formation of rock varnish in southern Tunisia

AbstractField observations, chemical analysis, electron and light microscopy and radiolabelling experiments provide information on the characteristics, formation and distribution of Fe and Mn rock varnishes in southern Tunisia. Unstable rocks are not surface‐coated, but when broken they commonly reveal internal microbial solution fronts. Stable rocks are coated in Fe, and more rarely Mn, rock varnishes. Radiolabelling indicates the presence of many respiring microbes on both Fe and Mn varnishes. Microscopy shows fungi and bacteria, both on the Fe varnishes and within the rock. Only rare, microcolonial fungi occur on Mn varnishes, suggesting that the microbes are within or beneath the varnish. The abundance of microorganisms supports a biogenic origin for rock varnishes. Different microbial communities appear to be involved in Fe and Mn varnish formation. Microbes appear to live beneath the surface of unstable rocks but coat stable rocks which cannot be penetrated, apparently to avoid u.v. light and desiccation. This explains the concentration of elements that form u.v. opaque minerals and clays. The origin of these elements is external to the substrate. These results suggest that rock varnish dating and paleoenvironmental techniques can be applied in southern Tunisia.

Measured effects of desert varnish on the mid-infrared spectra of weathered rocks as an aid to TIMS imagery interpretation

The thermal infrared spectral properties (7-12 mu m) of natural rock surfaces from Silver Lake, CA, are discussed. Although the reflectance of weathered rocks is largely a function of the quartz content in rocks, the presence of desert varnish (clay coating) on rocks reduces the reflectance and spectral contrast with features unique to the rock spectra persisting if varnish is thin. Thick varnish has a spectrum with a reflectance peak near 9.6 mu m, due to clays, and resembles the spectra of clay-rich playa surfaces. Comparison of laboratory reflectance spectra for varnish and weathered rock samples with Thermal Infrared Multispectral Scanner (TIMS) emissivity spectra for Silver Lake suggests that TIMS signatures for felsic rocks are dominated by weathered rock and rock debris. In contrast, it is likely that varnish plays an important role in the TIMS signatures of mafic rocks. >

Relict colluvial boulder deposits as paleoclimatic indicators in the Yucca Mountain region, southern Nevada

Early to middle Pleistocene boulder deposits are common features on southern Nevada hillslopes. These darkly varnished, ancient colluvial deposits stand but in stark contrast to the underlying light-colored bedrock of volcanic tuffs, and they serve as minor divides between drainage channels on modern hillslopes. To demonstrate the antiquity of these stable hillslope features, six colluvial boulder deposits from Yucca Mountain, Nye County, Nevada, were dated by cation- ratio dating of rock varnish accreted on boulder surfaces. Estimated minimum ages of these boulder deposits range from 760 to 170 ka. Five additional older deposits on nearby Skull and Little Skull Mountains and Buckboard Mesa yielded cation-ratio minimum-age estimates of 1.38 Ma to 800 ka. An independent cosmogenic chlorine-36 surface exposure date was obtained on one deposit, which confirms an estimated early to middle Quaternary age. These deposits have provided the oldest age estimates for unconsolidated hillslope deposits in the southwestern United States. We suggest that the colluvial boulder deposits were produced during early and middle Pleistocene glacial/pluvial episodes and were stabilized during the transition to drier interglacial climates. By comparison to modern periglacial environments, winter minimum monthly temperatures of -3 to -5 °C were necessary to initiate freeze-thaw conditions of such vigor to physically weather relatively large volumes of large boulders from the upper hillslopes of the Yucca Mountain area. These conditions imply that early and middle Pleistocene glacial winter temperatures were at least 1 to 3 °C colder than existed during the last Pleistocene glacial episode and 7 to 9 °C colder than present. We conclude that at least several early and middle Pleistocene glacial episodes were colder, and perhaps wetter, than glacial episodes of the late Pleistocene in the southern Great Basin. Geomorphic processes necessary to form these colluvial boulder deposits are not active on modern hillslopes in the southern Great Basin. In addition, the lack of young, relatively unvarnished colluvial boulder deposits on these hillslopes suggests that boulder-forming conditions did not exist during the late Pleistocene in this region. Modern semiarid hillslope processes primarily erode colluvium during infrequent high-intensity storms. The preservation of old, thin hillslope deposits and the less-than-2-m incision by hillslope runoff adjacent to these deposits, however, indicate that extremely low denudation rates have occurred on resistant volcanic hillslopes in the southern Great Basin during Quaternary time.

Emissivity of terrestrial materials in the 8–14 μm atmospheric window

Remotely sensed infrared radiance emitted by a surface is a function both of its kinetic temperature and its spectral emissivity. Consequently, assumptions are usually made about the emissivity of earth surface materials to allow their temperatures to be determined, or vice versa. To increase the accuracy of these assumptions, the directional hemispherical spectral reflectance of a wide range of natural earth surface materials has been measured and is summarized here. These include igneous, metamorphic, and sedimentary rocks, desert varnish, soils, vegetation, water, and ice. Kirchhoff's Law can be used to predict directional spectral emissivity from these data.