The Hawaiian Islands are an ideal location to study basaltic sand provenance in that they are predominantly composed of basaltic flows and pyroclastic deposits that are exposed in arid to humid climates on progressively older islands. The major components of Hawaiian beach sands are calcareous bioclasts, volcanic lithic fragments, and monomineralic grains of dense minerals and plagioclase. A small to moderate percentage of volcanic lithic fragments are altered to iron oxides and/or clay minerals. The relative proportions of bioclastic, unaltered, and altered volcanic grains in these samples are highly variable, but are generally a function of island evolution, reflecting the degree of reef development and the intensity and duration of weathering and erosion. Hawaiian volcanic lithic fractions consist of brown and black glassy volcanic fragments exhibiting vitric, microlitic, and lathwork textures and “other” holocrystalline volcanic lithic fragments. The proportions of these volcanic lithic types do not appear to vary systematically for sands with tholeiitic versus alkalic basalt provenance. Although these basaltic sands compositionally overlap sands from undissected magmatic arcs on traditional ternary plots (e.g., QFL, QmPK, LmLvLs), they can be compositionally and texturally distinguished from sands derived from island arc volcanic sources. In general, Hawaiian basaltic sands contain more olivine/pyroxene grains, fragments of tachylitic black glass, holocrystalline volcanic lithic fragments, and volcanic grains exhibiting lathwork textures.

Eocene-Oligocene strike-slip faulting along the Denali fault system in Yukon Territory produced several local strike-slip basins that were filled by coarse-grained alluvial, fluvial, and lacustrine sediment referred to as the Amphitheatre Formation. The Amphitheatre Formation provides an excellent opportunity to study in detail the composition of siliciclastic sediment in a major, accretionary strike-slip orogen. Petrographic analyses of sandstones and clast counts of conglomerates reveal a lithologically diverse provenance. On standard QFL and QmFLt diagrams, sandstones from the Burwash and Bates Lake basins are arkosic, overlapping the uplifted basement and dissected magmatic arc provenance fields of Dickinson and Suczek (1979). Conglomerates were derived from volcanic, plutonic, and medium- to low-grade metasedimentary rocks that currently are found in several accreted terranes associated with the Denali fault system. Wrangellia and the Alexander terrane provided voluminous volcanic, metavolcanic (greenstone), sedimentary, and low-grade metasedimentary material. The Yukon Crystalline terrane provided medium-grade (gneissic and schistose) material and plutonic material, and the Gravina-Nutzotin belt provided pelitic and metasedimentary material. These data support the contention that the Amphitheatre Formation was derived mainly from local, high-relief sources associated with major uplift in the eastern St. Elias Mountains, as well as the southern Yukon Crystalline terrane. The Eocene-Oligocene climatic event apparently had no significant effect on Amphitheatre sandstone composition. This is attributed to the proximal character of the sandstones studied; steep local relief kept soil residence times to a minimum in spite of climatic change. In addition, the persistence of coal- and stream-dominated alluvial fan facies throughout the entire stratigraphic section indicates that climate remained humid in southwestern Yukon Territory, possibly owing to local orographic effects. It is proposed that, in general, climate should not have a major impact on detrital sand composition in large strike-slip orogens, where sediment accumulates within the orogen itself and is subject neither to long distances of transport nor to long-term residence in lowland soil profiles where most compositional modification is expected.

Sandstone composition is a function of four complex and interrelated variables: provenance (itself a function of source rock, relief, and climate), transportation effects, depositional environment, and diagenesis. Documentation of source rock composition and comparison to derivative sandstone composition not only allow evaluation of sandstone provenance changes through time, they also provide evaluation of the relative importance of the variables controlling sandstone composition. Source rock composition is best determined by Gazzi-Dickinson point counts of the source rocks themselves, thus allowing direct comparisons between source rock and sandstone composition and taking into account source rock texture. This study uses this technique combined with study of sandstone composition to pursue two main objectives: to investigate the relationship between tectonic history and sandstone composition, and to document sandstone compositions of the Pennsylvanian-Permian of north-central New Mexico. Upper Paleozoic strata of north-central New Mexico were derived from Precambrian crystalline rocks and were associated with the Ancestral Rocky Mountain orogeny. They display subtle changes in detrital modes with age that can be correlated with the area’s Ancestral Rocky Mountain tectonic history. Periods of high tectonic activity resulted in deposition of sandstones with compositions closest to those of the Precambrian source rocks, so high tectonic activity allows source rock control to overwhelm other factors such as climate. During periods of low tectonic activity, other factors such as climate or transport/depositional environment were more important.

The post-Cambrian and pre-Upper Devonian Shoo Fly Complex represents the remnants of an ancient subduction system. The Quartz Hill chert, a thrust-fault-bounded, chlorite-grade greenschist facies unit, is included in the Shoo Fly Complex, and consists of about 25 m of interstratified radiolarite and argillite. Rare earth element (REE) data derived from 15 samples indicate that the Quartz Hill chert contains two chemically distinct groups of rock. Six Group 1 specimens display relatively flat REE/PAAS (post-Archean average Australian shale) distribution patterns, no Ce anomaly, and a variable positive Eu anomaly. In contrast, nine Group 2 specimens exhibit no Ce anomalies, a variable positive Eu anomaly, and display REE/PAAS values that increase from La to Eu, and then decrease from Eu to Lu. The REE, Th, and Sc characteristics of Group 1 samples are like those in Cretaceous, Tertiary, and Quaternary marine sediments containing particles derived from magmatic arcs. In contrast, the REE, Th, and Sc characteristics of Group 2 specimens are suggestive of a mixture of magmatic arc material and alkaline basaltic particulate matter derived from a seamount or ocean island. Data presented here are consistent with the results from previous petrological and geochemical studies of rocks in the Shoo Fly Complex, and indicate that the Quartz Hill chert was deposited on the margin of an oceanic plate adjacent to a magmatic arc and a seamount(s) or ocean island(s). Thus, the data here and in the literature suggest that the REE, Th, and Sc characteristics of chert/argillite sequences deposited in or adjacent to active subduction systems are controlled primarily by source rocks in adjacent magmatic arcs, and in seamounts or ocean islands located within subducting plates.

Measurements of Nd, Sr, and O isotopes were made on whole-rock Great Valley sandstones in order to determine the extent to which isotopes preserve the composition and weathering history of tectonically active sources. Previous work has shown that the Nd isotopic composition of sedimentary rocks provides an accurate record of the source and can be used to evaluate the effect of weathering and diagenesis on Sr-O isotope systematics. Variations in the whole-rock Nd, Sr, and O isotopic compositions are similar within and between petrofacies; this suggests that the isotopic composition is controlled by provenance. The Nd-Sr isotopic compositions are sensitive to the large west-east variation in crust formation ages of the igneous and metasedimentary components. The sandstones decrease from +7 to −5 in ε Nd and increase from 0.7045 to 0.7073 in 87 Sr/ 86 Sr with decreasing stratigraphic age. These values nearly encompass the range observed in the plutonic rocks and suggest that the Nd-Sr isotopic composition of the arc is preserved in the sandstones. In contrast, δ 18 O decreases from +19 to +9 with decreasing stratigraphic age; these values are significantly higher than δ 18 O values in the volcanic/plutonic arc source. Isotopic and petrographic variations are correlated; with increasing proportions of sedimentary and metasedimentary lithic fragments, values of ε Nd decrease, and values of 87 Sr/ 86 Sr and δ 18 O increase. The sedimentary and metasedimentary components decrease the whole-rock value by two to four ε Nd units and by one to 10‰, depending on the fraction and lithology of the recycled component. The metasedimentary component has a significantly lower Sr concentration than the igneous component and therefore has a smaller effect on the whole-rock 87 Sr/ 86 Sr value. Subtraction of the sedimentary-metasedimentary component from the whole-rock sandstone compositions yields igneous isotopic compositions that are directly comparable to the composition of the arc source. Calculated igneous compositions of the sandstones decrease in ε Nd from +7 to −5 with decreasing stratigraphic age and indicate that sediment sources were located at the eastward-migrating volcanic front. However, the calculated igneous components are +1 to +7‰ higher in δ 18 O than the igneous source. The high δ 18 O values are a measure of alteration of the source; increasing alteration in δ 18 O is correlated with increasing values of CIA. This suggests that high mechanical erosion rates of tectonically active sources do not preclude significant chemical weathering.

The rock fragment content (measured by log of polycrystalline/monocrystalline sand grains) of sand derived from five nearly identical granodiorite plutons, located in strongly contrasting climatic zones of the United States and Mexico, shows a much stronger correlation with source rock texture than with any environmental factor or combination of factors. These factors include mean annual temperature (range, −4° to 28°C), mean annual precipitation range (range, 201 to 1,566 mm), mean annual surplus of precipitation over evapotranspiration (range, 0 to 1,138 mm), average drainage slope (range, 0.29 to 0.61), or relief ratio (range, 0.17 to 0.70). Source rock texture is measured by intercrystalline area/volume of source rock and has a range of 7.99 to 25.08 mm −1 . Stepwise regression analysis indicates that, in coarse sand, 71% of variation in rock fragment content is accounted for by variations in source rock texture versus 16% accounted for by variations in temperature. In medium sand, 96% of the variation in rock fragment content can be attributed to texture versus 1% for slope and <1% for temperature. In fine sand, 92% of the variation in rock fragment content stems from texture versus 3% from temperature. The same analysis does not document a statistically significant relationship between precipitation and rock fragment abundance. Many previous studies that appear to document a relationship between rock fragment abundance and precipitation in modern and ancient sand have not accounted for variations in source rock lithology that are more plausible explanations for observed variations in rock fragment abundance in derived sand.

Orographic precipitation on the southern flank of the southeastern Koolau Mountains produces a pronounced precipitation gradient. The corresponding gradient in the intensity of the chemical weathering environment provides an opportunity to address the effects of varying chemical weathering intensity on the composition of clay-size weathering products in soils developed on basalt. In addition, little-modified remnants of the constructional surface of the Koolau Volcano, isolated by stream dissection, remain as facets on the southern ends of the parallel ridges of the study area. By comparing clay mineralogy of soils developed on these older geomorphic surfaces with those developed on the younger sharp-crested ridges and steep side slopes, the effects of weathering duration on clay mineralogy can also be addressed. Soil clays in this part of the Koolau Mountains are mineralogically complex; principal phases include smectite, kaolinite, and halloysite, but pure end member phases are uncommon. Rather, most phases contain some amount of mixed layering. Smectite may contain small (<5%) amounts of randomly interstratified halloysite. Similarly, kaolinite commonly contains a small proportion of halloysite interlayers. A complex halloysitic phase shows evidence of interstratification with both smectite and kaolinite. Nonphyllosilicates found in the clay fraction include gibbsite, goethite, rare quartz, and perhaps cristobalite. The gradient in precipitation is reflected in soil clay mineralogy by varying proportions of dominantly smectitic, kaolinitic, and halloysitic phases. In regions of relatively low precipitation (<2,000 mm/yr), soils are dominated by the smectitic and halloysitic phases. With increased precipitation (as much as ∼4,000 mm/yr), kaolinitic and halloysitic phases become the dominant clay minerals, and goethite and gibbsite become increasingly abundant. Older soils developed on geomorphic surfaces representing the original constructional surface of Koolau Volcano are markedly more leached than those from younger landscapes in the same precipitation regime. Although smectite may be present, kaolinite is the dominant phase, and accumulations of Fe and Ti occur in the uppermost soil levels. Enrichment of Zr and Ti in these soils, as compared to concentrations in the original basaltic parent material, indicates that as much as 75% of the parent material has been lost. Thus weathering duration may affect soil clay composition in the same way as weathering intensity. Because smectite and halloysite are expandable clay minerals, their presence in soils may decrease slope stability and influence the nature of slope processes. Soil avalanches occur on steep slopes throughout the study area, whereas slow-moving landslides appear to be restricted to gentler slopes in drier parts of the study area where smectite is abundant. The clay mineralogy of soils thus appears to influence the nature of slope processes in the southeastern Koolau Mountains.

Heavy mineral assemblages in rivers in the Apure River drainage basin of Venezuela and Colombia closely reflect the nature of the source regions, which lie in the Andean orogenic terranes to the west and northwest. The Caribbean Mountains, largely composed of greenschist-facies pelites, phyllites, carbonates, and metavolcanics, supply assemblages dominated by epidote and calcic amphibole. Minor amounts of the high-pressure index minerals glaucophane and lawsonite indicate the presence of blueschistfacies rocks, reflecting the origin of the Caribbean Mountains by subduction-related tectonism. The northern Mérida Andes, which comprise basement gneisses and granites overlain by unmetamorphosed to low-grade metamorphosed clastics, supply two types of assemblage reflecting these two lithological types: garnet-sillimanite-staurolite-amphibole suites from the basement rocks, and epidote-amphibole suites from the overlying cover sequence. The southern Mérida Andes supply stable heavy mineral suites reflecting recycling from the extensive unmetamorphosed sandstones that occur at outcrop. By considering suites from different physiographical provinces, the effects of short-term alluvial storage in the Llanos on heavy mineral assemblages have been evaluated. Weathering during alluvial storage appears to be effective in modifying the apatite-tourmaline ratio, which shows a steady, marked decline with distance from the mountain front, resulting from the removal of apatite during weathering. Clinopyroxene and garnet may also show evidence of loss through weathering, although the trends are poorly constrained statistically. Epidote and amphibole proportions remain essentially constant, possibly through a balance between loss through weathering and continual resupply from the breakdown of rock fragments. In general, the heavy mineral assemblages are less affected than the bulk mineralogy by alluvial storage on the Llanos.

The Permo-Triassic Sydney basin is located between a craton to the west and a volcanic source to the east. Systematic variations in sandstone composition, both in space and time, are characteristic patterns in the Sydney basin. A sharp change in sandstone composition is observed in the Late Permian from a quartzo-feldspathic subarkose to volcanic arenite. This change is attributed to the syndepositional development of a volcanic source along the eastern margin of the basin. Throughout the latter part of the Late Permian until Early Triassic time, the sediments were primarily derived from the volcanic source with a secondary mode from the craton. During this time, sandstone composition shows a gradual change in maturity from volcanic arenite to quartz arenite. This maturation was mostly related to chemical weathering in a temperate-humid climate in the source area. Petrographic data from the Sydney basin show that even temperate-humid climate can generate quartz arenite from a mixed cratonic-volcanic provenance and obliterate the plate tectonic signatures. In the Middle Triassic, the volcanic source was virtually shut off, and a quartz arenite unit was derived from the deeply weathered craton. Spatial variations in sandstone composition within correlative units and cyclicity within the broad maturity trend are also observed. Sandstones rich in volcanic grains to the east and sandstones rich in cratonic quartzose grains to the west, are related to the proximity of the respective source terrain. The cyclicity in compositional trends is related to a subtle variation in the duration of chemical weathering and fluvial dynamics.

A tumbler experiment using the 0.0625- to 4-mm fraction of four granodioritic grus samples was performed to investigate the nature of fracturing in the production of siliciclastic sand. Petrographical modal analysis was performed on 240 subsamples representing four major lithological constituents (quartz, potassium feldspar, plagioclase, and polymineralic rock fragments), four grain-size fractions (0.50 to 0.70, 0.35 to 0.49, 0.25 to 0.34, and 0.177 to 0.24 mm), and four time periods (untumbled grus; 2, 4, and 8 days). Each tumbling day represents a maximum of 25 km of transport in water. Tests of sample means and analysis of variance suggest that all subsamples are statistically homogeneous with respect to the number of quartz, potassium feldspar, and plagioclase grains among these size fractions. There is a highly significant difference in the number of rock fragments in the 0.50- to 0.70-mm fraction as compared to finer grained fractions, but no such differences occur within the medium- and fine-grained sand fractions. These results are inconsistent with the concept that lithic composition of the detrital light-mineral fraction is strongly size dependent within the 0.177- to 0.50-mm range. Where such compositional differences do occur, they may reflect sedimentological processes other than the comminution of grains from a single protolith. Grains from three grus samples examined for size changes displayed relatively rapid rates of disintegration during the first few days of tumbling, and then continued to fracture at reduced rates from days 4 through 8. From 8.9 to 16.2% of the relatively coarser grained fractions experienced comminution during 8 days (≃200 km) of transport. Of this amount, the sand fraction increased from 1.7 to 3.4%, whereas the silt plus clay fraction increased from 6.4 to 14.5%. For each sample, the weight percent of the fine- and/or medium-sand fraction remained nearly constant. Shape-ratio values (length of short axis/length of long axis of the maximum projection grain outline) as well as amplitude-ratio values for harmonics 2 and 3, as measured by Fourier analysis, were used to examine the character of quartz-grain fracture as evidenced in the 0.25- to 0.50-mm size fraction. Although some across-grain fracture occurred during the first day of tumbling, an overall increase in the frequency of smaller shape-ratio values indicated the dominance of grain-parallel fracture in all three samples. Scanning electron microscopy indicated that abrasion was not obvious even after 8 days (≃200 km) of transport. Average shape-ratio values were computed for at least 200 quartz grains in each of 49 samples from four high-gradient streams in southern California. Overall, the average shape-ratio values are remarkably consistent within each of the four streams, and closely resemble the value characteristic of the associated grus sample. Although significant decreases in average shape-ratio values occur along some local, very steep stream gradients due to high rates of grain-parallel fracture and/or shape sorting, the values reequilibrate after only 7 to 10 km of transport along lower gradient stream segments. Reequilibration probably reflects the dilution of more elongate by less elongate grains along lower stream gradients. No significant changes in the average shape-ratio values occur where small tributaries draining the same protolith enter the trunk streams, from sidewall erosion of the same source rock or across a contact with younger granitic source rock. Simple shearing is known to produce self-similar fracturing. Such fracturing is suggested by the lithological analysis as well as highly elongate quartz grains as observed in this and other studies. Self-similar fracturing may be an important mechanism in explaining the compositional and textural aspects of nascent siliciclastic sand derived from plutons subjected to emplacement and/or postcrystallization tectonism.

We studied the composition and roundness of medium sand from 18 small beaches of Elba Island. Six are pocket beaches less than 100 m long; the longest is 1.3 km long. The drainage basins of streams that supply the beaches are all less than 25 km 2 ; most are less than 5 km 2 . Beach sands range widely in composition owing to diverse source terrane. For the various drainage basins, comparison of the outcrop areas of the different types of bedrock with the compositions of beach sand grains yields the following conclusions: (1) the relative area of granodiorite outcrop is accurately represented by the amount of quartz + feldspar + quartzofeldspathic rock fragments in all beaches, although plagioclase in beach sand is significantly reduced relative to K-feldspar; (2) the relative areas of outcrop of ophiolitic and limestone bedrock are accurately represented by beach sand in pocket beaches, but are only moderately represented (ophiolitic rocks) or poorly represented (limestone) in other beaches; (3) the relative area of outcrop of metamorphic rocks is poorly represented in beach sand (metamorphic rock fragments + polycrystalline quartz) except in one anomalous beach supplied in part by mine tailings; (4) the relative area of bedded chert outcrop is poorly represented in beach sand because bedded chert does not break down into sand-size grains; and (5) shale bedrock is not represented or is only marginally represented in beach sand. For Elba beaches in general, the order of increasing roundness of grains, and thus increasing rate of abrasion, is: quartz < plagioclase < K-feldspar and igneous rock fragments (quartzo-feldspathic) < serpentine < metamorphic rock fragments < carbonate rock fragments (CRFs). There are no significant differences in roundness with beach length for quartz, K-feldspar, or CRFs. There are also no significant differences in roundness for the same three grain types from beaches of different size drainage basins, which indicates there is no perceptible rounding of grains by streams. First-cycle monocrystalline quartz grains of medium sand are unrounded, although coarser grains show minor blunting of edges. The roundness of quartz, K-feldspar, and CRFs are all greater on the eastern, more protected part of the island. This reflects a significant proportion of recycled quartz and K-feldspar in the eastern beaches, but CRFs may undergo more rounding in beaches of low to moderate wave activity than in high-energy beaches.

The Senio River drainage basin (northern Apennines, Italy) provides a good opportunity to study the effects of sedimentary recycling in a temperate climate. In fact, the bedrock of its upper portion is made of a single and homogeneous sedimentary unit, the Marnoso-arenacea Formation (Miocene), which consists of graded beds composed each of sandstone, siltstone, and mudstone. Composition of the soil sand-fraction within the upper portion of the Senio River drainage basin reflects the sandstone/siltstone composition of the underlying Marnosoarenacea Formation. Thus, pedogenic processes are generally not strong enough to affect siliciclastic sand grains formed by the alteration of the corresponding source lithology. Carbonate bioclasts are virtually the only sand-size detritus released from the poorly indurated mudstone of the Marnoso-arenacea Formation (i.e., T e divisions of turbidite beds and hemipelagic beds). Siliciclastic sand-fractions of fluvial sediments and soils are very similar, indicating that only minor modifications occurred during transport. On the other hand, a large population of carbonate clasts (as much as 35% of the sand fraction) is produced by the breakage of penecontemporaneous carbonate concretions generated mostly by vegetal activity along the upper reaches of the Senio River and its tributaries. In spite of their abundance, these carbonate grains are rapidly lost during transport because of their fragile and chemically labile nature; this implies that their preservation potential in the rock record is low. Marnoso-arenacea-derived sandstone and siltstone rock fragments are the only siliciclastic components of the detritus present along the Senio River which are modified during transport, decreasing progressively downstream probably because of breakage. The most useful indicators of sedimentary recycling within the sediments of the Senio River are the siltstone/sandstone lithic fragments, mono- and polycrystalline calcite grains due to breakage of the Marnoso-arenacea sandstone cement, and the mostly unworn fossil tests derived from the Marnoso-arenacea mudstones. This study confirms that composition of the sand shed off tectonically active source-areas composed of sedimentary rocks in conditions of temperate climate and high/moderate relief and short transport reflects the sediment source-rock, except for poorly indurated mudstone source rocks.

Differences in framework composition of first-cycle sandstones within coarse-grained delta systems of the Fountain and Minturn Formations (Pennsylvanian) near Colorado Springs and McCoy, Colorado, largely are a function of variable mechanical disaggregation and hydrodynamic sorting characteristic of different depositional environments within the deltas. Modification of composition occurred in spite of deposition in a tectonically active setting where rates of sediment supply and burial were relatively high. Within a wave-dominated delta in the Fountain Formation near Colorado Springs foreshore sandstones are most mature (Q 69 F 28 R 3 ) and offshore/transition sandstones are most immature (Q 50 F 48 R 2 ). Differences in maturity reflect shoreline reworking processes. Feldspar is mechanically broken and abraded in the foreshore due to swash/backwash processes. Smaller grains of feldspar are winnowed from the foreshore and transported in suspension to the offshore during storms. The average composition of shoreface sandstone (Q 62 F 34 R 4 ) closely resembles the composition of its precursor alluvial sandstone (Q 61 F 35 R 4 ), suggesting most shoreface sand was derived directly from the alluvial channels and underwent little or no compositional change. However, the overall variability in the composition of shoreface sandstones is greater than that of alluvial sandstones, suggesting a more complex derivational history. Some samples of shoreface sandstone are enriched in feldspar, presumably derived directly by winnowing from the foreshore; other samples are enriched in quartz, indicating total reworking of coarser mature sand from the foreshore. The Minturn Formation near McCoy contains facies of river-dominated deltas, fed by both meandering- and braided-river systems. Differences in depositional processes between the two systems caused distinctly different modification of the framework composition of sandstones deposited in the two systems. Sand delivered by meandering-fluvial systems presumably formed under more intense weathering conditions and contains up to 8% fewer rock fragments and as much as 12% more feldspar than braided-fluvial sandstones. Compositions of fluvial sandstones were subsequently modified by marine processes and the compositions of the sandstones in the braided and meandering fluviodeltaic systems diverged even further. The primary variation in composition is reflected by a decrease in abundance of rock fragments, with resulting enrichment of quartz and feldspar. In both systems, beach facies are very similar in composition to their parent alluvial sand, suggesting that sand in the Minturn deltas had low residence time in the beach and that the beaches experienced low wave activity. Overall, the marine-influenced fades of meandering-fluviodeltaic origin are more distinct in composition than those of braided-fluviodeltaic origin. Although variation in grain size between fades accounts for most of the observed difference in composition of sandstones of the meandering-fluviodeltaic system, weak correlation between grain size and QFR composition indicates that grain size by itself cannot explain all of the compositional variation in sandstones of the braided-fluviodeltaic system. Differences in compositional modification between the wave-dominated (Fountain) and river-dominated (Minturn) coarse-grained deltas were due largely to subtle differences in the composition and grain size of alluvial sand brought to the shoreline and in both the rigor and duration of reworking in the marine environments of the two types of deltas. Largely as a consequence of the latter, the composition of beach sandstones of the Fountain was more distinctly modified relative to other facies in the Fountain. Also, more modification occurred in Fountain beach sandstones than in shoreline sandstones of the Minturn Formation where full-scale development of beaches and their long-term existence were prevented because of the dominance of fluvial processes on the delta plains and delta fronts.

Distinctive compositional trends occur in sand suites collected from structurally diverse segments of Lake Tanganyika (East African Rift System). Climate (subhumid, tropical) and source lithology (mixed granitic and metasedimentary rocks) are generally invariant between the source areas studied. Therefore, differences in sand composition relate directly to differences in compositional modification during transport and deposition of the sands. Sand composition along contrasting segments of the lake basin can be related to the strong structural asymmetry of the basin. We point-counted medium sands from river mouth, deltaic, beach, and shallow-water environments of four structurally distinct, lake-margin settings: (1) escarpment margin (Pemba, Zaire site); (2) hinged margin (Nyanza Lac, Burundi, and Rumonge, Burundi sites); (3) accommodation-zone margin (Magara, Burundi site); and (4) axial margin (Ruzizi River site, Burundi). These four margin-types are typical of the structural morphology in strongly asymmetric half-graben rift basins. Using the Gazzi-Dickinson point counting methodology, margins two through four cluster tightly in QFL space, suggesting similar source lithology. Samples from the Pemba site (escarpment margin) are significantly depleted in feldspar. Because of the uncertainties in interpreting this difference, the escarpment margin sands were not included in subsequent analyses. Variations in sand composition occur between the remaining sites when using the traditional point-counting methodology. The suite of fluvial sands entering the lake basin along the accommodation-zone margin (Magara site) averages 38% rock fragments compared to an average of 15 to 20% and 18% rock fragments for the suite of fluvial sands entering the basin along the hinged or axial margins, respectively. This is a fundamental contrast that correlates directly to differences in drainage basin size. Reworking at the depositional site produces additional compositional differences across the structural margins. In comparing the sample suites for each site, it is the variability in the data that is more important than average composition in highlighting these differences. Minimal compositional modification occurs at the depositional site along both the accommodation-zone margin and the axial margin. In contrast, significant environmental overprinting occurs along the hinged margin sites. Limited compositional modification along the accommodation-zone margin results from the steep depositional gradient and the very narrow high-energy zone which limits reworking of the sediment. The limited reworking and the consequent minimal compositional modification along the axial margin are primarily a function of the high sedimentation rate. The compositional modification (enrichment in quartz) along the hinged margin is a function of low sediment accumulation rates coupled with low subsidence rates, leaving large regions of shallow-water substrate exposed to long periods of reworking. Chemical alteration of labile phases also occurs along the hinged margin through carbonate replacement of feldspar. Our results suggest that, with careful statistical analysis, it is possible to use sandstone composition to distinguish between tectonic margins typical of half-graben basins. Using this approach in ancient studies, it is necessary to collect from several coeval facies among different study sites and to constrain source lithology and climate. In addition, these results are most appropriate for humid-climate basins, where weathering processes are most extreme.

The Mediterranean Basin receives sedimentary contributions from several sources with different geological characteristics. The most important natural source of dust is the Saharan desert. Data from aerosol dust samples collected in a remote site in the Central Mediterranean Basin were used to investigate incursions of Saharan dust during a seven-month period (October through April). This work is an attempt to trace source areas and transport of dust plumes by means of grain-size, mineralogical, and chemical analyses. Two sets of aerosol samples typical of “desert-dominated” and “anthropogenic-dominated” air masses were studied. We propose a model for assessing the effects of latitude on the chemistry and mineralogy of the Central Mediterranean aerosol. Calculations and measurements of insoluble fluxes, in particular for Al (80 μg/cm 2 · yr −1 ) and Ca (28μg/cm 2 · yr −1 ), were performed, and results were compared with fluxes from the literature calculated in other stations in the Mediterranean Basin.