Hillert Ibbeken

Granulometric study of the Hanaupah Fan, Death Valley, California

We applied new granulometric techniques to the various surfaces of the Hanaupah Fan, Death Valley, California, namely the Q1 surface, with an estimated age of 800–490 ka, the younger Q2 (170–105 ka) and Q3 (50–14 ka) surfaces, the <14 ka deposits of the incised channel, and to a (c. 14 ka) Lake Manly shoreline deposit at the northern periphery of the fan. We used these techniques to generate quantitative information on surface clast grain-size distributions, clast sphericity, roundness, and clast orientation to provide a data set that could be used to define fan-segment surfaces, and to help interpret fan genesis. Grain-size analyses were carried out by photo-sieving of 139 surface pictures, by petrographic identification of samples taken in the incised channel, and by identification and measuring of the largest clasts (1452 measurements) on the Q3 surface. The results show that all fan-segment surfaces, regardless of age, have similar size distributions, with a well-defined gravel mode of −2·3 to −3·0 phi, and are poorly to moderately sorted. Samples from the incised channel have distributions that are very similar to each other, regardless of distance from the apex, but display reduced sorting compared to the fan surfaces (which largely lack fines, perhaps from winnowing by secondary overland flow). Only the shoreline deposit is different from the other elements, showing a much narrower, well-defined gravel mode (−3·0 phi), and is moderately well sorted. Sphericity and roundness of clasts on all surfaces show only minor differences, similar to the other sedimentary parameters, indicating a remarkable homogeneity of the surfaces of the sediment body. In addition, measurements of the largest clasts (>100 cm long axis) on the Q3 surface showed no discernible trend either with radial distance or with rock type. These data suggest large depositional episodes that produce extensive sedimentary units without differentiation relative to distance from the source. Of the examined parameters, clast orientation is the best predictor of relative age of fan surfaces. Clast orientation in the main channel is bimodal, i.e. the long axes of clasts are either at right angles or parallel to transport direction. This bimodality disappears with increasing age, and the preferred orientation becomes unimodal (long clast axes normal to transport direction) on the Q1 surface. Although the causes of this change are still in debate, use of this parameter as a relative-age dating tool seems possible.

Jointed source rock and fluvial gravels controlled by Rosin's law; a grain-size study in Calabria, South Italy

The primary distributions of the untransported source rocks are unimodally distributed and very positively skewed; the average sand fraction is only 4%. Only 6% of the distributions are lognormal; 94% follow the Rosin distribution, which is typical for products of technical breakage and milling processes. Twelve of the 19 river-mounth sediments are bimodally distributed with a primary mode in the gravel range, a marked valley in the 2 mm range, and a secondary mode in the sand range. The seven rivers with unimodal sediments also have their modes in the gravel range. Both unimodal bimodal sediments consist of, on the average, 74% boulder and gravel, 25% sand, and 1% silt and clay. Surprisingly, 77% of the gravel modes of the river sediments also follow the Rosin distribution. Only the sand fractions of both the unimodal and bimodal sediments are lognormally distributed. The Rosin distribution of the Calabrian river gravel is interpreted as a source-specific primary feature; the lognormality of the sand mode, in contrast, as a transport-specific secondary feature. Since the jointed and weathered source rocks contain only 4% sand they cannot be the only source of the 25% sand in the river sediments. The sand must also come from other sources such as soil building and transport wear.--Modified journal abstract.

A Simple Sieving and Splitting Device for Field Analysis of Coarse Grained Sediments

ABSTRACT Size analysis of coarse grained, unconsolidated sediments requires large samples which are difficult to transport for laboratory analysis. A sieving and splitting device described here enables the sedimentologist to sieve coarse gravel samples ranging from 16 to 250 mm in the field and to split the sediment fractions < 16 mm into a convenient laboratory-sample of the required volume. With the aid of this device it is possible to sieve up to 1 ton of gravel daily enabling field determinations of grain size distribution, petrographic composition and morphometry.

The Hanaupah-Fan shoreline deposit at Tule Spring, a gravelly shoreline deposit of Pleisticene Lake Manly, Death Valley, California, USA

The Hanaupah-Fan Shoreline Deposit (HSD) is an as yet undescribed occurrence of shoreline sediments of late Pleistocene Lake Manly in Death Valley, California. It is located in the southern part of Death Valley, at the northeastern periphery of Hanaupah Fan. The HSD is a gently sloping, WSW-ENE elongated ridge, about 600 m long, 165 m wide and 8 m high. Its surface extends from -12 to +28 m in elevation, i.e. it has a vertical range of 40 m. We interpret the deposit as a sediment body that extended from the Hanaupah Fan east into the lake. Rising lake level, and waves approaching both from the north and south eroded fan materials, and produced a sediment body with a complex architecture. Fetch for waves approaching from either direction was about 40 km. The sedimentary inventory consists of cross-stratified gravel beds of various size ranges, dipping towards the north, south, and east, and of horizontal berm gravel beds, and horizontal silt layers. A discordant gravel layer covers the entire surface of the deposit, probably produced by wave action during the last phase of lake regression. This uniform gravel layer forms a surface that is distinctly different from the surrounding fan surfaces. It is relatively fine grained, much better sorted, and densely packed. Rock varnish is very well developed, and imparts a dark color to the surface, which makes it easily recognizable on aerial photographs. No absolute age date is available as yet, but circumstantial evidence places the formation of the deposit at the peak of marine isotope stage 2 (Wisconsinan/Weichsellian glacial maximum)

Basins and Channel Network

We digitized the topographic map 1: 25000 using a 250·m geomatrix and quantified nearly all geomorphological properties of the area which comprises 103 basins and interfluves. We subdivided three main physiographic provinces, Aspromonte, Zomaroand SerreProvinces; 19 larger rivers were investigated more closely.

Rock Units and Their Regional Distribution

We digitized the Carta Geologica Della Calabria (CGDC) 1: 25000 using the 250-m geomatrix. Granitic rocks cover 24% of the basins 01 — 19, metamorphic rocks yield 37%, and 39% are made up by sedimentary rocks. We condensed the 55 lithological rock units of the CGDC to eight source rock types to which the provenance discussion of this book refers: granite (24% area), gneiss (7%), schist (30%), limestone (1%), conglomerate (9%), sandstone (9%), siltstone (12%) and argillite (8%). It goes without saying that source area geology is by far the most important variable controlling the composition of river-mouth sediments.

Erosion and Predicted Sediment Composition

The mean erosion rate, calculated as long-term (1 Ma) erosional budget instead of sediment yield, is 178mm/ka for the total area and 206 mm/ka if only the 19 basins are calculated, maximum values being 850 mm/ka. During the reference time of 1 Ma the entire area would have been able to produce a sedimentary deposit of 100 x 100 km, 55 m thick, with a calculated pore volume of 50%.

Water Budget and Dissolved Load

The mean effective precipitation in the 50-year period between 1921 and 1970 is 698 mm/a, the mean evaporation rate 48.6%. This results in an average annual runoff of 1278x 106 m3. The mean mineralization of the river waters is 322.6 ppm after the summer period and 221.4 ppm in late winter, 250 ppm being a reliable mean. About 47% of this dissolved load is regarded as being recycledor from atmospheric and anthropogenic input. The composition of the Calabrian river water corresponds fairly well to the worldwide mean of river water. Compared to the long-term (1 Ma) mean erosion rates of the 19 basins (206 mm/ka) the proportion of the present–day dissolved load is 16.3%; compared to mean actual effective erosion rates of 1527mm/ka consequently it is only 2.2%. Today the 19 basins release 319425 metric tons of dissolved load per year into the Ionian Sea, of which 169295 tons (53%) are geogenetic. This corresponds to a solid-rock cube of about 40 m side length.

Composition of Rocks and Soils

The igneous rocks of the Serre are mostly granodiorites. The majorityof the metamorphic rocks belong to the transition from greenschist to amphibolites facies, sometimes retrograde. Among the sediments the conglomerates are a distinct but controllable source of misleading results due to recycled gravels. The sandstones are mainly arkosic. The siltstone or olisthostrome complex contains only 20% sand (40% silt, 40% clay). The soils are dystric and eutric cambisols, orthic luvisols, calcaric regosols; we measureda mean of 71% sand, 19% silt and only 10% clay. Soils of granite and feldspathic sandstone show an arkosic composition (F/Rf and F/L> 1), while the samples from gneiss and schist show a lithic composition (F/Rf and F/L< 1). ZTR values of soil vary greatly; soils from metamorphic rocks contain 12% of ZTR and from granitic or sedimentary ones only 4%.

Development of the Chemical Composition of Sand

The chemical composition of the single types of source rocks and the sand fraction 63 — 500 μm of the soils, of the effective composition of the source rocks and soils, of the river-mouth sediments and of the longshore bar is compared. Gain and loss and constant-AI2O3 calculations reveal that the composition of the river-mouth sands comes closer to the source rocks than to the soils. Attrition during fluvial transport is the most important source of sand at this active plate margin, chemical weathering and soil formation being less effective.The chemical composition of the average Calabrian river-mouth sand fits almost perfectly that of active-margin sandstones; single rivers, however, deviate greatly from this mean and it may be hazardous to draw conclusions from only one or even a limited number of rivers.