Iron coating in recent terrace sequences under extremely arid conditions

CATENA

Vol. 9, 353-359

Braunschweig 1982

IRON COATING IN RECENT TERRACE SEQUENCES UNDER EXTREMELY ARID CONDITIONS D. Bowman, Beer-Sheva

SUMMARY Precise documentation of ferric oxide coating on calcareous sands of 13-15 recent alluvial fan terraces in the Dead Sea area, Israel, indicates a gradual ferric enrichment over the last 12,00 years. Airborne dust is suggested as the main source. The staining on the low terraces indicates that coating during short increments of time than hitherto generally accepted, up to periods of a few tenths of years, is distinguishable in an extremely arid environment. The results suggest an initial stage of alluvial red bed formation.

1. INTRODUCTION Recent desert sediments are slowly reddening as a result of diagenetic alteration which forms the desert varnish, a brownish-red ironoxide and manganese coating that stains rock surfaces, gravel and sand grains (TURNER 1980). The stain occurs on nearly all kinds of rocks, but is less common on limestone than on less calcareous rocks (HUNT 1954). Analyses show (HUNT & MABEY 1966, ENGEL & SHARP 1958) that the proportion of iron to manganese ranges from 1:1 to 10:1. The typical composition of the residue was found by POTTER & ROSSMAN (1977) to be 69% MnO2 + Fe203. The orange coat in contact with the soil on bottom of pavement stones gave residues consisting of 65% Fe203. The varnish layer is generally less than 100 kz thick (ALLEN 1978). KRUMBEIN (1969) isolated varnish microorganisms capable of precipitating manganese oxides without deliniating their role in varnish genesis. DORN & OBERLANDER (1981) presented evidence for a biological origin for black manganese-rich varnish by observing manganese concentrating bacteria on desert varnish with scanning electron microscopy and by laboratory replication of varnish. Microbiological genesis does however not exclude formation of natural varnish without biological assistance. The general cause for reddening is the gradual acquisition of coating with passage of time. Increase of the amount of coating in successively older deposits has been observed by many authors (JACKSON 1962, WALKER 1967, 1979, NORRIS 1969, DOLAN 1970, GLENNIE 1970, PERRY& ADAMS 1978). In some of these studies time was substituted by distance from the sediment source in down wind direction. In spite of the reddening of dunes with increasing age and distance of transport, WALKER (1979) suggested, because of the different amount of moisture, different availability of iron and type of clay minerals retained on the grains, that reddening is neither a reliable measure of age, nor a criteria for correlation. Nevertheless, varnish gradually became a tool for measurement of relative age within environments of similar field and climatic conditions. Relative age of gravel can be distinguished by the degree of stain, which became a yardstick for relating alluvial surfaces. Based on archeological records HUNT (1961) sugges-

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ted desert varnish as a useful tool in recent stratigraphy. He concluded that varnish permits separation of deposits and surfaces younger than 2000 years from earlier ones. He also suggested (1954) that the youngest varnish dated from the beginning of the Christian area. HOOKE et al. (1969) also suspected that staining is barely noticeable on 2000 years old artifacts, although varnish is forming in present times. The artifacts showed differences in concentration of oxides of the order of 20% within the varnish layer and suggested that concentration changes in excess of this value were significant. Archeologists often observe differential patination and arrange implements in a colour series. Patination is an accepted aid in assessing the relative age of industries in mixed assemblages (GOODWIN 1960). In all instances where stratified sites were later excavated the sequence indicated by patination has been confirmed. The aim of this study is to examine the varnish coating on calcareous sands in an UpperPleistocene to recent terrace sequence, as a tool for relative dating in an extremely arid environment. The study also examines whether each successive recent period of terrace entrenchment leaves enough time for differential staining of the young alluvial surfaces.

2.

STUDYAREA

The Jordan valley is part of the large Syrian-African rift valley system. The Dead Sea area (Fig. 1) is the lowest point on earth ( - 400 m below MSL) with an extremely arid and hot climate. Average annual precipitation amounts to 50 mm and summer temperatures are regularly in the 30°-40°C range. The low rainfall is caused mainly by the rainshadow position

Fig. 1: Location Map. The rift shoulders are indicated in the inset.

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of this region. The rift valley attained its structural form during late Pliocene to early Pleistocene. Morphologically its age is Upper Pleistocene to recent. Being well preserved and dated, the margins of the Dead Sea graben are ideal for morphological and pedological research. The largest, most developed and best preserved flight of terraces is located at the western fault scarp of the graben in the Zeelim fan, derived from the Zeelim catchment in the Judean desert. The second sequence of terraces studied is located in the Lot wadi. In the Zeelim wadi there are fifteen terraces entrenched in an alluvial fan over an altitudinal range of about 100 m, whereas in the Lot wadi thirteen terraces were recorded. The rock types which compose the alluvium of the terraces include mainly Cenomanian and Turonian carbonate and Dolomite and Campanian Flint. The sediments size range usually from silt and clay to coarse gravel. The Lisan lake was the last of a series of water bodies which occupied this section of the graben 50,000 to 12,000 years B.P. (NEEV& EMERY 1967, NEEV& HALL 1977, KAUFMAN 1971). These datings have pronounced significance in our study because they indicate that the entire sequences of the terraces were entrenched subsequent to the recession of the Lisan lake and hence show no morphological or sedimentary indication of a lacustrine environment. The pure fluviatile, post-Lisan origin of the terraces is best indicated by their braided surfaces. For the purpose of defining a terrace, only clear treads and risers were taken into account. The treads are boulder conglomerates which constitute a thin capping veneer, composed mainly of uncemented dolomite, limestone and flint showing an initial shallow reg soil formation. The height of the terrace-risers ranges from 0.5 to 17.5 m, with the average height being 4.4 m in the Zeelim and 6.8 m in the Lot wadi.

3.

PROCEDURES

Field observations over the entire flight of terraces did not discern gradational reddening of the calcareous sand. However, the sand of the highest terrace (no. 1) showed a markedly red pigment. One sample was taken from each of 10 terraces of the Zeelim. The variability of the staining along a single terrace was studied by taking 2-3 samples from every second terrace in the Lot sequence. For determining heavy minerals 4-5 additional samples per terrace were taken on six of the Lot terraces. A further set of 11-24 samples were taken from 9 terraces of the Lot for determining organic matter. All samples were collected from a depth of 10-20 cm below terrace surface, not exceeding the dusty red layer of 20-30 cm depth. For determining Fe, the sand fraction 0.5-2.0 mm was separated by wet sieving and individual heavy grains were removed under a binocular microscope. The varnish was separated from a 1 gr sub-sample ofunpowdered grains by boiling in HC1 + 4 drops of SnC12 10%. Fe was determined as Fe203 weight percent via atomic absorption spectrophotometer against standards. For determining heavy minerals, the 63-420/~ fraction was separated gravitationally in bromoform. For determinin organic matter, the samples were treated with hydrogen peroxide, heated 2-3 times and dried.

4.

RESULTS AND DISCUSSION

Both terrace sequences of the Lot and the Zeelim sites show a trend of enrichment in ferric oxides with time (Fig. 2, 3). Progressively older deposits disclose more staining. Iron content in the varnish is also a function of grain size and increases with decreasing grain size

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(TURNER 1980). Detailed grain size analysis (AMIT & G E R S O N - Personal communication) revealed no significant trend in abundance of silt and clay in the Zeelim terrace sequence. The a m o u n t of iron present in the pigmentary material in the study area is very similar to that found in the ancient red beds of the Sonoran desert 0.1-2.1% (WALKER &

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Fig. 4: Heavyminerals concentration in the terrace sequence of the Lot wadi. HONEA 1969). In the terrace sequence of the Lot wadi the 2-3 samples taken from the same terrace show some variability, which becomes greater with increase in the amount of staining (Fig. 3). Two likely sources are suggested for the iron: First, it could be argued that impurities of heavy minerals from the source rock account for the trend; secondly, the coating could be supplied mainly from external sources and hence it reflects time. It is hard to assume that impurities would have such a systematic variation over so many terraces. The exact reversed trend of the heavy minerals (Fig. 4), showing the highest values in the lowest terraces, indicates that variation in provenance is unrelated to the coating and that the impurities cannot have caused the trend. Post depositional coating of ferric hydroxides is thus the most likely explanation. The coating formed after deposition and was not removed because of the weak leaching process. The gradual increase in stain with time supports GIBBS (1967), that grain coating is formed in a weathering soil environment. PERRY & ADAMS (1978) suggested, after observing the lack of correlation between the composition of varnish and that of the substrata in Arizona, Utah and Idaho, that the iron in the varnish originated from air borne desert dust. This conclusion is further supported by HUNT (1954) and by POTTER & ROSSMAN (1977), who showed that the red pigment in the alluvial fan terraces forms in the soils where the moisture necessary for alteration of iron bearing grains, initially concentrates. In Israel most soils are infiltrated by dust deposits. YAALON et al. (1966) showed that the composition of silt and clay in southern Israel resembles that of the dust. GANOR (1975) found 2-5% of Fe203 and 0.1-5% heavy minerals in dust samples taken during storms. He estimated that only half of it accumulates in the soil profile, which fits the findings of this study. Oxygenic conditions, which are basic for development of the varnish, were provided in the study area by the continuous entrenchment, which caused the water table to lower and permitted successive new, well drained alluvial surfaces to form. The terrace surfaces thus completely dried out as the process proceeded. These oxidizing conditions do not lend themselves to accumulation of organic material, and indeed, the amount of organic matter in the Lot samples was low (1.2-2.9% by weight in the < 2.00 mm size fraction) and showed no trend within the terrace sequence. The overall setting is essentially that of a permeable red bed clastic alluvial fan wedge, flanking an rea of tectonic uplift.

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Computations of the rate of wadi entrenchment in the Dead Sea area in the Post-Lisan period, based on the Lisan dating and the post-Lisan vertical entrenchment, indicate an annual rate of 3-5 mm/year, similar to Gerson's 4,5 m m / y for stream entrenchment on Mt. Sedom (1972). NEEV & EMERY (1967) computed a faster entrenchment for the Jordan River - 10 mm/y. Our post-Lisan entrenchment rate would make the mean period for forming one terrace - 1400 years. Field evidence of rapid channel entrenchment during the winter floods suggest that the computed rate of entrenchment is far too low. It is a minimum rate which disregards long periods of non-activity or even periods of reversed trends. Furthermore, field evidence of initial staining on bars and on surfaces only somewhat elevated above the river channel showed, that stain develops within a few tenth of years if not quicker. This is also indicated by beginning of reg soil formation observed in inactive parts of broad wadi beds (DAN et al. 1981). It is therefore suggested that sediment particles in arid regions can initially redden within shorter periods than 2,000 or 1400 years. Varnish coating may cycle through periods of development and deterioration (ENGEL & SHARP 1958). Major climatic fluctuations may influence pigmentation rate or even reverse the trend by causing its removal. Disregarding the clear bend of the curve (Fig. 2), the trend of reddening in the Dead Sea area does not support rhythmic climatic fluctuations during the Dead Sea stage, claimed by NEEV & EMERY (1967) and NEEV & HALL (1977), based on stratigraphical record. The sample size and the scattering of the data points might be responsible for a poor climatic resolution. However, the terraces of the Zeelim and Lot wadis could as well have been detached continuously from channel water supply, avoiding in this way the control of climatic fluctuations.

ACKNOWLEDGEMENT The author gratefully acknowledges the constructive criticism and suggestions by IL Bryan and G. Szeicz, Scarborough College, University of Toronto. BIBLIOGRAPHY

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Anschrift des Autors: Dan Bowman, Ben-Gurion University of the Negev, Department of Geography, Beer-Sheva, Israel.