Open University Uranium-Series Laboratory    
Earth and Environmental Sciences, The Open University, Milton Keynes, UK    

Quaternary tufa deposition in the Naukluft Mountains, Namibia. Prof David Thomas and Heather Viles, Abi Stone, PGRS, IP/937/1106

 
Tufa deposits have great potential as palaeo-environmental indicators particularly in semi-arid to arid regions, which contain relatively little proxy evidence in geological archives. Tufas are also potentially amenable to U-series dating.

Fluvial tufa deposits in the ephemeral headwater streams that drain the Naukluft Mountains of the Great Escarpment, Namibia are the subject of this study. The tufas are biomediated freshwater continental carbonate deposits formed in non-thermal waters.

The Naukluft Mountains are part of the Great Escarpment separating the Namibian interior highlands from the coastal plain on the west coast. The flat-topped plateau rises to an elevation of 1960 m a.s.l. and is dissected by narrow valleys of the catchment streams, which drain west from the Great Escarpment into the Namib Sand Sea.

The outcropping Precambrian dolomite and limestone formations of the Naukluft complex have been incised and karstified, forming steep narrow valleys and extensive underground drainage systems. Discharge of groundwater in streams and springs has resulted in tufa .

The Naukluft tufa deposits are unusual in that their evolution appears to proceed in well-defined cycles: initially in steady-state, with semi-permanent surface flow forming cascades and barrages by turbulent degassing over the breaks of slope but with limited bedload entrainment. Subsequently, when cascades have grown to a critical size, water can pool and form banded tufa units. Finally, episodic high-flow conditions capable of carrying large bedrock boulders, result in the erosion and incision of barrage and cascade features. Subsequently, bedload becomes cemented by renewed tufa precipitation as the conditions return to the steady flow state, leaving clast-supported tufa facies (Figure 1). There is evidence of multiple such cycles in the various tufa deposits that are found at the Great Escarpment in Namibia.

 
  Naukluft barrage
Figure 1: Naukluft tufa barrage showing various evolutionary stages, including: uncemented bedload boulders in the incised channel, cemented bedload boulder bed, and 2 downstream prograding barrages with steeply forward-dipping fronts composed of ‘steady state’ tufa precipitates, Open spaces and vugs in the tufa layers are visible and these are the location of subsequent deposition of the ‘densely-cemented layered tufa’ used for most ‘U-series isochron’. Most units are separated by sub-horizontal gravel lags
Sample types:    

Within the barrage there is evidence for repeated periods of growth and erosion marked by the gravel lags, and within the vugs and spaces of many barrages, densely-cemented laminated tufa can be formed. For these to form water must have flown over and through the barrage features, rather than simply along the narrow and steeply incised channels, as observed today.

We refer to the laminated tufa facies as ‘banded’ and whilst banded samples are found very rarely within these catchments, we find three subtypes:

 
a pool-type banded facies suggested to form in low-gradient, slack-waters  
densely-cemented tufa-flowstone drapes, which coat the incised-channels and steep cascade fronts  
dense layers formed in vugs of large units of porous moss or reed  

Tufa samples with the best potential for dating are densely-cemented with negligible porosity to minimise the likelihood of open-systems behaviour, and pure-cements to reduce the influence of detrital contamination. The three banded facies types meet these criteria. Samples may not always remain chemically-closed systems and fabrics with high porosity have an increased likelihood of open systems behaviour. This includes post-depositional leaching of uranium, recrystallisation and/or neomorphism and secondary calcite precipitation.

 
Banded units are rare within the catchments and various other facies-types were also analysed.  
Moss tufa facies from two fluvial barrages.  
A moss facies with an outer coating of flowstone-type drape from the front of a cascade  
A gravel facies type from the top of the a cascade  
A reed and root facies from the front of a same barrage  

U-series disequilibrium dating methods are applied to a number of selected banded samples from the large and complex barrage deposits and to other tufa samples of various facies types.

The U-series dating exercise has revealed significant challenges for assigning ages to Naukluft tufa deposits. U-series secular equilibrium in a densely-cemented banded unit indicates that large, extensive complexes of tufa in the Naukluft Mountains are of considerable antiquity. Other such deposits yield ages at ~80 ka which suggest that some large barrage features are of at least MIS 5 age, while there is some evidence for Holocene age deposits as coatings in heavily incised channels. Many densely-cemented banded samples contain more than one allogenic component and standard ‘isochron’ mixing line protocols do not adequately correct for more than two end-members. Other densely-cemented banded samples exhibit have (230Th/234U)>1 and (234U/238U)>1activity ratios which cannot be the result of simple radioactive decay only but could be caused by leaching of 234U or ‘initial’ incorporation of 230Th. There is no petrographic evidence for uranium leaching in these samples, and leaching would also be indicated by (234U/238U)<1, which is not observed. It would appear that ‘unsupported’ 230Th was incorporated during tufa precipitation. This could be caused by leaching of both 230Th and to a lesser extent, 232Th from Naukluft limestone karst formations. Leached Th can be mobile at high PCO2 in alkaline groundwater and can then be incorporated during precipitation of the tufa

All four other facies types yielded high levels of detrital contamination, but have (230Th/234U)<1, and give finite age estimates in the range of 20-160 ka. Given the magnitude of the corrections for allogenic 230Th, micromorphological characteristics of these samples, the heterogeneity of cements types, and the porosity values of up to 40%, we cannot express great confidence in these data.

 
Various tufa deposits of the Naukluft Mountains contain similar suites of deposits in different channel catchments. This suggests that the hydro-climatic model of alternations between permanent steady river flow, episodic high glow conditions and quiescence, is robust for the region and this provides a first-order indication that the observed deposits relate to region-wide moisture availability, elevated groundwater levels and climatic forcing.
 
   
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© Peter van Calsteren
Last updated: 23 December, 2011 11:24