Stratigraphic and chemical correlation of volcanic ash beds in the terminal Proterozoic Nama Group, Namibia

• Published in: Geological magazine 2005-09, Vol.142 (5), p.519-538
• Main Authors: SAYLOR, BEVERLY Z., POLING, JANICE M., HUFF, WARREN D.
• Format: Article
• Language: English
• Subjects: Africa; basin analysis; basins; Cambrian; carbonate platforms; carbonate rocks; chemical composition; correlation; Correlation analysis; depositional environment; Ediacaran; electron probe data; Feldschuhhorn Member; Geochemistry; Geological time; Huns Member; igneous rocks; Kuibis Subgroup; lithostratigraphy; major elements; marine environment; metals; multivariate analysis; Nama Group; Namibia; Nasep Member; Neoproterozoic; Nudaus Formation; Original Article; paleoenvironment; Paleozoic; petrography; Precambrian; Proterozoic; pyroclastics; rare earths; rock, sediment, soil; Rocks; Schwarzrand Subgroup; sedimentary basins; sedimentary rocks; sedimentation; sequence stratigraphy; shelf environment; Southern Africa; Spitskop Member; Statistical analysis; Stratigraphy; tephrostratigraphy; trace elements; upper Precambrian; Urusis Formation; Vendian; volcanic ash; volcanic rocks; Volcanoes; Witputs Subbasin; X-ray diffraction data
• ISSN: 0016-7568; 1469-5081
• DOI: 10.1017/S0016756805000932

At least twenty silicified volcanic ash beds have been identified in the Kuibis and Schwarzrand subgroups of the terminal Proterozoic Nama Group of Namibia. Nineteen of the Nama ash beds are in the Schwarzrand Subgroup in the Witputs subbasin. Two of these are in the siliciclastic-dominated lower part of the subgroup, which consists of the Nudaus Formation and Nasep Member of the Urusis Formation and comprises two depositional sequences. Identification and correlation of these ash beds are very well known based on stratigraphic position. Sixteen ash beds are contained within the carbonate-dominated strata of the Huns, Feldschuhhorn and Spitskop members of the Urusis Formation. These strata comprise four large-scale sequences and eighteen medium-scale sequences. Ash beds have been found in three of the large-scale sequences and seven of the medium-scale sequences. Correlations are proposed for these ash beds that extend over large changes in facies and stratal thickness and across transitions between the seaward margin, depocentre and landward margin of the Huns-Spitskop carbonate shelf. A study of whole rock and in situ phenocryst compositions was conducted to evaluate the feasibility of independently testing sequence stratigraphic correlations by geochemically identifying individual ash beds. Whole rock abundances of Al, Fe, Mg, K and Ti vary inversely with Si, reflecting variations in phenocryst concentration due to air fall and hydrodynamic sorting. These sorting processes did not substantially fractionate whole rock rare earth element abundances (REE), which vary more widely with Si. REE abundances are higher in samples of the Nudaus ash bed than in samples of the Nasep ash bed, independent of position in bed, phenocryst abundance, or grainsize, providing a geochemical means for discriminating between the two beds. Variations in the position of chondrite-normalized whole rock REE plots similarly support suspected correlations of ash beds between widely separated sections of the Spitskop Member. Abundances of Fe, Mg and Mn in apatite plot in distinct clusters for Spitskop ash beds that are known to be different and in clusters that overlap for ash beds suspected of correlating between sections. Abundances of REE in monazites differ for the Nudaus, Nasep and Spitskop ash beds in which these phenocrysts were identified. Multivariate statistical analysis provided a quantitative analysis of the discriminating power of different elements and found that whole