Geochemistry of marine and lacustrine bands in the Upper Carboniferous of the Netherlands

Abstract

Geochemical studies on Upper Carboniferous marine bands showed that marked enrichment in redox-sensitive trace elements (uranium (U), vanadium (V), molybdenum (Mo)) mostly occur if they contain Goniatites. Goniatites indicate deposition in relatively distal and deep marine environments. In contrast, Westphalian marine bands found in the Netherlands predominantly show a Lingula facies, indicating deposition in a nearshore environment. These Lingula marine bands are mostly lacking significant trace element enrichments. The aim of this paper is to explain the mechanisms causing the differences in geochemical characteristics between distal (Goniatites facies) and proximal (Lingula facies) marine bands. Geochemical analyses (total organic carbon (TOC), sulfur (S), major and trace elements) were carried out on a selection of these marine bands. Furthermore, a comparison was made with some lacustrine bands which broadly show the same sedimentary development as the Lingula marine bands. The results show that the Lingula marine bands, in contrast to the Goniatites and lacustrine bands, are characterised by low organic carbon contents (1 – 2 wt.%). A relatively high input of siliciclastics probably prevented the accumulation of organic-rich layers (dilution effect). In turn, low organic carbon contents most likely prevented the effective scavenging of trace elements. Although the lacustrine bands are characterised by high TOC contents, here the limited availability of trace elements in fresh water forms the best explanation for low trace metal enrichments. Since marine bands form stratigraphically important horizons in the Upper Carboniferous, many attempts have been made to recognise marine bands using well logs (gamma-ray). The results from this study show that using gamma-ray devices (detecting U-enrichments), only marine bands in a Goniatites facies are clearly recognised while Lingula marine bands are not detected.

The Role of Seawater on the Trace Element Geochemistry of Some UK Coals and a Tribute to Goldschmidt

“…The enrichment ratios could be normalised to eliminate the effect of dilution using the detrital elements but the correction is minor and has little effect on the values for those elements that are enriched. Values above unity on Figure 1 are noted for the elements highlighted by Kombrink et al [26], namely V, Mo and U, but in addition there are other elements enriched including Ni, Cu, Zn, As, Pb and possibly Se, Cd, Sb and Tl. For the latter group values are closer to the analytical detection levels and percentage errors are higher.…”

“…In a study by Kombrink et al [26] on the geochemistry of marine and lacustrine bands in the Upper Carboniferous of the Netherlands the underlying coals were also analysed using both inductively coupled plasma-atomic emission spectrometry (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS). The main focus of their work was in establishing the geochemical differences between the goniatite bearing shales, deposited in a distal and relatively deep marine environment and the proximal Lingula facies.…”

“…Enrichment in redox-sensitive elements such as U, V and Mo was noted in the marine shales. Their discussion [26] covers the role of organic matter, the possible presence of anoxic bottom water and the rate of siliciclastic sedimentation. However, most pertinent to the present work are the important observations made on the geochemistry of the coals.…”

“…It was observed that higher Mo and U concentrations in the coals may result from percolating seawater from above, in accordance with the findings of Gayer et al [24], who, as noted above, also found enhanced U and Mo concentrations in marine influenced coals. The calculation of enrichment factors by Kombrink et al [26] involved using the mean Al concentration of the average shale [27] for the mudrocks deposited in different salinity environments. Aluminium was used to normalize data to take into account changing clay proportions.…”

“…On Figure 1 are plotted the enrichment factors for the marine shales in the work of Kombrink et al [26]. The enrichment factors are calculated here as a ratio of the median of the element concentration in the marine shales divided by the median of the element concentration in the lacustrine (non-marine) shales.…”