Carbon and sulphur geochemistry and clay mineralogy of the West Runton Freshwater Bed

JA Hannam, JE Andrews, SH Bottrell

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Weight % organic carbon and weight % total sulphur (C/S) in the Cromerian West Runton Freshwater Bed (WRFWB) of north Norfolk are not simply related to depositional environment as in the classic Berner and Raiswell, (1984) model. This is because the sediment is organic matter-rich, with total sulphur representing a variable mixture of pyrite sulphurs and organic sulphur. In addition, remobilisation of sulphur associated with post-depositional groundwater flow has modified depositional values. In the upper 20 cm of the bed, pyrite sulphur has been oxidised and largely removed. This sulphur was subsequently reprecipitated as later diagenic pyrite at about 40-60 cm in the bed under reducing conditions. Below 60 cm C/S ratios are probably close to depositional values and can be used as palaeosalinity indicators. As expected, C/pyrite-S ratios in this lower part of the bed indicate a freshwater depositional environment. The groundwater which oxidised pyrite in the top of the bed also destroyed organic carbon and probably dissolved aragonitic shell material. However, preservation of aragonitic shells below 60 cm in the WRFWB defines the extent to which groundwater penetrated the bed. Smectite is one of the dominant clay minerals in the WRFWB and potassium saturation suggests that the smectite may have had a volcanic orgin. The Eifel region of Germany is a possible source of fine-grained ash or dust at this time
Original languageEnglish
Pages (from-to)29-51
Number of pages23
JournalBulletin of the Geological Society of Norfolk
Publication statusPublished - 1996

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