Abstract

Following the investigations of Odin and others into the distribution of green granules, glaucony has been widely assumed to be a reliable indicator of a fully marine, open shelf environment with a low sedimentation rate. We have investigated the value of glaucony as a palaeoenvironmental indicator through an investigation of the pellets, and their distribution and reworking in the predominantly brackish to shallow marine Tertiary sediments of the Hampshire basin, together with a re-evaluation of the sedimentology.

Glaucony has apparently formed in situ in all lithofacies from shallow marine to estuarine. Of the three highest glaucony concentrations (all dominated by in situ glaucony) two occur within highstand system tracts, the third is at a sequence boundary. Several important surfaces do not have more than a few percent glaucony, with very variable proportions of mature and in situ pellets. The correlation between glaucony concentration and sequence stratigraphy is most obvious in the London Clay and Wittering Formations, where least reworking of pellets has occurred. In the Barton Group there are no major concentrations of glaucony at any of the important stratal surfaces, we believe this more random glaucony distribution is due to limited glaucony formation and reworking of older glaucony. In these sediments ideal conditions for glaucony formation are interpreted to have been: fully marine, 10–30 m water depth, a ‘warm’ temperature plus low sedimentation rate with periodic winnowing to concentrate the pellets. Although most of these conditions for glaucony formation occurred in the Selsey Formation and Barton Group, a factor or factors mitigated against glauconitization. We suggest that this was lowering of the water temperature. The London Clay and Wittering Formations were deposited relatively rapidly (50–60 m Ma −1) and include intervals of estuarine sedimentation, both factors that we believe inhibited glaucony formation.

Glaucony maturity reflects the minimum length of time spent in surface sediments, close to the oxic/sub-oxic interface. Point count data and chemical data for glaucony indicate widespread reworking and an overall increase in reworking with time, possibly due to uplift on the Isle of Wight monocline. The apparently wide range of conditions in which glaucony will form, and the frequency with which it is reworked, suggest that it is a less useful indicator palaeo-environmental indicator than is commonly supposed.

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