Mineralogical and geochemical characterisation of warm-water, shallow-marine glaucony from the Tertiary of the London Basin

Abstract

Glaucony is present in the Palaeocene sediments of the London Basin, from the Thanet Sand Formation to the gravel beds at the base of the Lower Mottled Beds of the Reading Formation. The Upnor Formation glaucony is a rare example of formation in warm, shallow, brackish water and this, combined with the ready availability of fresh material from boreholes, make this study important in developing our understanding of this mineral. Glaucony comprises up to 50% of the Upnor Formation, a grey to green sandstone, of variable thickness and composition, which was deposited in a warm, shallow, marine to estuarine environment, ∼55.6–56.2 Ma. Using morphological criteria, X-ray diffraction data and K+ abundance, the Upnor glaucony may be defined as evolved. The underlying shallow marine Thanet Sand contains <5% of nascent to slightly evolved glaucony. The rare earth element (REE) data for the Upnor Formation suggest more than one source for the sediment from which the Upnor glaucony formed, while the Thanet REE data are consistent with a large detrital clay component. In the Upnor Formation, the large proportion of glaucony that occurs as granule fragments rather than whole granules, and the high-energy estuarine to shallow-marine environment of deposition, are indicative of reworking. The Upnor glaucony is inferred to be intraformationally reworked, rather than derived from the Thanet Sand Formation. The glaucony may have formed in sediments deposited away from the main estuarine channel, and been subsequently reworked into higher-energy sediments. Warm seas with freshwater mixing are more typically characteristic of verdine formation than of glaucony. The shallow, brackish environment of deposition suggests that there is not a clear distinction between the environmental requirements of verdine (or odinite) and glaucony (or glauconite), as is often proposed. The highly fractured, delicate nature of some granules indicates that they have experienced some maturation in situ, after reworking. The oxygen and hydrogen isotopic compositions of Upnor Formation shark teeth and glaucony point to formation in low-salinity water at ∼23 ± 3°C, also consistent with formation in the Upnor Formation, rather than in a fully marine sediment and subsequent reworking. A higher than normal temperature of formation may have increased the rate of evolution of glaucony. Our multidisciplinary study considers many of the factors relating to depositional environment that must be considered when glaucony-rich facies are encountered in comparable palaeo-environmental settings elsewhere in the geological record.