Mineralogical and geochemical studies of Upper Eocene sediments in the Hampshire Basin of Southern England

Abstract

Sediments of the marine Barton Clay Formation, Barton Sand Formation, and the non-marine 'Lower Headon Beds® exposed along the coastal cliffs on mainland Hampshire and the Isle of Wight have been investigated mineralog- ically and geochemically. Sandy-clays and quartz-sand predominate and are dominated by quartz, clays and microcline feldspar with small amounts of anatase, goethite, pyrite, albite, oligoclase, biogenic calcite, aragonite and organic-carbon. The clay assemblage comprises degraded illite, smectite, kaolinite, illite- smectite and traces of chlorite. Geochemically the sediments are silica-rich but poor in alkali and alkaline-earths. Their trace element contents show strong association with clays and feldspars; whilst substantial concentrations of As, Ce, Cr, Cu, I, Mn, Pbj Sr, Zn occur with plant remains and/or carbonates. In general, the sediments show no significant facies-related compositional variation nor evidence for substantial diagenetic alteration. Support is provided for sediment derivation from Cretaceous sediments and intrabasinally-exposed Tertiary sediments on adjoining land areas in England and horst structures in the English Channel. Continuous low-scale tectonic movements and episodic eustatic sea-level fluctuations caused alternating periods of slow, clayey deposition and relatively shorter periods of rapid, sandy sedimentation. Palaeosols related to red-yellow podzols and hydromorphic swamps have been identified. These contain abundant authigenic kaolinite and goethite. Lepidocrocite, jarosite and gypsum occur in association with the hydromorphic palaeosols, although these are difficult to distinguish from Recent weather- ing products. Authigenesis of Fe- and Ca-rich phases was widespread. Freshwater limestones were formed, dominantly composed of micritic low-Mg calcites. Glauconitic-mica formed in the Barton Clay, predominantly within microfossil tests. Its time of formation appears to be substantially less than previously considered likely. Calcian-siderite ironstones and ferroan-calcite septarian concretions formed in early diagenesis at very shallow depths. The siderite shows between 1 and 10 mol% Ca2+ substitution. The substitution is facies- related, and greatest in marine and 'brackish® sediments. Ferroan calcite occurs in association with glauconie within marine sediments only. It is believed to form rather than siderite as a result of the early depletion of iron-oxide during glauconitisation. The formation of these low-Mg carbonate phases is highly unusual at shallow depths, and is believed to result from the high influx of iron-oxide and dissolved CaCOg. The clay assemblage, the red-yellow podzol palaeosols and the authigenic phases, together, suggest the prevalence of a warm, humid, probably sub- tropical palaeoclimate with moderate-intense weathering and active erosion.

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