Lacustrine and palustrine facies in the Bembridge Limestone (late Eocene, Hampshire Basin) of the Isle of Wight, southern England

Abstract

The carbonate lake system represented by the late Eocene Bembridge Limestone Formation (Isle of Wight, southern England) comprises two main facies: lacustrine and palustrine. These facies are arranged vertically in transgressive/regressive cycles, corresponding to a model of carbonate lakes of low gradient and fluctuating margins with extensive palustrine fringes. These lakes developed in a coastal plain of limited relief, where two paleogeographical domains predominate. A lacustrine domain is represented in northern and eastern sections in the Isle of Wight by cycles representing upward passage from central lacustrine sub-facies (marl/marly limestone) through marginal lacustrine sub-facies (biomicrite) to sub-facies indicative of exposure (brecciated-nodular limestone). A palustrine domain, predominant in west Wight, is represented by a predominantly pedogenic palustrine facies with clotted-peloidal to ooidal textures and some intercalated laminated (laminar calcrete) horizons. Within the pedogenic facies, textures have evolved from those of unaltered biomicrites through clotted-peloidal to dense ooidal to open ooidal textures. A third domain, developed in close geographical proximity to more saline waters, is represented by gypsiferous lake-margin facies found in north Wight in which microlenticular gypsum developed post-depositionally as a result of evaporation following subaerial sediment exposure. In the lacustrine domain, there appears to be an association between cyclicity and changes in base (sea) level reflecting a likelihood that the lakes were part of a paralic, coastal plain complex with distal marine connections. Alternatively, the cyclicity might have developed as a result of the alternation of wetter and drier climatic periods.