The Upper Eocene Bembridge Limestone Formation, Hampshire Basin, England

Resumen

The northwest-southeast oriented Hampshire DieppeBasin, approximately 280 km long (Hamblin etal., 1992), is a structural basin formed by Miocene inversion of the underlying Mesozoic Hampshire-DieppeHigh and the adjacent Jurassic-Lower Cretaceous extensional sedimentary basins (Chadwick, 1993; Hibschet al., 1993). The onshore portion in southern England, the Hampshire Basin (Figure 1)/ covers much of the county of Hampshire and extends into the adjacent counties of the Isle of Wight, Dorset, and West Sussex. Its southern boundary is formed by east-west, north youngingmonoclinal folds involving Upper Cretaceousand Paleogene strata, draped over reversed normal faults in Jurassic-Lower Cretaceous strata(Chadwick, 1993). The other boundaries correspond to the Paleogene-Upper Cretaceous (Chalk) junction andwere exposed as a result of post-Miocene erosion. The Paleogene succession (Daley, 1999) was depositedunconformably on weakly folded and eroded Upper Cretaceous (Campanian) Chalk (Melville and Freshney,1982) and subsequently uplifted and deformed duringmid-Paleocene (Laramide) crustal compression (Hamblinet al., 1992). The Paleogene strata dip steeply in the vicinity of the monocline, whereas to the north, dips are shallow in low-amplitude folds. At the beginning of Paleogene time, the British area was some 12 south of its present position (Irving,1967) and considerably warmer than at present (Reidand Chandler, 1933; Daley, 1972). Britain lay on the western margins of what has been called the Northwest European Tertiary Basin (Vinken et al., 1988). At the western end of this basin, the Tertiary North. Sea Basin extended northward between the British and Scandinavian land masses. The present Hampshire(tectonic) Basin was at the southeastern end of this369sedimentary basin. The Paleogene succession here is up to 652m thick (Edwards and Freshney, 1987a) (Figure 2).It begins with upper Paleocene coastal plain sediments and continues with lower to mid-Eocene (Ypresian-Bartonian,NP10-NP17), cyclically alternating marine, nearshore, and coastal plain sediments. Extreme shallowing and possibly subaerial exposure(Edwards and Freshney, 1987b) at the mid-upper Eocene (NP17-NP18) boundary preceded the onset ofpredominantly argillaceous, strongly river-influenced, shallow-marine, laguno-Iacustrine and lacustrine sedimentation(represented by the Solent Group), which continued at least into the early Oligocene (Rupelian,NP23). This major change in sedimentary style signifies the establishment of a better-defined sedimentary basin of unknown extent. The change coincided with gradual uplift of the Weald-Artois axis (coincidentwith the major Midi basement fault), linking extreme southeastern England to northeastern France (Hamblinet al., 1992). As a result of post-Miocene erosion, the middle and upper formations of the three comprising the Solent Group are now confined to the Isle of Wight.