Facies, environments and sedimentary cycles in the Middle Eocene, Bracklesham Formation of the Hampshire Basin: evidence for global sea‐level changes?

Abstract

The Bracklesham Formation is of Middle Eocene age and occurs throughout the Hampshire Basin of southern England. The basin is elongated east‐west and filled with Lower Tertiary sediments. Its southern margin is marked by either large, northward‐facing monoclines, or faults, both of which underwent differential movement, with uplift of the southern side throughout the Middle Eocene. The Bracklesham Formation, which is up to 240 m thick, shows pronounced lateral facies changes with dominantly marine sediments in the east passing to alluvial sediments in the west. Four principal sedimentary environments: marine, lagoonal, estuarine and alluvial are distinguished. Marine sediments comprise six facies including offshore silty clays and glauconitic silty sands, beach and aeolian dune sands, and flint conglomerates formed on pebble beaches. Offshore sediments predominate in the eastern part of the basin, as far west as Alum Bay, where they are replaced by nearshore sediments. Lagoonal sediments comprise four facies and formed in back‐barrier lagoons, coastal marshes and, on occasions, were deposited over much of the basin during periods of low salinity and restricted tidal motion. Five estuarine facies represent tidal channels, channel mouth‐bars and abandoned channels. These sediments suggest that much of the Bracklesham Formation was deposited under micro‐ to meso‐tidal conditions. Alluvial sediments dominate the formation to the west of Alum Bay. They comprise coarse to fine sands deposited on the point‐bars of meandering rivers, interbedded with thick sequences of laminated interchannel mudstones, deposited in marshes, swamps and lakes. Extensive layers of ball clay were periodically deposited in a lake occupying much of the alluvial basin. In alluvial areas, fault movement exposed Mesozoic rocks along the southern margin of the basin, the erosion of which generated fault‐scarp alluvial fan gravels. Locally, pisolitic limestone formed in pools fed by springs emerging at the faulted Chalk‐Tertiary contact. In marine areas, flint pebbles were eroded from coastal exposures of chalk and accumulated on pebble beaches and in estuaries. From other evidence it is suggested that older Tertiary sediments were also reworked. The Bracklesham Formation is strongly cyclic and was deposited during five marine transgressions, the effects of which can be recognized throughout the basin in both marine and alluvial areas. Each of the five transgressive cycles is a few tens of metres thick and contains little evidence of intervening major regression. The cycles are thought to represent small‐scale eustatic sea‐level rises (‘paracycles’) superimposed upon a major transgressive ‘cycle’ that began at the base of the Bracklesham Formation, following a major regression, and was terminated, at the top of the Barton Formation by another major regression. This major cycle can be recognized world‐wide and may reflect a period of rapid northward extension of the mid‐Atlantic ridge.

Early Eocene estuary at Strahan, Tasmania

“…The origin of plastic clays in estuarine environments has been discussed by several authors (e.g. Hyne et al 1979;Plint 1983), who hypothesised flocculation out of suspension in response to the mixing of fresh-and salt-water. Staub and Cohen (1979) noted the rapid flocculation of clay at the margins of a peat swamp as it comes in contact with highly acidic water from the swamp.…”

Graphic correlation and sequence stratigraphy in the Palaeogene of NW Europe

“…calcareous nannofossil studies by Aubry (1983Aubry ( , 1986, dinoflagellate cyst stratigraphy by Eaton (1976), and the sedimentologic work of Plint (1983). The sequence stratigraphic study of these outcrops by Plint (1988), correlates to the eustatic curve of Haq et af.…”

“…Our conclusions on the stratigraphy of the Palaeogene for the North Sea and NW Europe are that sedimentation in the Central North Sea occurs as a series of punctuated depositional pulses separated by time-correlative biostratigraphic data terraces, referred to as hiatal intervals. However, not all sequence boundaries that are observable in the physical stratigraphy are resolvable by graphic correlation, but the method serves to bracket packages that are defined in outcrop, by seismic, or by well log Plint (1983Plint ( , 1988, Knox et al (in press), Ali et al (1993), Eaton (1976), J. Riveline ( ers. comm.…”

“…Sequence stratigraphic interpretation of this section is modified from Plint (I%%), by correlating sequences described in other basins and using the original depositional environment descriptions of Plint (1 983 Graphic correlation plot of the dinoflagellate stratigraphy from Eaton (1976) of the Bracklesham succession at Whitecliff Bay. Lithologies and depositional environmoents are modified from Plint (1983), as is the sequence stratigraphy (Plint, 1988). Nannofossil picks come from Aubry (1983Aubry ( , 1986.…”

Sedimentology and structure of the Malvern Hills Coal Mine, Canterbury, New Zealand

“…Mancini & Tew 1993;Walker & Bergmen 1993), estuarine, lagoon and tidal channels (e.g. Plint 1983;Wood & Hopkins 1992). A paraautochthonous or allocthonous origin is suggested for the glauconite in this succession; grains have been swept from their place of origin by storm surges or tidal currents, and subsequently concentrated landwards within coeval deposits (Amorosi 1995(Amorosi , 1997.…”