The development and seismic expression of synsedimentary features within the Chalk of southern England

Abstract

Seismic reflection data acquired onshore to the north of Bournemouth, southern England, image clearly a series of prominent concave-up (troughs or depressions) and concave-down (mounded) structures within the White Chalk Subgroup. The Chalk lithostratigraphy in the area of study is established from borehole geophysical log correlations. These correlations then provide calibration of the seismic reflection data, indicating that the structures are concentrated at levels between the Lewes Nodular Chalk Formation and Tarrant Chalk Member, and towards the base of the Portsdown Chalk Formation. Onlap, downlap and truncation of reflectors are observed, with the most dramatic features forming a stacked series directly overlying faulting of Jurassic strata. Similar stratal geometries have been described from comparable levels in the Chalk of the North Sea, and in outcrop in Britain and France. To many they represent channels formed by submarine erosion and redeposition of the Chalk during relative sea-level falls, linked to tectonics. We also interpret a number of apparently incisive structures and slumped infill sequences described here as being of primary syndepositional origin. The slumping and the close association of these features with underlying faults, and proximity to nearby major crustal faults, may be indicative of submarine erosion of the Chalk caused by sea-level changes linked to syndepositional fault movements. However, evidence of toplap–downlap pairs is also suggestive of more aggradational mounding or laterally migrating surfaces or facies belts. The presence of any such structures implies rapid lateral variations in the Chalk that would have implications not only for aquifer management and future storage schemes but also during the processing of seismic reflection data and general studies and correlations of the Chalk across southern Britain.