Use of high-resolution stratigraphy and derived lithoclasts to document structural inversion: a case study from the Paleogene, Isle of Wight, UK

Abstract

The effects of structural inversion generated by the Pyrenean Orogeny on the southerly bounding faults of the Hampshire Basin (the Needles and Sandown faults) and on Eocene sedimentation in the adjacent regions were studied in outcrop by sedimentary logging, dip records and the identification of lithoclasts reworked from the crests of anticlines generated during inversion. The duration and precise age of the hiatuses associated with inversion were identified using bio- and magnetostratigraphy and compared with the Geologic Time Scale 2020. The succession on the northern limb of the Sandown Anticline (Whitecliff Bay) includes five hiatuses of varying durations, which together form a progressive unconformity developed during the Lutetian–Priabonian interval (47–35 Ma). Syn-inversion deposits thicken southwards towards the southern margin of the Hampshire Basin and are erosionally truncated by unconformities. The effects of each pulse of inversion are recorded by successively shallower dips and the age and nature of clasts reworked from the crest of the Sandown Anticline. Most individual hiatuses are interpreted as minor unconformities developed subsequent to inversion, rather than eustatically generated sequence boundaries or transgressive surfaces. By contrast, the succession north of the Needles Fault (Alum Bay) does not contain hiatuses of magnitude or internal unconformities. Subsidiary anticlinal and synclinal structures developed in the NW of the island as a response to Eocene inversion events by the reactivation of minor basement faults. The new dates of the Eocene inversion events correspond closely with radiometric ages (48–36 Ma) derived from fracture vein-fill calcites to the west in Dorset.