The age, origin and tectonic significance of Mesozoic sediment-filled fissures in the Mendip Hills (SW England): implications for extension models and Jurassic sea-level curves

Abstract

In the eastern Mendip Hills, on the northern margin of the Wessex Basin, SW England, the Carboniferous Limestone is cut by numerous fissures that are filled with Mesozoic sediments (sedimentary dykes, neptunian dykes). The fissures contain a record of Triassic–Lower Jurassic sediments that are only sparingly preserved in their normal stratigraphical position between the Carboniferous Limestone and the unconformably overlying Upper Inferior Oolite of Bajocian age. Detailed analysis of cross-cutting relationships, facies analysis, biostratigraphy, lithostratigraphy and strontium-isotope ages of relevant Mesozoic sediments has allowed the construction of an Upper Triassic–Lower Jurassic fissure-fill stratigraphy for the eastern Mendip area. Most fissures were clearly formed by rapid influx of unlithified sediment from the land surface or sea floor. Some smaller cavities, or larger cavities with restricted access to the unconformity, were apparently filled by sediment that trickled down into the fissure system. The vast majority of the Mendip fissures are interpreted as having formed as a response of the Carboniferous Limestone, north of major basin-bounding faults, to pulses of tectonic extension during Ladinian–Norian/Rhaetian, late Hettangian–early Sinemurian, late Sinemurian–early Pliensbachian, mid-Pliensbachian, late Pliensbachian and Bajocian times. Triassic–earliest Jurassic fissures have a broad spread of strike from E–W to NW–SE to N–S, accommodating extension in a roughly NE–SW direction. Younger Jurassic fissures show well-defined E–W and N–S trends with the former becoming dominant through time. Total extension of ∼4.7% N–S and ∼0.6% E–W was produced by the formation of Triassic–Jurassic fissures within the Carboniferous Limestone. Such patterns of extension are thought likely to be characteristic of the subsurface geology in much of southern England and Wales. Major implications of this study are that: (1) the presence of seismically unresolvable sediment-filled fissures in supposedly rigid fault blocks can lead to a significant underestimate of regional extension based on the restoration of motion on normal faults on seismic-reflection profiles, and (2) the isolation of pulses of tectonic activity with a temporal resolution of 10⁵–10⁶ years may provide a means of identifying a tectonic signal in relative sea-level curves derived from the Jurassic sedimentary record.