Fault activity and sedimentation in a marine rift basin (Upper Jurassic, Wessex Basin, UK)

Journal of the Geological Society, Jan 2000 by Newell, Andrew J

Abstract: Shallow-marine carbonates and siliciclastics of the Corallian Formation (Oxfordian-Early Kimmeridgian) accumulated on and around an intrabasinal high in the extensional Wessex Basin. Four sequences can be recognized. Sequences 1-3 accumulated under conditions of thermal subsidence on a ramp-type margin. The initial sequence was siliciclastic. Highstand sedimentation in this sequence reflects the supply of sandy mud from a recently emergent intrabasinal high. During transgression and regression this muddy sediment was reworked into cleaner sandstone bodies by landward or basinward migrating zones of shoreface erosion. Carbonates dominate the second and third sequences when rising sea level increased the area of carbonate production and reduced siliciclastic input. Oolite bodies developed as both transgressive barrier bars and highstand sheets. The forth sequence formed during the activation of major normal faults. This caused the breakdown of the ramp system, and patterns of sediment accumulation were strongly controlled by tectonic subsidence patterns.

Keywords: Jurassic, southern England, extensional tectonics, sequence stratigraphy.

Many rift basins develop by pulses of active extension and faulting separated by phases of thermal subsidence (e.g. Chadwick 1986), creating the potential for highly variable basin topography over time. This has important implications for understanding the basin fill because topography is a major control on the orientation, geometry and fades of a sedimentary deposit (Frostick & Steel 1993). The aim of this paper is to show the effects of a switch from thermal subsidence to active extension on mixed siliciclastic and carbonate sedimentation in a marine rift basin. The rocks described are the Upper Jurassic Corallian Formation of the Wessex Basin in southern England. This is a coherent stratigraphic package (a maximum of 150 m thick) enclosed within thick deep marine mudrocks of the Oxford and Kimmeridge Clay formations. The Corallian of the Wessex basin is considered to be a useful testing ground for models of marine sedimentation in extensional basins because: (i) the tectonic development of the basin is relatively well constrained from hydrocarbon-related subsurface data (Chadwick 1986); and (ii) most of the Corallian stratigraphy can be examined in continuous coastal outcrop and has been the subject of intense research providing good fades and biostratigraphical control (e.g. Arkell 1947; Wright 1986; Sun 1989; Coe 1995; De Wet 1998; Goldring et al. 1998).

Summary of basin history

This study follows Underhill & Stoneley (1998) in confining the Wessex Basin to the central and western parts of southern England and adjacent offshore areas (Fig. 1). The area formed part of a network of extensional sedimentary subbasins that covered much of NW Europe during the Mesozoic. The subsidence history of the Wessex Basin is complex, with shifting depocentres and pulsed episodes of active faulting (Chadwick 1986). Many of the depocentres were structurally inverted in the Cenozoic (Underhill & Paterson 1998) and this, together with extensive intra-Cretaceous erosion, has left a very fragmentary record of Upper Jurassic rocks with which to devise basin-scale models.

The major elements of the Wessex basin in the Jurassic were the Cornubian massif to the west, and the intrabasinal Central Channel and Hampshire-Dieppe highs to the south and north (Fig. 1). The Hampshire-Dieppe High formed the dividing zone between the Wessex and Weald basins, and is important in terms of this paper. It represents an array of footwall uplifts along the Purbeck-Wight fault system. The faults in this system are major structures that locally have a cumulative pre-Tertiary displacement in excess of 2 km (Underhill & Stoneley 1998). Fault movement was episodic. The Oxfordian was characterized by regional flexural subsidence with little syndepositional faulting (Chadwick 1986). Siliciclastic and oolitic-carbonate sediments (Corallian sequences 1-3 of this study) accumulated on a ramp-type margin that dipped toward the southwest. In the late Oxfordian/early Kimmeridgian, renewed normal faulting led to accelerated subsidence in the Wessex Basin. Sandstone (Corallian sequence 4) formed the initial fill of these tectonically active depocentres prior to the deposition of the Kimmeridge Clay. In the Weald Basin, coeval sandstones form hydrocarbon reservoirs (Sun 1992).

Corallian stratigraphy

There is still no agreement on whether the Corallian should be ranked as a formation (House 1989), or a group (Wright 1986). Formation is used here in conjunction with a slightly modified version of the well-established member divisions (Fig. 2). Wright (1986) described the lithostratigraphy of the Corallian around Weymouth Bay in south Dorset. At coastal outcrop, the lower boundary of the Corallian Formation is a sharp lithological break between the Nothe Grit Member and the underlying Oxford Clay Formation (Coe 1995). However, the upper boundary with the Kimmeridge Clay Formation is traditionally picked on biostratigraphical criteria at the Oxfordian-Kimmeridgian stage boundary (Arkell 1947). Unfortunately, this boundary does not correspond to a clear lithological break and is of limited use for defining a lithostratigraphical unit (Brookfield 1978). In this study, all sandy siliciclastic and carbonate strata between the more homogeneous mudstones of the Oxford and Kimmeridge Clay are included within the Corallian Formation. This includes Early Kimmeridgian (Baylei and Cymodoce ammonite zones) sandstones, which can reach 40 m thick in areas such as Abbotsbury. These sandstones represent the youngest of the Corallian sequences (Sequence 4) identified in this study. The inclusion of early Kimmeridgian sandstones within the Corallian of the Wessex Basin is consistent with the use of this term in the Weald Basin (Sun 1992).