Geometry and tectonic evolution of megasequences in the Plymouth Bay Basin, English Channel

Abstract

A complete Carboniferous-Permian-Triassic succession is found within the anomalously thick (10 km to acoustic basement) Plymouth Bay Basin (PBB). The evolution of this succession provides us with the most complete tectonic/ stratigraphic history of the late Palaeozoic to Triassic interval of the North Atlantic area. Deep and commercial seismic data from the PBB reveal the existence of four prominent seismic megasequences, bounded by strong and laterally persistent reflections. Isochore mapping and seismic facies analysis of each megasequence demonstrate that depocentres changed upward through the succession in a consistent manner, from E-W to elongate, NW-SE-orientated depocentres. This change may have occurred in the late Carboniferous or early Permian. The two-fold tectonic history of the PBB explains the thickness of sedimentary fill: an early basin formed above a deep crustal ramp, developed during Variscan compression along the Lizard-Start thrusts (and others); a later NW-SE early extensional or strike-slip basin depocentre is coincident with the earlier basin centre, creating further accommodation space.

The Rhenohercynian passive margin of SW England: Development, inversion and extensional reactivation

“…The lower two megasequences are ENE striking, overlain by 9 bright reflectors interpreted as latest Carboniferous-Early Permian lavas similar to those onshore, and are considered to have formed by extensional reactivation of thrusts defining the orogenic wedge and upper plate [77].…”

New evidence of Permo-Triassic rifting, onshore southern Ireland, and its implications for Variscan structural inheritance

“…Ruffell et al 1995). It is therefore reasonable to assume that any extension, particularly along a basinbounding fault, would have utilized existing (Variscan) basement fractures, lines of weakness and fabrics (Stein and Blundell 1990).…”

Late Paleozoic extensional reactivation of the Rheic–Rhenohercynian suture zone in SW England, the English Channel and Western Approaches

“…Farther south, in the southern parts of the Western Approaches Trough and Brittany Basin there are limited pre-Devonian well penetrations (e.g. Lenkett-1 Well, Ruffell et al 1995). These typically indicate rocks that can be correlated with Penteverian and Brioverian sequences from the Armorican Massif in NW France and the Channel Islands that were deformed during the Cadomian orogeny (590-540 Ma) (e.g.…”

“…2) is very strongly influenced by the reactivation of the Rheic-Rhenohercynian suture and associated ENE-WSW thrust faults and NNW-SSE transfer faults (Chapman 1989;Day et al 1989;Evans 1990;Hillis & Chapman 1992;Harvey et al 1994;Ruffell et al 1995). Subsidence typically accommodated 3-4 km of Permo-Triassic sediments (Evans 1990) but locally up to 9 km in the Plymouth Bay Basin which is developed above a prominent zone of SSE-dipping reflectors (Harvey et al 1994;Ruffell et al 1995). Early basin evolution is consistent with the extensional reactivation of thrusts and gives rise to a "sag-like" basin with progressive onlap (Fig.…”

“…For example, Well 87/12-1A (50 km SSE of Lizard Point) records over 1200 m of Permian and Triassic red beds overlying a 300 m thick Middle Devonian (Eifelian-Givetian) carbonate package with high acoustic impedance (Evans 1990). The direct well tie and character of the package suggests that it also forms a laterally extensive basement unit beneath the Plymouth Bay Basin (Harvey et al 1994;Ruffell et al 1995). The relative continuity of these reflectors indicates that the upper plate Devonian is comparatively undeformed and similar to the Middle Devonian limestones which overlie Late Ordovician to Silurian granites of the Mid-German Crystalline Rise in the Saar-1 borehole of the Saar-Nahe Basin (Oncken 1997).…”