Comparison between high-resolution seismic and sequence stratigraphic approaches applied to the upper Jurassic deposits of the Dover Strait area (Northern France)

Abstract

This study is an attempt to interpret very high resolution seismic data, usually devoted to shallow marine surveys, in terms of very high resolution seismic and sequence stratigraphy at the scale of the reservoir. Seismic data are compared with outcrop-based observations in order to discuss the geometrical relationships between rock bodies, and the nature of the seismic reflections in terms of seismic facies related to type lithologies. Two high-resolution seismic surveys, using a sparker, were performed in the Dover Strait area some 100s of metres away from the Boulonnais coastal cliffs. The aim was to look to the 130 m-thick, mixed siliciclastic-carbonate deposits of Upper Jurassic age (Kimmeridgian-Tithonian) cropping out at the sea floor and to compare them to the coastal cliffs exposures. The 100 m apart seismic profiles exhibit a 100 m deep penetration and 1 m in vertical resolution (upper 30 ms part of the profiles). Four seismic facies are recognised on the seismic profiles. They comprise ten seismic units which can be grouped into three broad families. Group 1 has a parallel/aggradational reflection configuration and conformable boundaries which may correspond to the transgressive and highstand systems tracts. The seismic facies exhibit either a high continuity-high amplitude (corresponding to clayey-mudstone lithologies of the mid ramp environment) or a high continuity-low amplitude (corresponding to shaly lithologies of the outer ramp environment). Group 2 has a sigmoid-progradational reflection configuration, bounded at the base by a downlap surface and at the top by a toplap surface. These boundaries correspond respectively in the field to a marine regressive surface of erosion and a marine transgressive surface of erosion. The seismic facies is cross-layered and exhibits a moderate continuity, low to moderate amplitude and moderate frequency. Group 2 corresponds to the bioclastic, sand-prone facies of the inner ramp environment. It is interpreted as lowstand sharp-based shorefaces. Group 3 exhibits a progradational, complex to chaotic configuration. It lies on an erosional unconformity with paleogullies. The seismic facies shows discontinuous reflections with moderate amplitude and high to moderate frequency. It corresponds in the field to cross-bedded sandstones in channel fill arrangements of fluvial origin (Wealdian deposits) marking the top of the section. The different systems tracts, sequence boundaries and depositional sequences identified onshore can be recognised as well on the seismic profiles, but the comparison demonstrates that seismic data complement the onshore sequence stratigraphic analysis by providing the geometrical relationships between systems tracts (onlap, downlap, toplap …) that cannot be observed onshore. The maximum flooding surfaces however present no specific seismic signature on such a low angle ramp profile. It can only be identified precisely in the field, which also help to relate lithologies to seismic facies. This study represents one of the first attempts to interpret very high resolution seismic data at the outcrop scale. It will be improved again in the future by the collection of impedance logs and synthetic seismic traces from cored sections on the coastal cliffs for comparison with the seismic data.