COMPARISON BETWEEN HIGH-RESOLUTION SEISMIC AND SEQUENCE STRATIGRAPHICAPPROACHES APPLIED TO THE UPPER JURASSIC DEPOSITS OF THE DOVER-STRAITAREA (NORTHERN FRANCE)

This study is an attempt to interpret very high resolution seismic dat a, usually devoted to shallow marine surveys, in terms of very high re solution seismic and sequence stratigraphy at the scale of the reservo ir. 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 spar ker, 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-t hick, mixed siliciclastic-carbonate deposits of Upper Jurassic age (Ki mmeridigian-Tithonian) cropping out at the sea floor and to compare th em to the coastal cliffs exposures. The 100 m apart seismic profiles e xhibit 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 groupe d into three broad families. Group 1 has a parallel/aggradational refl ection configuration and conformable boundaries which may correspond t o the transgressive and highstand systems tracts. The seismic facies e xhibit either a high continuity-high amplitude (corresponding to claye y-mudstone lithologies of the mid ramp environment) or a high continui ty-low amplitude (corresponding to shaly lithologies of the outer ramp environment). Group 2 has a sigmoid-progradational reflection configu ration, bounded at the base by a downlap surface and at the top by a t oplap surface. These boundaries correspond respectively in the field t o a marine regressive surface of erosion and a marine transgressive su rface of erosion. The seismic facies is cross-layered and exhibits a m oderate 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 f acies shows discontinuous reflections with moderate amplitude and high to moderate frequency. It corresponds in the field to cross-bedded sa ndstones in channel fill arrangements of fluvial origin (Wealdian depo sits) marking the top of the section. The different systems tracts, se quence boundaries and depositional sequences identified onshore can be recognised as well on the seismic profiles, but the comparison demons trates that seismic data complement the onshore sequence stratigraphic analysis by providing the geometrical relationships between systems t racts (onlap, downlap, toplap...) that cannot be observed onshore. The maximum flooding surfaces however present no specific seismic signatu re on such a low angle ramp profile. It can only be identified precise ly in the field, which also help to relate lithologies to seismic faci es. 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 comparis on with the seismic data. (C) 1998 Elsevier Science Ltd. All rights re served.