The role of uplifts in the rifting and sedimentation history of the N. Atlantic

The synthesis and integration of existing biostratigraphic and sedimentological information from exploration wells in the circum-Atlantic region with recent developments in genetic sequence stratigraphic techniques and computer modelling has allowed the documentation of uplifts and the rifting history of the North Atlantic during the Late Jurassic and Early Cretaceous and has led to a better understanding of the history of N.Atlantic extension. Correlation of the Late Mesozoic stratigraphy between the Lusitanian, Celtic Sea Area, Wessex, Jeanne d'Arc, Faeroe/Shetland Basins and the Rockall Trough area has resulted in the identification of the main unconformities/uplifts to have affected the N.Atlantic. This has permitted a new understanding for the evolution of the N.Atlantic and the mechanisms of rifting. Up to four major unconformities were identified along the Grand Banks/Iberian Margin during the Oxfordian to Valanginian period. The Late Kimmeridgian and Middle Berriasian unconformities caused the greatest amount of erosion and are associated with uplift events interpreted from vitrinite reflectance data, sedimentation and erosion patterns. These events are thought to be related to the sea floor spreading in the Tagus abyssal plain and the establishment of a new N.Atlantic rift axis in the Early Cretaceous. In the Celtic Sea Area and Wessex Basins a major intra-Cretaceous unconformity has been identified and it is interpreted to be associated with an uplift event which was heralded by a Late Jurassic/Early Cretaceous regression which climaxed in the Berriasian. The uplift, interpreted from subsidence, AFTA and erosion data, was centred on the granites of the Cornubian Platform and peaked after the initiation of rifting in the Late Jurassic and before the emplacement of oceanic crust in the Bay of Biscay and west of Goban Spur in the Aptian/Albian. Younger Aptian and Albian unconformities, to which the older event is normally linked, are also recognised but probably relate to minor rifting events prior to, and uplift associated with, the opening of the Bay of Biscay. A major intra-Cretaceous unconformity also exists in the Faeroe/Shetland to Rockall Trough area and is interpreted to be related to an uplift event in the Late Ryazanian centred on North Rockall, prior to Cretaceous rifting. The rifting in the proto-Rockall Trough propagated from south to north into the Faeroe/Shetland area following this uplift throughout the Early Cretaceous. It is proposed that this was the most significant rifting event to affect the Rockall Trough. From this study three different rift trends (Jurassic, Cretaceous and Tertiary) can be identified from the Late Jurassic to the present-day over the N.Atlantic area. These trends can be related to times of widespread plate reorganisation during the Tithonian, the Valanginian and the Cenomanian. Most areas underwent three phases of uplift peaking in the Kimmeridgian-Tithonian, Late Berriasian and AptianlAlbian prior to these plate reorganisations. These uplifts appear to have formed geographically ahead of the propagating rift systems and prior to localised extension, possibly due to tensional stress build-ups in the lithosphere. Evidence for the possible involvement of other localised uplift mechanisms such as asthenospheric upwellings and isostatic uplift has been inconclusive although these processes can not be completely ruled out as their effects have been recorded elsewhere. The main conclusion from this work is that many rift systems undergo uplift just prior to rifting, which is manifested by a sedimentary shallowing in the stratigraphic record during the uplift, and widespread erosion which is further exaggerated by rift flank uplift. In the case of an active system such as the Late Mesozoic N.Atlantic these uplifts occur roughly at the same time and coincide with major changes in the path of the propagating rift system and the development of new sea floor spreading centres.