Late Jurassic Extension Research Articles

Seismic sequence stratigraphy and tectonics offshore Namibia

Abstract High quality seismic data recently acquired offshore Namibia have revealed the complex interplay between sedimentation and rifting that accompanied the breakup of Gondwanaland. Seismic sequence analysis has demonstrated a complex series of superimposed phases of rifting. Jurassic early-rift sediments were deposited in a Basin & Range setting with marine embayments. Mid- to Late Jurassic extension probably culminated in acidic to intermediate volcanicity. Subsequently, late-rift fill sedimentary accumulations were terminated locally by a major period of plateau-forming effusive activity. This was coincident with the start of Early Cretaceous spreading in the South Atlantic. Shelf, slope and basinal facies are clearly defined in the post-rift succession. Although virtually undrilled, the area is considered to have very considerable petroleum potential. Seismic stratigraphy will continue to play a vital role in defining the critical juxtapositions of source rock, reservoir and seal.

Mesozoic extension in the North Sea: constraints from flexural backstripping, forward modelling and fault populations

The magnitude and distribuion of late Jurassic extension in the Northern Viking Graben has been investigated by (i) syn-rift forward modelling using the flexural-cantilever model of continental rifting; (ii) post-rift flexural backstripping of a series of cross-sections; and (iii) the analysis of fault-population statistics. Application of these three techniques indicates that Jurassic extension on the tilted fault-block terrains of the East Shetland Basin, Tampen Spur and western Horda Platform is on average c. 15% ( β = 1.15). In the graben axis the regional value of β rises to c. 1.3, perhaps locally rising to 1.4. On the eastern Horda Platform Jurassic extension is low, β = c. 1.05. Flexural backstripping of the post-rift part of a cross-section through the Central Graben yields similar estimates of extension. At the flanks of the Jurassic basin β is estimated to be 1.2, rising to a likely maximum of c. 1.3 in the basin centre. These estimates of extension lie at, or towards, the low end of the previously published range of estimates. The principal reasons for this are (i) the incorporation of the thick sequence of Triassic and Lower Jurassic sediments in the backstripping; and (ii) the use of flexural isostasy (rather than Airy isostasy) in both the backstripping and forward modelling. The estimates of Jurassic extension obtained in this study do not account for the observed crustal thinning and, therefore, point to there having been a significant pre-Jurassic extensional event in the North Sea, of probable Triassic age. While a residual thermal anomaly from this event may have made a small contribution to post-Jurassic subsidence, compaction of the Triassic-Middle Jurassic sequence has been a significant contribution of the Triassic event to the thickness of the Cretaceous/Tertiary basin.

Footwall uplift associated with Late Jurassic normal faulting in the northern North Sea

Tilted fault-blocks in the North Sea show varying degrees of erosion. Such footwall erosion is predicted by the domino model of extension, in which rigid fault-blocks rotate as extension proceeds. Profiles of three North Sea fields (Snorre, Brent and Heather) are analysed to estimate the amount of uplift that accompanied Late Jurassic normal faulting, and these estimates are compared with the footwall uplift predicted by the domino model. Agreement between observed and predicted uplift is excellent, varying from >1 km at Snorre to c . 100 m at Heather. For a given amount of extension, large fault-blocks tend to be bounded by large faults and to show a greater degree of erosion. The Brent Province of the northern North Sea is an area of prolific hydrocarbon discoveries. Almost all of the major oil-fields occur in tilted fault-blocks which were formed by Late Jurassic extension and subsequently buried by Cretaceous-Tertiary thermal subsidence ( Badley et al. 1988 ; Marsden et al. 1990 ). The crests of the tilted blocks are typically eroded or degraded, with varying amounts of Middle-Lower Jurassic and Upper Triassic section removed. This footwall erosion is commonly ascribed in the literature to pre-rifting doming ( Ziegler 1982 ), heterogeneous stretching ( Coward 1986 ), local transpression ( Beach 1985 ) or even regional compression ( Frost 1987 ). The purpose of this paper is to show that the uplift and degradation is a direct consequence of the Late Jurassic normal faulting and fault-block rotation. The degree of uplift is related in a simple way to the size (across strike) of