Basin Research - Volume 6, Issue 4, 1994

Two‐dimensional seismic modelling has been undertaken on an overall progradational succession of sloping mudstone and sandstone units from the Palaeogene of Spitsbergen. The modelling shows that the main geometric features of the section would be resolved at 1500 m depth (with frequencies below 60 Hz, which is common in seismic data at these depths). However, interference between the base and top of lithological units gives lateral amplitude variations and discrepancies between the seismic image and the geometrical model. This is particularly prominent in low‐frequency models. Terminations of reflectors, resembling toplap and onlap, may be interpreted, but are artefacts of the general convergence of lithological units present in the geometrical model. The geological section causes a seismic pattern resembling sigmoid progradational seismic facies. Two‐dimensional seismic modelling is an efficient tool in bridging the gap between outcrop observations and subsurface data. Hence, modelled outcrop sections are important as reference points' for improved seismic stratigraphic interpretation.

Multiple episodes of extensional tectonism dominated the formation of Mesozoic fault‐bounded basins on the Grand Banks of Newfoundland, the Irish Continental Shelf and the central North Sea. A range of structural and stratigraphic responses in the Jeanne d'Arc, Porcupine and Moray Firth basins support widespread synchronous tectonic controls on sedimentation during one of these episodes, the Late Cimmerian.

Rifting was preceded by a phase of related tectonism during which subsidence rates began to vary across broad areas but without significant fault block rotation. This Late Cimmerian ‘onset warp’ pattern of subsidence is considered to have been essential in the establishment of restricted anoxic basins from latest Oxfordian through Kimmeridgian (sensu gallico) time and the development of one prolific layer of organic‐rich source rocks.

The most prominent and widely recognized structural/lithostratigraphic response to Late Cimmerian rifting was the deposition of sediment wedges. Tithonian to early Valanginian strata generally thicken‐ into northerly trending faults in the Jeanne d'Arc and Porcupine basins, indicating that extensional stress was orientated WNW‐ESE across a very broad area. The misalignment of this regional Late Cimmerian extensional stress with local inherited structural fabric may be responsible for transpressional uplift of individual fault blocks in the Outer Moray Firth basin.

Sedimentological responses to Late Cimmerian rifting were varied, though a common lithofacies stacking pattern is recognized. Variably thick conglomerates and/or sandstones were widely deposited at the start of rift deformation, while palaeoenvironments ranged from alluvial and braid plain to submarine fan even within individual basins. The relatively coarse basal sediments fine upwards into a second layer of commonly organic‐rich shales and mark The widest variations in palaeoenvironments and sediment thicknesses occurred during the last phase of Late Cimmerian rift tectonism, though all three basins show evidence of decreasing water depths, increasing oxygen levels and increasing grain size. This lithofacies stacking pattern of relatively coarse to fine to coarse (reservoir/source/reservoir) and the development of bounding unconformities are largely attributable to progressive changes in rift‐controlled subsidence. Rift basin subsidence rates are interpreted to increase from a low at initiation of faulting to a mid‐rift peak, followed by slowing subsidence to the end of extension. A number of counteracting crustal mechanisms that may account for progressive variations in rift‐induced subsidence are considered.

The effect of various erosional processes on the relief development of a carbonate platform margin is documented from outcrops of the Southern Alps, northern Italy, by the occurrence of truncation surfaces and redistribution of remobilized sediments. The periplatform depositional history, with periods of intensive submarine erosion along the north‐western Trento plateau margin, is recorded by various carbonate deposits ranging in age from the Early Jurassic to Late Cretaceous with numerous gaps.

The first Early Jurassic period of submarine erosion is marked by truncation and extensive tectonic fracturing of lower Liassic oolitic skeletal periplatform deposits. These are overlain by pelmicritic sediments of late Hettangian to Toarcian age. The second period of submarine erosion during the late Early Jurassic resulted in almost complete truncation of the pelmicritic unit. Crinoidal to oolitic periplatform carbonate sands were subsequently deposited along the carbonate margin until the Aalenian/Bajocian. The third truncation surface was produced by partial current erosion of the crinoidal to oolitic periplatform deposits during the late Bajocian to Callovian. The fourth, and most prominent, truncation surface was produced by erosion during the Early Cretaceous cutting down from Aptian/Albian pelagic units to Toarcian periplatform deposits. The resulting submarine relief was completely buried during the late Maastrichtian by onlapping pelagic sediments.

The documentation of the depositional history during the Late Mesozoic of the north‐western Trento plateau pinpoints the main mechanisms responsible for the relief of the drowned carbonate platform margin. Extensional tectonic activity during differential subsidence and current‐induced erosional truncation, followed by gravitational downslope mass transport and rapid pelagic burial mainly determined the morphology of the drowned carbonate platform margin.

We investigate the effects of the cooling of intrusive and extrusive igneous bodies on the temperature history and surface heat flow of the Parana Basin. The Serra Geral igneous event (130–135 Ma) covered most of this basin with flood basalts. Associated with this event numerous sills and dykes intruded the sediments and basement, and extensive underplating may have occurred in the lower crust and upper mantle beneath the basin. We develop an analytical model of the conductive cooling of tabular intrusive bodies and use it to calculate temperatures within the sediments as a function of time since emplacement. Depending on the thickness of these igneous bodies and the timing of sequential emplacement, the thermal history of a given locus in the basin can range from a simple extended period of higher temperatures to multiple episodes of peak temperatures separated by cooling intervals. The cooling of surface flood basalts, sills and dykes is capable of maintaining temperatures abovc the normal geothermal gradient temperatures for a few hundred thousand years, while large‐scale underplating may influence temperatures for up to 10 million years. We conclude that any residual heat from the cooling of the Serra Geral igneous rocks has long since decayed to insignificant values and that present‐day temperatures and heat flow are not affected. However, the burial of the sediments beneath the thick basalt cap caused a permanent temperature increase of up to 50°C in the underlying sediments since the beginning of the Cretaceous.

Widespread bimodal‐calcalkaline magmatism and contemporaneously active faulting reflect the combined effects of late‐orogenic collapse and strike‐slip faulting during syn‐rift development of the late‐Variscan Saar‐Nahe Basin. The intrabasinal drainage systems experienced major but differing influences from three prominent styles of volcanic extrusives: extensive basic lava flows; localized acidic domes; and widespread pyroclastic units.

Sedimentary sections including basic to intermediate lavas show the transformation of pre‐eruptive meandering fluvial systems into lacustrine systems following lava eruptions. These relationships are interpreted in terms of damming of the drainage systems by the topographic barriers represented by the basic lavas themselves. Siliciclastic sediments re‐entered and ultimately filled the lakes through prograding, small‐scale crevasse channel/mouth bar complexes.

Uplift associated with the intrusive/extrusive emplacement of acidic domes affected both the accommodation space and the internal topography of the basin. Following an extrusive pulse, each dome collapse caused the progradation of voluminous volcaniclastic aprons onto the surrounding alluvial plain. Locally, the overload by large volumes of unconsolidated detritus converted the regional meandering drainage system into a braided one. The overall duration of acidic dome activity could have been longer than 10⁶ years, but was interrupted by repose periods varying probably between a few hundred years to several thousand years.

Pyroclastic activity accompanied dome emplacement and registers their episodic activity in the stratigraphic record. Fine‐grained ashes of phreatoplinian eruptions were spread widely within the basin and overloaded streams in the area of fallout. Rapid reworking of huge amounts of unconsolidated material supplied an abundance of volcanic detritus, which tended to fill topographic lows that were usually fault‐controlled.

The relationships between basin evolution, intrabasinal volcanism and contemporaneously active faulting, identified in the Saar‐Nahe Basin, are important. They emphasize and further the understanding of how similar volcanic products in sedimentary basins worldwide impacted on the depositional system.

Book reviewed in this article:

Diagenesis and Basin Development A. D. Horbury & A. G. Robinson (Eds)

Carbonate Sequence Stratigraphy: Recent Developments and Applications Robert G. Loucks & J. Frederick Sarg (Eds)