John R. Smallwood

Crust generated above the Iceland mantle plume: From continental rift to oceanic spreading center

Since the North Atlantic continental breakup in the early Tertiary, the process of rifting above a mantle plume has produced large thicknesses of igneous crust. We report results of an integrated offshore-onshore seismic study of the crust and upper mantle along a transect of the aseismic Faroe-Iceland Ridge, between the continental fragment beneath the Faroe Islands and the present-day spreading center in northeast Iceland. Normal-incidence seismic data provide an image of the uppermost crust, which is complemented by a velocity model from streamer refraction analyses. These data together image four sedimentary basins, up to 400 m deep, along the crest of the ridge. The streamer refraction velocity model in turn forms the uppermost section of a full crustal velocity model derived from travel time modeling of air gun and explosive data. The compressional to shear wave velocity ratio in the crust of the Faroe-Iceland Ridge is 1.77 ± 0.02, similar to that of eastern Iceland. The Moho beneath the Faroe-Iceland Ridge lies at a depth of 25–30 km. Crust of 25–30 km thickness requires an upper mantle potential temperature elevated 200–250°C above normal if formed by passive adiabatic decompression melting of the mantle or a somewhat less elevated temperature if, as is likely, there is a component of active convection in the underlying mantle plume core.

New technique for dating igneous sills

Three-dimensional seismic interpretation of Tertiary sills within the northeast Atlantic margin demonstrates that shallow-level ( 20 km long. The fill of the minibasin is dated biostratigraphically as 54.6–55 Ma, which fits well with radiometric dates for the timing of intrusion of sills around this basin (ca. 53–55 Ma). The recognition and dating of onlap-fill seismic reflector packages that are delimited by forced folds form a new and useful method for dating shallow-level igneous intrusions in sedimentary basins, a method that provides independent constraints on radiometric dating techniques. Similar forced folds are recognized from other basins that have shallow-level sills, suggesting that the technique presented here may be similarly applicable in comparable geologic settings.

The properties, morphology and distribution of igneous sills: modelling, borehole data and 3D seismic from the Faroe-Shetland area

Abstract An extensive suite of igneous sills, collectively known as the Faroe-Shetland Sill Complex, has been intruded into the Cretaceous and Tertiary sedimentary section of the Faroe-Shetland Channel area. These sills have been imaged offshore by three-dimensional (3D) reflection seismic surveys and penetrated by several exploration boreholes. Data from wireline log measurements in these boreholes allow us to characterize the physical properties of the sills and their thermal aureoles. The borehole data has been compiled to produce new empirical relationships between sonic velocity and density, and between compressional and shear sonic velocities within the sills. These relationships are used to assist in calculation of synthetic seismic traces for sills intruded into sedimentary section, in order to calibrate the seismic response of the sills as observed in field data. This paper describes how the seismic amplitude response of the sills can be used to predict sill thickness where there is some nearby well control, and use this technique to estimate the volume of one well-imaged sill penetrated by Well 205/10-2b. Since the sills have a high impedance contrast with their host rocks, they return strong seismic reflections. 3D seismic survey data allow mapping of the morphology of the sills with a high level of confidence, although in some instances disruption of the downgoing seismic wavefield causes the seismic imaging of deeper sills and other structures to be very poor. Examples of sub-circular and dish-shapes sills, and also semi-conical and sheet-like intrusions, which are highly discordant are shown. The introduction of intrusive rocks can play an important role in the subsequent development of the sedimentary system. An example is shown in which differential compaction or soft sediment deformation around and above the sills appears to have controlled deposition of a reservoir quality sand body. The positioning of the sills within sedimentary basins is discussed, by constructing a simple model in which pressure support of magma from a crustal magma chamber provides the hydrostatic head of magma required for intrusion at shallow levels. This model is made semi-quantitative using a simple equation relating rock densities to intrusion depth, calibrated to observations from the Faroe-Shetland area. The model predicts that sills can be intruded at shallower levels in the sedimentary section above basement highs, which agrees with observations detailed in this paper.

The rise and fall of the Faroe–Shetland Basin: evidence from seismic mapping of the Balder Formation

Seismic mapping of the Faroe–Shetland Basin from three-dimensional surveys provides striking evidence for uplift of the southern Faroe–Shetland Basin in the late Palaeocene and rapid subsidence in the early Eocene. The seismic reflector at the base of the Balder Formation follows a regional unconformity surface, which records erosion of a major branching drainage network into the underlying Palaeocene section, following a major base level fall. The topography of this surface was subsequently in-filled following marine transgression, and deep-water conditions returned to the basin in the early Eocene. We attribute base level changes to the influence of the proto-Iceland mantle plume beneath the lithosphere, providing support until continental break-up west of the Faroe Islands.

Seismic attenuation of Atlantic margin basalts: observations and modeling

Paleogene basalts are present over much of the northeastern Atlantic European margin. In regions containing significant thicknesses of layered basalt flows, conducting seismic imaging within and beneath the volcanic section has proven difficult, largely because the basalts severely attenuate and scatter seismic energy. We use data from a vertical seismic profile (VSP) from well 164/07-1 that penetrated 1.2 km of basalt in the northern Rockall Trough west of Britain to measure the seismic attenuation caused by the in-situ basalts. The effective quality factor Qeff of the basalt layer is found from the VSP to be 15–35, which is considerably lower (more attenuative) than the intrinsic attenuation measured on basalt samples in the laboratory. We then run synthetic seismogram models to investigate the likely cause of the attenuation. Full waveform 1D modeling of stacked sequences of lava flows based on rock properties from the same well indicates that much of the seismic attenuation observed from the VSP can b...

Alternatives in Paleocene exploration West of Shetland: a case study

Synopsis This paper illustrates oil exploration in the West of Shetlands area from 1995 to 2001 by documenting the history of Block 205/9, awarded in the UK 16th Licensing Round in 1995. Good quality Paleocene sands had been encountered in the 1989 well 205/9-1 in the down-dip part of the block, and equivalent sands were absent on the adjacent Flett Ridge to the SE, setting up the possibility of a pinchout play. The first well testing the play, 205/8-1, was drilled on the overall pinchout on 2D seismic data, but 3D seismic data were acquired across the area to better delineate the depositional systems. The first well drilled on the 3D data to test the sand pinchout, 205/14-3, failed to encounter sands in communication with those in 205/9-1, so attention turned to alternative stratigraphic traps highlighted by seismic amplitude anomalies. Detailed evaluation of the seismic data revealed that the attractive seismic amplitude response of one potential prospect was actually an artefact associated with overlying basalt. Further analysis of the 3D seismic attributes identified a tongue of sand south of 205/9-1, and analysis of magnetic fabrics from core data in 205/9-1 revealed that sand was input to this area from the NE, contrary to previous models. Unfortunately the seismic attributes, supported by fluid inclusion data, suggested that the sand was water-wet. As a result of the evaluation work, the block was relinquished in 1999 without further drilling. While there is undoubtedly scope for stratigraphic traps in the area to prove hydrocarbon-bearing, experience on the 205/9 block and other studies led to a refocusing on dip-closed structures, and a resulting discovery, the first in the Faroes, consisted of 170 m of hydrocarbon-bearing sands in the T10 Paleocene interval.

Seismic velocity structure of basalt flows

SUMMARY Basalt flows form a significant impediment to seismic imaging of structures beneath them. This arises both from the internal layering of the basalts, which may generate complex inter-bed multiples and interference patterns, and from the large impedance contrasts where high-velocity basalts overly low-velocity sediments. By making use of the wide-angle arrivals, both velocity and structural information can be derived from the refracted and reflected waves that pass through the basalts. We show examples of wide-angle seismic data recorded from the Atlantic volcanic margin, using two-ship acquisition to attain large offsets, and of normal incidence data from stacked lava flows in Iceland. Both are modeled using synthetic seismogram methods. We summarize the velocity structure and gradients of volcanic sequences by a compilation taken from sub-surface measurements around the world.

John Playfair on Schiehallion, 1801–1811

Abstract John Playfair first visited the Scottish mountain, Schiehallion, during Nevil Maskelynes 1774 plumbline deflection experiment, which was conducted to measure the density of the Earth. The mathematician Charles Hutton analysed the survey data from the experiment, reporting the mean Earth specific gravity as 4.5 in 1778. Playfair undertook a lithological mapping exercise in 1801, to improve the accuracy of Huttons estimate, and reported a range of 4.56–4.87 in 1811. The computation of the gravitational effect of topography with variable subsurface density effectively made him the creator of the first geophysical model. As such, not only was Playfairs Schiehallion contribution pioneering in itself, but it was representative of his more significant works in both mathematics and geology, in that he built on existing benchmark work with novel and valuable additions of his own. Although Playfairs map of the extent of the Schiehallion quartzite was quite accurate, the Societys fourth President, John MacCulloch, having visited Schiehallion, was dismissive of Playfairs representation of the subsurface density variation. MacCulloch spent several years searching Scotland for a more favourable site for a plumbline experiment, travels that allowed him to compile the data for his 1836 geological map of Scotland.

Attenuation of atlantic margin basalts using downhole VSP

A vertical seismic profile (VSP) from the Northern Rockall Trough, NE Atlantic Margin which penetrated 1km of basalt is used to estimate the effective quality factor, Q, a measure of seismic attenuation through the basalt. We filter the data to remove all but the downgoing waves then use the spectral ratio method on the direct arrivals to calculate Q. We show a robust result that measured Q is very low (15-35) throughout the basalt section. The result indicates that only a low frequency seismic source will be able to return significant energy from beneath such basalts.