T.R. Astin

Short Paper: Devonian and later movements on the Great Glen fault system, Scotland

On the Scottish mainland the Great Glen fault (GGF) displaces the Emsian to Frasnian Orcadian Old Red Sandstone (ORS) by only 25–29 km dextrally but net post–ORS dextral offsets in Shetland are much larger (120 km total). Most of the displacement (15–20 km) Permian initiation of the Inner Moray Firth Basin. It probably occurred between Frasnian cessation of Orcadian extension and accompanied transpressional inversion of the Orcadian Basin in the mid-late Carboniferous and/or possibly the late Devonian. Devono-Carboniferous transtension may also have occurred. The earlier history of the GGF includes late Caledonian sinistral motion which must have ceased by the late Emsian.

Braided stream and flood plain architecture: the Rio Vero Formation, Spanish Pyrenees

Early- to middle-Miocene fluvial sandstones of the Rio Vero Formation were studied, in an area around the town of Barbastro, south central Pyrenees Spain. The outstanding quality of outcrops in this area allows a three-dimensional study of architectural elements. Six architectural elements are recognised, described in detail, and interpreted from three key localities. Seven main lithofacies were identified and sub-divided into gravelly, sandy and fine-grained lithofacies. The architectural elements and lithofacies have been combined with a hierarchy of depositional bounding surfaces to fully interpret the evolution of the depositional system at the meso- and macro-scale. Not only the different architectural elements and lithofacies of the complete braided fluvial system, but also the lateral variation of the architectural elements were emphasised in this study. Differential tectonic movements, seasonal climate change, and their effect on vertical and lateral evolution of the area were the main control on basin sedimentation, channel interconnection, palaeocurrent patterns, and consequently the fluvial architecture. The presence of lateral ramp anticlines caused the fluvial system to be laterally restricted, with the main channel-belts being located in the areas of highest subsidence and lowest topography. Intervening topographic highs acted as both flood plains and lateral barriers between the main channel systems. The proposed depositional model comprises broad, low-sinuosity, perennial, but seasonal moderate-energy streams. The sandstone architecture is dominated by channel-fill and sheet sands, and associated simple and more complex bars. Adjacent to the main channel-belts fine-grained sandstones, siltstones and immature paleosols occur. The along-strike relationship between major fluvial systems and their outlets into a foreland basin has important implications for the infill of the basin and the modelling of fluvial systems along mountain belt fronts.

The Devonian lacustrine sediments of Orkney, Scotland; implications for climate cyclicity, basin structure and maturation history

The Orcadian Basin of northern Scotland (largely of Middle Devonian age) is a major lacustrine rift basin, with the widely correlateable, Achanarras/Sandwick lake bed extending for at least 800 km along the rift. Above this lake bed, there are about 540 m of cyclic lacustrine sediments, with lake sedimentation terminated by alluvial fan progadation. The Upper Stromness Flagstone Formation and the Rousay Flagstone Formation making up this lacustrine sequence are given revised and precise definitions within four reference sections from the Orkney Islands. The sequence consists of about 45 first-order cycles averaging about 12 m in thickness, which resulted from long-term climatic fluctuations in rainfall with a cycle time-scale of about 25 000 years. Longer climatic fluctuations of about 100000 and 400000 years are also present. These periods correspond to the precession and two eccentricity orbital cycles. Detailed knowledge of the stratigraphy constrains the structural and maturation history of the basin. The basin formed in a series of extensional half graben, and the Orkney area suffered later inversion. The lake sediments first reached the oil window at relatively shallow depths (c. 2000 m), probably in the early Carboniferous, implying very high geothermal gradients during the early part of the basin history as corroborated by the presence of Devonian volcanics in the region. In the Orkney area, uplift probably started in the late Carboniferous.

Experimental measurements of seismic attenuation in microfractured sedimentary rock

Ultrasonic compressional- and shear-wave attenuation in water-saturated Carrara Marble increase with increasing crack density and decreasing effective pressure. Between 0.4 and 1.0 MHz, empirical linear relationships between 1/Q and crack density CD were found to be:CD = 1.96 + or - 0.63 X 1/Q,for compressional waves andCD = 6.7 + or - 1.5 X 1/Q,for shear waves.

Climatic and tectonic controls on fluvial incision and aggradation in the Spanish Pyrenees

The influences of tectonic and climatic changes on upland river systems are investigated using data from Plio-Pleistocene terraces of the Rio Cinca river in the southern Pyrenees. This river runs transverse to the main thrust fault structures and is a major conduit for sediment delivery to the Ebro Basin. Detailed field mapping, combined with long-profile and palaeohydraulic reconstructions, yields a comprehensive picture of changes in palaeoriver character during the Plio-Pleistocene. As the area is over 150 km from the basin outlet in the Mediterranean Sea, changes in base level are unlikely to have influenced terrace development. Although tectonic activity has exerted a strong control on the position of the river, the main period of thrust propagation pre-dates the terraces and activity has waned from the Pliocene through to the present. It is concluded that the main control on incision in this area is climate, through its influence on sediment supply. Rivers which are starved of sediment by climate change will have the power to incise, whereas aggradational phases are linked to periods of increased sediment flux.

The Karoo sediments (Late Palaeozoic to Early Jurassic) of the Ogaden Basin, Ethiopia

Abstract The Ogaden Basin is a major sedimentary basin in East Africa located in the south and southeast of Ethiopia, covering an area of about 350,000 km 2 . It contains over 5,000 m of Phanerozoic sediments, as evidenced by recent drilling and seismic surveying. A sedimentological investigation has been carried out on cored sections of the Karoo. These are the oldest sediments in the basin, and occur only in the subsurface. Karoo sediments (Permian? to Lower Jurassic) in the Ogaden Basin have an average thickness of about 800 to 1,200 m. The sequence is divided into four formations, from oldest to youngest: the Calub Sandstone, the Bokh Shale, the Gumburo Sandstone and the Adigrat Sandstone. Continental Karoo sedimentation was ended by a progressive Mesozoic marine transgression which initiated the deposition of the overlying Hamanlei Limestone. The basal Calub Sandstone consists of arkosic and lithic conglomerate and sandstone of alluvial fan origin. It is overlain by lacustrine shale, siltstone and minor sandstone of the Bokh Shale. The remaining two formations have a transitional boundary, the Gumburo and Adigrat Sandstones, are fluviatile quartzitic sandstone deposited by braided and meandering rivers, respectively. Initially, Karoo sedimentation took place within an extensional rift, before becoming progressively more widespread during the deposition of the Gumburo and Adigrat Sandstone.

The effects of pore‐fluid salinity on ultrasonic wave propagation in sandstones

Petrophysical interpretation of increasingly refined seismic data from subsurface formations requires a more fundamental understanding of seismic wave propagation in sedimentary rocks. We consider the variation of ultrasonic wave velocity and attenuation in sandstones with pore‐fluid salinity and show that wave propagation is modified in proportion to the clay content of the rock and the salinity of the pore fluid. Using an ultrasonic pulse reflection technique (590–890 kHz), we have measured the P-wave and S-wave velocities and attenuations of 15 saturated sandstones with variable effective pressure (5–60 MPa) and pore‐fluid salinity (0.0–3.4 M). In clean sandstones, there was close agreement between experimental and Biot model values of dVP/dM, but they diverged progressively in rocks containing more than 5% clay. However, this effect is small: VP changed by only 0.6% per molar change in salinity for a rock with a clay content of 29%. The variation of VS with brine molarity exhibited Biot behavior in so...

Least-squares inversion of in-situ sonic Q measurements: Stability and resolution

We minimize the effect of noise and increase both the reliability and the resolution of attenuation estimates obtained from multireceiver full‐waveform sonics. Multiple measurements of effective attenuation were generated from full‐waveform sonic data recorded by an eight‐receiver sonic tool in a gas‐bearing sandstone reservoir using two independent techniques: the logarithmic spectral ratio (LSR) and the instantaneous frequency (IF) method. After rejecting unstable estimates [receiver separation <2 ft (0.61 m)], least‐squares inversion was used to combine the multiple estimates into high‐resolution attenuation logs. The procedure was applied to raw attenuation data obtained with both the LSR and IF methods, and the resulting logs showed that the attenuation estimates obtained for the maximum receiver separation of 3.5 ft (1.07 m) provide a smoothed approximation of the high‐resolution measurements. The approximation is better for the IF method, with the normalized crosscorrelation factor between the low‐...

The Effect of Degree of Saturation On Ultrasonic Velocity And Attenuation In Sandstones

Ultrasonic measurements of P-wave and S-wave velocity and quality factors (Qp, Qs) have been obtained from 9 sandstones, over a wide range of water saturations and confining pressures. The sandstones display a wide rang of porosity and permeability. Saturation state was varied in the sandstones by equilibrating the cores with various capillary pressures. Confining pressure varied from 5MPa to 60MPa. The values of the measured acoustic parameters were normalised to the fully saturated values to allow inter-sample comparison.

Ultrasonic velocity and attenuation in water and brine‐saturated sandstones

ULTRASONIC measurements of Pand S-wave velocity (V) and attenuation (1000/Q) have been made on fifteen different fully saturated sandstones over a range of effective pressures, using distilled water and two concentrations of brine as pore fluids. We found that the degree of change in Vp per molar change in pore fluid salinity (Vp/M) was dependent on the amount of clay present, and deviated from Biot predictions at levels of clay content greater than 5%. The differences between the experimental Vs data and Biot predictions were generally smaller. There was not found to be any significant difference between P-wave attenuation in water and brine saturated rocks, but S-wave attenuation in brine saturated rocks was slightly greater than, or equal to, those in water saturated rocks over a wide range of values. These data are consistent with the observation that fresh water reduces the permeability of rocks to a much greater extent than brine (Monicard, 1980). The results are applicable to field data, since pore water salinity may vary from zero in fresh water to over 300 000 ppm in some petroleum basins, where salinity generally increases with depth (Batzle and Wang, 1992).