bluuebunny/crossref_metadata_2025 huggingface dataset:
Vendobionta and Psammocorallia: lost constructions of Precambrian evolution
Authors: SEILACHER A.
Published: December 2007

The non-availability of biomineralized skeletons and low levels of predation led Vendian evolution along strange avenues. The Ediacara-type Vendobionta appear to represent a kingdom, in which foliate shapes, large sizes and the necessary compartmentalization were achieved by quilting of the skin rather than by multicellularity. Psammocorallia, in contrast, are interpreted as coelenterates that constructed an internal sand skeleton. Both were immobile soft-bottom dwellers that had high population densities, and both became preserved by obrutional accidents; thus they render ‘fossil snap shots’, in which the original distributional patterns, age structures and standing biomasses of populations are accurately recorded.
Nutrient-enriched waters and the early skeletal fossil record
Authors: BRASIER M. D.
Published: December 2007

This paper examines the role of stratified, oxygen-depleted and nutrient-enriched water masses (NEW) in transforming the fossil record over the Precambrian-Cambrian boundary interval. The following may serve as markers for the encroachment of NEW over low-latitude cratons: trace-element enriched black shales, ‘stone coals’ and hydrocarbon source rocks, phosphorites, phosphatized shells and organic matter, sponge spicule cherts, early appearance of silica and phosphatic skeletons, appearance of
Anabarites
and
Coleoloides
biofacies, peak diversity of phosphatic species, occurrence of black shale lagerstätten and light δ
13
C values of carbonates. Nutrient-depleted waters (NDW) are inferred over inner cratonic areas, and nutrient-starved waters (NSW) may well have developed over deeper waters at times through the later Cambrian, related to the effects of sea-level rise and salinity stratification. Carbon isotopic fluctuations are suggested to record the history of these water masses, with major changes in carbon burial and palaeoproductivity. The latter may well imply episodes of ‘greenhouse’ climate, with globally raised levels of carbon dioxide and relatively high ocean temperatures.
Burgess Shale-type faunas in the context of the ‘Cambrian explosion’: a review
Authors: CONWAY MORRIS S.
Published: December 2007

Burgess Shale-type faunas are now known from almost 40 lower and middle Cambrian localities, although those from British Columbia (Canada), Yunnan (China) and Peary Land (Greenland) are of paramount significance. The diverse faunas are dominated by animals with an otherwise negligible fossilization potential, and their stratigraphical position makes them of unique importance in documenting the major adaptive radiations at this time (‘Cambrian explosion’).

Important features of these faunas include the presence of taxa surviving from the late Precambrian Ediacaran faunas, the discovery of articulated material that throws new light on scleritome organization in various Cambrian skeletal taxa, and the significance of the degree of anatomical disparity among Cambrian animals. In addition, these soft-bodied faunas offer some hope of reconciling soft-part data with trace fossil occurrences. New discoveries of Burgess Shale-type faunas are inevitable, and such finds will continue to enhance our under­standing of early animal evolution.
The Precambrian–Cambrian boundary: seawater chemistry, ocean circulation and nutrient supply in metazoan evolution, extinction and biomineralization
Authors: TUCKER MAURICE E.
Published: December 2007

This paper reviews the evidence for changes in the global environment from the late Precambrian into the Cambrian, against which the evolution of many metazoan groups and the development of biomineralization should be seen. With higher carbon dioxide levels, Precambrian seawater was more supersaturated with respect to CaCO
3
than Phanerozoic seawater and carbonates were precipitated easily. From the late Precambrian to the early Cambrian, there was a decrease in the Mg/Ca ratio and an increase in the
a
CO
2
of seawater. Changes in global climate (icehouse to greenhouse) and increased plate tectonic activity resulted in major changes in ocean circulation and nutrient levels, a rise in global temperature, and the formation of extensive shallow seas. The Vendian-Cambrian radiation events and onset of biomineralization must have been strongly influenced, if not driven, by these global environmental changes.
Direction of fluid flow during contact metamorphism around the Burstall Granite, Australia
Authors: CARTWRIGHT I. , OLIVER N. H. S.
Published: December 2007

Marbles adjacent to the Burstall granite, Mary Kathleen, Australia, record up-temperature fluid flow during contact metamorphism. These fluids were water-rich and probably partially derived from devolatilization reactions. By contrast, Fe-rich skarns closer to the granite were formed by fluids emanating from the granite flowing down temperature. The rocks in the Burstall granite aureole record the operation of two contrasting fluid-flow regimes during contact metamorphism that is predicted by numerical models of fluid flow around cooling plutons, but which has rarely been documented around mesozonal plutons.
Pressure, temperature and time constraints on Himalayan metamorphism from eastern Kashmir and western Zanskar
Authors: SEARLE M. P. , WATERS D. J. , REX D. C. , WILSON R. N.
Published: December 2007

Himalayan metamorphism in the Kashmir and Zanskar sector of the High Himalaya resulted from thrust- and fold-related crustal thickening within the Indian plate following the collision of India and Asia at
c.
50 Ma. Interbanded metapelites, marbles, calcareous schists, amphibolites and quartzites represent metamorphosed equivalents of the Palaeozoic-Lower Mesozoic continental margin rocks. Granitic rocks include pre-collision K-feldspar megacrystic and biotite granites and post-collision two-mica ± garnet ± tourmaline granites. Average pressure-temperature conditions of equilibration using the self-consistent thermodynamic data-set of Holland & Powell (1990) are presented across a 45 km traverse of the eastern Kashmir-western Zanskar High Himalaya. Kyanite is the stable aluminosilicate phase across 35 km outcrop width in the middle of the slab. Complex microstructures indicate that prograde metamorphism up to kyanite grade and fabric development in the upper structural nappes is early and unrelated to the Main Central thrust. Diachronous metamorphism propagated southwards with the overall structural evolution. Early isograds and thrust-fold structures were carried passively in the hanging-wall of the Main Central thrust.

Peak metamorphism, based on
40
Ar/
39
Ar hornblende ages, occurred pre–30.7
±
2.0 Ma at the top of the slab, and in the middle of the slab was pre-22 ± 1.0 Ma, probably 25-28 Ma. The lower structural levels in Zanskar record peak metamorphic conditions around 700–750 °C and 8 kbar, reflecting depths of burial of 28–30 km. A new younger schistosity, which is not present in the higher structural levels, was developed under kyanite grade conditions. The regional distribution of high temperatures recorded by thermo-barometry does not support the concept of additional heat being supplied by frictional heating along the Main Central thrust or magmatic heat resulting from anatexis. Exhumation of the Himalayan metamorphic rocks was achieved by three major processes: erosion as a result of crustal thickening, uplift of the rocks along the hangingwall of the Main Central thrust above a major frontal ramp in the Kishtwar Window area, and extensional unroofing along the footwall of a large-scale, NE-dipping normal fault at upper crustal levels, probably synchronous with compression at depth.
Passive continental margin subsidence
Authors: BARR D.
Published: December 2007

The papers in the following thematic set arise from the 1991 William Smith Lecture Meeting of the Geological Society, convened by J. M. Cubitt and held at Burlington House on 3 April 1991.

The purpose of this meeting was to review our current understanding of passive margin subsidence. To this end an international cast of speakers was invited to a meeting centred on the William Smith Lecture by A.B. Watts of Oxford University. Four of the papers delivered are published here.

Passive margins form just over half of the present-day continental margin system and, although like the world's oceans all are relatively young (Mesozoic-Tertiary), former passive margins of Palaeozoic and Precambrian age are preserved in orogenic belts and plate interiors. It is now generally accepted that passive margin subsidence is a direct consequence of litho-spheric thinning, most of which took place prior to final separation of the continents and emplacement of an oceanic spreading centre. Intracontinental rifts that failed to develop oceanic crust are geodynamically very similar to passive margins, and in recent years lessons learned in one environment have been successfully applied in the other.

In both cases, subsidence is isostatically driven and results from thinning of the continental crust and its replacement by higher density mantle. Because extension and thinning tends to involve the whole lithosphere (125 km or so in thickness), and not just the top 30 km or so of continental crust, higher density lithospheric mantle is displaced by lower density astheno-sphere during stretching, partially canceling
Crustal structure and the mechanical properties of extended continental lithosphere in the Valencia trough (western Mediterranean)
Authors: WATTS A. B. , TORNÉ M.
Published: December 2007

Seismic refraction and reflection, gravity and well data have been used to constrain the structure and evolution of the Valencia trough: a young extensional basin in the western Mediterranean. Seismic refraction data reveal the present-day crustal structure, while backstripping of seismic reflection and well data reveal the structure
at the time of rifting.
By comparing the seismically constrained crustal structure to that inferred from backstripping, we have been able to evaluate whether the Moho is in the expected position, given the tectonic subsidence/uplift and sediment loading history of the trough. The best overall fit between the predicted and seismically constrained crustal structure in the trough is for a model in which the extended continental crust and lithosphere has acquired little or no strength following rifting. A low strength is in accord with the observed free-air gravity anomaly as well as results from other types of rift basins such as the North Sea and the continental margin off the east coast of the USA. There are discrepancies between the observed and predicted Moho depths beneath the flanks of the Valencia trough, but they are attributed to either the removal of crustal material during or following rifting (Iberian peninsula), or to orogenic loading of previously extended continental crust (Mallorca). The largest discrepancy (up to 10 km) correlates with the Ebro delta which has prograded into the trough by more than 60 km since the Pliocene. The discrepancy cannot be explained by the same large-scale processes that modified the Moho beneath the trough flanks and requires that the lithosphere beneath the Ebro delta region is associated with a high (60 <
T
c
< 80 km), rather than low, flexural strength. We do not believe, however, that the high values indicate that extended continental lithosphere is fundamentally strong. Rather, we attribute it to incomplete stress relaxation of the lithosphere during loading.
A simple magma-mixing model for sulphur behaviour in calc-alkaline volcanic rocks: mineralogical evidence from Mount Pinatubo 1991 eruption
Authors: MATTHEWS S. J. , JONES A. P. , BRISTOW C. S.
Published: December 2007

The increasing recognition of anhydrite as a primary magmatic mineral in volcanic products, including the important 1991 Pinatubo eruption, places new emphasis on the cycling of sulphur from deep sources including the upper mantle. We present a simple model for the supersaturation of sulphur during mingling and mixing in calc-alkaline magmas due to the quenching and oxidation of the mafic end member, which may have important consequences for predicting the annual flux of igneous sulphur. This model predicts the coexistence of anhydrite and sulphur dioxide gas, and explains the discrepancy between erupted magma volumes and sulphur gas emissions.

Although sulphur may exist as a number of species in volcanic emissions, one of its most important phases in volcanic rocks is primary anhydrite (CaS0
4
). A number of important historic eruptions of calc-alkaline volcanoes have now been found to involve anhydrite-bearing magmas. These include Mount Lamington in Papua New Guinea (Arculus
et al.
1983), Nevado del Ruiz in Colombia (Fournelle 1990), and El Chichon in Mexico (

Rose
et al.
1984

). The Ruiz eruption involved documented magma mixing, and both the Nevado del Ruiz and El Chichon eruptions were associated with very high sulphur emissions (both approximately 3.3 x 10
6
tonnes;
Krueger 1983
;

Williams
et al.
1990

). We have observed magma-mixing textures in detailed studies of two neighbouring calc-alkaline volcanoes in northern Chile, at Lascar (
Matthews 1992
) and Tumisa (
Gardeweg 1988
). Anhydrite has been found associated with these textures in pumice from Lascar Volcano. Both anhydrite and similar disequilibrium textures indicative of magma
A pre-caldera plateau-andesite field in the Borrowdale Volcanic Group of the English Lake District
Authors: PETTERSON M. G. , BEDDOE-STEPHENS B. , MILLWARD D. , JOHNSON E. W.
Published: December 2007

The 6 km thick Ordovician Borrowdale Volcanic Group is readily divisible into a lower 2.2–2.7 km thick predominantly pre-caldera succession dominated by basalt, andesite and dacite sheets, and an upper succession of caldera-related ignimbrites and volcaniclastic sedimentary rocks of
c.
3 km thickness. The lower Borrowdale Volcanic Group rocks, here included within a single lithostratigraphical unit, the Birker Fell Formation, affords a well-exposed section through a pre-caldera sequence.

The Birker Fell Formation is dominated by andesites which comprise 60% of the stratigraphy. Thin sequences of reworked volcanic detritus are commonly interbedded with the andesites; locally there are thicker units of volcaniclastic sedimentary rocks. Interpretation of the stratigraphy and eruptive history of the formation has been aided by the recognition of some distinctive lithological units including: (1) a 200-600 m thick sequence of weakly parallel-bedded basaltic tuffs resting on the pre-volcanic basement, (2) a 200-600 m sequence of single flow unit aa basalts, (3) dacite flows and ignimbrites up to > 1100 m thick in the middle and upper parts of the formation and (4) aphyric to pyroxene-phyric, simple and compound flows of basalt, 50-400 m thick, locally present in the uppermost parts.

Analysis of the facies comprising the Birker Fell Formation indicates that it was emplaced as a sub-aerial, plateau-andesite sequence, formed by the coalescence of products erupted from a number of centres or fissures in an extensional, subsiding volcano-tectonic rift zone. Volcano-tectonic faulting and eruption of thick ignimbrites locally influenced development of this field.
Carboniferous magmatism in the Iapetus convergence zone: evidence from deep seismic reflection profiles
Authors: SMITH KEVIN
Published: December 2007

An alternative interpretation of the North East Coast (NEC) deep seismic reflection profile is presented. Seismic reflectors in the lower crust of the UK sector of the North Sea, which were formerly attributed to the complex effects of Caledonian continental collision, are shown to be consistent with the inferred processes of Carboniferous igneous intrusion and magmatic underplating in the Iapetus convergence zone. The nature and distribution of Carboniferous magmatism in southern Scotland and northern England are briefly described. ENE-trending intrusions and chains of volcanic centres are widely recognized. Similarly trending aeromagnetic anomalies are used to identify where the offshore continuation of this igneous province intersects the NEC line. As a result, an area of crustal thickening in the northern part of the line is directly related to the presence of Carboniferous igneous intrusions beneath southern Scotland. In the same way, the single deep reflector on the NEC line which was previously identified as the Iapetus suture, is reinterpreted as a lower crustal intrusion associated with the emplacement of the end-Carboniferous Whin sill. The new seismic interpretation is incorporated in a speculative, regional crustal model of Carboniferous magmatism.
Petrogenesis of the Potato Hill pluton, Newfoundland: transpression during the Grenvillian orogenic cycle?
Authors: OWEN J. V. , GREENOUGH J. D. , FRYER B. J. , LONGSTAFFE F. J.
Published: December 2007

The Potato Hill pluton is a nested, composite, Grenvillian intrusion (
c.
1020 Ma) consisting of pyroxene-bearing quartz monzodiorite, equigranular biotite + hornblende + garnet granite, and K-feld-spar megacrystic hornblende + biotite quartz monzodiorite. All members are potassic with high Fe/Mg ratios, and have elevated concentrations of trace elements such as Ba, Zr, Nb and Y. Mass balance calculations successfully relate the concentration of major elements between the most primitive (megacrystic quartz monzodiorite) and evolved (equigranular granite) samples to the fractional crystallization of orthopyroxene, clinopyroxene, hornblende, biotite, garnet and plagioclase, and of trace elements (e.g. P, La and Zr) to the removal of accessory phases such as apatite, allanite and zircon. Lithologic zonation of the pluton is therefore attributed to in situ differentiation processes rather than varying degrees of crustal assimilation or the emplacement of discrete magma batches. This is supported by (1) contrasting δ
18
O values for the Potato Hill pluton (δ
18
O = +6.8 to +8.5º/
∞
) and the gneissic country rock (δ
18
O = +10.5 to + 12.1º/
∞
), and (2) uniformly low ɛ
Nd
values (= -1.6 to + 1.2) for the granitoid rocks.

Tectonomagmatic discrimination diagrams indicate a within-plate, late-orogenic setting for the Potato Hill pluton. Furthermore, trace element signatures closely resemble shoshonitic lamprophyres. Compositional criteria therefore suggest that the Potato Hill pluton is not directly subduction-related, but was formed in a transpressional regime following ocean closure. Younger (post-1 Ga) Grenvillian plutons in the area may record crustal thickening or the subsequent extensional collapse of the eastern Grenville Orogen.
The Palaeozoic history of an unusual intracratonic thrust belt in central Australia based on
40
Ar-
39
Ar, K-Ar and fission track dating
Authors: SHAW R. D. , ZEITLER P. K. , McDOUGALL I. , TINGATE P. R.
Published: December 2007

Intraplate thrusting, involving reactivation of a mid-Proterozoic province boundary within the basement metamorphic rocks of the Arunta block, occurs at the northern margin of the late Proterozoic to mid-Palaeozoic Amadeus Basin in central Australia. Earlier geological and geophysical studies show that the thrust system, referred to as the Alice Springs orogen, is 'thick-skinned' in the central part of the thrust belt and was dominated by the crust-cutting Redbank thrust zone. Several episodes of thrusting, defined stratigraphically, began about 400 Ma ago. Analysis and modelling of
40
Ar-
39
Ar age spectra on K-feldspar from the basement rocks, combined with regional geological constraints, point to cooling rates of 3–10 °C/Ma and moderate overall exhumation rates of 100–400 m/Ma during the Alice Springs orogeny, without appreciably increased heat flow. However, final uplift along the thrust front at 300–320 Ma was rapid, as the time of final closure of small domains in K-feldspar approximates apatite fission track ages. By using
40
Ar-
39
Ar age data as a constraint, together with available geological and geophysical data, crustal shortening across the thrust belt is thought to be approximately 20 to 30%. Overall convergence rates of 0.6–3 km/Ma are slow compared with rates of
≥
5 km/Ma for continent-continent collision zones.
Chemical cyclicity and correlation of Lower Lias mudstones using gamma ray logs, Yorkshire, UK
Authors: VAN BUCHEM F. S. P. , MELNYK D. H. , McCAVE I. N.
Published: December 2007

Natural gamma-ray spectrometry on coastal sections from Redcar and Robin Hood's Bay permits the radioactive characterization of five different mudstone units and the construction of an outcrop gamma-ray log for the Lower Lias. Correlation of these five units with their equivalents in the gamma-ray log of the Felixkirk Borehole, located 40 km to the west, has been established. Statistical comparison of tie points in the composite coastal section shows there to be a stratigraphic gap of 10 m between the two localities relative to the continuous section in the Felixkirk Borehole.

The distribution of Th (ppm), K (%) and U (ppm) are generally highly correlated with the total radioactivity. However, anomalies do occur: a peak in the Th/K ratio can be related to an iron-oolitic horizon marking a condensed sequence in the early/mid-Sinemurian; a sharp fall in the Th/K ratio followed by high U values coincides with the early-Pliensbachian transgression. Comparison with mineralogical (clays) and chemical (Si/Al and Al/K ratio, V, Total Organic Carbon) profiles of the same section allows for an evaluation of the significance of radioactive element distribution in terms of grainsize, run-off and oxygenation indicators.

A spline curve fitted through the total radioactivity data emphasizes low frequency cyclicity (wavelengths of between 10 and 30 m) in the outcrop section. These cycles appear to be correlatable with cycles of comparable wavelengths in the low-pass filtered gamma-ray log of the Felixkirk Borehole. Sedimentological evidence confirmed high radioactivity peaks correspond to transgressive phases. The gamma-ray signal of the Felixkirk borehole has also been analysed for high frequency cyclicity. With an overall rate of accumulation of about 13m/Ma variation in the Milankovitch band is marked by units 5 m or less in thickness. Evidence for eccentricity, obliquity and precession cycles has been found both in borehole and in outcrop. A typical shift from lower to higher frequencies occurs consistently in transgressive phases, and is related to increased sedimentation rates.
Active normal faulting, drainage patterns and sedimentation in southwestern Turkey
Authors: PATON STUART
Published: December 2007

The geomorphology of the rapidly extending area of western Turkey is principally controlled by large active normal faults. The graben formed by the faults are asymmetric and about 10–20 km wide. Measurements of the rotation of Neogene sediments in the footwall of the active faults and 2–D modelling of gravity data show the extension across each graben to be about 6–10 km, suggesting that the whole region has undergone only small degrees of extension (β ≈ 1.2–1.3). The faults bounding the grabens are segmented on a length scale of 5–10 km with the segments linked by some means other than single ‘transfer’ faults. Where the footwall lithology is resistant to mechanical weathering, axial drainage is important and rivers cut through the line of the fault at the end of fault segments, producing large fans. Where the footwall lithology is less consolidated, drainage is generally in linear valleys. Major graben can have more than one phase of faulting, with the main graben bounding fault stepping successively into the hanging wall.
The development of Early Palaeozoic global stratigraphy
Authors: MCKERROW W. S.
Published: December 2007

The major steps in the development of Early Palaeozoic stratigraphy are examined, with special emphasis on early
Journal
papers by Murchison and Sedgwick, and on their conception of systems and series, which permitted long-distance correlation. Unlike other periods, the Ordovician and Silurian were originally split into series; most stages have only been defined in the past 60 years. From 1880, Lapworth's graptolite zones have allowed much greater chronological precision. More recently, other methods have been developed for recognizing small time divisions, including studies in gradational evolution. A significant new advance is the correlation (by biostratigraphy) of short-lived physical events such as magnetic reversals and sea-level and climatic changes.
Petrogenetic implications of neodymium isotope data from the Kohistan batholith, North Pakistan
Authors: PETTERSON M. G. , CRAWFORD M. B. , WINDLEY B. F.
Published: December 2007

Neodymium data are presented for five granitoid (trondhjemite, granite and leucogranite) plutonic units from the Kohistan batholith aged 102 Ma to 29 Ma. These have present day
l43
Nd/
144
Nd ratios of between 0.512980 and 0.512734, calculated initial
143
Nd/
144
Nd ratios of between 0.512861 and 0.512705 and Nd values of between 6.91 and 2.04. There is a decrease in ɛNd with time which is inversely correlative with a similar increase in ɛSr.

Three plutonic units (Matum Das, Gilgit and Shirot) formed from a source enriched in Sm and depleted in Rb or radiogenic Sr relative to bulk earth, whilst a fourth unit (the Indus Confluence acid sheets), is only slightly enriched in radiogenic Sr. These four units define a temporal trend from the least evolved Matum Das pluton to the most evolved Indus Confluence acid sheets. This trend was produced as a result of subducting oceanic sediment or seawater-altered oceanic crust which melted or dehydrated and increasingly modified the isotopic composition of an original mantle protolith situated above the subducting plate. The data preclude any significant input to the magmatism from ancient crust. New data presented here, together with other published data, indicate an immature metavolcanic crustal source for the Matum Das pluton, and a plutonic basement crustal source for the Indus Confluence acid sheets. These crustally derived units retain a mantle isotopic signature indicating that the geochemical signature of Kohistan evolved by remobilization of recently formed arc crust in addition to new inputs of mantle-derived magma.

A fifth plutonic unit (the Parri acid sheets) shows a clear compositional break from the other units, being significantly enriched in radiogenic Sr
and Nd
with respect to bulk earth. The source to the Parri acid sheets is interpreted as metasedimentary rocks with a high Rb/Sr ratio and a crustal residence time of
c.
70 Ma.
Organisms and sediments: relationships and applications
Authors: GOLDRING R. , POLLARD J. E.
Published: December 2007

The sixth Lyell meeting of the Geological Society was held on 13 February 1992. Major themes included advances in the study of trace fossils with emphasis on ichnofabrics, and the use of skeletal accumulations (shell and bone beds), both in elucidating environmental change, and in sequence stratigraphy. Of the 12 papers read at the meeting, seven are published in this thematic set.

The papers presented cover topics which Lyell did not himself discuss at any length, with the exception of his classic study on the Temple at Puzzuoli. Nevertheless the theme of the meeting was aptly expressed by him: ‘Arrangements of fossils in strata: Each stratum in fact however far it may now lie beneath the surface was once in the state of shingle, loose sand or soft mud at the bottom of the sea, in which shells and other bodies easily became enveloped. By attending to the nature of these remains we are often enabled to determine whether deposition was slow or rapid, whether it took place in a deep or shallow sea, near the shore or far from land, and whether the water was salt, brackish or fresh’ (Elements of Geology, Chapter 3).

As well as honouring Lyell it was befitting to remember Robert W. Frey (University of Georgia, Athens) who died on 1 January 1992 following a long fight against cancer. Much of his considerable research output was directly in the fields covered at the meeting. Over the past 20 years he pioneered the study of animal-sediment
Description and analysis of bioturbation and ichnofabric
Authors: TAYLOR A. M. , GOLDRING R.
Published: December 2007

A new scheme for the description and analysis of bioturbation and the resultant ichnofabrics is proposed. This system can be used in core and field-based studies and consists of two parts. (1) A
bioturbation index
in which a descriptive grade is assigned to the degree of bioturbation. This integrates the sedimentology and ichnology, where the higher grades of bioturbation result from increased burrow overlap and the subsequent loss of the primary sedimentary fabric. (2) An
ichnofabric constituent diagram
which records the detail of the ichnofabric by graphically plotting the dimensional data of the ichnotaxa and their order of emplacement against their coverage.

This scheme differs from previous attempts to characterize bioturbation using semi-quantatitive methods and visually portrays the ichnofabric so that comparative studies can be carried out. This is of particular use in facies analysis, the establishment of the ichnocoenoses and tier diagrams, and within sequence stratigraphic studies so that hiatal surfaces can be recognized.
Sedimentation control on growth of stromatoporoid reefs in the Silurian of Gotland, Sweden
Authors: KERSHAW S.
Published: December 2007

Silurian stromatoporoid-dominated reefs of Gotland, Sweden, are bioherms and biostromes formed during phases of reduced clastic supply. Well-exposed examples are in the early Wenlock Högklint Formation and middle Ludlow Hemse Group. Reef development culminated in shallow water biostromes, laterally expanded over a stabilized substrate and associated with frequent erosion surfaces. Högklint reefs began as bioherms, to be replaced by biostromal phases as water shallowed, with vertically zoned communities. Biostromal phases are considered equivalent to the whole of individual stromato-poroid biostromes (without biohermal phases) of the Hemse Group, which developed in shallow, low to moderate energy water, on stabilized substrate. Low clastic supply associated with these reefs reduced not only the clay input, which is otherwise common in most non-reef environments on Gotland, but also is interpreted to have lowered available nutrient levels. Modern reefs develop best in conditions of low nutrients and also show (a) suppressed bioerosion, and (b) increased symbiosis between corals and algae which conserves nutrients in the reef biota. Högklint reefs show reduced bioerosion while Hemse reefs show almost complete lack of bioerosion. Symbiosis between stromatoporoids and corals, interpreted as a response to the need for nutrient conservation, occurs in the reefs, but is much more common in Hemse reefs. Both Högklint and Hemse reefs are regarded as having grown in nutrient-deficient conditions in line with modern reefs although the effect on Hemse reefs is believed to have been greater because they appear to have grown in quieter conditions on a broad shelf and may have had reduced access to nutrients in a regressive regime throughout the Ludlow. Reduced clastic supply while the Gotland reefs grew could have been due to regional tectonism controlling local sea-level change. However, the Högklint Formation and Hemse Group reef phases coincide with times of proposed eustatic sea-level falls. Reef growth on Gotland could therefore relate to eustatic tectonism by subduction-related or plate flexure processes, or climatic control. An oceanographic model for the Silurian proposed alternating wet and dry episodes, interpreted as CO
2
-driven, and could have controlled clastic input, permitting the development of fossiliferous reefs and the carbonate platforms on which they formed. These interpretations provide a revised framework for examining the reef growth.
The Maryport fault: the post-Caledonian tectonic history of southern Britain in microcosm
Authors: CHADWICK R.A. , EVANS D.J. , HOLLIDAY D.W.
Published: December 2007

The Maryport fault forms part of the faulted southern margin of the Northumberland-Solway Basin. Seismic reflection data reveal a complex history of repeated fault reactivation. Large oblique-normal displacements on the fault controlled early Carboniferous extensional basin development. Subsequently, Variscan basin inversion was accompanied by partial reversal of the fault. In Permian and Mesozoic times, renewed oblique-normal displacements strongly influenced basin subsidence. Further partial reverse displacement occurred as the Solway Basin was again inverted, probably in mid-Tertiary times in response to Alpine compression. The Maryport fault is most unusual, therefore, in providing direct evidence of the principal post-Caledonian tectonic events in southern Britain.
A buried granite batholith and the origin of the Sole Pit Basin, UK Southern North Sea
Authors: DONATO J. A.
Published: December 2007

Modelled gravity profiles, based upon the post-Carboniferous geological structure revealed by reflection seismic data, have been used to predict the gravity variation across part of the Southern North Sea Basin. When compared to the observed gravity field, these calculated values reveal a significant residual gravity low located to the east of the Sole Pit axis. This low may indicate the presence of a large buried granite body; a sister to the granite previously proposed beneath the East Midland Shelf to the SW of the Sole Pit axis. During periods of extensional tectonics, major subsidence has occurred within the Sole Pit Basin between these two granite-cored stable areas.
Volcanically influenced calcareous palaeosols from the Miocene Kiahera Formation, Rusinga Island, Kenya
Authors: BESTLAND E. A. , RETALLACK G. J.
Published: December 2007

The Miocene Kiahera Formation at its type locality on Rusinga Island in the Kenyan part of Lake Victoria consists of a sequence of alluvial sedimentary and pyroclastic strata and calcareous palaeosols. The 80 m thick formation was formed by braidplain aggradation and pyroclastic airfall in response to volcanism from the nearby Kisingiri volcano.

Numerous fining-upward sequences approximately 5 m to 15 m thick make up the bulk of the Kiahera Formation. A complete sequence contains sandy and conglomeratic beds at the base which fine upward to sandy siltstones, calcareous palaeosols and airfall tuffs. The fining-upward sequences may have been produced by aggradation during phases of volcanic eruption. During major eruptive episodes, scoriaceous nephelinite detritus flooded the alluvial system to create the lower, coarse-grained beds. As eruptive activity waned, airfall ash and overbank flood deposits accumulated on the landscape forming the upper, fine-grained beds of the sequence. Calcareous soils formed on these highly alkaline ashes between large ash fall events.

The calcareous palaeosols are grouped broadly into two main types: tuffaceous clayey Inceptisols with distinct Ck horizons (Chido palaeosols) and sandy Inceptisols consisting of nephelinitic rock fragments and subordinate amounts of tuff (Okoto palaeosols). Both types show only weak to moderate soil development interpreted to be a result of episodic rejuvenation by fresh airfall ash and flood deposits. Comparison of depth of the calcareous subsurface horizon from these palaeosols with those in calcareous modern soils from the Serengeti Plain that also developed on alkaline volcanic ash allow for rainfall estimates of between 550 mm a
−1
to 750 mm a
−1
for the Kiahera palaeosols. This together with root traces, soil structure, and major element chemistry of the two palaeosol types are evidence of a landscape with sandy, stream-side woodland soils (Okoto palaeosols) and clayey, floodplain grassy woodland soils (Chido palaeosols).
Aspects of basin inversion in southern Britain
Authors: CHADWICK R. A.
Published: December 2007

The Wessex Basin of southern England, a Permian to Cretaceous extensional basin, can be structurally divided into a set of constituent asymmetrical graben, bounded by major E–W-trending zones of en echelon syn-depositional normal faults. The graben were inverted in late Cretaceous and Tertiary times by compressive stresses oriented roughly north–south. Inversion structures fall into two related categories. Regional upwarps overlie earlier graben depocentres and were formed by bulk shortening of the graben-fill. Superimposed upon and geographically delimiting the regional upwarps, are roughly east-west trending linear zones of en echelon inversion structures. These coincide with the earlier graben-bounding faults and typically have the form of monoclinal or periclinal flexures each underlain by a partially reversed normal fault. The linear reverse fault/monocline inversion structures were a relatively inefficient method of basin shortening. Because upper crustal faults in the region steepen upward markedly, reversal of these faults under compression resulted in a shortening discrepancy at shallow depths. Locally, particularly in south Dorset, this was overcome by bucklefolding and low-angle reverse faulting which increased the amount of shortening attainable across the linear inversion structures. Elsewhere however, the shortening discrepancy was accommodated by bulk shortening and regional upwarp of the graben sedimentary-fill. This occurred preferentially in those graben containing young, poorly lithified and therefore weak sediments. Thus the early Cretaceous depocentres of the Weald and Channel basins were strongly upwarped, with axial uplifts of over 1000 m. Conversely, graben having older, more lithified sequences suffered little regional upwarp, shortening primarily by fault reversal along the linear inversion structures. The amount of crustal shortening which accompanied inversion was considerably less than the earlier crustal extension.
Sedimentary structures: Sorby and the last decade
Authors: ALLEN J.R.L.
Published: December 2007

Henry Clifton Sorby pioneered in the last century the description and especially the hydrodynamic interpretation of sedimentary structures, together with their use as palaeocurrent indicators. Research completed since the last syntheses were published a decade ago shows that work along these lines continues to be necessary and relevant, particularly as regards the physical explanation of structures, and to present significant challenges and opportunities. Perhaps the most pressing needs are for a better understanding of (1) bedforms in gravels, silts and carbonate sediments, (2) tidal and especially sand-wave bedding, (3) hummocky and swaley cross-stratification, and (4) soft-sediment and dewatering structures in turbidites. Many sedimentary structures present a little-exploited opportunity to quantify process-rates and define short time-periods from the rock record.
Unravelling dates through the ages: geochronology of the Scottish metamorphic complexes
Authors: ROGERS G. , PANKHURST R.J.
Published: December 2007

The paper by Giletti
et al.
1961 is seen as a major landmark in the evolution of dating techniques in polymetamorphic terrains. We consider certain critical issues from each of the main complexes of the Scottish Highlands studied by Giletti
et al.
to illustrate how subsequent developments in geochronological methodology have influenced our understanding of metamorphic belts. Lewisian examples focus on the formation of Archaean crust, and the age of the main high-grade metamorphism and the Scourie dyke swarm. The antiquity of Moinian sedimentation, its relationship to the Torridonian sandstones, and the timing of Precambrian metamorphism have been controversial issues. The timing and nature of Caledonian orogenesis, most clearly expressed in the Dalradian complex, have been the focal points for the refinement of radiometric investigation. These complexes have been subject to successive developments in methodology, with ever-tighter constraints from Rb-Sr and K-Ar mineral dating, through Rb-Sr and Pb-Pb whole-rock studies, U-Pb dating of bulk zircon fractions, and Sm-Nd whole-rock and mineral investigation, up to the latest technologies of single-grain zircon and ion microprobe analysis. The rocks have released their secrets reluctantly, and many of the questions posed in 1961 have still not been definitively answered. However, the hope of unambiguous solution leads towards greater efforts, ever more reliable data, and a clearer evolutionary picture.
Subsidence analyses from the North Sea ‘triple-junction’
Authors: WHITE N. , LATIN D.
Published: December 2007

When compared with theoretical subsidence curves calculated from the uniform stretching model, water-loaded subsidence data from the ‘triple-junction’ region of the North Sea suggest that the Jurassic–Cretaceous rifting event caused lithospheric thinning by a factor of
c.
2.0. Although somewhat larger than the stretching factors found elsewhere in the North Sea, this amount of thinning is anticipated from the overall geometry of the three-graben system and is consistent with the observed volume and elemental and isotopic composition of the Jurassic Forties volcanic province. Apart from in the southern North Sea, Permo-Triassic extension is thought to have been relatively minor in comparison to the later Jurassic-Cretaceous phase. The anomalously small amount of Late Jurassic syn-rift subsidence in those wells where local fault-controlled effects are minimal, supports the well-known idea of localized relative uplift or ‘doming’ in the triple-junction area (
c
. 10
4
km
2
) prior to and during the early stages of the Jurassic–Cretaceous rift phase. A time-dependent differential stretching model, in which the lithospheric mantle is initially stretched by a greater amount than the crust is stretched might provide an explanation. Such a model would require the total amount of stretching integrated over space and time to be the same for the lithospheric mantle and for the crust in order to avoid space problems. Alternatively, the same data could be explained by invoking a small transient thermal anomaly in the asthenosphere.
Stratigraphic and structural analysis of the Late Palaeozoic–Mesozoic of NE Wales and Liverpool Bay: implications for hydrocarbon prospectivity
Authors: WILLIAMS G. D. , EATON G. P.
Published: December 2007

Interpretation of seismic lines from the offshore NE Wales area has provided information on the structural and stratigraphic evolution of the southern part of Morecambe/Liverpool Bay. Basin-wide megasequence boundaries are present between the Dinantian and overlying sequences and between the Upper Carboniferous and Permo-Triassic megasequences. Geological information obtained from seismic lines has been used to construct an E–W geological cross section and a basin analysis summary chart for the NE Wales–Cheshire onshore area. Stratigraphic interpretations have been considered in an assessment of the petroleum potential of the region. The distribution of Carboniferous oil and gas prone source rocks is a key element in predicting hydrocarbon accumulations. Source rock maturity may have been reached in the offshore area prior to a phase of inversion and uplift during Tertiary times. Fault-controlled hydrocarbon traps generated during Triassic-Jurassic rifting were modified during Tertiary inversion and new structures were generated.
Early Silurian oceanic episodes and events
Authors: ALDRIDGE R. J. , JEPPSSON L. , DORNING K. J.
Published: December 2007

Biotic cycles in the early Silurian correlate broadly with postulated sea-level changes, but are better explained by a model that involves episodic changes in oceanic state. Primo episodes were characterized by cool high-latitude climates, cold oceanic bottom waters, and high nutrient supply which supported abundant and diverse planktonic communities. Secundo episodes were characterized by warmer high-latitude climates, salinity-dense oceanic bottom waters, low diversity planktonic communities, and carbonate formation in shallow waters. Extinction events occurred between primo and secundo episodes, with stepwise extinctions of taxa reflecting fluctuating conditions during the transition period. The pattern of turnover shown by conodont faunas, together with sedimentological information and data from other fossil groups, permit the identification of two cycles in the Llandovery to earliest Wenlock interval. The episodes and events within these cycles are named: the Spirodden Secundo episode, the Jong Primo episode, the Sandvika event, the Malmøykalven Secundo episode, the Snipklint Primo episode, and the Ireviken event.
Structural geology of Easter Ross, Scotland: implications for movement on the Great Glen fault zone
Authors: UNDERHILL J. R. , BRODIE J. A.
Published: December 2007

The integration of a recently-drilled exploration well, sited near the town of Tain, Ross-shire, with a seismic database and onshore exposures allows a new interpretation of the structure in the Easter Ross peninsula, NE Scotland. It is now evident that previous interpretations, which highlighted the importance of NE–SW-striking lineaments, need some modification. Instead, numerous N–S to NNE–SSW-trending folds and faults affect Devonian sequences and show evidence of compressional reactivation of structures which had previously-experienced extension. These inversion structures only appear to be dissected by NE–SW- and E–W-striking faults and extensional fractures in areas adjacent to the Great Glen fault zone. Integration of the onshore data with results from adjacent offshore areas suggests that the peninsula has undergone a complex structural history since the Early Devonian. It appears to have been characterized by Early Devonian (probably rift-related) extensional faulting and subsequent Mid-Late Devonian regional (thermal) subsidence. The restriction of similar structural relations to Devonian sequences on offshore seismic lines from adjacent areas suggests that the contractional and inversion structures, which similarly affect the Late Palaeozoic sequences in onshore areas, were probably related to Permo-Carboniferous structural events rather than younger deformation. Further integration of offshore data with the occurrence of Jurassic sequences on the coast suggests that the peninsula may be considered an integral part of the Inner Moray Firth basin and experienced a similar Mesozoic subsidence history to offshore areas before undergoing regional uplift and dissection due to reactivation of the Great Glen fault zone during the Cenozoic. There is no evidence to suggest that this fault zone had any significant control on basin development, sedimentary sequences or structural geometries during Palaeozoic or Mesozoic periods of extension, and it probably only had minor effect during Permo-Carboniferous inversion. Similarily, its role in tectonic events after the Early Cretaceous appears to have been limited to local deformation with little evidence for any large-scale strike-slip offset along its length. The unusual occurrence of Jurassic coastal exposures in Easter Ross appears to represent fault-bounded slices of sediment downfaulted to accommodate the formation of a ‘negative flower structure’ in response to limited, dextral strike-slip movement. The results suggest that the Great Glen fault zone was a strike-slip fault system effectively locked during phases of extension in the Inner Moray Firth, but was reactivated during phases of inversion when extensional slip vectors sub-paralleled the lineament.
Quaternary tectonics of the Argolikos Gulf and adjacent basins, Greece
Authors: VAN ANDEL T. H. , PERISSORATIS C. , RONDOYANNI T.
Published: December 2007

A single-channel seismic reflection survey of the Argolikos Gulf and Idhra Channel casts light on the tectonic history of this extensional basin of which the current subsidence phase is less than 0.5 Ma old. Although the Gulf has very low seismicity, Holocene subsidence on the western boundary faults increases southward from 50 to 100 cm ka
−1
. The eastern margin of the Gulf is offset eastward twice. For these offsets we postulate two left-lateral strike-slip faults accommodating a southward increase in the extension rate. These features can be traced on land where at least one of them shows evidence of left lateral movement. The Idhra Channel has a different tectonic pattern of E–W-trending extensional troughs separated by basement ridges. The latter diverge westward and continue into the southern Argolid as Mesozoic limestone ridges separated by ophiolite-floored tectonic valleys. The causes of this discordant pattern remain obscure.