Abstract The superposition of stratified rocks is an unmistakable manifestation of the history of sedimentary processes through deep time. However, the relationship between the preserved strata of the rock record and the passage of geological time, indisputable in principle, is unknowable in detail; incompleteness is an essential property of the record. That gaps exist at all scales in sedimentary successions is easily demonstrated from consideration of sediment accumulation rates, and expectations of continuity and completeness at any scale are correspondingly inadvisable. Locating and quantifying the gaps in the record is, however, very much less straightforward. Predictive modelling of strata – essential for their practical exploitation – requires such geohistorical understanding, yet over-simplified assumptions about how time is represented in rock can still lead to inadequate or even false conclusions. The contributions to this volume describe a range of practical studies, theoretical investigations, and numerical experiments in which the nature of the strata–time relationship is explored.

Abstract Sedimentary strata are the paramount source of geohistorical information. The ‘frozen accidents’ of individual deposits preserve evidence of past physical, chemical and biological processes at the Earth’s surface, while the spatial relationships between strata (especially superposition) yield successions of events through time. There is, however, no one-to-one relationship between strata and time, and the interpretation of the stratigraphic record depends on an understanding of its limitations. Stratigraphic continuity and completeness are unattainable ideals, and it is the departures from those ideals – the often cryptic gaps in the record – that provide both its characteristic texture and the principal challenge to its analysis. The existence of gaps is clearly demonstrated by consideration of accumulation rates, but identifying and quantifying them in the field is far more difficult, as is assessing their impact on the degree to which the stratigraphic record represents the environments and processes of the past. These issues can be tackled in a variety of ways, from empirical considerations based on classical field observations, to new ways of analysing data, to the generation and analysis of very large numbers of synthetic datasets. The range of approaches to the fundamental questions of the relationship between strata and time continues to expand and to challenge long-established practices and conventions. Superposed sedimentary strata are the most accessible routes into deep time, and acceptance of their historical significance was a major scientific breakthrough. Given that the study of strata has been undertaken in something like its modern form for over two centuries, stratigraphy as a scientific discipline might be expected to have stabilized, as perhaps is indicated by stratigraphy textbooks suggesting that the subject is widely regarded as boring. Yet if there is a problem with stratigraphy, it is the converse: its development is increasingly punctuated by paradigm shifts triggered by new theories (evolution; global tectonics; eustasy; orbital forcing of climate change) and technological breakthroughs (digital computing; continuous seismic profiling; isotopic methods in chronology and palaeoclimatology). With this accelerating progress, it has become increasingly clear that the stratigraphic record yields only snapshots of Earth’s past surface processes – the ‘frozen accidents’ that give the record its character and its enduring fascination. ‘Time is missing from sedimentary sequences on all scales … This discontinuity gives recorded planetary (geological) time a different architecture to human time’ ( Paola, C. 2003 . Floods of record. Nature , 425 , 459). Strata and Time: Probing the Gaps in our Understanding was the title of the Geological Society’s William Smith Meeting for 2012. Its aim was to explore the relationship between the preserved sedimentary rock record and the passage of geological time, identifying, evaluating and updating the models that lie behind current stratigraphic methods. This volume includes contributions by some of those who presented papers at the conference, together with two additional, related papers. The range of topics in these 15 papers is broad; from field-based studies to numerical modelling exercises, from theoretical considerations of the nature of the record to a study of hydrocarbon reservoir distribution. Critical to all of these studies is the relationship between sedimentary rock strata and geological time.

Abstract It has long been understood that the stratigraphic record is fragmentary. Barrell was the first to clearly understand the importance of accommodation, and the episodic way in which accommodation is created and removed by geological processes. He demonstrated that typically only a fraction of geological time is represented by accumulated sediment. This point was repeated in influential books by D. Ager. A significant feature of the sedimentary record is the correlation between the duration of a sedimentary unit and its sedimentation rate. Sedimentation rates range over more than eleven orders of magnitude. The durations of stratigraphic gaps, the distribution of layer thicknesses, and sedimentation rates have fractal-like properties, facilitating the integration of our knowledge of the processes of accommodation generation with data on varying sedimentation rates and the scales of hiatuses and processes operating over all time scales. This paper proposes the definition of a suite of Sedimentation Rate Scales to encompass the range of time scales and processes in the stratigraphic record. Assignment of stratigraphic units to the appropriate scale should help to clarify preservation mechanisms, leading to more complete quantitative understanding of the geological preservation machine, and a more grounded approach than earlier treatments of stratigraphic completeness.

Abstract Procedures used to define an international chronostratigraphic stage boundary and to locate and ratify a Global Boundary Stratotype Section and Point (GSSP) are outlined. A majority of current GSSPs use biostratigraphic data as primary markers with no reference to any physico-chemical markers, despite the International Subcommission on Stratigraphic Classification (ISSC) suggestion that such markers should be included if possible. It is argued that such definitions will not produce the high-precision Phanerozoic time scale necessary to understand such phenomena as pre-Pleistocene ice ages and global climate change. It is strongly recommended that all GSSPs should have physico-chemical markers as an integral part of their guiding criteria, and where such markers cannot be found, the GSSP should be relocated. The methods and approach embodied in oceanic stratigraphy – coring, logging, analysing and archiving of drill sites by numerous experts using a wide range of methods – could usefully serve as a scientific model for the analysis and archiving of GSSPs, all of which are on the present-day continents. The incorporation of many more stratigraphic sections into GSSP studies, the application of physico-chemical methods, and the replacement of old U–Pb dates by newer CA-TIMS U–Pb dates, together with the use of constrained optimization (CONOP) programs that produce a calendar of events from many sections, should lead to much more precise timescales for pre-Cenozoic time than are currently available.

Abstract Linear rates of sediment aggradation and fluvial incision are inverse functions of measurement interval, a generic consequence of unsteadiness in the underlying processes. This effect results from a one-dimensional approach–that is, vertical rates determined at a single location–and significantly complicates comparisons of rates at different timescales. Mass conservation imposes an important but underutilized constraint; sediment by-passing or eroded from one location must deposit somewhere else. Over the long term, sediment generation and deposition must balance. In principle, the effects of unsteadiness could be eliminated if the total volume of sediment eroded or deposited over different intervals could be measured. In practice, however, obtaining such three-dimensional data from an individual site is virtually impossible. Here, we advance from one- to two-dimensional rate data. We present two new global compilations of data: denudation rates of fluvial uplands; and lateral migration (progradation) rates of siliciclastic lowland and marine systems, from ripple to shelf-slope scale. Important new findings are: (1) upland denudation rates determined from specific sediment yield show little or no dependence of rate on time interval; (2) in the transfer zone between sediment source and sink, rates of erosion and deposition balance over all scales; and (3) progradation mirrors aggradation over all timescales. The product of progradation and aggradation is independent of timescale, implying that global sediment flux into the world’s oceans has been constant on the order of 10 0 m 2 /yr, from scales of months to tens of millions of years. Results show that global rates of denudation and accumulation are time invariant with appropriate spatial averaging; however, site-specific application remains a daunting challenge.

Abstract Conventional stratigraphic logging is biased towards human-scale units and is correspondingly suspect as a basis for statistical analysis of spatial layering relationships. AR analysis uses lithological sample series to define layers in terms of the ‘first returns’ of each sample value, yielding objective layer thickness inventories (LTI) covering several orders of magnitude. Bilogarithmic LTI plots, relating the return thicknesses to their numerical frequency in the section, reveal power-law relationships with non-integer exponents. Over two orders of magnitude, the geometric layering relationships are thus shown to be self-similar (fractal). The power-law exponents also relate to Hurst exponents that describe the variability in the data series and the negative long range dependence in the sampled records. Kilometre-scale gamma ray logs of continental, paralic, shallow marine and pelagic facies yield similar power-law exponents, regardless of the interpreted processes and rates of accumulation. This suggests that a ‘universal’ power law describes the self-similar layering of the stratigraphic record. This is inferred to mean that the power-law behaviours are the outcome of the threshold interactions between a sediment-transporting fluid and its bed. They suggest that the erosional operations that generate the record are characterized by self-organized criticality and involve a strange attractor.

Abstract This paper puts forward the proposition that sedimentation systems generally are in stasis. Three lines of evidence suggest the proposition is largely correct: (1) considerations of how sedimentation systems necessarily operate, (2) observations of active systems, and (3) a re-evaluation of ideas about sedimentation rates. There are of course systems to which the proposition cannot apply. A simulation exercise is used to address questions about the stratigraphic effects of stasis. The results show (1) that sedimentation systems that generally are in stasis can be of a variety of types, (2) that stasis is readily preserved in stratigraphic successions, (3) that successions produced by a system in which the time proportion of stasis is high are markedly more complete than successions produced by other systems of the same type, (4) that the proportion of stasis in a system cannot be estimated reliably from stratigraphic successions produced by that system, and (5) that the stratigraphic succession finally left behind by a system is necessarily a systematically biased and partial record of the history of that system. What is always missing is that part of the history before the oldest preserved horizon, which for systems that are in long-term balance will on average be half of the total time.

Abstract Hierarchies of cyclicity have been described from a wide variety of carbonate platform strata and are assumed to be a consequence of Milankovitch-forced variations in accommodation, although descriptions of hierarchical strata, including ‘cycles’ and what they constitute, are typically qualitative, subjective, and in some cases difficult to reproduce. One reason for this is the lack of any detailed definition of what constitutes a hierarchy, as well as the implicit and largely untested nature of the assumptions underpinning most interpretations of hierarchical strata. In this study we aim to investigate the response of depositional systems if they were to behave in the way implied by sequence stratigraphic (hierarchical) models, to clearly state the assumptions of these models, and illustrate the consequences of these assumptions when they are employed in a simple, internally-consistent forward model with plausible parameters. We define hierarchies, in both the time-domain (chronostratigraphic) and thickness-domain (stratigraphic), as two or more high-frequency sequences (HFSs) in which there exists a repeated trend of decreasing high-frequency sequence thickness such that within a single low-frequency sequence (LFS) each high-frequency sequence is thinner than the previous sequence. Based on this definition, results from 110 000 numerical model runs suggest that ordered forcing via cyclical eustatic sea-level oscillations rarely results in an easily identifiable hierarchy of stacked cycles. Hierarchies measured in the chronostratigraphic time-domain occur in only 9% of model run cases, and in 15% of cases when measured in the thickness-domain, suggesting that vertical thickness trends are probably not a useful way to identify products of ordered periodic external forcing. Variability in relative forcing periodicity results in significant variation in both HFS and LFS thickness trends making accurate identification of hierarchy and any forcing controls from thickness data alone very difficult. Comparison between model results and outcrop sections suggests that hierarchies are often assumed to be present despite a lack of adequate supporting evidence and quantitative analysis of these sections suggests that they are not hierarchical in any meaningful sense.

Abstract Searches for periodic signals (candidate Milankovitch cycles) in stratigraphic data often make use of methods of spectral analysis, some of which have come to be regarded as standard and therefore reliable. In this paper we highlight a problem with the application of filtering, or smoothing, to the data prior to spectral analysis. Such preprocessing is frequently used to amplify the signal-to-noise ratio at the expected wavelength. Using synthetic random data, we demonstrate how filtering generates artificial spectral peaks close to the characteristic wavelength of the filter. When applying the same preprocessing methods to actual stratigraphic data, we find that it is possible to replicate some recently published results claiming to show cyclicity. We recommend that filtering and other preprocessing techniques be used with great care: spectral peaks occurring close to the characteristic scale of the applied filter are likely to be artefacts of preprocessing.

Abstract The Milankovitch theory of climate change is widely accepted, but the registration of the climate changes in the stratigraphic record and their use in building high-resolution astronomically tuned timescales has been disputed due to the complex and fragmentary nature of the stratigraphic record. However, results of time series analysis and consistency with independent magnetobiostratigraphic and/or radio-isotopic age models show that Milankovitch cycles are recorded not only in deep marine and lacustrine successions, but also in ice cores and speleothems, and in eolian and fluvial successions. Integrated stratigraphic studies further provide evidence for continuous sedimentation at Milankovitch time scales (10 4 years up to 10 6 years). This combined approach also shows that strict application of statistical confidence limits in spectral analysis to verify astronomical forcing in climate proxy records is not fully justified and may lead to false negatives. This is in contrast to recent claims that failure to apply strict statistical standards can lead to false positives in the search for periodic signals. Finally, and contrary to the argument that changes in insolation are too small to effect significant climate change, seasonal insolation variations resulting from orbital extremes can be significant (20% and more) and, as shown by climate modelling, generate large climate changes that can be expected to leave a marked imprint in the stratigraphic record. The tuning of long and continuous cyclic successions now underlies the standard geological time scale for much of the Cenozoic and also for extended intervals of the Mesozoic. Such successions have to be taken into account to fully comprehend the (cyclic) nature of the stratigraphic record.

Abstract The Lower Jurassic Bridport Sand Formation records net deposition in the Wessex Basin, southern UK of a low-energy, siliciclastic shoreface that was dominated by storm-event beds reworked by bioturbation. Shoreface sandstones dip at 2–3° to define (subaerial?) clinoforms that pass distally into a near-horizontal platform, and then steepen again to form steep (2–3°) subaqueous clinoforms in the underlying Down Cliff Clay Member. The overall morphology indicates mud-dominated clinoforms of compound geometry. Compound clinoforms are grouped into progradational sets whose stacking reflects tectonic subsidence and sediment dispersal patterns, and also controls basin-scale reservoir distribution and diachroneity of the formation. Each shoreface clinoform set consists of an upward-shallowing succession that is several tens of metres thick with a laterally continuous mudstone interval at its base. The successions are punctuated by calcite-cemented concretionary layers of varying lateral continuity, which formed along bioclastic lags at the base of storm-event beds. Concretionary layers thus represent short periods of rapid sediment accumulation, while their distribution likely results from variations in storm-wave climate, relative sea-level, and/or sediment availability. The distribution of impermeable mudstone intervals that bound each clinoform set and concretionary layers along clinoform surfaces controls oil drainage in the Bridport Sand Formation reservoir.

Abstract The conventional Wheeler diagram aids the construction of a spatiotemporal framework of strata. The diagrams are created manually by studying outcrops, wells, or seismic data. For the latter case, automated methods now exist, which support the construction of 2D, as well as 3D Wheeler diagrams. Seismic data contains information in three dimensions, X , Y and Z , where ‘ Z ’ is either two-way time or depth. Seismic horizons are correlated surfaces that often follow geological time lines. In this case, a set of interpreted seismic horizons contains information in four dimensions ( X , Y , Z , and Geological Time). In the mapping from the structural domain to Wheeler space, information about Z (thickness) is lost. This means that one dimension is missing in the conventional Wheeler diagram. This paper describes a method to add information from the Z dimension to the Wheeler domain. It is done by computing stratigraphic thicknesses per sequence stratigraphic unit and displaying these as colour-coded overlays in the Wheeler domain. Thus displayed, thickness variations help in understanding changes in accommodation, sedimentation rate, and depositional trends. 3D Wheeler displays with colour-coded thickness information are referred to as 4D Wheeler diagrams. In this article, the method is described and applied to a case study from the southern North Sea.

Abstract Karstification produces a unique and spatially complex architecture of accommodation space for the accumulation of later sediments. The sedimentary record within caves can act as a repository for stratigraphic and palaeoenvironmental information that has been locally removed by subsequent surface erosion. Caves and karst also allow for the preservation of biota not usually found in the fossil record. Pennsylvanian palaeokarst from Illinois, USA, illustrate the potential of ancient caves as a home for ‘lost stratigraphy’. These palaeocaves have dissolutional features associated with contemporaneous sediment influx (paragenesis), indicating that speleogenesis and cave sediment deposition were synchronous. These features also provide evidence of changing water tables. The fill within the caves suggests multiple flood events on the surface. The enclosed biota contains rare upland plants, such as conifers, as well as scorpions. Both plants and animals preserve original organic constituents. The presence of charcoal, as well as diagnostic polyaromatic hydrocarbons, point to wildfires and thus dry episodes on the land surface. The cave fills are outliers from correlative formations in the region. The filled voids of these ancient caves thus fill palaeontological, palaeoenvironmental, and stratigraphic gaps.

Abstract Ths wht th fn-grnd mrine sdmtry rcrd rlly lks like.

Abstract To assess the extent to which it is possible to identify, characterize and quantify gaps in the stratigraphic record, two contrasting sections, the Silurian Pentamerus Beds of Shropshire, England, and the Ordovician Ribband Gp of County Wexford, Ireland, have been examined in the field. In the Pentamerus Beds the life assemblages contained in silty mudstones suggest slow to little/no deposition during fair weather periods. Interbedded lenticular calcirudites characteristically feature death assemblages of thick-shelled brachiopods and are interpreted as storm deposits, implying erosional breaks in accumulation. In contrast, the Ribband Gp’s regular ribbon banding of fine sandstones, siltstones and mudstones/silty mudstones suggests a more distal shelf environment. Fodinichnial trace fossils are witness to periods of more or less continuous sedimentation. The succession may have formed during seasonal alternations of stormy (thin cross-laminated sandstone beds) and fair weather periods (laminated mudstones); though longer-term climatic variations cannot be ruled out. There is minimal evidence for hiatuses in the accumulation.

Abstract The Lower and Middle Coal Measures of Langsettian (Westphalian A) and Duckmantian (Westphalian B) age (together equals Bashkirian in part) in Britain comprise an alternation of clastic sediments and coal deposited on coastal and alluvial plains over a period of 2–3.5 million years, depending on which time scale is accepted. In both the Pennine and South Wales Basins there are no obvious unconformities. Many of the clastic sequences show evidence of rapid sedimentation (burial of trees, bivalve escape burrows) that may suggest that a significant amount of time is taken up during the peat-forming intervals. Thickness data from a range of boreholes that record continuous sections through this time period were collated. Coal and sediment thicknesses, as well as coal to sediment ratios, are compared both within and across the basins. Data on the coals has allowed consideration of the time taken to deposit the peats. This work considers the compaction of the peat to coal, as well as a range of peat accumulation rates. Assuming the largest de-compaction rates and slowest accumulation rates of the coal formation, less than 50% of the allocated time can be accounted for. In addition however, calculations suggest that peat formation accounts for less than 25% of the total time taken for sediment accumulation. It is suspected that there are major time gaps in the sequences, most probably occurring between seat-earths and coal and within the coals, and it is believed that this finding has significance for the debate over short-term climate changes in the Carboniferous and the causes of peat and sediment alternations. Supplementary material: Borehole thickness data and references are available at http://www.geolsoc.org.uk/SUP18788

Abstract Time contained within coal seams is most commonly estimated using a volumetric approach that fails to take into account processes of carbon accumulation and loss during peat formation and coalification. A more appropriate approach for estimating the time contained within a coal seam is to use Holocene long-term carbon accumulation rates, accounting for carbon loss during coalification. Using this approach the thickness of coal corresponding to 10 kyr of carbon accumulation is calculated for coals of all ranks and latitudinal settings. To test the validity of this approach, latitudinal patterns of Holocene dust deposition are used in conjunction with estimated rates of carbon accumulation to calculate the concentration of titanium in coal. The result is a statistically significant correlation that is optimized when latitudinal variation in carbon accumulation rate is considered. Overall, the use of carbon produces far greater accountability of time within coal-bearing stratigraphic sequences and is not influenced by the presence of hiatal surfaces within the coal. Estimated coal seam duration increases considerably, often removing the need to infer substantial intra-seam hiatuses. On the basis of the results, a re-evaluation of coal and coal-bearing stratigraphic sequences is recommended.

Abstract Sixteen papers representing the petroleum geology of the Arctic, Russia and former Soviet Union were presented over the first day and a half of PGC VII. The region is huge, diverse and has generated a great deal of excitement and outside investment in the industry over the 20 years since the collapse of the Soviet Union. The Arctic region in particular has significance as perhaps the last great frontier hydrocarbon province on Earth. The region is large, approximately 5000 km across a polar view north of the Arctic Circle (Fig. 1 ). Importantly, from an oil and gas exploration perspective, the Arctic Ocean has the most extensive continental shelf area of any ocean basin ( c . 50% of offshore area). Much of this sits in the broad Russian offshore Arctic in water depths of less than 50 m. There are numerous sedimentary basins in the Arctic, some well known, but most poorly understood. Art Grantz (United States Geological Survey) and colleagues estimated resources at 114×10 9 barrels of undiscovered oil and 2000×10 12 standard cubic feet (SCF) of natural gas. If the estimates are correct, these hydrocarbons would account for more than a fifth of the world's undiscovered resources. This great prize, in a world of diminishing reserves, has recently brought territorial issues into focus between the five countries with claims in the Arctic Ocean (Russia, Norway, Denmark, Canada and the USA). All of this is taking place against a backdrop of increasing concern for the fragile Arctic environment.