ALBERT

Description: :  Stratigraphic architectures are fundamentally controlled by the interplay at different temporal and spatial scales of accommodation and sediment supply, modulated by autogenic responses of the sediment routing system and its constituent segments. The flux and caliber of sediment supply is a function of climate, catchment area, and tectonics in the source regions, and unraveling these forcing mechanisms from the observed stratigraphic architecture remains a key research challenge. The mid-to-late Eocene Escanilla sediment routing system had its source regions in the south-central Pyrenean orogen, northern Spain, and transported sediment from wedge-top basins along tectonic strike to marine depocenters. By constructing a volumetric budget of the sedimentary system, it has been demonstrated that there were marked changes in the grain-size distribution released from the sediment sources and also in the position of the gravel front, across three ~ 2.6 Myr time intervals from 41.6 to 33.9 Ma. Classical sequence stratigraphic interpretations would relate the movement of depositional boundaries such as the gravel front to changes of base level, either in isolation or in combination with sediment supply. Herein, we explore the possibility that the position of the gravel front was primarily driven by variability of grain-size distributions released from the source regions as a result of changes in catchment uplift rate and/or surface run-off. Using a simple model of sediment transport that captures first-order processes, we simulate the lateral movement of gravel deposition in the proximal part of the Escanilla sediment-routing system. Movement of the gravel front is a function of both accommodation generation and the transport capacity of the sediment routing system. We assume that the transport capacity is a linear function of the local slope and the water flux. By assuming that the observed thickness of deposits is equivalent to the accommodation available during deposition, we then use the stratigraphic architecture to constrain the change in catchment size and water flux over the three time intervals of the Escanilla paleo–sediment-routing system. Multiple scenarios are investigated in order to find the most plausible tectonic and climatic history. Model results indicate that during the mid-Eocene there was an increase in catchment length and sediment flux, most likely driven by tectonic uplift in the Pyrenean orogen. Subsequent marked progradation of the gravel front during the late Eocene was the consequence of reduced transport capacity due to a reduction in surface run-off. The latter model result is in agreement with records of pollen taxa that indicate increased climatic aridity in the late Eocene. The combination of a sediment transport model with a full sediment budget makes it possible to test the non-uniqueness of these results.

Description: 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.

Description: A bstract :  Distinguishing between forced and unforced regressive strata is important for identification of systems tracts, prediction of sediment bypass, and reconstruction of relative sea-level histories. Conventional sequence stratigraphic models distinguish between forced and unforced regressive strata through the presence of aggradational topset (no aggradation during forced regression) and style of shoreline trajectory (descending in forced regressive strata and flat to rising in unforced regressive strata). However, because present models contain implicit assumptions about sediment supply and the response of coastal-plain and fluvial deposystems to falling and rising relative sea level, it is probable that these two scenarios are an oversimplification of a more complex reality. This work investigates how topset aggradation might develop during relative sea-level fall using 1264 runs from a simple diffusional stratigraphic forward model. Topset aggradation is characterized for each model run by a topset/foreset volume ratio (t/f ratio). When topset strata are present the t/f ratio is 〉 0, when no topset strata are present the t/f ratio is zero. Multiple two-dimensional model runs (0.4 or 2 My duration, and constant sediment supply and discharge rates representative of medium-size river systems) suggest that sediment transport rate may be a key control on topset aggradation. Modeling a range of sediment transport rates for amplitudes of relative sea-level fall from 0 to 100 m shows that with relatively high rates of sediment transport, multiple model runs create strata with t/f ratios up to 0.08. Conversely, relatively low sediment transport rates lead to higher volumes of topset deposition reflected by t/f ratios ranging from 0.10 to 0.37. However, critically, high sediment transport with no relative sea-level fall leads to topset aggradation very similar to that resulting from low sediment transport and high-amplitude relative sea-level fall. This is an example of non-uniqueness, showing that topset aggradation can occur during falling relative sea level as well as steady to rising relative sea level, depending on rates of sediment transport. This result has been tested and verified with different rates of relative sea-level fall and with additional three-dimensional model runs, including runs in a different model. The results suggest that backstripped shoreline trajectories are likely to be a more reliable method of distinguishing forced from unforced regression, and that interpretation and prediction of sediment bypass history may be more complicated than current sequence stratigraphic models suggest.

Description: 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.

Description: The ideal cycle concept is poorly defined yet implicit and potentially useful in many stratigraphic analyses. A new method allows quantitative definition of ideal cycles and provides a simple but robust method to analyze stratal order and quantify stratigraphic interpretations. The method calculates transition probability (TP) matrices from a vertical succession of strata for all possible permutations of facies-class row numbering in the matrices. The ordering of facies classes that gives highest transition probabilities along diagonals of the TP matrix can be taken as a quantitative definition of an ideal cycle for the strata being analyzed. Application to a synthetic example shows how an ideal cycle can be identified, even in noisy strata, without any assumptions about or prior knowledge of cyclicity. Application to two outcrop examples shows how it can be useful to define the most optimal cycle and determine how much evidence is present for ordered and cyclical facies successions.

Description: :  Identifying order or pattern in strata on the basis of qualitative interpretation forms the basis for much current sedimentological and sequence stratigraphic analysis. Order can be usefully defined as some arrangement of facies or unit thickness that has a discernable trend or pattern that is unlikely to occur by chance because it requires some particular systematic process to form. Coarsening, fining, thickening, or thinning-upward trends, and arrangement of strata into cycles are examples of order. Qualitative interpretations of order often demonstrate little more than an implicit assumption of order. This paper defines a robust yet simple-to-apply quantitative method to identify order in strata and to indicate when order cannot be reliably demonstrated. The method is based on two calculated metrics, the Markov metric m derived from analysis of a vertical facies succession, and the runs metric r derived from analysis of observed thicknesses of stratal units. Most importantly, both metrics can be compared with equivalent metrics calculated for disordered strata composed of many randomly shuffled versions of the same lithological units. Probability values can then be calculated from the comparison between observed and randomly shuffled cases, and these p values indicate the degree of evidence present for order in the observed strata. Several test examples using synthetic strata show that the m and r values can define and identify different degrees and types of stratal order, and that the metrics are robust for both stationary and non-stationary successions with a range of different lengths and numbers of distinct facies. Analysis of four outcrop examples, two siliciclastic and two carbonate, demonstrate that ordered facies successions and thickness trends may be less common than typically assumed; none of the four examples analyzed show trends in thickness, and only the examples from the Book Cliffs, which represent a bedset scale composite of observations, show evidence for facies order. The examples demonstrate how a quantitative analysis can lead to better understanding of strata, either ordered or disordered, and can provide better insight into the validity of current stratigraphic interpretations and models. Absence of order in many of the analyzed 1D vertical successions may also indicate that we need to focus more on longer-term trends and analysis of 2D and 3D stratal geometries.

Description: Isolated carbonate buildups (ICBs) are commonly attractive exploration targets. However, identifying ICBs based only on seismic data can be difficult for a variety of reasons. These include poor-quality two-dimensional data and a basic similarity between ICBs and other features such as volcanoes, erosional remnants, and tilted fault blocks. To address these difficulties and develop reliable methods to identify ICBs, 234 seismic images were analyzed. The images included proven ICBs and other features, such as folds, volcanoes, and basement highs, which may appear similar to ICBs when imaged in seismic data. From this analysis, 18 identification criteria were derived to distinguish ICBs from non-ICB features. These criteria can be grouped into four categories: regional constraints, analysis of basic seismic geometries, analysis of geophysical details, and finer-scale seismic geometries. Systematically assessing the criteria is useful because it requires critical evaluation of the evidence present in the available data, working from the large-scale regional geology to the fine details of seismic response. It is also useful to summarize the criteria as a numerical score to facilitate comparison between different examples and different classes of ICBs and non-ICBs. Our analysis of scores of different classes of features suggests that the criteria do have some discriminatory power, but significant challenges remain.