William Helland-Hansen

Conceptual basis and variability in sequence stratigraphy: a different perspective

Abstract Sequence stratigraphic models are: (1) discussed from a theoretical point of view, with emphasis on a systematical discussion of the breakdown of depositional cycles produced by changes in relative sea-level and sediment supply into systems tracts and, (2) questioned by discussing and schematically showing alternative scenarios to the established ones. The “shoreline trajectory”, defined as the cross-sectional shoreline migration path along the depositional dip, is a useful building block for describing the internal architecture of the depositional cycles and their contained systems tracts. The shoreline trajectories can be grouped into discrete classes including accretionary and non-accretionary forced regression, normal regression, and accretionary and non-accretionary transgression. Depositional cycles formed as a response to successive rises and falls of relative sea-level should be divided into four, and not three systems tracts, which is the most common in the literature. Three key surfaces can encompass a complete cycle. These are the surfaces of maximum regression and transgression, and the subaerial unconformity formed during relative sea-level fall. The correlative conformity to the subaerial unconformity should correspond to the time of lowest relative sea-level. Variability of the lowstand wedge and highstand systems tracts can be treated together, since both are taking place during rising relative sea-level with sediment supply being larger than the accommodation being generated. The resultant progradation may or may not be interrupted by transgressive events. Three end-member scenarios for the transgressive systems tract can be envisaged: non-accretionary transgression; accretionary transgression; and backstepping by combined transgressions and normal regressions. Important controls on the variability of the forced regressive systems tract are the gradients of the shoreline trajectory and the fronting depositional foundation. Basin floor mass-gravity deposition may occur within all four systems tracts and will eventually take place both in a ramp and shelf-slope-basin setting if the receiving basin extends into deep waters and is oversupplied with sediments.

Strike variability of clastic depositional systems: Does it matter for sequence-stratigraphic analysis?

Current sequence-stratigraphic models are two-dimensional in nature and predict variability in depositional-dip sections. Strike and dip variability of sediment supply to clastic depositional systems causes significant changes to sequence architecture so that contrasting yet coeval stacking patterns result. These changes ultimately affect prediction potential based on fixed systems tract stacking patterns and relation to relative sea-level curves. Identification of key surfaces in sequences rather than stacking patterns is the most reliable indicator of relative sea-level history.

Impact of eustatic amplitude variations on shelf morphology, sediment dispersal, and sequence stratigraphic interpretation: Icehouse versus greenhouse systems

The Pliocene–Pleistocene icehouse stratigraphic record is characterized by distinct sediment distribution patterns and chronostratigraphic relationships resulting from high-frequency, high-amplitude changes in sea level. These modern relationships are often used as a template for sequence stratigraphic interpretation and for predicting sediment distribution in ancient greenhouse systems, deposited at times when sea-level fluctuations are known to have been different from the most recent period of geologic time. Numerical modeling suggests that the depth of the shelf rollover, and thereby also shelf accommodation, rapidly adapts to changes in eustatic amplitude along passive, constructional sedimentary margins. Lower amplitudes and perhaps also lower frequencies during greenhouse periods would allow across-shelf delta progradation and sediment delivery to the slope and basin floor during both third- and fourth-order highstands. Care should therefore be taken when extrapolating stratigraphic models between icehouse and greenhouse systems.

Sedimentation in Paleogene foreland basin, Spitsbergen

The upper part of the sedimentary succession in the Tertiary Central Trough on Spitsbergen, comprising the Hollendardalen, Gilsonryggen, Battfjellet, and Aspelintoppen formations, has been studied with respect to basin formation and basin filling. Thickness data from the lower marine part of this succession indicate an asymmetric, thickening-to-the-west distribution. Of the several theories on how space was generated to accommodate the sequence, tectonic subsidence driven by lithospheric loading from the flanking fold and thrust belt appears to be the most probable. The position of the basin between the front of the West Spitsbergen fold belt and the adjacent craton suggests a foreland basin setting. Most of the basin fill constitutes a major regressive sequence (Gilsonry gen, Battfjellet, and Aspelintoppen formations) deposited by the eastward progradation of a deltaic system. Evidence of eastward displacement of the depocenter through time, together with the incorporation of the orogenic flank of the basin fill into the deformation, suggests cratonward migration of the fold and thrust belt/foreland basin couplet. The present Tertiary Central Trough represents the uplifted remnants of the latest (Oligocene?) foreland basin.

A simulation of continental basin margin sedimentation in response to crustal movements, eustatic sea level change, and sediment accumulation rates

As eustasy, subsidence, and sediment accumulation vary, a 2D computer-based graphical simulation generates on-lapping and off-lapping geometries of both marine and near coastal alluvial deposits, reproducing timelines within sediment-bodies at basin margins. In the simulation, deposition is expressed by creation of new surfaces above previous ones. Thicknesses of layers are reduced by both erosion and compaction while their surfaces move vertically in response to tectonic change and loading. Simulation is divided into a series of equal time steps in which sediment is deposited as an array of en-echelon columns that mark the top of the previous depositional surface. The volume of sediment deposited in each time step is expressed as a 2D cross section and is derived from two right-angle triangles (sand and shale), whose areas are a 2D expression of the quantity of sediment deposited at that time step and whose length matches the width of the offshore sediment wedge seaward of the shoreline. Each column in the array is filled by both marine sediments up to sea level, and alluvial sediments to a surface determined by an “alluvial angle” that is projected landward from the shore to its intersection with the previous surface. Each time the area representing the sediment column is subtracted from the triangles, the triangle heights are reduced correspondingly. This process is repeated until the triangle heights match the position of sea level above the sediment surface, at which time the remaining area of the sediment triangle is deposited seaward as a single wedge of offshore sediments. This simulation is designed to aid interpretation of stratigraphic sequences. It can be used as a complement to seismic stratigraphy or can be used alone as an inexpensive test of stratigraphic models.

Source-to-sink systems on passive margins: theory and practice with an example from the Norwegian continental margin

Abstract Source-to-sink system analysis involves a complete, earth systems model approach from the ultimate onshore drainage point to the toe of related active deepwater sedimentary systems. Several methods and techniques have evolved in recent years, from experimental and numerical modelling through analysis of modern and recent systems, to analysis of ancient systems. A novel method has been developed, bridging between the previous approaches and dividing and analysing source-to-sink systems based on linked geomorphic segments along the source-to-sink profile. This approach builds on uniformitarian principles. The method is driven by the need to understand ancient, subsurface systems and still has high uncertainty but is an original, first-order approach to source-to-sink system analysis. In modern systems, entire onshore-to-offshore systems can be analysed with a higher degree of confidence than in ancient systems and semi-quantitative relationships can be established. Application in ancient systems is much more challenging but, in some cases, antecedent morphologies have been preserved onshore that can be matched with offshore known occurrences of, for instance, sandy submarine fan systems. Along the Norwegian North Sea and Norwegian Sea margins the Paleocene deep-marine reservoir of the giant Ormen Lange gas field is such an example. There, antecedent onshore drainage patterns which formed the feeder system to the offshore, deepwater fan system can be interpreted and aligned with onshore palaeogeomorphological evidence. Understanding the palaeogeomorphic development of basement regions such as the Fennoscandian shield is of high importance for understanding the offshore presence of deepwater sandstones.

Review and computer modelling of the Brent Group stratigraphy

Summary The depositional history of the Brent Delta system can be analysed on three different levels: (i) the individual formations and their facies distribution, used for the interpretation of the depositional processes; (ii) the vertical succession of formations in each well, indicating the temporal changes in environment; (iii) the two- or three-dimensional array of sequences illustrating both the temporal and spatial variations within the delta system. The Brent Group is essentially one regressive-transgressive megacycle. The regressive part comprises the Rannoch and Etive Formations (delta front) and the lower Ness Formation (delta plain), and the transgressive system contains the Tarbert Formation (delta front) and the (upper) Ness Formation (delta plain). With the aid of a computer program, the succession geometry can be analysed in terms of the relative importance of sediment supply, eustasy and tectonics. The change from regressive to transgressive conditions was probably initiated by an increase in subsidence rates, whereas regressive pulses in the overall transgressive Tarbert Formation can be explained in part by eustatic sea-level falls.

Sequence stratigraphy theory: remarks and recommendations

In this paper, depositional cycles formed by changes in relative sea-level and sediment supply are discussed from a theoretical point of view. The interplay between sediment supply and relative sea-level produces shoreline migration patterns that can be described in terms of the direction of their shoreline trajectories. Depositional cycles can be bracketed by one of three key surfaces. These are the surfaces of maximum transgression and maximum regression and the subaerial unconformity (and its correlative conformity) formed during relative sea-level fall. The two latter surfaces may be replaced by a transgressive surface of erosion along parts of their extent. It is proposed that depositional cycles formed in response to alternating falls and rises of relative sea-level can be divided into four systems tracts or segments and not three as has been previously suggested. The four systems tracts can be bounded between the levels of highest, lowest, maximum regressive and maximum transgressive shoreline positions within a relative sea-level cycle. Furthermore, it is claimed that the method of using superpositioned progradational and retrogradational parasequence stacking patterns as indicative of intervening sequence boundaries is dubious. The application of the Type-2 unconformity and Type-2 sequence boundary is problematic. It is recommended that these terms are redefined or taken out of use.

Linking an Early Triassic delta to antecedent topography: Source-to-sink study of the southwestern Barents Sea margin

Present-day catchments adjacent to sedimentary basins may preserve geomorphic elements that have been active through long intervals of time. Relicts of ancient catchments in present-day landscapes may be investigated using mass-balance models and can give important information about upland landscape evolution and reservoir distribution in adjacent basins. However, such methods are in their infancy and are often difficult to apply in deep-time settings due to later landscape modification. The southern Barents Sea margin of N Norway and NW Russia is ideal for investigating source-to-sink models, because it has been subject to minor tectonic activity since the Carboniferous, and large parts have eluded significant Quaternary glacial erosion. A zone close to the present-day coast has likely acted as the boundary between basin and catchments since the Carboniferous. Around the Permian-Triassic transition, a large delta system started to prograde from the same area as the present-day largest river in the area, the Tana River, which has long been interpreted to show features indicating that it was developed prior to present-day topography. We performed a source-to-sink study of this ancient system in order to investigate potential linkages between present-day geomorphology and ancient deposits. We investigated the sediment load of the ancient delta using well, core, twodimensional and three-dimensional seismic data, and digital elevation models to investigate the geomorphology of the onshore catchment and surrounding areas. Our results imply that the present-day

Evidence for Late Triassic provenance areas and Early Jurassic sediment supply turnover in the Barents Sea Basin of northern Pangea

We used detrital zircon fractions from the Late Triassic to Early Jurassic sedimentary succession in the Norwegian Barents Sea to constrain the role of eastern provenance areas in the basin infill history of the Northern Pangea Boreal basin. Geochronological data from sedimentary rocks in this succession reveal detrital zircon ages that are very close to the biostratigraphically defined maximum depositional age of the two lowermost intervals: The Norian to Rhaetian Fruholmen Formation show U-Pb minimum ages of 208.3 ± 4.2 Ma (discordant by -0.58) and 213.8 ± 5 Ma (discordant by 0.8), and the Rhaetian to Sinemurian Tubaen Formation is 200.6 ± 4.9 Ma (discordant by -3.99) at its minimum. These are the youngest ages thus far documented in the Norwegian Barents Sea, and they demonstrate that a provenance area was magmatically active while, or shortly before, these formations were being deposited. Such protolith ages have not been documented close to the study area, but based on the regional tectonic setting and paleogeography, we argue that the Novaya Zemlya protrusion of the northern Uralian orogen was the most likely provenance area in the region. The Sinemurian to Pliensbachian Nordmela Formation samples yielded, with an exception of a single detrital zircon age of 211 ± 4.3 Ma, a consistent 240–237 Ma minimum detrital zircon age, which suggests that either the magmatic activity or the sediment supply had come to an end by Sinemurian times. This turnover can be explained by a change in the hinterland drainage pattern. This study documents that eastern provenance areas were actively supplying sediments into the Norwegian Barents Sea Basin later than previously assumed, and our data offer age constraints for tectonic activity in the basin and its hinterland inferred from the changes in sediment supply to the basin.