Ronald J. Steel

Transgressive deposits: a review of their variability

Abstract Transgressive deposits accumulate with rising relative sea level during the landward migration of a coastline. Particularly at short time scales (e.g., 4th- to 6th-order cycles), transgressive deposits can be recognised through the evidence of a gradual or irregular landward shift of facies, or an upward deepening of facies that culminates in a surface or zone of maximum flooding. During transgression, the coastline moves landwards and the shelf area enlarges. This is accompanied by a tendency to have more sediment trapped in the alluvial and coastal-plain environments, a reduced sediment influx to the basin, and cannibalization (through ravinement) of previously deposited sediments, including those deposited in the early stages of transgression. The resultant deposits can be fully marine, estuarine/lagoonal or fluvial, and can include other facies such as coal and eolian deposits with a variability driven by changes in rate of sea-level rise, sediment supply, textural character of the sediments, shelf gradient or basin physiography. Transgression may be continuous or punctuated, the latter occurring by alternation of coastal retrogradation and regression despite a longer term, landward-stepping of the shorezone. This commonly results in shoreface retreat, barrier in-place drowning, or a variety of transgressive parasequences whose character depends on the balance between sediment supply and accommodation creation. Any classification effort based on driving force or sedimentary processes tends to be overidealised. We therefore propose a classification based on the recognition of distinctive surfaces (wave and tidal ravinement surfaces, transgressive surface) within the transgressive lithosome. Transgressive scenarios are presented from different settings. Five types of transgressive lithosome, with variable thickness, lateral extent and internal architecture are discussed. (1) Transgressive deposits developed below the lowest ravinement surface (termed T-A) are commonly coal bearing (back-barrier) and alluvial. They accumulate in low-gradient settings where there is divergence between the ravinement trajectory and the surface being transgressed. (2) Transgressive deposits developed above the tidal but below the wave ravinement surface (T-B) accumulate where tidal processes dominate over storm-wave processes. Here too there is, at least locally, a slight landward divergence between the two surface types, allowing the development of sandy estuarine lithosomes. The wave ravinement surface may be absent in the innermost part of transects or if the deposits represent purely tide-dominated high-energy settings (that are not dealt with in detail in this paper). (3) Transgressive deposits developed above the wave ravinement surface in low-gradient settings (T-C). Such deposits derive from shoreface erosion or from longshore drift. They are usually very thin, but thicken where the transgression was punctuated by regressive pulses or where offshore sand ridges form. (4) Transgressive deposits developed above the wave ravinement surface in high-gradient high-sediment supply settings (T-D). In this setting (commonly fault-bounded terraces, valley walls, or simply steep slopes), the transgressive deposits can be much thicker because all eroded and newly supplied sediment is deposited locally. (5) Transgressive deposits without evidence of ravinement surfaces (T-E). These can be characteristic of low-energy settings that are typically mud-dominated.

Architecture of Marine Rift-Basin Successions

Marine rift basins represent a continuum ranging from mixed nonmarine/marine through shallow marine to deep marine, or from partly emergent through partly submergent to completely submergent basin types. These rift basin types have strongly variable synrift sedimentary architectures because of temporal changes in relative sea level, accommodation creation, and sediment supply throughout the rift cycle. Accommodation changes are controlled mainly by local basin-floor rotation, basinwide background subsidence, and, to a lesser degree, by eustatic changes. Sediment supply determines how much of the accommodation is filled and in what manner, and is controlled by the distance to the main hinterland areas, and the size and sediment- yield potential of any local fault-block source area. Marine siliciclastic synrift successions, whether dominantly shallow or deep marine in nature, are classified in terms of sediment supply as overfilled, balanced, underfilled, and starved. Sediment-overfilled and sediment-balanced infill types are characterized by a threefold sandstone-mudstone- sandstone synrift sediment-infill motif; the sediment-underfilled type is represented by a two-fold conglomerate-sandstone-mudstone motif; and the sediment-starved type commonly is represented by a one-fold mudstone motif. The sequential development, linked depositional systems, and stratigraphic signatures of the early synrift, the rift climax, and the late synrift to early postrift stages vary significantly between these rift basin infill types, as do the tectonic significance (timing of initiation and duration) of stratal surfaces, such as footwall unconformities, nondepositional hiatuses, and marine condensed sections. The construction of the fourfold rift basin infill classification scheme provides a first basis and a strong tool for predicting the distribution and geometry of synrift reservoir and source rock types, despite the inherent variability of the marine synrift infills.

Shelf-margin deltas: their stratigraphic significance and relation to deepwater sands

Abstract Shelf-margin or shelf-edge deltas are a common constituent of Quaternary shelves, where their genetic link to falling and lowstand sea level has been well established, but they are rarely reported from older successions. Recognition of this delta type in the ancient record is nevertheless important, because (1) it helps to position the fossil shelf edge, (2) it provides insight into how sand budgets were partitioned between shelf edge, slope and basin-floor settings, and (3) it can contribute to the debate concerning three versus four systems-tract systematics and the positioning of a sequence boundary. Shelf-margin deltas create wide (tens of kilometers), high (hundreds of meters) and steep (3–6°) clinoforms, because they build across the relatively deepwater shelf margin. Such bodies tend to form strike-elongate wedges that initially thicken basinwards, attaining a maximum thickness of 50–200 m just outboard of the shelf edge, then thin on the middle/lower slope, whereas they commonly pinch out landwards by lapping back onto shelf shales. Prior to reaching the shelf edge, small-scale (tens of meters), tangential foresets (usually The key facies association in shelf-edge deltas is the mouth bar-to-delta front association. Mouth-bar facies, landwards of the shelf edge, consist mainly of thick, clean, flat to low-angle and ripple-laminated medium to fine sands. Basinwards from the shelf edge, such sands commonly alternate with heterolithic slumped unit, creating characteristic slumped to laminated couplets. Because the delta front is superimposed on a preexisting, steep and extended shelf margin, it commonly contains (beyond the mouth bars) thick successions (up to many tens of meters) of sandy, slope turbidites. Shelf-margin deltas differ from inner shelf deltas in showing: (1) an order of magnitude higher clinoforms (hundreds instead of tens of meters), and strike-elongated, locally pod-like sand bodies commonly affected and augmented by growth faulting; (2) paleoecological evidence of abrupt shallowing (foreshortened stratigraphy); (3) turbidite-prone delta-fronts; (4) larger scale and greater abundance of slope-controlled soft-sediment deformation and (5) the general absence of a paralic ‘tail’ along the trailing edge of the delta front. Shelf-margin deltas can be classified into two types. Stable shelf-margin deltas are usually tens of meters thick and are not associated with shelf-edge incision or major slope collapse/disruption features. Slope turbidites are common and occur as unconfined sheets and lobes. Such deltas form when relative sea level falls no lower than the level of the shelf platform or, after longer sea-level falls, when rivers reestablish at the shelf edge on the rise. Unstable shelf-margin deltas are associated with large-scale slope collapse, listric growth faults and sometimes with salt diapirs. Slides are common on the upper slope, whereas imbricated sediment packages and compressional ridges affect the slope toe. Slope turbidites can be ponded within structurally controlled mini-basins on the slope. Such deltas are often related to unusually great sand influx to the shelf edge, or simply to prolonged or large fall of relative sea level below the shelf edge. The diachronous erosional unconformity atop the shelf-to-shelf-edge deltaic wedge is viewed by some researchers as the most easily recognizable and persistent surface on the shelf and slope. This surface contrasts with the downlap erosional surface developed from the beginning of sea-level fall, which is viewed by other researchers as the key boundary surface within this complex. It should be noted that sea level can fall for a long period of time (tens of thousands of years) before deltas even reach the shelf margin and, therefore, before significant volumes of sand are delivered across the shelf break. Hence, the time of incision of the shelf edge and emplacement of deepwater sand is commonly long after the initial fall so the time of (maximum) relative lowstand of sea level may be a more practical choice for the timing of the sequence boundary. Recognition of shelf-margin deltas and analysis of their architecture help in the prediction of presence or absence of basin-floor fans. Deltaic complexes that are aggradational to backstepping, downlap onto disrupted or complex slopes, and overlie an incised shelf and shelf edge, predict that there should be basin-floor fans present. In contrast, delta complexes that show a prolonged and preserved progradational to downstepping architecture implies the presence of turbidite accumulations on the slope but not on the basin floor.

Thick turbidite successions from supply-dominated shelves during sea-level highstand

Emphasis on the association between relative sea-level lowstand and the formation of sandy deep-water fans has tended to downplay the significance of high sediment supply and its potential to create deep-water fans, even during sea-level highstands. The Lance– Fox Hills–Lewis shelf margin in southern Wyoming suggests that high supply was critical in causing the accretion of this moderately wide Maastrichtian shelf margin, at a minimum rate of 47.8 km/m.y., and the generation of large, sand-rich fans during every shoreline regression across the shelf. It is surprising that fans developed from shelf-margin clinoforms that show systematically rising shelf-edge trajectories (proxy for rising relative sea level) as well as from those that show flat trajectories (stable to falling relative sea level). However, the latter, producing more sediment bypass, resulted in bigger and thicker fans, whereas the former produced somewhat smaller and thinner fans. We term the former highstand fans and suggest caution in using the lowstand model for high-supply systems.

Principles of regression and transgression; the nature of the interplay between accommodation and sediment supply

ABSTRACT The rate of change of accommodation (at the shoreline) [A] and the rate of sediment supply [S] are the primary factors controlling regression and transgression in the geological record, or retreat and advance of a coastal depositional system. The generally accepted notion, here referred to as the A/S ratio concept, stating that the migration of a shoreline is controlled principally by the magnitude of the ratio of A to S is insufficient and somewhat misleading, partly owing to the problem of dimensional confusion. The interplay of A and S inevitably results in autoretreat of the shoreline, whereby the seaward advance of any shoreline is halted and is subsequently turned to landward retreat, provided there is a continuous rise of relative sea l vel. How effective the autoretreat process is depends, for a given period of relative sea level rising, upon the length of potential time period for the seaward advance of shoreline, and is proportional to A2/S and A1.5/S0.5 in two- and three-dimensional sediment dispersal modes, respectively. Autoretreat becomes more effective as A increases and/or S decreases: A functions more critically to the effectiveness than S does. The A/S ratio concept is only approximately applicable when autoretreat is less effective (i.e., with very low A and/or very high S). In this sense, the autoretreat theory and the A/S ratio concept are complementary to each other. The autoretreat mechanism is expected to be substantially useful not only in fluvial deltas but also in many other depositional systems (e.g., barrier islands) whose evolution has been considered in terms of the A/S ratio concept.

Tectonostratigraphy and sedimentary architecture of rift basins, with reference to the northern North Sea

Abstract A tectonostratigraphic model for the evolution of rift basins has been built, involving three distinct stages of basin development separated by key unconformities or unconformity complexes. The architecture and signature of the sediment infill for each stage are discussed, with reference to the northern North Sea palaeorift system. The proto-rift stage describes the rift onset with either doming or flexural subsidence. In the case of early doming, a proto-rift unconformity separates this stage from the subsequent main rift stage. Active stretching and rotation of fault blocks during the rift stage is terminated by the development of the syn-rift unconformity. Where crustal separation is accomplished, a break-up unconformity commonly marks the boundary to the overlying thermal relaxation or post-rift stage. Tabular architectures, thickening across relatively steep faults, characterize the proto-rift stage. Syn-rift architectures are much more variable. Depending on the ability of the sediment supply to fill the waxing and waning accommodation created during rotation and subsidence, one-, two- or three-fold lithosome architectures are likely to develop. During the post-rift stage, an early phase with coarse clastic infilling of remnant rift topography often precedes late stage widening of the basin and filling with fine-grained sediments.

In Defense of Shelf‐Edge Delta Development during Falling and Lowstand of Relative Sea Level

There is a continued debate about the sequence stratigraphic interpretation of deep‐marine sand accumulations. The conventional notion states that it is preferably (but not only) falling stage and lowstand of relative sea level that favor the formation of the deep‐marine systems because, at these times, the fluvial and deltaic transport system tends to extend relatively easily across a shallow‐marine shelf surface and hence has potential for sand delivery into deeper‐marine environments. It has been argued recently, however, that sand delivery across the shelf to the shelf edge is equally likely during highstand, as gauged from the estimated time periods for modern natural rivers to build their deltas to the shelf edge, during the modest relative sea level rise of the past 7000 yr. This argument and counterargument are examined here quantitatively by using, as a basis, the concept of “shoreline autoretreat.” The autoretreat theory predicts that under conditions of constant sediment supply ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Turbidite Variability and Architecture of Sand-Prone, Deep-Water Slopes: Eocene Clinoforms in the Central Basin, Spitsbergen

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