Tetsuji Muto

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.

Autogenic response of fluvial deltas to steady sea-level fall: Implications from flume-tank experiments

Flume-tank experiments on fluvial deltas, conducted with constant fall of relative sea level (rate A < 0), constant sediment discharge ( q S), and constant water discharge ( q W), reveal a built-in geomorphic process that inevitably causes valley incision (“autoincision”) after the beginning of the sea-level fall and forms paired stream terraces on the abandoned alluvial slope. Despite steady sea-level fall, deltas aggrade without major incision until the autoincision threshold is attained, at which time the aggradational regime is replaced by a degradational one. Results of the experiments imply that multiple valley incision and terrace formation can occur as an autogenic response of the depositional system to steady forcing by constant sea-level fall. Changes of A , q S, and q W, or changes in river energy, are not required to account for these geomorphic events. The understanding of the autogenic response, when combined with the theory of shoreline autoretreat, provides an alternative view of the geomorphic development of fluviodeltaic systems during base-level fall.

Autogenic attainment of large-scale alluvial grade with steady sea-level fall: An analog tank-flume experiment

A graded river conveys its sediment load without net deposition or erosion. The graded state is thought to represent the long-term response of alluvial rivers to steady external forcing. We show here that alluvial rivers building deltas can be in grade as an autogenic response to steady sea-level fall. Consider an antecedent graded river profile, the upstream end of which consists of an alluvial-bedrock transition, and the downstream end of which is a fixed overfall where constant sea level is maintained. The antecedent graded profile is then drowned by a jump in sea level, after which sea level drops. The result is a new river profile ending in a prograding delta that deposits on top of the antecedent profile. If the rate of sea-level fall is constant and the length of the antecedent reach is sufficient, the new profile eventually becomes parallel or quasi-parallel to the antecedent profile, maintaining grade as it progrades. In the experiments reported here, series of graded river profiles with prograding deltas are created by stacking fluviodeltaic systems; each graded profile and its associated delta is stacked on its immediate predecessor. For each fluviodeltaic system, a graded alluvial profile is attained with any constant rate of sea-level fall, provided that the antecedent profile is of sufficient length. Experiments suggest that this autogenic approach to grade is more rapid for higher rates of sea-level fall, lower rates of sediment supply, and higher water discharges.

A similarity solution for a dual moving boundary problem associated with a coastal-plain depositional system

Assuming that the sediment flux in the Exner equation can be linearly related to the local bed slope, we establish a one-dimensional model for the bed-load transport of sediment in a coastal-plain depositional system, such as a delta and a continental margin. The domain of this model is defined by two moving boundaries: the shoreline and the alluvial–bedrock transition. These boundaries represent fundamental transitions in surface morphology and sediment transport regime, and their trajectories in time and space define the evolution of the shape of the sedimentary prism. Under the assumptions of fixed bedrock slope and sea level the model admits a closed-form similarity solution for the movements of these boundaries. A mapping of the solution space, relevant to field scales, shows two domains controlled by the relative slopes of the bedrock and fluvial surface: one in which changes in environmental parameters are mainly recorded in the upstream boundary and another in which these changes are mainly recorded in the shoreline. We also find good agreement between the analytical solution and laboratory flume experiments for the movements of the alluvial–bedrock transition and the shoreline.

The Middle Jurassic Oseberg Delta, Northern North Sea: A Sedimentological and Sequence Stratigraphic Interpretation

The Aalenian Oseberg Formation (0-80 m thick) is an important reservoir unit in the Middle Jurassic Brent Group in the northern North Sea, consisting of multiple sets of sandy Gilbert-type deltas. Small-scale (1.5-10 m) fining-upward units seen in the gamma-ray log correspond with individual delta sets, as independently confirmed by steepening-upward trends seen in the dip log. Within each set, the steep foreset slopes typically show thinly bedded sandstone facies (avalanche grain flows), whereas the lower foreset slopes, toesets, and bottomsets are formed largely by massive sandstone facies (sandy debris flows). On an intermediate scale (up to 40 m), the gamma-ray logs show both fining-upward and coarsening-upward trends through stacked delta sets, and these trends, traceable between wells, are interpreted in terms of decelerating and accelerating rates of relative sea level rise, respectively. The relative abundance of the sandy debris-flow deposits reflects a periodic and significant instability of the deltas upper foreset slope, probably during times of increased water depth in front of the delta. The normal progradation of individual Gilbert-type sets, however, is likely to have been along a subhorizontal topography during periods of little or no change in water depth. The long-term change to produce the observed vertical stacking of deltaic sets was one of a generally rising relative sea level. Modeling of the sea level rise in a steplike manner can account for the rarity of topsets. The progressive eastward truncation of the underlying Drake Formation on the Horda Platform by the Oseberg Formation suggests tectonic uplift and a lowstand of sea level immediately prior to the development of the Oseberg deltas. The Oseberg Formation is thus seen as a lowstand prograding wedge that developed during an interval of relatively high sediment supply and a variably rising sea level, culminating in the latest Aalenian flooding seen in the base of the overlying Rannoch Formation. The dip-log data, integrated with cores and other logs, are critical to correctly interpreting the reservoirs internal architecture. The slope-readjustment model predicts that the development of stacked massive reservoir sandstones of Oseberg type will occur preferentially in local areas of high subsidence or syndepositional faulting, or where there is deeper water in the lee of already existing fault scarps.

Contributions to sequence stratigraphy from analogue and numerical experiments

The sequence stratigraphic model, although no longer focused on eustasy and accommodation, has been until recently based largely on observation and interpretation of outcrop and subsurface data. This approach may be restrictive if the current model places limits on what is observed and how observations are interpreted. To make progress in our understanding of strata, the sequence stratigraphic model and method should be tested against and fully incorporate theoretical and experimental results that provide new knowledge of (1) autogenesis, (2) intrinsic stratigraphic responses, (3) alluvial grade, and (4) scales appropriate to single depositional systems evolving with relative sea-level changes. More extensive inclusion of analogue and numerical experimental results could lead to significant modification and refinement of existing sequence stratigraphic models.