ALBERT

Description: We present a geometric, sediment mass-balance model for the interaction of axial and transverse alluvial systems in a subsiding basin. By comparing the model result with a flume experiment that employed a simplified half-graben tectonic geometry with axial and transverse sediment sources, we quantify rates of axial-transverse erosional sediment mixing. In the experiment, the lateral migration rate of the axial-transverse boundaries due to the sediment mixing scales with sediment supplies delivered by transverse drainages, but not with water (or sediment) discharge from the axial channel or with tectonic tilting rate. Using an empirical lateral erosion rate, the model shows how sediment supply partitioning among the axial, hanging-wall, and footwall drainages controls the width and the location of the axial-channel belt. Comparing the modeling results with field cases demonstrates that transverse sediment fluxes could slow the axial-channel migration or even reverse the movement against the tectonic forcing.

Description: Basinwide hiatal discontinuity has been generally accepted as firm evidence of a distinctive allogenic event such as tectonic movement or a complete eustatic cycle. Here we show through theoretical and experimental modeling that a largescale, hiatal discontinuous surface can be produced autogenically in fluviodeltaic successions under no changes in external dynamic forcing, i.e., constant relative sea-level rise (rate rslr) and constant sediment supply (rate qS per unit width). This "autogenic hiatus" occurs where (1) the hinterland slope is steeper than the subaqueous slope of the delta, and (2) the initial downstream length of the feeding alluvial river exceeds a critical magnitude Lcrt that is specified primarily by qSlrslr. In this topographic setting, the existing depositional system becomes transgressive and nondeltaic as soon as sea level starts to rise. The existing subaqueous surface is starved of sediment and progressively extends landward until alluvial length is reduced to Lcrt. After the retreating river has attained this critical length, the shoreline still retreats but the depositional system is restored to deltaic sedimentation. Because of this, the subaqueous surface, which was previously starved of sediment, becomes overlain by delta foreset deposits after a significant time gap. Thus a steady sea-level rise can produce all of the following: a) the strata underlying the hiatus, b) the hiatus itself, and c) the strata overlying the hiatus. Changes in rslr or qS are not required to account for the presence of a hiatus of this particular type. Numerical simulations suggest that autogenic hiatuses are likely to be produced in most natural river deltas if a sea-level rise such as occurred during latest Pleistocene to Holocene (i.e., 0.01 km/kyr for 10 kyr) is available. This further implies that autogenic hiatuses may well exist in stratigraphic records of river deltas, especially of Quaternary age. An understanding of autogenic hiatuses, when combined with the theory of shoreline autoretreat, provides an alternative view of the origin of some stratigraphic breaks.

Description: [1]  It is now generally accepted that deltas that prograde to the shelf edge are able to transport coarse sediment to deep water either with or without sea-level changes. However, it is still unclear how feeder rivers behave differently in the shelf-edge delta case to rivers found in a delta that progrades over the shelf. A series of nine shelf-edge delta experiments are presented to investigate the lateral mobility of the feeder channel at the shelf edge and the associated deep-water depositional system under a range of sediment supply rates and shelf-front depths. In the experiments, constant sediment supply from an upstream point source under static sea level led the fluviodeltaic system to prograde over the shallow shelf surface and advance beyond the shelf edge into deep water. The feeder river of the fluviodeltaic system became a bypass system once the toe of the delta front reached the shelf edge. After the delta front was perched at the shelf edge, a submarine fan developed in deep water although remaining disconnected from the delta. In this bypass stage, no regional avulsion or lateral migration of the feeder river occurred and all sediment from the upstream source bypassed the river, delta front, and shelf-front slope. The duration of the bypass stage is proportional to shelf-front depth and inversely proportional to sediment discharge. The combined duration of the shelf-transit phase of the fluviodeltaic system and the bypass phase is the characteristic time scale for the continental-margin to “anneal” transgression-inducing perturbation due to high frequency and/or high amplitude relative sea-level rise. The sequential evolution in the experiment compares favorably to the Eocene Sobrarbe Formation, a shelf-edge delta in Spain, although natural variations are noted. This comparison justifies the application of concepts proposed herein to natural systems and provides insight into interpreting processes from ancient shelf-edge delta systems.

Description: The distinction between depositional belts associated with the basin axis and flanking piedmont streams is a fundamental attribute of the stratigraphic architecture of intracontinental rift basins. Spatiotemporal distributions of these lithofacies associations are sensitive to a combination of factors, including basin geometry, subsidence rate, and sediment discharge; however, most studies have focused on one or two controls and one depositional component (axial or tributary) of the basin-fill succession. A new perspective on how these depositional belts develop under simple but precisely controlled boundary conditions of steady subsidence, sediment flux, and water discharge is achieved through the creation of an experimental stratigraphic succession. The Experimental EarthScape run in 2006 (XES06) focused on the geomorphic evolution of sedimentary successions within an asymmetrically subsiding basin, analogous to a simple half graben, containing four interacting supply points of sediment and water. Under the imposed conditions, the experimental system self-organized into an axial stream flanked by transverse fans. Imposition of various combinations of longitudinal and lateral sediment flux showed that the locations and widths of the axial and transverse systems were strongly controlled by relative sediment fluxes (“flux steering”), and less influenced by the location of the subsidence maximum and subsidence rate. The axial drainage was dominated by transversely sourced sediment through toe cutting of the transverse fans, except during the highest axial-sediment discharges. Footwall fans persisted even under conditions of very large axial-sediment discharge, aided by topographic inheritance of the steeper transverse depositional slopes.

Description: The stratigraphic architecture of intracontinental rift basins is defined by a dynamic relationship between depositional processes associated with the basin floor and flanking tributary streams. The resulting depositional belts are sensitive to a variety of factors, including basin geometry, subsidence rate, and sediment discharge. The Experimental EarthScape run in 2006 (XES06) examined the development of fluvial morphology and alluvial architecture as a function of subsidence and sediment flux in an experimental basin based on the form of a simple half graben. Sediments from tributary drainages were introduced into the axial stream through toe cutting and realignment of transverse drainage courses to parallel the prevailing axial-flow direction. Transverse sediment contributions to the axial stream were almost equally apportioned over a wide range of sediment discharges tested in the experiments. Sediment tracers showed a larger contribution of footwall-derived sediment into the axial belt, probably due to more frequent and aggressive toe cutting by axial streams. Changes in the axial–transverse deposit boundary to external forcing (by subsidence and sediment discharge), and relatively rapid intrastage stabilization of the depositional belts, resembles the large-scale self-organization observed in moving boundaries that define the morphology of fluviodeltaic systems. Basin sedimentation was matched to subsidence in order to maintain a constant base level, which made the location and width of the axial belt sensitive to the relative sediment fluxes from the transverse systems, rather than the axis of maximum subsidence. The asymmetrical subsidence pattern and the transverse-fan morphology influenced the preservation of sedimentary sequences. Stage-bounding stratigraphic lacunae were well preserved in the hanging-wall succession, providing a reliable record of basin development.

Description: The linkage between relative sea-level change, shelf-edge architecture, and evolution of Maastrichtian basin-floor fans in the Washakie Basin, Wyoming, has been investigated at the scale of lobes, lobe complexes, and submarine fans using 630 wireline logs. The basin-floor fan deposits of two adjacent clinothems form lobate shapes on the toe of slope and basin floor. The earlier lobe complexes of the two clinothems are only weakly developed (from no deposition to up to 3.9 km3 respectively in Clinothems 9 and 10), indicating small volumes of sandy sediment delivered to deep water. The lobe complexes (up to 6.4 km3 of each lobe complex) of Clinothem 9 aggraded with fixed slope channels and without strong basinward or lateral migration (40–170 m aggradation, 4–8 km progradation with 4 km lateral shift) and did so in concert with a highly aggradational shelf edge (50 m/100 ky with 5.5 km progradation) during a period of interpreted relative sea-level rise. In contrast, the deep-water lobe complexes (up to 11.5 km3 of each lobe complex) of Clinothem 10 prograded continuously for 15–18 km on the basin floor (with 60–210 m aggradation) coeval with a flattish shelf-edge progradation (25 km/100 ky with 25 m aggradation) and an interpreted minimal sea-level rise or stillstand. The depocenters of lobe complexes in Clinothem 10 switched laterally (7–14 km) by compensational stacking and slope-channel avulsions. During the late development of both clinothems, the deep-water lobe complexes became smaller (up to 1.9 and 6.1 km3 respectively in Clinothems 9 and 10) or retreated concurrently with shelf flooding. Washakie Basin deep-water fans thus evolved through stages of initiation, aggradation or progradation, and retreat of lobe complexes. The submarine-fan growth stages of these deep-water depocenters were surprisingly well linked to coeval changes in shelf-edge trajectory between successive, ca. 100 ky maximum flooding events on the shelf. We suggest that the close linkage of lobe-complex stacking pattern with shelf-edge behavior was because the Washakie Basin formed under greenhouse conditions with a continuously high, Laramide sediment discharge to the deep-water fans while the feeder deltas were at the shelf edge, despite significant sediment reworking of shelf-edge deltas by waves and tides.