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Shelf Transit Process Change Deepwater Sands Systems Tracts

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Shelf-Transiting Shorelines, Sequence Generation, and Shelf-Margin Accretion*

Ron Steel¹

Search and Discovery Article #40145 (2005)

Posted March 14, 2005

*Adapted from the 2004-2005 AAPG Distinguished Lecture presented by Dr. Steel

¹Professor & Davis Chair, University of Texas at Austin ([email protected])

Abstract

Basins that develop a shelf break and fill by means of clinoform accretion appear to require at least 150-200m of water depth. Ramp basins have shallow water throughout.

The landward reaches of both types of basin form a shelf platform across which shorelines make regressive (deltas, strandplains) and transgressive (estuaries, barrier-lagoon systems) transits. Numerical simulations of modern deltas on varying shelf width and gradient suggest that (a) the shoreline is more likely to reach the shelf edge (or far into the basin in the case of a ramp) if relative sea level is stable or falling rather than rising, because of the phenomenon of auto-retreat, and (b) regressive shelf-transit time only rarely exceeds 100ky for even the widest shelves. The latter has some consequences for the time scale of fundamental stratigraphic sequences.

Although high-sediment supply deltas may be somewhat insensitive to minor sea-level changes and may remain at the shelf margin for prolonged periods, small- and medium-sized deltas make repeated regressive and transgressive transits in response to accommodation changes on the shelf. There is some evidence of process-regime changes during such shelf transits, depending on whether regressive shoreline trajectories are slightly rising or are slightly falling. In the former cases, deltas are remarkably wave dominated throughout the transect, whereas in the latter case wave-influenced deltas often become tide dominated (ramp basins) or fluvial dominated (shelf-break settings). There is increasing evidence that some turbidite populations on the shelf margin and basin floor derive from the river-generated hyperpycnal flows at the shelf-edge. Transgressive coasts also have a clear tendency to be strongly tidally influenced.

A new generation of sequence stratigraphic models will be stronger and more predictive when clothed with process data.

Types of Data

Figure 1. Examples of the three types of data utilized in this study: seismic data (upper left--seismic line courtesy Statoil), large outcrops (middle right), and well transects (lower left, well data courtesy A2D/Devon Energy).

This study of shelf –transiting shoreline systems and generation of stratigraphic sequences in shelf-break basins, as opposed to ramp basins, has utilized seismic data, large outcrops, and well transects (Figure 1).

Key Questions

1. How are shelves constructed and how easily can shorelines reach the shelf edge?

2. What is the significance of shelf-transit time for the generation of sequences?

3. Are there changes in shoreline ‘type’ during the transit?

4. Is amplitude of fall/rise of sea level important?

5. Is the presence of deltas at the shelf edge a guarantee for deepwater sands?

How is sand delivered to the shelf edge and beyond?

Figures 2-5

	Figure 2. Delivery of sand to the shelf edge and beyond. A, B. Movement of sand on the shelf by waves and currents (B off southern Africa). C. Yellow River delta advancing across the shelf.
	Figure 3. Clinoforms in the shelf building (upper) and shelf/slope setting during highstand [A], falling stage [B], falling stage and lowstand [C] and transgression [D] (lower).
	Figure 4. Shelf construction during highstand and sea-level fall, with clinoforms during highstand and shelf-edge deltas during sea-level fall (seismic line courtesy Statoil).
	Figure 5. “The effect of the auto-rereat concept shown by a deltaic shoreline trajectory, as proposed by Muto and Steel (1992). If relative sea level is rising at a constant rate and if other external forces are constant, the shoreline undergoes a retreat after a relatively brief period of progradation. The retreat is due to the progressive increase in the effective surface area of the delta. A constant supply of sediment falls far short of allowing a steady accretion of clinoforms.” (Figure and caption from Muto and Steel, 1997, with kind permission of SEPM.)

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-Sand to the Shelf Edge

Waves, tides and currents can bring sand to shelf edge (Figure 2A and 2B), but difficulty increases with width of shelf.
Deltas are probably the most efficient mechanism (Figure 2C).

-‘Shelf’ successions are built from repeated shelf transits by shorelines (Figure 3).

-In shelf construction, controls on shelf-transit time and deltas attaining shelf edge (Figure 4) are:

shelf width and gradient
sea-level behavior
sediment flux from fluvial system
delta-front gradient

-Auto-retreat tends to prevent deltas from reaching the shelf edge. Shoreline regression can turn to transgression without any change in rate of sediment supply or sea-level rise (Figure 5).

Shelf-Transit Time

Figures 6-8

	Figure 6. Transit times for deltas to reach shelf edge (after Burgess and Hovius, 1998; Muto and Steel, 2002).
	Figure 7. A. Cross-section, Columbus basin: 4^th order sequence between maximum flooding surfaces below and above sand-rich interval defined by a basal unconformity (58) (from Sydow et al., 2003, with kind permission of GCSSEPM Foundation, Norman C. Rosen, Executive Director). Interval demonstrates growth across faults, with the largest at or near the shelf edge. B. Seismic Line A, East Venezuela shelf / Columbus basin (location in Figure 8): shelf-edge trajectory of the SEG reservoir interval shows lateral (progradation) and vertical (aggradation) components (from Sydow et al., 2003, with kind permission of GCSSEPM Foundation, Norman C. Rosen, Executive Director).
	Figure 8. A. Composite log of SEA area records repeated transgressive-regressive shelf transits on the East Venezuela shelf (from Sydow et al., 2003, with kind permission of GCSSEPM Foundation, Norman C. Rosen, Executive Director). B. Index map, also illustrating eastward advancement of shelf edge (from Sydow et al., 2003, with kind permission of GCSSEPM Foundation, Norman C. Rosen, Executive Director).

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Regressive transit of shelf is necessary for delivery system to reach shelf edge.

Auto-retreat prevents many deltas from attaining a shelf-edge location, when seal level (SL) is rising.

For stable or falling relative sea level, even for wide shelves, transit time rarely exceeds 100,000 years.

Orinoco transiting the east Venezuela shelf (Figures 7 and 8)

In cross-section
Repeated transgressive-regressive shelf transits from well data

Is There Process Change during the Regressive-Transgressive Transit?

(Figures 9, 10, 11, 12, 13, 14, and 15)

Figures 9-15

	Figure 9. A. Schematic cross-section illustrating the relation of the various systems tracts to shelf edges and to changes of relative sea level. B-D. Influence of dominant shoreline process (tide, river, wave) on shoreline configuration. B. Irrawaddy delta (Myanmar [Burma]). C. Yellow River delta, D. Niger delta.
	Figure 10. Schematic sections illustrating changes in influence of processes with changes in relative sea level: A. Open shelf setting. B. Ramp setting in shallow basin).
	Figure 11. Process change on shorelines with sea-level change (and within a sea-level cycle): examples of tidal influence and variation of it with time, change of relative sea level, and systems tract.
	Figure 12. Hogsnyta, Spitsbergen, characterized by shelf-edge river-dominated delta during lowstand and shelf-edge wave-dominated deltas at highstand.
	Figure 13. At Storvola, Spitsbergen, flat trajectory angle with different shoreline type than where the trajectory angle is rising.
	Figure 14. Seismic line (courtesy Statoil), showing expression of deltaic deposits during falling base level and during rising base level. Inset: Delta position and type on shelf in relation to sea level (F=fluvial-dominated; W=wave-dominated; T=tide-dominated).
	Figure 15. Sketch of important outcrop localities (Pallfjellet, Brogniartfjellet, Storvola, and Hyrenstabben) in Spitsbergen where the lower Eocene section shows relationships between shelf-edge trajectory and facies changes (after Steel and Olsen, 2002, with kind permission of GCSSEPM Foundation, Norman C. Rosen, Executive Director).

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-Shelf transit and shoreline type (Figure 10)

A. Open shelf setting

Wave influence on shelf increases during relative sea level (RSL) fall and transit of delta across shelf, due to reduced wave-energy attenuation as shelf width narrows (Suter, 2001)

B. Ramp setting in shallow basin

Tidal influence on ramp increases into the basis as RSL falls due to increased to increased fault topography and resonance changes (Martinsen, 2001)

-Process change on shorelines with sea-level change (Figure 11)

-Ancient example of change in delta regime with RS (Figure 12)

-Shoreline trajectory angle (reflecting sea-level behavior) affects shoreline type (Figure 13).

-Delta type varies with relative sea-level change (Figure 14).

-Spitsbergen database (of lower Eocene facies) has provided ideas on relationships between trajectory and facies changes (Figure 15)

What about Amplitude of Sea-level Change?

Figure 16. Sediment distribution under two conditions of sea level change. A. With small changes in sea level due to limited glaciation. B. widespread glaciation with high amplitude in sea level changes. (From Galloway, 2001.)

-It is becoming accepted that Icehouse amplitudes (>100m) are greater than Greenhouse ones (few 10s of m).

-It has been suggested that Icehouse shelf transits would, therefore, partition more of the sediment budget directly into deepwater areas.

-Greenhouse transits would allow more redistribution of the budget along the shelf, preserving more strandplain successions.

Galloway (2001) has shown contrasting sediment patterns under limited glaciation and associated sea-level change, as opposed to high-amplitude “glacioeustasy” (Figure 16).

Is the Presence of Deltas at the Shelf Edge a Guarantee for Deepwater Sands?

Figures 17-24

	Figure 17. A. Delta barely reaching shelf edge. B. Incision during sea level fall. C. Shelf-edge progradation during rise. D. Shelf-margin wedge without incision.
	Figure 18. Stratigraphic cross-section of regressive facies, transgressive and tidal facies, and sequence stratigraphic surfaces from Spitsbergen outcrop area (from Schellpeper and Steel, 2001; Steel et al., 2003, with kind permission of GCSSEPM Foundation, Norman C. Rosen, Executive Director). Inset same as Figure 17.
	Figure 19. Stratigraphic cross-section, Brogniartfjellet, Spitsbergen, showing architecture of incised shelf-edge to slope-canyon clinoforms: facies associations (from Mellere et al., 2003, with kind permission of GCSSEPM Foundation, Norman C. Rosen, Executive Director). Inset same as Figure 17.
	Figure 20. Delta-front sands below fluvial channelized sand on outcrop.
	Figure 21. Montage of sedimentary structures on outcrop, representing deposits from hyperpycnal flows, and stratigraphic cross section from Spitsbergen outcrop area with interpretation of processes, and upslope from channelized hyperpycnal flows, thin-bedded turbidites and initial lowstand wedge (courtesy Andy Petter, University of Texas).
	Figure 22. Outcrop at Storvola and Hyrnestabben, Spitsbergen, illustrates delivery of sands from deltas (D) through slope (BS) to basin-floor fans (BFF).
	Figure 23. Shelf edges on seismic line (courtesy Statoil) and on outcrop at Storvola (inset, which is same as outcrop illustration in Figure 1).
	Figure 24. Late prograding wedge is formed by shelf-edge delta during late lowstand.

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-Shelf-edge delta types (Figure 17)

-Shelf-edge delta without much sand by-pass into deep water (Figure 18)

-Deep fluvial erosion of the delta is necessary to produce significant sand by-pass (Figures 19, 20, and 22).

Reach shelf edge.
Become incised by their own distributary channels; eventually completely cannibalized.
Deliver sand via slope channels to basin floor.

-Type B deltas deliver sands through slope to basin-floor fans (Figure 22).

-How is the incised shelf edge setting recognized (Figure 23)?

-Shelf-edge deltas are established for a second time in late lowstand (Figure 24).

Process Changes, Systems Tracts, Shelf Trajectory, and Reservoir Prediction

Figures 25-27

	Figure 25. Seismic line (courtesy Statoil) showing transgressive shelf trajectory, regressive and falling shelf trajectory, and regressive and rising shelf trajectory (seismic line same as in Figure 23 ).
	Figure 26. Seismic line (uninterpreted and interpreted) illustrating highstand to falling stage, early lowstand, late lowstand, and trangressive stages (seismic line courtesy Statoil).
	Figure 27. Oblique photograph of outcrop at Storvola, with prominent shelf, shelf-edge, and slope settings (same as inset in Figure 23).

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-Shelf Trajectories (Figure 25)

Transgressive Shelf Trajectory

Generally strong tide influence

Regressive and Falling Shelf Trajectory

General fluvial dominance, but clear wave and tide influence

Regressive and Rising Shelf Trajectory:

General wave domination

-Models clothed with process information will aid greatly in reservoir prediction (Figures 26 and 27).

Highstand to Falling Stage

Fluvial-wave-tide interaction with increasing fluvial influence to shelf edge

Early Lowstand

Apparent fluvial domination, with great slope disruption

Late Lowstand

Fluvial-dominant in deeper reaches; wave influence in shallow reaches

Trangressive

Flood-dominant tidal currents in channels of shelf-edge estuaries

References⁺

Burgess, P.M., and N. Hovius, 1998, Rates of delta progradation during highstands; consequences for timing of deposition in deep-marine systems: Journal Geological Society London, v. 155, part 2, p. 217-222.

Dalrymple, R.W., B.A. Zaitlin, and R. Boyd, 1992, Estuarine facies models: Conceptual basis and stratigraphic implications: Perspective: Journal Sedimentary Petrology, V. 62, p. 1130-1146.

Galloway, W.E., 2001, Cenozoic evolution of sediment accumulation in deltaic and shore-zone depositional systems, Northern Gulf of Mexico Basin: Marine and Petroleum Geology, v. 18, p. 1031-1040.

Martinsen, Randi S., 2001, Wave dominated versus current dominated shorelines in the Cretaceous Western Interior Seaway (abstract): AAPG Bulletin, v. 85, no. 13. (Supplement).

Mellere, D., A. Breda, and R.J. Steel, 2003, Fluvially-incised shelf edge deltas and linkage to upper slope channels (Central Tertiary Basin, Spitsbergen), in Shelf-margin deltas and linked downslope petroleum systems, H.H. Roberts, N.C. Rosen, R.H. Fillon, and J.B Anderson, eds.: GCS-SEPM Foundation 23^rd Annual Research Conference, Houston (CD-ROM), p.231-266.

Muto, T., and R.J. Steel, 1992, Retreat of the front in a prograding delta: Geology, v. 20, p. 967-970.

Muto, T., and R.J. Steel, 1997, Principles of regression and transgression: the nature of the interplay between accommodation and sediment supply: Journal Sedimentary Research, v. 67, p. 994-1000.

Muto, T., and R.J. Steel, 2002, In defense of shelf-edge delta development during falling and lowstand of relative sea level: Journal Geology, v. 110, p. 421-436.

Petter, A., 2004, Eocene falling-stage deltas and associated upper slope channels; an outcrop study of a deepwater feeder system (central Spitsbergen basin) (abstract): GSA South-Central Section meeting, Abstracts with Programs, v. 36, no. 1, p. 23.

Schellpeper, M.E., and R.J. Steel, 2001, A shelf-edge delta-to-estuary couplet in the Eocene of Spitsbergen: AAPG Annual Meeting Expanded Abstracts, p. 179.

Steel, R.J., and T. Olsen, 2002, Clinoforms, clinoform trajectories and deepwater sands, in Sequence sratigraphic models for exploration and production: Evolving methodology, emerging models and application histories, J. M. Armentrout and N.C. Rosen, eds.: GCS-SEPM Foundation, 22^nd Annual Research Conference, Houston (CD-ROM), p. 367-380.

Steel, R.J., S. Porebski, P. Plink-Bjorklund, and M.E. Schellpeper, 2003, Shelf-edge delta types and their sequence stratigraphic relationships, in Shelf-margin deltas and linked downslope petroleum systems, H.H. Roberts, N.C. Rosen, R.H. Fillon, and J.B Anderson, eds.: GCS-SEPM Foundation 23^rd Annual Research Conference, Houston (CD-ROM), p. 205-230.

Sydow, J.C., J. Finneran, and A.P.Bowman, 2003,. Stacked shelf-edge delta reservoirs of the Columbus Basin, Trinidad West Indies, in Shelf-margin deltas and linked downslope petroleum systems, H.H. Roberts, N.C. Rosen, R.H. Fillon, and J.B Anderson, eds.: GCS-SEPM Foundation 23^rd Annual Research Conference, Houston (CD-ROM), p. 441-465.

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Types of Data

Key Questions

How is sand delivered to the shelf edge and beyond?

Text

Shelf-Transit Time

Figures 6-8

Text

Is There Process Change during the Regressive-Transgressive Transit?

Figures 9-15

Text

What about Amplitude of Sea-level Change?

Is the Presence of Deltas at the Shelf Edge a Guarantee for Deepwater Sands?

Figures 17-24

Text

Figures 25-27

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Text

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How is sand delivered to the shelf edge and beyond?

Is There Process Change during the Regressive-Transgressive Transit?

What about Amplitude of Sea-level Change?

Is the Presence of Deltas at the Shelf Edge a Guarantee for Deepwater Sands?

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