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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*
By
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])
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.
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Types of Data
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 Questions1. 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?
Text-Sand to the Shelf Edge
-‘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:
-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 TimeFigures 6-8
Text
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)
Is There Process Change during the Regressive-Transgressive Transit?(Figures 9, 10, 11, 12, 13, 14, and 15) Figures 9-15
Text-Shelf transit and shoreline type (Figure 10) A. Open shelf setting
B. Ramp setting in shallow basin
-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?
-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
Text-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).
-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 25-27Return to top.
Text-Shelf Trajectories (Figure 25)
Generally strong tide influence
General fluvial dominance, but clear wave and tide influence
General wave domination
-Models clothed with process information will aid greatly in reservoir prediction (Figures 26 and 27).
Fluvial-wave-tide interaction with increasing fluvial influence to shelf edge
Apparent fluvial domination, with great slope disruption
Fluvial-dominant in deeper reaches; wave influence in shallow reaches
Flood-dominant tidal currents in channels of shelf-edge estuaries
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 23rd 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, 22nd 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 23rd 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 23rd Annual Research Conference, Houston (CD-ROM), p. 441-465.
+Including abstracts of authors noted on figures.
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