Stratigraphic Architecture and Key Stratigraphic Surfaces Formed by Punctuated Flow Events - An Experiment on Fluviodeltaic Responses*

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1 Click to view movie-steady State Click to view movie-high Flow 1 Click to view movie- Low Flow 1 Click to view movie-high Flow Click to view movie-low Flow Note: If video clips do not function properly, you may not have the required codec; in this case, feel free to download newest versions from 7c9dff9a&BrowserMapId=1973&fn=baUG18cbS&adid=38938 Stratigraphic Architecture and Key Stratigraphic Surfaces Formed by Punctuated Flow Events - An Experiment on Fluviodeltaic Responses* Michael H. Hofmann 1, Albert Shultz, Craig Hill 3, and Chris Paola 3 Search and Discovery Article #8 (1) Posted October 1, 1 *Adapted from oral presentation at AAPG Annual Convention and Exhibition, New Orleans, Louisiana, April 11-1, 1 1 Subsurface Technology, ConocoPhillips, Houston, TX (michael.h.hofmann@conocophillips.com) 1 Subsurface Technology, ConocoPhillips, Houston, TX St Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN Abstract A tank experiment was conducted to gain understanding of the stratigraphic architecture and surface morphologies related to punctuated flow events involving high sediment yield. The experimental setup consisted of two 1 minute intervals with high sediment and water supply, each succeeded by a minute, low supply phase. Base level was held constant throughout the experiment. Early during the first high-supply pulse most of the sediment is stored in a large fan on the proximal delta plain close to the source. Most striking during this phase is the initiation of two incised valleys cutting into the proximal fan once the fan is established, limiting autogenic channel avulsion. During the first low-supply phase these valleys are further incised and the upper delta plain is essentially a zone of sediment bypass. These incised valleys also remain conduits for flow during the second high-supply pulse, although the valleys are partially filled early during this phase. Sediment deposition is most prevalent on the distal delta plain. During the second low-supply phase the valleys are again deeply incised, continuing to capture the flow in the upstream part of the delta plain. Although flow is confined to the same valley systems throughout most of the experiment, sediment transport to the marine portion of the experimental basin is very low and almost all of Copyright AAPG. Serial rights given by author. For all other rights contact author directly.

2 the sediment is stored on the delta plain. The most recognizable stratigraphic surface is a complex compound erosional surface formed by the alternate cycles of incision and partial filling of the valleys in the most proximal portions of the experiment. Most importantly, the deepest incisions along this surface are related to the flows during the low supply phase. Although this complex incision surface is controlled only by changes in supply, in the stratigraphic record it could be easily misinterpreted as a sequence boundary controlled by base-level fluctuations. In contrast to incised valleys formed by base-level cycles, almost no sediment was delivered to the deep receiving basin. These phases of alternating high and low supply, with the consequent formation of complex incised valleys in the proximal areas, would not be recognized by analyzing the stratigraphic record only in the more distal receiving basin.

3 Stratigraphic architecture and key stratigraphic surfaces formed by punctuated flow events an experiment on fluviodeltaic responses AAPG presentation by Michael Hofmann 1, Albert Shultz 1, Craig Hill, Chris Paola 1 ConocoPhillips, Subsurface Technology, Houston, TX St Anthony Falls Laboratory, Minneapolis, MN

4 Why this experiment? Gain understanding of the stratigraphic architecture and surface morphologies related to punctuated flow events involving high sediment yield Where on delta is sediment stored during high flow events versus low flow events? How does the stratigrapic architecture of flow pulses defer from other deposits in the experiment? What is the preservation potential of these flows (surfaces) in the stratigraphic record? Page

5 Experiment Set Up Delta Basin at SAFL Basin Dimensions m x m x 1cm Sediment mix: 7% sand, 3% coal No subsidence changing sediment supply (base supply:. l/s) changing water inflow (base supply:. l/s) Page 3

6 Page Experiment Build out 3 hr 3 min 1 st high supply pulse: 1 min 1 st low supply pulse: min nd high supply pulse: 1 min nd low supply pulse: min :3 :1 : : 3:3 3: 3: : :1 : :3 : : 3: 3: Time 3 1 3: Z, ocean Q multiplier Experiment Set Up base level and supply Experiment Set Up base level and supply Q,sed Z,ocean Q,sed Q,inflow Z,ocean Q,inflow

7 Steady State - Movie Time 3 1 Q multiplier Page

8 End Steady State - Topography Time 3 1 Q multiplier Page

9 High Flow 1 - Movie Time 3 1 Q multiplier Page 7

10 High Flow 1 - Isopach deposition End HIGH 1 Time 3 1 Q multiplier End Steady State Highest net deposition: proximal delta Highest net erosion: in sediment pathways (incised valleys) initiated during late pulse Page 8

11 Low Flow 1 - Movie Time 3 1 Q multiplier Page 9

12 Low Flow 1 - Isopach erosion End LOW 1 End HIGH 1 Time 3 1 Q multiplier Highest net deposition: distal delta and offshore Highest net erosion: in proximal sediment pathways (incised valleys) initiated during late pulse Page 1

13 High Flow - Movie Time 3 1 Q multiplier Page 11

14 3 1 deposition Time End LOW Pulse 1 Highest net deposition: in valleys and distal delta Highest net erosion: in sediment pathways (incised valleys) initiated during late pulse Page 1 End HIGH Pulse Q multiplier High High Flow Flow -- Isopach Isopach

15 Low Flow - Movie Time 3 1 Q multiplier Page 13

16 Low Flow - Isopach erosion End LOW Pulse End HIGH Pulse Time 3 1 Q multiplier Highest net deposition: distal delta and offshore Highest net erosion: in proximal sediment pathways (incised valleys) initiated during late high pulse Page 1

17 Erosion and Deposition on the delta plain % deposition on delta plain % erosion on delta plain % of total delta area % of total delta area high 1 low 1 high low high 1 low 1 high low experimental stage experimental stage Net erosion greatest during low flow pulses Erosion mainly in the proximal parts of the delta plain Erosion and deposition mainly confined to incised valleys in the proximal portion of the delta; more widely spread across the delta plain in the distal parts. Page 1

18 % sediment transported past shoreline and where does the sediment go (sediment storage)? steady state early fall 8 experiment 9 experiment. late fall 191. early rise late rise experimental stage Sediment volumes transported into deep basin greatest during low flow events Sediment volume transported to deep basin during the low flow events is similar to volume transported in the early stage of a base level cycle Minimum sediment transport into deep basin during high flow events Sediment volume transported past the shoreline is approximately the same as during regular steady state deposition High 1.. Low High. 7. Low base level [cm] supply multiplier Page 1

19 Stratigraphy Marine Record Incised Valley Record Early RS deposits FS deposits Steady state deposits Flow Pulse Record High deposits Low 1 deposits Page 17 Steady state deposits Formation of large, sandy, low angle clinoforms during low flow, similar to falling stage deposits during base level cycles Formation of muddy (coaly) deposits during high flows, more similar to steady state deposits

20 Morphologic differences between base level driven and supply driven incised valley formation Supply driven incision: Deepest incision proximal basinward Base level driven: Deepest incision distal Page 18

21 Continental Record - Preservation potential of surfaces and deposits Net deposition map High1 Net deposition of High pulse 1 deposits immediately after high supply Preserved strat map High1 Preserved deposits of High 1 at the end of the experiment Page 19

22 Continental Record - Preservation potential of surfaces and deposits End LOW Pulse End HIGH Pulse Flow pulses End LOW Pulse 1 End HIGH Pulse 1 End Steady State Flow pulses Marine Record Steady state High Low 1 Deposits preserved in proximal areas are mainly high pulse deposits Marine Record is dominated by low flow deposits Page

23 Continental Record - Preservation potential of surfaces and deposits Valley Fill End LOW 1 Supply driven: Proximal Valley fill - mainly sandy n/g >9% Mainly deposits of high flow pulse Page 1

24 Continental Record - Preservation potential of surfaces and deposits Timing of supply driven Incised Valley Formation End HIGH 1 End LOW End LOW 1 End LOW 1 End LOW End HIGH End Steady State Page

25 Summary Punctuated flow events create complex stratigraphic record The preservation potential of low flows is higher in marine record, while high pulses are more likely to be preserved in the continental record > % of surfaces/deposits are not present in the stratigraphic record and have been reworked by subsequent flows Main sediment storage during High flow pulses in alluvial fans. These fans are formed in progressively more distal positions on the delta plain Sediment transport (especially during low flow) is controlled by incised valleys spatially confined by High pulse fans Sediment transport past the shoreline is most efficient during low flow (reworking of high flow fans), similar volume as during the early stage of a base level cycle Marine Stratigraphic Record of flow pulses has similar architectural elements as steady state and falling stage deposits of base level driven systems but in a different succession Incised Valleys are mainly formed on proximal delta and have a complex basal surface that is time transgressive Valley fills are mainly sandy deposits of subsequent high flow pulse Page 3

26 High Flow 1 - Topography Time 3 1 Q multiplier Page

27 Low Flow 1 - Topography Time 3 1 Q multiplier Page

28 High Flow - Topography Time 3 1 Q multiplier Page

29 Low Flow - Topography Time 3 1 Q multiplier Page 7

30 Stratigraphy Valley Fill Base level driven: Distal Valley fill - low n/g (~ 8%) Supply driven: Proximal Valley fill - mainly sandy n/g >9% Mainly deposits of high flow pulse Page 8

31 Continental Record - Preservation potential of surfaces and deposits End LOW Pulse End HIGH Pulse End LOW Pulse 1 End HIGH Pulse 1 End Steady State Page 9