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Deepwater Channel Bypass and Backfilling Processes from the Eocene Ainsa Basin, South-Central Pyrenees, Spain*

By

Julian D. Clark1, Andrea Fildani1, and Nicholas J. Drinkwater2

 

Search and Discovery Article #50094 (2008)

Posted October 22, 2008

*Adapted from oral presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23

1Energy Technology Co., Chevron, San Ramon, CA. ([email protected])

2Energy Technology Co., Chevron, Houston, TX. 

 

Abstract

Deepwater channel outcrops in the Eocene Ainsa Basin have been previously well documented. The work presented here, however, focuses on evidence of turbidity current bypass and backfilling. Channel bases are characterized by a composite erosional surface with significant incision and scouring, and facies comprising scoured thin beds, fine-grained drapes, relatively coarse-grained bars and mud-rich debrites. Above this, channel fills are generally finer-grained thick-bedded sand facies. In the channel axes thick-bedded facies are commonly amalgamated, with dewatering and flame structures. Towards the margin these facies change laterally to thinner inter-bedded graded sandstones. These observations imply that numerous relatively high velocity turbidity currents were responsible for cutting the channels, with the majority of the sediment load bypassing down slope. Increases in flow velocity can either be related to changes in the staging area of the flows, or an increase in the channel floor gradient as channels attempt to establish equilibrium gradients on an irregular or dynamic slope. Seismic data from analogous subsurface systems suggests that the latter is a very common process in controlling channel architecture. In many channels, after the initial bypass phase, flows with a lower velocity backfill the channel resulting in rapid sand deposition. Debris flow deposits within channels are considered to be random events, but their common association with bypass facies may be related to the longevity of the bypass phase relative to the backfilling phase. Processes of bypass and backfilling operate at different scales, magnitudes and frequencies, resulting in a hierarchy of channelized stratigraphy with predictable facies associations. Generic models deriving from this work can be used to aid interpretation and modeling of analogous reservoirs.

 

 

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Selected Figures

 
Figure 1 Paleogeography, south-central Pyrenees, Spain (Marzo et al., 1998).


Figure 2 Channels and canyons of the Ainsa Basin.
Figure 3 Shelf to slope cross section.


Figure 4

Ainsa II stratigraphic channel hierarchy.  

 

Figure 5

Paleo-sea-floor interpretation.

 

Figure 5

Ainsa II channel system.

Conclusions

Bypass Facies and Processes

“Muddy Channel� interpreted to be complex-scale bypass facies association:

  • Composite erosional surface.
  • Scoured thin-bedded facies.
  • Debrites.
  • Coarse-grained channels with bars.
  • Aggradational levee with dunes (at base), fining-up sequence and localized slumps.

Debrites can be associated with bypass facies but not the process:

  • Sluggish flows infilling channels.

Stratigraphic Controls

Large scale erosional surfaces initiate channel complexes.

But what controls bypass?

  • External cyclic forcing (allogenic).
  • Autocyclic response of deepwater systems to bathymetry.

Supporting evidence:

  • Structurally active basin with syndepositional structures.
  • fill and spill processes should be expected.

  • Channel complexes appear to have gradual abandonments.
  • autocyclic waxing-waning flow cycles.

References

Clark, J.D., 1994, Architecture and processes in modern and ancient deep-marine channel complexes: PhD dissertation, Leicester University, U.K.

Falivene, O., A. Pau, A. Gardiner, G. Pickup, et al., 2006, Best practice stochastic facies modeling from a channel-fill turbidite sandstone analog (the Quarry outcrop, Eocene Ainsa Basin, northeast Spain): AAPG Bulletin, v. 90/7, p. 1003-1029.

Marzo, M., J.A. Munoz, J. Verges, M. Lopez-Blanco, et al., 1998, Sedimentation and tectonics; case studies from Paleogene, continental to deep water sequences of the South Pyrenean foreland basin (NE Spain): 15th International Congress of Sedimentology (IAS) Field Trip Guidebook, p. 199-204.

Mutti, E., M. Segurét, and M. Sgavetti, 1989, Sedimentation and deformation in the Tertiary sequences of the southern Pyrenees: AAPG Mediterranean Basins Conference Guidebook Fieldtrip No. 7, p. 157.

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