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Subsurface prediction of fluvial systems by Aislyn Barclay

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Subsurface prediction of fluvial systems by Aislyn Barclay

  1. 1. Subsurface Prediction of Fluvial Systems: Are Current Models Adequate? A Case Study of the Late Triassic Chinle Formation in Petrified Forest National Park Aislyn Trendell Barclay, Ph.D. Anadarko Petroleum Corporation Stacy C. Atchley, Ph.D. Lee C. Nordt, Ph.D. Baylor University
  2. 2. Talk Objective • To discuss various fluvial models and their applicability for subsurface interpretation. • To discuss the Chinle Formation at Petrified Forest National Park as a case study for fluvial system interpretation. 2
  3. 3. Fluvial Models • Two main types of fluvial systems: ▫ Tributary  Consist of tributary streams that connect into a trunk channel downstream  Ex. Mississippi River, Nile River ▫ Distributary  River enters into open basins from topographic highs where the downstream reaches of the channel are free to migrate laterally 3 Upstream Downstream Tarim Basin, China – River enters from south (Weissmann et al., 2011) Upstream Downstream
  4. 4. Tributary Fluvial Systems • Characteristics: 1. Tributaries transport water and sediment to the trunk channel and channel mouth 2. Increased discharge and channel size downstream 3. Commonly confined within a fluvial valley 4. Reworking of floodplain fines with fluvial migration • Classified into meandering, braided, or anastomosing based on in-channel transportation processes, architectural elements and stability/migration of the channel 4 Mississippi River Drainage Basin (USGS) Upstream Downstream Upstream Downstream
  5. 5. • Fluvial depositional models based predominantly on tributary systems • Found in continental basins and in degradational valleys ▫ Survey of rivers in fluvial models: 27% are in basinal settings (Weissman, 2011) • Basin Models ▫ Decreasing accommodation results in a system-wide evolution to increasingly more suspended load rivers as the fluvial equilibrium profile decreases slope 5 Tributary Fluvial Systems (Shanley and McCabe, 1994)
  6. 6. Distributary Fluvial Systems • Characteristics: 1. Rivers exit confinement from upland regions into open basins 2. Downstream reaches of the channel are free to migrate laterally (nodal avulsion common) 3. Large-scale fan-shaped (or pseudo-fan-shaped) package of sediment 4. River size and energy (and therefore grain size) decrease downstream • Fluvial fan models first documented in 1960s • Referred to as megafans, large alluvial fans, wet alluvial fans, fluvial distributary systems, and distributive fluvial systems in the literature 6 Tarim Basin, China – River enters from south (Weissmann et al., 2011) Upstream Downstream Upstream Downstream (Trendell et al., 2012)
  7. 7. Distributary Fluvial Systems • Recent studies have shown that distributive systems are found in almost all continental basins (Weissmann et al., 2011; Hartley et al., 2011). ▫ Distributive systems may be equally as important as tributary fluvial systems in continental strata in the rock record • Basin Models ▫ Decreased accommodation results in progradation of coarser grained sediments into the basin 7 Upstream Downstream Upstream Downstream Tarim Basin, China – River enters from south (Weissmann et al., 2011) (Trendell et al., 2012)
  8. 8. Distributary System Model • Differs from alluvial fans: Scale (up to 400km long); Grain size; Sediment transportation processes ▫ Fluvial transportation as opposed to gravity and mass transport processes • Commonly only have one active channel, but may have channel bifurcation or multiple active channels 8 (Trendell et al., 2012)
  9. 9. Distributary System Model Proximal • Low Accomodation • Immature Sediments • High equilibrium profile slope • High channel: overbank • Amalgamated-form meander beds 9 (Trendell et al., 2012)
  10. 10. Distributary System Model Medial • Lower Sediment load • Higher Accomodation • Slightly more reworked sediments • Shallower slope • Moderate channel:overbank • Simple-form meander belts 10 (Trendell et al., 2012)
  11. 11. Distributary System Model Distal • Low sediment load • High accomodation • More mature sediments • Low equilibrium profile slope • Low channel:overbank • Simple-form meander belts 11 (Trendell et al., 2012)
  12. 12. Distribution of Distributary Fluvial Systems 12 (Davidson et al., 2011 after Hartley et al., 2011) • ~400 large DFS (greater than 300km in length) identified globally • 1000s more smaller scale DFS
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  14. 14. Late Triassic Paleogeography • Fluvial, lacustrine, and palustrine deposits that are discontinuously exposed throughout southern US • Equatorial Pangea • Foreland basin • Drainage from present- day Texas to Nevada • Free of marine influence • Petrified Forest National Park • Geochronologically well-constrained record spanning 17 Ma (Ramezani et al. 2011) Background - Study Area - Results & Discussion - Conclusions
  15. 15. Study area - Stratigraphy of the Park (Raucci and Blakey, 2006) Background - Study Area - Results & Discussion - Conclusions
  16. 16. Study Area Map Background - Study Area - Results & Discussion - Conclusions
  17. 17. • Suspended- load • Inherited colors • Suspended- load • Poorly drained paleosols • Vernal ponds • Bedload • Moderately drained paleosools Background - Study Area - Results & Discussion - Conclusions • Suspended- load • Poorly drained paleosols Measured Sections Newspaper Rock Interval Measured Sections Upper Blue Mesa Member Measured Sections Lower Sonsela Member Measured Sections Lower Blue Mesa Member
  18. 18. Summary of Fluvial Characteristics • Newspaper Rock Interval ▫ Laterally migrating suspended-load system ▫ Upstream erosion of well-drained (oxidized) overbank  Drapes of red sediment on accretion surfaces • Blue Mesa (Lower and Upper) ▫ Small river sizes and overbank-dominated system  Predominantly overbank fines with rare crevasse and levees ▫ Poor overbank drainage (Blue and grey- colored paleosols)  Water table at or near the sediment-air interface • Sonsela Member ▫ Mixed-load fluvial system  Greater proportion of bedload deposits (downstream accretion) ▫ Increased drainage of overbank environments  Purple and red paleosols (in situ) 18 IncreasingAridity? Sequence Boundary?
  19. 19. Depositional Controls • Eustacy ▫ Petrified Forest National Park is located >500km from the paleoshoreline • Climate – Precipitation proxies indicate limited change during study succession despite paleosol changes Background - Study Area - Results & Discussion - Conclusions
  20. 20. Sandstone Composition Changes • Lithics (volcanogenic, igneous, and metamorphic), quartz, and minor feldspar • Lithic percentage varies but constituents remain the same • Systematic decrease in mineralogical maturity upsection Background - Study Area - Results & Discussion - Conclusions Blue Mesa MemberNewspaper Rock Sonsela Member Upsection
  21. 21. Subsidence • Subsidence ▫ High rates of subsidence in lower Blue Mesa Member ▫ Rates drastically decrease in upper Blue Mesa Member • Tributary System ▫  subsidence =  fluvial equilibrium profile as accommodation space is filled= competence and capacity =  suspended load transport • Distributive System ▫  subsidence = progradation and coarsening upward of system =  of system surface away from water table Background - Study Area - Results & Discussion - Conclusions (Trendell et al., 2012)
  22. 22. Chinle Depositional Model - Distributary System Background - Study Area - Results & Discussion - Conclusions • Characteristics consistent with a distributary system model – Coarsening upwards in decelerating subsidence – Increased drainage within stable MAP – Decreased mineralogic maturity within decreasing subsidence Newspaper Rock? Blue Mesa Member Sonsela Member
  23. 23. Modern Analog – Taquari River 23 (Hartley et al., 2012)
  24. 24. Conclusions • Progradation of a fluvial fan (distributive fluvial system) provides a mechanism that can account for depositional element trends, paleosol drainage changes within stable MAP, and decreased sandstone maturity in a decreasing subsidence regime ▫ Sonsela Member is interpreted as Medial Fan ▫ Blue Mesa Member is interpreted as distal fan ▫ Newspaper rock is interpreted as a larger channel within the distal fan • Changing paleosol colors are interpreted to represent elevation relative to water table, rather than early onset of aridity that occurs during latest Triassic and Jurassic • When examining continental strata that are divorced from marine influence, one must consider both types of fluvial systems in order to interpret and predict fluvial changes in the subsurface 24
  25. 25. References • Trendell, A. M., Atchley, S.C. and Nordt, L.C., Facies analysis of a probable large fluvial fan depositional system: the Upper Triassic Chinle Formation at Petrified Forest National Park, Arizona: Journal of Sedimentary Research, v. 83, p. 873-895 • Weissmann, G.S., Hartley, A.J., Nichols, G.J., Scuderi, L.A., Olson, M., Buehler, H., and Banteah, R., 2010, Fluvial form in modern continental sedimentary basins: Distributive fluvial systems: Geology, v. 38, p. 39 – 42, doi: 10.1130/G30242.1. • Weissmann, G.S., Hartley, A.J., and Nichols, G.J., 2011, Alluvial facies distributions in continental sedimentary basins - distributive fluvial systems, in Davidson, S.K., Leleu, S., and North, C.P., eds, From River To Rock Record: The Preservation of Fluvial Sediments and Their Subsequent Interpretation, SEPM, Special Publication 97, p. 327–355. • Hartley, A.J., Weissmann, G.S., Nichols, G.J., and Warwick, G.L., 2010, Large distributive fluvial systems: Characteristics, distribution, and controls on development: Journal of Sedimentary Research, v. 80, p. 167 –183. 25
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  27. 27. Fluvial Systems in Continental Basins • Kongakut River, Alaska (Davidson et al., 2011) 27
  28. 28. Fluvial Systems in Continental Basins • Helmand River, Afghanistan (Davidson et al., 2011) 28
  29. 29. Fluvial Systems in Continental Basins • Chaco Plain, Andean Foreland Basin (Weissmann et al. 2011) 29
  30. 30. Fluvial Systems in Continental Basins • Tarim Basin, China (Weissmann et al. 2011) 30
  31. 31. Fluvial Systems in Continental Basins • Pentanal Basin, Brazil (Weissmann et al. 2011) 31
  32. 32. Fluvial Systems in Continental Basins • Pentanal Basin, Brazil (Weissmann et al. 2011) 32

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