This document discusses fluvial depositional models and uses the Late Triassic Chinle Formation in Petrified Forest National Park as a case study. It finds that the Chinle Formation represents a distributary fluvial system rather than a tributary one. As subsidence decreased over time, the fluvial system prograded in a coarsening upward sequence. Paleosol changes reflected relative elevation to the water table rather than changes in aridity. Both tributary and distributary fluvial systems should be considered when interpreting continental strata.

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. 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. 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. 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. • 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. 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. 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. 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. 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. 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. Distributary System Model
Distal
• Low sediment load
• High accomodation
• More mature
sediments
• Low equilibrium
profile slope
• Low
channel:overbank
• Simple-form
meander belts
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(Trendell et al., 2012)

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

13. 13

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. Study area - Stratigraphy of the Park
(Raucci and Blakey, 2006)
Background - Study Area - Results & Discussion - Conclusions

16. Study Area Map
Background - Study Area - Results & Discussion - Conclusions

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. 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)
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IncreasingAridity?
Sequence
Boundary?

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. 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. 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. 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. Modern Analog – Taquari River
23
(Hartley et al., 2012)

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
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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.
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26. 26

27. Fluvial Systems in Continental Basins
• Kongakut River, Alaska (Davidson et al., 2011)
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28. Fluvial Systems in Continental Basins
• Helmand River, Afghanistan (Davidson et al., 2011)
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29. Fluvial Systems in Continental Basins
• Chaco Plain, Andean
Foreland Basin
(Weissmann et al. 2011)
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30. Fluvial Systems in Continental Basins
• Tarim Basin, China (Weissmann et al. 2011)
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31. Fluvial Systems in Continental Basins
• Pentanal Basin, Brazil (Weissmann et al. 2011)
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32. Fluvial Systems in Continental Basins
• Pentanal Basin, Brazil (Weissmann et al. 2011)
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