Subsurface prediction of fluvial systems by Aislyn Barclay

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

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.

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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

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Upstream

Downstream

Tarim Basin, China – River enters from south (Weissmann et al., 2011)

Upstream

Downstream

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

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Mississippi River Drainage Basin (USGS)

Upstream

Downstream

Upstream

Downstream

• 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

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Tributary Fluvial Systems

(Shanley and McCabe, 1994)

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

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UpstreamDownstream

Upstream

Downstream

(Trendell et al., 2012)

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

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UpstreamDownstream

Upstream

Downstream



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

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Proximal

• Low Accomodation

• Immature Sediments

• High equilibrium profile slope

• High channel: overbank

• Amalgamated-form meander beds

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Medial

• Lower Sediment load

• Higher Accomodation

• Slightly more reworked sediments

• Shallower slope

• Moderate channel:overbank

• Simple-form meander belts

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Distal

• Low sediment load

• High accomodation

• More mature sediments

• Low equilibrium profile slope

• Low channel:overbank

• Simple-form meander belts

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Distribution of Distributary Fluvial Systems

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(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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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

• Geochronologicallywell-constrained record spanning 17 Ma (Ramezani et al. 2011)

Background - Study Area - Results & Discussion - Conclusions

Study area - Stratigraphy of the Park

(Raucci and Blakey, 2006)


Study Area Map


• Suspended-

load

• Inherited

colors

• Suspended-load

• Poorly drained paleosols

• Vernal ponds

• Bedload

• Moderately

drained

paleosools


• Suspended-

load

• Poorly

drained

paleosols

Measured Sections

Newspaper Rock IntervalMeasured Sections

Upper Blue Mesa MemberMeasured Sections

Lower Sonsela MemberMeasured Sections

Lower Blue Mesa Member

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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Incr

easi

ng

Ari

dit

y?

Sequence Boundary?

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


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


Blue Mesa MemberNewspaper Rock Sonsela Member

Upsection

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



Chinle Depositional Model - Distributary System


• 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

SonselaMember

Modern Analog – Taquari River

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(Hartley et al., 2012)

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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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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Fluvial Systems in Continental Basins

• Kongakut River, Alaska (Davidson et al., 2011)

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• Helmand River, Afghanistan (Davidson et al., 2011)

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• Chaco Plain, Andean Foreland Basin(Weissmann et al. 2011)

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• Tarim Basin, China (Weissmann et al. 2011)

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• Pentanal Basin, Brazil (Weissmann et al. 2011)

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• Pentanal Basin, Brazil (Weissmann et al. 2011)

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Subsurface prediction of fluvial systems by Aislyn Barclay