Subsurface prediction of fluvial systems by Aislyn Barclay
-
Upload
daniel-matranga-rpl -
Category
Business
-
view
586 -
download
0
Transcript of 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.
2
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
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
• 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)
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)
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
7
UpstreamDownstream
Upstream
Downstream
Tarim Basin, China – River enters from south (Weissmann et al., 2011)
(Trendell et al., 2012)
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)
Distributary System Model
Proximal
• Low Accomodation
• Immature Sediments
• High equilibrium profile slope
• High channel: overbank
• Amalgamated-form meander beds
9
(Trendell et al., 2012)
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)
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)
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
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)
Background - Study Area - Results & Discussion - Conclusions
Study Area Map
Background - Study Area - Results & Discussion - Conclusions
• 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 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)
18
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
Background - Study Area - Results & Discussion - Conclusions
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
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)
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
SonselaMember
Modern Analog – Taquari River
23
(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
24
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
26
Fluvial Systems in Continental Basins
• Kongakut River, Alaska (Davidson et al., 2011)
27
Fluvial Systems in Continental Basins
• Helmand River, Afghanistan (Davidson et al., 2011)
28
Fluvial Systems in Continental Basins
• Chaco Plain, Andean Foreland Basin(Weissmann et al. 2011)
29
Fluvial Systems in Continental Basins
• Tarim Basin, China (Weissmann et al. 2011)
30
Fluvial Systems in Continental Basins
• Pentanal Basin, Brazil (Weissmann et al. 2011)
31
Fluvial Systems in Continental Basins
• Pentanal Basin, Brazil (Weissmann et al. 2011)
32