high frequency cyclical isostatic adjustments - significance for incised valleys - blum, 2009
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High Frequency Cyclical Isostatic Adjustments - Significance for Incised Valleys - Blum, 2009TRANSCRIPT
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High Frequency Cyclical Isostatic Adjustments: Significance for Incised Valleys*
Mike Blum1
Search and Discovery Article #30079 (2009) Posted March 26, 2009
*Adapted from oral presentation at AAPG International Conference and Exhibition, Cape Town, South Africa, October 26-29, 2008 1Geology and Geophysics, Louisiana State University, Baton Rouge, LA ([email protected] )
Abstract Isostatic adjustment to changes in water and sediment loads are rarely considered in high-resolution stratigraphic interpretations. This presentation uses Gulf of Mexico examples to illustrate two possible high-frequency (Milankovitch-scale) cyclical isostatic adjustments that may be intrinsic to fluvial systems and incised valleys. First, geophysicists recognize that isostatic adjustments must accompany sea-level change. It is also known that sea-level fall forces channel extension across emergent shelves, and the formation of incised valleys, whereas sea-level rise forces river mouth backstepping, flooding of the shelf, and valley filling. New 1D steady-state modeling of the Texas Gulf of Mexico shelf illustrates that isostatic uplift in response to sea-level fall will impact river long profiles, and may serve as the driving force for incision itself. With a shelf width of ~100 km, and sea-level fall of 100 m, isostatic uplift may be 20-30 m at the shelf margin, with flexural effects extending 10’s of km upstream from the highstand shoreline. Sea-level rise and flooding of the shelf will have the opposite effect. Second, deltaic loading and subsidence are widely recognized, but development of incised valleys results in sediment unloading as well, which likely produces a cyclical pattern of isostatic uplift and subsidence. New 1D steady-state and 3D visco-elastic modeling of the Mississippi delta region indicates that sediment volumes removed and replaced were sufficient to induce >12 m of uplift in the valley center, and >9 m along valley margins, followed by subsidence of the same magnitude, with flexural effects extending ~150 km along the Gulf of Mexico coast. Cyclical uplift and subsidence should amplify valley incision and filling, whereas spatial patterns of uplift and subsidence should play a key role in development of valley fill architecture.
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MississippiRiver
Colorado River
High Frequency Cyclical Isostatic
Adjustments:
Significance forIncised Valleys
Mike BlumDepartment of Geology and Geophysics
Louisiana State UniversityBaton Rouge, Louisiana
Current address: ExxonMobil Upstream Research Company
Houston, Texas
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Topics of Discussion:
• Incised-valley evolution
• Isostatic adjustments from sediment unloading and loading (large river phenomenon)
• Isostatic adjustments from sea-level change (hydroisostasy)
• Implications/speculation
MississippiRiver
Colorado River
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GLOBAL SEA LEVEL CHANGE: 1-0 MA
data from Miller et al. (2005)
Falling Sea Level55% of Time
Rising Sea Level45% of Time
Sea
Leve
l (m
)Fr
eque
ncy
(# o
f 5 k
yr b
ins)
Freq
uenc
y(#
of 5
kyr
bin
s)
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INCISED VALLEYS AND VALLEY FILLSForm from Fluvial-Deltaic Transit of the Shelf
highstandshoreline
shelf-slope break
Last Interglacial Highstand
0 20 40 60 80 100 120kyrs BP
0
-40
-80
-120sea
leve
l (m
)highstandalluvial plain
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highstandshoreline
shelf-slope break
coastal-plain and cross-shelf incised valley
Last Glacial Period Falling Stage
0 20 40 60 80 100 120kyrs BP
0
-40
-80
-120sea
leve
l (m
)
exposed shelf
shelf-phase deltas
INCISED VALLEYS AND VALLEY FILLSForm from Fluvial-Deltaic Transit of the Shelf
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highstandshoreline
shelf-slope break
Last Glacial Period Lowstand
0 20 40 60 80 100 120kyrs BP
0
-40
-80
-120sea
leve
l (m
)
coastal-plain and cross-shelf incised valley
exposed shelf
shelf-margin deltas
INCISED VALLEYS AND VALLEY FILLSForm from Fluvial-Deltaic Transit of the Shelf
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highstandshoreline
shelf-slope break
Holocene Highstand
0 20 40 60 80 100 120kyrs BP
0
-40
-80
-120sea
leve
l (m
)
highstandalluvial plain
submerged shelf
shelf-margin deltas
INCISED VALLEYS AND VALLEY FILLSForm from Fluvial-Deltaic Transit of the Shelf
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RIVER LONG PROFILE RESPONSESTO SEA-LEVEL CHANGE
after Blum and Tornqvist (2000)
net degradational reach where long profiles reflecttectonically- and isostatically-controlled incision rates
Long profile cross-overand upstream limits
of sea-level influence
lowstand depositionalshoreline break
onlap distanceduring highstand
channel extensionduring falling stage
∆SLhighstand depositional
shoreline break
Lowstand floodplainLowstand depth of scour
Highstand floodplainHighstand depth of scour
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HOW DO YOU FORM AN INCISED VALLEY?initial channel incision
and cross-shelf extensionwith sediment bypass
lateral migration withcontemporaneouschannelbelt formation
• Step-wise incision, followed by lateral migration and channelbelt construction widens the incised channel into a valley-scale feature
• What are the mechanisms that promote the step-wise incision?
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Vicksburg
Baton Rouge
New Madrid
Memphis
New Orleans
Mississippiincised valley
Yazoo Basin
Isostatic Adjustments to Sediment Unloading and Loading
• Example from the Mississippi River incised valley (a large river phenomenon)
• Valley incision during the last glacial period sea-level fall and lowstand
• Valley filling during the present transgression and highstand
Baton RougeBaton Rouge
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LOWER MISSISSIPPI INCISED VALLEYValley Cross-Section at N 30 r
modified from Saucier (1994)
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LOWER MISSISSIPPI VALLEY AND DELTA1D Isostatic Effects of Unloading and Loading at N 30r
Steady-State Solution - Symetrical Uplift and Subsidence
after Blum et al. (2008)
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Cyclical uplift and subsidence is plausible…….. what about the timing??
LOWER MISSISSIPPI VALLEY AND DELTA1D Isostatic Effects of Unloading and Loading at N 30r
Steady-State Solution - Symetrical Uplift and Subsidence
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LONG PROFILE EVOLUTION, LOWER MISSISSIPPI VALLEY AND DELTA
Constrains the Magnitude and Timing of Valley Unloading and Loading
after Blum et al. (2008)
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FLEXURAL MODELING PARAMETERSFLEXURAL MODELING PARAMETERSFLEXURAL MODELING PARAMETERSFLEXURAL MODELING PARAMETERSFLEXURAL MODELING PARAMETERSFLEXURAL MODELING PARAMETERSANU 3d Visco-Elastic Glacial Rebound Model
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after Blum et al. (2008)
FLEXURAL DEFORMATION OF THE NORTHERN GULF OF MEXICO SHORELINE (LAST 30 KYRS)
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FLEXURAL DEFORMATION OF THE NORTHERN GULF OF MEXICO SHORELINE (LAST 30 KYRS)
10203040
0
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Vicksburg
Baton Rouge
New Madrid
Memphis
New Orleans
Mississippiincised valley
Yazoo Basin
Isostatic Adjustments to Sediment Unloading and Loading
• Large-scale cyclical uplift and subsidence in response to valley incision and valley filling for mega river systems
• Isostatic response is rapid, with less than 2000 yrs lag between load change and response
• Feedback process that should enhance incision and deposition
Baton RougeBaton Rouge
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NORTHWEST GULF OF MEXICO MARGIN:Topography and Bathymetry
AA’
Colorado-Brazos Delta
highstandshoreline shelf
margin
0 50 100 150 200 250 300 350
0
-500
-1000
Distance (km)
Elev
atio
n (m
) SL
A A’
bedrockto alluvialtransition
Hydroisostatic effects?•Magnitude of uplift that results from sea-level fall to the shelf margin
•High-amplitude sea-level change with a broad shelf
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COLORADO-BRAZOS INCISED VALLEYLast 100 kyr Glacial-Interglacial Cycle
Colorado mixedbedrock-alluvial
valley
Brazos mixed bedrock-alluvial
valley
Colorado Delta Brazos Delta
coastal-plainincised valley
0 20 km
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INCISED-VALLEY FILL ARCHITECTUREColorado River, Texas Coastal Plain
0 6 km
Caney Creek(ca. 1000 yrs ago)
San Bernard River(ca. 5000 yrs ago)
Laminated Mud
Estuarine Mud
Crevasse-Splay Sand
Channelbelt Sand
Mature Paleosol
Slickensided Mud
Massive Mud
Weakly Laminated Mud
10
0
-10
-20
-
Colorado River20 km
after Blum and Aslan (2006)
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LONG PROFILE, LOWER COLORADO RIVER,TEXAS COASTAL PLAIN AND SHELF
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LONG PROFILE, LOWER COLORADO RIVER,TEXAS COASTAL PLAIN AND SHELF
coastal plain incised valley
cross-shelf incised valley
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LONG PROFILE, LOWER COLORADO RIVER,TEXAS COASTAL PLAIN AND SHELF
Steady-State Solution - Hydroisostatic Uplift from Sea-Level Fall
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LONG PROFILE, LOWER COLORADO RIVER,TEXAS COASTAL PLAIN AND SHELF
coastal plain incised valley
much deeper cross-shelf incised valley
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HOW DO YOU FORM AN INCISED VALLEY?initial channel incision
and cross-shelf extensionwith sediment bypass
lateral migration withcontemporaneouschannelbelt formation
• Large-scale cyclical uplift and subsidence from unloading and loading of shelves in response to sea-level change
• In regional-scale rivers, step-wise incision may reflect hydroisostatic uplift in response to sea-level fall !!
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CONCLUSIONS• Evolution of incised valley systems very likely co-occurs with 2 distinct
types of large-scale cyclical isostatic adjustments.
• For large river systems, large-scale flexural uplift and subsidence results from evolution of the incised valley itself, and the change in mass distribution from sediment unloading and loading. • Magnitude of uplift and subsidence will correlate to scale of river
system and incised valley.• Timing of uplift and subsidence will reflect the forcing mechanisms that
drive incision and filling, and need not correlate from system to system
• For river systems that discharge to broad shelves, large-scale flexural uplift and subsidence results from hydroisostatic effects that accompany sea-level change. • Magnitude will correlate to shelf width and amplitude of SL change.• Timing of effects will correlate to SL change…..regional signal
• Isostatic effects should be incorporated in models for incised-valley evolution, and variability in likely isostatic effects should be explored (low vs. high gradient systems, greenhouse vs. icehouse worlds).
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References Blum, M.D., H. Jonathan, A. Purcell, and R.R. Lancaster, 2008, Ups and downs of the Mississippi Delta: Geology Boulder, v. 36/9, p. 675-678. Blum, M.D. and A. Aslan, 2006, Signatures of climate vs. sea-level change within incised valley-fill successions; Quaternary examples from the Texas Gulf Coast: Sedimentary Geology, v. 190/1-4, p. 177-211. Blum, M.D. and T.E. Tornqvist, 2000, Fluvial responses to climate and sea-level change; a review and look forward: Sedimentology, v. 47, supplement 1, p. 2-48. Miller, K.G., J.D. Wright, and J.V. Browning, 2005, Visions of ice sheets in a greenhouse world, in Ocean chemistry over the Phanerozoic and its links to geological processes, v. 217/3-4, p. 215-231. Saucier, R.T., 1994, Evidence of late glacial runoff in the Lower Mississippi Valley: Quaternary Science Reviews, v. 13/9-10, p. 973-981.