Nearshore along-strike variability: Is the concept of the ... · PDF fileNearshore...
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Nearshore along-strike variability: Is the concept of the systems tract unhinged? Nearshore along-strike variability: Is the concept of the systems tract unhinged? Ashley D. Harris, Andrew S. Madof, and Sean D. Connell Chevron Energy Technology Company, Houston, TX, 77002-7308, U.S.A 1. Abstract Sequence stratigraphic models assume that nearshore strata have relatively consistent and laterally persistent stacking at the systems tract scale and therefore may not fully describe the three-dimensional stratigraphic architecture in areas displaying marked nearshore along-strike variability. A stratigraphic model of nearshore deposits is presented that corrects for this assumption by explaining variations in along-strike stratal geometries in terms of a systematic change in the orientation of a shoreline trend or clinoform rollover, a scenario comparable to deflection around a hinge. The model defines hinge zones that are both fixed and moving with respect to time, and was created from outcrop, well-log, and seismic reflection data. Model end members predict contemporaneous progradational, aggradational, and retrogradational stacking bounded by surfaces displaying significant along-strike changes in architecture, implying that sequence stratigraphic surfaces can be diachronous. We advocate examining the impact of stratigraphic variations parallel to depositional strike by testing for the presence of uniform versus differential progradation, phenomena responsible for creating unhinged and hinged nearshore depositional systems, respectively. Understanding these differences will improve subsurface predictions and provide a more complete understanding of the stratigraphic evolution of sedimentary basins. 2. Stratal stacking patterns and the depositional sequence A: Stacking patterns used to define systems tracts B: Conventional interpretations imply consistent lateral stacking (i.e., no significant strike variability) LST TST EHST LHST A Stacking patterns Aggradational Progradational Retrogradational B Depositional sequence 4 3 2 1 4 3 2 1 4 3 2 1 LHST Late highstand TST Transgressive LST Lowstand Sb Sequence boundary Mfs Maximum flooding surface Coastal-plain sandstones & mudstones Shallow-marine sandstones Incised valley fill (fluvial & estuarine sandstones) Deep-water sandstones Shelf & slope mudstones & thin sandstones Shelf mudstones & thin sandstones EHST Early highstand Sb Sb Mfs LHST Truncation Onlap Downlap Toplap Relative age Older Younger Parasequence no. 1 1 Depositional facies Systems tracts Surfaces 3. Conceptual depositional models A: Uniform progradation results in equal and coincident rollover movement, as illustrated in cross-sections x, y, and z (on right). Where projected onto the same plane, uniform progradation manifests as a single point. B: Nearshore lateral variability creates along-strike differences in rollover position. Superimposing rollover trends in cross-sections x, y, and z illustrates increased progradation away from the intersection of rollover 1 and 2 that defines a hinge to the left of line x. A Uniform progradation B Differential progradation x y z Forestepping Backstepping 2 Rollover (Time 2) Rollover (Time 1) 1 Progradation direction (Time 1) Progradation direction (Time 2) Map view 2 1 Onshore Offshore z y x = + + x z Cross section Differential Map view Onshore Offshore z y x 2 1 = + + Cross section Uniform A: Map view illustrating how backstepping and forestepping result in clinoform retreat and advance, respectively. B: Oblique view illustrating along-strike changes in stacking: retrogradational—transgressive systems tract (TST), aggradational—early highstand systems tract (EHST); and progradational—late highstand systems tract (LHST). Depositional facies include coastal plain (green), shallow marine (yellow), and shelf (gray). Mfs—maximum flooding surface. C: Hypothetical well-log cross section (wx-wy-wz) highlighting strike-parallel changes in stacking. Lines drawn at the top of shallowmarine sandstone intervals represent physical stratigraphic surfaces, whereas lines at the bases delineate facies changes. 4. Hinge Model wz wy wx A Map view B Oblique view C Cross section Onshore Offshore Backstepping Backstepping z' z Forestepping Forestepping x' z' y' Mfs Mfs TST EHST LHST y x x' y' y 4 3 2 1 4 3 2 1 4 3 2 1 Hinge z Retrograde Aggrade Prograde Maximum flooding surface x wz wy wx A: Generalized stratigraphic columns and age control for parasequences 1–6 in the Cozzette Sandstone. Deposits include shallow-marine (yellow), paleovalley fill (PVF, pink), coastal plain (green), and marine (gray) facies. Red lines denote sequence boundaries and blue lines denote flooding surfaces. Cretaceous (Campanian) ages were defined using ammonites (Didymoceras nebrascense, D. stevensoni, and Exiteloceras jenneyi; see Madof et al., 2015). B: Map view of rollover trends associated with the Cozzette parasequences illustrates locations of the projected hinge zones. C: Spatial variations in stratigraphic architecture across cross-section panels x-x’, y-y’, and z-z’. Relative sea-level curve (right) was created by plotting rollover positions through time. The gap in x-x’ is caused by stratigraphic pinch-out of parasequence 5 west of y-y’. Postdepositional tectonic tilting is interpreted for the Cozzette (far right), highlighting back-tilted stratal geometries caused by flattening on the downlap surface. LHST—late highstand systems tract; EHST—highstand systems tract; TST—transgressive systems tract. 5. Fixed and moving hinges for the Cozzette Sandstone (Book Cliffs, Piceance Basin, Colorado, USA) A Stratigraphy B Map view y' x' C Oblique view Rollins Sandstone Corcoran Sandstone Campanian Late 74 Ma 72 Ma 76 Ma 3 2 4 PVF 1 PVF 2 Cozzette Sandstone Mount Garfield Formation 6 5 1 2 3 4 5 0.00° 0.02° 0.04° 6 * N z' z y' y x' 39°N 108°W = Grand Junction, Colorado * = Book Cliffs escarpment x y z' z IVF 1 IVF 2 Time 1 Time 2 Time 3 Flattened Tectonic tilting TST EHST LHST Cross section Western interior seway 1 2 3 4 5 1 2 3 6 1 0 m +30 m 0 m +30 m 0 m +30 m Datum 2 1 5 m Fixed hinge - Cretaceous shoreface Hinge x 1 3 2 4 6 5 10 km 10 m 5 km 6. Moving hinge: Marambaia Formation (Santos Basin, Brazil) A: Generalized stratigraphic columns and age control for sequences 1–4 in the Marambaia Formation (see description of A). Eocene (Ypresian) ages were determined using planktonic foraminifers (Morozovella acuta, M. aequa, and Acarinina pseudotopilensis; Alicia Kahn, 2015, personal communication). B: Map view of locations of the projected hinge zones, Marambaia sequence. C: Architectural changes in the Marambaia and differences in relative sea level resulted from changes in source area. Blue arrows depict the direction of active outbuilding for sequences 1–3. LLSSS—late lowstand sequence set; ELSSS—early lowstand sequence set. A B C 55 Ma Ypresian 49 Ma 53 Ma 51 Ma 1 0° 2° 4° 6° Time 1 Source change Map view Time 3 4 3 2 1 2 3 4 Marambaia Formation Time 2 N Hinge 1 24°S 42°W x' x y' y z Atlantic Ocean Stratigraphy Map view Oblique view ELSSS LLSSS LLSSS ELSSS Hinge 2 1 2 3 4 1 2 3 y' x' x y z' z -175 m 0 m -175 m 0 m -175 m 0 m 2 1 3 50 m Moving hinge - Eocene shelf z' 1 3 2 10 km 2.5 km 250 m 7. Along-strike variations in rollover position Plots are projected onto a single plane and illustrate progradation distance versus aggradation thickness. Arrows denote vectors projected from x, y, and z, and show intraparasequence and sequence variability. The shaded area highlights the magnitude of progradation and aggradation between adjacent parasequences and sequences. Arrows (left) are used to calculate migration rates (right), based on average cycle length (i.e., duration/cycles). A: Differential progradation in the fixed hinge parasequences of the Cozzette Sandstone shows an increase in along-strike variability. B: The moving hinge sequences of the Marambaia Formation show a decrease in progradation magnitude with time. B A Fixed hinge - shoreface parasequences Moving hinge - shelf sequences West -100 0 -200 0 2 Aggradation (m) Progradation (km) 4 6 8 10 12 x y z x z East 2 3 1 z z +10 +20 +30 0 0 10 20 30 40 50 Aggradation (m) Progradation (km) x East West 2 3 4 5 6 x y 1 1 2 3 4 5 6 160 0 Parasequence Migration rate (m/kyr) 80 80 1 2 3 0 Sequence 1.0 0.5 Time Time Migration rate (m/kyr) 9. Conclusions Stratigraphic hinge models provide simple geometric tests of uniform or spatially variable sedimentation in nearshore systems: -Fixed hinge: roughly stationary area defined by convergence of shoreline or rollover trends. -Moving hinge: two or more hinges that are spatially separated or move through time. Hinge zones can predict stacking patterns through interpolation of shoreline or rollover trends between or beyond data points. Always be vigilant of the “3D menace” (or opportunity) in interpretation! 10. References Fulthorpe, C.S., and Austin, J.A., 2008, Assessing the significance of along-strike variations of middle to late Miocene prograding clinoformal sequence geometries beneath the New Jersey continental shelf: Basin Research, v. 20, no. 2, p. 269– 283, doi: 10.1111/j.1365-2117.2008.00350.x. Gill, J.R., and Cobban, W.A., 1973, Stratigraphy and geologic history of the Montana Group and equivalent rocks, Montana, Wyoming, North and South Dakota:U.S. Geological Survey Professional Paper 776, 37 p. Madof, A.S., Christie-Blick, N., and Anders, M.H., 2015, Tectonically controlled nearshore deposition: Cozzette Sandstone, Book Cliffs, Colorado, U.S.A.: Journal of Sedimentary Research, v. 85, p. 459–488, doi: 10 .2110 /jsr .2015 .26. Madof, A.S., Harris, A.D., and Connell, S.D., 2016, Nearshore along-strike variability: Is the concept of the systems tract unhinged?: Geology, v. 44, no. 4, p. 315–318, doi: 10.1130/g37613.1. Wehr, F.L., 1993, Effects of variations in subsidence and sediment supply on Parasequence stacking patterns, in Weimer, P., and Posamentier, H.W., eds., Siliciclastic sequence stratigraphy: Recent developments and applications: American Asso- ciation of Petroleum Geologists Memoir 59, p. 369–379, doi:10 .1306 /M58581C14 8. Observations from other localities Older Younger Gill and Cobban (1973) Fulthorpe and Austin (2008) Wehr (1993) New Jersey margin, U.S.A. Montana and Wyoming, U.S.A. Alberta, Canada © 2016 Chevron U.S.A., All Rights Reserved
Transcript of Nearshore along-strike variability: Is the concept of the ... · PDF fileNearshore...
Nearshore along-strike variability: Is the concept of the systems tract unhinged?Nearshore along-strike variability: Is the concept of the systems tract unhinged?Ashley D. Harris, Andrew S. Madof, and Sean D. Connell
Chevron Energy Technology Company, Houston, TX, 77002-7308, U.S.A
1. Abstract Sequence stratigraphic models assume that nearshore strata have relatively consistent and laterally persistent stacking at the systems tract scale and therefore may not fully describe the three-dimensional stratigraphic architecture in areas displaying marked nearshore along-strike variability. A stratigraphic model of nearshore deposits is presented that corrects for this assumption by explaining variations in along-strike stratal geometries in terms of a systematic change in the orientation of a shoreline trend or clinoform rollover, a scenario comparable to de�ection around a hinge. The model de�nes hinge zones that are both �xed and moving with respect to time, and was created from outcrop, well-log, and seismic re�ection data. Model end members predict contemporaneous progradational, aggradational, and retrogradational stacking bounded by surfaces displaying signi�cant along-strike changes in architecture, implying that sequence stratigraphic surfaces can be diachronous. We advocate examining the impact of stratigraphic variations parallel to depositional strike by testing for the presence of uniform versus di�erential progradation, phenomena responsible for creating unhinged and hinged nearshore depositional systems, respectively. Understanding these di�erences will improve subsurface predictions and provide a more complete understanding of the stratigraphic evolution of sedimentary basins.
2. Stratal stacking patterns and the depositional sequence
A: Stacking patterns used to de�ne systems tracts
B: Conventional interpretations imply consistent lateral stacking (i.e., no signi�cant strike variability)
LSTTST
EHSTLHST
A Stacking patterns
Aggradational
Progradational
Retrogradational
B Depositional sequence
4321
4321
4321
LHST Late highstand
TST Transgressive
LST Lowstand Sb Sequence boundaryMfs Maximum �ooding surface
Coastal-plain sandstones & mudstones
Shallow-marine sandstones
Incised valley �ll (�uvial & estuarine sandstones)
Deep-water sandstones
Shelf & slope mudstones & thin sandstones
Shelf mudstones & thin sandstones
EHST Early highstand
Sb
Sb
MfsLHST
Truncation Onlap Downlap Toplap
Relative age
Older Younger
Parasequence no. 11
Depositional facies
Systems tracts
Surfaces
3. Conceptual depositional models
A: Uniform progradation results in equal and coincident rollover movement, as illustrated in cross-sections x, y, and z (on right). Where projected onto the same plane, uniform progradation manifests as a single point.
B: Nearshore lateral variability creates along-strike di�erences in rollover position. Superimposing rollover trends in cross-sections x, y, and z illustrates increased progradation away from the intersection of rollover 1 and 2 that de�nes a hinge to the left of line x.
A Uniform progradation B Differential progradation
x
y
z
Forestepping Backstepping2 Rollover (Time 2)
Rollover (Time 1)1 Progradation direction (Time 1)Progradation direction (Time 2)
Map view
2
1
Onshore
O�shore
zyx=
+
+
x
z
Cross section
Di�erential
Map view Onshore
O�shorezyx
2
1=
+
+
Cross section
Uniform
A: Map view illustrating how backstepping and forestepping result in clinoform retreat and advance, respectively.
B: Oblique view illustrating along-strike changes in stacking: retrogradational—transgressive systems tract (TST), aggradational—early highstand systems tract (EHST); and progradational—late highstand systems tract (LHST). Depositional facies include coastal plain (green), shallow marine (yellow), and shelf (gray). Mfs—maximum �ooding surface.
C: Hypothetical well-log cross section (wx-wy-wz) highlighting strike-parallel changes in stacking. Lines drawn at the top of shallowmarinesandstone intervals represent physical stratigraphic surfaces, whereas lines at the bases delineate facies changes.
4. Hinge Model
wzwywxA Map view B Oblique view C Cross section
Onshore
O�shore
Backstepping
Backstepping
z'
z
Forestepping
Forestepping
x'
z'
y'
Mfs
Mfs
TST
EHST
LHST
y
x
x' y'
y
4321
4321
4321
Hinge
z
Retr
ogra
de
Aggr
ade
Prog
rade
Maxim
um �ooding surface
x
wz
wy
wx
A: Generalized stratigraphic columns and age control for parasequences 1–6 in the Cozzette Sandstone. Deposits includeshallow-marine (yellow), paleovalley �ll (PVF, pink), coastal plain (green), and marine (gray) facies. Red lines denote sequence boundaries andblue lines denote �ooding surfaces. Cretaceous (Campanian) ages were de�ned using ammonites (Didymoceras nebrascense, D. stevensoni,and Exiteloceras jenneyi; see Madof et al., 2015).
B: Map view of rollover trends associated with the Cozzette parasequences illustrates locations of the projected hinge zones.
C: Spatial variations in stratigraphic architecture across cross-section panels x-x’, y-y’, and z-z’. Relative sea-level curve (right) was created by plotting rollover positions through time. The gap in x-x’ is caused by stratigraphic pinch-out of parasequence 5 west of y-y’. Postdepositional tectonic tilting is interpreted for the Cozzette (far right), highlighting back-tilted stratal geometries caused by �attening on the downlap surface. LHST—late highstand systems tract; EHST—highstand systems tract; TST—transgressive systems tract.
5. Fixed and moving hinges for the Cozzette Sandstone (Book Cli�s, Piceance Basin, Colorado, USA)
A Stratigraphy B Map view
y'
x'
C Oblique view
Rollins Sandstone
Corcoran Sandstone
Cam
pan
ian
Late
74 Ma
72 Ma
76 Ma
3
2
4
PVF 1
PVF 2
Cozz
ette
San
dst
one
Mou
nt G
ar�e
ld F
orm
atio
n
6
5
1
23
4 50.00°
0.02°0.04°
6
*
N
z'
z
y'
y
x'
39°N
108°W
= Grand Junction, Colorado*= Book Cli�s escarpment
x
y
z'z IVF 1IVF 2
Time 1
Time 2
Time 3
Flattened
Tectonic tilting
TST
EHST
LHST
Cros
s sec
tion
Western interior seway
1
2 34 5
1
2 3
6
1
0 m +30 m
0 m +30 m
0 m +30 m
Datum21
5 m
Fixe
d h
ing
e - C
reta
ceo
us
sho
refa
ce
Hinge
x
1
3
2
4
65
10 km
10 m
5 km
6. Moving hinge: Marambaia Formation (Santos Basin, Brazil)
A: Generalized stratigraphic columns and age control for sequences 1–4 in the Marambaia Formation (see description of A). Eocene (Ypresian) ages were determined using planktonic foraminifers (Morozovella acuta, M. aequa, and Acarinina pseudotopilensis; Alicia Kahn, 2015, personal communication).
B: Map view of locations of the projected hinge zones, Marambaia sequence.
C: Architectural changes in the Marambaia and di�erences in relative sea level resulted from changes in source area. Blue arrows depict the direction of active outbuilding for sequences 1–3. LLSSS—late lowstand sequence set; ELSSS—early lowstand sequence set.
A B C
55 Ma
Ypre
sian
49 Ma
53 Ma
51 Ma
10°2°
4°6°
Time 1Source change
Map
vie
w
Time 3
4
3
2
1
2 3 4
Mar
amb
aia
Form
atio
nTime 2
N
Hinge 1
24°S
42°W
x'
x
y'
y z
Atlantic Ocean
Stratigraphy Map view Oblique view
ELSS
S
LLSS
SLL
SSS
ELSS
S
Hinge 2
1 2 3 4
1 2 3
y'
x'x
y
z'z
-175 m 0 m
-175 m 0 m
-175 m 0 m
2
1
3
50 m
Mov
ing
hin
ge
- Eo
cen
e sh
elf
z'1
32
10 km
2.5 km250
m
7. Along-strike variations in rollover position
Plots are projected onto a single plane and illustrate progradation distance versus aggradation thickness. Arrows denote vectors projected from x, y, and z, and show intraparasequence and sequence variability. The shaded area highlights the magnitude of progradation and aggradation between adjacent parasequences and sequences. Arrows (left) are used to calculate migration rates (right), based on average cycle length (i.e., duration/cycles).
A: Di�erential progradation in the �xed hinge parasequences of the Cozzette Sandstone shows an increase in along-strike variability.
B: The moving hinge sequences of the Marambaia Formation show a decrease in progradation magnitude with time.
B
A Fixed hinge - shoreface parasequences
Moving hinge - shelf sequencesWest
-100
0
-2000 2Ag
grad
atio
n (m
)
Progradation (km)4 6 8 10 12
x y
z
x
z
East
2 31
z
z+10
+20
+30
00 10 20 30 40 50Ag
grad
atio
n (m
)
Progradation (km)
x
EastWest
2 3 4 5 6
x y
1
1 2 3 4 5 6
160
0
ParasequenceMig
ratio
n ra
te (m
/kyr
)
8080
1 2 30
Sequence
1.0
0.5
Time
Time
Mig
ratio
n ra
te (m
/kyr
)
9. ConclusionsStratigraphic hinge models provide simple geometric tests of uniform or spatially variable sedimentation in nearshore systems:
-Fixed hinge: roughly stationary area de�ned by convergence of shoreline or rollover trends. -Moving hinge: two or more hinges that are spatially separated or move through time.
Hinge zones can predict stacking patterns through interpolation of shoreline or rollover trends between or beyond data points.
Always be vigilant of the “3D menace” (or opportunity) in interpretation!
10. ReferencesFulthorpe, C.S., and Austin, J.A., 2008, Assessing the signi�cance of along-strike variations of middle to late Miocene prograding clinoformal sequence geometries beneath the New Jersey continental shelf: Basin Research, v. 20, no. 2, p. 269– 283, doi: 10.1111/j.1365-2117.2008.00350.x.
Gill, J.R., and Cobban, W.A., 1973, Stratigraphy and geologic history of the Montana Group and equivalent rocks, Montana, Wyoming, North and South Dakota:U.S. Geological Survey Professional Paper 776, 37 p.
Madof, A.S., Christie-Blick, N., and Anders, M.H., 2015, Tectonically controlled nearshore deposition: Cozzette Sandstone, Book Cli�s, Colorado, U.S.A.: Journal of Sedimentary Research, v. 85, p. 459–488, doi: 10 .2110 /jsr .2015 .26.
Madof, A.S., Harris, A.D., and Connell, S.D., 2016, Nearshore along-strike variability: Is the concept of the systems tract unhinged?: Geology, v. 44, no. 4, p. 315–318, doi: 10.1130/g37613.1.
Wehr, F.L., 1993, E�ects of variations in subsidence and sediment supply on Parasequence stacking patterns, in Weimer, P., and Posamentier, H.W., eds., Siliciclastic sequence stratigraphy: Recent developments and applications: American Asso-ciation of Petroleum Geologists Memoir 59, p. 369–379, doi:10 .1306 /M58581C14
8. Observations from other localities
Older
Younger
Gill and Cobban (1973)
Fulthorpe and Austin (2008) Wehr (1993)
New Jersey margin, U.S.A.
Montana and Wyoming, U.S.A.
Alberta, Canada
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