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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