Northern IllinoisUniversity

Ramps, Flats, and Rubble Zones: Case Studies of Deformation beneath Allochthonous Salt in the Flinders Ranges, South Australia

Lillian R. Lueck and Mark P. FischerNorthern Illinois University

Institute forthe Study of

NORTHERN ILLINOISUNIVERSITY

FLUIDS &STRUCTURES

(b)

(b)

(a)

(a)

salt

subsalt strata

suprasalt strata

ramps

�ats

feeder

sea �oor

ramp

sea�oor

�at

salt

Figure 3. (a) Seismic data of allochthonous salt sheet underlying the Sigsbee Escarpment in the Gulf of Mexico. Subsalt �ats (green) and ramps (orange) are highlighted for comparison with the schematic drawing in (b). (b) Schematic cross-section of an allochthonous salt sheet showing suprasalt and subsalt strata. Figure adapted from Hudec and Jackson (2006). Flats (green) and ramps (orange) are highlighted in the subsalt strata.

Motivation and Signi�canceSalt bodies are signi�cant traps for hydrocarbons throughout the world. The viability of these traps most often depends on near-salt pinchouts and deformation, many of which occur less than 300 m from the salt-sediment interface, a scale below the resolution of most seismic data. Deformation beneath allochthonous salt is especially challenging to predict because it can originate by carapace slumping or halokinetic processes. Numerical models aim to predict the nature and extent of deformation beneath allochthonous salt, but o�er constrasting results. Nikolinakou et al. (2018a) predicts substantial subsalt deformation, whereas Li and Fischer (2018) show little strain in the subsalt strata (Figure 1).

This project uses �eld work to characterize the deformation beneath allochthonous salt and tests the hypothesis (Figure 2; Williams et al, 2019) that the deformation near allchthonous salt will vary with structural position (i.e. ramps v �ats, Figure 3). We provide new data on deformation adjacent to an allochthonous salt sheet in the Flinders Ranges of South Australia, with special emphasis on the contrasting subsalt deformation between ramps and �ats. See presentation EGU2020-21148 by Wegmann et al. in this same session for additional case studies of subsalt deformation elsewhere in the Flinders.

depending on mechanical coupling between carapace and salt, carapace composition, rheology and anisotropy, spatial variability of

compaction from increasedgravity loading beneath salt

any near-salt deformation during thistime may be preserved and later found

adjacent to a tertiary weld

footwall deformation heremay be preserved and later

found beneath a tertiary weld

depending on location, sediment composition, rheology and anisotropy, subsalt shearing may occur in a variety of styles

asymmetricfolding

foliatedshear zones

brittlefaulting

opening mode joints or veins extensional faulting tectonic foliation contractional faulting

Conceptual Model of Deformation Patterns During Submarine Advance of Allochthonous Salt

Figure 2. Deformation patterns associated with allochthonous salt advance hypothesized by Williams et al. (2019).

100m

Feed=400m

(Nikolinakou et al., 2018a)

(Li and Fischer, 2018)

Contours ofplastic strain

50,000 yearsof salt advance

salt

salt

salt

sediments

sediments

sediments

Orientations of principalplastic strains

Feed=400m

Mohr-Coulombplasticity used forsediment rheology

Sediments modeled as porous,elastoplastic, critical-state material

No slip along salt-sediment interface

Figure 1. (a) Left; Contours of plastic shear strain in the sediments surrounding an advancing salt sheet after 78 m.y. and 102 m.y. The model predicts high shear strain at distances up to ~2km beneath and advancing salt sheet. (b) Right; Magnitude and orientation of principle plastic strains beneath a salt sheet that has advanced over a planar surface. This model predicts subsalt shearing will be concentrated in a <50m wide zone beneath the salt. Strain orientations imply structures with unique orientations will be concentrated in this zone.

80

7070

60

15

40

80

6650

000m

N

320000mE

N

Ulupa Siltstone

Nuccaleena Fm.

Amberoona Fm.

Yankaninna Fm.

Tapley Hill Fm.

Bolla Bollana Tillite

Fitton Fm.

Nd2

Soda Leucogranite

Callanna

Nsu

Nsu

Nsn

Nsn

Nib

Nib Niy

Niy

Niy

Nnt

Nnt

Nnt

Nyb

Nyb

Nyb

Nyb

Nyf

Nyf

NyfNyf

Nd2

Nd2

CD1

CD1

br

br

br

Fortress Hill Fm.

Wonoka Fm.

Bonney Sandstone

Nd1

Myrtle Springs Fm.

Skillogalee Dolomite

Nef

Nef

NefNww

Nww

Nww

Npb

Npb

Nms

Nd1

Nd1

Ndm

Ndm

Ndm

Nms

White, medium-grained soda leucogranite

Sandstone, �ne to medium grained, �aggy to medium bedded, silty and feldspathic, cross bedded, ripple marks, mudcracks

Shale, gray, calcareous; �aggy dolomite, limestone and silt

Dolomite, thin, laminated and micritic

Siltstone, shale, green-gray and purple

Siltstone, gritty; dolomitic lenses and cobbles

Shale, green, �nely laminated, silty; purple and gray silty shale

Siltstone, gray-green, thinly bedded, calcareous with lenses of dolomite

Siltstone, gray to black, dolomitic and pyritic grading upwards to calcareous, thinly laminated, locally cross-bedded; dolomite, gray, �aggy to massive

Tillite, massive, high boulder content, gritty, sub-graywacke matrix, minor quartzite

Conglomerate, arkose, pebbly; white, massive quartzite interbedded with silty shale

Gray-green dolomitic siltstone; minor thin, blue dolomite

Quartzite, feldspathic, medium to coarse

Siltstone, green; minor quartzite and dolomite

Dolomite; marble with magnesite mud-pellet conglomerates

Breccia, undi�erentiated

�eld map locationFig. 5

SouthAustralia

FlindersRanges5 km

Tourmaline Hill Field Area Map

Figure 4. Regional geologic map and stratigraphic column of the Tourmaline Hill area based on the Umberatana Quadrangle. The black outline represents the detailed �eld area in Figure 5.

6650

000m

N

320000mE

63

69

75

79

74

62 80

47

33

22

61

77

70

56

74

Nms

Ndm

Ndm

Nyf

Nyf

Nyb

Nyb

Nnt

Nnt

Nnt

Niy

Niy

Nib

Nib

Nsn

Nsu

Niy

Nms

CD1

br

br

subsalt

supra

salt

N

Ulupa Siltstone

Nuccaleena Fm.

Amberoona Fm.

Yankaninna Fm.

Tapley Hill Fm.

Bolla Bollana Tillite

Fitton Fm.

Myrtle Springs Fm.

Skillogalee Dolomite

Leucogranite

Breccia (salt)

Detailed Study Area

Bedding Traces

Nsu

Nsn

Nib

Niy

Nnt

Nyb

Nyf

Ndm

Nms

CD1

br

SouthAustralia

FlindersRangesmeters

10005000

Tourmaline Hill Field Map

Figure 5. Geologic �eld map of the Tourmaline Hill study area. Dashed blue line shows general bedding trends based on collected structural data. Dashed red line represents the locations of detailed studies of subsalt deformation.

bedding trends

collapsed salt feeder

potential salt feeder

ramp transect

map

�at transect

map

100m

Callanna (br)Tapley Hill (Nnt)

0 meters

180 meters

SU002 - 3

SU002 - 2

SU002- 1

69

75

79

70

74

Nyf

Nyb

Nnt

Niy

Nib

Nsu

Tourmaline Hill Field Map

Figure xx. Geologic �eld map of the Tourmaline Hill study area. Dashed blue line shows general bedding trends based on collected structural data. Dashed red line rep-resents the locations of detailed studies of subsalt defor-mation.

ramp transect

map

Subsalt Flat Transect

0

90

180

Stratig

raphy

Lithology

Stratig

raphic

Meters Below Salt

Age

Tapl

ey H

ill F

m.

br

Stur

tian

SU002- 1

SU002- 2

SU002- 3

*heavy lines indicate average orientation*

Outcrop Location

Transect Line

Unit Contact

Undi�erentialed breccia (salt)

Dolomitic siltstone, thinly laminated, cross-bedded

Bedding

Joints Veins

Figure 6. Close-up aerial photo and lithostratigraphic column of the transect location in the subsalt �at showing the station locations (yellow dots) where data were collected. The transect line (purple) is oriented perpendicular to bedding. The transect starts at the salt-sediment interface, and spans 180 meters of stratigraphic section through the Tapley HIll Formation.

0

180

77

75

142

174

134

Stratig

raphy

Lithology

Outcrop D

etail

AgeTa

pley

Hill

Fm

.br

Stur

tian

Stratig

raphic

Meters Below Salt

SU00

2 - 3

SU00

2 - 2

SU00

2 - 1

Tourmaline Hill Ramp Detail

Legend

Bedding

Brecciated Salt

Massive Tillite

JointsScapolite

Disharmonic Folding

Gouge Vein

Quartz Vein

North Arrow

Figure 7. Lithostratigraphic column and deformation details of stations located along the subsalt �at transect. A) Outcrop example showing the abundance of scapolite mineralization and the cross-cutting relationship of gouge veins at station SU002-1. B) Well indurated massive siltstone with quartz vein at SU002-2. Note that scapolite is not present. C) Detail of disharmonic folding (slumping?) within the bedding of SU002-2. D) Detail of quartz vein oblique to bedding orientation at Station SU002-3.

A

B

D

C

A

B

C

D

Subsalt Ramp Transect

100 m

SU001-1

0 meters

SU001-2

SU001-3

SU001-4

450 meters

*heavy lines indicate average orientation*

Outcrop Location

Transect Line

Unit Contact

Massive boulder tillite, quartzite, and siltstone

Pebbly conglomerate arkose, interbedded shales and quartzites

Bedding

Joints VeinsFigure 8. Close-up aerial photo and lithostratigraphic column of the transect location in the subsalt showing the high resolution drone image and station locations (yellow dots) where data were collected. The transect line (purple) is oriented perpendicular to bedding. The transect starts at the salt-sediment interface, and spans 750 meters across the Bolla Bollana Tillite, ending in the Fitton Fm.

0

225

450

Stratig

raphy

Lithology

Stratig

raphic

Meters Below Salt

Age

Bolla

Bol

lana

Till

iteFi

tton

Fm

.br

Stur

tian

SU001- 1

SU001- 2

SU001- 3

SU001- 4

Callanna (br)

Bolla Bollana Tillite (Nyb)

Fitton Fm. (Nyf )

62

74

Tourmaline Hill Ramp Detail

0

238

253

123

113

Stratig

raphy

Lithology

Outcrop D

etail

AgeBo

lla B

olla

na T

illite

Bolla

Bol

lana

Till

itebr

Stur

tian

Stratig

raphic

Meters Below Salt

Legend

Bedding

Brecciated Salt

Massive Tillite

JointsHematite

Carbonate Vein(siderite?, calcite?)

Quartz Vein

Figure 9. Lithostratigraphic and deformation details of stations located along the subsalt ramp transect. A) Quartz veins at station SU001-1 oriented oblique to bedding. B) Siderite veins at station SU001-1 oriented perpendicular and oblique to bedding. C) Quartz in a siderite vein at station SU001-2. D) Quartz veins at station SU001-2 oriented oblique to bedding.

A

B

C

D

SU00

1- 2

SU00

1- 1

A

B

C D

North Arrow

Tourmaline Hill Ramp Detail

342

418

443

318

Stratig

raphy

Lithology

Outcrop D

etail

AgeBo

lla B

olla

na T

illite

Fitt

on F

m.

Stur

tian

SU00

1- 3

SU00

1- 4

Legend

Bedding

Arkose, Quartzite

Massive Tillite

North ArrowJointsHematite

Quartz Vein

Carbonate Vein(siderite?, calcite?)

Figure 10. Lithostratigraphic column and deformation details of stations located along the subsalt ramp transect. A) Siderite mass comprising station SU001- 3. B) Detail of large siderite crystals and calcite? in mass at SU001- 3. C) Vein of siderite and calcite? at station SU001- 4 oriented oblique to bedding. D) Siderite and calcite vein at SU001- 4 oriented oblique to bedding.Stra

tigraphic

Meters Below Salt

AB

C

D

A

B

C D

Summary

Special Thanks to...

Fractures are generally nonsystematic and abundant near the salt contact and become systematic and less abundant with distance away from salt. Scapolite is abundant near the salt-sediment interface, and rare or absent in rocks more than 100m away from the salt-sediment interface.

Salt-Sediment Interaction Research Consortium (SSIRC)American Association of Petroleum Geologists (AAPG)Geological Society of America (GSA)

Madison Schweitzer, for assisting in the �eldMark Rowan, Carl Fiduk, and Josep Anton, for in-�eld discussions/debatesChris McHugh, for hospitality at Umberatana Station

Subsalt Ramps

-decameter scale folding-abundant mineralized fractures-�uid migration (accumulation?)

Subsalt Flats

-strata-bound, decimeter scale folding-soft-sediment deformation of slumped carapace?-few mineralized fractures

Prompts for Discussion

References

Numerical models have predicted a phenomenon some call “stem push” near where allochthonous salt emerges from a feeder or where salt diapirs are near the surface of the sea�oor. What are some of your thoughts about this concept? Have you seen evidence of it? How would you test for its existence?

What is the rheology/strength of sediments/rocks when allochthonous salt is advancing over them?

Do subsalt deformation patterns correlate to the distribution of ramps and �ats beneath an advancing allochthonous salt sheet? Should we expect more or less intense deformation at ramps?

Li, Z. and Fischer, M. P. (2018) Stress and derformation during the emplacement of allochthonous salt: results of numerical models. [Thesis] Northern Illinois University

Nikolinakou, M.A., Heidari, M., Hudec, M.R., Flemings, P.B., 2018a, Geomechanical modeling of stress and deformation associated with salt-sheet advance. In: Proceedings of the 52nd US Rock

Mechanics/Geomechanics Symposium. American Rock Mechanics Association Paper ARMA 18-637.

Williams, N.J., Fischer, M.P. Canova, D.P., 2019, Structural evolution and deformation near a tertiary salt weld, Willouran Ranges, South Australia: Marine and Petroleum Geology, 102, 305-320, doi:

10.1016/j.marpetgeo.2018.12.035.