systems tract & sequence stratigraphic surfaces_sydney basin

Author: Kim Purcell 1163 Spring Creek Rd, YARRAWONGA NSW 2850

Mobile: 0433 335 452 [email protected]

UNIVERSITY OF NEWCASTLE

GEOS3150 Basin Analysis (c3154659)

Systems Tract & Sequence Stratigraphic Surfaces: Evidence Sydney

Basin, Eastern Australia is an incised valley with good source rock

potential for petroleum exploration

mailto:[email protected]

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ABSTRACT

High-resolution sequence stratigraphy is the analysis of the sedimentary response to changes in

base level, and depositional trends that emerge from the interplay of accommodation and

sedimentation (Catuneanu, 2006). Field observations and outcrops allow the physical attributes

of stratigraphic units and bounding surfaces at these scales to be observed and classified as

‘sequence stratigraphy’. A sequence is defined as a “cycle of change in accommodation or

sediment supply defined by the recurrence of the same types of sequence stratigraphic surface

through geologic time”. Sediment supply may fluctuate in response to both allogenic and

autogenic factors, and may control the timing of any sequence stratigraphic surface that forms

during stages of positive accommodation (Zechhin & Catunearu, 2013). This study focuses on

an 800 m thick Permian depositional sequence of the southernmost part of the Sydney Basin.

This study found the vertical succession of successions reflects lateral changes of depositional

environment from fluvial to outer shelf, from lowstand systems tract (LST), to falling stage

systems tract, predominantly under a transgression. In the case of Sydney Basin, the sediment

accumulation in the east are thick and sandy and considered good reservoir rocks with good

source rock potential and worth exploration.

Keywords: Sequence Stratigraphy, Sydney Basin, Fluvial, Transgression, Systems Tract.

1. INTRODUCTION

The Sydney-Gunnedah-Bowen Basin is 2000 km north-south trending foreland basin from Batemans Bay in

NSW to North Queensland, East Australia. The basin is between the Lachlan Fold Belt to the west and the

orogen of the New England Fold Belt to the east (Eyles et al. 1998). The system develops eastward, with

northeast younging rocks. This report will focus on the sequence stratigraphy of the Permian depositional

sequence of the Southern Sydney Basin starting with the basement known as the Wagonga Beds at Myrtle

Beach, to the Nowra Sandstone Formation at Dolphin Point, south of Ulladulla. The system is highly

asymmetrical, dominated by 600 m of transgressive rocks. Suggesting the basins was under extension and

subjected to prolonged periods of subsidence. There is strong evidence to indicate Sydney Basin is an incised

valley, this is based on observed sequence stratigraphy and geometry, characterised by the fluvial rocks being

approximately 100 m thick, laterally restricted to a few kilometres and contains continental, marginal and deep

marine rocks.

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2. METHODOLOGY

Field work was conducted east of Ulladulla, on the New South Wales south coast. The depositional environment

was identified by direct observations of exposed outcrops of the seven lowermost formations of the Lachlan

Fold Belt which form the southernmost Sydney Basin, from the Wangonga Beds and the Wasp Head Formation

at Myrtle Beach to the Wandrawandian Siltstone and Nowra Sandstone at Dolphin Point. These exposures are

nearly continuous in a NE direction, each unit was directly observed, and interpreted. Observations such as:

sedimentology, texture, composition, biota, depositional structures, stratigraphic patterns, basin geometry,

associated facies and depositional processes were observed to identify systems tracts and sequence stratigraphic

surfaces.

3. FACIES ASSOCIATIONS

3.1 Wagonga Beds

Description

The Wagonga beds are a metamorphosed unit of predominately chert and shale. The rock unit is highly

deformed, it has been uplifted, tilted and is highly folded, creating an erosional surface, or non-conformity

separating it from the overlying sedimentary unit (Figure 1a, page 3).

Interpretation

This is interpreted as the basement, the beginning of the Lachlan Fold Belt within the southern Sydney Basin.

The non-conformity separating the underlying metamorphosed unit from the sedimentary rocks above

represents a sub-aerial unconformity (SU).

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Figure 1: A) Wagonga Beds (Devonian), showing the underlying metamorphosed rock unit, dominated by chert and shale, the unit is highly folded (yellow), creating an erosional surface or subaerial unconformity (red) separating it from the overlying sedimentary unit. B) Wasp Head Formation (Permian), showing imbrication of textually immature coarse sand to boulder sized clasts in a chaotic breccia.

Figure 2: A) One of three cyclic phases of fining upward sandstone and chaotic breccia within the Wasp Head Formation, and B) Parallel stratification (red) and elongated ‘bullet-shaped’ clasts above horizontal pavement (yellow), indicating palaeocurrent direction of east.

3.2 Wasp Head Formation

Description

The Wasp Head Formation represents the Lower Permian Talaterang Group exposed at Myrtle Beach, near

Durras (Tye et al., 1996). It is a 100 m thick sedimentary unit, the lower-most section of the unit displays

imbrication of textually immature, coarse sand to boulders (Figure 1b), and displays at least three cyclic facies,

or stacked channel successions, of fining-up sandstone and chaotic breccia (Figure 2a). Sedimentary structures

include horizontal stratification, planar lamination, trough cross-bedding, planar cross-beds alternating between

sandstone and conglomerate, and unidirectional palaeocurrent, and bullet shaped clasts (Figure 2b).

Chaotic Breccia

Chaotic Breccia

Chaotic Breccia

Wagonga Beds

Subaerial Unconformity

Chaotic Breccia

Chaotic Breccia

Wasp Head Formation

Chaotic Breccia

Chaotic Breccia

Wasp Head Formation

Chaotic Breccia

Chaotic Breccia

Wagonga Beds

Elongated clast

Horizontal pavement Palaeocurrent direction E

20 cm

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While the upper part of the unit displays continuing finer grained sand, increasing mud ratio, with large scale

cross-bedding, with over-banks becoming common (Figure 3).

Interpretation

This is interpreted as a fluvial system such as a braided river. The lower part of the unit displays an alternating

flow regime from low to high velocity and debris flows indicating a cyclic migrating channel. Compelling

evidence to support this includes: i) bullet shaped clasts within the breccia units, indicating transport over basal

shear zone just above depositional surface, ii) parallel stratification of elongated clasts within sandstone unit

(Figure 1d) over a horizontal pavement, and iii) planar cross-beds alternating between sandstone and

conglomerate, which is unique to fluvial rocks. While the upper unit displays evidence of migrating dunes and

channels, within a losing energy fluvial system.

Figure 3, Upper Wasp Head Formation: Displaying fine grained sandstone with unidirectional large scale cross-bedding, indicating migrating dunes, evidence of a fluvial environment.

10 cm

Large scale cross-bedding

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3.3 Pebbly Beach Formation (lower)

Description

Overlying the fluvial rocks is large amounts of symmetrical shells that lack structure (Figure 4a), abundant

dropstones (Figure 4b) and hummocky cross-stratification common indicating a change of environment. The

overlying sedimentary unit is approximately 100 m thick in total and broken into three distinct sub-units. The

lower most unit, displays a coarsening upward muddy unit. The second, was a muddier unit, approximately 15

m thick, displaying ball and pillow structures (Figure 5a), mud drapes (Figure 5b), a high frequency, but low

diversity trace fossil assemblage, the unit also displayed structures of lenticular, wavy and flaser bedding (Figure

5c). Separating this unit from the overlying unit is a well-developed tidal ravinement surface (TRS). While, the

upper most unit is dominated by coarser sand, displaying retrogradational, coarsening upward channelized

succession, with cross-cutting relationship. Also within this unit is was heterolithic planer cross-beds overlain

by reactivation surface (Figure 5e) with an oblique palaeocurrent of 340o whilst the dominant current direction

is east with inclined heterolithic bedding above. This unit also displays a laterally extensive bioturbation and

vertical burrows (Figure 5f).

Interpretation

The lower-most unit is interpreted as the deepest part of a delta, or the bay head delta. The bay head delta forms

a progradational coarsening upward system within a larger transgressive fining upward system and marks the

lower part of the estuarine unit. Above the Bay Head Delta is the Central Estuary which is separated from the

overlying Estuary Mouth Complex by a tidal ravinement surface (TRS). The heterolithic planer cross-beds

overlain by reactivation surface with an oblique palaeocurrent within the Estuary Mouth Complex represents a

point bar and is strong evidence of a meandering stream within an estuarine environment.

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Figure 4, the MRS: A) Overlying the fluvial rocks of the Wasp Head Formation is an extensively bioturbated unit, and B) Abundant large dropstones. The MRS marking a change of environment from fluvial to estuary.

Figure 5, Pebbly Beach Formation: A) Ball and pillow structures; B) Mud drapes, C) Heterolithic bedding (lenticular, wavy & flaser); D) A tidal ravinement surface, a within trend flooding surface; E) Heterolithic planer cross-beds overlain by reactivation surface with

oblique palaeocurrent (representing a point bar), and F) Laterally extensive bioturbation. These are all evidence to suggesting a meandering stream in an estuarine system. Source: Kim Purcell (Author)

A B

A

B

D

C

E

F

Ball & Pillow Structure Mud drapes

Heterolithic bedding Tidal ravinement surface

Heterolithic planer cross-bedding

Reactivation surface

Oblique bedding

Dropstone

Extensive bioturbated

MRS

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3.4 Pebbly Beach Formation (middle)

Description

A dense shelly accumulation displaying onlapping and laterally extensive dismembered marine fauna dominated

by Eurydesma separates the underlying estuary unit from the overlying unit. The overlying sedimentary unit is

a 5 m thick, is sand dominated (approximately 90% sand matrix) and fining upwards. Sedimentology structures

within the unit include hummocky and swaley cross-stratification (Figure 6a) as well as lenticular and flaser

bedding (Figure 6c). There are also numerous tempestite surfaces with pebble to cobble size clasts contained

within (Figure 6d); this unit ends with an abrupt change.

Interpretation

The dense shelly accumulation is interpreted as a wave ravinement surface (WRS), while the laterally extensive

dismembered marine fauna indicates high energy and is strong evidence to suggest the system has moved from

an estuary to upper shoreface environment. Sedimentology structures within the overlying sandy unit such as

hummocky and swaley cross-stratification and tempestites indicating repeated storm events. These is strong

evidence signifying the system has moved from an estuarine system to an upper shoreface environment.

Figure 6, Pebbly Beach Formation: A) Hummocky and swaley cross-stratification, C) lenticular, wavy and flaser bedding,

and D) Tempestite surface with pebble size clasts contained within, evidence of an upper shoreface environment.

Lenticular

Wavy

Flaser

Herringbone

cross-bedding

A

D C

B

Tempestite surface

Pebble clast Heterolithic bedding

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3.5 Pebbly Beach Formation

Description

The sedimentary unit is approximately 20 m thick. Exhibits a fining upward trend, with tidal cross-bedding,

mud drapes with herringbone and sigmoidal cross-stratification (Figure 6b). The unit also displays lenticular,

wavy and flaser bedding, within increasing sand/mud ratio. Distal tempestite with lag deposits, amalgamated

sandstone units, hummocky and cross-stratification and vertical burrows.

Interpretation

The cross-bedding, lenticular and flaser bedding and herringbone structures are also indicative of a tidally

influenced environment. Suggesting the system has moved from upper shoreface to lower shoreface.

3.6 Snapper Point Formation

Description

Above the WTFS is a thick fining upward sand dominated unit approximately 280 m thick, displaying

sedimentary structures such as swaley and hummocky cross-stratification, tempestites and ripples. The unit also

displays intense shell accumulations and bioturbation of mud and sand, undulating lamination (Figure 7)

Interpretation

These features are indicators the system has moved to the inner shelf. Evidence includes sand dominated and

storm related structures such as swaley and hummocky cross-stratification, tempestites and ripples, which

require both storm and high amplitude waves. Additional indicators include intense shell accumulations and

bioturbation of mud and sand.

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Figure 7, Snapper Point Formation: A) Swaley and hummocky cross-stratification, B) Bioturbation, C) Tempestite

surface, and D) Ripple marks. The unit also displays intense shell accumulations of mud and sand, undulating

lamination (not pictured). Indicating an inner shelf depositional environment.

3.7 Wandrawandian Siltstone Formation

Description

This sedimentary unit is approximately 120 m thick, with more mud. In the middle of the unit are two units of

shells, separated into two distinct units. A lower, more condensed shell unit displaying back-lapping, and an

upper, less condensed unit displaying down-lapping. Between the two shell units is a gap with increasing

bioturbation and sand.

A

D

B

C

Hummocky Cross stratification

Swaley Cross stratification

Bioturbation

Samd

Tempestite surface Ripple Marks

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Interpretation

The lower more condensed shell unit are the backlap shell beds (BSD), followed by the maximum flooding

surface (MFS), then the less condensed downlap shell beds (DSB), they are also related to sediment starvation

and contain animals in living or near living position. The system tract below the MFS is under transgression,

while above the MFS the systems tract is under highstand normal regression (HSNR). Overlying the down-lap

shell beds is another WTFS, the downlap surface (Figure 8). This in interpreted as the outer shelf.

Figure 8, Wandrawandian Siltstone Formation: Photograph highlighting a condensed backlap shell beds (under transgression), the maximum flooding surface separating the overlying less condensed downlap shell beds (under high stand normal regression).

3.8 Nowra Sandstone Formation

Description

Further up sequence is another boundary, overlying the boundary is a sedimentary unit approximately 140 m

thick, this unit becomes more sandier, and displays coarsening upward, prograding trend.

Transgressive systems tract

Highstand normal regression

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Interpretation

This unit is interpreted as an upper shoreface depositional environment. The boundary is interpreted as the

regressive surface of marine erosion (RSME), marking the end of HSNR and the beginning of falling stage

systems tract (Figure 9).

Figure 9: Photograph highlighting the Wandrawandian Siltstone Formation, considered the outer shelf, under highstand normal regression, separated by the regressive surface of marine erosion (RSME) from the overlying Nowra Sandstone Formation (upper

shoreface) under forced regression.

4. HIGH-RESOLUTION SEQUENCE STRATIGRAPHIC SURFACES

High-resolution sequence stratigraphy studies stratigraphic stacking patterns and changes thereof in a temporal

framework. Sequence stratigraphic surfaces may be used as systems tract boundaries; an important characteristic

that separates them from any other surface. Within-trend facies contacts are lithological discontinuities within

systems tract and suitable for lithostratigraphic or allostratographic analyses (Zecchin & Catuneanu, 2013).

Below is a list and brief explanation of the observed sequence stratigraphic surfaces, within-trend facies contacts

or sequence boundaries observed within the studied depositional sequence. Followed by map showing the

location of sequence stratigraphic surfaces within the studied area (Figure 10).

RSME

Nowra Sandstone Formation, Upper Shoreface, under forced regression.

Wandrawandian Siltstone Formation, Outer Shelf Downlap Surface; another WTFS, under highstand normal regression.

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4.1 Subaerial Unconformity

The subaerial unconformity (SU) was observed in the field separating the Wagonga Beds (basement) from the

overlying Wasp Head Formation (fluvial rocks). The SU is typically associated with erosion, non-deposition

and therefore it is typified by temporal hiatus. The SU commonly is well defined in the field, and may be

associated with sharp channelized truncations of the underlying units, or by fluvial erosion (Catuneanu, 2006).

4.2 Maximum Regressive Surface

The maximum regressive surface (MRS) was observed in the field overlying the Wasp Head Formation (fluvial)

underlying the Pebbly Beach Formation (estuary). It was distinguished by a laterally extensive shelly unit,

bioturbation and abundant dropstones, indicating a shift from a fluvial system, to a tidally influenced

environment. The MRS separates regressive deposits below from transgressive deposits above. In marine

settings, the MRS is commonly marked by a conformable shift from a progradational stacking pattern

(coarsening, and shallowing upward) to a retrogradational stacking pattern (fining, and deepening upward)

(Zecchin & Catuneanu, 2013).

4.3 Ravinement Surface

In the field the tidal ravinement surface (TRS) and wave ravinement surface (WRS) were observed within the

Pebbly Beach Formation. The TRS was observed between the Central Estuary and the Estuary Mouth Complex,

and the WRS was observed between the upper and lower shoreface. A ravinement surface (RS) is an erosional

surface cut during transgression by tidal currents in estuarine settings, or waves in shallow-marine settings

(Zecchin, 2007).

4.4 Maximum Flooding Surface

In the field the maximum flooding surface (MFS) was observed within the middle of Wandrawandian Siltstone

Formation (outer shelf) between the backlap shell beds and the downlap shell beds, as the systems tract change

from transgression to highstand normal regression. The MFS corresponds to the seafloor at the time of maximum

shoreline transgression, and marks a change between transgressive and normal regressive shoreline trajectories

(Helland-Hansen and Martinsen, 1996; Catuneanu, 2006).

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4.5 Regressive Surface of Marine Erosion

In the field the regressive surface of marine erosion (RSME) was observed at the interchange of the upper most

Wandrawandian Siltstone Formation (outer shelf) and the Nowra Sandstone Formation (upper shoreface)

marking the beginning of forced regression. RSME is produced by wave erosion in the lower shoreface during

relative sea-level fall, and it marks the base of forced regressive shorefaces. This surface is commonly

recognizable by a sharp contact between fine-grained shelf sediments below and sandy to gravelly shoreface

sediments above. The RSME may be associated with lags (Pattison, 1995), gutter casts (Plint & Nummedal,

2000) and Glossifungites ichnofacies (Pemberton et al., 1992).

Figure 10: Vertical succession showing the systems tract and sequence stratigraphic surfaces observed within the studied area from the Wagonga Beds (basement) to the Nowra Sandstone Formation (upper shoreface).

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5. DISCUSSION

There is strong evidence to suggest the depositional sequence of the studied area follows a full systems tract,

dominated by transgressive deposits. Beginning with the Wagonga Beds where the overlying subaerial

unconformity signifies the end of forced regression and the beginning of lowstand systems tract. The Wasp

Head Formation shows evidence the system developed into a lowstand normal regression, moving toward a

transgressional system; initially progradational then aggradational, with a general fining upward trend and on-

lapping, caused by relative sea level rise. The thick back-stepping estuary deposits of the lower Pebbly Beach

Formation indicate aggradation in an early to mid-transgressive systems tract. While the upper unit bound by

two within trend flooding surfaces indicating an abrupt change or deepening of the system, but still within

transgressive system. Moving up sequence, the Snapper Point Formation is the thickest transgressive unit, and

is approximately 260 m thick. Above is the Wandrawandian Siltstone Formation, within the middle of the unit

is two shelly units, a lower more condensed shell unit considered the backlap shell beds, followed by break,

suggesting the maximum flooding surface, then the less condensed shelly unit, interpreted as the downlap shell

beds, they are related to sediment starvation and contain animals in living or near living position. The system

tract below the maximum flooding is still under transgression, while above the maximum flooding surface the

systems tract is under highstand normal regression. Overlying the down-lap shell beds is another within trend

flooding surface, known as the downlap surface. While the overlying Nowra Sandstone Formation represents

the beginning of forced regression.

Published reports also suggest the Pebbly Beach Formation formed in coastal and nearshore marine

environments, with sequences dominated by the transgressive systems tract, and preserve the lowstand

systems tract (Bann et al., 2004; Fielding et al., 2006; Gostin & Herbert, 1973 & Tye et al., 1996).

Moreover, Eyles et al. (1998) states the presence of heterolithic bedding is a definite indication of

shallow marine estuarine environment and the abrupt deepening is evidence the basin is under

transgression, and suggests the Snapper Point Formation and Wandrawandian Siltstone Formation

sediments deposited in a quiet muddy mid-outer shelf based on the presence of the Cruziana and

Zoophycos Ichno Ichnofacies assemblages and were subject to episodic scour during large storms.

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In summations, approximately 700 m of transgressive rocks were observed, yet published models

report sea level fluctuations of only 100 m to 30 m, and thus does not explain the 700 m of transgressive

rocks. Published models suggest a eustatic drop of 200 m during the mid-Permian; e.g. a change in

water depth, from 200 m (outer shelf) to 20 m (upper shoreface). This large change in the eustasy does

explain the forced regression and the shallowing upward trend. Therefore, the system must have been

dominated first by tectonic activity, then by eustatic changes.

6. CONCLUSION

In conclusion, the vertical succession of facies reflects lateral changes of depositional environment from fluvial,

all the way to the outer shelf. The sequence stratigraphy displays sedimentary strata bounded by unconformities

and erosional surfaces that follow a full systems tract cycle from LSNR, transgression, to HSNR to the onset of

FR. The thick transgressive rocks and asymmetrical geometry of the basin indicates the system was under

extension with prolonged subsidence. The fluvial rocks are approx. 100 m thick, laterally restricted to a few

kilometres and contains continental, marginal and fully marine rocks. This compelling evidence of an incised

valley, with deep submarine fan sediments, suggesting Sydney Basin has good source rock potential and worth

exploration.

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7. REFERENCES

Catuneanu, O. 2006. ‘Principles of Sequence Stratigraphy’. Elsevier, Oxford. Ebook, Available at:

http://site.ebrary.com.ezproxy.newcastle.edu.au/lib/newcastle/reader.action?docID=10169804

[Accessed October 11, 2015]

Eyles, C.H., Eyles, N. & Gostin, V.A. 1998. ‘Facies and allostratigraphy of high-latitude, glacially

influenced marine strata of the Early Permian southern Sydney Basin, Australia’. Sedimentology.

Vol. 45, pp. 121-161

Fielding, C.R., Bann, K.L., Macearchern, J.A., Tye, S.C. & Jones, B.G. 2006. ‘Cyclicity in the

nearshore marine to coastal, Lower Permian, Pebbley Beach Formation, southern Sydney Basin,

Australia: a record of relative sea-level fluctuations at the close of the Late Palaeozoic Gondwanan

ice age’. Sedimentology. Vol. 53, pp. 435-463.

Helland-Hansen, W. & Martinsen, O.J. 1996. ‘Shoreline trajectories and sequences: description of

variable depositional-dip scenarios’. Journal of Sedimentary Research. Vol. 66, pp. 670-688.

Rygel, M.C., Fielding, C.R., Bann, K.L., Frank, T.D., Birgenheier, L. & Tye, S.C. 2008. ‘The Lower Permian Wasp Head Formation, Sydney Basin: high-latitude, shallow marine sedimentation

following the late Asselian to early Sakmarian glacial event in eastern Australia’. Sedimintology.

Vol. 55, pp. 1517-1540.

Pattison, S.A.J. 1995. ‘Sequence stratigraphic significance of sharp-based lowstand shoreface

deposits’. AAPG Bulletin. Vol. 79, pp. 444-462.

Pemberton, S.G., MacEachern, J.A. & Frey, R.W. 1992. ‘Trace fossil facies models: environmental

and allostratigraphic significance’. In: Walker, R.G. & James, N.P. (Eds.) Facies Models:

Response to Sea Level Change. Geological Association of Canada. Vol. 89, pp. 47-72.

Plint, A.G. & Nummedal, D. 2000. ‘The falling stage systems tract: recognition and importance in

sequence stratigraphic analyses. In: Hunt, D., Gawthorpe, R.L. (Eds.), Sedimentary Responses to

Forced Regressions. Geological Society Special Publication. Vol. 172, pp. 1-17

Tye, S.C., Fielding, C.R. & Jones, B.G. 1996. ‘Stratigraphy and sedimentology of the Permian

Talaterang and Shoalhaven Groups in the southernmost Sydney Basin, New South Wales.

Australia’. Earth Science. Vol. 43, pp. 57–69.

Zecchin, M. 2007. ‘The architectural variability of small-scale cycles in shelf and ramp clastic

systems: the controlling factors’. Earth-Science Reviews. Vol. 84, pp. 21-55.

Zecchin, M. & Catuneanu, O. 2013. ‘High-resolution sequence stratigraphy of clastic shelves II:

Controls on sequence development’. Marine and Petroleum Geology. Vol. 39, pp. 1-25

http://site.ebrary.com.ezproxy.newcastle.edu.au/lib/newcastle/reader.action?docID=10169804

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