- Email: [email protected]
Chapter 8.3 Stratigraphy of Stromatolite Occurrences in Carbonate Lakes of the Coorong Lagoon Area, South Australia
8. RECENT MODELS FOR INTERPRETING STROMATOLITE ENVIRONMENTS
Chapter 8.3 STRATIGRAPHY OF STROMATOLITE OCCURRENCES IN CARBONATE LAKES OF THE COORONG LAGOON AREA, SOUTH AUSTRALIA Christopher C. uon der Borch
INTRODUCTION
Geologically Recent stromatolites have been described from several areas of modern carbonate sedimentation, the most notable of which is Shark Bay in Western Australia (Logan, 1961; Davies, 1970b). Other important areas include the Persian Gulf (Kendall and Skipwith, 1968; Kinsman et al., 1971), the Bahamas (Monty, 1967,1972), Bermuda (Gebelein, 1969) and the Great Salt Lake, Utah (Carozzi, 1962). In addition t o the above, a somewhat restricted but nevertheless interesting occurrence of stromatolites has been described from two ephemeral carbonate lakes associated with the Coorong Lagoon in South Australia (Walter et al., 1973). These stromatolites occur in carbonate muds composed of the minerals hydromagnesite, aragonite, dolomite and calcite (Alderman and Von der Borch, 1960, 1961; Skinner, 1963; Peterson and Von der Borch, 1965; Von der Borch, 1965). Small amounts of amorphous silica are associated with these carbonates. This assemblage has its analogues in the geologic record, particularly in the Precambrian (e.g. the Skillogalee Dolomite Formation of South Australia, Preiss, 1973a) where silicified dolomitic stromatolite-bearing rocks are relatively common. Because of this, as an aid in recognizing equivalent depositional environments in the ancient rock record, the sedimentological history and stratigraphy of the Coorong stromatolite association will be described in some detail. GEOLOGICAL SETTING
The Coorong Lagoon (Fig. 1)is the dominant physiographic feature in the area of the stromatolite lakes in question. It is located landward of a modern calcareous barrier island known as Younghusband Peninsula and is approximately 100 km in length and 3 km in width. There is a single marine pass at the northwestern extremity, where water depth in the lagoon reaches 10 m,
414
C.C.
VON DER BORCH
Fig. 1. Locality map, showing Coorong Lagoon and ephemeral carbonate lakes. The localities of the detailed maps in Fig. 2 are indicated.
however water depth throughout most of the area averages only 2-3 m. Water salinity is generally higher than that of normal seawater, reaching values of 60 parts per thousand in southeastern areas. The Coorong Lagoon is the most recent of several that formed across a broad coastal plain during the Pleistocene. Several stranded barrier islands, now represented by calcreted calcareous eolianite ridges, occur subparallel to the present coastline and extend inland up to 65 km from the coast. These and their associated lagoonal deposits have been progressively stranded by a combination of glacially induced sea-level oscillation and gentle regional upwarping (Hossfeld, 1950; Sprigg, 1952). The landward shoreline of the Coorong Lagoon is formed in most areas by one of these Pleistocene calcreted barrier-beach-ridge complexes. Occasional inliers of a Pleistocene eolianite ridge occur within the present barrier of Ypunghusband Peninsula, particularly in northern areas (Brown, 1965). The Coorong Lagoon itself occupies one and sometimes two interdunal depressions between these ridges. Towards the southeastern extremity of the Coorong Lagoon, in the vicinity of the two stromatolite-bearing lakes (Fig. 2), a rather complex calcreted
415
ENVIRONMENTAL MODELS
I
COORONG
I
LAGOON
A
L
J
IY
B
M
E C CALCAREOUS SMDS OF mDEWI BARRIER MD BEACH RIOGES. SILICEOUS SAND RlOGE NEAR SALT CREEK,
$$$$
MICROCRYSTALLINE WLORITE. CALCITE. ARAGONITE AND HYDROMGNESITE SEDIIILNTS OF LAKES AND ELEVATED FLATS. (LAYTS SHOW IN WHITE)
CALCAREOUS SANDS OF TIDAL DELTA FACIES. 0
CALCRETED PLEISTOCENE EOLlANlTE
S T R W l O L I T E OCCURKNCES ROADS
Fig. 2. Details of the two ephemeral carbonate lakes which contain stromatolites. A. Northern stromatolite lake area. B. Southern stromatolite lake area. C. Generalized cross section X-Y, showing typical stratigraphic relationships. The 120,000 and 80,000 year old barriers are indicated.
416
C.C. VON DER BORCH
eolianite topography exists, due possibly to a combination of Pleistocene dune blow-outs superimposed on the normal beach-ridge-barrier lineation. Low-lying portions of these areas were inundated between 3,000 and 5,000 years ago when the Holocene transgression reached its maximum. GEOLOGICAL HISTORY
The sedimentological evolution of the Coorong Lagoon and marginal ephemeral lakes is most clearly understood by reference to the sea-level curve of the last 200,000 years of Veeh and Chappell (1970) constructed from a study of Late Pleistocene coral reef terraces in New Guinea (Fig. 3). Assuming that the degree of upwarping in the Coorong region has been essentially negligible for that time, it is evident that the last time the sea was near its present level was 120,000 years ago during the Sangamon interglacial. During this stage the presently calcreted beach-ridge-barrier system that forms the landward shoreline of the Coorong Lagoon was probably built (D. Schwebel, pers. comm., 1975). This system forms the basement of the stromatolite lakes shown in Fig. 2. A subsequent sea level approached t o within several metres of the present sea level about 80,000 years ago and this may have been responsible for the barrier island which now forms the eolianite inlier within the Younghusband Peninsula barrier. Next in sequence followed the major sea-level retreat of the Late Wisconsin Glaciation after which the Holocene transgression returned the sea to its present level, partly inundating the older barriers to form the initial stages of the modern Coorong Lagoon.
mi
0
I
40
I
I 120
80 YEARS
I
MO
200
10’
Fig. 3. Sea-level curve of the last 200,000 years, based on New Guinea data of Veeh and Chappell(l970). Relevant dates are as follows: A = high sea level of 120,000 years ago corresponding t o Sangamon interglacial: formation of older barrier that now forms most of the Pleistocene basement in the area: B = relatively high sea level of 80,000 years ago: formation of eolianite inlier within modern barrier. C = Holocene transgression: formation of modern barrier.
A t this early stage the lagoon water was about l m above its present level as evidenced by widespread stranded lagoonal sediments (Brown, 1965). It is uncertain whether this higher level was due to a sea level slightly higher than at present or was related t o unrestricted access of the ocean through numerous passes. However, this was the time during which low-lying areas of the
ENVIRONMENTAL MODELS
417
Pleistocene topography marginal t o the lagoon, including the stromatolite lakes, were flooded by waters with oceanic affinities. As sea level stabilized in the Late Holocene, the Younghusband Peninsula barrier built over the older ridge causing the lagoon to become increasingly restricted until ultimately its southern waters reached their present hypersaline state. Lagoon waters at that time, possibly 3,000 years ago, receded l m to their present level which averages that of mean sea level, causing the stranding of the marginal stromatolite lakes. These lakes now have floors about 1m above present high lagoon level and receive their annual charge of water during winter months by local and regional groundwater seepage from surrounding calcareous eolianite ridges and deeper Tertiary aquifers. This water reaches a maximum depth of about 0.5 m in late winter and evaporates to dryness in early summer. The salts in the lake waters are derived in part by leaching from marginal flats, possibly augmented by magnesium ions from breakdown of high-magnesian calcite in the calcareous aquifers. It is in this environment that various carbonate minerals and the stromatolites are forming at the present day. STROMATOLITE DESCRIPTIONS AND STRATIGRAPHY
The Coorong stromatolites have been described in detail by Walter et al. (1973). Three morphological forms were described: globular, stratiform and crenulate (Fig. 4). They occur in marginal areas of the two shallow ephemeral lakes shown in Fig. 2 and are associated with a carbonate mineralogy of hydromagnesite, aragonite, dolomite and calcite. During winter and early spring months pH of the associated lake waters varies from 8.2 to 9.9 (Von der Borch, 1965) with salinities reaching a minimum of about loo/, . At this stage a sparse growth of the aquatic grass Ruppia maritima occurs on lake floors. Complete desiccation generally occurs during ensuing arid summer months. The actual stromatolite structures persist throughout the dry period with varying degrees of preservation, however during the wet season small myriapod arthropods burrow into them and destroy some of their features. Sediment cores taken through the stromatolite lakes to depths of several metres (Fig. 5) show a stratigraphy that reflects the evolutionary history of the Coorong Lagoon and its associated marginal lakes. In both cases basement is Pleistocene calcrete, which is developed on calcareous eolianite. Overlying this is a shallow-marine sedimentary unit (Protected Marine Phase, Fig. 5), consisting of skeletal grainstones and packstones containing a variety of bivalves such as Katelysh spp. and Venerupis sp. This was formed during and immediately following the Holocene transgression, before significant build-up occurred of the Y ounghusband Peninsula barrier. This grades upwards into pelletized organic-rich aragonite and magnesian calcite muds rich in a lagoonal fauna comprising the small gastropod Coxiella confusa, the
418
C.C. VON DER BORCH
Fig. 4. Coorong stromatolite morphologies. A. Globular stromatolites, southern stromatolite lake (coin 23 mm wide). B. Desiccation polygons in stratiform stromatolites, southern stromatolite lake (scale 30 cm long). C. Crenulate stromatolites on the margins of desiccation polygons of stratiform stromatolites, southern hydromagnesite lake (pencil 18 cm long).
foraminifera Ammonia beccarii and a variety of ostracods (Lagoonal Phase, Fig. 5). This change in sediment-type reflects a period of increasing restriction due to barrier development. Upper portions of cores, finally, are composed of white, microcrystalline carbonate muds and pellet packstones composed of the minerals hydromagnesite, calcite, aragonite and dolomite (Ephemeral Lake Phase, Fig. 5). This unit, which developed in response to the l m drop in lagoon level, contains stromatolite structures both at the surface and occasionally at depth. GEOLOGICAL SIGNIFICANCE
Stromatolites are rare in modem carbonate sediments, largely because of the browsing on algal mats by organisms such as crustacea. The fact that stromatolites occur in two lakes in the Coorong area is therefore somewhat sur-
419
ENVIRONMENTAL MODELS NORTHERN STROM. LAKE
0
-
SOUTHERN STROM. LAKE WITH NANTLY HYDROMAGNESITE, ARAGONITE,
1-
2 SKELETAL GRAINSTONE AND PACKSTONE W I T H B I V A L V E S K A T E L Y S I A AND V E N E R u P I S .
vl Y t
2
.......J PLEISTOCENE CALCRETE
3PLEISTOCENE CALCAREOUS E O L l A N l T E
Fig. 5. Generalized stratigraphic columns from northern and southern stromatolite lakes. Terminology partly adapted from Brown (1965).
prising, particularly in view of their general absence in apparently comparable lakes nearby. One contributing factor may be that the two stromatolite lakes, due to greater localized ground-water influx, do not become as thoroughly desiccated as other lakes. This higher ground-water discharge in turn may lead t o the formation of the carbonate mineral hydromagnesite which is associated with the stromatolites. It would be quite possible for sediments formed under conditions similar t o the above to be preserved in the geologic record, particularly in structurally negative areas. Such occurrences have been described in the literature (Peterson, 1962). The deposits would be typified by an association of carbonate cycles similar to the one described from the Coorong sediment cores. A single upward-fining cycle would ideally measure a few metres in thickness, beginning at the base with a shallow marine carbonate unit. This would grade upwards into an organic-rich pelletized lagoonal carbonate unit and would culminate in a siliceous microcrystalline carbonate with associated stromatolite structures. Mineralogy of the upper unit would be variable in both the lateral and vertical sense and could comprise combinations of the minerals magnesite, dolomite or calcite. The carbonate units themselves would be lenticular and laterally discontinuous and would be closely associated with shoestrings of calcareous barrier sands.
420
C.C. VON DER BORCH
ACKNOWLEDGEMENTS
Sediment coring in the Coorong area has been supported by grants from the Australian Research Grants Committee and Flinders University. The manuscript was read by D. Schwebel. Photographs of the Coorong stromatolites were provided by M.R.Walter.
Chapter 8.3 STRATIGRAPHY OF STROMATOLITE OCCURRENCES IN CARBONATE LAKES OF THE COORONG LAGOON AREA, SOUTH AUSTRALIA Christopher C. uon der Borch
INTRODUCTION
Geologically Recent stromatolites have been described from several areas of modern carbonate sedimentation, the most notable of which is Shark Bay in Western Australia (Logan, 1961; Davies, 1970b). Other important areas include the Persian Gulf (Kendall and Skipwith, 1968; Kinsman et al., 1971), the Bahamas (Monty, 1967,1972), Bermuda (Gebelein, 1969) and the Great Salt Lake, Utah (Carozzi, 1962). In addition t o the above, a somewhat restricted but nevertheless interesting occurrence of stromatolites has been described from two ephemeral carbonate lakes associated with the Coorong Lagoon in South Australia (Walter et al., 1973). These stromatolites occur in carbonate muds composed of the minerals hydromagnesite, aragonite, dolomite and calcite (Alderman and Von der Borch, 1960, 1961; Skinner, 1963; Peterson and Von der Borch, 1965; Von der Borch, 1965). Small amounts of amorphous silica are associated with these carbonates. This assemblage has its analogues in the geologic record, particularly in the Precambrian (e.g. the Skillogalee Dolomite Formation of South Australia, Preiss, 1973a) where silicified dolomitic stromatolite-bearing rocks are relatively common. Because of this, as an aid in recognizing equivalent depositional environments in the ancient rock record, the sedimentological history and stratigraphy of the Coorong stromatolite association will be described in some detail. GEOLOGICAL SETTING
The Coorong Lagoon (Fig. 1)is the dominant physiographic feature in the area of the stromatolite lakes in question. It is located landward of a modern calcareous barrier island known as Younghusband Peninsula and is approximately 100 km in length and 3 km in width. There is a single marine pass at the northwestern extremity, where water depth in the lagoon reaches 10 m,
414
C.C.
VON DER BORCH
Fig. 1. Locality map, showing Coorong Lagoon and ephemeral carbonate lakes. The localities of the detailed maps in Fig. 2 are indicated.
however water depth throughout most of the area averages only 2-3 m. Water salinity is generally higher than that of normal seawater, reaching values of 60 parts per thousand in southeastern areas. The Coorong Lagoon is the most recent of several that formed across a broad coastal plain during the Pleistocene. Several stranded barrier islands, now represented by calcreted calcareous eolianite ridges, occur subparallel to the present coastline and extend inland up to 65 km from the coast. These and their associated lagoonal deposits have been progressively stranded by a combination of glacially induced sea-level oscillation and gentle regional upwarping (Hossfeld, 1950; Sprigg, 1952). The landward shoreline of the Coorong Lagoon is formed in most areas by one of these Pleistocene calcreted barrier-beach-ridge complexes. Occasional inliers of a Pleistocene eolianite ridge occur within the present barrier of Ypunghusband Peninsula, particularly in northern areas (Brown, 1965). The Coorong Lagoon itself occupies one and sometimes two interdunal depressions between these ridges. Towards the southeastern extremity of the Coorong Lagoon, in the vicinity of the two stromatolite-bearing lakes (Fig. 2), a rather complex calcreted
415
ENVIRONMENTAL MODELS
I
COORONG
I
LAGOON
A
L
J
IY
B
M
E C CALCAREOUS SMDS OF mDEWI BARRIER MD BEACH RIOGES. SILICEOUS SAND RlOGE NEAR SALT CREEK,
$$$$
MICROCRYSTALLINE WLORITE. CALCITE. ARAGONITE AND HYDROMGNESITE SEDIIILNTS OF LAKES AND ELEVATED FLATS. (LAYTS SHOW IN WHITE)
CALCAREOUS SANDS OF TIDAL DELTA FACIES. 0
CALCRETED PLEISTOCENE EOLlANlTE
S T R W l O L I T E OCCURKNCES ROADS
Fig. 2. Details of the two ephemeral carbonate lakes which contain stromatolites. A. Northern stromatolite lake area. B. Southern stromatolite lake area. C. Generalized cross section X-Y, showing typical stratigraphic relationships. The 120,000 and 80,000 year old barriers are indicated.
416
C.C. VON DER BORCH
eolianite topography exists, due possibly to a combination of Pleistocene dune blow-outs superimposed on the normal beach-ridge-barrier lineation. Low-lying portions of these areas were inundated between 3,000 and 5,000 years ago when the Holocene transgression reached its maximum. GEOLOGICAL HISTORY
The sedimentological evolution of the Coorong Lagoon and marginal ephemeral lakes is most clearly understood by reference to the sea-level curve of the last 200,000 years of Veeh and Chappell (1970) constructed from a study of Late Pleistocene coral reef terraces in New Guinea (Fig. 3). Assuming that the degree of upwarping in the Coorong region has been essentially negligible for that time, it is evident that the last time the sea was near its present level was 120,000 years ago during the Sangamon interglacial. During this stage the presently calcreted beach-ridge-barrier system that forms the landward shoreline of the Coorong Lagoon was probably built (D. Schwebel, pers. comm., 1975). This system forms the basement of the stromatolite lakes shown in Fig. 2. A subsequent sea level approached t o within several metres of the present sea level about 80,000 years ago and this may have been responsible for the barrier island which now forms the eolianite inlier within the Younghusband Peninsula barrier. Next in sequence followed the major sea-level retreat of the Late Wisconsin Glaciation after which the Holocene transgression returned the sea to its present level, partly inundating the older barriers to form the initial stages of the modern Coorong Lagoon.
mi
0
I
40
I
I 120
80 YEARS
I
MO
200
10’
Fig. 3. Sea-level curve of the last 200,000 years, based on New Guinea data of Veeh and Chappell(l970). Relevant dates are as follows: A = high sea level of 120,000 years ago corresponding t o Sangamon interglacial: formation of older barrier that now forms most of the Pleistocene basement in the area: B = relatively high sea level of 80,000 years ago: formation of eolianite inlier within modern barrier. C = Holocene transgression: formation of modern barrier.
A t this early stage the lagoon water was about l m above its present level as evidenced by widespread stranded lagoonal sediments (Brown, 1965). It is uncertain whether this higher level was due to a sea level slightly higher than at present or was related t o unrestricted access of the ocean through numerous passes. However, this was the time during which low-lying areas of the
ENVIRONMENTAL MODELS
417
Pleistocene topography marginal t o the lagoon, including the stromatolite lakes, were flooded by waters with oceanic affinities. As sea level stabilized in the Late Holocene, the Younghusband Peninsula barrier built over the older ridge causing the lagoon to become increasingly restricted until ultimately its southern waters reached their present hypersaline state. Lagoon waters at that time, possibly 3,000 years ago, receded l m to their present level which averages that of mean sea level, causing the stranding of the marginal stromatolite lakes. These lakes now have floors about 1m above present high lagoon level and receive their annual charge of water during winter months by local and regional groundwater seepage from surrounding calcareous eolianite ridges and deeper Tertiary aquifers. This water reaches a maximum depth of about 0.5 m in late winter and evaporates to dryness in early summer. The salts in the lake waters are derived in part by leaching from marginal flats, possibly augmented by magnesium ions from breakdown of high-magnesian calcite in the calcareous aquifers. It is in this environment that various carbonate minerals and the stromatolites are forming at the present day. STROMATOLITE DESCRIPTIONS AND STRATIGRAPHY
The Coorong stromatolites have been described in detail by Walter et al. (1973). Three morphological forms were described: globular, stratiform and crenulate (Fig. 4). They occur in marginal areas of the two shallow ephemeral lakes shown in Fig. 2 and are associated with a carbonate mineralogy of hydromagnesite, aragonite, dolomite and calcite. During winter and early spring months pH of the associated lake waters varies from 8.2 to 9.9 (Von der Borch, 1965) with salinities reaching a minimum of about loo/, . At this stage a sparse growth of the aquatic grass Ruppia maritima occurs on lake floors. Complete desiccation generally occurs during ensuing arid summer months. The actual stromatolite structures persist throughout the dry period with varying degrees of preservation, however during the wet season small myriapod arthropods burrow into them and destroy some of their features. Sediment cores taken through the stromatolite lakes to depths of several metres (Fig. 5) show a stratigraphy that reflects the evolutionary history of the Coorong Lagoon and its associated marginal lakes. In both cases basement is Pleistocene calcrete, which is developed on calcareous eolianite. Overlying this is a shallow-marine sedimentary unit (Protected Marine Phase, Fig. 5), consisting of skeletal grainstones and packstones containing a variety of bivalves such as Katelysh spp. and Venerupis sp. This was formed during and immediately following the Holocene transgression, before significant build-up occurred of the Y ounghusband Peninsula barrier. This grades upwards into pelletized organic-rich aragonite and magnesian calcite muds rich in a lagoonal fauna comprising the small gastropod Coxiella confusa, the
418
C.C. VON DER BORCH
Fig. 4. Coorong stromatolite morphologies. A. Globular stromatolites, southern stromatolite lake (coin 23 mm wide). B. Desiccation polygons in stratiform stromatolites, southern stromatolite lake (scale 30 cm long). C. Crenulate stromatolites on the margins of desiccation polygons of stratiform stromatolites, southern hydromagnesite lake (pencil 18 cm long).
foraminifera Ammonia beccarii and a variety of ostracods (Lagoonal Phase, Fig. 5). This change in sediment-type reflects a period of increasing restriction due to barrier development. Upper portions of cores, finally, are composed of white, microcrystalline carbonate muds and pellet packstones composed of the minerals hydromagnesite, calcite, aragonite and dolomite (Ephemeral Lake Phase, Fig. 5). This unit, which developed in response to the l m drop in lagoon level, contains stromatolite structures both at the surface and occasionally at depth. GEOLOGICAL SIGNIFICANCE
Stromatolites are rare in modem carbonate sediments, largely because of the browsing on algal mats by organisms such as crustacea. The fact that stromatolites occur in two lakes in the Coorong area is therefore somewhat sur-
419
ENVIRONMENTAL MODELS NORTHERN STROM. LAKE
0
-
SOUTHERN STROM. LAKE WITH NANTLY HYDROMAGNESITE, ARAGONITE,
1-
2 SKELETAL GRAINSTONE AND PACKSTONE W I T H B I V A L V E S K A T E L Y S I A AND V E N E R u P I S .
vl Y t
2
.......J PLEISTOCENE CALCRETE
3PLEISTOCENE CALCAREOUS E O L l A N l T E
Fig. 5. Generalized stratigraphic columns from northern and southern stromatolite lakes. Terminology partly adapted from Brown (1965).
prising, particularly in view of their general absence in apparently comparable lakes nearby. One contributing factor may be that the two stromatolite lakes, due to greater localized ground-water influx, do not become as thoroughly desiccated as other lakes. This higher ground-water discharge in turn may lead t o the formation of the carbonate mineral hydromagnesite which is associated with the stromatolites. It would be quite possible for sediments formed under conditions similar t o the above to be preserved in the geologic record, particularly in structurally negative areas. Such occurrences have been described in the literature (Peterson, 1962). The deposits would be typified by an association of carbonate cycles similar to the one described from the Coorong sediment cores. A single upward-fining cycle would ideally measure a few metres in thickness, beginning at the base with a shallow marine carbonate unit. This would grade upwards into an organic-rich pelletized lagoonal carbonate unit and would culminate in a siliceous microcrystalline carbonate with associated stromatolite structures. Mineralogy of the upper unit would be variable in both the lateral and vertical sense and could comprise combinations of the minerals magnesite, dolomite or calcite. The carbonate units themselves would be lenticular and laterally discontinuous and would be closely associated with shoestrings of calcareous barrier sands.
420
C.C. VON DER BORCH
ACKNOWLEDGEMENTS
Sediment coring in the Coorong area has been supported by grants from the Australian Research Grants Committee and Flinders University. The manuscript was read by D. Schwebel. Photographs of the Coorong stromatolites were provided by M.R.Walter.