Fossil Flora - Silcrete Plant Fossils; Evidence for Climate Change - Part Two

Hartley, 1967 stated “The past is a foreign country”

The validity of this statement becomes readily apparent looking across the vista of breakaways, dry sandy terrain, silcrete-covered ridges and dry gibber desert.  A stark contrast exists between the present day environment and the evidence presented by fossil flora of the very different conditions that prevailed here, when the fossils were deposited.  What had previously been a wet, lush, and green environment that supported a complex mosaic of tropical and temperate rainforest, is now devoid of all but the most resilient vegetation.

The content of this post, because of its scientific nature is rather specialised, however, I have attempted to succinctly explain the region’s local geology and provide evidence to how the fossils found in the area can be used to interpret climatic change.  To view an image larger, double click the image.

Geology

The region’s geology has been poorly studied, despite there being considerable interest because of the record of surface environments in the Eyre basin over the past 40 million years, and the famous silcrete floras and mammalian megafauna.  

LEFT:  A satellite view of the region (courtesy Google Maps) clearly showing the drainage lines (green), gibber plains (light brown) and breakaways (dark brown).

The geology is complex, in part because of the number of stratigraphic units involved and the interrelationship between lacustrine, fluvial, and aeolian sediments.  It’s this interrelationship and the variable development of duricrusts which has recently propelled the region’s interest, as these environments provide good scientific analogues for the surface of Mars.

The basement rock is comprised of marine deposited shale laid down during the Cretaceous Period when much of what is now inland Australia was covered in seawater.  The break-up of Gondwanaland and resultant rearrangement of tectonic plates gently deformed and duricrusted this shale, named the Bulldog Shale, to form the gibber plain uplands of the Stoney Desert.  The breakaways, prominent sentinels in today’s environment, are comprised of fluvial and lacustrine-derived sediments deposited during the Upper Triassic and Lower Tertiary, and were formed by erosion from several ephemeral streams including Coopers Creek.

Sedimentology and Palaeoenvironments   

I’ve attempted to provide a short introduction to the sedimentary units involved in the formation of the palaeochannel in which the fossils have been deposited. This interpretation is quite broad in content and further study and work needs to be done to accurately assign dates to the various stratigraphic units.  

Palaeoenvironment

The nature of the palaenvironment is well understood; a fluvial system with well-established oxbow lakes and shallow lakes surrounded by a mosaic of riparian tropical to temperate rainforest in a warm semi-tropical climate.

LEFT:  Indistinct to the untrained eye, this sandstone rock exhibits tell-tai sedimentary structures which indicate that the region was once covered in water.  The ripple marks can be used, amougst ither things, to interpreete water depth and current direction.

During the mid-Tertiary, the environment began to experience seasonality with fluctuating watertable levels; increased periods of semi-aridity began to alter the rainforest mosaic until species survived only around permanent water channels and lakes.  Eventually, as Australia and Antarctica separated and Australia moved north, aridity became more widespread and taxa became locally extinct.

The Watchie Sandstone Unit

The Watchie Sandstone is a lacustrine sequence comprising; in upwards succession, a local channel-sand facies and widespread lag deposits, a transgressive fine-grained lacustrine facies and regressive strandline deposits with a wavebase lag.  

Lag deposits indicate a low-gradient surface and strandlines incorporating pedogenic silcrete clast provide evidence of former soil horizons.    The palaeocurrent evidence is consistent with longshore processes and bar accretion onto a foreshore and larger bedforms indicate storm events.  Ridges have formed only along the eastern shoreline, indicating a strong westerly airstream and silicification and ferruginisation alternated during lacustrine regression.  

Insects (including ants and termites) lived in developing soil profiles and rhizonodules indicate that plants grew on these soils.   

The Willalinchina Sandstone Unit

This unit was deposited in a fluvial channel environment and is interpreted as a broad, shallow meandering to braided channel system which abuts a floodplain to a lacustrine palaeoenvironment.  Storm and associated flood events have also deposited bar, levee and floodplain deposits which interfinger with fluvial-derived sediments.   

LEFT:  Partly eroded to show depth of the deposit, the structures clearly are indicative of sequential mud cracks that provide evidence of a drying environment.

Field observations support this interpretation:  A thin lens of basal conglomerate has been overlain by several meters of cross-bedded fine to medium-grained sandstone containing minor lenses of course-grained sandstone.  Silicification, caused by alternating watertable levels, has occurred in several places.    

Silcrete and Silicification

Silification provides evidence of a fluctuating watertable and, in general, a marked seasonality toward aridity, which inland Australia experienced during the mid Cenozoic. 

LEFT:  Reed mould or ant's nest silcrete - a possible interpretation is reed casts or methane gas escape structures from rotting vegetation.

The silcrete is defined by closely spaced vertical structures which have been referred to in past literature as “ants nest or reed mould silcrete”.  The vertical structures have been interpreted as reed casts, however, there is no evidence of organic matter found within the Willalinchina Sandstone Unit.  A possible alternative is soft sediment deformation caused by gas escaping from rotting vegetation.  This scenario would explain why the gas escape structures are not evident throughout the sandstone unit, but are preferentially preserved. 

Another interesting sedimentary structure preserved within the silcrete are circular mound-like features (below left) which have not been successfully interpreted. One theory suggests that the features are formed when liquid silcrete bubbled its way toward the surface, akin to how artesian water bubbled to the surface in nearby artesian spring moulds.

Silcrete is an indurated soil duricrust formed when silica is dissolved and resolidifies as a cement. It is a hard and resistant material, and though different in origin and nature, appears similar to quartzite.  Silcrete is relatively common in inland Australia, often forming the resistant cap rock on features like breakaways.

Stratigraphic Analysis and Age    

Sediments from the Palaeochannel have been placed in the Watchie and Willalinchina Sandstone units although most of the palaeochannel area is located within the later.  

Preliminary palaeobotanical work suggests that the Willalinchina Sandstone correlates with the youngest phase of the Eyre Formation in the Lake Eyre Basin and is Eocene in age.  The Watchie Sandstone has been interpreted as Miocene and correlates with the Billa Kalina Basin.  However, recent stratigraphic analysis may indicate that the Willalinchina Sandstone could be Miocene to Pliocene in age.  Further investigation needs to be made in this area before a definite date can be attributed to the palaeochannel.   

Floral Record - Interpreting Climatic Change   

The Australian Tertiary plant fossil record is very poor.  Studies indicate that tropical to temperate rainforests occupied  south eastern and south western Australia for much of the early-Tertiary with a contraction of these rainforest communities in the mid to late-Tertiary.   

LEFT: One of many example of the fossil flora found in the area.  The state pf preservation is amazing and identification, in some cases, to species level is possible.

The discovery of the fossil flora confirm the palaeochannel is one of the richest, most extensive Tertiary plant fossil localities in Australia, if not globally.   

The botanical significance of silcrete fossils may be limited because silcrete formation is notoriously difficult to interpret and date with precision; however, the presence of extensive silcrete, caused by the fluctuating watertable levels does provide evidence consistent with marked climatic seasonality that occurred in inland Australia during the Cenozoic.  Therefore, floral assemblages could preserve evidence of the effect of climate change on the vegetation of inland Australia.

Initial Analysis    

Initial analysis of floral elements indicate a mosaic of plant communities dominated by sclerophyllous woodlands (Eucalypt spp.) and interspersed with riparian rainforests and deciduous marginal monsoon forests.  These plants grew along the watercourses where permanent water enabled them to survive seasonal dry periods.   

Interpretation of Fossil floras and Palaeoclimate     

Observational studies in tropical and temperate forests have demonstrated a direct relationship between leaf form (foliar physiognomy) and local climate.  For example; leaf length to leaf width and stomata number and size correlate to rainfall and ambient temperature.  

LEFT:  Ripple marks showing bifurcation of ripple crests indicate wave-formed ripples that have formed by wind blowing across shallow water.  The ripple marks are so clear, it's difficult to realize that they were formed approximately between 24 and 5 million years before present.

Mean canopy leaf size is also strongly correlated with mean annual temperature.   Palaeobotanical investigations of the botanical specimens found within the palaeochannel (Willalinchina Sandstone) provides valuable information on the mid-Tertiary distribution of Eucalyptus spp. and other plants.  

To date 245 leaf types, 47 fruit and seed types and 2 major wood types have been identified.   The sclerophyllous component (identified from linear to lanceolate leaf forms) dominated the flora and would have grown on the exposed drier plains in the more open forest areas of the floodplain.  

Many of the fossil leaves are indiscernible from extant Eucalyptus leaves and the minimal variation in leaf form suggests that Eucalyptus has existed in the area for ~15 million years.  The ecological niche of this species, based on this information, appears to be similar to that of extant Eucalyptus.    

LEFT:  An excellent example of Banklsieaeformis praegrandi.  If you open this image (double click) you can see the preserved intricate patterns of the veins that have been preserved.  The red colour is caused by iron oxide that has percolated through the sediments.

The rainforest component of the flora grew in areas that provided a permanent water resource and are not present in the area today; their absence indicative of a changing climate and environment. As the climate became increasingly drier, these plants died out to eventually become confined to the present monsoonal and tropical rainforests of northern Australia and became locally extinct.   

Ancient Eucalyptus are not the only plant fossils that have been uncovered; numerous other plant species are likely to be extinct representatives of extant genera.  As with the Eucalyptus, similarity in leaf forms between fossil leaves and extant leaves have made identification problematic and it’s highly likely that many of these species were intermediate rainforest / sclerophyllous species sandwiched between the effects of climate change.     

One species that appears to have been positively identified from its deeply incised proteaceous leaf form is Banklsieaeformis praegrandi.  This fossil plant has been found in low numbers within the palaeochannel and has been linked to the extant species Banksia chamaephyton which is restricted to a small area of heathland in Western Australia (Greenwood, 1997).  

Access  

Access to this area is STRICTLY PROHIBITED without express permission from the land owner, property station manager, and approval from an educational facility such as University or museum.   Removal of fossils or collecting is NOT ALLOWED. 

LEFT:  Preferential erosion erodes the former land surface to leave the more harder and durable silcreted surface, in a formation called a breakaway; a term uniquely Australian.  The breakaways can reveal the surface of an environment that often is completely changed to that of today.  It's often difficult to grasp that approximately 70 million years ago these surfaces were the floor of a vast inland sea.

Next Journal Post

In the final post on this topic, we'll look at a number of photographs of the fossils and discuss the best method to photograph them in situ.

References   

Krieg, G. W., Rogers, P. A., Callen, R. A., Freeman, P. J., Alley N. F. and Forbes, B. G., 1991.  Explanatory Notes Curdimurka South Australia. 1:250,000 Geological Series Geological Survey of South Australia, Peacock Publications, pp 35 – 38.  Rowett, A., 1997.  Earthwatch '96.  MESA, Journal 5, pp 27-29. 

Fossil Flora - Searching For & Photographing Fossils To Document Climate Change - Part One

Dead things are usually not on my list to photograph, however, I jumped at the opportunity when tasked to photograph fossils at a location in far northern South Australia.  The fossils were deposited during the Tertiary Period (63 million years ago to 1.8 million years ago).  During this time the environment was very different to what it is today and much of Australia was blanketed in a mantle of green with humid temperatures; a far cry from the hot dry gibber deserts and sand dunes seen today.

LEFT:  Tracks are indistinct in gibber country.

The location of the fossil beds is remote, and a fully equipped four wheel drive vehicle is needed to negotiate the many small, unmarked tracks that eventually lead to the “jumping off” point to reach the fossils.  From this central location, where a base camp was erected, I explored via foot a number of low lying ridges and gullies that previously had been identified as being geologically suitable for fossil preservation.

This will be the first of two/three posts describing the 3 week camping trip in a relatively remote part of Australia.

Part of the Kidman Empire

Access to the fossils is via a number of badly maintained and eroded station tracks.  The tracks initially were constructed to allow pastoralists’ access to cattle which were run on the property.  At one stage, the property I was driving through was part of the vast Kidman Empire and was the largest cattle station (ranch) in the world, covering an area of 34,000 sq. km (6 million acres). Although the original property has since been sub-dived into smaller cattle stations, the area I was on is still 8,000 sq. km larger than its nearest rival in the Northern Territory of Australia, Alexandria Station. By contrast, the largest American cattle station “ranch” is 3,000 sq. km.  

The area I wanted to visit had been serviced by a station track, however, inactivity, sand migration and recent rains had caused the track to be indistinct; in several areas it had petered out completely.   Several creek beds and steep gullies needed to be traversed; the main concern being that sharp-sided rocks would puncture a tyre.  

Changing Environment

I had marked on a map the location I wanted to camp, this area being where I had established a base camp on an earlier visit several years ago.  I remembered the area was well protected from wind and several large Eucalyptus shaded the camp from the intense sun. 

LEFT:  This dry creek bed adjacent to the gibber plains was home for two weeks. 

However, floodwater and sand migration had altered the environment and the only way to get the vehicle to the campsite was to traverse a creek bed which was in-filled with deep, loose sand; on my previous visit the creek was loose rock and pebbles.  It's not uncommon for tracks and creek beds to be covered in moving sand in the desert areas of Australia; their mid-latitude produce constant seasonal winds that entrain sand and dirt and depositing it many kilometers from its origin.

I decided that bogging the vehicle in the sand wasn’t a good idea, for despite the four wheel drive having a winch, there wasn’t a suitable attachment point to connect the cable and winch to.  Therefore, a less than ideal camp site was chosen, with easier access, further along the creek line.

The camp site chosen was amongst a small grove of Eucalyptus growing alongside an ephemeral stream.  Bordering on both sides were vast gibber plains and low lying hills, which glistened during the heat of the day with the tell-tail shimmering of a heat mirage.

Exploring the Gibbers

Finding fossils in the desert is akin to finding a needle in a hay stack and a knowledge of geology helps identify areas prone to fossil preservation (I have post graduate studies in palaeontology).  Stony rises, coloured red-brown from oxidized iron are good places to look.  These rises, at the time of deposition millions of years ago, were flood plains, small lakes, or creek backwaters; ripple marks and other sedimentary structures preserved in the rock are evidence that support this.

Get Out & Walk….Warm Temperatures

I have always remembered a comment made by a legendary geologist during my university training “get out of the car and walk!”  “The rocks are waiting to tell you their stories” This advice seemed pertinent at this juncture as fossils can be small in size and usually can’t be seen from a moving vehicle.  Further, gibber desert environments are particularly sensitive to erosion and driving a heavy vehicle across gibber plains can compress the stones causing wheel ruts; ruts that will remain visible for many years to come.  This is a legacy I did not want to be responsible for.  Therefore, several days were spent carrying photographic and geological equipment, sometimes several kilometers each day, to explore the plains and low-lying gibber hills.

Walking was not uncomfortable in the early and late afternoon, but during the middle of the day when temperatures soured, we scrambled under what shade we could find, leaving the gibber surface to the ants which seemed to have immunity against the heat. 

One of Australia’s earliest explorers remarked that gibber environments are the most inhospitable places in Australia and in summer temperatures can reach well above 69 Degrees Celsius (in the sun).  During my visit, which was in early spring, the temperature on several days was 44 Degrees Celsius. 

Gibber Formation

The name gibber comes from the Aboriginal word for stone.  Gibber deserts are formed from the effect of constant seasonal winds that remove all sand and dust, but leave behind a highly eroded environment.  Rocks in this environment, often called gibber stones, appear highly polished, are wind-sculptured and exhibit a red-coloured desert varnish.

LEFT:  Gibber desert with ephemeral stream and vegetation.  You walked over gibber plains such as this for several kilometers each day to reach the fossilferous locations, often walking on a compass bearing with a GPS fix.

The red colouring comes from iron-rich or silica-rich material that has leeched over time, while the varnish is from constant polishing and scouring by sand entrained winds (similar to sand-blasting).  The gibber stones act as a "rock armour" protecting the delicate soil horizons beneath the stones.

Those gibber stones that are strongly wind-sculptured are called venifacts and are often used by earth scientists to determine paleowind direction; the side of the eroded and sculptured rock points toward the direction that the wind has blown for many thousands of years.

Desert Life - Gibbers

Although the ambient temperatures are extreme during the summer months, a number of animal species have made the gibber environment their home.  Spiders and ants can be found beneath stones; the spiders protect themselves from dedication by retreating into deep burrows emerging only to hunt at night and during overcast days when it’s cooler.  

LEFT:   The endemic Lozenge-marked Dragon (A. vadnappa) sunning itself on a venifact. 

On the gibber stones, the only reptile I found  was A. vadnappa basking in the midday sun.  The lizard remained motionless for over an hour; locomotion requires energy and in this harsh environment, if you don't need to move you don't!  A. vadnappa is endemic to this area and is not found elsewhere.

Along dry creek beds brown snakes were observed warming themselves on the warm sand.  Snakes and lizards are some of the more commonly found animals in inland Australia and vigilance is required to ensure you do not accidentally tread on a venomous snake.

Vegetation on the gibber plains is very sparse and acacias, desert peas and Eucalyptus spp. only grow in perfusion where the surface of the gibber plains has lost its protective "desert armour", such  as when eroded by the water from an ephemeral water course. 

Red Sand Dunes

Adjacent to the gibber plains is red sand dune country.   During the day you only see the tracks and sand excavations  of various animals in the shape of burrows, shelters and holes, however, at night the dune country comes alive with the glow of many eyes - testament to the large number of species that live in the soft sand or within the low lying scrub, which preferentially grows in the dune country.

Walking in the late afternoon and early morning I observed the Central Netted Dragon (Amphibolurus nuchalis), a large monitor lizard (varanus spp.), and several invertebrates such as the red grasshopper, red-sand spider and sand scorpion.  

Birds

Birds are obvious inhabitants of many environments and the Eucalyptus-lined creek beds provide an ideal habitat for many species.  Following extended rainfall and flood events, bird populations increase dramatically to take advantage of the additional food supply that seasonal water brings.  Lake Eyre, Australia’s largest salt lake had recently flooded and many bird species had extended their range to take into account the recent floodwater.

LEFT:  The ornate and beautifully patterned Central Netted Dragon (Amphibolurus nuchalis) was seen feeding on ants in the early morning.

Near the camp site a flock of corellas had made their home along with zebra finches, sulphur-crested cockatoos, ring-necked parrots, black shouldered kites and two peregrine falcons.  Large wedge-tailed eagles soared high on thermals in search of smaller mammals and unwary reptiles.

Recent Flooding (La Nina)

The climate in Australia waxes and wanes in response to the cycles of El Nino and La Nina; evidence for the latter, and its severity, being the flood debris piled some three-meters high against tree trunks, in what were now dry and dusty creek beds.  

Rainfall, although sparse in desert regions is not unheard of and torrential rain causing flooding can occur every decade or so.  During this time the gibbers are awash with water (sheet flow) and creeks are raging torrents of flood water entrained with sediment.  The water rejuvenates the area and Eucalyptus and Acacia seeds, long buried in the sand, begin to sprout and grow along with other native species such as grasses and small flowering perennials.  

Location, Access & Collecting

I purposely have not documented the actual location of this site.  Access is limited only for scientific research and  proof of research must be given to the landholder to gain permission.  Any extraction and collecting of fossils is forbidden.

Fossils - Photographing & Importance

Hopefully this post has set the scene. 

In two upcoming posts, we’ll describe the local geology, examine some of the fossils and discuss their interpretation and importance in relation to climate change in Australia.  We'll also look at various methods used to photograph the fossils.