Tumblagooda sandstone
Encyclopedia
The Tumblagooda sandstone is a geological formation deposited during the Silurian
or Ordovician
periods, around four to five hundred million years ago, and is now exposed on the west coast of Australia
, straddling the boundary of the Carnarvon
and Perth
basin
s. Visible trackway
s may prove to be the earliest evidence of terrestrial animals.
overlies a Proterozoic
basement
. The formation is divided into four facies
associations (FAs), numbered stratigraphically
, that is from bottom to top. These lithified sediments portray an environment dominated by high-energy braided streams, flowing into the sea in places; ephemeral pools were also common.
, deposited by broad, high-energy braided rivers, which formed the outwash plain of an alluvial system
. Trace fossil
s are virtually absent, because the high depositional energy meant burrowing organisms could not survive. The downslope flow was to the north west.
of FA1 sediments. Beds are on the whole thin, planar and well sorted. Beds about 5 centimetres (2 in) thick form 2 metres (6.6 ft) units of "bedded sandsheets" -- layers of sand blown by the wind -- which form the dominant lithology of this facies.
Low angle (<20°), cross-stratified sandstones form units up to 50 centimetres (19.7 in) thick, sometimes reaching thicknesses as much as 2 metres (6.6 ft). The current's direction here is to the south east - up slope - and reinforces their interpretation as aeolian dunes.
A further suite of layers contain a dense trace fossil
assemblage; other layers bear current ripple marks, which likely formed in shallow streams, with flooded hollows probably hosting the creators of the trace fossils.
Cyclicity is absent, suggesting that, rather than being seasonal events, the occasional innundation was based on unpredictable events such as storms, a varying water table, and changing stream courses.
, with an abundance of Skolithos
- a fossil typically found in marine environments.
It has been interpreted as an inter-distributary
bay, or alternatively as a sandy coastline featuring wave-generated bars, perhaps with tidal influence; braided fluvial streams often reworked the sediments.
) and with virtually no body fossils, its age is very difficult to constrain. It was first thought to have formed around , during the Cretaceous
period, on the basis of stratigraphy; current estimates place it far earlier, in the lower Ordovician
, . It is hoped that a new technique based on the uranium-thorium dating
of diagenetic
monazite
crystals may produce a more precise estimate of the age, but initial attempts have failed to extract sufficient monazite from the unit.
Such a method would be of great value, as previous attempts to date the unit have been rather inconsistent. The initial Cretaceous estimate was soon reviewed with a "mid-Cambrian to early Ordovician" (~) estimate based on trace fossils, and was shortly afterwards replaced by a mid-Silurian
age based on spore
s and acritarch
s. This was apparently confirmed by the identification in the overlying beds of a conodont
fauna with a recognizably Silurian character, but when the palæomagnetism of the area was studied, an early Ordovician age was deduced. A single conodont element, again drawn from overlying sediments, was taken to support a late Cambrian to early Ordovician age, but this was refuted by the observation that the trace fossil assemblages bore great similarity to well constrained lower Silurian assemblages from Antarctica. The current early Ordovician age estimation is based on a much more diverse and numerous -- therefore more securely dated -- assemblage of conodonts, again from overlying sediments.
ic rocks, the Tumblagooda has undergone minimal tectonic activity since its formation. Therefore faulting is rare, and units continue laterally for great distances. Jointing is the dominant control of the landscape, with meandering rivers enhancing joint locations. Miocene
uplift has resulted in the formation of deep (approximately 60m/200 ft) gorges exposing large cliff sections; with the sparse vegetation characteristic of arid Western Australia
, this means the unit is exceptionally exposed, making detailed study easy. Despite this, the unit was not studied until 1948, due to its inaccessibility - the 600 miles (1000 km) from Perth were mainly dirt tracks until the 1970s.
ic arthropod
) has been found in the Tumblagooda, mainly due to the large clast size and the abundance of predatory and burrowing organisms. (This meant that oxygen could penetrate to good depths in the sediment, permitting decomposing organisms to decay anything that burrowing animals had not eaten too rapidly for fossils to form.) Since Kalbarria had 11 pairs of legs, it can be tentatively matched to some Protoichnites arthropod trackway
s of the same size. Protoichnites is abundant in subaerial facies in FA2-4. Marks which can only have been made on exposed wet sand are seen: for example "splurges" where the legs of the organism flipped sand out behind them. The marks vary in crispness and character according to the wetability of the underlying sediment; this is particularly marked where the traces cross ripples, with the lee slopes recording a trace markedly different in appearance to those in the troughs, and the stoss slopes recording no trace at all. Behaviour can be inferred from these traces; in places, they parallel features which modern observation notes forming at the edge of a wind-blown pond, just on the landward side of the shore. This behaviour has been interpreted as a feeding trace; presumably the trace-maker dined on organic matter blown out of the pool, or detritus left as the pool had shrunk. Further tracks can be traced across dunes; a slow walk up turns into a skid as the organism slid down the lee slope and into the pool on the other side. Another instance shows the trackways of two organisms converging, then becoming one trackway, before one individual swerves away to the left, leaving the other to walk onwards.
These trackways are the earliest evidence of terrestrial animals. Due to the poor dating of the unit, it is currently impossible to speculate whether the plants, which colonised the land in the mid-Ordovician, got there first.
Aquatic trace fossils are also abundant. Two major ichnofacies
are observed, bearing close resemblance to assemblages found in Antarctica and demonstrating proximity of western Australia and Antarctica at the time of deposition. One is dominated by Skolithos
, suggesting marine deposition. The fabric of the other is dominated by Heimdallia
, a strange planar trace that does not have a circular cross-section; it is interrupted in places by Beaconella
, a wide trace thought to be constructed by a burrowing arthropod ploughing through the sediment for food, leaving a mound of piled sediment at the end of each trace. This may suggest the Beaconella organism feeding on Heimdallia. Daedalus
, a trace fossil resembling a giant garlic, is also present in this assemblage.
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
or Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
periods, around four to five hundred million years ago, and is now exposed on the west coast of Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, straddling the boundary of the Carnarvon
Carnarvon Basin
The Carnarvon Basin is a geological basin located in the north west of Western Australia. This is the main geological feature that makes up the North West Shelf...
and Perth
Perth basin
The Perth Basin is a thick sedimentary basin in Western Australia. It lies beneath the Swan Coastal Plain west of the Darling Scarp, representing the western limit of the much older Yilgarn Craton, and extends further west offshore...
basin
Basin (geology)
A structural basin is a large-scale structural formation of rock strata formed by tectonic warping of previously flat lying strata. Structural basins are geological depressions, and are the inverse of domes. Some elongated structural basins are also known as synclines...
s. Visible trackway
Trackway
A trackway is an ancient route of travel for people or animals. In biology, a trackway can be a set of impressions in the soft earth, usually a set of footprints, left by an animal. A fossil trackway is the fossilized imprint of a trackway. Trackways have been found all over the world...
s may prove to be the earliest evidence of terrestrial animals.
Sedimentology
The Tumblagooda is over 1,400 m (4,500 ft) deep: the bottom has never been found, but seismic data suggests it unconformablyUnconformity
An unconformity is a buried erosion surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval of time before deposition of the younger, but the term is used to describe...
overlies a Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
basement
Basement (geology)
In geology, the terms basement and crystalline basement are used to define the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin...
. The formation is divided into four facies
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
associations (FAs), numbered stratigraphically
Stratigraphy
Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....
, that is from bottom to top. These lithified sediments portray an environment dominated by high-energy braided streams, flowing into the sea in places; ephemeral pools were also common.
Facies Association 1
The lowest facies association in the unit is dominated by trough cross-stratificationCross-bedding
In geology, the sedimentary structures known as cross-bedding refer to horizontal units that are internally composed of inclined layers. This is a case in geology in which the original depositional layering is tilted, and the tilting is not a result of post-depositional deformation...
, deposited by broad, high-energy braided rivers, which formed the outwash plain of an alluvial system
Alluvial fan
An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial...
. Trace fossil
Trace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...
s are virtually absent, because the high depositional energy meant burrowing organisms could not survive. The downslope flow was to the north west.
Facies Association 2
These facies reflect a quieter environment; the unit is occasionally interrupted by lensesLens (geology)
In geology a lens is a body of ore or rock or a deposit that is thick in the middle and thin at the edges, resembling a convex lens in cross-section. Adjective: "lenticular"....
of FA1 sediments. Beds are on the whole thin, planar and well sorted. Beds about 5 centimetres (2 in) thick form 2 metres (6.6 ft) units of "bedded sandsheets" -- layers of sand blown by the wind -- which form the dominant lithology of this facies.
Low angle (<20°), cross-stratified sandstones form units up to 50 centimetres (19.7 in) thick, sometimes reaching thicknesses as much as 2 metres (6.6 ft). The current's direction here is to the south east - up slope - and reinforces their interpretation as aeolian dunes.
A further suite of layers contain a dense trace fossil
Trace fossil
Trace fossils, also called ichnofossils , are geological records of biological activity. Trace fossils may be impressions made on the substrate by an organism: for example, burrows, borings , urolites , footprints and feeding marks, and root cavities...
assemblage; other layers bear current ripple marks, which likely formed in shallow streams, with flooded hollows probably hosting the creators of the trace fossils.
Cyclicity is absent, suggesting that, rather than being seasonal events, the occasional innundation was based on unpredictable events such as storms, a varying water table, and changing stream courses.
Facies Association 3
This facies is much like FA1, with an increased supply of clastic material represented in the sedimentary record by coarse-grained, poorly sorted, upwards-fining (i.e. largest grains at the bottom of the unit, becoming progressively finer towards the top), pebbly trough cross-bedded units up to four metres thick. Trace fossils are rare. Sheet-like braided rivers are inferred as the dominant control on sedimentation in these facies.Facies Association 4
The uppermost facies association appears to reflect an environment on the fringes of the sea. Fining-up is observed on 0.5 metres (1.6 ft) to 2 metres (6.6 ft) scales, with trough cross bedding at the bases of units overlain by current ripples. Fine sandstones and green shales are also present. The upper units are strongly bioturbatedBioturbation
In oceanography, limnology, pedology, geology , and archaeology, bioturbation is the displacement and mixing of sediment particles and solutes by fauna or flora . The mediators of bioturbation are typically annelid worms , bivalves In oceanography, limnology, pedology, geology (especially...
, with an abundance of Skolithos
Skolithos
Skolithos is a common trace fossil ichnogenus whose original form consisted of approximately vertical cylinders. One well-known occurrence of Cambrian trace fossils is the famous 'Pipe Rock' of northwest Scotland...
- a fossil typically found in marine environments.
It has been interpreted as an inter-distributary
Distributary
A distributary, or a distributary channel, is a stream that branches off and flows away from a main stream channel. They are a common feature of river deltas. The phenomenon is known as river bifurcation. The opposite of a distributary is a tributary...
bay, or alternatively as a sandy coastline featuring wave-generated bars, perhaps with tidal influence; braided fluvial streams often reworked the sediments.
Age
Since the Tumblagooda sandstone comprises a sedimentary succession with no volcanic layers (which could be dated radiometricallyRadiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
) and with virtually no body fossils, its age is very difficult to constrain. It was first thought to have formed around , during the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
period, on the basis of stratigraphy; current estimates place it far earlier, in the lower Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
, . It is hoped that a new technique based on the uranium-thorium dating
Uranium-thorium dating
Uranium-thorium dating, also called thorium-230 dating, uranium-series disequilibrium dating or uranium-series dating, is a radiometric dating technique commonly used to determine the age of calcium carbonate materials such as speleothem or coral...
of diagenetic
Diagenesis
In geology and oceanography, diagenesis is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration and metamorphism. These changes happen at relatively low temperatures and pressures...
monazite
Monazite
Monazite is a reddish-brown phosphate mineral containing rare earth metals. It occurs usually in small isolated crystals. There are actually at least four different kinds of monazite, depending on relative elemental composition of the mineral:...
crystals may produce a more precise estimate of the age, but initial attempts have failed to extract sufficient monazite from the unit.
Such a method would be of great value, as previous attempts to date the unit have been rather inconsistent. The initial Cretaceous estimate was soon reviewed with a "mid-Cambrian to early Ordovician" (~) estimate based on trace fossils, and was shortly afterwards replaced by a mid-Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
age based on spore
Spore
In biology, a spore is a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycles of many bacteria, plants, algae, fungi and some protozoa. According to scientist Dr...
s and acritarch
Acritarch
Acritarchs are small organic fossils, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.-Definition:In general, any small, non-acid soluble Acritarchs are small organic fossils, present from...
s. This was apparently confirmed by the identification in the overlying beds of a conodont
Conodont
Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...
fauna with a recognizably Silurian character, but when the palæomagnetism of the area was studied, an early Ordovician age was deduced. A single conodont element, again drawn from overlying sediments, was taken to support a late Cambrian to early Ordovician age, but this was refuted by the observation that the trace fossil assemblages bore great similarity to well constrained lower Silurian assemblages from Antarctica. The current early Ordovician age estimation is based on a much more diverse and numerous -- therefore more securely dated -- assemblage of conodonts, again from overlying sediments.
Tectonic history
In common with most of the Australian cratonCraton
A craton is an old and stable part of the continental lithosphere. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates. They are characteristically composed of ancient crystalline basement rock, which may be covered by...
ic rocks, the Tumblagooda has undergone minimal tectonic activity since its formation. Therefore faulting is rare, and units continue laterally for great distances. Jointing is the dominant control of the landscape, with meandering rivers enhancing joint locations. Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
uplift has resulted in the formation of deep (approximately 60m/200 ft) gorges exposing large cliff sections; with the sparse vegetation characteristic of arid Western Australia
Western Australia
Western Australia is a state of Australia, occupying the entire western third of the Australian continent. It is bounded by the Indian Ocean to the north and west, the Great Australian Bight and Indian Ocean to the south, the Northern Territory to the north-east and South Australia to the south-east...
, this means the unit is exceptionally exposed, making detailed study easy. Despite this, the unit was not studied until 1948, due to its inaccessibility - the 600 miles (1000 km) from Perth were mainly dirt tracks until the 1970s.
Palæontology
Only one body fossil, Kalbarria (an early euthycarcinoidEuthycarcinoid
Euthycarcinoida is a group of amphibious freshwater arthropods that until recently were only known from the Carboniferous onwards. A single Ordovician/Silurian individual was identified in the Tumblagooda sandstone in 1993; a Devonian example was added from the Rhynie chert in 2003, and most...
ic arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
) has been found in the Tumblagooda, mainly due to the large clast size and the abundance of predatory and burrowing organisms. (This meant that oxygen could penetrate to good depths in the sediment, permitting decomposing organisms to decay anything that burrowing animals had not eaten too rapidly for fossils to form.) Since Kalbarria had 11 pairs of legs, it can be tentatively matched to some Protoichnites arthropod trackway
Fossil trackway
A fossil trackway is a type of trace fossil, a trackway made by an organism. Many fossil trackways were made by dinosaurs, early tetrapods, and other quadrupeds and bipeds on land...
s of the same size. Protoichnites is abundant in subaerial facies in FA2-4. Marks which can only have been made on exposed wet sand are seen: for example "splurges" where the legs of the organism flipped sand out behind them. The marks vary in crispness and character according to the wetability of the underlying sediment; this is particularly marked where the traces cross ripples, with the lee slopes recording a trace markedly different in appearance to those in the troughs, and the stoss slopes recording no trace at all. Behaviour can be inferred from these traces; in places, they parallel features which modern observation notes forming at the edge of a wind-blown pond, just on the landward side of the shore. This behaviour has been interpreted as a feeding trace; presumably the trace-maker dined on organic matter blown out of the pool, or detritus left as the pool had shrunk. Further tracks can be traced across dunes; a slow walk up turns into a skid as the organism slid down the lee slope and into the pool on the other side. Another instance shows the trackways of two organisms converging, then becoming one trackway, before one individual swerves away to the left, leaving the other to walk onwards.
These trackways are the earliest evidence of terrestrial animals. Due to the poor dating of the unit, it is currently impossible to speculate whether the plants, which colonised the land in the mid-Ordovician, got there first.
Aquatic trace fossils are also abundant. Two major ichnofacies
Ichnofacies
An ichnofacies is an assemblage of trace fossils that provide an indication of the conditions that their formative organisms inhabited.-The concept:...
are observed, bearing close resemblance to assemblages found in Antarctica and demonstrating proximity of western Australia and Antarctica at the time of deposition. One is dominated by Skolithos
Skolithos
Skolithos is a common trace fossil ichnogenus whose original form consisted of approximately vertical cylinders. One well-known occurrence of Cambrian trace fossils is the famous 'Pipe Rock' of northwest Scotland...
, suggesting marine deposition. The fabric of the other is dominated by Heimdallia
Heimdallia
Heimdallia is an ichnogenus comprising a strange planar trace that does not have a circular cross-section; its maker may have been fed upon by Beaconella.The trace was probably made by a small crustacean....
, a strange planar trace that does not have a circular cross-section; it is interrupted in places by Beaconella
Beaconella
Beaconella is an ichnogenus comprising a wide trace thought to be constructed by a burrowing arthropod ploughing through the sediment for food, leaving a mound of piled sediment at the end of each trace....
, a wide trace thought to be constructed by a burrowing arthropod ploughing through the sediment for food, leaving a mound of piled sediment at the end of each trace. This may suggest the Beaconella organism feeding on Heimdallia. Daedalus
Daedalus (trace fossil)
Daedalus is an ichnogenus, describing burrows with the shape of a garlic clove....
, a trace fossil resembling a giant garlic, is also present in this assemblage.