Planet Ipswich : A bridge between the Ipswiches of the world

Ipswich Basin, West Ipswich Fault, Ipswich Coal Measures & Ipswich Microflora, Queensland, Australia  

The city of Ipswich, Queensland has given its name to geomorphological features in this part of Australia.

During the Permian and Triassic periods, almost all the Earth’s land mass was concentrated into a single supercontinent called Pangaea.  In the Late Permian or Early Triassic (about 250 million years ago) stresses built up along what is now the eastern Australian coastline when tectonic plate movements created subduction zones on the margin of the Pangaea land mass.  The stress resulted in what is known as the West Ipswich Fault, running northwest to southeast, causing extensive shearing of the crust along the fault-line (known as a strike-slip fault).  This was followed by the gradual subsidence of the crust in the Middle Triassic (from about 240 million years ago).  This formed the intermontane Ipswich Basin southwest of Brisbane straddling the Queensland-New South Wales border and extending about 8000 sq. km (3088 sq miles), bound in the north and southeast by earlier Carboniferous block massifs, with the Ipswich Fault to the west.  The eastern extent of the basin is not well known but probably covers an offshore area of about 1000 sq. km (386 sq. miles) under Moreton Bay at Brisbane.

The basin-fill is dominated by sandstones, shales, conglomerates and coals deposited in alluvial, fluvial and lacustrine environments, with some interbedded volcanics.  However, the greatest characteristic is the thick accumulation of coal measures – the Ipswich Coal Measures – that became an important resource in the Ipswich Basin and gave rise to the coal mining industry there.  These coal measures were mostly laid down during the Carnian Age of the Late Triassic (228 to 216 million years ago).  Sedimentation began in the Middle Triassic in the slowly subsiding basin with the surrounding mountains providing sediment that was deposited by streams.  Numerous small lakes and swamps dotted the landscape.  In moist areas, leaf litter and plant remains accumulate and, mixed with mud and silt, when compressed they eventually form coal beds.  A sequence of sandstones and coal measures was repeated several times during this cycle of deposition.  The Ipswich Coal Measures comprise a number of sub-groups forming different coal seams. The coal originated from a peat-swamp in an alluvial plain with meandering channel systems.  The coal seams are separated into three main groups based on the mean seam thickness and the manner in which they were deposited.  Group 1 contains the oldest seams and shows a very high thickness, which indicates the replacement of water by peat in a shallow flood basin as a result of tectonic subsidence.  Group 2 seams have moderate thickness, indicating that peat accumulated in progressively shallower, swampy wetlands with a rising water table.  Group 3 seams are the youngest and thinnest, and show that they were deposited in quiet depositional environments with the prolonged existence of extensive peat-swamp conditions.  There is an abundance of fossil flora and fauna and the noticeable feature about these fossils is the complete absence of all traces of fish and marine life, indicating that the Triassic sedimentation of the Ipswich Basin was entirely terrestrial.

Around 205 million years ago, at the beginning of the Jurassic period, tectonic movements began breaking Pangaea into two continents (Australia being part of the southern continent of Gondwana), and the Ipswich Basin experienced uplift.  This tectonic activity was accompanied by extensive volcanic extrusions, and subsequent erosion of the surface of the Ipswich Basin.  This was followed by rifting and subsidence that created the adjacent Nambour and Clarence-Moreton basins to the east and south of the Ipswich Basin.  This resulted in the eroded upper parts of the Ipswich Coal Measures being overlain by thick layers of conglomerates, shales, sandstone, siltstone and clay formed in extensive fluvial and lacustrine conditions (the Bundamba and Marburg deposits, named after localities around Ipswich), indicating that the conditions were now less swampy, but still terrestrial.    

As the Jurassic progressed the tectonic plates were reactivated and began the process of separation of Gondwana, with the major continents of today beginning to drift away from each other.  It was at this time that the Australian continent was delineated.  By the close of the Jurassic and beginning of the Cretaceous, 144 million years ago, Australia was the only continent still joined to Antarctica, and about 80 million years ago the New Zealand land mass separated from Australia.  The rifting basically created the present east coast of Australia, but at the beginning of the Cretaceous period, the seas began to rise and large areas in northern and central Australia became inundated.  That is when the Great Artesian Basin was formed in central Queensland, but the southern part of Queensland formed a large island separate from the rest of Australia.  That is why no Cretaceous material is present in the Ipswich Basin.

The seas eventually drained away, leaving the landscape more or less as it is today.

Fossils of the Ipswich Coal Measures: Abundant plant fossils representing “Dicroidium” flora are present in the Ipswich Coal Measures along with rare freshwater bivalves, conchostrachans, and a variety of insects.  The two major fossil sites are the Denmark Hill Conservation Park and Haig Street Quarry Bushland Reserve (see Ipswich as a Scientific Name page) .

The Denmark Hill Conservation Park: This park is within walking distance from central Ipswich.  In the 1890s many well preserved insect fossils were discovered here, including those of dragonflies, locusts and mantids from the Upper Triassic period, in what became known as the Denmark Hill Insect Bed.  The fossils were found in the Blackstone Formation of the Ipswich Coal Measures, dated to the Carnian age (228 - 216 million years ago).  Also found in the Blackstone Formation are the only two definite reports of dinosaurs in the Triassic Period in Australia.  In 1964 several three-toed dinosaur tracks were discovered at the Rylance colliery in Dinmore, a suburb of Ipswich.  These were identified as those of a large theropod.  In 1983 at least six further footprints were found in siltstone and shale deposits of the Blackstone Formation at the colliery.  These have been attributed to the small bipedal theropod “Grallator”.  The existence of these tracks in a remote intermontane basin of eastern Gondwana indicates that theropod dinosaurs had achieved an extensive, possibly global, range as early as the Carnian age, whereas most other dinosaurs only evolved during the following Jurassic period.  These and other fossil exhibits are now on show in the area known as Triassic Park.

Haig Street Quarry Bushland Reserve, Brassall, Ipswich:  Important fossil flora occurs in a disused quarry at the end of Haig Street, Brassall, located 4 km (2½ miles) northwest of Ipswich city centre.  Formerly a sand and gravel quarry, this site was handed to the City of Ipswich in 1989 and converted into an open eucalypt forest reserve and environmental park.  Two rock units are exposed within the reserve.  The Late Triassic Tivoli Formation is exposed on the eastern wall of the quarry and contains rich fossil flora.  The site is located close to the West Ipswich Fault forming the western margin of the Ipswich Basin, and extensive movement along this fault has caused strata in the quarry to be tilted at a high angle.  Dip slopes of beds in the eastern wall of the quarry host an array of leaf impressions.  The fossil flora of the Tivoli Formation at this site is different from the Corystospermaceae foliage (Dicroidium) found elsewhere in the Ipswich Basin, in that it is dominated by leaves of conifers (Heidiphyllum), Petriellales (Rhochipteris), Ginkgoales (Ginkgoites) and Matatiellales (Dejerseya).

Ipswich Microflora: This is a specific term in palynology (the study of pollen, spores and other microscopic organisms) to describe a particular environment found in the Upper Triassic sediments laid down between 237 to 228 million years ago.  This term was applied by J H Dolby and B E Balme in 1976 when they became the first palynologists to demonstrate that the distinct differences between microscopic flora found in Triassic sediments were determined by the latitudes that they inhabited at that particular epoch.  Their studies were based on the fossil microscopic flora found in sedimentary deposits in Australia around Onslow, Western Australia, and Ipswich, Queensland.  

Dolby and Balme showed that a number of taxa (populations or groups of populations of organisms that share common characteristics) found in deposits in north west Australia and Timor, which they termed “the Onslow Microflora”, were basically the same as those found in northern Madagascar and western Europe, whereas taxa found in deposits of the same age in eastern Australia, termed “the Ipswich Microflora”, were different but the same as taxa found in deposits in Antarctica, New Zealand and southern Madagascar.  Not only did this provide further evidence that these lands were once united in a single continental mass, but it showed that the “Onslow Microflora” represented warm, temperate rain-forests found in latitudes between 30°S and 35°S, while the “Ipswich Microflora” represented a cooler, temperate climate found at higher latitudes of between 50°S and 70°S.  Mixed populations have been found in the intermediate latitudes.  The “Onslow Microflora” were deposited in marine and coastal margins of the continent, whereas the “Ipswich Microflora” represent sedimentation of a continental nature, deposited in intramontane basins in which “Onslow Microflora” is notably absent.      

Further studies elsewhere confirmed these findings, and the terms became accepted to describe the two major regional flora realms that existed in the Upper Triassic epoch.  Thus, similar environments found in Patagonia, South Africa, Antarctica and New Zealand are referred to as “Ipswich Microflora” regardless of the fact that the deposits are not laid down in proximity to that city. 

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