The Origin Of The Twelve Apostles Section,in Australia

The Twelve Apostles as a well known landmark,were located within the Port Campbell National Park,south coast of Victoria,Australia.The stratum of coastal cliffs is generally considered to be deposited in a moderate to high-energy continental shelf environment during the Miocene Period.They indeed show weathering characteristics of Karst,such as stalactite,stalagmite and clint in some locations.However,aeolian pedogenic features are observed from this sequence,such as:1)Plant root fossils are commonly found in such beds;2)Many CaCO3 nodules were deposited underneath these beds.In order to understand the origin and sedimentary environment of Port Campbell Limestone,one section from the Twelve Apostles was sampled.Rock magnetic parameters,grain size,morphology of quartz grains and rare earth elements are measured and studied.The following conclusions can be drawn:(1)The results of rock magnetism show that the magnetic susceptibility of the Twelve Apostles section is relatively low.In the 30-35m of the strata,the magnetic susceptibility is relatively higher but the values of S_ratio and F300 are very low.These demonstrate that the weak magnetic minerals are predominant in the 30-35m of the strata,mainly hard magnetic minerals(goethite,etc.).The rest of the layer of magnetic susceptibility is relatively low but the values of S-ratio and F300 are relatively high,which indicate anti-magnetic minerals(calcium carbonate)and paramagnetic minerals are dominant and only contains a small amount of magnetic minerals(magnetite)in the rest of the strata.(2)Particle size,morphology of quartz grains,Rare Earth Elements(REE)were measured and compared with those of typical Quaternary aeolian loess-palaeosols and modern beach sand.The particles size of the whole section shows uniform fine feature and smaller than 100μm.Silt fractions of Port Campbell Limestone are constantly dominant with the median grain size varying from 3.014 to 20.702μm.The C-M,A-M and L-M plots of grain size distribution of the Twelve Apostles section are similar to those of loess and palaeosols.Its particle characteristics of Port Campbell Limestone differs from those of modern beach sand,however very close to those of loess and palaeosols according to Sahu’s empirical judgement equation(the Y-values of eolian deposits using the equation should be smaller than-2.7411).Scanning Electron Microscopy(SEM)analysis demonstrations that most of the quartz grains from Port Campbell Limestone have irregular and angular shapes and many are characterized by sharp edges and conchiform fractures.These morphology features of quartz grain are considered as wind-blown characteristic.The REE patterns of Port Campbell Limestone show extreme homogeneity with those of Chinese loess-paleosol in Xifeng.These samples enriched LREE and fairly flat HREE profiles,are also the REE properties of the upper continental crust(UCC).These results indicate that the sediments of Port Campbell Limestone were all derived from well-mixed sedimentary protoliths,as are characteristic of eolian deposits.(3)Previous studies revealed that climatic condition surround Lake Eyre Basin had a clear change from Middle Miocene to Late Miocene,marked by vegetation reduction.Aridification in Lake Eyre Basin during this period may provide an aeolian source to deposit silt dust in Port Campbell and down wind area.16.0 Ma to 5.3 Ma dust record from the Twelve Apostles,indicating the aridification in Australia began before 16.0 Ma.(4)Although Port Campbell Limestone indeed show weathering characteristics of Karst,We thus suspect the Port Campbell Limestone was unlikely deposited under an environment of continental shelf,but developed in continent as topsoil environment mainly.Some beds are tilted to follow paleotopography,indicating their original topsoil position.Since Miocene,climate change between warm and cool has occurred many times,leading sea-level changes.This could be one of the reasons to interpret the aeolian sediment and pedogenic developed by coast also containing shells and well-preserved shallow or open-marine microfauna in some strata positions.