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Sea level controls on the geomorphic evolution of Geographe Bay, south- west Australia

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  • TotalEnergies

Abstract and Figures

High-resolution shallow seismic profiles collected along the inner shelf in Geographe Bay (south-west Australia) illustrate a highly-variable buried architecture. Three main acoustic units, separated by unconformities, correspond to different geological facies, deposited under various sea-level conditions. The acoustic basement (Unit B) belongs to the Lower Cretaceous Leederville Formation; the middle unit is attributable to the Tamala Limestone (Unit P, Mid-to Late Pleistocene) and the top unit (Unit H) is Holocene. Combining the seismic data with high-resolution bathymetry and sediment grabs, several surficial and buried morphological features are revealed, including sandbars, palaeochannels and ridges. The shore-oblique sandbars have been directly influenced by local hydrodynamics including mean wave direction and currents, benthic habitats such as seagrass, and sediment grain size. The palaeochannels (buried and surficial) are the expression of previous sea-level lowstands. Two sets of shore-parallel, low-relief ridges, at depths of ~7 m and ~20 m, are relict landforms that are most likely regressive beach ridges and sub-littoral deposits, belonging to the Tamala Limestone. These structures were formed during Late Pleistocene relatively high sea-level stages (late Marine Isotope Stages 5e and 5c, respectively, between 120 and 100 thousand years ago), cemented when the sea level was lower and subsequently subject to transgressive erosion. The newly acquired seismic datasets shows that the inner shelf is mostly covered by a veneer of sediment (with average thickness of 50 cm) above the Pleistocene hard surface, whereas sandbars can be up to 6 m thick.
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83
Journal of the Royal Society of Western Australia, 102: 83–97, 2019
© Royal Society of Western Australia 2019
Sea level controls on the geomorphic evolution of Geographe Bay, south-
west Australia
GIADA BUFARALE 1*, MICHAEL J O’LEARY 2 & JULIEN BOURGET 2
1 Department of Applied Geology, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
2 School of Earth Sciences, University of Western Australia, Crawley, WA 6009, Australia
* Corresponding author: giada.bufarale@postgrad.curtin.edu.au
Abstract


          

               
             
           

          
    
    
  
    

 

           
        

Keywords: Geographe Bay, Sandbars, Palaeochannels, Low-relief ridges, Sea level oscillations
Manuscript received 6 December 2018; accepted 27 August 2019
INTRODUCTION
       
       
       
       
      et al
    
      
       
      

        
     
      et al  
relict siliciclastic grains within the inner shelf are a
        

    

      
      
        

such as sea-surface temperature, nutrient availability
         

    
et al
     
        
       
et al
      
 
   

         
      
      
      
      
   
 
    
  


Journal of the Royal Society of Western Australia, 102, 2019
        
     
       
     et al  
      
       
et alet al
         
     
   et al  
     
      
       
         
     
      
et al     
         
100 km north of the survey area) by Skene et al 
      
        
     
       
       
    
      
  
       
        
        
       
     

 

REGIONAL SETTING
Coastal physiography
          
      

      
         
    
  
  

      
       
        
       
        
       
       
       
Figure 1.           et al   
      



               
          
 

85
et al
       
  Posidonia sinuosa  Amphibolis
antarctica      
      
et al
     

et al
       
      
          
       
        
    
  
        
     
   et al   
       
          
         
 et al      
       


et al
    
       
      
  et al    
     
      
      
       
    et al   
      
       
      et al
et al
Coastal Geology and Geomorphology
      
      et al
      
       
 
      
       
     et al 
et alet al
et alet al
     
      
        
marine, with minor shallow-water near-shore marine
  et al    
et al
     
         
      
      
         
      
      
       
     
       et
al      
 
      
     

     
     

  
        
et al
  ,     
       
         
        
         
        
         
       

      
et al
METHODS
Seaoor mapping
 
     
      
     
       
          
        

        
through a visual interpretation of the bathymetry,
      
      
    


which areas within the bay to carry out the following
        
      
        

         
   
         
        

High-resolution shallow-seismic data acquisition
        

  
      
        
86
Journal of the Royal Society of Western Australia, 102, 2019
       
     

   
     


  
      
        
      

     
      
      
     


       
      

      
        
       
        
       
         
et al
         
      
        


Sediment sampling and analysis
        
       
      
        


shaker, over 63 μm, 125 μm, 250 μm, 500 μm, 1 mm
        
      
        



RESULTS
Shallow stratigraphy
     
       
       
     


       



         
       
      
          
        
        
        
       
     
        

        


Figure 2.       
        
        
          
          
   
     
      
      
          
      
     
         
   
       
          

87
et al

         
        
       
        
         
        
       
      
   
       
        
        
  
          
        



         
           
     
       

      
        


       


Seabed features
High-resolution bathymetry highlights three main
     
      


 
        
          
       

        
        

       
          

          
       


     
       
   
        
        
Figure 3.  

  

Figure 4.       
        
     
      
        
     
  
        
       



88
Journal of the Royal Society of Western Australia, 102, 2019
       
         



 
        

      
          
       which are


       
  
         
      
       
       

Sediment distribution
        
        
       
         
of biogenic skeletal grains that are highly variable in
          
      
     
        


        
  
       
    
       
Figure 5.      

       
         
     
      
      
  
          
        

Figure 6.       
       
         
        
         
      
   
   
       
          
        
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et al
       
       

         
      
      
 

        

         
       
        
        
        
      
     
      



   


       
     

     
       

        


STRATIGRAPHIC INTERPRETATION
       
        
     
        
        
      
        
Figure 7.  
  
  
    
   
   
    
   
    
     


Figure 8.  
  
  

   
    
    
ranges mainly from coarse
     
     
    
represent stations where
   
   
line marks the location of the
    
     

90
Journal of the Royal Society of Western Australia, 102, 2019
       
       
      
   
et al
    
       
       
      
        
        

   
      
the shallow stratigraphy, up to 50 m below the Australian
      
    



Unit B: Leederville Formation
      
      
       
       
    
as the     
      
et al
      

     
      
      
         
      
           
    
      
      
       
       
      

Unit P: Tamala Limestone
        
         
      
       
       


        
    et al    


      
        
        et al
      
       
     

      
     et al 

et al
      
       
 
et al       

    

     
      


Unit H: Holocene unit

    
Figure 9.     
   
           
    
        
           
          
      
      
 

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et al
  

       
   et al    
      
   
et al
21 locations along southern Western Australia, from
       
        
       
  
        
         
   
composite Holocene sea-level curve for the Houtman
et al      
        
        
    


of relict siliciclastic grains, with a variable component of
   
     



        

   
        
    

        
        
       
       

        

     
      in situ biogenic
      

et al   et al     

       
        
 
      
       
        
  
       
et al   
      
et al 
      

GEOMORPHOLOGICAL FEATURES
         
        
     
      
 et al    
  et al      
          et
al       et al 

       
      
      
       
palaeochannels or topographic lows whereas finer
     
        
     
     
   


        
     
       
       
     
    
Figure 10.  
   
  
   
    
    
palaeochannel incisions are
   

92
Journal of the Royal Society of Western Australia, 102, 2019
      
        
        
       
       
       
      

Sandbars
       
         
     
          
      
 
 
      
     
        

      et
al         

   
      



       
       
        
       
        

          
     
       
      
 
       
     et al  
      

Palaeochannels
       
      
      
et al     
      
     
et al
       
      
     
        

    
    et al
      et al  
     
   
      

       
     
     
       
       
          
      
   
 
   
   
          
        
           
         
    
       
       
      
       


Figure 11.      

      

Figure 12.

   
         
   

93
et al
    
bathymetric composite images, are more numerous
        
     
        
        
       


         
        
et alet al
    
      

      
         
       
      
  
       
   
    
       
      
        
 
et al
Submerged low-relief ridges
     
       

        
      
       
    
          
et al
      

       et al 
     et al   
       
     
       
    
     et al  
      
     
        
          

 
 
       
       
       


          
       
 
   
        

Figure 13.
    

  
   
    
    

     
  
    
     
     
   
   
   
  

    
   


Journal of the Royal Society of Western Australia, 102, 2019
  
 
      et
al. 
        
       
     
       
analogous manner, when the sea level was 20–30 m below
  et al   et al  
  
    



    
      
        
        et al
      
        
      

       
        


       
       

CONCLUSIONS
     
    
       

      
       
     
     

      

 

        

       

      
     
       
  
       
     
        
    
       
   in situ accumulation
      

       
are:
     
      

    
     
      
features mostly lie in the south-western portion
        
       
       
     
     
     
Figure 14.        

        
      18  
      et al  
      
      


  
   
         
     
          
        
et alet al
95
et al
influencing the geomorphology in the inner
       

      
         


     
   
  
 
     

      
plains, an abrasion surface characterises the top
         




      


ACKNOWLEDGEMENTS
       
     
      
       
      Alec Hansen
III       
      
       
     
      
      
      


       
      

REFERENCES

        
Marine Geology 141
           
  
        
Marine Geology 163
         
        
         
Quaternary International 83–85, 257–273
            
      
      Journal of
Coastal Research 42
         in 
      
20th  
  th      
Engineers Australia.
     
    
   Australasian Coasts & Ports 2017:
1037
         
         
   Palaeogeography, Palaeoclimatology,
Palaeoecology 187
        
       
       
      
International Journal of Remote Sensing 31
           
       


Quaternary Science Reviews 93
            
     
     
    Continental
Shelf Research 134

     
      Marine
Geology 231
        

Marine Geology 359

      
Catena
153
       
   

         
       
  Journal of the Royal Society of Western
Australia 38


 
Earth and Planetary Science Leers 141,

         
     
     
   Italian journal of geosciences 131,

     


  
        
Geological Survey of Western Australia Annual Report 1972,

         
     
     
Australian Journal of Earth Sciences 31
          
    
Sedimentary Geology 60
           
    

Quaternary International 145
96
Journal of the Royal Society of Western Australia, 102, 2019
       
  Western Australia Geological
Survey
     
Australian Geomechanics Journal 38
           

       Quaternary Science
Reviews 163
       
      

         
      
Geological Survey of Western Australia 26
     
   


   Proceedings of Acoustics: Research to
     

         
     

        Physics and
Chemistry of the Earth 4
     
      
      


Australian Geomechanics Journal 42
          
        
       
Continental Shelf Research 24
         
     
Australian Journal of Earth Sciences 44,
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 
Quaternary Research 60,

           
  
    Quaternary Science Reviews 26,


  Geological Survey of Western Australia,
17
    
   Geological Survey of Western
Australia25
   
        
Sedimentology 60
    
        
Journal of Sedimentary Research 69,

Neritic carbonate sediments in a temperate
realm: southern Australia  
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An introduction to geophysical
exploration. 
          
       
Quaternary Science Reviews 10
 
Science 292,
    
         Nature 419,
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
      
      
     Australian
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   
     
      
Sedimentology 58
        
J           
   Journal of the Royal
Society of Western Australia Royal 80
          
       
Freshwater Biology 48,
         
      
Marine Geology 211
  
    
    International Association of
Sedimentologists Special Publication 44
Munsell soil color chart.

           

Continental
Shelf Research 24

Australian
Journal of Earth Sciences 36
            

   Continental Shelf Research
31
    
  
      

         
     
    
     

        
     
       
       

    
       
  Proceedings of Coasts and Ports 2011,

          
       
     
 Proceedings of Coasts and Ports 2015, 

       
        
    Australasian Coasts &
Ports 2017: Working with Nature
          
   in    
97
et al
    Managing the Coast 

      
Geological Survey of Western Australia Bulletin 124
        
    in    
    Geology and Hydrogeology of Carbonate
Islands, Developments in Sedimentology
          
      
    Quaternary Science
Reviews 20

    Geological Survey of Western
Australia

      
     
Underwater Technology 33
 Sedimentary environments: processes, facies and
stratigraphy
             
      
         
 
Sedimentary Geology 201
      

 Petroleum Geology 62,

         
      
     
HG31
            
       
Marine Geology 233
            
     
Geoscience Australia2005/10, 88
           
      
        
   Earth and Planetary Science Leers 135,

          
   Marine Geology 29 

          D J
       
        
Marine Geology 348
           
      
Marine Geology 275,

           
      
      

         
Western Australia Geological Survey,
Annual Report
         
       
Western Australia Geological Survey1982/2
          
      

        
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Geological Survey124
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39
... While it is becoming well documented that nonreefal accumulations, such as stacked aeolianites and beachrocks, also have the ability to form bathymetric highs on the modern seafloor (e.g., Brooke et al., 2017;Bufarale et al., 2019;Green et al., 2020;Lebrec et al., 2022aLebrec et al., , 2022bO'Leary et al., 2020;Passos et al., 2019) and can misleadingly exhibit reefal morphologies in seismic-reflection data (Bubb & Hatlelid, 1977;Salzmann et al., 2013), pre-Quaternary carbonate aeolianites and other relict coastal features are rarely documented in the geologic literature (e.g., Abegg & Handford, 2001;Dodd et al., 2001;Kindler & Davaud, 2001;McKee & Ward, 1983;Smith et al., 2001), and non-reefal carbonate buildups are seldom described by seismic interpreters. This is particularly puzzling given the ability of drowned coastal features to exhibit buildup morphologies and to form both carbonate and siliciclastic barrier complexes-composed of beachrocks, aeolianites and other coastal sedimentary deposits preserved through early cementation-forming seafloor ridges enclosing lagoons, bays or estuaries (e.g., Alcántara-Carrió et al., 2013;Brooke et al., 2010;De Falco et al., 2015;Gardner et al., 2007;Lebrec et al., 2022a;Locker et al., 1996;Mellett et al., 2012;Passos et al., 2019;Sade et al., 2006;Wenau et al., 2020). ...
Article
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Linear buildups formed in tropical carbonate environments are often interpreted as bioconstructed reefs. Nevertheless, coastal processes can also form extensive sedimentary ridges exhibiting buildup morphologies. This study investigates two Miocene ridges developed along the Australian North West Shelf using 3D seismic and well data. Ridge 1 is ca. 30 m thick and >60 km long, and it is made of foraminiferal pack‐grainstones. It protects a lagoon with pinnacle morphologies. Ridge 2 is ca. 150 m thick and >80 km long. It is composed of quartz sand forming lobes. Both ridges have a continuous curvilinear front and are in a mid‐shelf setting. They mimic the modern Australian coastline. It is then proposed that Ridge 1 is either: (1) a barrier reef developed on a drowned shoreline, or (2) stacked carbonate aeolianites and beachrocks acting as a barrier. Ridge 2 is interpreted as stacked deltaic sands. This study demonstrates that lithified and buried coastal features of carbonate and siliciclastic nature can form extensive ridges exhibiting buildup morphologies. It is proposed that ridges formed by stacked coastal features are overall continuous with a curvilinear front, while reefal ridges are more discontinuous and exhibit deeper and more stable passes.
Article
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The Busselton area in southwestern Australia is characterised by three distinct coastal plains along the foot of Whicher Range that formed mainly by marine attrition during progressive sea-level falls through the Cenozoic and, to a lesser extent, erosion by long-lived rivers. None of the geological and geomorphic units, all of which overlie the Lower Cretaceous Leederville Formation, show evidence of tectonic tilting so, in the absence of masking dune systems, this area offers a reference for Cenozoic global sea levels. The oldest landform, a marine erosional surface at 112–166 m ASL on Whicher Range, is a remnant of the Blackwood Plateau capped by in situ laterite of likely Eocene age. The Whicher Scarp, with relief of about 120 m, formed by marine erosion removing much of the Leederville Formation during a progressive Eocene–Miocene sea-level fall (~43–13 Ma). At 72–83 m ASL, the Yelverton Bench represents a probable Miocene stillstand during this fall. The scarp below this bench is characterised by a piedmont laterite lithologically and spatially distinct from the older plateau laterite on Whicher Range. The toe-line at 41 m ASL marks the base of the Whicher Scarp and the beginning of the coastal plains—it represents the geomorphic expression of a buried Pliocene (~2.8 Ma) erosional surface at 29 m ASL. At 21–41 m ASL, the Ambergate Plain is a terrestrially re-sedimented marine erosion surface covered by continuous strand facies of the upper Pliocene Yoganup Formation, which in turn is overlain by lateritized clay that may correlate with the Pliocene–Pleistocene Guildford Formation. The main heavy-mineral strands formed as ancestral shorelines and are embedded within the Yoganup Formation across the entire Ambergate Plain. The Cemetery Scarp, with a relief of 11 m and associated erosion surface at 5 m ASL, cuts into the Ambergate Plain and probably formed during an early Pleistocene interglacial highstand, possibly MIS 11 (~400 ka). The Ludlow Plain at 3–5 m ASL is covered by low eolian swales and ridges of shelly calcareous sand up to 6 m thick containing coral fragments of possible MIS 5e (~124 ka) age attributed to the Tamala Limestone, which marks the beginning of marine platform carbonate production. The Busselton Wetland Plain was formed during a gentle recession after the Holocene highstand at 7.5 ka following the last glacial maximum and is recognised from a low scarp on the seaward edge of the Ludlow Plain. Although the Capel River has a history spanning the last 30–40 Ma, most rivers draining the scarp postdate the Pliocene. The build-up of barrier beach dunes during the last 7500 years next to the present coast has diverted rivers on the Wetland Plain and forced outlets to Geographe Bay to migrate laterally. Lateritization was episodic, principally in the Eocene, Miocene and Pliocene, but after the deposition of the Guildford Formation, did not extend through the Pleistocene or Holocene.
Article
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Palaeoshorelines that lie submerged on stable continental shelves are relict coastal depositional and erosional structures formed during periods of lower sea level. An analysis of the well-dated Late Quaternary (0–128 ka) sea-level record indicates that the most persistent (modal) lower sea levels were at 30 – 40m below present, which occurred between 97 and 116 ka and at approximately 85 ka and 10 ka. A secondary modal position was at 70–90m that occurred mostly during a period of fluctuating sea level between 30 and 60 ka, as well as at around 87 ka (70 – 80m only) and 12−15 ka. For the tectonically stable Australian continental shelf, we show that a range of shorelines formed at each of these sea level modal positions and their morphology and degree of preservation depends on composition (carbonate vs siliciclastic) and oceanographic setting (wave, tide and wind energy). These ancient coasts record a range of oceanographic and geological regimes that existed during relatively long periods of lower sea level and provide a guide to the general depth zones in which similar features likely occur on other shelves globally. Australian palaeoshorelines represent distinctive benthic habitats that strongly influence the distribution of biodiversity across the shelf. Accurate mapping of these features provides a robust geospatial framework for investigations of marine species distributions and environmental change monitoring. These data also enable the better targeting of relict coastal areas that potentially include sand resources and sites of human occupation during periods of lower sea level.
Conference Paper
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Coastal infrastructure projects such harbours and marinas require the construction of breakwaters to maintain a stable entrance channel but they also interrupt the alongshore transport of sand resulting in accumulation of sand on the upstream side of the breakwater. Some developments have installed sand bypass systems which artificially pump sand across the entrance. There are many successful projects which have been constructed and operational using this approach. In some cases they have been unsuccessful and remedial measures had to be undertaken and lessons learnt. This paper presents a case study of an unsuccessful system constructed on the above principles. Port Geographe, a marina and residential canal estate development was developed in the early nineties. The breakwaters were designed to prevent sand bar formation at the harbour entrance channel with a concentrated water flow through the channel to maintain navigable depth, and also to avoid high wave action within the channel itself. A sand bypass system was designed to pump sand across the harbour entrance from the west (upstream) to nourish the beaches in the east. However, the artificial bypassing has become a challenging issue due to the presence of sea grass wrack, which was more efficiently trapped by the sand trap than sand. Up to ~100,000 m 3 of wrack is trapped on the beach to depths of several meters and 1-2 km in length. Wrack accumulation along the breakwater and the adjacent beaches resulted in severe environmental problems (e.g. odour, beach use) on the western side of the development and erosion of beaches to the east at Wonnerup. Solutions to alleviate this problem were developed through the development of a seagrass wrack transport model linked to an existing morphological model (MIKE-21). An optimum configuration for the coastal structures, to promote natural bypassing of sand and sea grass wrack, included: an angled groyne on the western side of Port Geographe as a replacement to the existing breakwater, which is perpendicular to the shoreline and removal of groynes associated with the pocket beaches by a foreshore seawall. The model simulations guided a reconfiguration of the marina entrance which was implemented in 2014.
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Estimates of marine isotope stage (MIS) 5c and 5a global mean sea level (GMSL) based on marine terraces and coastal indicators range from 15–37 m and 9–30 m below present, respectively. Tectonic displacement and glacial isostatic adjustment (GIA) complicate efforts to refine this range. We revisit this issue using numerical predictions of post-glacial sea-level change and updated estimates of site-specific tectonic signals based upon these predictions. Laurentide and Cordilleran peripheral bulge dynamics and variations in the gravitational effect of these ice masses dominate the GIA signal along the east and west coasts of North America. Published compilations suggest that MIS 5 sea-level indicators extending from Virginia to the Caribbean lie on the outer flank of the peripheral bulge, while those along a transect from Oregon to Baja California sit on the inner flank of the bulge. Our GIA modeling reconciles these data by adopting a significantly weaker (i.e., lower viscosity) upper mantle in analyzing data on the Pacific coast relative to the Atlantic, an inference supported by seismic tomography. We present a sensitivity analysis that compares the observed elevation of globally distributed high stand markers to a suite of GIA simulations that vary the Earth model and GMSL from the Last Interglaciation (MIS 5e, ∼130-115 ka) to 70 ka. We conclude that GMSL peaked at −8.5 ± 4.6 m (1σ) during MIS 5a and −9.4 ± 5.3 m during MIS 5c. A more restrictive analysis yields slightly wider bounds corresponding to −10.5 ± 5.5 m and −11.1 ± 6.6 m, respectively.
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High-resolution seismic profiles were conducted across the metropolitan area of the Swan River estuary (Perth, Western Australia) to explore the sub-surficial stratigraphic architecture, down to a depth of about 40 m below the river bed. The acoustic profiles revealed a complex system of palaeochannels where three main unconformities (R1, R2, R3) bound as many seismic units (U1, U2, U3), over the acoustic basement. Integrating these data with sediment borehole analysis, LiDAR data and available literature of the geology and stratigraphy of the area, it was possible to determine the development of these stratigraphic units, in response to Late Pleistocene and Holocene sea level fluctuations and conditioned by pre-existing topography and depositional palaeoenvironments during the last ~ 130,000 years. The deepest unit (U3) can be interpreted as the Perth Formation, which consists of interbedded sediments that were deposited in a large palaeo-valley downcutting into the underlying acoustic basement (bedrock: Tamala Limestone and Kings Park Formation), under a fluvial to estuarine setting, existing between ~ 130 and 80 ky BP (in the Last Interglacial).
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Groundwater is important to the city of Perth for public water supply, irrigation of parks and gardens and horticulture and about one in four houses has its own bore for garden watering. The superficial aquifer receives stormwater drainage and is easily contaminated from accidental spills or waste disposal. It supports perennial wetlands, and has been drained in areas of urban development with shallow water tables. The confined aquifers are used mainly for public water supply and are used conjunctively with the unconfined groundwater and surface water. The Gnangara Mound, which is the main resource in the superficial aquifer and also recharges the confined aquifers, is protected from urban development and is covered by native woodland or pine forest. Groundwater in the crystalline rocks of the Darling Range is limited to low yielding bores and wells for gardens or orchards.
Book
This book is the first comprehensive documentation and interpretation of modern neritic carbonate sediments on the southern Australian continental margin, the largest cool-water carbonate depositional system on the globe. The approach is classical but the information is new. A brief chapter of introduction is followed by a section that describes the setting of the continental margin in terms of the regional geology, its evolution through time, the climate, and the complex oceanography. The setting is further explored in chapter 3 that outlines the Pleistocene history of sedimentation in this region. This is particularly important since many of the surficial sediments have a partial older history. The following section on the carbonate factory describes in detail the nature of the animals and plants that determine the nature of the sediments and the environmental conditions that control their distribution. The shelf itself cannot be discussed in isolation and thus a short chapter on the marginal marine environment is presented. The core of the book comprises two chapters that document the suite of depositional facies and their composition and then the suite of depositional environments where these sediments are found. The variety of deposits in this vast area is such that three chapters are devoted to the character of the materials on the southwestern shelf the south Australian sea and the southeastern shelf. The diagenesis that affects these sediments is tackled in a chapter after all the attributes are documented because they are intimately linked to different controls. The book finishes with a summary chapter that also addresses the various controls on sedimentation and models the effects to be expected when these are changed outside those present in the current realm. Audience: The book is an invaluable source of information about this vast region and will be a critical reference for researchers, graduate students, and professionals engaged in marine and environmental research. It will be of particular importance for geologists interpreting the ancient rock record.
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Geographe Bay, southwestern Australia, is a shallow open embayment with sandy substrata dominated by the seagrass Posidonia sinuosa, which has greater than 60% cover of the bay. The surrounding agricultural catchments are sandy and low-lying, with extensive drains contributing anthropogenic nutrients (255 tonnes of N and 34 tonnes of P seasonally) to Geographe Bay. Potential for eutrophication of this system and the subsequent effects on the biota, particularly the health of seagrass communities, has been of concern by local residents. This study documented physical parameters in the water column and the distribution and status of the benthic biota from 1993 to 1995. There were seasonal changes in water temperature (ranging between 14.8°C in winter to 21.6°C in summer), and irradiance (ranging between 0 in winter to 850 μmol m-2 s-1 in summer). These changes followed expected seasonal patterns in a mediterranean climate. Above-ground biomass of the dominant seagrass Posidonia sinuosa (115 to 470 g m-2) was similar to that reported for unpolluted systems in southern Australia, as was epiphyte load (0.1 to 26 g m-2). Geographe Bay showed few symptoms of eutrohication despite seasonal increases in nutrient loads.
Article
There is a similarity in the pattern of Holocene sea-level change at Rottnest Island and a number of other locations in southern Western Australia. Rottnest Island was one of the field study areas that Fairbridge (1961) used to develop his fluctuating Holocene sea-level curve. Eight 14C ages obtained from fixed shell material of Serpulid tubeworms from tightly constrained inter-tidal zones are presented from Rottnest Island, and thirteen more from a transect extending 1000 km along the coast of southwest Australia. The age-elevation profiles show a consistent mid- to late-Holocene sea-level fluctuation over the entire southwest Australian region, which argues against a strong local tectonic or a broader hydro-isostatic influence. A comparison of the southwest and southeast coasts of Australia, based on fixed inter-tidal biological indicators obtained by similar methods, indicates a broadly similar mid- to late-Holocene oscillating sea-level curve up to two metres higher-than-present. Peak levels are preceded and followed by sharp ∼1 metre falls, which appear to be associated with cooling sea temperatures at ∼5200 and ∼3800 cal years BP. Both rising sea levels and warmer-than-present sea surface temperatures occur at all locations at ∼4200 and ∼6500 cal years BP, with a net sea-level fall to the present, but a slight warming beginning at ∼1400 cal yr BP. There also appears to be a common pattern of species change suggestive of synchronous climatic variations. These mid-latitude fluctuations and environmental changes on the Australian coastline are similar to some events over the same period in the equivalent far-field locations of southern Brazil and southeast Asia. The synchronicity of these oscillations and their climatic underpinnings, in mid-latitude sites far from the complications of glacial isostasy, is not inconsistent with Fairbridge's (1961) basic thesis of world-wide eustatic Holocene sea-level change.