• Abstract and Figures
• Public Full-text

Rocks and Landscapes of Maryborough & Hervey Bay Warwick Willmott FOUNDATIONS OF THE FRASER COAST Rocks are not particularly obvious around Maryborough, Stretching and fracturing of eastern Queensland seems but they do occur below the surface and they have to have stopped about 90 my ago, but parts of the crust controlled the landscape and important economic resources east of the fracture zone sank to form the current offshore of the district, so it is important to understand their story. Queensland and Marion Plateaus, with associated small The rocks also provide a rare and fascinating insight into basins within them. In contrast, stretching continued east a period when the eastern side of the Australian continent of New South Wales, leading to the opening of the Tasman was beginning to fracture and break apart, preceding the Sea and the creation of oceanic crust on its floor. When this opening of the Tasman and Coral Seas. opening eventually extended northwards about 60 to 50 1. FIRST, THE BIG PICTURE my ago, the stretching and fracturing was to the east of the older zone, and formed the Cato Trough and Coral Sea, to The rocks around Maryborough were deposited in a the east of the Queensland and Marion Plateaus. sinking depression known as the Maryborough Basin, which To the west of Childers are various older rocks of the eastern extended eastwards from Childers and Tiaro to well offshore side of the continent – these have a complex history and are of Fraser Island. This was about 140 to 100 million years beyond the scope of this particular leaflet. ago (my ago) in the late Jurassic to early Cretaceous periods. Before then the Australian continent extended much farther eastwards than at present. Then about 140 million years ago something began to stir deep beneath the crust, possibly an upwelling of material or heat from the Earth’s mantle. Certainly there was sufficient heat to melt parts of the crust to form blobs of granitic magma, which rose upwards and higher in the crust to solidify again as small bodies of granitic rocks. These were intruded particularly in the Townsville-Whitsunday-Mackay hinterland, and in a line extending from north of Bundaberg through Mount Bauple and Noosa to the Brisbane Valley. They were of varying compositions and were later exposed at the surface by erosion of overlying materials. The next stage seems to have been the fracturing of the crust into sinking ‘fault’ blocks along an elongate zone extending from east of Cairns to at least southern New South Wales, as the crust began to be stretched apart. The sinking of the Maryborough Basin was part of this process. The fracturing was accompanied by extensive heating and melting to produce magmas that were erupted at the surface in violent volcanic explosions, from 130 to 90 my ago. Examples are the volcanic rocks of the Whitsunday Islands, islands offshore of Rockhampton, volcanic rocks in areas west of Rockhampton, and the volcanic Grahams Creek Formation, which composed the first rocks laid down in the Fracturing of the eastern side of Queensland in two stages Maryborough Basin. from 140 to 50 million years ago. 140 to 120 million years ago. Stretching and melting of crust, 120 to 110 million years ago. Subsidence of Maryborough intrusion of granitic bodies, eruption of volcanic rocks. Basin continues, sediments deposited in it. 2. THE INTRUSIONS southeast, implying land still existed to the east. The small intrusions of the Jurassic-Cretaceous granitic The total thickness of rocks in the Basin is at least 6500 m, rocks form a northwesterly trending line just to the probably as a result of the deep sinking of the basin along west of the margin of the Maryborough Basin. They a fractured continental margin. This represents one of the extend from Gunalda in the south, through Bauple and thickest accumulations of sediments of this age in Australia. Mungar to southeast of Childers. They have quite variable The rocks have been affected by later compression and compositions but many are grey hornblende micro-diorites crumpling from a northeasterly direction, and now outcrop and micro-granodiorites, with some micro-granite as well. in several large and many minor anticlines (up folds) and Micro refers to their small grain size. They are usually synclines (down folds). Some of these can be seen at Point resistant to erosion and form small hills, but these are not Vernon (see title photo). Over a long period of time after easily accessible for inspection by the public. The Mount the rocks were exposed at the surface they were eroded and Bauple Syenite for example outcrops only on the higher planed off to a subdued surface sloping gently eastwards. slopes of that mountain. Because of their compositions they 4. A LATER COVERING give fertile soils, and many of the hills are clothed in dry rainforest or vine scrub. Most of the rocks of the Maryborough Basin are obscured by relatively thin layers of later rocks. The most significant 3. ROCKS OF THE MARYBOROUGH BASIN of these is the Elliott Formation, which comprises The Grahams Creek Formation encompasses the first rocks conglomerate, sandstone, siltstone, silty mudstone and shale to be deposited in the Basin as it began to subside in latest up to 34 m thick, which were apparently deposited on a Jurassic to early Cretaceous times. Volcanic rocks were river or delta plain sloping east. The age of these sediments erupted in fault blocks along the western margin, and remains uncertain; most seem to be younger than ~23 my, their vents may have been associated with the intrusions of but some layers could be as old as 55-65 my. Some people granitic rocks just to the west, although this is not certain. consider them to be the onshore fringe of the marine Andesite seems to have been common in the south (such Capricorn Basin, one of several small basins which subsided as can be seen in road cuttings at Glenwood), but rhyolite down the length of southeastern Queensland in the early and rhyolitic welded tuff (from violent explosive eruptions) Paleogene (early ‘Tertiary’, 65 to 55 my ago). This was as a predominated in the north. There were also tuffs, which result of tension in the crust related to the opening of the settled from ash from explosive eruptions, and various Coral Sea to the east. The Capricorn Basin is now situated sediments washed from the volcanic terrain into adjacent on the edge of the continent north of Fraser Island. Basalt streams. The rocks outcrop in a belt along the western lavas were erupted into some of these basins and some of margin of the Basin, and discontinuously in the Bauple- these outcrop as red soils at Nikenbah southeast of Pialba, Glenwood area where they probably flowed down an old and southwest of Burrum Heads. valley. They did not continue to the eastern margin of the 5. DEEP WEATHERING Basin as they are absent under Fraser Island. However they extended at least as far south as Double Island Point near To complicate matters, both the rocks of the Maryborough Rainbow Beach, where hard grey tuffs and lavas of andesite Basin and of the Elliott Formation (and some older rocks composition form the headland. as well) have been subjected to episodes of deep weathering more than once during the Cenozoic. Such weathering Maryborough Formation. There was a hiatus after the results in a reddish-brown cap of iron-oxide rich material, volcanic episode, but soon sediments began to be deposited sometimes hardened, developed over a soft, bleached, white along river valleys, and then under shallow marine or white-red mottled layer. This is aduricrust or laterite conditions as the Basin sank and the sea advanced from the ‘profile’, and it forms by wetting and evaporation over east. The sediments consolidated to mudstone, siltstone, sandstone, conglomerate and limestone, The Burrum Coal Measures began to be deposited after the sea retreated again and conditions changed to river deltas. Rocks deposited at first were sandstone, siltstone and mudstone, and then finer-grained sediments with seams of coal were formed. These were later mined in the Burrum Coalfield around Howard. Textures in the rocks suggest 65 to 23 million years ago. Deposition of blanket of sands that the sediments were sourced from the northeast and and gravel across landscape on margin of Capricorn Basin. Thick iron-oxide rich surface horizon of a laterite, or deep- Laterite weathering profile developed on lower land surface weathering profile, on an old higher land surface at Childers. on Maryborough Formation, Yengarie, Mungar road. repeated seasonal cycles, with insoluble iron and aluminium have been at their peak, much of the floor of Hervey Bay oxides remaning at the surface, and other elements being and Great Sandy Strait has been dry land. However at times leached out at depth by groundwater. of warm climate and high sea levels, marine sand washed up The oldest and best developed profile seems to have the coast and into Hervey Bay has been moved shorewards developed on an old higher-level land surface, the remnants to build up sets of sand ridges behind the beaches. of which now form the Childers plateau, extending to A set of such ‘beach ridges’ which accumulated during Cordalba to the north. Here a thick ferruginous surface the last high sea level 120 000 years ago (Pleistocene) is layer has formed above a bleached layer developed in a preserved behind the coast at Woodgate and Burrum Heads. variety of older rocks. The plateau continues to be eroded A second set of ridges related to the present sea level, which by gullies around its fringes.

Load more

  6

Copy Link