Rocks and Landscapes of Maryborough &

Warwick Willmott

foundations of The Fraser Coast Rocks are not particularly obvious around Maryborough, Stretching and fracturing of eastern 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 , 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 . 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 -Whitsunday-Mackay hinterland, and in a line extending from north of through Mount Bauple and Noosa to the 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 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 , islands offshore of , 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 in the south, through Bauple and thickest accumulations of sediments of this age in Australia. 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 TheGrahams 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 , Glenwood area where they probably flowed down an old and southwest of . 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 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 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. peaked about 7000 years ago, lines the shore line in front of A lower, thinner and probably younger laterite profile the older ridges (the present sea level is about 1-2m lower is developed on the Elliott Formation and rocks of the than the former and hence the older ridges are preserved). Maryborough Basin to the east. Areas where the ferruginous Younger ridges also line the shore line at Dundowran and cap remains are shown as ‘duricrust’ on the attached map, Torquay. but where the capping has been removed many of the rocks The Pleistocene ridges are considerably eroded and are now are still leached and softened to white or mottled clays, and merely a sand plain covered by coastal heath and sugar cane. it is difficult to discern the parent material beneath. The younger ones are more recogniseable as ridges separated 6. LATER VOLCANIC ROCKS by hollows (swales) of swampy vegetation. There was substantial volcanic activity just offshore from the On Fraser Island a major sand mass has built up through district about 30 my ago later in the Cainozoic, as volcanic many cycles of sea-level change. As seas were rising, sand rocks of this age (trachyte and breccia) are exposed on from the continental shelf was washed landwards, where it Waddy Point on Fraser Island and have been encountered in built up behind the anchoring headland of Waddy Point. a bore sunk beneath Sandy Point. These may be related to From there strong winds blew it up into high dunes, a ‘hot-spot’ volcano active as Australia drifted northwards. particularly in drier windier times. The island is now a Other small areas of basalt were erupted later at Dundowran complex of overlapping and partially reworked dunes of and near Childers (~10 my ago) as well as near Goodwood, various ages. More on the history and landscapes of the without any clear reason for the activity. island is given in the Society’s book Rocks and Landscapes of 7. GREAT ACCUMULATIONS OF SAND the National Parks of Southern Queensland. Along the major rivers alluvium has accumulated in several Over the last 400 000 years or so there have been stages shown by terraces of differnt heights. Near the coast considerable fluctuations in sea level resulting from global estuarine sediments interfinger with this alluvium. Ice Ages tying up sea water in ice sheets. When the Ice Ages Landscapes Much of the Grahams Creek Formation along the western on the extent of deep-weathering profiles. Similar margin of the district has not been greatly affected by the landscapes are developed on the soft rocks of the Burrum deep weathering profiles, and forms gently undulating Coal Measures which do not outcrop except on wave- cut country. Because of the compositional variability of its platforms around Point Vernon at Pialba. volcanic rocks, the soils also tend to be variable. In the The old Childers-Cordalba plateau is relatively flat but is south around Glenwood and Bauple where andesite being dissected around its margins. The soils on its thick predominates soils are quite fertile and support vine scrubs, ferruginous capping have supported sugar cane in the but where rhyolite is dominant the fertility of the soil is past (some loss of fertility was experienced) but now are much less. increasingly used for macadamias, avocados and vegetables. In contrast the Maryborough Formation forms flat, poorly The covering of the Elliott Formation over much of the drained plains, except for several prominent ridges on coastal plain forms low, gently sloping country of little beds of hardened siltstone south of Hervey Bay between relief, from Tin Can Bay to Maryborough and Howard. Dundowran and River Heads. These elevated hills with Soil fertility is closely connected to the presence or absence panoramic views have been popular for closer settlement. of a ferruginous capping in the weathering profile and its Elsewhere soils are of poor to moderate fertility depending thickness. Soils on these layers contain more iron and other thickness. Soils on these layers contain more iron and other capping are pale and far less fertile. Some are poorly elements and are better drained, and as a result have been drained. the mainstay of the sugar cane and horticultural industries The older, degraded beach ridges near the coast also have (although they are still not as fertile as reddish soils soils of reasonable fertility and have been used for sugar developed on basalt). Soils on areas beneath the ferruginous cane. useful materials Thin coal seams occur in the middle section of the Burrum Rock for crushed aggregate and road gravels is quarried Coal Measures and were worked in several underground at two sites in the basalt near Dundowran, and one near mines at Howard and from 1866 to 1997. The Goodwood east of Childers. Rock is also produced from coal was used mainly in the old Howard power station and andesite of the Grahams Creek Formation at Antigua south for gas making. There has been renewed exploration interest of Mungar. for this coal in recent years. Sand and gravel were previously dredged from the Mary The thick accumulation of sediments in the Maryborough River near Maryborough. Fine sand is obtained from the Basin has encouraged petroleum exploration and some wells old sand ridges near the coast between Pialba and Burrum have been drilled in the past. Although the sediments are Heads. Clay for brick and tile making was obtained from considered mature for petroleum generation and traces of weathered rocks of the Burrum Coal Measures, leached gas have been recorded, the rock sequence does not appear zones of weathering profiles and alluvium west of the city, to contain suitable reservoirs. but the brickworks are now closed. looking at the rocks Because of soft rocks and deep weathering profiles, composition. South of Bauple on the fresh outcrops of typical rocks are rare, and even road cuttings andesite is exposed in several major cuttings near Glenwood reveal mainly weathered materials. However the few places but these are too dangerous to inspect. descibed below give some appreciation of what the rocks are TheMaryborough Formation does not usually outcrop, but like. cuttings and excavations commonly reveal beds of indurated Volcanic rocks of the Grahams Creek Formation can be seen (hardened) white siltstone dipping at gentle to moderate in road cuttings west and south of the Oakhurst railway angles. South of Oakhurst railway crossing on the Mungar crossing west of the city. On the road about road the old quarry now hosting the Yengarie Transfer 10 km west of the railway, a cutting shows finely bedded, Station shows a laterite profile (photo, page 3) developed friable, purple tuffs or tuffaceous sediments of andesitic on siltstone. Fresh purple-grey siltstone is exposed in the composition on the east, overlain by thicker, coarser fine drain at the junction with the main road. Inclined, bedded sandstone, and then by a thick, dark green-grey andesite siltstone is well exposed in the Bruce Highway interchange lava on the west This has irregualar small patches of ?zeolite with the Biggenden road but is too dangerous to inspect. throughout, and shows typical onion-skin weathering. Here it is slightly leached with the begining of a ferruginous The next significant cutting (past the road) shows the layer on top. dark andesite on the east, abutting a light grey, coarse tuff in Bedded white siltstone is exposed in cuttings on the the remainder of the cutting. This tuff shows aggregations Maryborough-Hervey Bay road north of the about 2 cm across that may have been caused by raindrops and north of the Nikenbah turnoff. It is also exposed on falling through the ash/tuff cloud. The junction with the the Urangan-River Heads road just past the turnoff to andesite is vertical and may be a fault. Booral East (photo). At the ferry landing at the end of this On the Mungar road, closely cleaved fine-grained purple road beds of fine-grained sandstone outcrop around the rhyolite is exposed in a cutting past houses nos. 862 & shoreline just above the mangroves. This sandstone contains 863. Just past Pine Hills Road on the right a lower cutting irregular flecks and fragments of black shale (photo) and exposes weathered cream tuff or welded tuff of rhyolitic is interpreted to be the result of ‘bioturbation’, or mixing

Thinly bedded andesitic tuff or tuffaceous sediments of Andesite of Graham Creek Volcanics on Biggenden road Grahams Creek Formation on Biggenden road. showing onion-skin weathering. Bedded hard siltstone of Maryborough Formation on River Fine-grained sandstone of Maryborough Formation showing Heads road. ‘bioturbation’. At ferry landing, River Heads. of sediments on the sea floor by marine organisms. The limb of the major Urangan Syncline. The varying trends beds strike (trend) parallel to the peninsula showing that of the beds are well displayed on the Google Earth image the strata are defining the topography in this area. Bedded of Point Vernon, which seems to have been taken at mid- white siltstone is also revealed in a cutting at Takura tide. East of the boat ramp there is a small cliff at highwater Heights on the Takura-Torbanlea road and in cuttings on mark, whose vertical face displays finely interbedded the Dundowran- road. siltstone and carbonaceous shale (photo). TheBurrum Coal Measures do not outcrop except around Very few exposures of the Elliott Formation are evident. the shoreline of Point Vernon at Pialba. Typical rocks are However on Bidwill Road, just off the Tin Can Bay road exposed at Gatekers Bay where finely bedded siltstone and east of Maryborough, a low cutting on the east exposes a shale predominate. On the western side of the boat ramp coarse sandy conglomerate (hardened gravel, see photo). the rocks dip (slope) gently to the east, but on the eastern Thebasalt at Dundowran is weathered deeply and despite side the rocks have been bent into a fold, with the western being quarried in two major operations is not well exposed limb of this dipping steeply to the west, and the eastern along the roads. Some is evident along the Dundowran- limb dipping more gently to the east. (photo on page 1). Toogoom road in the second cutting west of Dundowran The boat ramp has been constructed in the corresponding Road. syncline. These folds are a minor ‘crinkle’ on the western

Thinly bedded siltstone of Burrum Coal Measures with streaks Coarse conglomerate of the Elliott Formation on Bidwill Road of carbonaceous shale at Point Vernon. east of Maryborough.

© Geological Society of Australia Qld Div 2016 Geological sketch of Maryborough-Hervey Bay district. Simplified from Maryborough 1:250 000 Geological Map, Geological Survey of Queensland, 1992.