DEPARTMENT OF MINERALS AND ENERGY BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS DEPARTMENT OF MINES, STATE OF QUEENSLAND GEOLOGICAL SURVEY OF QUEENSLAND 1:250000 GEOLOGICAL SERIES-EXPLANATORY NOTES GILBERTON QUEENSLAND SHEET SEj54-16 INTERNATIONAL INDEX COMPILED BY J. SMART AUSTRALIAN GOVERNMENT PUBLISHING SERVICE, CANBERRA, 1973 DEPARTMENT OF MINERALS AND ENERGY MINISTER: THE HON. R. F. X. CoNNOR, M.P. SECRETARY:SIR LENOX HEwrrr, O.B.E. BUREAU OF MINERAL RESOURCES, GEOLOGY AND GEOPHYSICS DIRECTOR:N. H. FISHER ASSISTANTDIRECTOR,GEOLOGICALBRANCH:J. N. CASEY DEPARTMENT OF MINES, STATE OF QUEENSLAND MINISTER: THE HON. R. E. CAMM, M.L.A. UNDER-SECRETARY:E. K. HEALY, I.S.O. GEOLOGICAL SURVEY OF QUEENSLAND CHIEF GOVERNMENTGEOLOGIST:J. T. WOODS Published for the Bureau of Mineral Resources, Geology and Geophysics by the Australian Government Publishing Service Printed bl/ Graphic Services P17 Ltd. 60 Wl/att Street. Adelaide. S.A. 5000 Explanatory Notes on the Gilberton Geological Sheet 2nd EDITION Compiled by J. Smart The Gilberton 1:250 000 Sheet area is bounded by latitudes 19·00'S and 20·OO'S, and longitudes 142°30'E and 144°00'E; it contains the mineral fields of Gilberton, Percyville, and Woolgar, and part of the Great Artesian Basin. The area is divided into three distinct geological and geographical units: the Gregory Range, which crosses the area from northwest to southeast, and joins the Great Dividing Range a few kilometres east of the Sheet boundary; the Claraville Plain, covering the western half of the Sheet area; and the Georgetown lnlier in the northeast corner. There are no towns, main roads, or railways in the area; a network of station tracks provides access into the west; roads in the Gregory Range are few and access is difficult. In the northeast there is reasonable access along tracks to Gilber• ton and percyvilIe homesteads. Some homesteads maintain airstrips. Air-photographs at a nominal scale of 1: 80 000 taken by Adastra Surveys in 1967 provide a complete coverage of the area. Topographic maps at 1:250000 scale are available from the Division of National Mapping, Canberra, and cadastral maps at 4 miles to 1 inch (1 :253440) from the Queensland Department of Lands, Brisbane. The area was geologically mapped in 1956 and 1958 by a combined party from the Bureau of Mineral Resources (BMR) and the Geological Survey of Queensland (GSQ); the 1st Edition geological map was published in 1962. In 1969, another combined geological party remapped the Mesozoic and younger formations and re-examined the Precambrian in the southeast. These notes incor• porate the work of both parties. In 1970, BMR carried out shallow stratigraphic drilling in the south (Needham et al., 1971). Previous investigatioM Daintree (1870, 1872) made the first geological reconnaissance of the Gilberton Sheet area; in 1872 he produced the first geological map of Queensland, which showed Lower Silurian 'metamorphic-mica schist, intruded by dykes of elvanite, diorite, hornblende rocks, etc.', in the Gilberton Mineral Field. In 1898 Maitland attempted to delineate the artesian water area west of the Gregory Range, and provided the first comprehensive account of the hydrology. Cameron (1900) and, later in more detail, Ball (1915) investigated the mines and mineral deposits in the PercyvilIe and Woolgar areas. Saint-Smith (1922) re• ported on the Woolgar Goldfield and provided a comprehensive account of the gold mining. Jensen (1923) made a broad reconnaissance of the Cairns hinterland which included the Precambrian part of the Gilberton Sheet area. No further geological surveys were made until 1945, when Morton surveyed the scheelite deposit at Percyville, and Ridgeway the agate deposit at Agate Creek. 3 Whitehouse (1955) and Ogilvie (1955) studied the geology and hydrology of the Great Artesian Basin for the Queensland Co-ordinator General's Department. In 1953 and 1954 the Land Research and Regional Survey Section, Common• wealth Scientific and Industrial Research Organization (CSIRO), Canberra, carried out a land-use survey of the Leichhardt-Gilbert area, which included the Gilberton Sheet area (Perry et aI., 1964). A combined geological party from BMR and GSQ began systematic regional mapping of the Georgetown, Gilberton, Einasleigh. Clarke River, and Atherton Sheet areas in 1956. Mapping of the Gilberton Sheet area was completed in 1959 and the results recorded by White (1962, 1965). The igneous rocks and cauldron subsidence structures were described by Branch (1966a, b). Age dates of the acid igneous rocks were published by Richards et a1. (1966a, b). Isbell et a1. (1968), in explaining Sheet 7 of, the Atlas of Aus• tralian soils, gave descriptions which in effect outlined the superficial lithology of the Sheet area. In 1955, BMR carried out airborne scintillometer surveys of the Precambrian succession in the east. Anomalies were found to coincide with granite (White & Hughes, 1957); no significant uranium mineralization was located. A Bouguer anomaly map at 1:250000 scale was published by BMR in 1969 (E54/B2-16). PHYSIOGRAPHY In the Gilberton Sheet area the six main physiographic units are: Oaraville Plain, Gregory Range, Gilberton Plateau, Strathpark Plain, Newcastle Range, and Chud• leigh Plateau (Fig. 1). The Claraville Plain (Twidale, 1956, 1966) is a depositional plain decreasing in height from 300 m in the east to 200 m in the. west. It is composed of soft sandstone and claystone of the late Cainozoic Wyaaba Beds, and much of it consists of thin outwash sands. At present the central and eastern parts of the plain are being dissected to expose Mesozoic sandstone and mudstone. The Gilberton Plateau covers a large area in the southeast; its height ranges from 600 m to 900 m and its surface has been tilted between to and 10 to the west, southwest, and south by late Cainozoic uplift. The plateau consists of flat-lying Mesozoic sandstone; its sandy surface is derived in part from the Mesozoic rock and in part from the products of a deep-weathering zone developed during a climatic regime different from that of the present. It is being eroded at its margins, particularly in the north and west. To the northwest, dissection is more extensive and the plateau form is lost. The Gregory Range is essentially the dissected continuation of the Gilberton Plateau. The plateau and range topography developed after uplift relative to the western half of the Sheet area in late Cainozoic time (Doutch et aI., 1970), and is due to erosion by the Gilbert and its tributaries flowing north, and to the Clara and Norman Rivers flowing west. 4 GILBERT RIVER CLARA RIVER NORMAN RIVER CHUDLEIGH PLATEAU JUNTALA DOME .... 0& "-"0~• ~ ,"" e:t> ~"SI""~,. t.Il 1*~1Wallumbilla Formation I::':;:::"d Gilbert River Formation ~;mm\'!:.~Loth Formation I:::: : IHampstead Sandstone :;;:::::~ Paloeozoic granite ~ Precambrian E54/AJ6/il Fig. 1. Physiographic units GUbertoD 1:250 000 Sheet area. The Strathpark Plain, a sand plain in which few rivers rise, is apparently underlain by ferricrete. It may have been part of the same surface as the Gilberton Plateau before disruption by late Cainozoic tectonism. It is lower than the plateau, but higher than the Claraville Plain. Headwater erosion of the Saxby River into the Strathpark Plain is exposing the quartzose sandstones beneath it. The Newcastle Range is the southern end of the range. It is developed on resistant, tightly folded Precambrian sedimentary, granitic, and metamorphic rocks. The sedimentary rocks are eroded to rough strike ridges and gullies, the granites and metamorphics to smooth undulating rises. The relief varies from 500 m to 800 m from south to north. The Chudleigh Plateau was formed by Cainozoic basalts extruded from vents in the eastern flank of the Juntala Dome east of the Gilberton Sheet area. DRAINAGE Drainage In the Gregory Range area the courses of many rivers are controlled by faults or joints; the Norman and Woolgar Rivers are the most prominent examples. In the northwestern part of the Gilberton Plateau several ancient superimposed drainage patterns have been recognized (Doutch et al., 1970). It appears that the original meandering river flowed northwards, and its course was modified several times by faulting and tilting of blocks associated with uplift immediately east of the eastern margin of the Sheet area, the Juntala Dome being a local culmination. STRATIGRAPHY Stratigraphy Precambrian and Palaeozoic igneous and metamorphic rock~ cropping out over wide areas in the northeast and east are part of the 'Georgetown Massif' (Hill, 1951) or the 'Georgetown Inlier' (White, 1961). Inliers of Archaean and Palaeo• zoic rocks within the Mesozoic sandstones are present in the south in the WooIgar River and Loth Creek areas. OutHers of freshwater Devonian to Carboniferous sedimentary rocks occur in the Georgetown Inlier near Gilberton homestead and in the Woolgar River and Stawell River areas. Mesozoic and Cainozoic sedimentary rocks cover the greater part of the Sheet area. Cainozoic basalt rests on Palaeozoic and Mesozoic rocks in the southeast. The stratigraphy is summarized in Tables 1-4. ARCHAEAN?-PROTEROZOIC The oldest rocks, of possible Archaean age, consist of high-grade regional meta• morphics of the almandine-amphibolite and granulite facies. These are the Einas• leigh Metamorphics, which crop out south of Gilberton homestead, in the Percy• ville-Welfern area, and in the headwaters of the Woolgar River. They are intruded by the Proterozoic Robin Hood and Forsayth Granites, and the Upper Silurian to Lower Devonian Dumbano Granite. The metamorphic rocks exhibit retrograde and contact metamorphism which was probably produced by the intrusions. 6 Proterozoic sedimentary rocks of the Etheridge Geosyncline (White, 1961) are unconformably overlain by Mesozoic sediments near Ortona Mine. The cal• careous Bernecker Creek Formation partly intertongues with, and partly conform• ably overHes, the shale and siltstone of the Etheridge Formation. Paddys Creek Formation quartzite is preserved as a roof pendant in the Dumbano Granite.