Includes reports: - Geochronology of stratigraphically significant rocks from South Australia. Progress report no. 5. - Geochronology of stratigraphically significant rocks from South Australia. Progress reports 6 and 7. - Geochronology of stratigraphically significant rocks from South Australia. Progress reports 8 and 9 - Geochronology of stratigraphically significant rocks from South Australia. Progress reports 10 to 13, 1975-76. - Geochronology of stratigraphically significant rocks from South Australia. Progress reports 14 to 20, 1976-77. - Geochronology of stratigraphically significant rocks from South Australia. Progress report no. 21. Samples of rocks from the Adelaidean sequence in the Torrens Hinge Zone and Stuart Shelf were obtained from drill cores from SADM Wokurna 6 and Australian Selection SAS-1, and submitted for Rb-Sr isotopic dating. The four samples from Wokurna 6 were red-brown siltstones from the equivalent of the Brachina Formation (Wilpena Group), while eleven samples of basalt from the one flow were taken from SAS-1, this flow being one of several which make up a thickness of almost 200 m below the Tapley Hill Formation. The stratigraphic position of the basalts indicates that they are older than the Marinoan glacial period and have a minimum age of early Sturtian. These samples and those from the Brachina Formation should therefore provide a maximum and minimum age respectively for the Marinoan glaciation. - SADME / Amdel Project 1/1/126 : Geochronology of stratigraphically significant rocks from South Australia. Progress report no. 22. This report discusses the results of petrographic screening of 22 altered volcanic rock samples from the Stuart Shelf, to determine their compositional suitability for geochronology. Most of the rocks are too highly altered, commonly by calcite and ferruginous material; however, three samples from Delhi's Beda Arm DDH 2 are carbonate-free, and would be suitable for whole-rock rubidium-strontium or potassium-argon isotopic dating. These samples are all taken from the one lava flow, which is distinguished from others in the drilled section by the absence of amygdales (which are carbonate-filled in these other rocks). The three North Broken Hill drillhole samples from the Port Pirie region are even more altered than the Beda Arm rocks, and tend to show a wider range of secondary minerals (e.g. muscovite, chlorite and tourmaline) possibly associated, in part, with mineralisation. - Geochronology of stratigraphically significant rocks from South Australia. Progress report no. 24. Rubidium-strontium dating was carried out on selected samples of Brachina Formation shale taken from Woomera 1 diamond drillhole, and on samples of a foliated granodiorite taken from Bungalow BD2 drillhole. The Woomera 1, Stuart Shelf samples had a very restricted range in Rb/Sr ratios (1.0 to 1.3), and although geochronological analyses were made on 6 of the samples, the close grouping of the plotted isotopic values produced a large error in the isochron age. Regression of the data gave a Model 3 isochron of 676 /- 204 Ma with an initial strontium 87 / strontium 86 ratio of 0.7183 /- 0.0101. Although this result is imprecise, the mean age is considerably older than that determined on the Brachina Formation in Torrens Hinge Zone drillhole Wokurna no. 6 (503 /- 56 Ma), and is close to the age expected for a unit that post-dates the Marinoan glaciation. Samples of a foliated granodiorite intersected by drilling of a magnetic anomaly lying to the north-west of Cowell are similar in appearance to the Middlecamp Granite, located south-west of Cowell. The Middlecamp Granite has been dated at 1650 /- 35 Ma. Rb-Sr analyses were made on 10 samples of the Bungalow BD2 drill core, and all but one of these analyses defined a Model 1 isochron of 1601 /- 14 Ma with an initial strontium 87 / strontium 86 ratio of 0.7148 /- 0.0008. Although the age is significantly younger than that determined for the Middlecamp Granite, both intrusions belong to the syn-orogenic phase of the Kimban Orogeny, and the high initial isotopic ratios of both granites (and also of the Carpa and Narridy Creek Granites) indicate that significant assimilation of sialic crust occurred in the Cowell region during the generation and emplacement of granitic magma between 1650 and 1600 Ma ago. - Geochronology of stratigraphically significant rocks from South Australia. Progress report no. 25. This report discusses the results of petrographic screening of 8 volcanic rock samples from the Stuart Shelf, to determine why their Rb/Sr isotopic ratios show a general variation with depth. The author contends that at greater depths rubidium contents tend to decrease and strontium contents tend to increase, causing a concommitant decrease in the Rb/Sr ratio. This variation can be correlated with the observed petrographic features, and in particular, the degree of alteration. - Geochronology of stratigraphically significant rocks from South Australia. Progress report no. 27. Six samples of fine-grained shaley sediments of the Corunna Conglomerate, obtained from diamond drill cores retrieved from Uno mapsheet area drillhole DDH AU-U6 between 856 and 921 feet depth, had Rb-Sr isotopic analysis performed on them for absolute age determination. Regression of the plotted analysis results produced a Model 3 isochron of 1473 /- 20 Ma, with an initial strontium 87 to strontium 86 ratio of 0.7119 /- 0.0023. The Corunna Conglomerate contains boulders of acid volcanic rocks similar to those of the Gawler Range Volcanics, and plugs of acid volcanics are known to intrude the conglomerate. Because of this field evidence, the conglomerate has been considered to be penecontemporaneous with the Gawler Range Volcanics. The volcanics were extruded over an interval of time between 1525 and 1500 Ma, while late stage granitic rocks and rhyolite dykes were emplaced at 1478 and 1458 Ma ago respectively. Geochronological investigations into the later stages of the volcanic activity of this region are continuing, and it is possible that evidence of volcanism between 1500 and 1460 Ma may be found. Thus the age of the Corunna Conglomerate can be bracketed by the oldest and youngest events so far dated for the Gawler Range Volcanics, viz. 1525 and 1460 Ma. - Geochronology of stratigraphically significant rocks from South Australia. Progress report no. 28. 10 selected drill core samples from the depth interval 266.17 to 281.89 metres KB in Murnaroo 1 were submitted for Rb/Sr total rock dating. A restricted range of Rb/Sr ratios in the samples has lent uncertainty to the Late Proterozoic isochron obtained, such that a possible Cambrian age cannot be discounted. - Geochronology of stratigraphically significant rocks from South Australia. Progress report no. 31. 5 samples of glauconite from the Blanche Point Formation, cored in a series of shallow geotechnical drillholes put down by Maunsell and Partners at the Morphett Street Bridge and Brougham Place construction sites in Adelaide during 1964, were submitted for Rb-Sr age dating. The aim was to check a previously derived date obtained by K-Ar dating of other samples taken from the same 'Hantkenina primitiva' biostratigraphic zone, in order to give a precise age to the Eocene P.15/P.16 palynozone boundary. - SADME / Amdel Project 1/1/126 : Geochronology of stratigraphically significant rocks from South Australia. Progress report no. 32. Fine-grained sericite separated from 3 samples of drill core taken from between 178 and 208 metres depth in Roopena DDH6 were submitted for Rb-Sr dating to determine a minimum age for the Moonabie Formation. Also, 10 samples of fine-grained, slatey rocks from the Middleback Range area were submitted for petrographic examination and Rb-Sr geochronology. - Geochronology of stratigraphically significant rocks from South Australia. Progress report no. 33. 8 selected samples of outcrop of the Burkitt Granite were submitted for petrographic descriptions and screening for their suitability for Rb-Sr or K-Ar isotopic age determination. A high mobility of both potassium and calcium was evident during three stages of the history of the granite suite, and all the samples were found to have low Rb/Sr ratios due mainly to their high strontium contents. Six of the samples were subsequently chosen for Rb-Sr geochronology. Their isotopic analyses produced a Model 1 regression isochron, plotting well within the range of experimental error, that corresponds to an age of 1655 +/- 61 Ma, with an initial strontium 87 to strontium 86 ratio of 0.7073 +/- 0.0011.