Revisions to the stratigraphy of Hay River, Huckitta and Tobermory 4-mile Sheets, Northern Territory

This report outlines corrections and additions to previous mapping on the Huckitta, Hay River and Tobermory Four-mile Sheets, Northern Territory, and presents all available data on water bores drilled in those areas. On the May River Sheet, Upper Cambrian and Lower Ordovician sediments on the western limb of the Toko Syncline are faulted against Upper Proterozoic sediments along a complex north-west trending fault zone named the Toomba Structure. The maximum throw of the main fault exceeds 2,000 feet, and there are smaller, parallel strike faults which affect Middle Ordovician sediments. A section of 460 feet was measured in a ? Ordovician sandstone sequence which overlies the Middle Ordovician Mithaka Beds. Samples obtained from several water bores along the Field River show the presence of a locally non-outcropping sequence, probably the Tarlton Formation, overlying the Upper Proterozoic Field River Beds. On the Tobermory Four-mile Sheet, the effects of lateritisation on the mixed lithologies of the Upper Cambrian and Lower Ordovician units were unravelled, and it was shown that the change from the Tomahawk Beds of the Huckitta area to the Ninmaroo and Kelly Creek Formations of the Toko Range is a facies variation. A. small disconformity was established between the Nora Formation and the Carlo Sandstone in the Tarlton Range. Numerous measurements in the Tarlton Range show that during the deposition of the Carlo Sandstone currents came from the south-east. Further plant fossils were collected from the Tarlton Formation; they indicate a probable Triassic or early Jurassic age. On the Huckitta Four-mile Sheet, two sections of about 750 feet were measured in the predominantly dolomitic Arthur Creek Beds, in the area west and South west of Huckitta homestead; in one section, trilobites and brachiopods were discovered 85 feet, stratigraphically, above archaeocyathids in the top of the Lower Cambrian Mount Baldwin Formation. Three new fossiliferous horizons were found in the Upper Cambrian Arrinthrunga Formation; one is near the base and contains trilobites of lower Upper Cambrian age; the other two horizons are near the top of the formation and contain trilobites and brachiopods. Revised structural mapping and measurement of new sections show that the Arrinthrunga Formation is about 3,200 feet thick. The Middle Ordovician Nora Formation which occurs only in the southern foothills of the Dulcie Range, has been mapped as a separate unit from the Tomahawk Beds which formerly included it .Additional samples of Devonian fish were collected from a new locality in the Point Spring area.