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Stratigraphy and Sedimentation
Published in Supriya Sengupta, Introduction to Sedimentology, 2017
In the case of a gradual marine transgression over an unconformity, the younger layers overlap the older ones and are onlapped by the still younger sediments. The younger layers in this case are progressively displaced away from the centre of the basin. In a typical case of marine transgression on land, this results in a fining-upward sequence. In the event of a gradual marine regression, the sea is progressively displaced towards the basin centre and the younger layers are shifted away from the land area. The relationship between the successive sedimentary layers in this case is offlapping. Pinchouts are common in onlaps but truncations occur frequently in offlaps (Fig. 8.3).
Geological setting of the Moorowie Formation, lower Cambrian Hawker Group, Mt Chambers Gorge, eastern Flinders Ranges, South Australia
Published in Australian Journal of Earth Sciences, 2020
T. J. Mount, J. B. Jago, N. R. Langsford, C. R. Dalgarno
The Cambrian succession of the Mt Chambers area features near-shore sediments, such as shallow platform carbonates, alternating with distal marine deposits that pass up to saline sabkha and continental red-bed environments higher in the succession. This pattern suggests that the marine transgression into the Arrowie Basin that had produced the thick siliciclastic-carbonate succession of the lower to middle Hawker Group (e.g. Gravestock & Shergold, 2001; Zang, 2002) was followed by a marine regression and an increasing continental influence into the overlying Lake Frome Group. Fluctuating sea-levels during the regression produced alternations of carbonate platform facies with deeper marine basin facies through the Hawker Group. Regional patterns of sedimentation were overprinted locally by sea-level changes owing to emerging evaporite stocks. The causes of regression in the Moorowie area need not be global (eustasy) or even regional in scale and may relate only to local uplift (basement faulting, diapirs and halokinetics). Further studies are required to extend the concept, especially in adjacent areas such as Mt John, Mt Roebuck, Reaphook Hill, Ten Mile Creek and Wirrealpa.
Intraplate volcanism on the Zealandia Eocene-Early Oligocene continental shelf: the Waiareka-Deborah Volcanic Field, North Otago
Published in New Zealand Journal of Geology and Geophysics, 2020
James M. Scott, James D. L. White, Petrus J. le Roux
After breakaway from Gondwana at 84 Ma, New Zealand’s sedimentary record shows this continental mass became largely submerged. The peak of marine transgression occurred in the Early Oligocene (e.g. Forsyth 2001), with later marine regression and uplift of the North and South Islands initiated by mid-Oligocene propagation of the Australia-Pacific plate boundary through the centre of the continent. Zealandia has throughout its history been punctuated by volcanism. While the most prominent volcanism, forming the volcanoes of the Taupo Volcanic Zone, is arc-related, Zealandia has a long history of intraplate volcanism (e.g. Mortimer and Scott 2020) and one product of this history is the topic of this paper: mid-Cenozoic volcanic rocks near Oamaru in North Otago in the South Island. The volcanic rocks around Oamaru were historically considered to comprise two separate formations with the older Waiareka Volcanic Formation separated from the younger Deborah Volcanic Formation by the Totara Limestone (e.g. Park 1918; Uttley 1918, 1920; Benson 1943; Coombs and Dickey 1965). This limestone, subsequently renamed as Ototara, was deposited during and between times of volcanic activity but the inference that there is an older and a younger member separated by a hiatus involving regional carbonate formation is a major oversimplification. Instead, there were many local periods of non-volcanic deposition that produced limestone and marine mud- and silt-stone units (Coombs et al. 1986; Edwards 1991; Andrews 2003; Maicher 2003). The two volcanic formations are now grouped as part of the Alma Group (Forsyth 2001) and following Coombs et al. (1986) we refer to the volcanic rocks collectively as the Waiareka-Deborah Volcanic Field.