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The Continental Sedimentary Environment
Published in Aurèle Parriaux, Geology, 2018
It often happens that the tongue of a glacier ends in a lake (Fig. 8.64), where it forms a glacier front delta. The lake depression receives only fine particles, like the bottom-set beds we described in the lacustrine environment. There are two notable differences between the glaciolacustrine and the lacustrine sediments: The presence of pebbles caught in fine cyclical stratifications, called dropstones, or fallen pebbles, they are dispersed by icebergs that float on the lake with a veneer of moraine stuck to the ice;Absence or rarity of faunal or floral remains because of the very cold climate.
The Continental Sedimentary Environment
Published in Aurèle Parriaux, Geology, 2018
It often happens that the tongue of a glacier ends in a lake (Fig. 8.63), where it forms a glacier front delta. The lake depression receives fine particles only, like the bottom-set beds we described in the lacustrine environment. There are two notable differences between the glaciolacustrine and the lacustrine sediments: The presence of pebbles caught in fine cyclical stratifications; these are dropstones, that is fallen pebbles, dispersed by icebergs that float on the lake with a veneer of moraine stuck to the ice;Absence or rarity of fauna or flora remains because of the very cold climate.
Glacial geology
Published in Barry G. Clarke, Engineering of Glacial Deposits, 2017
Figure 2.6 shows the transport and deposition processes in a pro-glacial lake in which glaciolacustrine deposits are formed. The deposition process is a function of the density profile within the lake, which is a function of the suspended load of the meltwater entering the lake and the temperature profile within the lake (Figure 2.15). The variation in surface temperature and density leads to seasonal changes, which affects the deposition of the glacial debris (Figure 2.16). Most sediments are formed as either deltaic sediments or lake bottom sediments formed as topsets, forests or bottom sets. Deltaic sediments are typically sand and gravel as they are nearest to the source of the sediment. Deltaic sediments include deltas, delta moraines, De Geer moraines, shorelines deltas, areas of debris slumping and sedimentation of fine sediment. Delta moraines are a product of glaciofluvial deposition in front of a stationary ice margin creating shorelines (e.g. Glen Roy, Scotland). As the lake drains, the shoreline remains. Lake bottom sediments can be laminated couplets of silt and clay representing summer and winter deposition. They may contain glacial debris including dropstones released from icebergs. Three types of varved deposits exist; those in which the thicker clay layers are separated by thin silt layers; those in which the clay and silt layers are equal in thickness; and those which are deposited near to the ice margin forming thick silt layers separated by thin layers of clay. The sources of these materials include subglacial, englacial or supraglacial sediment and glacier melt streams.
U–Pb geochronology reveals evidence of a Late Devonian hydrothermal event, and protracted hydrothermal–epithermal system, within the Mount Painter Inlier, northern Flinders Ranges, South Australia
Published in Australian Journal of Earth Sciences, 2020
S. B. Hore, S. M. Hill, A. Reid, B. Wade, N. F. Alley, D. R. Mason
Sample 2381597 (Figure 6c) was collected from an outcrop exposed in a historic drill pad area on the otherwise steep slopes near Mt Gee. The weathered and poorly layered sandstone outcrop (Figure 6d) contains occasional rounded polished lonestones, which may be glacial dropstones. This exposure is similar to other outcrops of the RRB with crude layering and an attitude following the general topography of the area. After deposition and burial, low-grade recrystallisation of the rock produced fine-grained non-foliated sericite + minor hematite matrix replacement; this occurred at P–T conditions equivalent to the lower greenschist facies and may be due to the intruding late-stage MGS noted in outcrop.
Glaciations at high-latitude Southern Australia during the Early Cretaceous
Published in Australian Journal of Earth Sciences, 2020
N. F. Alley, S. B. Hore, L. A. Frakes
Livingston Tillite Member at ‘Recorder Hill’ type section. (a) Recorder Hill showing the unconformity between the Cadna-owie Formation and bedrock (yellow line) and the position of the Livingston Tillite Member tillite facies (shaded grey area between the black lines). Person in lower right arrowed. (b) Cross-bedded, bouldery sandstone underlying the tillite facies, the more lithified sediment above geological pick handle. Boulder of granite to left of pick and large quartzite boulder lower left from pick. Note the sloping disconformity between the tillite facies and underlying sandstone, which becomes an interfingering relationship upslope. (c) Large subrounded pink sandstone boulder at base of the Cadna-owie Formation overlying weathered bedrock. (d) Tillite facies with finer-grained matrix. Small clasts dominated by siltstone from the local bedrock, in this area an unnamed unit of the Billy Springs Formation. Blade of knife 6.5 cm in length. (e) Boulders and pebbles in sandier facies of the tillite dominated by local siltstone (grey clasts) and rounded quartzite pebble, upper right from two-dollar coin. (f) Section through sandier tillite exposing a boulder of Pepegoona Porphyry. Magnetic pen ∼15 cm length. (g) Strongly bioturbated upper surface of the sandy tillite facies. Ten cent coin for scale. (h) Calcareous tillite lens within mudstone at the 7–8 m level. Large pink sandstone clast to right of spade. Between spade and clast is a thin lens of HCS sandstone. (i) Photo showing stratigraphic position of the tillite lens (black arrow) and the tillite facies (yellow arrow). (j, k) are consecutive views of the same bed from left to right. (j) Dropstone of quartzite within the Cadna-owie Formation at approximately the 10–11 m level. Note how the boulder penetrates the sediments, typical of dropstones. A thin layer of gravel and clay rip-up clasts extends to the right of the boulder. Pen measures 13.5 cm in length. (k) A thin layer of gravel and rip-up clasts to right of boulder. HCS sandstone within and below the layer. (l) Westerly looking view of very large lonestone of quartzite (spade for scale) derived from the Cadna-owie Formation. Lower part of the Trinity Well Sandstone and the transition from the underlying Cadna-owie Formation exposed in the east-facing escarpment in the middle distance. Upper part of the Trinity Well Sandstone exposed in a west-facing escarpment along the ridge top in the distance.