9 research outputs found
Deposition and preservation of fluvio-tidal shallow-marine sandstones: A re-evaluation of the Neoproterozoic Jura Quartzite (western Scotland)
The 2 to 5 km thick, sandstoneâdominated (>90%) Jura Quartzite is an extreme example of a mature Neoproterozoic sandstone, previously interpreted as a tideâinfluenced shelf deposit and herein reâinterpreted within a fluvioâtidal deltaic depositional model. Three issues are addressed: (i) evidence for the reâinterpretation from tidal shelf to tidal delta; (ii) reasons for vertical facies uniformity; and (iii) sand supply mechanisms to form thick tidalâshelf sandstones. The predominant facies (compound crossâbedded, coarseâgrained sandstones) represents the lower parts of metres to tens of metres high, transverse fluvioâtidal bedforms with superimposed smaller bedforms. Ubiquitous erosional surfaces, some with granuleâpebble lags, record erosion of the upper parts of those bedforms. There was selective preservation of the higher energy, topographicallyâlower, parts of channelâbar systems. Strongly asymmetrical, bimodal, palaeocurrents are interpreted as due to associated selective preservation of fluviallyâenhanced ebb tidal currents. Finerâgrained facies are scarce, due largely to suspended sediment bypass. They record deposition in lowerâenergy environments, including channel mouth bars, between and down depositionalâdip of higher energy fluvioâebb tidal bars. The lack of waveâformed sedimentary structures and low continuity of mudstone and sandstone interbeds, support deposition in a nonâshelf setting. Hence, a sandârich, fluvialâtidal, currentâdominated, largely subâtidal, delta setting is proposed. This new interpretation avoids the problem of transporting large amounts of coarse sand to a shelf. Facies uniformity and vertical stacking are likely due to sediment oversupply and bypass rather than balanced sediment supply and subsidence rates. However, facies evidence of relative sea level changes is difficult to recognise, which is attributed to: (i) the areally extensive and polygenetic nature of the preserved facies, and (ii) a large stored sediment buffer that dampened response to relative seaâlevel and/or sediment supply changes. Consideration of preservation bias towards highâenergy deposits may be more generally relevant, especially to thick Neoproterozoic and Lower Palaeozoic marine sandstones
OligoceneâMiocene carbonates in the Reed Bank area, South China Sea, and their tectono-sedimentary evolution
Indicators of relative completeness of the glacial record of the Port Askaig Formation, Garvellach Islands, Scotland
The Port Askaig Formation (PAF) is a diamictite-bearing succession in the Dalradian Supergroup of Scotland that provides an excellent archive of a Cryogenian glaciation in the Garvellach Islands and Islay, Argyll. The formation is ~1100 m thick, comprises 5 members and includes 47 diamictite beds, interbedded with siltstones, dolostones and sandstones. Here we document seven features of the PAF that indicate its relative stratigraphic completeness. There are gradual, progressive changes up-section in the lithologies of the diamictites, their interbeds, and clast lithologies. The sharp basal surfaces of the diamictites each show the same, repeated pattern of environmental change, from non-glacial to glacial. Many of the top surfaces of the diamictites show evidence of periglacial conditions. The succession in the PAF records a total of 76 climatically-related stratigraphic episodes: 28 glacial episodes, 25 periglacial episodes and 23 non-glacial episodes. Parts of Member 1 (Diamictites 1-12 and Diamictites 16-18) and Member 2 (Diamictite 31 to the base of Member 3) are most compete on the east coast of Garbh Eileach. The PAF in the Garvellach Islands occurs within a succession that is several kilometres thick, as newly revealed by sea-floor mapping. Compared with other Cryogenian and Phanerozoic glacial successions, the PAF is exceptional in its combination of formation thickness, the number of climatically-related stratigraphic episodes, and the considerable thickness of its host supergroup. Furthermore, these indicators of relative stratigraphic completeness provide evidence that the base of the PAF on the east coast of Garbh Eileach is a succession without a major break in deposition, supporting the account of the strata at and below the base of the PAF in the companion article b
Indicators of relative completeness of the glacial record of the Port Askaig Formation, Garvellach Islands, Scotland
The Port Askaig Formation (PAF) is a diamictite-bearing succession in the Dalradian Supergroup of Scotland that provides an excellent archive of a Cryogenian glaciation in the Garvellach Islands and Islay, Argyll. The formation is ~1100 m thick, comprises 5 members and includes 47 diamictite beds, interbedded with siltstones, dolostones and sandstones. Here we document seven features of the PAF that indicate its relative stratigraphic completeness. There are gradual, progressive changes up-section in the lithologies of the diamictites, their interbeds, and clast lithologies. The sharp basal surfaces of the diamictites each show the same, repeated pattern of environmental change, from non-glacial to glacial. Many of the top surfaces of the diamictites show evidence of periglacial conditions. The succession in the PAF records a total of 76 climatically-related stratigraphic episodes: 28 glacial episodes, 25 periglacial episodes and 23 non-glacial episodes. Parts of Member 1 (Diamictites 1-12 and Diamictites 16-18) and Member 2 (Diamictite 31 to the base of Member 3) are most compete on the east coast of Garbh Eileach. The PAF in the Garvellach Islands occurs within a succession that is several kilometres thick, as newly revealed by sea-floor mapping. Compared with other Cryogenian and Phanerozoic glacial successions, the PAF is exceptional in its combination of formation thickness, the number of climatically-related stratigraphic episodes, and the considerable thickness of its host supergroup. Furthermore, these indicators of relative stratigraphic completeness provide evidence that the base of the PAF on the east coast of Garbh Eileach is a succession without a major break in deposition, supporting the account of the strata at and below the base of the PAF in the companion article b
Sedimentary structures and sequences within a late Proterozoic tidal shelf deposit: the Upper Morar Psammite Formation of northwestern Scotland
Facies characterization and sequential evolution of an ancient offshore dunefield in a semi-enclosed sea: Neuquén Basin, Argentina
Crustaceans from a bitumen clast in Carboniferous glacial diamictite of Oman extend fossil record of copepods
Copepod crustaceans are extremely abundant but, because of their small size and fragility, they fossilize poorly. Their fossil record consists of one Cretaceous (c. 115âMa) parasite and a few Miocene (c. 14âMa) fossils. In this paper, we describe abundant crustacean fragments, including copepods, from a single bitumen clast in a glacial diamictite of late Carboniferous age (c. 303âMa) from eastern Oman. Geochemistry identifies the source of the bitumen as an oilfield some 100â300âkm to the southwest, which is consistent with an ice flow direction from glacial striae. The bitumen likely originated as an oil seep into a subglacial lake. This find extends the fossil record of copepods by some 188âMa, and of free-living forms by 289âMa. The copepods include evidence of the extant family Canthocamptidae, believed to have colonized fresh water in Pangaea during Carboniferous times