摘要:

AbstractThe Pleistocene Knocknasilloge Member, an upward coarsening sequence of massive and laminated muds to cross‐laminated fine sands and silts, was deposited in a series of shallow lacustrine basins formed at the retreating margin of the Late Devensian Irish Sea ice‐sheet. Sedimentary successions mark increased proximity to a sediment input source and a shift from lake floor suspension sedimentation to bottom traction in low‐angled prodelta foresets and distributary barmouth channels. The microfauna contained within the sequence is derived from preexisting climatic stages and there is no evidence to support either anin situinterglacial or glaciomarine origin, as previously pro

ISSN:0072-1050    

DOI:10.1002/gj.3350220202

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractTwo right‐way‐up Moine lithostratigraphic units—the Shiaba (older) and Assapol (younger) Groups—are distinguished in the Ross of Mull. These were intruded in the west by the post‐tectonic, Ross‐of‐Mull granite complex at 414 ± 3 Ma. Apparently undisturbed inclusions of Moine metasediments within the granite permit the boundary between the lithostratigraphic units to be followed westward almost to the supposed trace of the Moine thrust in the Sound of Iona. The Shiaba and Assapol Groups which have a transitional, albeit attenuated, stratigraphic contact are correlated with the Morar and Glenfinnan Divisions of Inverness‐shire, the Sgurr Beag ductile thrust (slide) normally found at the Morar/Glenfinnan Division boundary being absent. This implies that the stratigraphic relationship between these two divisions, which is elsewhere obscured by the thrust, is uniquely preserved on Mull.Within a local, D1‐D4 sequence, D2 and D3 structures are dominant. Originally subhorizontal D2 folds are intensely curvilinear on all scales about an originally flat‐lying, NNW‐SSE trending stretching lineation (L2). In sections parallel to L2, D2 minor folds display a constant sense of vergence throughout the Ross‐of‐Mull Moine Rocks, overturning generally NW‐NNW. The present day structure is dominated by the almost upright, SSW‐plunging D3 Assapol synform which overprints all earlier structures. Tentative correlation of the deformation sequence with that of Inverness‐shire, suggests that the D1‐D2 structures of Mull, with accompanying moderate‐to‐high‐grade metamorphism, may be Precambrian, while the D3 Assapol synform may be Caledonian.The presence of migmatites of kyanite grade means that metamorphic grade, established during MS1‐MP2, is anomalously high for Moine rocks lying close to the supposed Caldonian front. This suggests that they may lie within one of the higher Caledonian thrust nappes of the North Highland Moine—possibly the Knoydart nappe, where metamorphic grade is comparable. The greenschist facies metamorphism and single phase of deformation affecting the ‘Torridonian’ rocks of Iona presents a significant contrast to the Moine rocks of the Ross of Mull. A major fault in the Sound of Iona is implied, but

ISSN:0072-1050    

DOI:10.1002/gj.3350220203

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractThe largely concealed sub‐Flandrian bedrock surface in the Severn Estuary, chiefly known from borings and chart soundings, has the form of a valley within a valley. In the aftermath of an unusual storm in March 1986, which removed from many places the obscuring modern sediments normally present, periglacial structures were recognized on the floor and upper margins of the upper valley at ten widely scattered localities. The structures occur only where the valley is formed on relatively soft Triassic and Jurassic mudrocks; they involve gravels and/or red‐brown sands belonging to the Main Terrace sequence, but in some places incorporate local gravels. There are no completely convincing indications of cold conditions where the relatively strong Silurian and Old Red Sandstone rocks and the Carboniferous Limestone form the valley floor. The altitudinal relationships of the involutions and wedges, together with the scale of the outer valley, suggest that this sub‐Flandrian rock bench was merely re‐excavated during the Late Devensian. Then permafrost developed while the river lay confined within the much narrower inne

ISSN:0072-1050    

DOI:10.1002/gj.3350220204

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractThe Mohr circle is a well known representation of two‐dimensional strain. It is commonly used to illustrate strain algebra, or derive a strain ellipse from particular strain data. The established sign convention is positive (anticlockwise) shear strain on the upward ordinate, after Brace. However, in practice, confusion may easily arise from inconsistent conventions in standard text books.A Mohr‐circle convention is proposed here which represents clockwise shear strain on the upward γ ordinate (i.e. upside‐down). It simplifies the Mohr circle by representing both single and double angles in their natural sense. Single angles may be traced from rock to Mohr circle directly which is not the case for the Brace circle. Deformed brachiopods and stretched belemnites are used to illustrate this simplified Mohr circle.The pole to the Mohr circle is a useful addition which allows strain data to be represented in their true orientation. The pole method only works on the γ clockwise‐up Mohr circle.The Mohr circle is useful to demonstrate simple‐shear geometry and algebra. Successive Mohr circles with poles may be used to map strain across a heterogeneous simple

ISSN:0072-1050    

DOI:10.1002/gj.3350220205

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractThe Yr Arddu Tuffs (Ordovician) are a sequence of predominantly welded acid ash‐flow tuffs in the outlier centred on Yr Arddu (North Wales). The tuffs accumulated above a NNE‐trending fracture, the Yr Arddu Fracture, which had previously influenced sedimentation. The heterogeneity of the tuffs and their restricted development suggest that they represent proximal accumulations from small or suppressed (boiling‐over) eruption columns. The eruptions and emplacement developed, at least in the initial stages, in a submarine environment. Lithological, geochemical and palaeontological evidence indicates that the tuffs represent the earliest eruptive phase of the Lower Rhyolitic Tuff Formation. They were later intruded by comagmatic rhyolite domes, whose alignment reflects the continued influence of the early fra

ISSN:0072-1050    

DOI:10.1002/gj.3350220206

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

摘要:

AbstractA large quarry at Billinge Beacon in Lancashire, northwest England, has provided an excellent exposure of part of the Lower Westphalian A (Upper Carboniferous) Coal Measures sequence which was deposited in a lower delta plain setting. The main horizon exposed, known as the Dyneley Knoll Flags, is interpreted as the proximal reaches of two crevasse‐initiated minor delta complexes, into the lower of which has been incised a sinuous minor distributary channel.The channel, which was approximately 120 m wide, built a 160 m wide channel belt by limited lateral accretion with time; vertical accretion appears to have been the dominant infilling mechanism. This produced a stacked series of epsilon cross‐strata, made up of rhythmically interbedded sandstone and silty claystone. The peculiar style of infilling of the channel is ascribed to deep incision of the channel into minor delta deposits, and possibly the influence of a seasonal clim

ISSN:0072-1050    

DOI:10.1002/gj.3350220207

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350220208

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350220209

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350220211

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350220212

出版商:John Wiley&Sons Ltd    

年代:1987

数据来源: WILEY