摘要:

AbstractSedimentary sequences through the uppermost Ordovician (Hirnantian), Kosov Formation, are described. The Kosov Formation is a shale‐dominated formation with two thin horizons of sandy diamictites near the base and two ‘flysch’ units separated by shales forming the main part of the formation, which is overlain abruptly by Silurian graptolitic shales. The diamictites are interpreted as of glaciomarine origin, probably deposited from coastal winter ice. The flysch units have many beds with hummocky cross‐stratification and wave‐ripples indicating deposition from storm generated currents above storm wave‐base. The shales between the two flysch units have thin interbedded sandstones and siltstones with mainly unidirectional current structures, but also some wave‐ripples, suggesting deposition around storm wave‐base.The sequence shows clear evidence of two regressive phases, the second of which is the more pronounced, followed by rapid transgression near the Ordovician/Silurian boundary. The regressions and subsequent transgression are interpreted as being caused by Hirnantian glacioeustatic changes rather than being tectonic in origin.The importance of the glaciomarine diamictites is that they record the onset of cold climatic conditions in the region as being early Hirnantian. The disappearance of most of the fauna at about the same time might reflect the influence of low temperatures on faunal diversity, and could have significance for the end Ordovi

ISSN:0072-1050    

DOI:10.1002/gj.3350240302

出版商:John Wiley&Sons Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractA tectonic study of the Newberry Crater region of central Oregon has been based on the interpretation of Landsat Thematic Mapper imagery. Two major faults, the Brothers‐Tumalo and Eugene‐Denio Faults, pass NW‐SE through the region and step to the right at the eastern margin of the Cascades Range. Dextral wrench faulting on these structures during the Tertiary controlled the formation of the La Pine Basin, a pull‐apart structure containing Tertiary and Quaternary sediments and volcanics. Tertiary wrench faulting appears to have been associated with rotations of crustal blocks at a plate margin, but was superseded in the Quaternary by extensional faulting of the Basin and Range province. Newberry Crater and other major bimodal volcanic centres in the NW Cordillera (Crater Lake, Medicine Lake, Mt. St. Helens) seem to have a similar tectonic setting in crustal pull‐aparts. A relationship between magma type and fault trend at Newberry and Medicine Lake is

ISSN:0072-1050    

DOI:10.1002/gj.3350240303

出版商:John Wiley&Sons Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractPrevious studies of meandering gravel‐bed rivers have illustrated a wide range of bar types. The River Tywi of South Wales shows that significant variations of accretionary style can also occur within a single river. There is a downstream decrease in the proportion of lateral bars to point bars and changes in the morphological characteristics of these point bars. Three types are recognized: simple, linguoid and multi‐unit point bars. Sedimentation on the concave sides of meander bends is locally important. The changes of bar type are accompanied by different styles of channel behaviour.The River Tywi is interpreted to have deposited multilateral gravel sheets, composed of partially reworked and abandoned bars and dissected by palaeochannels and sloughs. Bar deposits consist of parallel‐bedded gravel, inclined laterally‐accreted gravel, local angle‐of‐repose foresets and inclined lenses of heterolithic beds. The proportion of the various sedimentary structures and the geometry of the abandoned bars varies along the Tywi valley because of the patterns of bar distribution and channel behaviour.The deposits of this river have strong affinities with Tertiary sequences in the Italian Apennines, previously interpreted as the deposits of meandering gravel‐bed rivers. This type of river is not readily distinguished from ‘Scott type’ braided streams in the geological record, unless exposures are particularly good. In this respect, the presence of abundant, inclined heterolithic wedges and lenses may be a useful dia

ISSN:0072-1050    

DOI:10.1002/gj.3350240304

出版商:John Wiley&Sons Ltd    

年代:1989

数据来源: WILEY

摘要:

AbstractThe Cherty Rock is a laterally persistent horizon that extends across the Inner Moray Firth Basin, northeastern Scotland. The southern exposures of the Cherty Rock near Elgin reveal an indurated carbonate and silica‐rich horizon (0·75‐10m thick) developed in the uppermost part of the largely aeolian Lossiemouth Sandstone Formation. Laminated, pisolitic, and brecciated textures within the Cherty Rock confirm that the horizon represents an analogue to Recent calcretes of semiarid areas. Textures characteristic of silcretes are widespread in the Cherty Rock but these originate from silica replacement of calcite. The carbon and oxygen stable isotopic composition of micrite in the Cherty Rock (mean σ13C = ‐6·9%°, mean σ18O = ‐5·7%°, both relative to PDB) is comparable to that of other Permo‐Triassic calcretes and modern soil carbonate, suggesting that similar processes were responsible for their formation. The σ13C and σ18O values for calcite in the Cherty Rock may indicate that evaporative processes contributed to its development. σ18O data from spar (mean σ18O = ‐9·8%° PDB) and macroquartz (mean σ18O = 19·7% SMOW) indicate that these are later (burial) cements.The Cherty Rock is a useful stratigraphical and palaeoclimatic indicator, and its presence in the Inner Moray Firth basin suggests a period of tectonic stability in the basin

ISSN:0072-1050    

DOI:10.1002/gj.3350240305

出版商:John Wiley&Sons Ltd    

年代:1989

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350240306

出版商:John Wiley&Sons Ltd    

年代:1989

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350240307

出版商:John Wiley&Sons Ltd    

年代:1989

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350240308

出版商:John Wiley&Sons Ltd    

年代:1989

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350240309

出版商:John Wiley&Sons Ltd    

年代:1989

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350240311

出版商:John Wiley&Sons Ltd    

年代:1989

数据来源: WILEY

ISSN:0072-1050    

DOI:10.1002/gj.3350240312

出版商:John Wiley&Sons Ltd    

年代:1989

数据来源: WILEY