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fbtwitterlinkedinvimeoflicker grey 14rssslideshare1
Chidlaw, Nicholas
Languages: English
Types: Doctoral thesis
Subjects: QE
The formations were examined along the Cotswold scarp\ud (160km), and subcrop data were also utilised. The spatial\ud and temporal characteristics of the formations allow\ud sedimentological patterns to be related to structures in\ud the pre-Permian basement and in the overlying Middle\ud Jurassic strata. Sedimentation was strongly controlled by\ud an actively subsiding block faulted basement, which formed\ud part of the North Atlantic Rift system. The generalised\ud Pliensbachian-Bajocian model of Sellwood and Jenkyns (1975)\ud is supported by evidence in the Cotswolds.\ud Both formations show cyclic sedimentation characterised by\ud upward changes in grain size, mineralogy, thickness,\ud sedimentary structures and fauna. Spatial patterns\ud reflect the N-S structures of the basement. Primary\ud controls on the cyclicity are shown to be tectonic rather\ud than eustatic. Five facies are recognised in the Marlstone\ud Rock Bed Formation.\ud The stratigraphic interpretation of the formations is\ud refined. There was a break in sedimentation at the end of\ud the Pliensbachian. The base of both formations is\ud diachronous, and spread from the centre of the basin outwards\ud to both E and W margins. Randomly-interstratified\ud illite-smectite in these rocks is interpreted as a\ud weathering product of illite, while smectite was produced\ud by alteration of air-fall volcanic ash. Both were derived\ud from adjacent land areas.\ud Ferruginous ooids probably formed through mechanical and/\ud or. algal accretion in temporary reducing conditions on the\ud sea bed. The iron-rich sediments were formed at the\ud boundary between siliciclastic and carbonate regimes.\ud True ironstones are virtually absent as a result of\ud rapidly changing patterns of sedimentation within the rift.\ud Widespread 'wavy' bedding is shown to be mostly diagenetic\ud pseudo-bedding, although some appears to have been\ud produced by wave rippling or by compaction alone.
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    • 17. Marlstone Rock Bed Formation: clay mineralogy.
    • 18. Dyrham Silt Formations: clay mineralogy.
    • 19. Marlstone Rock Bed Formation: iron content (weight %).
    • 20. Dyrham Silt Formation: iron content (weight %).
    • 21. Marlstone Rock Bed Formation: Hand specimen descriptions: Lithology and Sedimentary Structures.
    • 22. Dyrham Silt Formation: Hand specimen descriptions: Lithology and Sedimentary Structures.
    • 23. ~Iarlstone Rock Bed Formation: Hand specimen descriptions: Fauna and Flora.
    • 24. Dyrham Silt ~ormations: Hand specimen descriptions: Fauna and Flora.
    • 25. Comparison between hand specimen classification and Particle Size Analysis (Picard) for Dyrham Silt Formation Mudrocks.
    • 26. Marlstone Rock Bed Pormation Facies. Thin Section Petrography: Mean Values.of Components. A. Chamositic Silty Sandstone Facies (I). B. Shelly Charnositic Grainstone Facies (II). C. Grainstone Facies (III). D. Chamositic And Sideritic Grainstone Facies (IV). E. Limoni te-001i te Pseudospar Facies (v).
    • 27. Marlstone Rock Bed Formation: Textural Divisions of Facies.
    • 28. Sources of data for Fig. 50 (l\larlstone I~ock Bed Formation Facies Map).
    • 29. Sources of data for Fig. 51 (Dyrham Silt Formation - Thickness and Facies Maps).
    • 30. tvildcat oil well logs in the Cotswolds passing through the Pliensbachian (Department of .energy) •
    • 31. Sources of data for Fig. 52 (Marlstone I~ock Bed Isopachyte Map).
    • 32. Onshore oil well logs and BGS boreholes - Wessex Basin and offshore areas.
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