Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. The sedimentology of a vertical succession of alternating beds of massive and fissile diamictons on a 6Þorisjökull plateau icefield outlet foreland is employed to assess the evolution of subglacial traction tills 7 at the margins of active temperate glaciers with deformable substrates. Lodged boulders display strong 8A-axes and surface striae alignments which parallel surface flutings, indicating that fluting construction 9 and till emplacement was related to moulding by consistent glacier flow from the SSW during the Little 10 Ice Age. In contrast, clast macrofabrics at the sub-boulder size, not unlike those reported from other 11Icelandic tills, are not as strong as would be expected in a subglacially sheared medium, indicating shear 12 strains too low for a steady state strain signature. This separation of fabric data has isolated the strain 13 signatures of the lodgement and deformation components of subglacial traction till, whereby the 14 orientations of the largest, lodged clasts record high cumulative shear strains and those of the sub-15 boulder sized clasts record greater susceptibility to deformation of their enclosing matrix. This is likely 16 due to the effect of clast collisions in clast rich till and the perturbations set up by the numerous large 17 boulders, consistent with observations on till fabrics in flutings and around lodged clasts.
Marine driftwood bored by teredinid bivalves (‘shipworms’) is common in the London Clay Formation (Ypresian, Eocene) of southern England. The London Clay of Sheppey, Kent has yielded six genera of teredinids and is the most diverse fossil assemblage of the family known. The presence of several genera that are known exclusively from mangroves at the present day demonstrates the potential value of teredinids in the recognition of ancient mangroves. The rapid sea‐floor burial of logs containing numerous individuals resulted in the borings becoming completely or partially sealed by the plug‐like pallets. In this exceptional mode of preservation, valves, pseudofaeces and pallets are found in life position, and early cementation prevented compaction of the wood and borings. The bored wood acted as centres of carbonate concretion formation. Cement compositions, mineral associations and δ 13 C and δ 18 O stable isotope data indicate that most precipitation occurred in the iron reduction zone, with sulphate reduction also being an important source of carbonate for the Sheppey concretions. Fractures and their infilling cements are demonstrably contemporaneous with the cement zones in which they terminate. It is likely that rapid changes in cement chemistry are a consequence of sudden release of fluid by dewatering events, whilst more gradual shifts in chemistry were controlled be evolution through the oxic, Fe, Mn and S reduction zones. Teredinid boring linings and the cements which infill them may have recrystallized by a dissolution:reprecipitation process which has preserved the chemistry of successive cements and has probably not significantly changed δ 18 O values.
The anatomical features and morphology and lining of the borings of woodboring bivalves are evaluated as taxobases. The aragonitic valves and pallets have poorer preservation potential than the calcitic boring linings that some forms secrete. These linings may be septate, containing retrusive caps.
1997 09 15: Late Jurassic soft-bodied wood epibionts preserved by bioimmuration. Lethaia, Vol. 30, pp. 185-189. Oslo. ISSN 0024-1164.While the encrustation of floating driftwood by pseudoplankton has attracted much debate, the utilization of benthic xylic substrata by sessile organisms has received scant attention. Here we record a benthic woodground fauna, including weakly mineralized and entirely soft-bodied taxa, which have been preserved within the cement of an overgrowing oyster. This process, bioimmuration, is ubiquitous in marine hard-substrate communities but is recorded here on a xylic substrate for the first time. Comparison of bioimmured communities will allow investigation of changes in woodground fauna through time and offers the potential for a fuller understanding of the effect of substrate texture on community composition. OBioimmuration, wood-ground, ctenostome bryozoans.Sidn Evans [S.EvansOnhm.ac.uk],
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