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The erratic rocks of the Upper Cretaceous Chalk of England: how did they get there, ice transport or other means?

Jeans Christopher V., Platten Ian M.

Acta Geologica Polonica

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2021

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Vol. 71, no. 3

287--304

EN

Rare erratic clasts - extraneous rock types - occur in the Upper Cretaceous Chalk, including a local basal facies, the Cambridge Greensand. The underlying Upper Albian Gault Clay and the Hunstanton Red Chalk Formations have also yielded erratics. The discovery of these erratics, their description and the development of hypotheses to explain their origins and significance are reviewed. They became the subject of scientific interest with the interpretation of a particularly large example “The Purley Boulder” by Godwin-Austen (1858) as having been transported to its depositional site in the Chalk Sea by drifting coastal ice. Thin section petrography (1930–1951) extended knowledge of their diverse provenance. At the same time the Chalk Sea had become interpreted as warm, so drifting ice was considered out of context, and the preferred agents of transport were entanglement in the roots of drifting trees, as holdfasts of floating marine algae, or as stomach stones of marine reptiles or large fish. Reconsideration of their occurrence, variable nature and sedimentary setting suggests that there are three zones in the English Chalk where erratics may be less rare (1) near the base of the Cenomanian in the Cambridge area, (2) the Upper Cenomanian-Middle Turonian in Surrey, and (3) the Upper Coniacian and Lower Santonian of Kent. The assemblage from each level and their sedimentary setting is subtly different. Present evidence suggests that the erratics found in the Upper Albian-Lower Cenomanian and the Upper Cenomanian-Middle Turonian zones represent shallow water and shoreline rocks that were transported into the Chalk Sea by coastal ice (fast-ice) that enclosed coastal marine sediments as it froze. The Upper Coniacian and Lower Santonian erratics from Rochester and Gravesend in Kent are gastroliths.

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Redox conditions, glacio-eustasy, and the status of the Cenomanian-Turonian Anoxic Event: new evidence from the Upper Cretaceous Chalk of England

Jeans Christopher V., Wray David S., Williams C. Terry, Bland David J., Wood Christopher J.

Acta Geologica Polonica

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2021

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Vol. 71, no. 2

103--152

EN

The nature of the Cenomanian–Turonian Oceanic Anoxic Event (CTOAE) and its δ13 C Excursion is considered in the light of (1) the stratigraphical framework in which the CTOAE developed in the European shelf seas, (2) conclusions that can be drawn from new detailed investigations of the Chalk succession at three locations in England, at Melton Ross and Flixton in the Northern Province where organic-rich ‘black bands’ are present, and at Dover in the Southern Province (part of the Anglo-Paris Basin) where they are absent, and (3) how these conclusion fit in with the present understanding of the CTOAE. The application of the cerium anomaly method (German and Elderfield 1990) at Dover, Melton Ross and Flixton has allowed the varying palaeoredox conditions in the Chalk Sea and its sediments to be related to the acid insoluble residues, organic carbon, δ18O (calcite), δ13C (calcite), δ13C (organic matter), Fe 2+ and Mn2+ (calcite), and P/TiO2 (acid insoluble residue). This has provided evidence that the initial stages of the δ13C Excursion in England were related to (1) a drop of sea level estimated at between 45 and 85 metres, (2) influxes of terrestrial silicate and organic detritus from adjacent continental sources and the reworking of exposed marine sediments, and (3) the presence of three cold water phases (named the Wood, Jefferies and Black) associated with the appearance of the cold-water pulse fauna during the Plenus Cold Event. Conditions in the water column and in the chalk sediment were different in the two areas. In the Northern Province, cerium-enriched waters and anoxic conditions were widespread; the δ13C pattern reflects the interplay between the development of anoxia in the water column and the preservation of terrestrial and marine organic matter in the black bands; here the CTOAE was short-lived (~0.25 Ma) lasting only the length of the Upper Cenomanian Metoicoceras geslinianum Zone. In the Southern Province, water conditions were oxic and the δ13C Excursion lasted to the top of the Lower Turonian Watinoceras devonense Zone, much longer (~1.05 Ma) than in the Northern Province. These differences are discussed with respect to (1) the Cenomanian–Turonian Anoxic Event (CTAE) hypothesis when the ocean-continent-atmosphere systems were linked, (2) limitations of chemostratigraphic global correlation, and (3) the Cenomanian-Turonian Anoxic Event Recovery (CTOAER), a new term to define the varying lengths of time it took different oceans and seas to recover once the linked ocean-continent-atmosphere system was over. The possibility is considered that glacio-eustasy (the glacial control hypothesis of Jeans et al. 1991) with the waxing and waning of polar ice sheets, in association with the degassing of large igneous provinces, may have set the scene for the development of the Cenomanian-Turonian Anoxic Event (CTAE).

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Sulfur isotope patterns of iron sulfide and barite nodules in the Upper Cretaceous Chalk of England and their regional significance in the origin of coloured chalks

Jeans C. V., Turchyn A. V., Hu X.-F.

Acta Geologica Polonica

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2016

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Vol. 66, no. 2

227--256

EN

The relationship between the development of iron sulfide and barite nodules in the Cenomanian Chalk of England and the presence of a red hematitic pigment has been investigated using sulfur isotopes. In southern England where red and pink chalks are absent, iron sulfide nodules are widespread. Two typical large iron sulfide nodules exhibit δ34S ranging from -48.6‰ at their core to -32.6‰ at their outer margins. In eastern England, where red and pink chalks occur in three main bands, there is an antipathetic relationship between the coloured chalks and the occurrence of iron sulfide or barite nodules. Here iron sulfide, or its oxidised remnants, are restricted to two situations: (1) in association with hard grounds that developed originally in chalks that contained the hematite pigment or its postulated precursor FeOH3, or (2) in regional sulfidization zones that cut across the stratigraphy. In the Cenomanian Chalk exposed in the cliffs at Speeton, Yorkshire, pyrite and marcasite (both iron sulfide) nodules range in δ34S from -34.7‰ to +40.0‰. In the lower part of the section δ34S vary from -34.8‰ to +7.8‰, a single barite nodule has δ34S between +26.9‰ and +29.9‰. In the middle part of the section δ34S ranges from +23.8‰ to +40.0‰. In the sulfidization zones that cut across the Cenomanian Chalk of Lincolnshire the iron sulfide nodules are typically heavily weathered but these may contain patches of unoxidised pyrite. In these zones, δ34S ranges from -32.9‰ to +7.9‰. The cross-cutting zones of sulfidization in eastern England are linked to three basement faults – the Flamborough Head Fault Zone, the Caistor Fault and the postulated Wash Line of Jeans (1980) – that have affected the deposition of the Chalk. It is argued that these faults have been both the conduits by which allochthonous fluids – rich in hydrogen sulfide/sulfate, hydrocarbons and possibly charged with sulfate-reducing bacteria – have penetrated the Cenomanian Chalk as the result of movement during the Late Cretaceous or Cenozoic. These invasive fluids are associated with (1) the reduction of the red hematite pigment or its praecursor, (2) the subsequent development of both iron sulfides and barite, and (3) the loss of overpressure in the Cenomanian Chalk and its late diagenetic hardening by anoxic cementation. Evidence is reviewed for the origin of the red hematite pigment of the coloured chalks and for the iron involved in the development of iron sulfides, a hydrothermal or volcanogenic origin is favoured.