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

Abstract

:Jeans, C.V, Turchyn, A.V. and X-F. Hu 2016. 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. Acta Geologica Polonica, 66 (2), 227-256. Warszawa.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 δ 34 S 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 FeOH 3 , 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 δ 34 S from -34.7‰ to +40.0‰. In the lower part of the section δ 34 S vary from -34.8‰ to +7.8‰, a single barite nodule has δ 34 S between +26.9‰ and +29.9‰. In the middle part of the section δ 34 S 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, δ 34 S 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.

The sequestration of trace metals preserved in pyritized burrows

“…The Blue Lias Formation, exposed at Hock Cliff, Fretherne, has alternating mudstones, shales and limestones, with vertical pyritized burrows preserved in dark, grey shale beds (Simms et al, 2004). Southerham Grey Pit, Lewes, has abundant 'worm' tubes, which are pyritized or have pyritized nodules spatially associated with them, found in the Zig Zag Chalk Formation (Jeans et al, 2016).…”

Acta Geologica Polonica

“…18) was part of the Upper Pink Band prior to its sulfidization in latest Cretaceous or early Cenozoic times. This sulfidization was associated with iron sulphide nodules with 34 S/ 32 S values up to as heavy as 40‰ (Jeans et al 2016). Related to this was the regional precipitation of calcite cement in the Northern Chalk Province responsible for the high specific gravity of the chalk.…”

“…There is no evidence that the whole or parts of the Transitional Province was subaerially exposed, but there is little doubt that it restricted interchange between the waters of the Northern and Southern Provinces. The strata of the Transitional Province, as well as those in the most southerly part of the Northern Province, are conspicuously different from those to the north and south -unless they have been affected by late stage non-intrinsic diagenesis during the Cenozoic (Jeans et al 2016). At Tetford, South Thoresby and West Row (Text-fig.…”

“…The regional distribution of iron in Northern and Southern Chalk provinces is complex. The black bands sequences of the Northern Province with their very low Fe 2+ calcite content of less than 50 ppm pass southwards and laterally into pink chalk of the Transitional Province (Text-figs 5, 16) with an Fe 2+ (calcite) content of 200-300 ppm (Jeans et al 2014) that contain a fine-grained hematitic pigment possibly of hydrothermal or volcanogenic origin (Jeans et al 2016).…”

A marker band folgen stratigraphy for the Cenomanian Chalk of England and its extension to northen Germany and France

“…At Speeton, the Upper Inoceramus Bed is 0.8 m thick, consisting of nodular chalk with impersistent marl seams and abundant inoceramid fragments. This bed contains the Brown Flint Band (Jeans 1980;Jeans et al 2016)…”