Microbial crust with Frutexites(?) and iron staining in chalks: Albian–Cenomanian boundary, Hunstanton, UK

Abstract

AbstractNew petrographic observations and stable isotope data help reinterpret the complex sedimentology of the Albian–Cenomanian boundary exposed in the famous red and white chalks of the cliff section at Hunstanton in Norfolk, UK. Thin-section analysis of a prominent crust at the top of the Hunstanton Red Chalk Formation reveals fossil microstructures attributable to the microbe Frutexites. These Red Chalk microstructures are less bushy that Frutexites sensu stricto, and poor preservation, in part caused by later diagenetic iron migration, means they are identified only tentatively as Frutexites. Stable oxygen isotope values from the crust are similar to those from early diagenetic nodular chalks immediately below the crust, and to partially altered chalks elsewhere in Norfolk. The δ18O data are interpreted as Albian seafloor depositional values albeit slightly altered by subsequent meteoric diagenesis. The microbial affinities of Frutexites are not yet proven; thus, the presence of Frutexites alone is not diagnostic of either photic zone or deep-water sedimentation. However, the presence of Frutexites(?) suggests that the red colour of the Hunstanton Red Chalk is due, at least in part, to the mediation of iron-fixing microbes in the accumulating chalk sediment at a dysoxic–anoxic interface. Centimetre-scale columnar and nodular structures above the ‘Frutexites crust’ that project upwards into the basal Paradoxica Bed of the overlying Ferriby Chalk Formation are sites of localized syndepositional iron staining. These nodules are not stromatolitic or microbial and are not evidence for deposition in shallow-water or intertidal settings.