Palaeoenvironmental controls on the uranium distribution in an Upper Carboniferous black shale (Gastrioceras listeri Marine Band) and associated strata; England

The mudrocks associated with the Gastrioceras listeri (G. listeri) Marine B and contain 0 to 200 ppm authigenic uranium. Several geochemical (degree of pyritisation (DOP), C/S ratios and framboidal pyrite size distribution) an d palaeontological indicators (oxygen-restricted biofacies (ORB) analysis) suggest that the highest authigenic uranium concentrations are within marin e sediments associated with the most oxygen-restricted biofacies (ORBs 2 an d 3). The uranium peaks tend to occur at the transition between biofacies r ather than within the middle of more persistently anoxic intervals. These h orizons may have been deposited when the oxygen minimum zone impinged on th e seafloor. Brief oxygenation of an otherwise anoxic environment was conduc ive to francolite precipitation, which then scavenged dissolved uranium. Sl ow sedimentation rates are also important in concentrating uranium and fran colite. Thus, enriched U values are only encountered under specific conditi ons of low, but fluctuating, oxygen regime and extremely slow sedimentation rates. Truly euxinic facies. lacking any fossils, and a uniformly small fr amboid population do not contain high concentrations of francolite and are not U-rich. (C) 2001 Elsevier Science B.V. All rights reserved.