| Abstract: | Phosphatized limestones from the Pourtales Terrace, Straits of Florida, have undergone fresh-water diagenesis, as shown by several selective petrologic features such as dissolution of aragonitic skeletal material and preservation and recrystallization of Mg-calcite. This interpretation is supported by the fact that greater than 94% of the uranium in these samples is in the U(VI) oxidation state. On the deeper Pourtales Escarpment, phosphatized limestones show no alteration features indicative of fresh-water diagenesis. The uranium in these samples is a mixture of U(IV) and U(VI) with oxidation-state ratios similar to other sea-floor phosphorites. Our interpretation of these data is that uranium in the terrace samples was oxidized when that area was emergent. The karsted surface of the terrace, therefore, is a result of subaerial exposure of this portion of the Floridan platform. The escarpment samples were never above sea level, and so more closely retain their original uranium oxidation-state ratios. |
