Structure of a silica diagenetic transformation zone: the Gjallar Ridge, offshore Norway

Three-dimensional seismic data from the Gjallar Ridge were used together with X-ray diffraction data, scientific boreholes and wireline logs to analyse the seismic structure of a silica diagenetic transformation zone. The following features were identified: (i) an interval some 150 to 300 m thick that contains anomalously high-amplitude reflections; (ii) a strong reflection event at the top of this interval which cross-cuts stratigraphy, interpreted as the transformation boundary between opal-A-rich and opal-CT-rich sediment; (iii) amplitude variations on stratigraphic reflections within the interval, attributed to variations in the proportions of opal-A and opal-CT; and (iv) a second, deeper, cross-cutting reflection event within the interval, interpreted as the transformation boundary between opal-CT-rich and quartz-rich sediment. The base of the interval containing the anomalously high-amplitude reflections is interpreted as a stratigraphic reflection demarcating the base of the silica-rich strata. On a stratigraphic reflection within the interval of high amplitudes, roughly circular regions of anomalously high amplitude with diameters of 0.8 to 2.5 km are separated by lower amplitude regions. This pattern is similar to the cells previously identified at the opal-A to opal-CT transformation boundary and probably results from more complete transformation of opal-A to opal-CT. All of these observations provide the first recognition from seismic data that silica diagenetic transformations are not always narrow boundaries represented by single cross-cutting seismic reflection events, as implied previously, but can be heterogeneous and hundreds of metres in thickness, as observed at outcrop.