Deepwater dolomites and associated hiatuses in Gulf of Cadiz contourites

Deep-marine regional unconformities are common and significant features of the contourite depositional systems in the Gulf of Cadiz. Some of these unconformities are marked by the development of fine-crystalline dolomite layers, as observed at Sites U1387 and U1391. The timing and mechanism of the dolomite formation are not fully understood.
The mineralogy and geochemistry of these dolomites and adjacent sediments were investigated. Transmitted light microscopy, SEM, X-ray tomography, XRF and stable isotope geochemistry were employed to understand the mechanisms of the dolomite formation.
Although the dolomite layers at these two sites occupy similar stratigraphic position, the associated unconformities are considerably different in duration. At Site U1391, the unconformity represents the early Pliocene discontinuity of 0.05 Ma, separating the initial drift stage below from the transition stage. The unconformity at site U1387 represents combined two hiatuses (late Pliocene and early Quaternary discontinuities) with a time gap of 1.4 Ma. This unconformity separates the initial stages of the drift (Pliocene mixed system) from the modern drift stage (mainly Pleistocene contourites), with the transitional drift stage absent at this site.
The dolomites from both sites show replacement textures with subordinate relicts of the silicate minerals. Furthermore, the dolomites at site U1387 exhibits complex zoning and geochemical pattern compared to those of U1391, indicating a complex history of formation. Low oxygen conditions were deciphered from the foraminiferal assemblage in the associated sediments. Bacterial activities in a cold seep environment may have triggered the development of the dolomites and maintained alkaline medium for a prolonged time. However, the lithostratigraphic and structural regime may have played a considerable role in the dolomitization processes.