New technique for dating igneous sills

Three-dimensional seismic interpretation of Tertiary sills within the northeast Atlantic margin demonstrates that shallow-level (<1 km) intrusion of igneous sills below the seafloor can deform the interface between water and sediment during its early burial stage by hydraulic elevation of the overburden. We show that a group of interconnected sills forcibly "jacked up" the seafloor, contributing to the creation of a minibasin 5 km wide, 250 m deep, and >20 km long. The fill of the minibasin is dated biostratigraphically as 54.6-55 Ma, which fits well with radiometric dates for the timing of intrusion of sills around this basin (ca. 53-55 Ma). The recognition and dating of onlap-fill seismic reflector packages that are delimited by forced folds form a new and useful method for dating shallow-level igneous intrusions in sedimentary basins, a method that provides independent constraints on radiometric dating techniques. Similar forced folds are recognized from other basins that have shallow-level sills, suggesting that the technique presented here may be similarly applicable in comparable geologic settings.