Ocean Drilling Program, United States

Forging a link between seismic data and the 936.9 m of core data at Ocean Drilling Program Site 1276 is essential to understand the tectonic and sedimentary history of the wider Newfoundland Basin. Logging data traditionally bridge the gap between core and seismic data by measuring in situ geophysical properties of borehole lithologies at a scale intermediate between seismic and core data. Unfortunately, such data could not be collected at Site 1276 because of unstable hole conditions. In lieu of logging data, we employed shipboard laboratory measurements of density and velocity to create synthetic seismograms in order to tie seismic data to core data at Site 1276. The upper 800 m of sediment at Site 1276 were not cored or logged. To establish a link between the top of the cored section and two-way traveltime, we determined velocities in the upper 800 m by analyzing multichannel seismic reflection data and refined the link by comparing the reflection characteristics of seismic data and synthetic seismograms beneath 800 meters below seafloor. We demonstrate that the prominent U reflection, which is observed throughout much of the Newfoundland Basin, is at least locally created by the shallower of two postrift igneous sills encountered at the base of Site 1276. A brighter reflection that lies beneath the U reflection is generated by the strong impedance contrast between very low-velocity, low-density sediments (interpreted as undercompacted) and the deeper postrift sill. Additionally, we describe linkages between other seismic stratigraphic horizons, such as Horizon A (super U) , and the boundaries between primary lithologic units in Site 1276.