The Newfoundland margin constitutes the western half of the well-studied Newfoundland–Iberia nonvolcanic margin pair. This conjugate margin system is a type example of nonvolcanic rifting wherein continental breakup is associated with little if any synrift magmatism. Previous Ocean Drilling Program (ODP) legs on the Iberia margin (e.g., Legs 103, 149, and 173), together with coincident geophysical data, have revealed that the final stages of rifting were in part accommodated by the exposure of subcontinental mantle at the seafloor between thinned continental crust and oceanic crust (Boillot et al., 1987; Chian et al., 1999; Krawczyk et al., 1996; Sawyer, Whitmarsh, Klaus, et al., 1994; Whitmarsh, Beslier, Wallace, et al., 1998).
To understand the entire rift system, a characterization of the conjugate Newfoundland margin is also necessary (Fig. F1). Several key questions about this margin have remained unresolved. One of the most important is the origin of an enigmatic section of basement (hereafter called transitional basement) that lies between unambiguous continental crust and the earliest presumed oceanic crust in the region of magnetic anomalies ~M3 and M0 on the Newfoundland margin (Keen et al., 1987; Srivastava et al., 2000; Tucholke et al., 1989). Three interpretations for transitional basement have been proposed:
In seismic reflection sections, this crustal domain appears seismically transparent and is overlain by a set of bright reflections that include the U reflection, a seismic horizon observed throughout the Newfoundland Basin over both continental and transitional basement (Fig. F2). This horizon has been used to constrain the rifting history (Tucholke et al., 1989, in press). Learning the nature and age of these bright reflections is important for attaining a better understanding of both margins.
In addition to recording some of the tectonic history of the Newfoundland margin, the stratigraphic record encountered at ODP Site 1276 also contains information on the paleoceanography of this region. For example, another target of drilling at Site 1276 was Horizon AU, which has been interpreted to be associated with the onset of strong abyssal circulation in the North Atlantic Ocean (Miller and Tucholke, 1983; Tucholke and Mountain, 1979). This paleoceanographic event is thought to represent erosion along the margins of the North Atlantic, followed by deposition of large sedimentary drifts (Jansa et al., 1979; Tucholke and Mountain, 1979; Wold, 1994). The AU reflection is observed throughout the Newfoundland Basin, so learning its age and nature is important for understanding regional paleoceanography. Further analysis of core data and work on the seismic stratigraphy of this part of the Newfoundland Basin since ODP Leg 210 has revealed that there are two possible candidates for the AU reflection in the vicinity of Site 1276 (Tucholke and Sibuet, this volume) (Fig. F2, AU1 and AU2 reflections). By establishing a link between core and seismic data at Site 1276 for this interval, we can place some constraints on this aspect of the postrift history.
To investigate these features prior to drilling, new geophysical data were collected during July and August, 2000, on the Newfoundland margin in positions conjugate to Iberia drilling and geophysical transects, based on the reconstructions of Srivastava et al. (2000) (Funck et al., 2003; Hopper et al., 2004; Lau et al., 2006a, 2006b; Shillington et al., 2006; Van Avendonk et al., 2006) (Fig. F1). Velocity models created from wide-angle seismic data acquired during the Studies of Continental Rifting and Extension on the Eastern Canadian Shelf (SCREECH) experiment suggest that transitional basement is likely thinned, possibly intruded continental crust in its landward part and exhumed mantle in its seaward part (Lau et al., 2006b; Van Avendonk et al., 2006). SCREECH geophysical data were used to select and justify drilling targets on the Newfoundland margin. Drilling on the Iberian margin during ODP Legs 103, 149, and 173 had delineated the nature of basement within the zones of thinned continental crust and exhumed subcontinental mantle by sampling basement highs. To complement this information, the proponents of Leg 210 drilling proposed a deep hole away from basement highs on the Newfoundland margin. The hole was intended both to recover an extensive sedimentary record over transitional basement (from the upper Oligocene through the oldest sediments) and to sample the transitional basement. The deep sedimentary record should provide data that can be used to constrain the timing and character of events associated with late stages of rifting and early seafloor spreading.
Drilling at Site 1276 recovered 936.9 m of core dating from the early Oligocene to the earliest Albian–latest Aptian between 800.0 and 1736.9 meters below seafloor (mbsf), but it did not reach basement (Shipboard Scientific Party, 2004). Two diabase sills were encountered in the lowermost recovered sedimentary section at 1612.7–1623.0 mbsf and at 1719.2 to >1736.9 mbsf; minor, thin (3–31 cm thick) sills occur 7–14 m above the lower sill. The sills are separated by 100 m of sediment that included an ~17-m-thick section of sediments with very low velocities (~1.7 km/s) and densities (~2.05 g/cm3). This layer has been interpreted as undercompacted, and its properties might be related to the emplacement of the postrift sills (Karner and Shillington, 2005; Shipboard Scientific Party, 2004). The upper sill is ~10 m thick, and drilling terminated ~18 m into the lower sill, so its full thickness is unknown. Postcruise work has demonstrated that the sills are postrift (Albian, ~105–98 Ma) (Hart and Blusztajn, 2006; Karner and Shillington, 2005). From preliminary analyses during and immediately following Leg 210, the interval containing sills and undercompacted sediments was estimated to lie at the approximate depth of the U reflection and other bright reflections overlying transitional basement, although synthetic seismograms did not establish a definitive link between either the sills or undercompacted sediments and the U reflection (Shipboard Scientific Party, 2004).
The thick section of cored sediments and the recovery of two postrift sills at Site 1276 present an excellent opportunity to better constrain the history of the Newfoundland–Iberia conjugate margins. A necessary first step to extrapolating detailed information on age and lithology at Site 1276 to the remainder of the Newfoundland Basin is to make a link between core data and coincident seismic reflection data. Logging data are traditionally used to link these data sets by measuring in situ geophysical properties of borehole lithologies at a scale intermediate between seismic and core data. However, logging data could not be collected at Site 1276 because of unstable hole conditions. In lieu of logging data, we present results that link Site 1276 to coincident seismic reflection data by the creation of synthetic seismograms from core physical property measurements. We use the reflectivity method, which includes the contribution of interbed multiples and frequency-dependent interaction in the presence of thin layering (see Kennett, 1983, for a full description). These effects are potentially important given the presence of thin layers associated with large velocity variations at Site 1276. Excellent core recovery (average = 85%) throughout Site 1276 afforded measurement of a detailed physical property data set to use for this purpose. Such data have been successfully applied to the creation of synthetic seismograms for other ODP sites (Bloomer and Mayer, 1997; Edwards, 1998; Norris et al., 2001; Zühlsdorff and Spiess, 2001). Our method includes the following steps: