AGE CONSTRAINT AND INTERPRETATIONS OF A HIATUS IN THE NEWFOUNDLAND-IBERIA RIFT BASIN, ODP LEG 210

Ocean Drilling Program Leg 210 cored sediments from 800 to 1725.16 meters below sea floor (mbsf) at Site 1276 in the Newfoundland Basin.  The uppermost cored interval (800 to 885.37 mbsf) contains an assemblage of Oligocene to Eocene microfossils.  Shipboard analyses of calcareous nannofossils, planktonic foraminifers, and organic-walled dinocysts revealed the presence of a significant hiatus that lasted approximately 5.1 to 10.4 m.y. during the middle Eocene, correlating with a sedimentary disconformity at 864.72 m depth, and near a strong seismic reflector at ~960 mbsf.  Post-cruise calcareous nannofossil examination has redefined this event as a condensed interval, possibly containing a hiatus spanning a 3.6 to 6.9 m.y. time period.  The base of this condensed interval occurred at 47.3 Ma, determined by the last occurrence of Rhabdosphaera inflata (Subzone CP12b).  The top of the interval is dated by the last occurrence of Chiasmolithus solitus at 40.4 Ma (Subzone CP14a). 

A change in abundance of Discoaster and Sphenolithus species across this section implies the institution of a new hydrologic regime with higher velocity, colder, more corrosive bottom currents that would result from the tectonic opening of an Arctic gateway.  In addition, linear sedimentation rates at ODP Site 1276 are almost directly proportional to those at Deep Sea Drilling Project Site 398 located on the Iberia side of the rift margin.  Potential causes of the distinct change in sedimentation rate in this interval are 1) relation to Horizon A^u, a seismic reflector representing a regional unconformity caused by increased bottom-water velocity following the subsidence of the Greenland-Iceland-Faroe Ridge, 2) a basin-wide, tectonically forced sea level fluctuation, or 3) a global sea level regression from glacial/interglacial cycles.