Paleoflow directions of Albian basin-floor turbidity currents in the Newfoundland Basin (2007)

Albian turbidites and intercalated shales were cored from ~1145 to 1700 meters below seafloor at Site 1276 in the Newfoundland Basin. Strata at this level dip ~2.5° seaward (toward an azimuth of ~130°) based on seismic profiles. In contrast, beds dip an average of ~10° in the cores. This higher apparent dip is the sum of the ~2.5° seaward dip and a measured hole deviation of 7.43°, which must be essentially in the same seaward direction. Using the maximum dip direction in the cores as a reference direction, paleocurrents were measured from 11 currentripple foresets and 11 vector means of grain fabric in planar-laminated sandstones. Five of the planar-laminated sandstone samples have a grain imbrication ≥8°, permitting specification of a unique flow direction rather than just the line-of-motion of the current. Both ripples and grain fabric point to unconfined flow toward the north-northeast. There is considerable spread in the data so that some paleoflow indicators point toward the northwest, whereas others point southeast. Nevertheless, the overall pattern of paleoflow suggests a source for the turbidity currents on the southeastern Grand Banks, likely from the long-emergent Avalon Uplift in that area. On average, turbidity currents apparently flowed axially in the young Albian rift, toward the north. This is opposite to what might be expected for a northwardpropagating rift and a young ocean opening in a zipperlike fashion from south to north. 1Hiscott, R.N., 2007. Paleoflow directions of Albian basin-floor turbidity currents in the Newfoundland Basin. In Tucholke, B.E., Sibuet, J.-C., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 210: College Station, TX (Ocean Drilling Program), 1–27. doi:10.2973/ odp.proc.sr.210.103.2007 2Earth Sciences Department, Memorial University of Newfoundland, St. John’s NF, A1B 3X5, Canada. rhiscott@mun.ca Initial receipt: 5 January 2006 Acceptance: 14 July 2006 Web publication: 14 May 2007 Ms 210SR-103 R.N. HISCOTT PALEOFLOW DIRECTIONS OF ALBIAN BASIN-FLOOR TURBIDITY CURRENTS 2 INTRODUCTION The source of deep-sea turbidites can be inferred from majorcomponent petrography (e.g., Dickinson et al., 1983; Zuffa, 1985; Marsaglia, this volume), heavy minerals (Hubert, 1971), trace element or rare earth element geochemistry (e.g., McLennan et al., 1990), and/ or paleocurrent analysis (Potter and Pettijohn, 1963). The latter technique might not point unambiguously to the source region if flows took a circuitous path to the site of deposition (e.g., Zuffa et al., 2000). This paper employs paleocurrent analysis using flow directions determined from ripple foresets and grain-fabric studies of Albian sandstone turbidites recovered in Hole 1276A east of the Grand Banks of Newfoundland (Fig. F1). No downhole geophysical logs were obtained in Hole 1276A, preventing the reorientation of core samples to a modern geographic reference frame using, for example, high-resolution Formation MicroScanner (FMS) images. In the absence of this type of straightforward way to reorient samples, a different approach was used: seismic dips, bedding dips in cores, and the availability of a single measurement of the amount of the borehole deviation (but not its direction) were all considered in order to infer the azimuth of the dip line in core pieces. The dip line was then used as a reference direction to reorient the core samples and paleocurrent measurements. Site 1276 is located in 4560 m of water east of the Grand Banks and south of Flemish Cap. Coring began 800 meters below seafloor (mbsf) and proceeded to approximately the Aptian/Albian boundary at 1739 mbsf. The sedimentation rate was generally <5–7 m/m.y. from the earliest Oligocene to the Cenomanian but was ~20–100 m/m.y. during the Albian. As a consequence, the Albian section predominates and extends from ~1145 to 1700 mbsf (Fig. F2). This interval consists of ~ hemipelagic bioturbated mudrocks, ~ mud-dominated gravity-flow deposits, and volumetrically minor but numerous black shales rich in terrestrial organic carbon. The Albian succession belongs to previously defined lithologic Subunits 5B and 5C (Shipboard Scientific Party, 2004a). The proportion of black shales decreases below ~1400 mbsf, whereas the proportion of thick bedded to very thick bedded muddy turbidites, some as graded units thicker than 2 m, increases below ~1500 mbsf (Fig. F3). The thicker turbidites are characterized by ductile soft-sediment deformation and pseudonodule development. Sorting in these beds is poor, and there are common plant fragments and mud clasts. The Albian succession corresponds to seismic Sequence B (Shipboard Scientific Party, 2004a), which is characterized by mostly parallel reflectors that typically extend for tens to hundreds of kilometers. In its upper part, minor local downlap and onlap suggest some depositional relief. There is a slight seaward dip over a broad area around Site 1276. This dip is crucial to unraveling paleocurrents in the Albian succession and will be discussed more fully in “Data and Methods,” p. 4. During the Albian, a narrow seaway had opened between the Grand Banks and the western margin of Iberia (Fig. F4). The central segment of the North Atlantic Ocean was much wider; northwest Africa had separated from Nova Scotia in the Middle Jurassic. By the Albian, the African plate had slid along the south Newfoundland transform margin to a position well east of the Grand Banks. Hence, the narrow seaway at Site 1276 faced a wide ocean basin to the south. Potential source areas for the Albian turbidites include Flemish Cap, the southern Grand Banks, and the more northerly extension of the propagating North At54°W 52° 50° 48° 46° 44° 42° 40° 5000 4000 300