Eocene calcareous nannofossil biostratigraphy and sediment accumulation of turbidite sequences on the Iberia Abyssal Plain, ODP Sites 1067-1069

Abstract

Six sites were drilled on the southern Iberia Abyssal Plain during Ocean Drilling Program (ODP) Leg 173. Three holes (1067A, 1068A, and 1069A) recovered Eocene sediments consisting of thinly bedded turbidite deposits with interbedded hemipelagic sediments (Bouma sequence Te) deposited near the calcite compensation depth. The hemipelagic sediments are barren of nannofossils, necessitating the use of the turbidite deposits to erect an Eocene biostratigraphy for these holes. Moderately preserved, diverse assemblages of nannofossils were recovered from silty clays (Bouma sequence Td) and poorly preserved, less diverse assemblages were recovered from sandy/silty clays (Bouma sequence Tc). Hole 1067A has a continuous record of sedimentation (Subzones CP9a–CP14a) and Holes 1068A and 1069A have similar continuous records (Subzones CP9a–CP12a), although all holes contain barren intervals. Holes 1067A, 1068A, 1069A, 900A (ODP Leg 149), and 398D (Deep Sea Drilling Project Leg 47B) display a similar increase in mass accumulation rates in the lowermost middle Eocene. A reliable Eocene biostratigraphy has been erected using nannofossil data from turbidite sequences, allowing for correlation between Iberia Abyssal Plain sites. 1McGonigal, K.L., and Wise, S.W., Jr., 2001. Eocene calcareous nannofossil biostratigraphy and sediment accumulation of turbidite sequences on the Iberia Abyssal Plain, ODP Sites 1067–1069. In Beslier, M.-O., Whitmarsh, R.B., Wallace, P.J., and Girardeau, J. (Eds.), Proc. ODP, Sci. Results, 173, 1–35 [Online]. Available from World Wide Web: . [Cited YYYY-MM-DD] 2Department of Geological Sciences, Florida State University, Tallahassee FL 32306, USA. Correspondence author: roessig@gly.fsu.edu Initial receipt: 20 October 1999 Acceptance: 30 August 2000 Web publication: 30 April 2001 Ms 173SR-008 K.L. MCGONIGAL AND S.W. WISE JR. EOCENE CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY 2 INTRODUCTION The western continental margin of the Iberian peninsula extends from Cape Finisterre in the north to Cape Saint Vincent in the south. The southern Iberia Abyssal Plain (IAP) is bound by a steep continental shelf to the east (Fig. F1), the Vasco da Gama Seamount to the north, and by the Estremadura Spur to the south (Whitmarsh, Beslier, Wallace, et al., 1998). Six sites were drilled during Ocean Drilling Program (ODP) Leg 173 on the West Iberia continental margin to continue the eastwest transect across the ocean-continent transition begun by ODP Leg 149. In addition to investigating the mechanisms of thinning and breakup of the continental lithosphere, a key objective of Leg 173 was to investigate the early sedimentary history of the rifted margin (Whitmarsh, Beslier, Wallace, et al., 1998). Sedimentary sequences overlying structural highs were recovered using the rotary core barrel system at Sites 1065, 1067, 1068, 1069, and 1070. At Holes 1067A, 1068A, and 1069A, we cored continuous middle to lower Eocene upward-darkening turbidite sequences of calcareous siltstone/sandstones, calcareous silty claystones, and claystones (Fig. F2). Pelagic sediments in these holes contain a poor nannofossil record, necessitating the use of nannofossil datums from turbidite sequences to establish a reliable biostratigraphy. Leg 173 sediments were deposited near the calcite compensation depth (CCD) and are similar to sediments recovered during Leg 149 at nearby Site 900 (Shipboard Scientific Party, 1998a, 1998b, 1998c). Site 398 near the Vigo Seamount (Deep Sea Drilling Project [DSDP] Leg 47B) also recovered middle to lower Eocene sediments consisting of fine-grained current and/or turbidite deposits (Sibuet, Ryan, et al., 1979). Turbidites are generally unfit for the construction of an accurate biostratigraphy because of the large number of reworked nannofossils. Studies of Quaternary turbidites on the Madeira Abyssal Plain (MAP), off the east coast of Africa, however, have shown that an accuracy of 50–500 k.y. can be obtained (Weaver and Thomson, 1993) when constructing a nannofossil biostratigraphy from turbidite deposits. Weaver (1994) used synthetic mixtures of nannofossils and compared them to actual deposits to show that turbidite sediment source bodies are tabular in shape and contain sediment with an age range of 50–500 k.y. Additionally, Weaver and Thomson (1993) were able to show that turbidity flows are only slightly erosive in their transport to the abyssal plain, with less than 12% of the flow being derived from erosion of underlying sediments. This technique has since proven useful in Miocene and Paleogene turbidites on the MAP (Howe and Sblendorio-Levy, 1998), where pelagic interbeds were barren of nannofossils. The purpose of this study is to document the calcareous nannofossils from Eocene turbidite sediments on the IAP recovered during Leg 173. Additionally, the use of turbidite sequences in constructing a reliable biostratigraphy in Eocene sediments outside of the MAP has been tested. The nannofossil biostratigraphy has been used to construct agedepth plots, to calculate linear sedimentation and mass accumulation rates, and to establish an early–middle Eocene regional correlation between Holes 1067A, 1068A, 1069A, 900A, and 398D. 1067 1068 1069 398 900 Portugal Iberia Abyssal Plain l in VdG VS