ODP Leg 173 : Return to Iberia

Objectives

The objective of ODP Leg 173 was to drill an east-west transect of Sites on the West Iberian margin to study the ocean-continent transition zone (OCT). A similar transect had already been drilled during ODP Leg 149, which determined the landward and oceanward limits of the OCT. Leg 173 Sites (Figure 1) were chosen to complement this transect by coring crystalline basement from several topographic highs, principally within the OCT. The main aim was to characterize the tectonic and magmatic processes that dominate the transition from continental to oceanic crusts and to determine the role of detachment fault tectonics in the evolution of the margin and the role and extent of syn-rift magmatism in the OCT.

Crystalline basement was sampled at several Sites during the Leg 173. Unfortunately, bad hole conditions prevented the passage of the logging tools to basement depths. Therefore, no downhole measurements were made of crystalline basement or across the sediment-basement interface. The main scientific results of the Leg 173 can be found at the Ocean Drilling Program and in Beslier M.-O., Whitmarsh R.B. and Wallace P. et al. (in preparation).

Logging

Logging data were acquired in ODP Holes 1065, 1068 and 1069 during Leg 173 (Table 1). The Triple Combo tool string was used in each Hole while the Formation MicroScanner (FMS) / Sonic tool combination was used only in the first Hole. The Triple Combo included the phasor induction tool (DITE), the integrated porosity-lithology tool string (IPLT) and the Lamont temperature tool (TLT). The IPLT consists of a spectral gamma-ray tool (HNGS), a porosity tool (APS) and a litho-density tool (HLDS). The FMS/Sonic string included the FMS, the GPIT which provides geographical orientation for the FMS images, the array sonic (SDT) and a natural gamma-ray tool (NGT).

In Hole 1065A, geophysical logging data overlapped with a 350 meters cored interval of pre/syn-rift (? post rift) sedimentary rocks. In this zone core recovery was low (12 %) so logging data helped determine the lithostratigraphic units identified from core observations. Logging data also provided more detailed sedimentary information, identifying layers dominated by clay, sand and calcareous fractions and their relative cementation state. Also, in the upper part of the hole, which was not cored, lithologic units described from Hole 900 cores (Leg 149 ; Sawyer, D., Whitmarsh, R.B. and Klaus, A. et al., 1994.) can be inferred from logging data. Unfortunately only poor quality FMS images were recorded in Hole 1065A, however, sedimentary dips and azimuths were calculated in two zones. Between 360-570 mbsf dips ranged from 10°-20°, with azimuth varying from 045° to 090°. Below 570 mbsf, average dips increased to 20°-40°, with azimuths changing from 000°-020° at 586 mbsf, to 100°-130° at 595 mbsf, and 080°-110° at 600-605 mbsf.

Site 1068 is located 600 m to the west of Site 900 on a topographic high which was previously drilled during Leg 149. Site 1069 is located 43 miles to the west of Site 1068 on the adjacent topographic high. At both Sites (1068 and 1069) the overlap between logging data and cores is restricted to an interval of 60 meters. Gamma-ray and resistivity logging data are also available at Site 900 at comparable depths. Figure 2 presents a comparison of gamma-ray data in Holes 900, 1068 and 1069 from east to west which clearly shows the lateral continuation of sedimentary units between Sites. Observed depth variations between correlated features is probably a function of varying sedimentation rates between the Sites. Differences between Sites 1068 and 1069 suggest a greater rate during the Miocene at Site 1069 (top of the logged section), a slightly lower rate during the upper Eocene, and a similar rate during the middle to lower Eocene (below 600 mbsf) relative to Site 1068. To the contrary, sedimentation rates seem to be very similar at Sites 1069 and 900.

Core-FMS images correlation

During Leg 173 the DMT digital CoreScan system was used to collect whole round, 360 degree circumferential and split core images. Unfortunately, high resolutionFMS images were not recovered during Leg 173 that would have provided measurements on the borehole wall at the core scale, allowing accurate core-log integration. However, excellent microresistivity images of the borehole wall were collected during Leg 149 at Site 900 (Figure 3). These images clearly show the thinly bedded turbidite/contourite sequences (Unit IIB) that were cored during both Legs. Future work using automated digital image processing techniques should enable the production of synthetic core scale physical properties logs that can help constrain and calibrate the geophysical log response through these units where recovery was low.

Conclusion

Bad hole conditions prevented the acquisition of logging data in crystalline basement during Leg 173. Therefore, the main logging objectives of Leg 173 were not reached. However, a comprehensive log dataset now exists for the sedimentary sequences of the Iberia margin, which should provide and insight into the pre-, syn- and post-rift sedimentary processes of this region.

Logging Scientists:
Véronique Louvel, Laboratoire de Mesures en Forage, IMT, Marseille, France

Adrian Newton, Leicester University Borehole Research Group, Leicester, UK

Bibliographic references

Beslier, M.-O., Whitmarsh, R.B. and Wallace, P. et al. (in preparation). Proc. ODP, Init. Repts., 173: College Station, TX (Ocean drilling Program).

Sawyer, D., Whitmarsh, R.B. and Klaus, A. et al., 1994. Proc. ODP, Init. Repts., 149: College Station, TX (Ocean drilling Program).