Leg 160 (Mediterranean I)
Leg 160 was the first of a two leg program in the Mediterranean Sea to investigate the tectonic and paleoceanographic history of the area. During March-April 1995 the JOIDES Resolution recorded for ODP FMS (FMS) and geomagnetic (GHMT) downhole log data for the first time in the Mediterranean Sea and also the standard log data set (Quad Combo logs). Sites 965, 966, 967, 968, 970, 971 were successfully logged and Site 973 partially logged.
Downhole logs obtained at Sites 965, 966, 967, 968, (Eratosthenes Seamount), 970 , 971 (Milano and Napoli mud volcanoes) and 973 (Mediterranean Ridge), seven of the eleven Leg 160 sites, provided valuable data for the interpretation and understanding of the geology investigated at those sites. Due to overall low core recovery at those sites, logging data constitute a unique source of information and are a essential tool to interpret the geology.
The Eratosthenes Seamount-Cyprus margin transect of holes (965 to 968), across an active plate boundary between Africa and Eurasia, located to the south of Cyprus, was designed to test different tectonic hypothesis about this structural feature. Logging data provided information on the rock composition, lithostratigraphy and sedimentary units and also on fracturing. Formation microscanner along with conventional log data allowed to determine the presence of a sedimentary breccia (not undoubtedly identified on cores) at Hole 966f and of non cored evaporites at Hole 967E and to identify fractures and their orientation at Sites 965, 966, and 967. A preliminary shipboard interpretation of the downhole logs provided the location of boundaries between main geologic units and also the characterization of sedimentary facies, recognizable in base of a particular log response (electrofacies). This information is specially valuable due to low core recovery at these "tectonic sites".
At all the Eratosthenes Seamount transect (ESM) sites, both the quad combination (consisting of the natural gamma ray spectrometry, density-photoelectric effect with caliper, neutron, induction and sonic velocity tools) and FMS (FMS) tool strings were successfully run. Additionally, in Holes 966F and 967E the geomagnetic tool (GHMT Geological High-Resolution Magnetic Tool) was used and geochemical logs (GLT Geochemical Logging Tool) were obtained at Hole 967E.
Starting the transect at Site 965 the quad combination tool string, along with the FMS were run at Hole 965A and log data recorded over 180 m depth interval (73.3-253.3 mbsf). Preliminary shipboard interpretation of the quad combination logs combined with the FMS preliminary processed images allowed us to reconstruct the main features of a poorly core-recovered carbonate sequence, of probable Miocene age. The more relevant logging results at this hole are that the rather homogeneous carbonate section can be subdivided into 3 Logging Units that these would correspond to sedimentary or diagenetic facies recognized in base of a different log response. FMS data indicate that the dips are nearly horizontal throughout the logged interval. The lower part of the succession includes apparent cross lamination (recognized on FMS images), while the upper part is essentially planar bedded. High-angle faults can be recognized also on the FMS images.
At Site 966, second site logged on the Eratosthenes Seamount, the quad combination, FMS, and GHMT toolstrings logged over more than 280 m of a Miocene to Quaternary (77.1-357.4 mbsf) sequence. A preliminary shipboard interpretation allowed us to differentiate four Logging Units within the Hole 966F logged section. The Logging Units were characterized on the basis of changes in composition and texture of the sediments recorded by the logging tools as changes in log shape and values. The boundaries between units, placed where major changes occur, have been interpreted as related to main sedimentary discontinuities and facies changes. Logging Unit I corresponds to muddy sediments (lower Pliocene). Units II and III are made up of limestones of Miocene age and Unit IV of limestones of probable Eocene age. FMS images made it possible to confirm that the breccia recovered on top of the Miocene limestones was originally sedimentary (not a drilling breccia) and also allowed us to recognize features of non recovered sediments, like bedding and lamination (Figure 1) in Unit III carbonates or fracturing in Units II and III and intense fracturing within Unit IV (high angle and low angle faults).
Figure 1. Shipboard processed FMS images of Hole 966F at 120 mbsf. The slim hole FMS tool used gives a coverage of about 20% of the borehole wall. From left to right the images recorded by the four pads of the tool. 0 to 360 indicates pad azimuth. A blue line in the image indicates the reference pad (pad 1). Structures and textures, in this case bedding and laminations, are recognizable by color changes that correspond to more resistive the sediments, the lighter the color, or more conductive the darker the color.
The logging data recorded at Hole 967E with the quad combination tool string, FMS, GLT, and GHMT tool strings from 72.9 to 598.6 mbsf, provided valuable information on the geology over several intervals of poor core recovery. On the basis of the log response and in a preliminary interpretation, six Logging Units were characterized within the Cretaceous to Quaternary section drilled and logged at Hole 967E. The boundaries between Logging Units have been interpreted as disconformities. Figure 2 illustrates the interpreted Logging Units and the most relevant features exhibited by the logs These units have a good correspondence with the main units that were defined on the basis of of core lithological and paleontological data. Bedding, laminations, rock textures, and fault zones recognizable on FMS images helped significantly to reconstruct and interpret the geology of a poorly recovered section at Hole 967E. Logging data from the uppermost part of the hole (Logging Unit I, Figure 2) indicate that the occurrence of sapropels is associated with the presence of highly radioactive levels (uranium rich, according to the radioactivity spectral log data). Post-cruise analysis will confirm whether radioactive levels are coincident with the sapropel layers or just associated to sapropel-rich intervals. The sapropel layers were recognizable as well on FMS images due to differences in resistivity compared to the adjacent layers. Preliminary analysis of the quad combination logs, while logging, revealed the presence of an interval about 4 m thick within Logging Unit II (Figure 2), made up mainly of gypsum, observable a as well on the FMS images and detected also by the geochemical logs. The evaporites were not recovered while coring and thus their existence was revealed for the first time by the logging data. The results obtained later on by the geochemists from core samples analyses supported the presence of evaporites where detected by the logs. According to its stratigraphic position and the existing paleontological data the evaporites detected in Hole 967E would correspond to the Messinian event, an important episode in the paleoceanography and evolution of the Mediterranean sea. Logging Units III and IV (Figure 2) correspond respectively to Eocene and Cretaceous pelagic limestones. In base of the logging data, these units can be subdivided into subunits most likely related to facies changes. Sedimentary facies characterization corresponding to the eletrofacies identified will be made post cruise integrating the study of core samples. Logging Unit V corresponds to shallow water carbonates and Logging Unit VI to brecciated carbonates, according to core descriptions. This is also in good agreement with the features recognizable on FMS images. Shipboard processed FMS data showed the "in situ" evidence of the existence of fractured zones and of sets of low and high angle faults, most probably indicating different episodes of faulting. Both open fractures, recognizable by their highly conductive nature that makes them appear on the FMS images as dark colored and sealed (cemented) fractures that appear as bright colored features, could be identified.
Figure 2. Hole 967E Interpreted Logging Units I through VI. From left to right, natural gamma radioactivity (SGR), density (RHOB) logs and a preliminary shipboard reprocessed sonic log (DT), illustrate the changes in log character, related to changes in composition and texture of the sediments. Radioactive intervals related to sapropels, evaporites and other log remarkable features are indicated by the arrows. Va and Vb are two subunits within unit V.
At Site 968, the quad combination and the FMS toolstrings were run in Hole 968A. The quad combination logs were recorded between 71 and 298 mbsf but a bridge encountered around 150 mbsf prevented from logging with the FMS below that depth. A preliminary shipboard interpretation of the logs allowed to differentiate six logging units within the logged section (Miocene? to Pliocene) dominated by clayey lithologies. Changes in the log shape and values occur at boundaries between units and are related to changes in sediment composition. The comparison with cores revealed a good correspondence between sapropel-rich intervals and high radioactivity (uranium rich) levels within Logging Unit I (subunit Ia). Graded sequences were interpreted to exist within Logging Units IV and VI in base of gradual changes and trends recognizable on log data Abrupt changes recognized at the base of Logging Units I, III, IV, and V were interpreted as relevant changes that might be related to sedimentary discontinuities.
Sites 970 and 971 were planned to investigate the origin and evolution of active mud volcanoes in the Mediterranean Ridge. At both Sites, 970 (Milano Mud Volcano) and 971 (Napoli Mud Volcano), the quad combination and the FMS tool strings were run and log data successfully recorded. Hole conditions were poor, especially at Hole 971B, and log data quality is locally negatively affected. The log data obtained at Sites 970 and 971 are specially valuable as far as they recorded the in situ physical properties of the sediments involved in these recent and still active geological features, poorly understood so far. These data will be an essential tool in the research and studies involving the mechanisms and evolution of the mud volcanoes of the Mediterranean Ridge.
Figure 3. Hole 970A interpreted Logging Units I and II. The boundary between them is underlined by a change in log shape and values. From left to right, natural gamma radioactivity (SGR), induction-resistivity (ILD, SFLU, ILM) and density (RHOB) logs. The stripped lines within Logging Unit II indicate the location of the boundaries between subunits IIa through IIe.
At Hole 970A log data were acquired between 64.25 and 202.2 mbsf. In base of the quad combo log features (Figure 3) two Logging Units were characterized. Unit I suffered from extreme poor core recovery, logs have been thus essential to characterize this interval, a rather homogenous unit (Figure 3), made up of pebbly mud to mud breccia, recognizable on shipboard processed FMS images (Figure 4). Light-dark spots recognizable on the images would indicate the presence of pebbles of different composition (different resistivity). Variability in pebble size is also recognizable on the images. Towards the base of the unit a boulder of about 55 cm across was intersected by one pad (Figure 4) of the FMS tool in two successive passes. FMS images showed the presence of layered-laminated intervals and intervals of pebbly mud within Unit II (Figure 3). 5 subunits were identified in base of a changing log response and have an overall good correspondence to changing lithologies observed in the cores. The sapropel-rich intervals are related to highly radioactive (uranium-rich) intervals.
Figure 4.Shipboard processed FMS images of Hole 970A at 131 mbsf. Pebbles appear mostly as light spots (higher resistivity) imbedded by pad 1 (reference pad identified in the image by a blue line). This image was consistent in two passes of the FMS tool and thus can be held as a real feature. Poor pad contact gives rise to unreliable information.
Log data acquired at Site 971 are of low quality due to overall bad hole conditions. The hole conditions were deteriorating while logging and the first quad combination tool string recorded between 59.5 and 171 mbsf but the FMS tool string could be run only to a depth of 128 mbsf. Preliminary shipboard processing of the FMS data allowed a rough recognition of the rock textures of the sediments that made up the mud volcano within the logged interval and the presence of pebbly intervals at the bottom of the logged interval.
At the Ionian transect sites (Mediterranean Ridge) only Site 973 was logged. A reduced set of logs was obtained (induction, sonic and natural gamma ray spectrometry logs) due to time constraints. Nevertheless, these data will allow to better understand the poorly core recovered section of Hole 973D. A preliminary analysis allowed to interpret the presence of a turbidite-rich interval, related to gradual progressive changes in the radioactivity profile of the gamma-ray logs. Downhole logs suggest also the presence of alternations of relatively coarse- and fine-grained sediments. High uranium content levels recognized in the downhole logs, similar to those described for Sites 967, 968 and 970, were interpreted to be related to the presence of sapropel-rich layers.