46. GEOLOGICAL SETTING OF SITE 372 46.1. GEOLOGICAL AND GEOPHYSICAL SETTING OF DSDP SITE 372 (WESTERN MEDITERRANEAN) A. Mauffret, L. Montadert, M. Lavergne, and C. Willm, Institut Français du Pétrole 92502 Rueil Malmaison, France INTRODUCTION Rifting Two surveys were carried out by I.F.P.-C.N.E.X.O. During and after the thermal swelling, rifting occur- north and west of Menorca Island in preparation for red, resulting in the creation of horst and graben Leg 42A. These two surveys enable us to interpret the structures. results of Site 372 within a precise geological frame- The rifting in the eastern part of the Gulf of Valen- work. cia is marked by the formation of such features along the Catalonia Margin (Figure 1). The rifting was probably also responsible for the creation of the Valles Geological Framework Penedes graben in Spain. The Mallorca continental Site 372, east of Menorca, is in a transitional area margin is bounded on the northeast by a significant between the Balearic-Provencal Abyssal Plain, the Gulf northwest-southeast trending fault. Prolongations of of Valencia, and the Balearic Rise. Mallorca and Ibiza this feature are seen also between Menorca and Mal- islands belong to the Betic belt which was highly lorca and on the Catalonia Margin itself. On land in tectonized during the middle Miocene (Apostolescu et the region of Barcelona, a dextral strike-slip fault may al, 1974; Bizon et al., this volume; Biju-Duval et al., also be related to this feature. The fault may be a this volume). The Gulf of Valencia and Menorca (?) transform fault linked to a displacement of Menorca. belong to the foreland which was hardly affected by In the Oliogocene, the late-Hercynian conjugate this Miocene orogeny. The recovery of undisturbed fault systems (northeast-southwest and northwest- Burdigalian deposits just east of Menorca (Site 372) southeast trends) were probably reactivated. These supports this conclusion. We may thus, with some structural trends have been active during the Miocene confidence, compare the geology of the eastern margin and Pliocene-Quaternary and are related to the open- of Menorca with the Gulf of Valencia. The ages of the ing and subsidence of the deep western Mediterranean Gulf of Valencia and the Balearic-Provencal Basin are basins. probably similar (Oliogocene to early Miocene) (Le The acoustic basement, as recorded in the area Pichon et al., 1971), and the two early opening stages between the Catalonia Margin and Menorca, is corre- (thermal swelling and rifting) can be recognized in lated with the volcanic layer drilled in DSDP Hole 123 both areas. (ash and volcanic glass dated at 20 ±2 m.y.B.P.; Ryan, Hsü, et al., 1973). North of Menorca, a large magnetic anomaly has been interpreted as evidence of another fracture zone, Thermal Swelling and the Cenozoic volcanoes present in the area may Oil industry data (Stoeckinger, 1976) show that the also be related to this feature. On the "Spartacus Miocene sediments lie directly on Mesozoic sediments S.eamount" (Figure 1), olivine basalt has been on the northwest continental shelf of the Gulf of dredged (Bellaiche et al., 1974). The submarine volca- Valencia, whereas towards the center of the Gulf, the noes associated with Columbretes Island are the super- Mesozoic rocks disappear and the basement is mainly ficial manifestation of a deep linear fracture (Galdeano Paleozoic rocks. and Rossignol, in press). On the other side of the Balearic margin Paleozoic Volcanic rocks were emplaced in the attenuated rocks were dredged (Bourrouilh and Mauffret, 1975). continental crust between the islands of Mallorca and On the Balearic Islands, the Mesozoic is exposed Ibiza and the Valencia Margin (Hinz, 1972). locally. During the Oliogocene, Mesozoic clastic sedi- The Gulf of Valencia was probably formed at the ments were trapped in grabens (Stoeckinger, 1976) same time as the deep Balearic-Provencal Basin (Olig- and continental deposits and evaporites are known ocene to early Miocene), and this aborted rift with a from the Ebro Basin. The lack of Mesozoic rocks in the thin continental crust, is probably linked either to the center of the Gulf may have been caused by erosion drift of the Corsica-Sardinia block or to the opening of linked to this thermal swelling. the North Algerian Basin (Biju-Duval et al., this A. MAUFFRET, L. MONTADERT, M. LAVERGNE, C. WILLM Height or horst Low or graben Old Fault Recent Fault Magnetic anomaly Sampling 50 km BRUTUS "S VOLCANO SPARTACUS DR 21* V'M VOLCANO y W- EBRO DELTA ©CASTELLON B1 © AMPOSTA COLUMBRETESf ISLANDS 1° 2 Figure 1. Structural map of the Gulf of Valencia and adjacent areas. volume). However, the geological evolution of the Gulf Sedimentary Cover of Valencia did not stop during Miocene time. Continu- ous volcanic activity, particularly marked during One profile, J 211 (Figure 3), shows two distinct Messinian and early Pliocene times, occurred up to the depressions filled by a lower, probably clastic unit. The present (Mauffret, 1976). same unit also fills the half graben illustrated in profile J 212 (Figure 4). This series lies on the relief of the ANALYSIS OF THE GEOLOGICAL AND acoustic basement and is directly succeeded by upper GEOPHYSICAL SETTING OF SITE 372 Burdigalian deposits. These were not reached by Hole 372, but they probably are Oligocene continental Structures sediments as occur in the Gulf of Valencia. We also Some of the tectonic events responsible for the suspect that evaporites are present, as indicated by the creation of the Balearic-Provencal Basin and the Gulf nigh-salinity interstitial water measured at the bottom of Valencia are recognized along the eastern and of Hole 372 (Site 372 Report, this volume). Such a northern margin of Menorca. hypothesis is supported by the presence of evaporites On the northern margin an east-west trend is linked in the sediments just below the Miocene deposits of the to the rifting of the Gulf of Valencia. The eastern Valles Penedes graben (Anonymous, 1963). Upper margin contains several northwest-southeast trending Miocene continental sediments were described in horst and graben structures (Figure 2) which are DSDP Hole 133 (Figure 5) on the western Sardinia probably related to the opening of the Balearic-Pro- margin (Ryan, Hsü, et al., 1973). It now seems likely, vencal Basin. Several of the grabens are more than from a re-examination of seismic data, that these series 2000 meters deep. Thus there is good evidence for are lower Miocene to Oligocene and were deposited large-scale rifting in the area east of Menorca. Clearly, during rifting of the margin. as previously indicated, these structures have been Site 372 reached lower Burdigalian sediments (Pro- active up to the present time. Also note that the file J 204, Site 372 Report, this volume) characterized continental slope is parallel to these structural features. by a transparent acoustic layer. The calculated sedi- 890 GEOLOGICAL AND GEOPHYSICAL SETTING OF SITE 372 mentation rate was very high and is probably linked to unit, which can be followed basinward until the ap- the creation of a new topography in Aquitanian times. pearance of the salt layer. More than 500 meters of The upper Burdigalian to Tortonian series is thinner. upper Burdigalian to Messinian deposits have been It is represented in Profile J 212 (Figure 4) as a well- eroded and the upper evaporites may lie directly on stratified acoustic unit covering all the other units. This top of the acoustic basement (Profile J 203, Figure 6). large-scale transgressive unit is also known on land Locally the acoustic basement may also have been (Balearic Islands and Iberian Peninsula). The emer- eroded (Profile J 211, Figure 3). We believe that this gent land areas were consderably eroded during early important erosional event must have occurred on Burdigalian time, then partially submerged during late emergent land. Thus the erosional episode may have Burdigalian time. Thus, the sedimentation rate was been even greater than that which appears on Profile J reduced. The acoustic unit may represent the same 204 or from the results of Site 372 where marine deposit as that existing below the salt layer in the Messinian and upper Tortonian are missing. We should abyssal plain. note in this respect that the Messinian and Tortonian On the Menorca continental rise this unit (I) is on Menorca are represented by a shallow-water car- irregularly covered by thin sedimentary accumulations bonate facies (Bizon et al., 1973) and that the same (Profile J 204, Site 372 report, this volume). Messinian late Miocene erosional surface is present there (Bour- marine sediments have been cored on one of these rouilh, 1973). features (Bizon et al., 1975; Bizon et al., this volume). The abyssal plain salt series is discussed elsewhere We also noted a large erosional surface (Montadert in this volume (Montadert et al.). We note here, et al., this volume), just on top of the well-stratified however, that the evaporites lay at an average depth of 40° 30' 39° 30 4° 30' SCALE 1/250 000 INSTITUT FRANCAIS DU PETROLE Figure 2. Detailed map of the Site 372 area. 891 A. MAUFFRET, L. MONTADERT, M. LAVERGNE, C. WILLM 40°30• Isopachs of infra-evaporitic layers in meters Basement outcrop = I Limit of Pliocene - evaporites overburden = Salt pinch out = IE 40c 40° 30' Basement isobaths in meters 40c J L J 1 I I L •>° • ^ l I 4°30' Figure 2. (Continued). 892 GEOLOGICAL AND GEOPHYSICAL SETTING OF SITE 372 SW MENORCA NE W 3 km 4 [V] Upper Evaporite H Salt Layer W7Å Upper Miocene - Late Burdigalian Vertical exaggeration ×2.5 rm Lower Burdigalian |777?| Acoustic Basement ll~~l Messinian-Tortonian of Menorca Seismic profile I.