Abstract: This paper provides for the first time a detailed vertical and spatial representation of Quaternary evolution of the contourite depositional system (CDS) in the Gulf of Cadiz, based on the results of careful morphological, structural and stratigraphic analyses using high-resolution seismic reflection profiles as well as oil company borehole data, and piston and gravity cores. Different drifts observed on the stratigraphic architecture allow us to propose a regional Quaternary evolution for the whole system, in which three major stages can be identified. (1) In the Early Pleistocene to Mid-Pleistocene, the CDS was mainly dominated by depositional processes, where the upper and lower cores of the Mediterranean Outflow Water (MOW) generated the mounded elongated Cadiz-Faro-Albufeira drift in the transition between the middle and upper slope, and the equivalent Huelva-Guadalquivir drift on the middle slope. During this stage the main erosive features were established close to the Strait of Gibraltar. (2) In the Mid-Pleistocene to Late Pleistocene, two important changes in the CDS took place. One occurred at the transition between the middle and upper slope, related to a change in the upper branch of the MOW, when a mixed drift began to develop, burying the eastern part of the Cadiz-Faro-Albufeira mounded elongated and separated drift. The second change is observed on the central area of the middle slope, related to the lower branch of the MOW, where a large contourite channel (the Guadalquivir channel) progressively eroded the western part of the mounded Huelva-Guadalquivir drift. Laterally an extensive sheeted drift buried the previous mounded deposits. (3) In the Late Pleistocene to Holocene, in the northern area of the CDS, a plastered drift started to be developed in the transitional zone between the upper and middle slope. On the middle slope, the mounded elongated Huelva-Guadalquivir drift was not developed and more erosive processes became dominant as the lower core of the MOW intensified. In the sector close to the Straits of Gibraltar, a field of broad seabed forms was generated. These three evolutionary stages have been controlled by tectonics, including recent diapiric movement, Guadalquivir Bank uplift, and reactivation along several fault systems and anticline-syncline structures. Tectonics has been a key factor in the sea-floor morphological changes, which has caused new pathways for the core and branches of the MOW, and consequently has produced the contourite stratigraphic and architectural changes. Superimposed on these tectonic changes, both climatic and eustatic changes during the Quaternary (but especially from the Mid-Pleistocene) have controlled the development of vertical contourite stratigraphy. The general conclusion of this study is that the contourite depositional system of the Gulf of Cadiz has changed from a dominantly depositional system to a dominantly erosive one during the Quaternary.

Abstract The distribution patterns and internal geometries of recent shelf-margin wedges off the Guadiana River, Gulf of Cadiz margin, were studied in order to discern the effects of varying trends of falling sea-level and lowstand on upper slope progradation. A seismic-sequence stratigraphic analysis was conducted, based on the interpretation of a dense grid of high-resolution seismic profiles. Five major shelf-margin wedges deposited during late Quaternary sea-level fall and lowstand periods were documented. Most of the studied shelf-margin wedges produce upbuilt-outbuilt upper slopes. The analysis of their internal geometries reveals two distinct types of configuration: the four older shelf-margin wedges are mainly composed of forced regressive deposits developed during stepped sea-level falls. The scarceness of lowstand deposits suggests abrupt sea-level fall to rise transitions; and the most recent shelf-margin wedge shows both forced regressive deposits and low-stand deposits significantly preserved. This architecture probably resulted from the occurrence of a long-lived sea-level lowstand after a prolonged period of gentle sea-level fall. Two types of shelf-margin wedges have been recognized: wedges with elongate parallel depocentres, laterally constant thickness and uniform seaward shelf-break migration, related to linear source supply and high lateral sediment redistribution during gently falling sea level; and wedges with (multi) lobate depocentres with laterally variable thickness, leading to uneven shelf-break migration during periods of faster sea-level fall, due to delta lobe switching and significant shelf valley incision.

Abstract The northern margin of the Gulf of Cadiz is swept by Mediterranean Outflow Water between about 500 and 1000 m water depth. This warm, saline, thermohaline, bottom current attains velocities in excess of 1 m s −1 through the narrow and relatively shallow Gibraltar gateway, and then descends and slows as it moves towards the north and west around the Iberian margin. It was established in its present form in the latest Miocene, following tectonic re-opening of the Gibraltar gateway, and has since helped to sculpt the slope region in conjunction with downslope processes and diapiric intrusion. The principal area of contourite deposition, up to 600 m in thickness, is the Faro–Albufeira drift complex in a mid-slope setting some 30 km south of Faro. This comprises an elongate low-mounded drift (Faro–Albufeira) and adjacent broad sheeted drifts (Faro and Bartolomeu Dias Planaltos), flanked and partly dissected by deep, erosional, bottom-current channels and buried channels. The seismic character is one of progradational-aggradational depositional units with laterally extensive sub-parallel reflectors, widespread discontinuities and a large-scale cyclicity in seismic facies. The upper 10 m of cored section comprises muddy, silty and sandy contourites of mixed terrigenous and biogenic composition, that show small-scale cyclicity in grain size and associated sedimentary features. Rates of accumulation varied from < 1 to 14.5 cm ka −1 (cores), and 3.5 to 29.5 cm ka −1 (seismics). The large and small-scale cyclicity noted can be related to fluctuation in bottom current velocity related to climate and sea-level changes, although the precise correlation between these events remains uncertain.

Abstract Single-channel, very high-resolution seismic profiles allow detailed study of the Late Quaternary stratigraphic architecture of the Gulf of Cádiz continental margin, Southern Spain. The Late Quaternary stratigraphy of this area comprises fourth-order Type 1 composite depositional sequences, generated by asymmetric relative sea-level changes of 100–110 ka duration. The composite fourth-order sequences consist of forced regressive, lowstand, transgressive and highstand systems tracts. Volumetrically, the forced regressive and lowstand systems tracts are the most important components. The fourth-order composite sequences are themselves comprised of composite fifth-order sequences formed in response to asymmetric relative sea-level changes with a duration of 22–23 ka. Sediments within the forced regressive and lowstand systems tracts dominate the 5th-order sequences; their transgressive and highstand deposits are either (i) perched above present-day sea-level and so not recorded in marine seismic data, (ii) restricted to outer-mid-shelf positions, or (iii) may be absent from the shelf altogether at the resolution of this study (e.g.<0.5 m thick). The fifth-order sea-level falls were themselves modulated by minor cycles, generating very high-frequency (sixth-order) sequences. These very high-order sequences are recognized for the last 80 ka bp, and their development is attributed to asymmetric relative sea-level cycles operating on time scales of: 10–15 ka (Heinrich events), 4–4.5 ka. (P cycles), 2.3–0.97 ka. (Dansgaard-Oeschger oscillations h cycles) and 500–50 a. (c cycles). We have developed a depositional model that accounts for the very high-frequency hierarchy of Late Quaternary depositional sequences observed in the Gulf of Cadiz marine seismic record and incorporates the age of well-constrained highstand coastal deposits that are exposed along the southern Iberian coastline. The model developed serves to illustrate the evolution and importance of depositional systems during falling relative sea-level and forced regression. Development of the forced regressive systems tract appears to be particularly significant within Quaternary strata. This is because the Quaternary was strongly influenced by a high-amplitude, high-frequency glacioeustatic signal characterized by rapid sea-level rises, very short highstands, and gradual relatively long-term sea-level falls suggesting that forced regressive deposits are likely to predominate in continental margin successions subject to low rates of subsidence.

Abstract Hydroxyl reactivity of synthetic goethite, gibbsite, and two series of silica-alumina gels having a wide range of Si/Al composition (one series having characteristics of allophane) were studied by phosphate adsorption and “ammonium oxalate reactivity” (Ro), where Ro = amount of hydroxyls released in the reaction of amonium oxalate with hydroxylated surfaces at pH 6.3 over 25 min. Ro ranged from 14 mmole/100 g for goethite to 61 and 106 mmole/100 g for gibbsite prepared at pH 4.5 and 6.5, respectively. In the silica-alumina gels, Ro ranged from 90 to 442 mmole/100 g. In both series of gels, Ro reached a maximum at a SiO 2 /Al 2 O 3 ratio near 1. At this composition the Al and Si that dissolved during the reaction had the highest Al/Si ratio. Ro was also determined on samples heated to 105°C. In all but one sample of the allophanic series, Ro decreased on heating, the largest decreases being observed for the more reactive samples. In the most Si-rich member of the series (SiO 2 /Al 2 O 3 = 1.5), however, Ro increased on heating by 24 mmole/100 g. This result may explain the Ro values observed for Andisols containing Si-rich allophanes. Here, Ro increased on heating to 105°C. Active OH groups on goethite, as determined by Ro, were found to be in agreement with those determined by phosphate adsorption. The maximum phosphate adsorption for the allophanic gels was displayed by samples having the highest Ro values.