Gemma Ercilla

Gas-charged sediments and large pockmark-like features on the Gulf of Cadiz slope (SW Spain)

Abstract Gassy sediments and large seafloor pockmarks are identified from seismic profiles in the Gulf of Cadiz slope. Gas-charged sediments are represented by zones of acoustic turbidity and other acoustic anomalies on seismic profiles. Most of the gas is believed to be biogenic in origin, resulting from the decay of organic matter contained within rapidly developed regressive shelf-margin deltas. A group of large asymmetric seafloor pockmarks also occurs downslope of the gassy area, both on the seafloor surface and deeply buried in the sediment column. Sediment sorting by the Mediterranean Undercurrent seems to act as a controlling factor for the development of each type of gas-originated feature. The area where the strong Mediterranean Undercurrent outflow impinges the seafloor is covered by sandy sediments of high porosity which allow the interstitial gas to escape through the sediment pores, whereas the areas affected by weaker currents are covered by clays, and the gas tends to accumulate in the sediment beneath impermeable horizons.

Contourite processes associated with the Mediterranean Outflow Water after its exit from the Strait of Gibraltar: Global and conceptual implications

We characterize the eastern Gulf of Cadiz, proximal to the Strait of Gibraltar, using a multidisciplinary approach that combines oceanographic, morphosedimentary, and stratigraphic studies. Two terraces (upper and lower) were identified along the middle slope. They are composed of several associated morphologic elements, including two large erosive channels, which allow us to determine a new and more detailed understanding of the Mediterranean Outflow Water (MOW) pathway and its deceleration upon exiting the Strait of Gibraltar. There is evidence for along-slope circulation and additional secondary circulation oblique to the main flow. The present upper core of the MOW flows along the upper terrace and the lower core flows along the lower terrace. However, the lower terrace shows larger and better defined erosive features on the seafloor than does the upper terrace; we attribute this to a denser, deeper, and faster MOW circulation that prevailed during past cold climates. Development of the present features started ca. 3.8–3.9 Ma, but the present morphology was not established until the late Pliocene–early Quaternary (3.2 to older than 2.0 Ma), when the MOW was enhanced, coeval with global cooling, a sea-level fall, and an increase in thermohaline circulation. We propose a direct link between the MOW and the Atlantic Meridional Overturning Circulation and therefore between the MOW and both the Northern Hemisphere and global climate. Our results have enabled a better understanding of a major overflow related to an oceanic gateway, and are of broad interest to geologists, climatologists, oceanographers, and petroleum geologists.

POTENTIAL GEOLOGIC HAZARDS ON THE EASTERN GULF OF CADIZ SLOPE (SW SPAIN)

Abstract Geologic hazards resulting from sedimentary, oceanographic and tectonic processes affect more than one third of the offshore Gulf of Cadiz, and are identified by interpreting high-resolution seismic profiles and sonographs. Hazards of sedimentary origin include the occurrence of slope instability processes in the form of single or multiple slumps occupying up to 147 km 2 mainly concentrated in the steeper, upper slope area. Besides the presence of steep slopes, the triggering of submarine landslides is probably due to seismic activity and favoured by the presence of biogenic gas within the sediment. Gassy sediments and associated seafloor pockmarks cover more than 240 km 2 in the upper slope. Hazards from oceanographic processes result from the complex system of bottom currents created by the interaction of the strong Mediterranean Undercurrent and the rough seafloor physiography. The local intensification of bottom currents is responsible for erosive processes along more than 1900 km 2 in the upper slope and in the canyons eroded in the central area of the slope, undermining slopes and causing instability. The strong bottom currents also create a mobile seafloor containing bedforms in an area of the Gulf that extends more than 2500 km 2 , mostly in the continental slope terraces. Hazards of tectonic origin are important because the Gulf of Cadiz straddles two major tectonic regions, the Azores–Gibraltar fracture zone and the Betic range, which results in diapir uplift over an area of more than 1000 km 2 , and in active seismicity with earthquakes of moderate magnitude. Also, tsunamis produced by strong earthquakes occur in the Gulf of Cadiz, and are related to the tectonic activity along the Azores–Gibraltar fracture zone.

History of mud diapirism and trigger mechanisms in the Western Alboran Sea

Abstract The distribution and morphology of the mud diapirs in the Western Alboran Basin were studied using multichannel and high resolution seismic profiles (airgun). The diapirism in the Western Alboran Basin forms diapiric ridges and mud volcanoes. Three types of contact relationships between the diapirs and the sedimentary cover have been identified: (1) inverse-piercing contacts; (2) normal-fault contacts; and (3) subvertical contacts. These geometric relationships and the study of the sedimentary cover composed of six seismic units (Lower Miocene to Quaternary) allow us to establish the timing and the geodynamic framework under which the diapirism has evolved. The diapir distribution was controlled through the geodynamic evolution of the Alboran Basin related to extensive, compressive and strike-slip processes. Four phases of diapirism are proposed in order to explain the evolution of diapirism: reactive, active, passive and collapse phases. We establish an evolutionary model of diapirism from the Langhian to the present. Diapirism started in the Langhian-lower Serravallian controlled by extensional processes that allowed a reactive phase to develop. Upper Serravallian-lower Tortonian diapirism was characterized mainly by active diapirism induced by compressional and strike-slip processes. During the upper Tortonian-lower Messinian passive diapirism was predominant until the upper Messinian, when an active diapiric phase developed under a transtensive tectonic setting. Extensional processes continued acting during the Pliocene and Lower Quaternary in the Western Alboran Sea, inducing reactive diapirism though it was punctuated by several active phases. The Upper Quaternary was characterized by a generalised collapse phase linked to extensional processes that developed in many mud ridges, although mud volcanism and active diapirism was also favoured by differential loading produced by a contouritic drift.

Sedimentary evolution of the northwestern Alboran Sea during the Quaternary

Five depositional bodies occur within the Quaternary deposits of the northwestern Alboran Sea: Guadalmedina-Guadalhorce prodelta, shelf-edge wedges, progradational packages, Guadiaro channel-levee complex, and debris flow deposits. The sedimentary structure reflects two styles of margin growth characterized: 1) by an essentially sediment-starved outer, shelf and upper slope and by divergent slope seismic facies; 2) by a prograding sediment outer shelf, and parallel slope seismic facies. Eustatic oscillations, sediment supply, and tectonic tilting have controlled the type of growth pattern, and the occurrence of the depositional bodies. Debris flows were also controlled locally by diapirism.

Structure and geodynamic evolution of the central Bransfield Basin (NW Antarctica) from seismic reflection data.

Abstract The Bransfield Basin is a young active rift basin located between the northern margin of the Antarctic Peninsula and the South Shetland Islands margin. Deception and Bridgeman islands divide the Bransfield Basin in three subbasins, the western, central and eastern. Specific morpho-tectonic features and sediment fill differentiate each subbasin. The structure and geodynamic evolution of the Central Bransfield Basin, which is in a stage of incipient seafloor spreading, have been investigated in detail from a dense grid of single-channel seismic reflection data. The Central Bransfield Basin is dominated by two families of normal faults which are oriented northeast and northwest. The NE-trending faults define three graben systems that are roughly parallel to the basin axis. In an across-basin direction, the mean trend of this family of faults ranges from N71 (the graben system nearest to the Antarctic Peninsula) over N64 (the intermediate graben system), to N53 (the graben system nearest to the South Shetland Islands). The NW-trending family of faults is responsible for the deepening of the basin from southwest to northeast. Both families of faults define the overall Central Bransfield Basin structure, resulting in a complex division of the basin floor. Additionally, tens of volcanic edifices are located on the basin floor, the larger ones being associated to the NW-trending faults. Interaction of tectonics and sedimentation give place to the differentiation of three tectonostratigraphic units, TU1, TU2 and TU3 (from oldest to youngest). TU1 occupies the southernmost graben system, and it is affected by the NE-trending bounding normal faults. TU2 extends further northwestwards than TU1 and essentially fills the intermediate graben system. TU3 represents a further extension of the sediment infill over most of the Central Bransfield Basin, and marks the initiation of the infill of the northernmost graben system. Faults bounding this graben also determine the straight and abrupt morphology of the South Shetland Islands margin. The observed arrangement of the graben systems and the filling tectonostratigraphic units reveal a migration of extensional tectonics and associated depocentres from the Antarctic Peninsula margin to the South Shetland Island margin. The left-lateral rotation from N71 to N53 in the mean trend of the three successive graben systems could have been produced by the oblique subduction of the Phoenix plate and the effect of the sinistral strike-slip movement of the South Scotia Ridge, 200 km northeastwards of the Central Bransfield Basin.

Post-Calabrian sequence stratigraphy of the northwestern Alboran Sea (southwestern Mediterranean)

Abstract The post-Calabrian sedimentary column of the northwestern Alboran Sea comprises three depositional sequences. The two older depositional sequences are defined by lowstand systems tracts (shelf-margin deltas, slope, base-of-slope, and basin deposits, and the Guadiaro channel-levee complex). In contrast, the most recent depositional sequence also includes transgressive (relict shelf facies) and high-stand (the Guadalmedina-Guadalhorce prodelta and hemipelagic facies) systems tracts. The stratigraphic architecture of these depositional sequences is controlled by the synchronism between high frequency sea-level changes, variations in sediment supply, and sedimentary processes. The configuration of the depositional sequences is variable and their distribution is complex, as a result of the relative importance played by sea-level changes and tectonism through the area. The sequence boundaries are represented by polygenetic surfaces in the proximal margin, and by monogenetic surfaces in the distal margin and basin. Each polygenetic surface results from the interaction between the sequence boundary with the lowstand erosional truncation surface and the transgressive surface, both developed during the previous sea-level cycle. The monogenetic surfaces correspond to unconformities and their correlative conformities, formed during sea-level lowstands. This pattern of depositional sequences developed in the margin and basin of the northwestern Alboran Sea shows differences with the Exxon Sequence Stratigraphy Model as traditionally applied: sea-level change control is essentially recognized through lowstand systems tracts, and sequence boundary coincides with lowstand erosional truncation surface and transgressive surface, both developed during the previous sea-level cycle.

Pleistocene progradational growth pattern of the northern Catalonia continental shelf (northwestern Mediterranean)

The Pleistocene sedimentary growth pattern of the northern Catalonia continental shelf is characterized by the vertical stacking of seaward downlapping regressive deposits. These deposits are characterized by a progradational development, with oblique clinoforms of low angle in the middle continental shelf, that become more inclined seaward in the outer continental shelf and shelfbreak. Eustatic sea level fluctuations controlled the development of this sedimentary pattern, whereas sediment supply conditioned the nonuniform progradation along the continental shelf and subsidence due to both sediment loading and tectonics controlled its preservation through and along the continental shelf.

Holocene depositional history of the Fluviá—Muga prodelta, northwestern Mediterranean Sea

Abstract Seismic analysis of the Fluvia-Muga prodelta, located on the Gulf of Rosas continental shelf, northwestern Mediterranean Sea, is a prograding oblique-sigmoidal depositional wedge. The internal seismic pattern of this wedge records its sedimentary history developed during the Holocene sea-level highstand. Three major types of seismic facies have been identified within the prodeltaic wedge: (1) continuous stratified facies, that characterizes the proximal prodelta environment; (2) discontinuous stratified facies, that describes the middle prodelta environment; and (3) a transparent facies representing the distal prodelta environment. The growth pattern of the prodelta is chiefly controlled by the seasonal character and low water discharge of the Muga and Fluvia rivers, the multiple changes in their mouth locations, the local shelf-current regime developed in the Gulf of Rosas embayment and the predominance of gravity-induced sedimentary processes.

Turbidity current sediment waves on irregular slopes: observations from the Orinoco sediment-wave field

Abstract The Orinoco sediment–wave field covers an area of at least 29 000 km2 on the southern margin of the Orinoco Valley, at a water depth of 4400–4825 m. Wave dimensions are highly variable across the wave field, with wavelengths of 110–2600 m, and wave heights of 1–15 m. Slope gradients are also very variable, with values of 0.14–0.48°. Overall, the sedimentary sequence on the upslope wave flanks is about 40% thicker than that on the downslope flanks, leading to upslope wave migration in the manner of antidunes. In addition, reflectors on the upslope flanks generally display higher reflectivity than those on the downslope flanks, suggesting that a higher proportion of coarser sediment occurs on the upslope flank. An unconfined turbidity current origin is proposed for the Orinoco sediment waves, based upon detailed analysis of regional stratigraphic/seismic facies, and sediment wave distribution, morphology and dimensions. Sediment waves are not related to flows passing along (or spilling out of) the Orinoco Valley, or to bottom currents flowing parallel to the slope. Turbidity currents responsible for wave generation are interpreted as originating from slope failures on the adjacent Venezuela, Guyana and Suriname continental margins. Simple numerical modelling has enabled turbidity current flow characteristics across the Orinoco sediment waves to be estimated: internal Froude number=0.7–1.1, flow thickness=24–645 m, and flow velocity=31–82 cm s−1. A key finding of this study is that there appears to be a close relationship between the changes in slope gradient and those in wave dimensions across the wave field. The irregular gradient of the present-day wave field is partly a reflection of the irregular bounding surface of the sediment waves, which is represented by mass-flow deposits and associated mud diapirs. The changes in slope gradient along this lower boundary lead to variations in the flow thickness and flow velocity of passing turbidity currents, which in turn control the wave dimensions. Generally, on lower gradients beyond minor breaks of slope, flow thickness increases and flow velocity decreases, leading to an increase in wavelength and a decrease in height.