Jesús M. Soria

Differential features of alluvial fans controlled by tectonic or eustatic accommodation space. Examples from the Betic Cordillera, Spain

Abstract The creation of accommodation space in subaerial sedimentary environments is closely linked to uplift the source area (along basin–margin faults) or to a rise in base level. Depending (amongst others) on the relative importance of these two factors, the margins of terrestrial sedimentary basins (where alluvial fans develop) produce aggradational, progradational or retrogradational stratigraphic architecture. We studied a total of 43 Quaternary alluvial fans in the Granada and Bajo Segura basins (Betic Cordillera, Spain). The analysis of their stratigraphic architecture and sedimentary facies has revealed the existence of three groups characterised by distinct stratal stacking patterns, which is the result of differences in eustasy and tectonics. The qualitative and quantitative analysis of these alluvial fans and their drainage basins has allowed us to define certain characteristic geomorphological patterns related, in each case, to the main mechanism operative in the creation of accommodation space. These patterns are described as follows. (1) Alluvial aggradation is the main result of high tectonic subsidence with the development of quite open, but small fans with a constant steep slope lacking incised channels and headward-eroding gullies. (2) In contrast, when the principal mechanism in creating accommodation space is low tectonic subsidence, progradation occurs and the fans are narrower and larger. Moreover, the slopes of such fans are less steep in relation to the drainage area than in the above case, being constant or even concave in longitudinal profile. They also have short, deep incised channels, culminating downfan in a depositional lobe. In inactive zones, long, deep headward-eroding gullies develop. (3) Areas of extremely low tectonic subsidence and base level rise produce very open fans in the sedimentary basin, although with a pronounced mountain embayment, which gives them a characteristic “mushroom” shape in plan view. In the latter case, retrogradation is the typical stratigraphic stacking pattern and the fans are quite extensive with respect to their drainage areas, usually showing a segmented longitudinal profile with a low gradient. The feeder channels do not surpass stream order 3, and various extremely long and shallow incised channels can appear. (4) Contrary to the case of fans developing under a regime of subsidence, when the accommodation is due to base level rise, the fan slopes bear no relation to the extent of the catchment basins. In such a case, the overall fan slope is strongly influenced by the extent of the mountain embayment because it determines the distributions of debris and sheet-flows over the alluvial surface.

Late Miocene stratigraphy and palaeogeographic evolution of the intramontane Guadix Basin (Central Betic Cordillera, Spain): implications for an Atlantic–Mediterranean connection

Abstract The Guadix Basin is an intramontane basin located in the central sector of the Betic Cordillera. Its Late Miocene stratigraphic record is divided into three depositional sequences limited by unconformities representing tectonic and/or eustatic events. Depositional sequence I (late Tortonian) is composed by two systems tracts. The lower marks the beginning of marine sedimentation in the basin, and comprises three retrograding lithological complexes or depositional systems (coastal, shallow platform and pelagic basin), and is interpreted as a transgressive systems tract. The upper systems tract consists of a shallow shelf prograding into a pelagic basin. This systems tract exhibits a typically regressive pattern and was deposited during high sea level, and is therefore interpreted as a highstand systems tract. Depositional sequence II (late Tortonian) is composed of both continental (alluvial cones) and shallow marine (Gilbert-type delta, shallow shelf, and reef) depositional systems. The deposits of this sequence record a significant fall in sea level and are interpreted as a lowstand systems tract. Depositional sequence III (late Turolian–Ventian or late Tortonian–Messinian) was formed of solely retrograding continental depositional systems (alluvial fan, lacustrine fan delta, mud flat, and lacustrine). With this sequence begins the continental sedimentation in Guadix Basin in an endorheic context. The palaeogeographic evolution of the Guadix Basin during the Late Miocene can thus be divided into three stages. The first corresponds to depositional sequence I and is characterised by the development of shallow marine environments on the basin margins and deep water deposits in its centre. During this stage the basin was connected to the west with the Atlantic Ocean via the Granada and Guadalquivir basins and toward the east with the Mediterranean Sea through the Almanzora and Campo Coy basins. In the second stage, represented by depositional sequence II, most of the basin was occupied by shallow marine environments. During this stage the connection with the Atlantic and Mediterranean was only open through the Guadalquivir and Campo Coy basins, respectively. Finally, the third and last stage, corresponding to depositional sequence III, is characterised by the development of continental environments throughout the basin. This stage records the definitive closing of the connections with the Atlantic Ocean and the Mediterranean Sea.

Asymmetrical soft-sediment deformation structures triggered by rapid sedimentation in turbiditic deposits (Late Miocene, Guadix Basin, southern Spain)

SummarySoft-sediment deformation structures in Tortonian turbiditic deposits of the Guadix Basin (southern Spain) have been described. The most common structures are asymmetrical pillow structures and elongated sets of loadcasts. The structures are metric in scale and have been interpreted as the result of liquefaction and/or fluidization processes triggered by the rapid sedimentation of single high concentration turbidites.Final morphology of soft-sediment deformation structures is related to two main driving force systems: unstable density gradient and lateral shear stress. The latter is probably induced by the downslope component of the sediment weight. The asymmetry of deformational structures (in horizontal and vertical cross-section) allows a clarification of the relationship between morphology of deformation and direction of lateral shear stress: this relationship seems ambiguous and confused in the literature. The interpretations both of deformation mechanism and trigger agent have been supported with:-field analyses;-calculations on the liquefaction processes induced by rapid sedimentation;-qualitative models in laboratory.

Late Miocene–Pleistocene tectono-sedimentary evolution and subsidence history of the central Betic Cordillera (Spain): a case study in the Guadix intramontane basin

The Guadix Basin became established as an intramontane basin in the central sector of the Betic Cordillera at the beginning of Late Miocene time. Its geodynamic evolution starts with a unit of Tortonian marine sediments and is completed by a unit of Late Turolian-Pleistocene continental sedi- ments. In the two units, six depositional sequences have been differentiated whose boundaries, in most cases, coincide with tectonic events. Geohistorical diagrams show the results of quantitative analyses of subsidence in the northern sector of the basin and permit correlation of the main events with signif- icant changes in the history of subsidence and uplift. A period of strong subsidence occurred at the beginning of Tortonian time causing the formation of a marine basin 800 m deep. The mid- and end- Tortonian tectonic events involved periods of uplift leading to shallowing in the basin. After continen- talization at the end of the Tortonian, the basin was uplifted continuously from Late Turolian to Late Pleistocene times, finally accumulating sediments at a height of 1000 m.

A Mammalian Lost World in Southwest Europe during the Late Pliocene

Background Over the last decades, there has been an increasing interest on the chronology, distribution and mammal taxonomy (including hominins) related with the faunal turnovers that took place around the Pliocene-Pleistocene transition [ca. 1.8 mega-annum (Ma)] in Europe. However, these turnovers are not fully understood due to: the precarious nature of the periods fossil record; the “non-coexistence” in this record of many of the species involved; and the enormous geographical area encompassed. This palaeontological information gap can now be in part bridged with data from the Fonelas P-1 site (Granada, Spain), whose faunal composition and late Upper Pliocene date shed light on some of the problems concerning the timing and geography of the dispersals. Methodology/Principal Findings This rich fossil site yielded 32 species of mammals, among which autochthonous species of the European Upper Villafranchian coexist with canids (Canis), ovibovines (Praeovibos) and giraffids (Mitilanotherium) from Asia. Typical African species, such as the brown hyena (Hyaena brunnea) and the bush pig (Potamochoerus) are also present. Conclusions/Significance This assemblage is taxonomically and palaeobiogeographically unique, and suggests that fewer dispersal events than was previously thought (possibly only one close to 2.0 Ma) are responsible for the changes seen around 1.9–1.7 Ma ago in the fauna of the two continents.

Onshore and offshore compressional tectonics in the eastern Betic Cordillera (SE Spain)

Abstract This study concerns the northern terminal splay of the Eastern Betic Shear Zone (Betic Cordillera, SE Spain), where the Bajo Segura Basin lies, and its offshore prolongation along the western Mediterranean margin. We have integrated seismic reflection profiles with gravimetry, seismicity, wells and outcrop data from the Bajo Segura Basin and the Alicante shelf in order to determine the current geodynamic setting of the area. The results indicate that the same compressional structures in Upper Miocene–Quaternary rocks are observed both on- and offshore. In the onshore Bajo Segura Basin, there are ENE–WSW growth folds related to reverse faults in the basement. In the Alicante shelf, the main structure is an ENE–WSW anticlinorium which deforms Upper Miocene–Quaternary syntectonic deposits. These compressional structures are still active at present, as shown by the offshore seismicity. From the structural analysis and focal mechanisms of the earthquakes we conclude that the Bajo Segura Basin and its adjacent shelf have been subject to NNW–SSE compression since Late Miocene until the Present. Folding and reverse faulting of the Upper Miocene–Quaternary sedimentary cover and of its basement have accommodated this compression. The main active structures onshore, located in the Bajo Segura Basin, extend eastwards into the Mediterranean Sea. This fact is interpreted as the Eastern Betic Shear Zone continues offshore to the east.

Quantitative subsidence-uplift analysis of the Bajo Segura Basin (eastern Betic Cordillera, Spain): tectonic control on the stratigraphic architecture

The Bajo Segura Basin is located in the eastern Betic Cordillera, at present connected with the Mediterranean Sea to the east. It has a complete stratigraphic record from the Tortonian to the Quaternary, which has been separated into six units bounded by unconformities. This paper is concerned with the northern edge of the basin, controlled by a major strike‐slip fault (the Crevillente Fault Zone, CFZ), where the most complete stratigraphic successions are found. The results obtained (summarised below) are based on an integrated analysis of the sedimentary evolution and the subsidence-uplift movements. Unit I (Early Tortonian) is transgressive on the basin basement and is represented by ramp-type platform facies, organised in a shallowingupward sequence related to tectonic uplift during the first stages of movement along the CFZ. Unit II (lower Late Tortonian) consists of shallow platform facies at bottom and pelagic basin facies at top, forming a deepening-upward sequence associated with tectonic subsidence due to sinistral motion along the CFZ. Unit III (middle Late Tortonian) is made up of exotic turbiditic facies related to a stage of uplift and erosion of the southern edge of the basin. Unit IV (upper Late Tortonian) consists of pelagic basin facies at bottom and shallow platform facies at top, defining a shallowing-upward sequence related to tectonic uplift during continued sinistral movement on the basin-bounding fault. Units V (latest Tortonian‐Messinian) and VI (Pliocene‐ Pleistocene p.p.) consist of shallowing-upward sequences deposited during folding and uplift of the northern margin of the basin. No definitive evidence of any major eustatic sea-level fall, associated with the ‘Messinian salinity crisis’, has been recorded in the stratigraphic sections studied. q 2001 Elsevier Science B.V. All rights reserved.

Engineering-geological model of the Segura River flood plain (SE Spain): a case study for engineering planning

This article presents a sedimentological and geotechnical study of the surficial sediments in the Segura River valley (SE Spain). We formulate an engineering-geological model consisting of four zones, each characterized by its geotechnical properties and by various geotechnical problems (namely low bearing capacity, significant ground settlement and liquefaction of sandy sediments). The model quantifies the geotechnical properties and potential problems in each zone. It serves as a useful tool for preliminary geotechnical investigations. The model also enables a better design of field surveys as well as optimal selection of geotechnical investigation techniques for future civil engineering works.

Quaternary deformation of the Bajo Segura blind fault (eastern Betic Cordillera, Spain) revealed by high-resolution reflection profiling

The blind reverse Bajo Segura Fault is located at the eastern extreme of the Trans-Alboran shear zone (Betic Cordillera, southeast Iberian Peninsula). The surface expression of recent activity of this blind ENE–WSW fault is represented by coseismic surface anticlines and growth synclines on both sides of the anticlines. In the synclines, the deformation of the most recent Quaternary materials is obscured by a sedimentary unit more than 30 m thick which was deposited during the later part of the Late Pleistocene and the Holocene. The present study reports three high-resolution seismic profiles made in the northern growth syncline, which was the one developed most by the Bajo Segura Fault. In these seismic profiles we recognize the boundary between pre-growth strata and growth strata. This marker, Early Pliocene in age, dates the start of the activity of this blind reverse fault. The geometry observed in the seismic profiles of the syntectonic strata, dating from the Late Pliocene and Quaternary, indicates a limb rotation folding mechanism. On seismic profile 2, the complex geometry of the Benejuzar anticline forelimb can be attributed to several splay faults close to the surface of Bajo Segura Fault.

Correlative Lowstand Deltaic and Shelf Systems in the Guadix Basin (Late Miocene, Betic Cordillera, Spain): The Stratigraphic Record of Forced and Normal Regressions

ABSTRACT Marine deposits that crop out in the Guadix Basin (Spain) comprise three upper Tortonian stratigraphic units that are bounded by unconformities representing tectonic and/or eustatic events. The two upper units (II and III) have been interpreted as highstand and lowstand systems tracts, respectively. This work focuses on Unit III, which consists of two shallow-marine depositional systems: one a Gilbert-type delta on the basin margin and the other a shelf system with local reefal patches in its center. Both of these systems lie over slope and deep pelagic basinal depositional systems of Unit II. The bathymetric difference between Units II and III indicates a relative sea-level fall related to a rapid stage of tectonic uplift of the basin. This sea-level fall produced a displacement of the coastline toward the basin interior, resulting in a tectonically induced forced regression, and provided the context for deposition of lowstand systems tract of Unit III. A detailed analysis of the deltaic depositional system of Unit III reveals at least five progradational phases represented by sigmoidal conglomerate lithosomes bounded by erosional surfaces. Overall, these lithosomes are characterized by a reduction in thickness of foresets and by upward-decreasing distances between successive topsets and bottomsets. The five phases of deltaic progradation can be correlated with five phases of aggradation in the time-equivalent shelf depositional system, each of which is represented by thickening- and coarsening-upward successions (TCU); these successions comprise a stacking of marl-sand cycles. The thickness of the TCU successions decreases upward, coinciding with the upward decrease in thickness in the deltaic system. Taking into account the dominant basinward displacement of the brinkpoints of the deltaic clinoforms, with a slight upward component, we have deduced that the progradation of Unit III took place in a context of slightly rising relative sea level and represents a normal regression.