Unconformity-bounded Stratigraphic Units Research Articles

Structural Control at Monte Somma and Vesuvio during the Last 5600 Years through Time and Space Distribution of Volcanic Vents

Vesuvio is likely the most if not one of the most dangerous volcanoes in the world. It is an active volcano, quiescent since 1944. The activity of the Monte Somma and Vesuvio volcanic complex is commonly referred to as two central volcanic edifices, namely Monte Somma and Vesuvio. Nevertheless, the opening of numerous eruptive fissures and related vents have characterized Monte Somma and Vesuvio throughout their lives. Spatter cones, spatter ramparts, and related eruptive fissures are disseminated downslope of Vesuvio’s main cone and on the southern slopes of the volcano. Similarly, cinder cones, spatter cones, and welded spatters are distributed in the sequence cropping out on the Monte Somma cliff and on the northern slopes of Monte Somma. In this work, a total of 168 eruptive vents have been identified and characterized in a GIS environment in which field data have been merged with relevant information from historical maps and documents. These vents have been arranged into units bounded by unconformities (Unconformity Bounded Stratigraphic Units) defining the eruptive history of the volcano. Alignments of vents and eruptive fissures within each unit have been compared with regional tectonic elements and the volcano-tectonic features affecting Monte Somma and Vesuvio during the last 5600 years, thus inferring that different structural trends were active in the different stratigraphic units. In particular, we show that the N300°–320° regional, Apennine, left-lateral, strike-slip fault system, the N040°–055° Torre del Greco direct fault system, the N70° and the EW fault system, and the generally NS oriented group of local brittle elements, all analyzed here, were differently active during the investigated time span. These tectonic trends might control the position of the eruptive fissures and vents in case of future unrest of the volcano.

Stratigraphic and tectonic architecture of the Middle Jurassic-Upper Cretaceous at the southern front of the Central High Atlas (Morocco): a cartographic revision

ABSTRACT The Middle Jurassic (Bathonian)-Upper Cretaceous (Maastrichtian) deposits exposed at the southern front of the Central High Atlas (CHA, Southern Morocco) include thick fluvial successions largely undifferentiated in previous studies. These continental strata record a long period of relief formation due to crustal shortening and subsequent denudation that predated the Cenozoic, syn-collisional, tectonic inversion of the CHA. This study, carried out through mapping along more than 270 km between Ouarzazate and Errachidia towns, firstly proposes a rock-stratigraphic unified scheme based on a framework of unconformity-bounded stratigraphic units, including sub-units, differentiated for their specific depositional meaning. Secondly, the geological maps and related cross-sections illustrate the evidence of syn-depositional shortening with distinct deformation styles, outlining pre- and syn-collisional tectonic inversion.

Palaeosols in an Upper Pliocene fluvial to shallow marine succession (Valdelsa Basin, Central Italy): A sequence-stratigraphic perspective

The present study provides an example of how palaeopedology and facies analysis may be integrated for the interpretation of a cyclothemic succession in the Upper Pliocene (Piacenzian) of the Valdelsa Basin (central Italy). The stacking of facies and intervening bounding surfaces, including palaeosols, outline a hierarchy of elementary (EDS) and composite (CDS) depositional sequences within the unconformity-bounded stratigraphic units (S4-S6 synthems) which compose the succession. The focus of the study is on synthem S4 and the transition to synthem S5. Synthem S4 records the development of a distal alluvial plain dominated by floodbasin mudstone with subordinated channelized sandstone (S4 1 ), followed by the incision of a fluvial valley aggraded by the cyclical stacking of braided and low sinuosity channelized conglomerate and sandstone (S4 2 ). Synthem S5 includes lower shoreface sandstone and inner shelf mudstone related to a major transgression which affected the study area during the late Piacenzian. Evidence of soil-forming processes is well preserved within sub-unit S4 1 where five palaeosols (PS4a-e) are stacked within the facies architecture of EDS1 c-d , both being defined as transgressive to high stand tracts. Increasing palaeosol development was observed within both EDSs, while palaeosols were generally thicker and more closely stacked within EDS1 d . Palaeosols were interpreted as marking minor cycles of accommodation space creation within EDS1 c , while marking a better defined high stand tract in EDS1 d . Increasing upward soil development was observed within each EDS, with generally better developed soils in EDS S41d. The unconformable transition from S4 to S5 is marked by a thin veneer of slope deposits, bearing pedogenic carbonates reworked from a missing PS4f palaeosol, formed during the shaping of the erosional surface separating sub-units S4 1 and S4 2 . The results of this study indicate that: 1) the sedimentary and pedogenic processes recorded in the channelized facies and palaeosols in the floodbasin facies of sub-unit S4 1 , are coherent with a palaeoenvironmental setting dominated by seasonal climate; 2) the facies-palaeosol architecture of S4 synthem corresponds to those described in sequence-stratigraphic models applied to continental successions. The pedo-sedimentary signature of the major sea-level fluctuation recorded in the transition between S4 and S5 synthem differs from these models. In this case a well-developed and drained palaeosol, expected to record the maximum regressive surface shaped during the falling stage of sea level, is missing. This difference is related to a rapid fall and subsequent rise of relative sea level which marked the transition between S4 and S5 synthems. • Palaeosol and facies analysis coherently restore a Piacenzian fluvial system. • A seasonal climate controlled sediment dispersal and weathering. • Palaeosols are key-surfaces in the cyclical fluvial facies architecture. • Cyclothemic stack of palaeosols partially fits sequence-stratigraphic models. • Major sequence boundary lacks a well developed palaeosol.

Characteristics and significance of intravolcanic saprolite paleoweathering and associate paleosurface in a silicic effusive volcano: The case study of Monte Amiata (middle Pleistocene, Tuscany, Italy)

A paleoweathering saprolite mantle and an associated paleosurface have been recognized into the stratigraphic volcanic succession of the silicic effusive (trachydacite) middle Pleistocene Monte Amiata volcano (Italy). This element has been used for applying UBSU (Unconformity Bounded Stratigraphic Units) subdivision to the volcano stratigraphy. All the lavas emplaced in the onset of the volcanic activity (Bágnore Synthem, BAS) have been indistinctly affected by a saprolite in-situ alteration. Saprolite facies varies from poor lithoid consistency but with conservation of the original texture, to fractured rock with spheroidal weathering, to incoherent rock consisting of a prevalent sandy fraction, up to accumulations of residual lava blocks (core-stones, mainly in coincidence with the arcuate ridges on top of the main lava flows). Unaltered lavas of the younger phase of volcanic activity (Monte Amiata Synthem, MAS) have preserved the paleosurface within its own saprolite. Where the MAS cover is absent, the saprolite mantle represents a relict paleosurface on the present landscape. Weathering of the silicic lavas was favored by the vitrophyric texture of the parent rock and by moderate topographic reliefs formed by extensive lava plateaus positioned at various elevations. The palaeowathering saprolite is indicative of warm and relatively humid temperate climatic conditions and relative tectonic and morphogenetic stability.

The geological and structural evolution of the long-lived Miocene-Pleistocene La Hoyada Volcanic Complex in the geodynamic framework of the Central Andes, Argentina

The long-lived La Hoyada Volcanic Complex (LHVC) is located in the east of Cordillera de San Buenaventura in Argentina. This region is important for understanding of the geodynamic evolution of the Southern Puna, since it is located in its southern edge, where the transition between normal and low-angle subduction occurs. The aim of this work is to contribute to the knowledge of the volcanism of the Southern Puna through the analysis of the stratigraphy and geological evolution of LHVC. Fieldwork and image analysis have led to a new lithostratigraphic scheme, that identifies lithosomes and unconformity-bounded stratigraphic units defining the stratigraphic architecture of the LHVC. New 40Ar/39Ar ages further constrain the geological evolution of this complex. The resulting stratigraphic scheme combines the lithostratigraphic units into lithosomes and synthemic units, with the identification of at least six superimposed eruptive centers and their products during the LHVC evolution. A new stratigraphic framework for the eastern Cordillera de San Buenaventura, composed by four supersynthems separated by unconformities that reflect periods of quiescence of the volcanic activity, catastrophic destructive events and changes in the magmatic plumbing system, is proposed: 1) San Buenaventura supersynthem (ca. 9–7 Ma), including mainly explosive rhyolitic volcanism; 2) Lower La Hoyada supersynthem (ca. 7–<4.63 Ma), comprising andesitic-dacitic lava flows and domes characterized by a pervasive compressive tectonic deformation; 3) Upper La Hoyada supersynthem (4.63–1.37 Ma), mainly represented by arc-related, calc-alkaline, andesitic-dacitic effusive products and subordinated pyroclastic and epiclastic deposits; and 4) Purulla supersynthem (<1.37 Ma), which comprises volcanic products with bimodal composition related to a Pleistocene extensional volcanotectonic fault system. The application of unconformity-based stratigraphic units provides a powerful tool for understanding the volcanic evolution of eroded long-lived volcanic complexes, with an effective reconstruction of the temporal and spatial relationships among constructive and destructive phases of the volcano history, correlated with the regional volcanic, magmatic and tectonic evolution of the southern Puna.

Quaternary marine and continental unconformity-bounded stratigraphic units of the NW Sicily coastal belt

ABSTRACTIn the coastal sector of NW Sicily, the regional correlation of relevant unconformities recognised within the Quaternary sedimentary successions allowed the mapping of seven unconformity-bounded stratigraphic units (UBSUs). The regional unconformities are marine or subaerial erosional surfaces, as well as non-depositional surfaces, locally marked by paleosoils. The erosional surfaces were produced from marine abrasion, surface water overland/concentrated flow, river erosion, karst solution, mass movement, or wind erosion. The main lithofacies of the Quaternary UBSUs consist of: (a) marine and coastal bioclastic calcarenites, (b) aeolian sandstones, (c) river deposits, (d) colluvial deposits, (e) talus slope deposits, (f) landslide deposits, and (g) chemical carbonates (travertines and speleothems). Quaternary environmental changes, due to tectonics, climate, and sea-level oscillations, are the causes that favoured the development of erosion/deposition processes responsible for the genesis of unconformities and deposits. As a result, through the UBSU map of the NW Sicilian coastal belt, it is possible to: (i) recognise stratigraphic units controlled by tectonic, climatic, and environmental processes (and their interplay) and (ii) detect Quaternary sedimentary evolution.

3D geological modeling and visualization of the subsurface data of the Piemonte plains derived from the Geodatabase of digital Geological map of Piemonte

Three-dimensional (3D) subsurface modelling and visualization of seven regional unconformity-bounded stratigraphic units, (synthems) of the Messinian-Quaternary succession of the Po Plain Basin and Savigliano-Alessandria Basin are here described. 3D model is developed using the EnterVol Ctech® software environment, grounding on information stored in GeoPiemonte Map database. This modelling allows to visualise the geological data and to establish topological relationships between the analysed objects, coupling the data processing capabilities of GIS with 3D modeling and sharing the result on 3D WebGIS service of ARPA Piemonte Geoportal.

Volcanology of the Southwestern sector of Vesuvius volcano, Italy

ABSTRACTHere we present a new volcanological map of the Southwestern portion of the Vesuvius volcano at a scale of 1:10,000. Mapped units were subdivided into Unconformity-Bounded Stratigraphic Units. This work constitutes a significant development with respect to previous maps, particularly for the medieval lavas. It also includes a series of volcanic–tectonic lineaments, which have never been reported in any of the previous cartographic works. Archeological and historical data were used for mapping the lava emitted during the last 2000 years. All the historical lava flow paths and tephra deposits of the last 4000 years that were reported on the map and evidence the intimate relationship between volcanological and morphological features, and the development of human activity on this very densely populated sector of the volcano.

Geology of the middle and upper Pleistocene marine and continental terraces of the northern Tavoliere di Puglia plain (Apulia, southern Italy)

The Tavoliere di Puglia is a large alluvial plain located in southern Italy (Foggia Province, Apulia Region). From a geological perspective view, it represents the northern part of the Bradanic trough located between the southern Apennine Chain and the Apulian Foreland. A detailed geological survey of the middle-late Pleistocene terraces has been performed in this area and a new geologic map, at 1:150,000 scale, is here described. Criteria to distinguish these units are based on the recognition of basal unconformity surfaces: seven different synthems (UBSU, Unconformity-bounded Stratigraphic Units) have been discriminated. The data are presented in a synthesis map in which the geometrical relationships between the different sedimentary units are shown. New results for this sector of the Bradanic trough are significantly different from previous literature data. Furthermore, the map allows a reliable reconstruction of the Tavoliere di Puglia paleogeography during the Late Quaternary. Moreover, a detailed ma...

Geology of the middle and upper Pleistocene marine and continental terraces of the northern Tavoliere di Puglia plain (Apulia, southern Italy)

The Tavoliere di Puglia is a large alluvial plain located in southern Italy (Foggia Province, Apulia Region). From a geological perspective view, it represents the northern part of the Bradanic trough located between the southern Apennine Chain and the Apulian Foreland. A detailed geological survey of the middle-late Pleistocene terraces has been performed in this area and a new geologic map, at 1:150,000 scale, is here described. Criteria to distinguish these units are based on the recognition of basal unconformity surfaces: seven different synthems (UBSU, Unconformity-bounded Stratigraphic Units) have been discriminated. The data are presented in a synthesis map in which the geometrical relationships between the different sedimentary units are shown. New results for this sector of the Bradanic trough are significantly different from previous literature data. Furthermore, the map allows a reliable reconstruction of the Tavoliere di Puglia paleogeography during the Late Quaternary. Moreover, a detailed map of the Pleistocene terraces is fundamental to analyze and solve environmental problems associated with soil loss and desertification processes and river flood events.

Pliocene to Quaternary evolution of the Ofanto Basin in southern Italy: an approach based on the unconformity-bounded stratigraphic units

The Ofanto Basin is an actively evolving intra-chain basin of the Southern Apennines, Italy. It has an elongated shape, about 7 km large and 45 km long, and is E-W striking, representing a marked bend in the NW-SE regular orientation of the south-Apennines morphostructures.The basin is filled up by Pliocene to Quaternary clay, sandstone, and conglomerate, deposited in both marine and continental environments. The main sedimentary deposits are grouped into six units bounded by stratigraphic discontinuities marked by unconformities and abrupt lithological variations. Three of those discontinuities are recognisable on a regional scale and represent the physical boundaries of three supersynthems, in turn subdivided into synthems and subsynthems by basin-scale unconformities. The sedimentary evolution of the basin is herewith reported. Facies analyses and architecture of the sedimentary bodies revealed that each unit formed different alluvial and deltaic depositional systems located on the southern and northern margins of the basin. The morphology of the northern slope was probably steeper than the southern one. The oldest units were deposited in the Western sector of the Ofanto basin. Starting from the late Zanclean to the early Gelasian, the sedimentary bodies underwent an Eastward shift in their deposition, suggesting a basinward relocation of the depositional systems. Such variations in time and space seem to reflect relevant changes in accommodation space and sedimentary supply during the tectonic evolution of the basin. Eventually, correlations among adjacent basins and the meaning of the discontinuities as former erosional surfaces have been pointed out.

Growth and erosion: The volcanic geology and morphological evolution of La Fossa (Island of Vulcano, Southern Italy) in the last 1000 years

The Island of Vulcano (Aeolian Islands, Southern Italy) consists of several volcanic edifices whose formation overlapped in time and space beginning 120ka ago. The most recent volcano is the La Fossa cone, a 391m-high active composite cone that began to erupt 5.5ka ago. Eruptive activity at the La Fossa cone occurred in several cyclic phases separated by prolonged periods of erosion.The last 1000years of eruptive activity and morphological variations in the cone and its surrounding area were investigated through a stratigraphic reconstruction. This was based on 139 natural cuts, 26 machine-excavated and 5 hand-dug trenches in the volcaniclastic succession.The revised stratigraphy of the volcanic and volcaniclastic sequence was compared with geological maps based on the Unconformity-bounded Stratigraphic Units criteria compiled in 2006–2010. It was found that the last 1000-year period can be divided into (in hierarchical order) Eruptive Clusters and Units. Several unconformities of different hierarchical order were also identified (erosional surfaces and/or palaeosols). Stratigraphic relationships with the Vulcanello products and with rhyolitic tephras related to the eruptions of Mt. Pilato (the last-formed volcanic edifice of the Island of Lipari) were fundamental in assigning a calendar age to most of the tephra units in the studied sequence. The morphological evolution of the upper part of the cone was also reconstructed in order to assess the average cone growth rate. This work suggests a new stratigraphic and chronological interpretation of the evolution and “cyclic” activity of the La Fossa cone in the last 1000years. Several eruptions occurred in two main clusters. The stratigraphic record and morphological features reveal that the areas around the cone were affected by the deposition of reworked materials, with large amounts of tephra deposited on the steep slopes and within the major streams.

Unconformity-Bounded Stratigraphic Units (Ubsus) In An Italian Alluvial-Plain Area: Recognizing and Dating

Abstract We studied the coastal zone of the Tavoliere di Puglia plain, (Puglia region, southern Italy) with the aim to recognize the main unconformities, and therefore, the unconformity-bounded stratigraphic units (UBSUs; Salvador 1987, 1994) forming its Quaternary sedimentary fill. Recognizing unconformities is particularly problematic in an alluvial plain, due to the difficulties in distinguishing the unconformities that bound the UBSUs. So far, the recognition of UBSUs in buried successions has been made mostly by using seismic profiles. Instead, in our case, the unavailability of the latter has prompted us to address the problem by developing a methodological protocol consisting of the following steps: I) geological survey in the field; II) draft of a preliminary geological setting based on the field-survey results; III) dating of 102 samples coming from a large number of boreholes and some outcropping sections by means of the amino acid racemization (AAR) method applied to ostracod shells and 14C dating, filtering of the ages and the selection of valid ages; IV) correction of the preliminary geological setting in the light of the numerical ages; definition of the final geological setting with UBSUs; identification of a “hypothetical” or “attributed time range” (HTR or ATR) for each UBSU, the former very wide and subject to a subsequent modification, the latter definitive; V) cross-checking between the numerical ages and/or other characteristics of the sedimentary bodies and/or the sea-level curves (with their effects on the sedimentary processes) in order to restrict also the hypothetical time ranges in the attributed time ranges. The successful application of AAR geochronology to ostracod shells relies on the fact that the ability of ostracods to colonize almost all environments constitutes a tool for correlation, and also allow the inclusion in the same unit of coeval sediments that differ lithologically and paleoenvironmentally. The treatment of the numerical ages obtained using the AAR method required special attention. The first filtering step was made by the laboratory (rejection criteria a and b). Then, the second filtering step was made by testing in the field the remaining ages. Among these, in fact, we never compared an age with a single preceding and/or following age; instead, we identified homogeneous groups of numerical ages consistent with their reciprocal stratigraphic position. This operation led to the rejection of further numerical ages that deviate erratically from a larger, homogeneous age population which fits well with its stratigraphic position (rejection criterion c). After all of the filtering steps, the valid ages that remained were used for the subdivision of the sedimentary sequences into UBSUs together with the lithological and paleoenvironmental criteria. The numerical ages allowed us, in the first instance, to recognize all of the age gaps between two consecutive samples. Next, we identified the level, in the sedimentary thickness that is between these two samples, that may represent the most suitable UBSU boundary based on its lithology and/or the paleoenvironment. The recognized units are: I) Coppa Nevigata sands (NEA), HTR: MIS 20–14, ATR: MIS 17–16; II) Argille subappennine (ASP), HTR: MIS 15–11, ATR: MIS 15–13; III) Coppa Nevigata synthem (NVI), HTR: MIS 13–8, ATR: MIS 12–11; IV) Sabbie di Torre Quarto (STQ), HTR: MIS 13–9.1, ATR: MIS 11; V) Amendola subsynthem (MLM1), HTR: MIS 12–10, ATR: MIS 11; VI) Undifferentiated continental unit (UCI), HTR: MIS 11–6.2, ATR: MIS 9.3–7.1; VII) Foggia synthem (TGF), ATR: MIS 6; VIII) Masseria Finamondo synthem (TPF), ATR: Upper Pleistocene; IX) Carapelle and Cervaro streams synthem (RPL), subdivided into: IXa) Incoronata subsynthem (RPL1), HTR: MIS 6–3; ATR: MIS 5–3; IXb) Marane La Pidocchiosa–Castello subsynthem (RPL3), ATR: Holocene; X) Masseria Inacquata synthem (NAQ), ATR: Holocene. The possibility of recognizing and dating Quaternary units in an alluvial plain to the scale of a marine isotope stage constitutes a clear step forward compared with similar studies regarding other alluvial-plain areas, where Quaternary units were dated almost exclusively using their stratigraphic position. As a result, they were generically associated with a geological sub-epoch. Instead, our method allowed a higher detail in the timing of the sedimentary processes: for example, MIS 11 and MIS 5.5 deposits have been recognized and characterized for the first time in the study area, highlighting their importance as phases of sedimentation.

I vulcani nella Carta Geologica d’Italia: i risultati della sperimentazione basata sull’uso di UBSU e di altre unità stratigrafiche.

Within the framework of the national Geological Mapping Project at 1:50.000 scale (CARG), since the late 1980s the Geological Survey of Italy (GSI) financed the realization of 55 sheets where volcanic deposits are present with various extensions. The main areas interested by the 1:10.000 surveys are the active volcanic districts of Etna, Vesuvius, Phlegrean Fields and Ischia island, as well as those interested by Pleistocene volcanism. Moreover, ancient volcanism of Alps and Sardinia, also covered by the surveys, contributed to renovate this geological cartography.With the aim to produce an up-to-date, useful, innovative, official Geological Map, at the beginning of the CARG Project, GSI asked to the CNR Commission for Geological Cartography and Geomorphology (CG) to edit the Guide-books for Cartography in volcanic areas (Servizio Geologico Nazionale, 1992). In particular, referring to detailed studies by Ferrrrari et alii (1989a, b), Neri et alii (1991), Calvari et alii (1994), Coltelli et alii (1994), CG indicated the Unconformity-Bounded Stratigraphic Units (UBSU) be the stratigraphic units to be used in the cartographic representation of volcanic areas. CG indeed considered UBSU the best units to reconstruct both the eruptive history of a volcano and the geometric and stratigraphic relationships between the deposits coming from other volcanic centres, as well as the significant correlations with the adjacent different depositional environments.During the work-in-progress of the CARG Project, GSI has supported the CG's Guide-lines (APAT, 2005); however this new methodology needed an experimental application on field in order to test its potentialities and limits. So, considering the necessity of some flexibility in the application during the experimental stage, GSI allowed the Authors of the maps, to diversify this methodology, in accordance with the various volcanic contexts. Now most of the maps interesting the whole case histories of Italian volcanism are almost complete and it is possible to focus the results. As a consequence, we have more knowledge of critical expects and together with the Scientific Community we can make a careful study of the problems in order to obtain mature and, as much as possible, sharing standard methodologies, both in cartography and in terminology.In this paper we show some examples of the new official cartography in volcanic areas in which the UBS units and Lithosomes have been used for the first time, together with the traditional lithostratigraphic units, through a generally integrated approach. In the realization of the CARG sheets, the Authors of the cartography of the volcanic areas used three main stratigraphic approaches. These have been analyzed in order to evidence their advantages and good results, as well as the stratigraphic problems which need some study yet, such as the criteria of lithostratigraphic formalization of volcanic units, the criteria of both hierarchical assignment to UBS units and exportability of their unconformities among different geological contexts and, finally, the use of the lithosomes in volcanic areas.In conclusion, although some questions are still open, we point out the originality of this cultural operation that involves some key question and investigation for the future prospects.

Modern geological mapping and subsurface lithostratigraphic setting of the Venice Lagoon (Italy)

In the last decade, the knowledge of the Late Pleistocene and Holocene morphostratigraphic setting and geological evolution of the Venice lagoon area (north-western Adriatic Sea, Italy) has largely improved thanks to the analysis of new data from cores and the interpretation of a large number of very high resolution seismic (VHRS) profiles acquired in the Lagoon and Gulf of Venice. With reference to this topic, the Sheet 128 “Venezia” and the Sheet 148–149 “Chioggia-Malamocco” of the new Geological Map of Italy at the 1:50,000 scale (Carta Geologica d’Italia alla scala 1:50.000) represent the most important geological synthetic documents realized in the last years. They show age, distribution, sedimentological characteristics, and depositional environments of the exposed and buried sedimentary bodies, classified using the unconformity-bounded stratigraphic units. The analysis of data from deep cores, supported by the interpretation of VHRS profiles acquired along channels, has allowed the identification of the stratigraphic setting of the Venice lagoon subsoil generally down to about 30 m from the ground surface, and the reconstruction of the Late Pleistocene and Holocene evolution of the study area. More recently, a new VHRS system, implemented for investigations in water depth <1 m, has provided clear images of the subsurface also in the lagoon shallows; consequently, it has allowed the acquisition of new information about depositional geometries, internal bounding surfaces, and stratal relationships.

Dynamics and driving factors of late Holocene gullying in the Main Ethiopian Rift (MER)

The issue of determining the driving factors in gullying, apart from land use, is somewhat lagging in comparison with the study of their physical modelling and control technology. Available information focuses on the basic ideas of climatic control, anthropic determinism and internal “authigenic” dynamics. High resolution chronology of cyclic systems, common in extensively gullied areas, can provide a clue to the weight of each factor. This paper reports a study of this kind, focusing on two gully catchments in the Main Ethiopian Rift (MER), but backed by an extensive regional survey. By integrating tracing and correlation of unconformity-bounded stratigraphic units and soils with radiocarbon dating, a detailed chronology was obtained for the last 5000 years. This could be compared with proxy climate reconstructions of similar detail. Clear evidence of climatic control emerged; gully filling is triggered by decreased stream transport capacity and increased sediment supply during transitions towards drier climate phases, while gully entrenchment appears to take place at the start of moist intervals, for the reverse reasons. A broader consideration of geological setting, however, puts forward a more general interpretation. These gullies actually formed, in the beginning, as part of the land surface response to sudden, very recent tectonic events, which created accommodation space for temporary sediment stores. They should then be seen in the frame of the Discontinuous Ephemeral Stream (DES) concept; as such, they are intrinsically non-linear and complex phenomena, whose response is linearized by a strong climatic–vegetational forcing, acting on both channel flow and sediment supply.

The last interglacial pedocomplexes in the litho- and morpho-stratigraphical framework of the central-northern Apennines (Central Italy)

Recently in Italy, a great deal of effort has been devoted to the mapping of Quaternary continental deposits. Several Unconformity Bounded Stratigraphic Units (UBSU) have been defined. The unconformities indicate important phases of changing dynamics linked to Quaternary climatic changes, and have been related to valley downcutting simultaneous with paleosol formation during interglacials. However, glacial periods are characterised by deposition of thick continental sequences. We suggest that the UBSU's belonging to the late Middle Pleistocene (MIS 6) and Late Pleistocene can also be defined under a pedostratigraphic point of view. Locally at the top of the UBSU attributed to MIS 6, buried or relict paleosols are preserved. The paleosols developed in calcareous fluvial and moraine gravels with minor flints. The pedocomplexes are made of three paleosols separated by erosion surfaces and/or stonelines. The older paleosol (MIS 5e) is severely truncated and is characterised by a Ck horizon overlain by strongly leached reddish Bt horizons with flinty rock fragments (with a diagnostic wavy lower boundary). In some sites, subsequent secondary precipitation of carbonates transformed this horizon into a Btk. This paleosol is buried by a younger paleosol (MIS 5c) with less leached reddish Btk horizons, quartz of aeolian origin, and very small and scarce calcareous clasts. The main pedofeatures are associated with colluvial processes, biological activity, and carbonate precipitation. This paleosol is also truncated and buried under a similar paleosol (MIS 5a). Therefore, the lower paleosol of the pedocomplex can be attributed to the Eemian, and constitutes a valid marker for the definition and mapping of the underlying and overlying sedimentary units. In other parts of the Apennines, no sedimentary units have been found that correspond to the unconformities separating different pedostratigraphical units. In order to map the different UBSU's, due to the poor lateral continuity of the preserved paleosols, morpho- and litho-stratigraphical investigations and correlation are always recommended.

FACIES ANALYSIS, STRATIGRAPHY AND PALAEONTOLOGY (MOLLUSCS AND VERTEBRATES) IN THE UPPER PLIOCENE SANDY FLOOD-BASIN DEPOSITS OF THE UPPER VALDARNO BASIN (NORTHERN APENNINES)

The Upper Valdarno Basin, one of the most investigated Neogene–Quaternary basins of the Northern Apennines, developed during three main phases, as testified by the occurrence of three unconformity-bounded stratigraphic units (UBSUs) in the basin infill. Despite numerous studies having been carried out, biochronological, paleoecological and stratigraphical issues in the lower portion of the Montevarchi Synthem (second phase) have yet to be understood. Sandy deposits (Montecarlo Sand and Silt Unit), stratigraphically located in this portion of the Montevarchi Synthem, are the focus of this multidisciplinary study. These deposits conformably overlie sandy fluvio-eolian sediments and are, in turn, capped by fluvio-palustrine deposits through a progressive unconformity. Facies analysis suggest a sandy flood-basin environment for these deposits, characterised by variations in water discharge and flood event energy. Mollusc and fish remains, pointing to quiet or slow-moving shallow waters, have been affected by transport processes before final burial in overbank areas. Fish remains of the primary marine family Mugilids highlight a connection between the basin and the sea that was previously only supposed. Small mammal remains, referred to the rodent Mimomys polonicus , are coherent with a water-rich environment. Cyclic variations in shell content and sedimentological characteristics testify to the occurrence of short-term climatic oscillations during this warming phase. This study fits with paleomagnetic and radiometric datings and mammal biochronology, in indicating that the Montecarlo Sand and Silt Unit belongs to a time interval preceding the Reunion paleomagnetic event. The depositional evolution of the Montecarlo Unit was driven by climatic change from arid to humid conditions, related to a global increase in temperature that occurred between 2.4 and 2.2 Ma.

Episodic Cenozoic tectonism and the development of the NW European ‘passive’ continental margin

The North Atlantic margins are archetypally passive, yet they have experienced post-rift vertical movements of up to kilometre scale. The Cenozoic history of such movements along the NW European margin, from Ireland to mid-Norway, is examined by integrating published analyses of uplift and subsidence with higher resolution tectono-stratigraphic indicators of relative movements (including results from the STRATAGEM project). Three episodes of epeirogenic movement are identified, in the early, mid- and late Cenozoic, distinct from at least one phase of compressive tectonism. Two forms of epeirogenic movement are recognised, referred to as tilting (coeval subsidence and uplift, rotations <1° over distances of 100s of Kilometres) and sagging (strongly differential subsidence, rotations up to 4° over distances <100 km). Each epeirogenic episode involved relatively rapid (<10 Ma) km-scale tectonic movements that drove major changes in patterns of sedimentation to find expression in regional unconformity-bounded stratigraphic units. Early Cenozoic tilting (late Paleocene to early Eocene, c. 60–50 Ma) caused the basinward progradation of shelf-slope wedges from elongate uplifts along the inner continental margin and from offshore highs. Mid-Cenozoic sagging (late Eocene to early Oligocene, c. 35–25 Ma) ended wedge progradation and caused the onset of contourite deposition in deep-water basins. Late Cenozoic tilting (early Pliocene to present, <4±0.5 Ma) again caused the basinward progradation of shelf-slope wedges, from uplifts along the inner margin (including broad dome-like features) and from offshore highs. The early, mid- and late Cenozoic epeirogenic episodes coincided with Atlantic plate reorganisations, but the observed km-scale tectonic movements are too large to be accounted for as flexural deflections due to intra-plate stress variations. Mantle–lithosphere interactions are implied, but the succession of epeirogenic episodes, of differing form, are difficult to reconcile with the various syn-to post-rift mechanisms of permanent and/or transient movements proposed in the hypothetical context of a plume beneath Iceland. The epeirogenic movements can be explained as dynamic topographic responses to changing forms of small-scale convective flow in the upper mantle: tilting as coeval upwelling and downwelling above an edge-driven convection cell, sagging as a loss of dynamic support above a former upwelling. The inferred Cenozoic succession of epeirogenic tilting, sagging and tilting is proposed to record the episodic evolution of upper mantle convection during ocean opening, a process that may also be the underlying cause of plate reorganisations. The postulated episodes of flow reorganisation in the NE Atlantic region have testable implications for epeirogenic movements along the adjacent oceanic spreading ridge and conjugate continental margin, as well as on other Atlantic-type ‘passive’ margins.

The Ziway–Shala lake basin (main Ethiopian rift, Ethiopia): a revision of basin evolution with special reference to the Late Quaternary

The Ziway–Shala basin, in the Main Ethiopian Rift (MER), is a reference site for regional to global paleoclimatic reconstructions. We undertook and interpreted a stratigraphical, pedological and geomorphological study, including a new geological map scale 1:250,000, to provide a Late Quaternary-centred revised geological history of the basin. 1 The map can be found at the back of this issue. 1 We mapped several Late Quaternary sedimentary units and arranged them in four major unconformity-bounded stratigraphic units (synthems), recording equivalent phases of geomorphic change. A new, extensive, soil survey allowed us to establish a pedostratigraphic unit, the T’ora geosol, as a distinctive marker of landscape stability and instability in the area during the Holocene. Climate change was a major control on geo-morphologic evolution of this area during the intense climate fluctuations of the last 100,000 years. Extensive lake systems developed during relatively humid Last Glacial interstadials and in the early-mid Holocene; this last was characterized by short, but high-amplitude, regressions during arid pulses. Major lakes’ lowering occurred in the terminal Pleistocene and in the last 5000 years. Evidences for high or very high terminal Pleistocene lake levels suggest possible non-climatic controls on changes in lakes’ extension and volumes between Late Pleistocene and Holocene. We suggest that modifications of hydrological thresholds, due to activity of structures parallel and transversal to the MER, established new lakes’ boundaries between terminal Pleistocene and early Holocene, setting the maximum level of Holocene lake systems at about 1670 m a.s.l. The integrated analysis of lacustrine, fluvial, slope and soil systems provided a basis for a general interpretation of relations between climatic changes and geomorphic processes at a basin scale.