Non-cohesive Debris Flows Research Articles

Long runout distance of a block breccia density flow along a half-graben axis: latest Jurassic rift climax in East Greenland

A basinally isolated breccia–pebbly sandstone couplet occurs in the axial part of the latest Jurassic rift-climax half-graben in Wollaston Forland, East Greenland. Synrift breccias are otherwise restricted to a narrow zone along the scarp of the basin-margin Dombjerg Fault. Towards the north the Dombjerg Fault sidesteps en echelon towards the east where it continues in the Thomsen Land Fault. The breccia part of the couplet was transported by a noncohesive debris flow and the pebbly sandstone by a high-density turbidity flow. The couplet is located about 15 km east of the Dombjerg scarp and flowed southwards along the half-graben axis. It is interpreted to have been derived from the southern end of the Thomsen Land Fault. This gives a runout distance of at least 25 km. Other isolated breccias have been reported towards the south along the half-graben axis, giving a runout distance of up to 40 km along the roughly horizontal half-graben axis. The angularity of the clasts is remarkable and reflects derivation directly from the fault scarp during a single catastrophic event. The occurrence of breccias thus cannot be taken as evidence of short transport and deposition at the foot of a scarp or steep slope.

Facies architecture of the Plio-Quaternary alluvial fan deposits from south-eastern margin of the Saïss foreland basin (Morocco): Paleoclimatic and neotectonic implications

Eight Plio-Quaternary alluvial fans from south-eastern margin of the Saïss foreland basin were the subject of a thorough sedimentological study based upon detailed facies analysis. This study allowed distinguishing 16 sedimentary facies. The typology of the latter, their spatial distribution and sedimentary organization afforded the classification of the studied fans into three groups. Group 1 is dominated by non-cohesive debris flow and cohesive to very poorly-sorted blocky conglomerate deposits that are associated with silt-clayey and calcrete. The setting up of the deposits of this group occurred in a tectono-karstic depression whereby a WNW progradation of the deposits and an important incision into the previous consolidated deposits would have been controlled. Group 2 is characterized by a predominance of the muddy fraction, presumably linked to the large surface area of the watershed and the river activity. Group 3 is dominated by coarse deposits with rare mudflows interpreted to be the direct consequence of an important neotectonic activity which is expressed by localized incremental pulses of the Agourai-El Hajeb-Ain Cheggag faults system and Middle Atlas directional faults (i.e., NE-SW) along toe fan during the Plio-Quaternary period. Groups 2 and 3 are also characterized by deposits whose the related processes are dominated by cohesive and non-cohesive debris flows, mud-rich debris, channelized debris flows and non-cohesive flows, hyper-concentrated flows and sheet-flood flows. The morphogenesis of these alluvial fans and the emplacement of their deposits are neotectonic- and climate-controlled. Tectonics have played a major role in the rejuvenation of the Middle Atlas relief and, through controlling the base-level, created the space available for the establishment and development of alluvial fans onto the south-eastern margin of the Saïss basin. This extensive tectonics has resulted in normal faultings and layer tiltings. The climate played an important role in the formation of facies by controlling (i) the mechanical properties of paleoflows, (ii) sedimentary dynamics and (iii) the formation of paleosols and calcrete.

Lahar deposits along the Nigorikawa River in the eastern foot of Zao Volcano, Japan, during the last 8000 years

Lahar phenomena and the accompanying eruptions at Zao Volcano, NE Japan, have been recorded historically from the 13th to 20th centuries. However, no studies have been conducted on lahar deposits. This study focused on the lahar deposits of the last ca. 8000 years from fluvial terraces (terraces I, II, and III in descending order of elevation) distributed along the Nigorikawa River at the eastern foot of the volcano. The lithologic, granulometric, and component features of the lahar deposits revealed gravelly non-cohesive debris flow, sandy hyperconcentrated flow, and muddy cohesive debris flow lahar deposits. The sandy and muddy matrix of the lahar deposits mostly originated in the scoriaceous magmatic and phreatic eruption deposits and/or phreatic-eruption-related products in the Zao youngest stage. The clasts of older lavas and basement rocks of all the lahar deposits were entrained during transportation through the river. Radiocarbon (14C) dating indicates the depositional ages of the lahar deposits in terraces I, II, and III to be ca. 8–6 ka, ca. 3.5–2.5 ka ca. and 1700–1900 CE, respectively. The two gravelly and three muddy lahar units of terrace I, two muddy units of terrace II, and at least two gravelly and three muddy units of terrace III correlate with the reported magmatic and phreatic eruptions. Based on chronostratigraphic positions and lithology, the 1895 CE phreatic eruptions may have triggered the two uppermost muddy units in terrace III. However, the gravelly and sandy units of terrace II revealed the thickest deposits, and are widely distributed among the sections of the study area, with no corresponding magmatic eruption deposits in the proximal area. Unknown large-scale magmatic eruptions or crater lake outbursts could have triggered the lahars that formed the sequence. This study revealed that lahar events occurred repeatedly during the past 8000 years and flowed down to distal area. These results indicate the likely occurrence of lahars, especially during and after the eruption at Zao Volcano.

Post-eruptive lahars related to the 1913 eruption in La Lumbre Ravine, Volcán de Colima, Mexico: The influence of ravine morphometry on flow dynamics

A detailed historic record on Volcán de Colima, Mexico, of lahar activity and related phenomena dating back to the sixteenth century has proven that debris flows have caused more human casualties in the area than any other volcanic activity including pyroclastic flows. In this paper we present a multi-approach study of a relevant lahar (a non-cohesive debris flow with the largest runout and one of the most voluminous of the volcano) at Volcán de Colima, Mexico. Observations and information relevant to the lahar deposit and event including componentry, textural, sedimentological, stratigraphic, morphometric and geochronologic analyses and oral testimonies from eyewitnesses enable us not only to confirm a relationship between the studied outcrops and the 1913 Plinian eruption of the Colima Volcano (the most intense and large in volume eruption of this volcano of the last century), but also to find a clear and direct correlation among morphometric features such as slope, ravine width, curves and kinematics, flow patterns, flow transport mechanisms, volume, shape of clasts, granulometry and sub-unit formations. In particular, we maintain the slope has a marked control on the thickness of the deposits, deposited volume, longitudinal changes, and patterns of downstream changes in particle morphology and granulometry. All this information will be useful for generating or updating hazard maps, taking these lahars as a worst-case scenario.

Controls on deep-water slope channel complex fill, propagation, stacking, and orientation in the Middle Eocene-Oligocene Kırkgeçit Formation, Elazığ, eastern Turkey

Deep-water channels are well-exposed along the northern margin of the east-west oriented Late Paleocene to Oligocene Elazığ Basin of eastern Turkey. The deep-water slope channel complexes are characterised by their association with prominent normal faults, where they seem to have controlled their geometries at the northern margin. The channels were fed by coarse-grained sediment shed by subaqueous gravity flows such as high and low-density turbidity currents, and both cohesive and noncohesive debris flows, from elevated hinterland across narrow shelves and through canyons and gullies into the channels. The faulting and subsequent folding along the northern basin margin created a significantly irregular deep-water slope, and these gravity flows were deflected around, and ponded against this topography. The nature of the impact of seafloor topography, created both before and during channel activity, was examined to create a scheme that documents the range of such effects. Detailed field mapping in this semidesert area shows that these slope channel complexes were strongly controlled by block fault topography generated during early basin evolution in the Late Paleocene, and again during fold development from thrust nappe emplacement during basin filling in the Late Middle Eocene when the Hazar- Maden Basin closed in the south. Four styles of slope channel complex-fold interaction have been recognised: deflection, blocking, diversion, and confinement. In each style of interaction, folding and faulting controlled both channel complex planform geometry and architectural style of the channel fills. In addition to seafloor topography inherited from these earlier phases of compression, subsequent gravitational collapse on the northern basin margin also created normal fault-bound blocks that locally controlled turbidity current pathways and channel complex orientation. It has also been shown that the orientation of the slope channel complexes within mudprone slope sequences has later controlled the orientation of younger fold axis in the Middle Eocene. The study illustrates very well-exposed examples of complex seafloor topography generated by compressional and extensional tectonics, and the impact this has had on slope channel complex evolution.

Sedimentary and diapiric mélanges in the Skrzydlna area (Outer Carpathians of Poland) as indicators of basinal and structural evolution

The Dukla Nappe in the Skrzydlna area exposes two types of mélange reflecting two different phases of basinal and tectonic evolution of the Outer West Carpathian orogen in its Polish sector. The Oligocene-age sedimentary mélange (olistostrome) is related to growth of the accretionary wedge, whereas the Miocene-age diapiric mélange postdates the orogenic thrusting. Textural and structural features of the very coarse-grained sedimentary mélange suggest non-cohesive debris flows and high-density turbidity currents as predominant emplacement mechanisms. Growth strata, associated with progressive unconformities, and facies contrast between the underlying fine-grained unit and the overlying olistostrome reflect a considerable uplift of the source area and rotation of the adjacent part of the basin floor. The olistostrome and the overlying turbidite succession form a retrogressive sequence interpreted as a submarine canyon infill grading to a small submarine fan. The diapiric mélange, injected into the Oligocene-age succession of the Dukla Nappe, contains the Early and Late Cretaceous-age blocks and matrix derived from the underlying Silesian Nappe. The features reflecting diapiric emplacement include matrix proportion increase and block content decrease towards the mélange margins, scaly fabric and shear zones. Both mélanges, interpreted in the past as chaotic bodies, upon detailed examination reveal genesis-related subtle internal organization.

Paleoenvironment and age constraints of Paleoproterozoic alluvial fans in the Sao Francisco Craton, Brazil

The Ouricuri do Ouro Formation represents sedimentary deposits of the Statherian period in São Francisco Craton. Previous works have focused just on the Formation mapping, interpreting the depositional system only in general aspects. However, this study focuses on detailed facies analysis and geochronological investigations of the Ouricuri do Ouro Formation in the central area of São Francisco Craton, Brazil. Four facies associations are recognized: i) Proximal deposits dominated by non-cohesive debris flow; ii) Proximal sheetfloods; iii) Intermediate sheetfloods; and iv) Sandy plains of distal floods. Paleocurrent directions derived from cross-bedding indicate a radial pattern to NE-N. The vertical stacking of distal deposits overlying the proximal facies association suggests a retrogradation of the alluvial fan system, indicating a change in the dynamic of the depositional system. Retrogradation might have been caused by a decrease in the regional gradient declining of the tectonic activity or the retreat of the basin's margin faults system. Furthermore, the presence of higher frequency fining-upward cycles also suggests a climatic control in those deposits. These cycles could be results of the progressive decrease in flow capacity and competence, associated with a decrease of superficial runoff over time. Allied with the dominant paleocurrent, the geochronological data of four tuffaceous sandstones reveal the contribution of Rhyacian to Mesoarchean zircons, suggesting that they came with sediments transported from SW and S of the studied area during the Statherian. The transported sediments came from the southern portion of Gavião Paleoplate. The contribution of volcanic zircons recognized in the tuffaceous sandstones is related to syndepositional magmatism of Statherian Rift of Espinhaço Supergroup. The tuff layer at the base of drill hole (Sample FLV-61 A) contains a zircon age with dominant fraction represented by Statherian zircons defining the syn-sedimentary age of 1722 ± 13 Ma at the basal section of the Ouricuri do Ouro Formation. The present results and available geochronological data constraints the depositional interval of Lower Espinhaço Basin or Megasequence (in sense of Chemale et al., 2012; Guadagnin et al., 2015a, b) between <1.80 and >1.68 Ga in the Chapada Diamantina Region.

Rheologic survey of mass transport events from the geologic record of an Andean Precordilleran slope

Gravitational processes are the most substantial mechanisms of continental sediment transport. Sedimentary deposits produced by these processes record evidence of the sediment transport mechanism and flow regime, as well as rheologic characteristics. This study aims to analyze sedimentary deposits at the Andean Precordillera in order to interpret the sediment transport mechanism and understand the rheologic behavior of the mass flows. The studied area, situated in Neuquén Province, Argentina, comprises a small-scale drainage basin and an alluvial fan. The study approach includes the description and analysis of fractures, landslide head scarps, texture, and geometry of sedimentary deposits. In the drainage basin, we identified planar landslide head scarps associated with deposits of talus and marginal levees. Sedimentary deposits record rockfall, non-cohesive debris flow, and catastrophic and normal stream flow mechanisms of sediment transport. The triggered mechanism is highly controlled by the high slope and climate conditions. A turbulent flow regime with Newtonian rheologic behavior was interpreted for the stream flow events, whereas a laminar flow regime with plastic (non-Newtonian) rheologic characteristics was related to the non-cohesive debris flows.

Alluvial fan deposition along a rift depocentre border from the Neuquén Basin, Argentina

The interaction between hangingwall block rotation and alluvial deposition is examined from Late Triassic–Early Jurassic successions exposed along the Catán Lil half-graben border fault system in the Neuquén Basin, Argentina. Analysis of transport and depositional processes, clast composition and rock body geometry allowed the identification of three distinctive fan-shaped alluvial units. The contrasting lithologic nature of the basement (igneous-metamorphic) and syn-rift fill (volcanic and volcanic-derived) permits detailed studies of clast provenance. The origin of each alluvial system (footwall- vs. hangingwall-derived) can thus be verified. A simple method was implemented to establish the geometry of each alluvial unit by comparing the stereographic projection of its bedding to that of an idealised fan shaped body. Results show that the three alluvial systems occupied the same relative location in the rift depocentre. Unit 1 is interpreted as an alluvial fan orientated transverse to the depocentre border fault system and fed from the footwall. Non-cohesive debris flow deposition was the dominant process in this environment. Unit 2 is interpreted as a mainly hangingwall-fed alluvial fan, parallel to the depocentre border fault system and shows an upward decrease in footwall-derived clasts. Hyperconcentrated flow was the principal transport process. Unit 3 represents a fan delta, parallel to the depocentre border fault system. Its components are completely hangingwall-derived and hyperconcentrated flow deposition was the dominant process. Differences in grain-size, composition, transport directions and fan body geometry are proved to be directly linked to variations in ground tilting induced by the direction of hangingwall block rotation in an endorheic rift depocentre.

Depositional architecture and sequence stratigraphic correlation of Upper Ordovician glaciogenic deposits, Illizi Basin, Algeria

Abstract Upper Ordovician glaciogenic deposits are profoundly important as hydrocarbon reservoirs across North Africa, such as within the Illizi Basin of SE Algeria. In this study we present a new sedimentological and sequence stratigraphic model for Upper Ordovician glaciogenic deposits based on the analysis of core descriptions and wireline logs from 25 wells in the Tiguentourine Field. Within the glaciogenic succession, two ice advance–retreat cycles can be defined, consisting of glaciomarine ice-contact fan deposits and tillites. Deposits of the marine ice-contact fan systems generally show a retrogradational stacking pattern from ice-proximal to ice-distal deposits. This pattern is attributed to the deposition in front of a retreating ice sheet. The proximal marine ice-contact fan deposits consist of massive or low-angle cross-bedded pebbly sandstone. They are interpreted as the deposits of turbulent, high-energy plane-wall jets, emerging from subglacial meltwater conduits. These jet-efflux deposits are up to 60 m thick and interbedded with deposits of cohesive and non-cohesive debris flows. The jet-efflux deposits are overlain by fine-grained, thick-bedded massive sandstone. These mid-fan deposits build up the bulk of the glaciomarine fans and are interpreted as deposits of underflows, generated at the point of flow-detachment, where marine meltwater jets become buoyant and large volumes of sediment fall-out from suspension. In the upper part of the fan succession massive sandstones pass upwards into mud-prone massive sandstones, interpreted as deposits of cohesive sandy debris flows. The most ice-distal deposits are muddy sandstones and mudstones deposited by waning low-density turbulent flows and suspension fall-out. The best reservoir properties within the glaciogenic succession are attributed to the proximal and medial deposits of the ice-contact fans such as coarse-grained jet-efflux deposits and sustained high-density turbulent flow deposits. However, the mud content within the massive sandstones is highly variable and influences the reservoir quality. Both glacial depositional sequences infill 60–175 m deep, elongated depressions, which are interpreted as subglacial tunnel valleys. These tunnel valleys acted as depocentres for the glaciomarine fan deposits. After final deglaciation and post-glacial transgression, organic-rich shale was preferentially deposited in underfilled tunnel valleys.

Debris Flows Resulting From Glacial-Lake Outburst Floods in Tibet, China

During the last 70 years of general climatic amelioration, 18 glacial-lake outburst floods (GLOFs) and related debris flows have occurred from 15 moraine-dammed lakes in Tibet, China. Catastrophic loss of life and property has occurred because of the following factors: the large volumes of water discharged, the steep gradients of the U-shaped channels, and the amount and texture of the downstream channel bed and bank material. The peak discharge of each GLOF exceeded 1000 m3/s. These flood discharges transformed to non-cohesive debris flows if the channels contained sufficient loose sediment for entrainment (bulking) and if their gradients were >1%. We focus on this key element, transformation, and suggest that it be included in evaluating future GLOF-related risk, the probability of transformation to debris flow and hyperconcentrated flow. The general, sequential evolution of the flows can be described as from proximal GLOFs, to sedimentladen streamflow, to hyperconcentrated flow, to non-cohesive debris flow (viscous or cohesive debris flow only if sufficient fine sediment is present), and then, distally, back to hyperconcentrated flow and sediment-laden streamflow as sediment is progressively deposited. Most of the Tibet examples transformed only to non-cohesive debris flows. The important lesson for future hazard assessment and mitigation planning is that, as a GLOF entrains (bulks) enough sediment to become a debris flow, the flow volume must increase by at least three times (the "bulking factor"). In fact, the transforming flow waves overrun and mix with downstream streamflow, in addition to adding the entrained sediment (and thus enabling addition of yet more sediment and a bulking factor in excess of three times). To effectively reduce the risk of GLOF debris flows, reducing the level of a potentially dangerous lake with a siphon or excavated spillway or installing gabions in combination with a downstream debris dam are the primary approaches.

Description and numerical modelling of the October 2000 Nora debris flow, Northwestern Italian Alps

The Nora alluvial fan, which is located in the Orco River valley (Piedmont region, Italy), was affected by a debris flow of approximately 10 000 m3 in October 2000. The event was triggered by heavy rainfall that affected the whole Piedmont region at that time. The mobilization of the water-satured altered gneissic bedrock and colluvium in the upper zone of the Nora basin resulted in the detachment of a debris slide, which rapidly evolved into a channelized noncohesive debris flow. As information from post-event surveys and two pre-event digital elevation models (DEMs), with different resolutions, were available for the study site, the dynamics of the event has been numerically back-analyzed using the numerical code RASH3D. The obtained results have shown both the capability of the code to simulate the dynamics of a noncohesive debris flow, including the final deposit depth distribution, and the importance of on-site surveys and digital elevation model (DEM) resolution in interpreting numerical results and the associated calibrated rheological parameters.

Evidence for carbonate platform failure during rapid sea-level rise; ca 14 000 year old bioclastic flow deposits in the Lesser Antilles

Bioclastic flow deposits offshore from the Soufriere Hills volcano on Montserrat in the Lesser Antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr. The volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world, which occurred on Montserrat in 2003. These flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south-west flank of Antigua or the east flank of Redonda; adjacent islands that are not volcanically active. The bioclastic flow deposits are relatively coarse-grained and either ungraded or poorly graded, and were deposited by non-cohesive debris flow and high density turbidity currents. The bioclastic deposit often comprises multiple sub-units that cannot be correlated between core sites; some located just 2 km apart. Multiple sub-units in the bioclastic deposit result from either flow reflection, stacking of multiple debris flow lobes, and/or multi-stage collapse of the initial landslide. This study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka. Few large submarine landslides have been well dated, but the slope failures that have been dated are commonly associated with periods of rapid sea-level change.

The ~ 2500 yr B.P. Chicoral non-cohesive debris flow from Cerro Machín Volcano, Colombia

Cerro Machín Volcano (CMV) is located in the central part of the Colombian Andes (2750 m asl), 150 km southwest of Bogotá. It is considered the most dangerous active volcano of Colombia. CMV has experienced at least six major explosive eruptions during the last 5000 years. These eruptions have emplaced many types of pyroclastic deposits with associated lahars that have traveled more than 100 km. One of these lahars is called Chicoral Debris Flow Deposit (DFD2). This deposit is exposed as discontinuous terraces (3–20 m thick) along the Coello and Magdalena rivers up to 109 km from the source. The DFD2 covers a minimum area of 62 km 2 and has a minimum volume of 0.57 km 3. It comprises two dacite-rich volcaniclastic units. Grain-size analysis reveals that the matrix content and sorting increase with distance while the average grain size decreases. The clay content of the DFD2 matrix is approximately 1%, thus categorizing it as a non-cohesive debris flow. Radiocarbon dates obtained from underlying and overlying paleosols yielded ages of 2505 + 65 and 1640 + 45 yr B.P., respectively. These dates suggest that DFD2 is related to the ~ 2600 yr B.P. El Guaico eruption of CMV. This eruption produced a block-and-ash flow that filled and blocked the Toche River up to 5 km from the volcano. Subsequent remobilization of this loose material by runoff water generated a massive debris flow that traveled 91 km along the Toche and Coello rivers and continued across the Espinal Alluvial Fan debouching into the Magdalena River where it continued another 18 km prior to its transformation into a sediment-laden flow. Because the last eruption of the volcano occurred ca. 900 years ago, no historic activity of CMV is known among inhabitants of the region. Hence the region has developed without awareness of volcanic hazards. Therefore an assessment of volcanic hazards is essential for understanding and evaluating the vulnerability and risk to which people are exposed in case of a future eruption. Such assessment is critical for urban planning, development, contingency, emergency and education planning.

Evidence of different contractional styles along foredeep margins provided by Gilbert deltas: examples from Bowser Basin, British Columbia, Canada

Abstract The northern margin of the early Middle Jurassic Bowser Basin in British Columbia contains large, solitary shelf-type Gilbert deltas and smaller, multistorey Gilbert deltas stacked against inferred active reverse faults. Analysis of the deltas provides evidence of tectonic structures that can no longer be directly observed. Bowser Basin developed as a foredeep in response to overthrusting of Stikinia by Cache Creek Terrane (oceanic crust) during accretion of Stikinia to the western margin of North America. The terrane boundary is currently represented by the King Salmon and associated thrust faults. The shelf-type Gilbert deltas consist of accretional conglomerate packages up to 100 m thick, that are overlain by and laterally associated with cyclothemic shelf facies. Gilbert deltas responded to low accommodation to sediment supply ratios by prograding to the shelf break of the basin, where they fed large volumes of gravel to slope gullies and the deep basin beyond. At one location, fault-block rotation resulted in oversteepening of foresets and internal discordances. Deposition was dominated by non-cohesive debris flows (50% to 60% of foresets) and intervening muddy debris flows. The coarse-grained sediments are texturally and compositionally mature, consisting predominantly of spherical radiolarian chert clasts derived from the overthrust Cache Creek accretionary prism. A spectacular succession at least 700 m thick near Cartmel Lake records the aggradational stacking of 30 small Gilbert deltas. Like the shelf-type Gilbert deltas, sediment gravity flows predominated. Creation of accommodation space for accumulation of the Gilbert delta stack was repeated many times; the locus of accommodation remained in much the same place over time. This implies a high degree of tectonic control over the geometry of the stacked Gilbert deltas. Accommodation to sediment supply ratios for this kind of Gilbert delta were high. Based on their thickness, geometry, and general association with shelf lithofacies, Bowser Basin shelf-type Gilbert deltas probably required relatively large drainage catchments, such as those associated with thrust ramps, fault transfer zones or blind thrusts. In one example, delta progradation direction was at a low angle (15 –27°) to the main structural trends and in this particular case we favour delta accumulation at a thrust fault transfer zone. In contrast, the stacked Gilbert deltas are inferred to be footwall deltas that accumulated close to an active, reverse fault where the stacking geometry indicates little or no basinward shift in the locus of deposition. Drainage basins were relatively small and were frequently rejuvenated. The Bowser Basin Gilbert deltas accumulated at the margin of a foreland thrust wedge, outboard of a major Early to Middle Jurassic, crustal-scale thrust system that carried oceanic Cache Creek terrane rocks in its hanging wall. Coarse-grained sediment, derived either from northwards erosional back-stepping of King Salmon Fault or from older Bowser Basin sediment, was cannibalized and redeposited in the Gilbert deltas and associated lithofacies. Either case provides a mechanism to account for the reworking necessary to produce the high clast sphericity and the monomict chert composition.

Arquitetura e mecanismos deposicionais em sistemas aluviais holocênicos de clima árido no oeste argentino e comparação com depósitos neocretáceos brasileiros

The Pre-Cordillera in the northwest of Argentine is characterized by geomorphological features formed during Miocene Andes Cordillera uplifting. The arid weather and compressive tectonic regime propitiated the origin of countless sedimentary basins filled by alluvial fans and playa-lakes deposits. Because of the outcrops abundance and their qualities these basins are important sites for direct observation of depositional alluvial architectures and facies analyses. The description of some deposits in the Rio Jáchal valley (neighborhood of San Jose de Jáchal), and Pie de Palo desert alluvial plain in San Juan, allowed the recognition of main depositional mechanisms in these environments. The analyses were based in the longitudinal cross sections that show intertonguing of the alluvial fans, playa-lakes and eolian deposits. It was characterized the internal and external geometries of the bodies by hierarchical surfaces and macroforms analyses to make the interpretation of the sedimentary process and the depositional dynamics. The facies variation from proximal to distal deposits recorded the evolution of confined or unconfined non-cohesive debris flow to turbulent and laminar flows, resulting by the transport stream energy decrease of ephemeral torrential currents associated with storm periods. Due to the annual low pluviosity (70 mm/y), the sedimentation rate is very low, mainly suggested by the vegetation and soil developed. Although this study has been made in a restrict area with a very recent sedimentary history initiated only in the Late Pleistocene to Holocene, the facies distribution and the depositional mechanisms conditioned by geomorphologic positions in the alluvial fan systems and the sedimentologic characteristics conditioned by the arid climate allow to compare these deposits with the Brazilian neocretaceous, in the Bauru Basin, that shows similar sedimentologic and architecture characteristics.

Tectono sedimentary evolution of the Umm Ghaddah Formation (late Ediacaran-early Cambrian) in Jordan

The terrestrial Umm Ghaddah Formation of late Ediacaran-early Cambrian age was deposited in NE–SW elongated intracontinental rift system basins and sub-basins bounded by active listric half-graben faults. Basin fill consists of conglomerate facies association A, deposited in a fault-controlled transverse alluvial fan system that drained northwestward and graded laterally into sandstone facies association B, deposited by a braided river system flowing northeastward axial to the rift basin. The alluvial fan facies association was deposited by rock falls and non-cohesive debris flows of sediment gravity flow origin, and by sheetflood processes. The Umm Ghaddah Formation is dominated by a large-scale fining upward succession interpreted to reflect a gradual cessation of the Pan African Orogeny. Within this large-scale trend there are also minor fining and coarsening upward cycles that are attributed to repeated minor tectonic pulses and autocyclic shifting of the system. The distribution pattern of the Umm Ghaddah Formation and the underlying Ediacaran Sarmuj Conglomerates, Hiyala Volcaniclastics and Aheimir Volcanics in Jordan and adjacent countries in isolated extensional half-grabens and grabens formed during the extensional collapse phase of Arabia associated with the Najd Fault System seems to be unrelated to the present day Wadi Araba-Dead Sea transform fault system.

Late Pleistocene tectonic-geomorphological development within a passive margin — The Cariatá trough, northeastern Brazil

Studies on the geomorphological evolution of the South American passive margin have been based on the pediplanation model, which predicts that its morphology is a response to regional uniform uplift and concomitant development of erosion surfaces. We combined remote sensing, geological mapping, lithostratigraphic and facies analyses, and luminescence dating in the Cariatá trough, northeastern Brazil, in order to determine how brittle tectonics and climate influenced colluviation and the shaping of local landforms in the Quaternary. Our work indicates that Cariatá is an asymmetrical trough ∼ 40 km long, ∼ 25 km wide, 250–550 m deep, and delimited by ENE–WSW-trending faults to the north and south. We recognized an ENE–WSW-oriented compression related to a strike-slip faulting regime, which corresponds to the present-day stress field in the region. This faulting event led to the deposition of colluvial fans, shed from adjacent uplifted crustal blocks, in a tectonically controlled depression. The colluvial succession is ∼ 45 m thick and presents two facies assemblages that filled the southern and, in particular, the northern borders of the trough: non-cohesive debrisflow and mudflow deposits. Optically stimulated luminescence dates of the sedimentary infill yielded ages at 224–128 ka and 45–28 ka, dominated by debrisflow and mudflow deposits, respectively. These ages may be over-estimated due to poor bleaching of colluvium, but they and our field data suggest that the margins of the trough were tectonically uplifted and eroded twice in the Late Pleistocene. The spasmodic colluvial accretion reflects the occurrence of high-magnitude, low-recurrence episodes probably associated with climate shifts in a semi-arid hillslope system. It follows that the present-day low-lying piedmont in which the Cariatá trough occurs is a juxtaposition of surfaces of various ages. This trough may have counterparts across the region. These conclusions do not corroborate the application of the cyclical pediplanation hypothesis in the area.

Submarine fan and slope-apron deposition in a Cretaceous Forearc Basin: The Gürsökü Formation (Kavak–Samsun, N. Turkey)

The Late Cretaceous Gürsökü Formation represents the proximal fill of the Sinop–Samsun Forearc Basin that was probably initiated by extension during the Early Cretaceous. The succession records sedimentation in two contrasting depositional systems: a slope-apron flanking a faulted basin margin and coarse-grained submarine fans. The slope-apron deposits consist of thinly bedded turbiditic sandstones and mudstones, interbedded with non-channelized chaotic boulder beds and intraformational slump sheets representing a spectrum of processes ranging from debris flow to submarine slides. The submarine fan sediments are represented by conglomerates and sandstones interpreted as deposited from high density turbidity currents and non-cohesive debris flows. The occurrence of both slope apron and submarine fan depositional systems in the Gürsökü Formation may indicates that the region was a tectonically active basin margin during the Late Cretaceous.

An experimental investigation of sand–mud suspension settling behaviour: implications for bimodal mud contents of submarine flow deposits

AbstractThe settling behaviour of particulate suspensions and their deposits has been documented using a series of settling tube experiments. Suspensions comprised saline solution and noncohesive glass‐ballotini sand of particle size 35·5 μm &lt; d &lt; 250 μm and volume fractions, φs, up to 0·6 and cohesive kaolinite clay of particle size d &lt; 35·5 μm and volume fractions, φm, up to 0·15. Five texturally distinct deposits were found, associated with different settling regimes: (I) clean, graded sand beds produced by incremental deposition under unhindered or hindered settling conditions; (II) partially graded, clean sand beds with an ungraded base and a graded top, produced by incremental deposition under hindered settling conditions; (III) graded muddy sands produced by compaction with significant particle sorting by elutriation; (IV) ungraded clean sand produced by compaction and (V) ungraded muddy sand produced by compaction. A transition from particle size segregation (regime I) to suppressed size segregation (regime II or III) to virtually no size segregation (IV or V) occurred as sediment concentration was increased. In noncohesive particulate suspensions, segregation was initially suppressed at φs ∼ 0·2 and entirely inhibited at φs ≥ 0·6. In noncohesive and cohesive mixtures with low sand concentrations (φs &lt; 0·2), particle segregation was initially suppressed at φm ∼ 0·07 and entirely suppressed at φm ≥ 0·13. The experimental results have a number of implications for the depositional dynamics of submarine sediment gravity flows and other particulate flows that carry sand and mud; because the influence of moving flow is ignored in these experiments, the results will only be applicable to flows in which settling processes, in the depositional boundary, dominate over shear‐flow processes, as might be the case for rapidly decelerating currents with high suspended load fallout rates. The ‘abrupt’ change in settling regimes between regime I and V, over a relatively small change in mud concentration (&lt;5% by volume), favours the development of either mud‐poor, graded sandy deposits or mud‐rich, ungraded sandy deposits. This may explain the bimodality in sediment texture (clean ‘turbidite’ or muddy ‘debrite’ sand or sandstone) found in some turbidite systems. Furthermore, it supports the notion that distal ‘linked’ debrites could form because of a relatively small increase in the mud concentration of turbidity currents, perhaps associated with erosion of a muddy sea floor. Ungraded, clean sand deposits were formed by noncohesive suspensions with concentrations 0·2 ≤ φs ≤ 0·4. Hydrodynamic sorting is interpreted as being suppressed in this case by relatively high bed aggradation rates which could also occur in association with sustained, stratified turbidity currents or noncohesive debris flows with relatively high near‐bed sediment concentrations.