Terrace T4 Research Articles

Luminescence chronology of the Sankosh river terraces in the Assam- Bhutan foothills of the Himalayas: Implications to climate and tectonics

Himalaya is an active fold and thrust belt formed due to continent-continent collision between the Eurasian and Indian plates. It comprises a 3000 km long chain of mountains that span ∼1000 km across, with major boundary thrusts viz., Main Central Thrust (MCT), Main Boundary Thrust (MBT) and the Main Frontal Thrust (MFT). MFT is marked as mountain front and is the most active thrust; however, evidence of tectonic activity along MCT and MBT also exists. Tectonic activity along MFT created uplifted terraces which now serve as geomorphic archives of past tectonic events. The present study focussed on a glacial-fed river Sankosh that originates in northern Bhutan, and crosses MCT, MBT and MFT before joining the Brahmaputra River in Assam. Due to tectonic uplift, the river shows a deflection at MFT, incising and thus forming four levels of strath terraces. Luminescence chronology, geomorphic studies and analysis of satellite images suggest four levels of terraces T4 (highest level, 195 m asl), T3, T2 and T1 (lowest level, 120 m asl). The quartz was found insensitive for luminescence dating, and thus fading corrected Infra-Red Stimulated Luminescence (IRSL) ages on feldspar minerals were measured that provided ages of 143-77 ka (T4), 65-36 ka (T2) and 35-14 ka (T1), respectively. The T3 terrace was present only on the right bank of the river and could not be accessed. These ages accord with other studies at the Chalsa and Malbazar, North Bengal (west of the study area) and this regional disposition of similar ages suggest that these formed during glacial-interglacial periods. The strath terraces indicate a time-averaged tectonic uplift with a 0.5 mm/year rate over the past 150 ka.

Response of terrace deposit thickness to climate change and tectonic deformation: An example of the Liyuan River in the Northeast Tibetan Plateau

AbstractThe relationships between tectonics, climate and sedimentation are important for understanding landscape evolution. In order to elucidate these relationships, we conducted a detailed survey of the thickness of terrace deposits and the altitude of terrace surfaces along the Liyuan River valley, in the Northeast (NE) Tibetan Plateau. We have dated the formation ages of terraces T3 and T4 to 42 ± 3.2 and 93.7 ± 7.5 ka, respectively, using optically stimulated luminescence (OSL) method. By compiling the deposit thickness of terraces, palaeoclimate records and local tectonism, we found that variations in precipitation account for the temporal differences in thickness/rates of sediment deposition in terraces T4 and T3. However, the spatial distribution of terrace deposit thickness was determined by tectonism, including faulting and folding, which changed the local base level. In order to maintain a foreland‐ward‐dipping gradient, the Liyuan River adjusts its sedimentary processes via spatially differentiated aggradation.

Late Quaternary Kinematics and Deformation Rate of the Huoyanshan Structure Derived From Deformed River Terraces in the South Piedmont of the Eastern Chinese Tian Shan

The deformation pattern and strain partitioning in the Eastern Chinese Tian Shan are poorly known because of the lack of quantitative study of the kinematics and deformation rate of the major structure. Here we report a late Quaternary shortening rate for the most active reverse fault-and-fold in the Eastern Chinese Tian Shan. We quantified the kinematics and late Quaternary shortening rate of the Huoyanshan structure based on detailed high-resolution remote sensing image interpretations, field investigations and geological mapping. Six generations of folded terraces along the Tuyugou valley that showed the progressive folding process by the Huoyanshan structure were identified. A kinematic model of curved thrust fault propagation and folding allowed us to describe the terrace deformation pattern and subsurface fault geometry and calculate shortening across this structure. Combined with a regional age control of terrace T4 in the Tuyugou valley, a late Quaternary shortening rate of 2.0–3.2 mm/yr of the Huoyanshan structure was obtained. This is a relatively high shortening rate in the whole Eastern Chinese Tian Shan (roughly east of 88 E). This shortening rate of the Huoyanshan structure highlights that the ongoing India and Eurasia collision has affected the entire Tian Shan but shows two strain partitions: the main strain-absorption belt is located within the Eastern Chinese Tian Shan interior, but strain also occurs at the range-front foreland in the Western Tian Shan.

Spatial variations of river incision rate in the northern Chinese Tian Shan range derived from late Quaternary fluvial terraces

Abstract The rate of erosion is critical for characterizing landscape evolution and understanding the interactions between tectonic activity, climate, and surface processes in active orogenic belts. This work focuses on the spatial pattern of river incision rate in the northern Tian Shan range, an active orogenic belt in NW China, based on fluvial geomorphic investigations. Both remote and field-based observations have been used to define the terrace sequences of four selected rivers preserved at the mountain front. For each analyzed river, one of the defined terraces (i.e. terrace T4 of the Kuitun River, terrace T5 of the Jingou River, terrace T5 of the Manas River, and terrace T4 of the Urumqi River) is used as a reference surface from which to reconstruct the paleogeomorphic surface (i.e. the previous riverbed) which was abandoned as a terrace as a result of fluvial incision. By comparing the present-day topography with the reconstructed (pre-incision) landscape, and after a porosity correction, the volume of material eroded by river incision and the depth of incision have been estimated. When combined with the formation age of each reference terrace, the average rates of river incision in the mountain range have been estimated at ~4.1 ± 1.5 mm/yr (Kuitun River), ~3.5 ± 1.4 mm/yr (Jingou River), ~3.1 ± 1.5 mm/yr (Manas River), and ~1.9 ± 1.3 mm/yr (Urumqi River). The above rates display a notable decreasing trend from the Kuitun (west) to the Urumqi (east). Our analyses indicate that lithology and climate might not be the main factors controlling the spatial variation of incision rates in the northern Chinese Tian Shan range. Instead, the observed pattern is in close agreement with changes of slope, relief, and N-S crustal shortening across the range. We thus propose that tectonic activity is the primary factor controlling the erosion and landscape evolution of the northern Tian Shan range, NW China.

Evolution of the Upper Yellow River as Revealed by Changes in Heavy-Mineral and Geochemical (REE) Signatures of Fluvial Terraces (Lanzhou, China)

Despite decades of study, the factors that controlled the formation and evolution of theupper reaches of the Yellow River, including uplift of the northeastern Tibetan Plateau, Pliocene-Pleistocene climate change, and autogenetic processes are still poorly constrained. The stratigraphicrecord of such paleogeographic evolution is recorded in the sequence of nine terraces formed duringprogressive incision of the Yellow River in the last 1.7 Ma. This article investigates in detail forsediment provenance in terraces of the Lanzhou area, based on heavy-mineral and geochemical(REE) signatures. Two main provenance changes are identified, pointing each to a majorpaleogeographic reorganization coupled with expansion of the upper Yellow River catchment andenhanced sediment fluxes. The first change took place between the deposition of terrace T9 (formedaround 1.7 Ma) and terrace T8 (formed around 1.5 Ma), when rapid fluvial incision point to tectoniccontrol and active uplift of northeastern Tibetan Plateau. The second change took place betweendeposition of terrace T4 (formed around 0.86 Ma) and terrace T3 (formed around 0.14 Ma), duringa period of low incision rates and notably enhanced sediment fluxes as a response to enhanced EastAsian Summer Monsoon and consequently increased precipitations, pointing instead chiefly toclimatic control.

Constraints on deformation kinematics across the Yumu Shan, NE Tibetan Plateau, based on fluvial terraces

Abstract In compressive ranges, the kinematics of thrusts and related folds are the key to understanding mountain building processes; however, relatively little attention has been paid to folding on the hanging-wall of a thrust fault which has broken the surface. On the hanging-wall of the Yumu Shan thrust fault, which lies on the north side of the Yumu Shan at the NE edge of the Tibetan Plateau, we found that significant folding deformation had occurred simultaneously with faulting at the surface. To determine the kinematics of the folding and faulting, we surveyed deformed fluvial terrace surfaces across the fold and fault. The geometry of the surface of terrace T4 of the Liyuan River exhibits a faulting displacement of 55 ± 11 m at the fault trace, while in the hinterland, there is an active anticline with a wave length of ~4000 m. Limb rotation on the forelimb is recorded by the terrace geometry. Based on terrace geometry and the deformation of bedrock strata, we infer the geometry of the fault and fold, and conclude that the forelimb was formed by simple-shear deformation above the fault ramp, and that the backlimb was formed by the down-lowering of the fault dip angle. Using the geometry of the fault and fold and previous age control for the terraces, an average rock uplift of 1.2 ± 0.1 mm/a is estimated across the range, which produces a total fault slip rate of 1.8 ± 0.4 mm/a below the Yumu Shan. This new uplift rate indicates an onset age of 2.5 ± 0.5 Ma for the uplift of the Yumu Shan, suggesting that this is a young mountain range. This result suggests that the outward accretion of the NE Tibetan Plateau occurred at ~3 Ma, and it provides an insight into the kinematics of deformation within relatively recently deforming structures, suggesting that folding is a significant process in accommodating the crustal shortening where the thrusting has broken the surface.

Geoarchaeology of the Cobrinhos site (Vila Velha de Ródão, Portugal) - a record of the earliest Mousterian in western Iberia

Cobrinhos (Vila Velha do Ródão, central eastern Portugal) is a Mousterian site found during factory construction in 2014. This area is located in the Lower Tejo valley, which is characterized in terms of geomorphology by six river terraces, numbered downwards (T1 to T6), with Palaeolithic industries associated only with T4 to T6. Terrace T4 was recently dated as spanning ca. 340 ka to 155 ka, with Acheulean in the basal and middle levels and early Mousterian in the uppermost levels. The geological context at Cobrinhos is a colluvial unit that links to the top of T4. It has evidence for palaeoweathering with the same characteristics as seen in T1 to T4, considerably different from that seen in T5 and T6. Despite disturbance by ploughing, the site shows a uniform distribution of sizes and shapes of lithic artefacts, with thousands of implements <30 mm and a coherent Mousterian assemblage including Levallois and discoidal reduction pieces, Levallois flakes, blades and points, pseudo-Levallois points, notches, denticulates, sidescrapers, and an absence of Acheulean or Upper Palaeolithic tools. The available data suggest that the colluvial unit is coeval with the topmost T4 deposits and that the Cobrinhos industry is in its original geomorphological context. Although the colluvial unit cannot be dated directly, from the combination of site data and available published luminescence ages for T4, we suggest a probable age of ca. 165 to 155 ka for this industry. These results are of relevance in the investigation of the demise of archaic Pleistocene human populations and the proliferation of Neanderthal groups in Iberia.

Identifying the source of fluvial terrace deposits using XRF scanning and Canonical Discriminant Analysis: A case study of the Chihshang terraces, eastern Taiwan

The source of fluvial deposits in terraces provides important information about the catchment fluvial processes and landform evolution. In this study, we propose a novel approach that combines high-resolution Itrax-XRF scanning and Canonical Discriminant Analysis (CDA) to identify the source of fine-grained fluvial terrace deposits. We apply this approach to a group of terraces that are located on the hanging wall of the Chihshang Fault in eastern Taiwan with two possible sources, the Coastal Range on the east and the Central Range on the west. Our results of standard samples from the two potential sources show distinct ranges of canonical variables, which provided a better separation ability than individual chemical elements. We then tested the possibility of using this approach by applying it to several samples with known sediment sources and obtain positive results. Applying this same approach to the fine-grained sediments in Chihshang terraces indicates that they are mostly composed of Coastal Range material but also contain some inputs from the Central Range. In two lowest terraces T1 and T2, the fine-grained deposits show significant Central Range component. For terrace T4, the results show less Central Range input and a trend of decreasing Central Range influences up section. The Coastal Range material becomes dominant in the two highest terraces T7 and T10. Sediments in terrace T5 appear to have been potentially altered by post-deposition chemical alteration processes and are not included in the analysis. Our results show that the change in source material in the terraces deposits was relatively gradual rather than the sharp changes suggested by the composition of the gravels and conglomerates. We suggest that this change in sources is related to the change in dominant fluvial processes that controlled by the tectonic activity.

Geomorphic Records along the General Carrera (Chile)–Buenos Aires (Argentina) Glacial Lake (46°–48°S), Climate Inferences, and Glacial Rebound for the Past 7–9 ka

We present geomorphic, stratigraphic, and chronological data acquired along the General Carrera-Buenos Aires (GCBA) glacial lake located along a major morphological incision across the Andes. Complementing relevant available data, relative chronology of morpho-climatic records together with 18 10Be Cosmic Ray Exposure (CRE) ages allow constraining the timing of the Patagonian ice-sheet fluctuations since the LGM. It improves the knowledge of the Patagonia climate evolution in the 46-48°S area, and allows documenting the uplift rates (glacial rebound) for the past ~7-9 ka. The first major ice lobe retreat occurred after 17.3 ± 0.6 ka and has likely continued during the ACR from ~12.9 to 14.5 ka. Between ~12.9 ka and ~10.9 ± 1.3 ka, the General Carrera Lake evolved as an endorheic basin. Terraces T4 to T1 (top to bottom) have recorded abrupt lake regressions likely controlled by rainfall deficit. They have accumulated in the time interval ~17.3-12.3 ka (maximum limits). Two glacial re-advances at ~10.9 ± 1.3 and ~7.9 ± 1.1 ka marked a major climate change that led the lake to be ice-dammed again. A major transgression occurred subsequently that have flooded the previously accumulated terraces. Since then, a pervasive regression has steered the GCBA Lake to the situation at Present. The highest shoreline of the transgression is used as a passive marker in order to quantify the magnitude and character of the regional deformation. At 72°30' W, the GCBA Lake area uplifted (glacial rebound) at a rate between 15 to 33.5 mm.yr--1 during the past ~7.9 ± 1.1 ka. We infer that the high uplift rate mainly originates from the North Patagonian icefield ice loss.

A long-term rock uplift rate for eastern Crete and geodynamic implications for the Hellenic subduction zone

The island of Crete in the forearc of the Hellenic subduction zone has a rugged topography with local relief exceeding 2km. Based on the elevation of marine shorelines, rates of rock uplift during the Late Holocene were previously estimated to range between 1 and 4mm/a in different parts of the island. These rates may, however, not be representative for longer timescales, because subduction earthquakes with up to 9m of vertical coseismic displacement have affected Crete in the Late Holocene. Here we use a well preserved sequence of marine terraces near Kato Zakros in eastern Crete to determine the rate of rock uplift over the last ∼600ka. Field investigations and topographic profiles document a flight of more than 13 marine bedrock terraces that were carved into limestones of the Tripolitza unit. Preliminary age constraints for the terraces were obtained by 10Be exposure dating of rare quartz-bearing sandstone clasts, which are present on some terraces. The 10Be ages of these samples, which have been corrected for an inherited nuclide component, yielded exposure ages between ∼100ka and zero. Combined with geomorphologic evidence the two oldest 10Be ages suggest that the terraces T4 and T5, with shoreline angles at an elevation of ∼68 and ∼76m above sea level, respectively, formed during the marine isotope stage 5e about 120ka ago. The correlation of the higher terraces (T6 to T13) with regional sea-level highstands indicates sustained rock uplift at a rate of ∼0.5m/ka since at least ∼600ka. As normal faulting has dominated the tectonics of Crete during the last several million years, upper crustal shortening can be ruled out as a cause for rock uplift. We argue that the sustained uplift of the island results from the continuous underplating of sediments, which are transferred from the subducting African plate to the base of the crust beneath Crete.

Late Quaternary uplift rate inferred from marine terraces, Shimokita Peninsula, northeastern Japan: A preliminary investigation of the buried shoreline angle

After estimating tectonic uplift rates along the northern part of the northeast Japan forearc (the overriding plate in the northeast Japan subduction zone) by mapping the elevation of the inner edges of marine terrace surfaces, we refined this estimate through elevation measurements of the buried shoreline angle beneath well-dated marine terrace surfaces, from which we could derive more accurate paleo-sea levels. The uplift rate initially inferred from the inner edge of marine terrace T4, correlated with marine isotope stage MIS 5e by tephrochronology, increases eastward from 0.11–0.22mky−1 around the backarc volcanic front to 0.17–0.32mky−1 in the forearc on the peninsula of Shiriyazaki. We refined the uplift rates for T4, on the basis of the shoreline angle elevation, from the reconstructed profile of the paleo-sea cliff and wave-cut platform on a rocky coast and the reconstructed profile of the swash zone sediments and terrace deposits on a sandy coast. The refined uplift rates were 0.14–0.25mky−1 on the rocky coast and 0.14–0.23mky−1 on the sandy coast, slightly slower than the rates we inferred from the height of T4 and about one-half to three-fourths of previously reported rates. By extrapolation from the example of the sandy coast, the refined uplift rate around the volcanic front was 0.09–0.18mky−1. The vertical deformation across the forearc of the Shimokita Peninsula since MIS 5e is possibly associated with regional isostatic uplift of 0.09–0.18mky−1 and anticlinal deformation by an offshore fault, interpreted from acoustic profiles, of 0.05–0.07mky−1.

10Be exposure dating of river terraces at the southern mountain front of the Dzungarian Alatau (SE Kazakhstan) reveals rate of thrust faulting over the past ~ 400 ka

The mountain belts of the Dzungarian Alatau (SE Kazakhstan) and the Tien Shan are part of the actively deforming India–Asia collision zone but how the strain is partitioned on individual faults remains poorly known. Here we use terrace mapping, topographic profiling, and 10Be exposure dating to constrain the slip rate of the 160-km-long Usek thrust fault, which defines the southern front of the Dzungarian Alatau. In the eastern part of the fault, where the Usek River has formed five terraces (T1–T5), the Usek thrust fault has vertically displaced terrace T4 by 132±10m. At two sites on T4, exposure dating of boulders, amalgamated quartz pebbles, and sand from a depth profile yielded 10Be ages of 366±60ka and 360 +77/−48ka (both calculated for an erosion rate of 0.5mm/ka). Combined with the vertical offset and a 45–70° dip of the Usek fault, these age constraints result in vertical and horizontal slip rates of ~0.4 and ~0.25mm/a, respectively. These rates are below the current resolution of GPS measurements and highlight the importance of determining slip rates for individual faults by dating deformed landforms to resolve the pattern of strain distribution across intracontinental mountain belts.

Tectonic controls for transverse drainage and timing of the Xin-Ding paleolake breach in the upper reach of the Hutuo River, north China

The upper reach of the Hutuo River flows along the Xin-Ding basin and cuts a transverse drainage through the Xizhou Mountain and the Taihang Range into the North China Plain. Previous studies showed that the Xin-Ding basin was occupied by a lake during the early-middle Pleistocene. However, the timing of the paleolake breach and the mechanism for the creation of the transverse drainage are unknown. We constructed the fluvial terrace sequence in the upper reach of the Hutuo drainage combined with thermoluminescence (TL) and optically stimulated luminescence (OSL) dating, as well as the timescale of the overlying loess–paleosol sequence. Our results reveal that (1) five terraces (T5–T1) developed along the upper reach of the Hutuo River, amongst which terraces T4–T1 were formed synchronously at ~600, ~120–130, ~20–26 and ~6–7ka, respectively; (2) the creation of the transverse drainage and breach of the Xin-Ding paleolake occurred between ~600 and ~130ka; and (3) the mechanism for the creation of the transverse drainage is via river piracy of paleostreams on both sides of the drainage divide. Localized differential uplift and associated tilting of the Xizhou Mountain block during the middle Pleistocene result in the formation of the transverse drainage and breach of the Xin-Ding paleolake.

Uplift differential of active fold zones during the late Quaternary, northern piedmonts of the Tianshan Mountains, China

On the north piedmont of Tianshan Mountains, China, the Kuytun River and Manas River transverse the Dushanzi and Manas folds, resulted in 7-level and 6-level pedestal terraces, respectively, which are the tectonomorphic marks of folding and uplift of the Dushanzi and Manas anticlines since the late Quaternary. We have collected samples from deposits of all terraces for OSL (optically stimulated luminescence) geological dating using the SMAR (single-multiple-aliquot-regeneration) method on fine grains. We have also performed dating using the 14C method on the samples from the deposit of terrace T1 along the Kuytun River. The results show that all these deposits are of the later phase of the late Pleistocene. Comparison of terrace dating and climate change since 200 ka suggests that the terraces T1, T2, T3, T4 and T5 along the Manas River formed in 6, 8.5, 10, 14 and 32 ka, respectively. The incision time of Kuitun River’s T1, T2, T3, T4, T5, T6 and T7 terraces were 1.7, 14, 20, 25, 32, 50 and 100 ka years ago, respectively. The terrace T4 along the Manas River and T2 along the Kuytun River were formed during the late part of the late Pleistocene, i.e., 14 ka. Since 14 ka, the incision of the Manas River has generated three levels of pedestal terraces, while that of the Kuytun River has only produced one level of such terraces. The latest folding and uplift of the Dushanzi anticline took place in 1.7 ka, while that of the Manas anticline occurred in 5 ka. Since 14ka or the later time of the late Pleistocene, the Dushanzi and Manas anticlines have risen by 40 and 95 m, respectively, implying uplift rates 2.7 and 6.8 mm/a for each.

Geomorphological and sedimentological comparison of fluvial terraces and karst caves in Zhangjiajie, northwest Hunan, China: an archive of sandstone landform development

The Zhangjiajie Sandstone Peak Forest Geopark (Zhangjiajie World Geopark) of northwest Hunan, China hosts a well-preserved sequence of fluvial terraces and karst caves. In this contribution, a comparative study of fluvial terraces with karst caves along the middle-lower Suoxi River in Zhangjiajie World Geopark is presented to improve the understanding of the development of striking sandstone landscape in the upper Suoxi River. By integrating geomorphological, sedimentological, and geochronological techniques, the possible correlation between fluvial terraces and karst caves, as well as their climatic and tectonic implications is investigated. The available electron spin resonance and thermo-luminescence numerical ages coupled with morphostratigraphic analysis indicate that aggradation of fluvial terrace levels occurred at ca. 347 ± 34 ka (T4), 104.45 ± 8.88 to 117.62 ± 9.99 ka (T3), 60.95 ± 5.18 ka (T2), and Holocene (T1), followed by the stream incision. Fluvial terrace levels (T4 to T1) correlate morphologically with the karst cave levels (L1 to L4), yet the proposed chronology for the fluvial terrace levels is a bit later than the chronological data obtained from karst caves. In northwest Hunan, where a unique sandstone peak forest landscape was extensively developed, the fluvial terrace sequences as well as the cave systems are the important archives for studying the evolution of the sandstone landscape. The beginning of the sandstone landscape development must be earlier than the aggradation of the fluvial terrace T4, allowing this unique landscape to occur in the Middle Pleistocene.

Raised coastal terraces along the Ionian Sea coast of northern Calabria, Italy, suggest space and time variability of tectonic uplift rates

A detailed study of uplifted Middle–Late Pleistocene marine terraces on the eastern side of northern Calabria, southern Italy, provides insights into the temporal and spatial scale variability of vertical displacement rates over a time span of ∼400 ka. Calabria is located in the frontal orogen of southern Italy above the westerly-plunging Ionian slab, and a combination of lithospheric, crustal, and surface processes concurred to rapid Late Quaternary uplift. Eleven terrace orders and a raised Holocene beach were mapped up to ∼480 m a.s.l., and were correlated between the coastal slopes of Pollino and Sila mountain ranges across the Sibari Plain, facing the Ionian Sea side of northeastern Calabria. Precise corrections were applied to the measured shoreline angles in order to account for uncertainty in measurement, erosion of marine deposits, recent debris shedding, and bathymetric range of markers. Radiometric (ESR and 14C) dating of shells provides a crono-stratigraphic scheme, although many samples were found to be resedimented in younger terraces. Terrace T4, whose inner margin stands at elevations of 94–130 m, is assigned to MIS 5.5 (∼124 ka), based on new ESR dating and previous amino acid racemization estimations. The underlying terraces T3, T2 and T1 are attributed to MIS 5.3 (∼100 ka), 5.1 (∼80 ka) and 3 (∼60 ka), as inferred from their relative position supplemented by ESR and 14C age determinations. The age of higher terraces is poorly constrained, but conceivably is tracked back to MIS 11 (∼400 ka). The reconstructed depositional sequence of terraces attributed to MIS 5.5 and 7 reveals two regressive marine cycles separated by an alluvial fanglomerate, which, given the steady uplift regime, points to minor sub-orbital sea-level changes during interstadial highstands. Based on the terrace chronology, uplift in the last ∼400 ka occurred at an average rate of 1 mm/a, but was characterized by the alternation of more rapid (up to ∼3.5 mm/a) and slower (down to ∼0.5 mm/a) periods of displacement. Spatial variability in uplift rates is recorded by the deformation profile of terraces parallel to the coast, which document the growth of local fold structures.

Estudio sedimentológico de un sector del río Guadalquivir en las proximidades de Andujar (provincia de Jaén) los depósitos de la terraza + 6m (T4)

The characteristics of the current river channel of the Guadalquivir river and deposits of the terrace T4 ( + 6 m) near Andujar are described. The study of morphological parameters evidence that they are balanced; this indicates that morphology and channel pattern are contolled mainly by local causes. Changes in discharge and bedload are responsible for gradual variations of channel patterno In contrast, overflow causes sharp changes. A general displacement of the river course towards the S. and SW. was deduced from careful study of maps, air photographs, etc., corresponding to various dates. T4 ( + 6 m) terrace deposits exhibit 2 typical vertical sequences (called 1 and 2 respectively) of channel and flood-plain facies. A model of terrace growth is proposed which is domonated by point bar deposits.