Alluvial Phases Research Articles

Renewed incision and complex response of the North Fork Toutle River following the eruption of Mount St. Helens in 1980

The upper North Fork Toutle River (NFTR) has been evolving in a complex way since being disrupted by a 2.5-km3 debris avalanche deposit during the catastrophic eruption of Mount St. Helens in Washington State, USA in 1980. In this study, we improved the alluvial phase space diagram (APSD), which classifies channel adjustment into different stages based on bed elevation and cross-sectional area changes, and applied it to the NFTR at annual time scale in the 1980s. The results showed that in the early 1980s, the upstream narrow single-threaded reaches were degraded, while the downstream wide anabranching reaches were aggraded, with the middle transitional reach acting as the hinge zone. Gradually, the upstream channel reaches armoured, widened, bed slope and excess stream power decreased in the late 1980s, and quasi-equilibrium occurred locally and during short time periods. Sediment load decreased at the lower reaches, resulting in bed degradation and a transition from sediment sink to source. This reversal seemed to have triggered renewed incision, which migrated upstream. The renewed incision was characterised by Stage 1/2 (bed incision with limited bank erosion) followed by Stage 2 (degradation and widening) in the improved APSD. The degradation rate accelerated in the 1990s, especially due to the big flood in 1996, and then slowed down after ∼2000. Channel adjustment rates of the renewed incision were much smaller compared to those of the initial rapid degradation. The change rates of cross-sectional area and valley width decreased by ∼58% and ∼85%, respectively. The comparison between this study and existing studies highlights the significant role played by the time scale, at which the APSD is applied to study channel morphology adjustment.

Sedimentology and Mineralogy of Surface Formations of the Mossendjo Area (Republic of Congo): Paleoenvironmental and Paleoclimatic Implications

Studies undertaken at several sites in the Congolese basin and in the Chaillu massif are helping to reinterpret the sedimentological formations known as cover formations, which are widely exposed in Central Africa. This article presents the results of granulometric, morphoscopic, mineralogical and chemical analyzes obtained at three sites in the Chaillu. Gronulometric analysis indicates that the sands at the three sites are rated medium to high. The cumulative curves are essentially sigmoidal with regular and spreading slopes. This facies indicates that the deposition took place due to a banal and moderate variation in the competence of the carrier current. The morphoscopy of the quartz grains shows that they are mainly angular and shiny. These data reflect a relatively close source and transport by water. The synthesis of lithological, granulometric, petrographic and geochemical analyzes allowed us to highlight three episodes of deposits in the Mossendjo area: the first is characterized by a channel-type fluvial system marked by deposits from the hospital borehole and from the station; the second, which is alluvial, corresponds to the clogging of the channel. These two alluvial phases are overcome by a third, fluvial, channel-type episode, the end of which is marked by the establishment of the yellow formation. The vertical and lateral organization at the outcrop of this yellow formation has the same characteristics as those described earlier and in the sub-region where it was attributed to the wind-generated cover formation. This study thus highlights the existence of a wet phase during which two fluvial systems of non-contemporary channel types developed which overlap.

Multidecadal Geomorphic Evolution of a Profoundly Disturbed Gravel Bed River System—A Complex, Nonlinear Response and Its Impact on Sediment Delivery

AbstractA 2.5‐km3 debris avalanche during the 1980 eruption of Mount St. Helens buried upper North Fork Toutle River valley and reset the fluvial landscape. Since then, a new drainage network has evolved. Cross‐sectional surveys repeated over nearly 40 years at 16 locations along a 20‐km reach of river valley document channel evolution. We analyze spatial and temporal changes in channel morphology using two new metrics: (1) a shape index that defines the degree of U‐shaped or V‐shaped valley geometry and (2) an alluvial phase space diagram that relates bed degradation or aggradation to increases or decreases in cross‐sectional area. Unlike a simple, linear response model previously proposed, our analysis reveals channel development has been distinctly nonlinear and nonsequential. Rather than following a sequential trajectory of (1) channel initiation and incision, (2) aggradation and widening, and (3) episodic scour and fill with little change in bed elevation, long‐term channel evolution has been more complex with vertical and lateral adjustments intertwined throughout. Our analysis reveals channel evolution has followed a complex trajectory that has migrated nonsequentially through several phase space domains including degradation and aggradation with widening and narrowing, bed‐level fluctuations with little change in cross‐section area, and changes in cross‐sectional area with little change of bed elevation. Persistent channel widening and reworking of the channel bed are responsible for maintaining elevated sediment delivery from this basin. Elevated sediment delivery is likely to persist until valley floor widths greatly exceed that of the channel migration zone, and/or channel slopes and valley walls stabilize.

The climate-triggered western shift of the confluence between the Dora Baltea and Po rivers (north-western Italy) during the late Holocene

The study on the migrations of the confluence between the rivers Po and Dora Baltea was based on a detailed observation of aerial photographs, field surveys and sediment stratigraphy. The age of the sediments and morphological features was based on radiocarbon datings, on the presence of archaeological settlements and ancient artefacts and on historical data. The aerial photographs of the GAI 1954 flight of the Italian Air Force were used because in the year 1954, some low terraces were still clearly visible, while today are almost completely obliterated because of the works for the improvement of the rice fields. A succession of nine fluvioglacial and fluvial terraces and many abandoned riverbeds have been identified: the interpretation of the morphological features made it possible to identify the Po and Dora Baltea evolution during the late Holocene. The study established that during the last 3000 years, the confluence of the Dora Baltea into the Po has constantly migrated to the west and that this migration occurred during alluvial phases dating back to the Iron Age, 2nd century BC–1st century AD, 5th century AD, 6th–8th century AD and 15th–19th century AD. The alluvial phases occurred during periods of increased floods in northern Italy and advances of the Alpine glaciers in northwestern Italy and Switzerland. Neither tectonic deformations nor anthropic actions seem to have influenced fluvial evolution in the last 3000 years. The westward migration of the confluence between the rivers was therefore triggered by climatic changes, but caused by the different responses of the catchments of the Dora Baltea and the Po rivers to climatic changes, and by the greater slope of the Dora Baltea flood plain.

Origin and Holocene geomorphological evolution of the landslide-dammed basin of la Narse de la Sauvetat (Massif Central, France)

This work is the first geomorphological analysis of La Narse de la Sauvetat, a hydromorphic basin located in the southern Limagne plain (French Massif Central) which had never been studied despite its great palaeoenvironmental interest. We explore the potential of its sedimentary archives to provide valuable data on Holocene geomorphic dynamics and their sensitivity to local and regional hydro-climatic changes. In order to characterize the nature and the morphostratigraphic and pedosedimentary evolution of the basin, we used an integrative approach involving geomorphological mapping, hand auger stratigraphic cross-sections, topographic analysis based on LiDAR data and stereophotogrammetric reconstruction, geophysical prospection and radiocarbon dating. Results revealed a palaeotopography fossilized under 6 m of sedimentary infilling, with a clear shift from fluvial to hydromorphic conditions circa 2800 cal BCE, separated by a level with colluvial and lacustrine features. A detailed analysis of the eastern border of the basin also suggests the existence of a former open valley in place of the current closed depression, now locked in by a topographic threshold forming a dam. The sedimentary aggradation of the basin started with an alluvial phase during the Upper Pleistocene and probably continued into the Middle and Late Holocene, consistently with regional alluvial dynamics. In the early Subboreal, a massive landslide occurred on the western slopes of the Puy-de-Corent volcanic plateau, thereby damming the valley and forming a closed depression. This episode coincided with the mid-Holocene climatic shift and an associated phase of slope instability and increased landslides in Western Europe due to changing climatic conditions. This depression evolved into a hydromorphic basin with several high energy hydro-sedimentary episodes after 2800 cal BCE; however hydro-sedimentary conditions gradually became more stable towards the Late Holocene. Around 500 cal CE, the drainage deteriorated towards palustrine conditions and caused the formation of a marsh and its consequent evolution into a shallow lake throughout the Middle Ages. This lake was drained in the late XVIIIth Century. The long-term sedimentary record of La Narse de la Sauvetat, sensitive to climatic instability periods of the Middle Holocene, makes this landslide-dammed basin a valuable site with an important potential to further development of palaeoenvironmental and geoarchaeological studies.

Coarse sediments in Northern Apennine peat bogs and lakes: New data for the record of Holocene alluvial phases in peninsular Italy

This research was conducted in the Northern Apennines, in the upper valleys of the Cedra River and Parma River, occupied by glaciers during the last glacial maximum. The stratigraphy, the dating and the interpretation of the environmental significance of alluvial deposits interbedded between lacustrine sediments are reported. The data provide an overview of the periods of enhanced alluvial activity that occurred after the glacial retreat in the Apennine chain. The alluvial phases and the periods of environmental stability varied in frequency and length during the Holocene. In particular, starting from the beginning of the Apennine Neoglacial (about 4.2 kyr BP), the events became much more frequent but shorter. The elapsed time between alluvial phases (i.e. the length of the phases of stability) was greater during the Early Holocene, probably because of the values of insolation: when the insolation in July was lower, the length of the phases of stability was greater. Despite the uncertainty in the dating of some events, it is likely that many alluvial phases, later than about 8.5 kyr BP, started during ice rafted debris events in the North Atlantic. The overall analysis of the alluvial sedimentation indicates that periods of more frequent floods are associated with times of rapid climate change and not only with cool periods. The current increase in floods, linked to the ongoing climate change, confirms the data obtained from Holocene sediments.

The organic and mineral matter contents in deposits infilling floodplain basins: Holocene alluviation record from the Kłodnica and Osobłoga river valleys, southern Poland

The work examines the timing and environmental conditions of floodplain sedimentation in the valleys of the upland Kłodnica and piedmont Osobłoga rivers in the Upper Odra River basin. A distribution of 52 14C-ages shows relatively high floodplain sedimentation at the Late Glacial–Holocene transition, more stable floodplain environments since the Early (in the Kłodnica Valley) and Middle Holocene (in the Osobłoga Valley) and a gradual increase in floodplain deposition in the Late Holocene (since <3.4kyr BP). Organic matter [OM] and mineral matter [MM] fluctuations were correlated with variables responsible for the activation of erosion (i.e. vegetation changes, human impact and hydrological events) as well as factors affecting the local record of sedimentation (i.e. valley morphology, hydrologic conditions and episodes of local erosion). A clear relationship is shown between an increase in alluviation and climate- or human-induced extension of unforested areas. The deposition of mineral-rich sediments increases rapidly during periods characterized by non-arboreal pollen values exceeding approximately 8% in pollen diagrams. On the other hand, the results obtained do not confirm significant interactions between Holocene changes in forest composition and alluviation. Despite the settlement of agrarian groups, the sedimentary record of human activity in the Osobłoga catchment is very poor during the Neolithic and early Bronze Age. A large-scale alluviation of the Osobłoga and Kłodnica valleys was initiated during the settlement of people of the Lusatian culture from the middle Bronze Age and escalated in the early Middle Ages and Modern Times. The deposition of products of soil erosion was limited to between ca. 1.9–1.2kyrBP, probably due to demographic regression during the Migration Period. Comparison of OM/MM fluctuations with phases of increased fluvial activity does not show a relationship between Holocene wetter phases and catchment sediment yield. Sedimentary episodes in the Upper Odra basin also show a low degree of correlation with the probability density curve of the 14C-ages. The results obtained in the Kłodnica and Osobłoga valleys indicate a strong to moderate correlation between the spatial distribution of the study sites and the origin of MM-rich deposits, but a weak correlation between the spatial distribution of the study sites and TOC content. Such a pattern suggests that OM/MM fluctuations relate predominantly to the changes in sediment yield, although morphological conditions have a significant impact on the capture potential of sedimentary basins during phases of alluviation. Additionally, high OM content is not a simple function of an increase in wetness of the sedimentary environment. On the other hand, hydrologically-conditioned hiatuses as well as erosion episodes impoverish the sedimentary record, complicating the consideration on the geochronology of deposits and making it difficult to calculate reliable accumulation rates. However, they do not reduce the value of OM/MM fluctuations as an indicator of alluviation events for a preserved series of sediments.

Correlating Alpine glaciation with Adriatic sea‐level changes through lake and alluvial stratigraphy

AbstractWe compare lake and alluvial stratigraphy along a frame connecting the southern Alpine foothills and the Adriatic Sea, with the aim of matching the effects of Alpine glaciation and sea‐level changes on sedimentation during the last glacial cycle. The palynostratigraphy of Lake Fimon provided proxies for regional vegetation and climate change and was coupled with sediment petrography, loss on ignition and magnetic susceptibility, disentangling alluvial phases from fluvioglacial activity related to culminations of the southeastern Alpine glaciers. The Fimon area was not reached by alluvial fans during the penultimate glacial maximum, nor by the sea transgression during the last interglacial, but a closed lake soon developed at the Eemian onset due to enhanced rainfall. Sea‐level fall at glacial inception triggered the entrenchment of the drainage network in the plain reaching the outer Fimon Basin. Slow aggradation, but no sign of fluvioglacial activity, lasted to 38.2 ± 1.45 cal. ka BP, when a major forest withdrawal took place, coeval to the spread of alluvial fans. By 27.5 ± 0.5 cal. ka BP the Fimon Basin was dammed by the Brenta outwash system. The main step of of forest recovery commenced at around (15.8) cal. ka BP, when apex trenching of the outwash fans was triggered by the glacier's decay. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Environment–human relationships in historical times: The balance between urban development and natural forces at Leptis Magna (Libya)

The relationships between human modification of the environment and natural events in the Roman city of Leptis Magna (UNESCO world heritage), western Libya, are analyzed. For the first time, the history of Leptis Magna is tested against a geomorphological and stratigraphical reconstruction and radiocarbon dating. Historical and archaeological interpretations or analyses indicate the occurrence of different extreme natural events as the cause of the town’s decline: earthquakes, flooding, and tsunami. Geological and geomorphological surveys investigated the dynamics of the nearby Wadi Lebda, a major dryland stream that forms the depositional and erosional systems of the settlement area. Alluvial phases were studied by applying traditional stratigraphic analyses of outcrops and hand-cores. Additionally, the mapped flights of inset terrace surfaces provided insights into the human modifications of the natural depositional/erosional environment during historical times and the following alluvial phases affecting the Leptis Magna harbor. The results integrate the archaeological knowledge by providing some independent chronological constraints, and indicate that Leptis Magna history was tightly linked to the Wadi Lebda. Aware of the hazards related to devastating flooding, the Romans were able to cope with the threat posed by the wadi by performing engineering defensive hydraulic works around the town (dam and artificial channels). Once the economic decay began and the society could no longer guarantee the ongoing maintenance of these structures, the decline of the settlement started and the occurrence of destructive floods reclaimed the populated areas. Conversely, the occurrence of a large earthquake (365 CE), or of a tsunami that caused the disruption of the hydraulic systems and the infill of the harbor, has been discarded as primary cause of the decline of Leptis Magna.

Alluvial terrace systems in Zhangjiajie of northwest Hunan, China: Implications for climatic change, tectonic uplift and geomorphic evolution

This paper reports the latest details from two comprehensive investigations of alluvial terrace sequences in Zhangjiajie, northwest Hunan Province, China. Seven alluvial terrace units along the Maoxi River and four terrace sequences along the Suoxi River record significant regional geomorphic history. Rates of regional Quaternary uplift and climate change are reconstructed using topographic and stratigraphic evidence from terrace and adjacent cave deposits, along with Electron Spin Resonance (ESR) and Thermo-luminescence (TL) dating controls. Between 928 ka and 689 ka the time-averaged uplift rate (or incision rate) was 0.16 m/ka. The rate decreased to 0.05 m/ka between 689 ka and 347 ka, and then increased slightly to 0.11–0.14 m/ka after 347 ka. The inferred incision rate increased roughly from 0.21–0.32 m/ka to 0.51 m/ka from the Late Pleistocene to present. The seven alluvial phases (T 7 − T 1) and their associated chronology are consistent with climatic variations at regional and/or global scales, suggesting that these terraces represent climate-driven pauses imprinted atop the record of long-term tectonically induced incision by rivers. Insights from these alluvial terrace staircases and cave features indicate that the spectacular sandstone peak forest landscape of the study area has emerged since the middle period of the Middle Pleistocene.

Stratigraphic correlation of the Neogene basins in southwestern Anatolia: Regional palaeogeographical, palaeoclimatic and tectonic implications

The extensional intramontane grabens that formed in southwestern Anatolia in the Late Cenozoic bear the unique tectono-sedimentary, palaeontological and palynological record of a region that underwent rapid transition from the last compressional pulses of the Alpine orogeny to the tectonic phase of orogen collapse and the onset of neotectonic regime. The change in tectonic regime was accompanied by regional climatic changes, recorded by the sedimentary environments of the basins and evidenced by palaeontological and palynological data. The late Early to mid-Middle Miocene was characterized by a warm and humid subtropical climate with densely forested wetlands. The late Middle to Late Miocene witnessed a change to arid climatic conditions, with grass-dominated steppe ecosystems. The Pliocene climate was warm and humid, with savannah-type open habitats. In terms of their stratigraphic development, the southwestern Anatolian basins form two distinct groups. One group is represented by the Denizli, Söke and Yatağan basins, which commenced their development in the Early Burdigalian and underwent similar phases of environmental change: (1) the Burdigalian phase of alluvial environment and redbed deposition; (2) the Late Burdigalian–Langhian phase of alluvial sedimentation accompanied by the formation of “terminal” lakes; (3) the Serravallian–Middle Tortonian phase of the formation of shallow perennial lakes; (4) the Middle Tortonian–Messinian phase of lake expansion and deepening, followed by shrinking and shallowing; (5) the Pliocene phase of lake expansion and deepening; and (6) the erosional Quaternary phase of two-stage alluvial sedimentation separated by a high-relief erosional unconformity. The other, younger group of tectonic grabens is represented by the Karacasu, Bozdoğan, Çameli and Eşen basins, which commenced their development in the earliest Tortonian and underwent common environmental changes: (1) the Tortonian phase of alluvial environment and redbed deposition, coeval with the phase of lake drying in the Denizli, Söke and Yatağan basins; (2) the Messinian phase of the formation of “terminal” lakes, coeval with the re-establishment of shallow lakes in the three other basins; (3) the Zanclean phase of shallow perennial lakes, coeval with the expansion and deepening of lakes in the other basins; (4) the Piacenzian phase of deep lacustrine environments; (5) the Gelasian phase of shallow lacustrine environments; and (6) the erosional Quaternary phase of two-stage alluvial sedimentation, similar as in the other basins. In spite of the time offset in their development, the two basin groups have been strongly modulated by climate and hence show correlative responses to regional climatic changes. Despite the time offset in their tectonic development, the two basin groups showed correlative environmental changes imposed by regional climatic conditions. The basins were environmentally unified by the Quaternary alluvial phase of their development. In the Late Cenozoic, Anatolia was at the crossroads of fauna migration routes between Africa, Asia and Europe, and the palaeontological data from the southwestern Anatolian basins shed more light on this phenomenon. The study as a whole contributes to a better understanding of the Late Cenozoic environmental, climatical and biogeographical changes in the Eastern Mediterranean.

Types and spatial combinations of Danxia landform of Fangyan in Zhejiang Province

The prominent types of Danxia landform in Fangyan include enclosed valleys, mesas, peaks, stone columns and grooves etc. Their spatial combinations have regular configurations along the northwest-southeast direction: typical grooves and caves are located in the northwestern Wufengshuyuan mainly; abundant fresh collapsed stones may be observed in central Jimingfeng and Taohuafeng; stone drums and stone columns are in the southeastern Shiguliao particularly; enclosed valleys are encircling joints of peaks and plains from three directions east, west, and south. Their spatial combinations reflect that the developments of Danxia landform have undergone stages of geomorphic cyclical erosion in the form of weathering, collapse, transportation, sedimentation and other processes, together with the “sculpture” of external forces mainly as tectonic uplift. The picturesque Danxia landform began its formation at that point. Danxia landform developed mainly in the strata of Fangyan Formation (K1f) caused by the alluvial fan-braided river phase of anterior fan in the late period of the Early Cretaceous. Regular patterns of weathering of stones and features of braided alluvial phase sediments may be verified by the analysis of three groups of experimental data. Danxia landform of Fangyan is a unique representative of the “adolescent” development type in the application of the World Natural Heritages status in China, by virtue of its outstanding universal aesthetic and scientific value.

Fluvio–sedimentary response of the monsoon-fed Indian rivers to Late Pleistocene–Holocene changes in monsoon strength: reconstruction based on existing 14C dates

The primary objective of the present study is to identify major phases of alluviation in the Indian region since the abrupt Deglacial intensification of the monsoon (∼15 cal ka BP) on the basis of analysis of 68 radiocarbon dates from two major hydro-geomorphic regions of India: the Central Ganga Basin (CGB) and the Deccan Peninsula (DP). The recognition of main phases of alluviation and incision has been achieved by evaluating the temporal distribution and clustering of the radiocarbon dates from alluvial sequences. The clusters were detected on the basis of the interpretation of the summed probability distribution plots derived by using OxCal version 4.0.1 and CALPAL (version May 2006) software packages. The summed probability plots reveal that periods of alluviation in the CGB, represented by three clusters (13.9–12.3, 11.9–11.2 and 9.8–9.0 cal ka BP) occur roughly before the onset of Early Holocene monsoon optimum phase. Two other clusters occur in the intervals 3.6–2.8 and 1.1–0.9 cal ka BP. The peak monsoon period generally lacks clusters of radiocarbon dates implying fluvial erosion and channel incision. This period also shows clustering of radiocarbon dates of the abandoned channels. In comparison, 14C dates from DP alluvial units form clusters at 16.4–14, 12.8–11.2, 10.8–8.9, 8.1–6.7 and 5.1–3.9 cal ka BP, indicating an association with the Deglacial–Early Holocene humid phase. Alluviation in the DP appears to have continued, more or less, uninterrupted till the middle of the Holocene epoch. The beginning and end of the discernible gap in the radiocarbon dates of CGB (9.0–3.6 cal ka) broadly corresponds with the two well-established short-term events of the Holocene, 8.2 and 4.2 ka cal BP. In comparison, the prominent gap of DP radiocarbon dates (3.9–2.1 ka cal BP) approximately begins with the 4.2 ka cal BP short-term event (onset of aridity) and ends with the 2.0 ka cal BP enhanced monsoon event. Notwithstanding the inter-regional differences in the fluvio–sedimentary response in the India region, the clusters of radiocarbon dates indicate that the century to millennium scale variations in fluvial activity in the Indian subcontinent were intimately linked to long-term fluctuations in the monsoon strength during the Late Quaternary.

Late Holocene fluvial chronology of Spain: The role of climatic variability and human impact

A database of published and unpublished radiocarbon dates from Late Holocene fluvial environments in Spain was critically analysed in order to obtain a fluvial chronology for this period. The principal types of depositional environment represented in the record were slackwater flood deposits (28 dates), alluvial overbank facies (12 dates) and alluvial channel facies (7 dates). The sedimentary context of each date was classified in order to identify those samples that specifically dated change in the fluvial environment. Analysis of the different depositional environment data-sets using summed probability plots identified phases of alluviation at 2800–2350 and 800–500 cal. BP. Increased frequency of large magnitude floods occurred at 2850–2500, 1000–800 and 520–250 cal. BP. The results indicate that in Mediterranean regions slackwater flood deposits are valuable archives of climatic variability even during periods of increased human impact.

Geoarchaeological simulation of meandering river deposits and settlement distributions: A three-dimensional approach

Fluvial processes have the potential to obscure, expose, or even destroy portions of the archaeological record. Floodplain aggradation can bury and hide archaeological features, whereas actively migrating channels can erode them. The archaeological record preserved in the subsurface of a fluvial system is potentially fragmented and is three-dimensionally complex, especially when the system has been subjected to successive phases of alluviation and entrenchment. A simulation model is presented to gain insight into the threedimensional subsurface distribution, visibility, and preservation potential of the archaeological record in a meander-floodplain system as a function of geomorphic history. Simulation results indicate that fluvial cut-fill cycles can strongly influence the density of archaeological material in the subsurface. Thus, interpretation of floodplain habitation based solely upon features visible in the shallow subsurface (through traditional techniques such as aerial photography and geophysical prospection) can be misleading. In the examples, the loss of archaeological record by channel migration ranges between 45% and 90% over 12,000 years for channel belt-dominated systems, decreasing to 10 to 30% for rivers where the floodplain width is a multiple of channel belt width. The modeling presented can be used to test excavation strategies in relation to hypothesized scenarios of stratigraphic evolution for archaeological sites. © 2006 Wiley Periodicals, Inc.

Late-Holocene alluvial events in the Central Apennines, Italy

A stratigraphic sequence was identified in the Gran Sasso Massif (Central Apennines, Italy), consisting of alluvial sediments and buried soils. The soils, dated by the radiocarbon method, enabled various alluvial phases that have occurred in the last 2500 years to be chronologically framed. These phases were then correlated with the frequency of the Tiber floods at Rome and with the progradation of its delta. The phases of soil burial by alluvial sediments coincide with the periods of intense flooding of the Tiber at Rome (between the second century Bc and the second century AD, between the sixth and the ninth centuries AD, and between the fifteenth and the nineteenth centuries AD) and with periods of strong progradation of the river delta. On the basis of the chronological correlation between alluvial phases, glacial variations, and oscillations in the levels of Lakes Fucino and Trasimeno, it was then possible to establish that, corresponding to the alluvial phases, the hydrological balance of the lakes was positive and, in two of the three cases, the increased flood frequency coincided with the phases of advance of the only glacier located in the Apennines.

Evolution of the Minle and Chaoshui Basins, China: Implications for Mesozoic strike-slip basin formation in Central Asia

The Minle and Chaoshui Basins of northern China are pull-apart basins that originated during Jurassic to Early Cretaceous strike-slip and extensional deformation in the western North China block (Alashan), and the region to its southwest, known as the Hexi Corridor. The basins are separated by the Longshou Shan, which has been a positive topographic feature since Jurassic time. Lower Cretaceous successions in the Minle and Chaoshui Basins display similar rift-related, fining-upward motifs. These comprise alluvial conglomerates and sandstones that pass upward into lacustrine mudrocks, followed by a final fine-grained alluvial phase with extensive paleosol development. The upper part of each succession includes a second lacustrine interval, which is related either to a short-lived humid climatic period, or downstream damming of the exit for the fluvial systems of both basins. North-south–trending normal faults and rare synsedimentary rotation and slump structures affect the basin fills. The Minle and Chaoshui Basins are only two of a series of Late Jurassic–Early Cretaceous strike-slip–related basins in this region that nucleated on three basement blocks, Tarim, Qilian Shan, and Alashan. All of the basins are characterized by a nonmarine, alluvial-lacustrine fill, controlled by a combination of strike-slip and normal faults. From an analysis of the distribution, orientation, and motion sense of subsurface and exposed faults we conclude that three types of strike-slip–related basins are present: pull-apart basins (Minle, Huahai-Jinta, Chaoshui), transtensional basins (Southwest Badanjilin, northern Wuwei), and a third type that developed between divergent strike-slip faults of opposing motion senses. We term this type “extrusion fault-wedge basins” (Jiuquan, Bayanhaote, western Wuwei). The cause of Late Jurassic to Early Cretaceous strike-slip and extension in this region is interpreted to be the Lhasa block–Asia collision. Compression arising from this event is speculated to have caused eastward extrusion of the crust of the Hexi Corridor and Alashan. Deformation was accommodated by a 25° counterclockwise rotation of the Alashan with respect to the remainder of the North China block; thrusting occurred in the intervening Helan Shan. The extension in the Hexi Corridor occurred at the same time as vast foreland basins formed to the west (Tarim) and east (Ordos), and while extension affected much of eastern Mongolia and northeastern China.

The geoarchaeology of Holocene environments and land use at Kazane Höyük, S.E. Turkey

Geoarchaeological investigations at Kazane Höyük, S.E. Turkey demonstrate a record of Holocene environmental change, fluctuating agricultural potential, and human environmental impact. A mid-Holocene alluvial phase with seasonal swamps and steady channel flows suggests a moister climatic regime than at present. This was a suitable landscape for raising pigs and cattle (reflected in the fauna of archaeological levels from this period), and high-yield cultivation of cereals. These environmental conditions continued from the Chalcolithic period through the Early Bronze Age. Subsequent desiccation led to downcutting of the streams and drying out of the marshy environs around the site, although the occupation continued well into the Middle Bronze Age. Alluviation was renewed again in the Medieval period. The lowermost of these Late Holocene deposits indicate fine-grained floodplain and levee development suggestive of a climatic amelioration with later indications of sediment disturbances and colluviation related to human land use and deforestation in Medieval times. ©1997 John Wiley & Sons, Inc.

The geoarchaeology of Holocene environments and land use at Kazane Höyük, S.E. Turkey

Geoarchaeological investigations at Kazane Höyük, S.E. Turkey demonstrate a record of Holocene environmental change, fluctuating agricultural potential, and human environmental impact. A mid-Holocene alluvial phase with seasonal swamps and steady channel flows suggests a moister climatic regime than at present. This was a suitable landscape for raising pigs and cattle (reflected in the fauna of archaeological levels from this period), and high-yield cultivation of cereals. These environmental conditions continued from the Chalcolithic period through the Early Bronze Age. Subsequent desiccation led to downcutting of the streams and drying out of the marshy environs around the site, although the occupation continued well into the Middle Bronze Age. Alluviation was renewed again in the Medieval period. The lowermost of these Late Holocene deposits indicate fine-grained floodplain and levee development suggestive of a climatic amelioration with later indications of sediment disturbances and colluviation related to human land use and deforestation in Medieval times. ©1997 John Wiley & Sons, Inc.

Late quaternary stratigraphy and environments of the west Mexican coastal plain

Abstract The geomorphic events that formed the west Mexican coastal plain and the stratigraphy and chronology for those events based on soils, pollen zones, and radiocarbon ages are discussed. Three phases of geomorphic development are recognized. The Alluvial Phase represents deposition of the coastal bajada during the Neogene; oxisols and saprolites have formed on this surface. The Teacapan Peninsula Phase began during the Sangamonian Inter‐glacial with deposition of the spit; an ultisol and an alfisol have developed on the peninsula. In the Prograding Beach Phase, beginning about 13,000 B.P., four sequences of beach ridges were deposited during eustatic sea level rise and periodic uplift. On the resulting ridge and swale topography are four soil units, three inceptisols on the initial, early, and middle beaches and an entisol on the late beach. The pollen stratigraphy and chronology are based on samples from eleven cores, twelve radiocarbon dates, and calculated deposition rates. The alternation in dom...