Top Terrace Research Articles

The coupling between tectonic uplift and climate change recorded by the Yellow River terraces during the Zoige basin excavation in the northeastern Tibetan Plateau

Understanding the processes of fluvial geomorphic evolution is based on thorough evaluation of how rivers respond to tectonic activity and climatic change, with river terraces representing a key archive of alternating deposition and incision in response to such forcing, enabling an excellent means for correlation with tectonic and climatic records. An example presented here points to a difference in fluvial response to climate fluctuations along a trunk river and its tributary: the Yellow River and its tributary the Xike River, which has incised the Lajia Gorge to pirate the Zoige Basin, in the eastern Tibetan Plateau. Based on Accelerator Mass Spectrometry 14C dating, the chronological sequence of basin fill top and fluvial terraces below has been established, defining the drainage reorganization of the Yellow River excavation through the Zoige Basin since the Last Glacial Maximum. Spatial and stratigraphic comparison between the two fluvial sequences, combined with correlation with high-resolution climatic records, indicates that the tributary Xike River formed more terraces in response to trunk incision (base-level variation), climatic fluctuations, and underlying weak bedrock than the Yellow River. Thus, our results not only demonstrate a combined archive of climatic fluctuation and tectonic uplift recorded in the fluvial terrace sequences, but also suggest that the generation of fluvial terraces may be influenced by the underlying bedrock within the same tectonic and climatic settings.

Application of Multi-Channel Analysis Surface Waves and Electrical Resistivity Tomography Methods to Identify Weak Zones at University of Mosul, Northern Iraq

Cracks have been developed in a large lecture hall on the University of Mosul Campus. These cracks exist along two sides of the building. Multi Analysis Surface Wave and Electrical Resistivity Tomography surveys were conducted at the surrounding building to investigate the nature and distribution of shallow subsurface soil/rock. The MASW’s data show three layers: the first layer is characterized by low shear wave velocity ranges between 210 and 420 m/s which is attributed to infill materials, the next layer is attributed to a river terrace (fairly competent rock) which has Vs ranging from 430m/s to 840 m/s. It is recognized by a gradual increase in mechanical characteristics as depth increases. In-depth between about 2.5m and 6.0 m the shear wave velocity drops in the top contact of this layer. The causes of low velocity may be due to weathering of the medium because of a sinkhole. The Electrical Resistivity Tomography profile shows four electrical zones, the first zone has 50-70 Ω.m. with a variable thickness from 0.5-1.5 m which indicates infill materials. The second zone has very low resistivity value, and a depth ranges 0.5m - 3.5 m, which might be interpreted to be increased clay, silt and water content (high water-saturated zone). The third zone is characterized by a high-resistivity value (>100 Ω.m) that could be related to a dry conglomerate rock and gravels belong to river terraces. The fourth zone has a low-resistivity value (<20 Ω.m) associated with a water-saturated marl bed. We could be definitively correlated the resistivity and velocity anomalies to sinkhole activity in were identified and characterized using combined geophysical methods. The Multi Analysis Surface Waves and Electrical Resistivity Tomography were shown to complete each other in the evaluation. The variations in Vs (low velocity) and resistivity (conductive zone) within the river terrace were detected and proposed to be indicative of dissolution and the subsidence responsible for structural damage implied by the change in the velocity and resistivity. The roughness of the top terrace surface strongly influences the nature of the velocity and resistivity values. This roughness is suggestive of dissolution or erosion.

Late Quaternary terrace formation from knickpoint propagation in the headwaters of the Yellow River, NE Tibetan Plateau

AbstractUpstream knickpoint propagation is an essential mechanism for channel erosion, carrying changes in base level, tectonics and climate across the landscape. Generally, the terraces on cross‐sections at steady‐state conditions have been widely reported. However, many landscapes in the field appear to be in a transient state. Here, we explore the mechanism of knickpoint initiation and fluvial evolution in a transient setting in the northeastern Tibetan Plateau. Analysis of channel profiles and terrace correlation indicates that the Yellow River is adjusted to match the increase in differentiated fault activity and climate change in a regional setting of continuous uplift. Consequently, a series of terraces were formed, and the number of terrace steps increased downstream, in the headwaters of the Yellow River. All terraces were dated using the optically stimulated luminescence method. The top terrace, distributed continuously in the whole basin with a gradient, was deposited during a cold period and abandoned at the climatic transition from cold to warm state, at approximately 14.6–9.5 ka. After that, one terrace formed at around 4.2 ka in the upper reach. In correlation with the continuous topographic gradient surface of this terrace, three terrace steps were formed in the down reach during the period from 9.5 ka to 4.2 ka. This phenomenon might indicate multiple phases of continuous headward migration of fluvial knickpoint waves and terrace formation during the downcutting. It was caused by fault activity and tectonic uplift of the gorge at the outlet of the basin, under influence of the gradual integration of the Yellow River from downstream. This phenomenon shows that the fluvial incision in a transient state along the high relief margin of the orogenic plateau can be caused by fault activity, in addition to widespread surface uplift, climatically driven lake spillover and the establishment of external drainage.

Initial incision of the Jinshan Gorge of the Yellow River, China, constrained by terrestrial in situ cosmogenic nuclides chronology

How post-burial terrestrial in situ cosmogenic nuclides (TCN) production is build-up in slow burial condition remains uncertain. In this study, we dated high fluvial terrace sediments that overlain by loess with a slow deposition rate that is based on the assumption of steady accumulation rate via 26Al/10Be burial dating method. For sample in deep and rapid buried condition prior to slow overlying, post-burial production shows low component (4–6%) in measured concentration. However, post-burial production occupied 39–79% of measured 26Al and 10Be concentrations, which suggests approximately 0.2–0.3 Ma underestimation in simple burial dating mode if post-burial production is neglected. Corrected by post-burial production, we yielded burial ages of two top terraces (ca. 1.7–2.5 Ma and 1.5 Ma, respectively) below the Tangxian planation surface along the Jinshan Gorge of the Yellow River, implying that the Jinshan Gorge initially incised at ca. 1.7–2.5 Ma, corresponding well to regional tectonic activities, such as extension in the Fenwei Graben, accelerative uplift of the Tangxian planation surface, vigorous dissection of North China, and significant tectonic deformation in northeastern Tibetan Plateau. The slow burial mode is probably a potential method for plateau paleo-elevation estimation taking advantage of height sensitivity of TCN production rate if overlying loess magnetostratigraphic age is known.

Effect of Topography on Characteristics, Fertility Status and Classification of the Soils of Almora District in Uttarakhand

Topographic position is one of the most dominant factors that govern soil development in the hilly areas. Ten soil profiles representing different landform units i.e. hill top, side slope, broad valley and narrow valley terraces were chosen to investigate the soil morphological, physical and chemical characteristics and their relationships with topography in Almora district of Uttarakhand. Soil properties varied remarkably among the studied profiles. The soils were moderately shallow to very deep in depth (32–160 cm), nonsaline (EC 0.03–0.45 dS m−1), acidic in nature (pH 5.1–6.6), low to high in organic carbon (OC) (0.03–2.91%) with low to moderate CEC [1.6–19.4 cmol(p+)kg−1]. Base saturation ranged from 45.9 to 75.1% in which calcium (Ca2+) and magnesium (Mg2+) were the dominant cations on exchangeable sites followed by potassium (K+) and sodium (Na+). The studied soils were dominated by silt fraction followed by clay, except in Dhamas (P4), Dhaili (P5) and Kosi (P10) which showed high sand content. Hill top profiles showed the highest OC. The upper soil layers of hill top and broad valley terrace profiles showed higher availability of soil macronutrients, whereas micronutrients were higher than agronomic critical limits in all profiles. Three soil profiles were classified as Entisols and keyed out as Typic Udifluvents (Kosi), Lithic Udorthents (Dhamas) and Typic Udorthents (Dhaili). Majkhali (P7) soils were placed under Inceptisol soil order and were classified as Dystric Eutrudepts. The remaining soils were placed under Alfisol soil order and keyed out as Hapludalf at great group level. Among the Alfisols, Someshwar (P9) was classified as Oxyaquick Hapludalfs, Sitlakhet (P1) was classified as Ultic Hapludalfs, Salla Routela (P2) and Chaubattia (P3) were classified as Mollic Hapludalfs, while Kausani (P6) and Hawalbagh (P8) soils were keyed out as Typic Hapludalfs. The soils in Almora district of Uttarakhand exhibited wide variations with topographic positions and proper soil management will help in maintaining sustainable production of this area.

Reprint of “Ion-mediated growth of ultra thin molybdenum disulfide layers on highly oriented pyrolytic graphite”

Van der Waals (vdW) heterostructures composed of different two-dimensional (2D) materials are at the center of many novel devices. To prepare vdW heterostructures which are of the highest quality and suitable for applications, chemical vapour deposition (CVD) can be used to grow the 2D materials directly on top of each other and thus build the vdW heterostructure in a bottom-up fashion. However, obtaining layers of uniform quality by precisely controlling their growth poses a severe challenge. The aim of our work is to understand the growth mechanisms and we have chosen MoS2 layers on highly oriented pyrolytic graphite (HOPG) as a model system for the MoS2-graphene interface. In our model system we observe, that MoS2 layers do not grow on the HOPG terraces but are more likely to grow at HOPG edges, one-dimensional defects, which obviously acts as growth seeds. In graphene however, step edges are absent and the ever-improving quality of commercially available CVD graphene yields less and less defects per unit area. While this is clearly an advantage for most devices, in the light of our findings it constitutes a major disadvantage for the bottom-up preparation of vdW heterostructures. To overcome this obstacle we artificially introduce defects into the HOPG surface by highly charged ion irradiation. In this way we induce an easily controllable number of quasi zero-dimensional defects before the chemical vapour deposition of MoS2 takes place. We show that this treatment results in MoS2 island growth on top HOPG terraces.

Assessing Soil Erosion by Agricultural and Forestry Production and Proposing Solutions to Mitigate: A Case Study in Son La Province, Vietnam

Son La is a province in the Northwest region of Vietnam; this province has diverse terrains with elevation ranging from 100 m to 2,900 m. Due to lack of cultivating lands, farmers still cultivate lands with steep slope, even above 25o. Consequently, the soil erosion has occurred hugely and the time for productive cultivation has been shortened. Therefore, it is necessary to assess the actual soil erosion, analyzing causes and proposing solutions to mitigate soil loss in the Son La province. Application of GIS technology and the Universal Soil Loss Equation (USLE) to predict soil erosion showed that the soil eroded at a very low level (0–1 ton·ha−1·year−1) accounted for the largest proportion with 56.49% of the area. Low level (1–5 ton·ha−1·year−1), medium level (5–10 ton·ha−1·year−1), and high level (10–50 ton·ha−1·year−1) eroded areas account for 8.52%, 7.88%, and 1.41%, respectively, of the area. Soil is eroded in very high level (>50 ton·ha−1·year−1), accounting for 25.7% of the study area. Research on solutions to mitigate soil erosion in Mai Son district showed that, for the cultivation of perennial crops (coffee), when planting on following contour lines, the amount of soil lost annually due to erosion is 63.37 ton·ha−1, while the cultivation following level top bench terrace soil loss is 39.55 ton·ha−1. If more grass strips were planted, the amount of soil lost annually due to erosion was reduced by 71% compared to traditional solutions. For this solution, when intercropping soybean, the soil erosion was reduced by 63–76% compared to the traditional methods. For cultivation of annual crops (maize) where the traditional methods of cultivation, namely, burning, ploughing, and seeding, are followed, the average loss of soil annually is 64.06 ton·ha−1. This value was 45.04 ton·ha−1 and 41.96 ton·ha−1, respectively, as the minimum soil tillage measure and no-tillage measure were used. When using the stems of maize after harvesting to cover lands following contour lines and terraces, soil erosion decreased by 38–59% compared to traditional solutions, while intercropping with legumes, soil loss due to erosion decreased 50–68% compared to traditional measures.

Ion-mediated growth of ultra thin molybdenum disulfide layers on highly oriented pyrolytic graphite

Van der Waals (vdW) heterostructures composed of different two-dimensional (2D) materials are at the center of many novel devices. To prepare vdW heterostructures which are of the highest quality and suitable for applications, chemical vapour deposition (CVD) can be used to grow the 2D materials directly on top of each other and thus build the vdW heterostructure in a bottom-up fashion. However, obtaining layers of uniform quality by precisely controlling their growth poses a severe challenge. The aim of our work is to understand the growth mechanisms and we have chosen MoS2 layers on highly oriented pyrolytic graphite (HOPG) as a model system for the MoS2-graphene interface. In our model system we observe, that MoS2 layers do not grow on the HOPG terraces but are more likely to grow at HOPG edges, one-dimensional defects, which obviously acts as growth seeds. In graphene however, step edges are absent and the ever-improving quality of commercially available CVD graphene yields less and less defects per unit area. While this is clearly an advantage for most devices, in the light of our findings it constitutes a major disadvantage for the bottom-up preparation of vdW heterostructures. To overcome this obstacle we artificially introduce defects into the HOPG surface by highly charged ion irradiation. In this way we induce an easily controllable number of quasi zero-dimensional defects before the chemical vapour deposition of MoS2 takes place. We show that this treatment results in MoS2 island growth on top HOPG terraces.

Polarization rotation Bragg grating using Si waveguide with top surface groove grating and terrace

We report a silicon-waveguide-type polarization rotation Bragg grating with high diffraction efficiency to be used in a wavelength filter. An antisymmetric grating structure formed on the top surface of the waveguide with a terrace is used to generate polarization rotation Bragg diffraction. The finite difference time domain method is used to simulate the device. We found that introducing a side terrace of 25 to 75 nm can attain a high diffraction efficiency. The diffraction efficiency can be more than 20% larger when using this structure. A 300-nm-thick waveguide used in foundries is beneficial for fabricating a device showing large diffraction and main-subpeak separation.

Impacts of dominated landscape types on hydrogen and oxygen isotope effects of spring water in the Hani Rice Terraces

Analysis of hydrogen and oxygen stable isotopes is an effective method to track the water cycle in watershed. Impact of landscape pattern on the isotope effects of spring water is a new interdisciplinary topic between landscape ecology and isotope hydrology. Taking the Quanfuzhuang River basin located in the core area of UNESCO World Cultural Heritage of Honghe Hani Rice Terrace as the object, collecting the monthly samples of 78 points of spring water and 39 precipitation at altitude of 1500 m (terraces), 1700 m (terraces) and 1900 m (forest) from March 2015 to March 2016, we analyzed the hydrogen and oxygen stable isotopes of water samples under the different landscape types. The results indicated that the dominated landscape types were forests and rice terraces, being 66.6% and 22.1% of the whole landscape area respectively, and they had a spatial vertical pattern of forest located at the mountain top and rice terraces at the down-slope. The correlation analysis showed that the spring water not only came from the precipitation, but also from other water sources which had a more positive δ18O and δD values, the spring water in up-slope forests mainly came from precipitation, while that in down-slope rice terraces came from precipitation, ri-ver water, rice terrace water and under ground water. Therefore, the mixing effects of spring water δ18O and δD were more significant in rice terraces. The overall altitude effect of the hydrogen and oxygen stable isotopes in spring water was obvious. The linear decreasing rates of δ18O and δD values were -0.125‰·(100 m)-1and -0.688‰·(100 m)-1, respectively. The deuterium surplus value increased with the altitude because of the impacts of landscape pattern and the local cycle of water isotopes. In summary, the dominant landscape types had a significant impact on the hydrogen and oxygen isotopes of spring water, which could be used as response indicator to reveal the impacts of landscape pattern on hydrological process.

History of the great Kanto earthquakes inferred from the ages of Holocene marine terraces revealed by a comprehensive drilling survey

We measured the emergence ages of four marine terraces in the Chikura lowland, which lies to the southeast of the Boso Peninsula, in eastern Japan, to reevaluate the history of the great earthquake occurrences along the Sagami Trough over the past 10,000 years. The dates of the marine terraces are measured via radiocarbon dating of shell fossils obtained from the marine deposits. The sampling method employed in this study collects core samples using a dense and systematic drilling survey, which increased the reliability when correlating shell fossils with marine terraces. In addition, radiocarbon dating was performed with accelerator mass spectrometry, which produces more highly accurate measurements than those measured in previous studies. Moreover, we explored the surface profiles of the terraces with detailed digital elevation model (DEM) data obtained using LiDAR. The maximum emergence ages of the marine terraces were dated at 6300 cal yBP, 3000 cal yBP, and 2200 cal yBP from the top terrace excepting the lowest terrace (which was estimated at AD1703). In addition, another previously unrecognized terrace was detected between the highest and the second terrace in both the dating and the geomorphological analyses and was dated at 5800 cal yBP. The newly obtained ages are nearly a thousand of years younger than previously estimated ages; consequently, the intervals of the great earthquakes that occurred along the Sagami Trough are estimated to be much shorter and more varied than those of previous estimations. This result revises the data used in the current assessment of the probabilities of earthquakes along the Sagami Trough, which could devastate the Tokyo metropolitan area. Furthermore, it demonstrates that the current approach could be a powerful tool to increase the accuracy of assessments of the other areas with depositional marine terraces.

Sharing Our Library Facility: Prescott Valley Arizona Public Library

ABSTRACTThe present Prescott Valley Public Library facility was created in 2009 as a space that was meant to be shared. The building was designed to be aesthetically unique and people friendly. Key elements include an auditorium for Town Council, a multifunctional meeting room with a kitchen and multimedia projection capabilities, a roof top terrace, outside patios, PC lab, Digital Media Lab, and a variety of smaller gathering spaces. Additionally, we share our facility with a satellite campus of Northern Arizona University, which affords frequent opportunities to collaborate in novel ways to connect with our community. Sharing is a value we learn early in life and it is the hallmark of professional librarianship. It is challenging, sometimes frustrating work. But the rewards far outweigh the risks. Those rewards extend to every citizen in our shared, county network—Yavapai Library Network. Throughout this article, we present a variety of practical aspects related to our experience, which may guide and inspire others to share their library space within their community.

Step-Induced Oxygen Upward Diffusion on Stepped Cu(100) Surface

Surface defects such as step edges play an important role in determining the surface properties, affecting immensely the growth mechanisms and morphologies of the nanostructures in epitaxial film growth processes. Here, we probe the dynamics of the oxidation on stepped Cu(100) using molecular dynamic simulations in conjunction with a reactive force field, and we elucidate the mechanisms and energy barriers affecting the oxidation process. Molecular dynamic simulations show that the adsorbed oxygen adatoms are unevenly distributed on the stepped surface, favoring the top terrace. We show that this behavior is due to Ehrlich–Schwobel (ES) barrier effect. However, differently from the reduced interlayer self-diffusion in descending a step as in a conventional ES barrier effect, we find instead that the ES barrier reduces the ascending diffusion barrier for oxygen, promoting its transport across the step edge and enhancing oxidation of the upper terrace. Additionally, we find that the ES barrier is step-heigh...

Controlled formation of GeSi nanostructures on periodic Si (001) sub-micro pillars

Self-assembled GeSi nanostructures on periodic Si (001) sub-micro pillars (SMPs) are systematically studied. Different GeSi nanostructures, including circularly arranged quantum dots, quantum rings and quantum dot molecules can be readily obtained at the edge of the pillars by controlling the growth temperatures and the diameter of the pillar. These phenomena are explained by taking into account the surface chemical potential around the top terrace of SMPs, which is considerably affected by the formation of {113} facets. Our results demonstrate a feasible route to obtain novel periodic Si pillars embedded with the desired GeSi nanostructures, which have promising applications in optoelectronic devices.

Influence of fcc Underlayer Facet on Microstructure of Co Thin Film

Co epitaxial thin films are prepared on (111)-oriented fcc-Au and fcc-Ag underlayers with island-like surfaces consisting of wide top (111) terraces surrounded by narrow side {111} and {100} facets by using an ultrahigh vacuum molecular beam epitaxy system. The influences of side facet of underlayer on crystallographic properties are investigated by pole figure X-ray diffraction and cross-sectional transmission electron microscopy. fcc-Co crystals grow epitaxially not only on top Au(111) terraces, but also on side Au{111} facets, involving stacking faults along the directions normal to the top Au(111) terraces and the side Au{111} facets, respectively. hcp-Co crystals are also formed on side Au{100} facets, where the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis of hcp-Co crystals is parallel to the side Au{100} facets. On the contrary, hcp-Co crystals grow epitaxially on top Ag(111) terraces and side Ag{111} facets, whereas fcc-Co crystals are formed on side Ag{100} terraces. As the surface roughness of underlayer increases, the Co film growth on the side facet is promoted. It is demonstrated that surface roughness reduction of underlayer is important in the preparation of well-oriented magnetic thin films.

Binding of In and Pb surfactants on Cu{1 1 1} surfaces

Surfactants generally affect both diffusion and nucleation on surfaces, and their function depends on their spatial distribution. Using density-functional-theory based ab initio calculations, the authors have determined the binding energies of In and Pb surfactants at various surface sites of Cu{1 1 1}. The calculation results show that In surfactant prefers incorporation inside/near surface steps or inside top terrace layer, in contrast to at adatom positions. The relative preference among step and terrace sites depends on the availability of surface vacancies, which form either during deposition without sufficient diffusion or by thermal activation. The preference of In surfactant inside top terrace layer makes it effective in the generation of strain on terrace and the slow-down of Cu adatom diffusion. In contrast, incorporation of Pb surfactant inside top terrace is energetically less preferable. As a result, Pb surfactant is less effective in the generation of strain on terrace.

Formation and characterization of the Cu 2O overlayer on Zn-terminated ZnO(0 0 0 1)

Low-energy electron diffraction, X-ray photoelectron spectroscopy and synchrotron-radiation-excited angle-resolved photoelectron spectroscopy have been used to characterize Cu-oxide overlayers on the Zn-terminated ZnO(0 0 0 1) surface. Deposition of Cu on the ZnO(0 0 0 1)–Zn surface results in the formation of Cu clusters with (1 1 1) top terraces. Oxidation of these clusters by annealing at 650 K in O 2 atmosphere (1.3 × 10 −4 Pa) leads to an ordered Cu 2O overlayer with (1 1 1) orientation. Good crystallinity of the Cu 2O(1 1 1) overlayer is proved by energy dispersion of one of Cu 2O valence bands. The Cu 2O(1 1 1) film exhibits a strong p-type semiconducting nature with the valence band maximum (VBM) of 0.1 eV below the Fermi level. The VBM of ZnO at the Cu 2O(1 1 1)/ZnO(0 0 0 1)–Zn interface is estimated to be 2.4 eV, yielding the valence-band offset of 2.3 eV.

Growth kinetics of disk-shaped copper islands in electrochemical deposition

The ability to independently dictate the shape and crystal orientation of islands in electrocrystallization remains a significant challenge. The main reason for this is that the complex interplay between the substrate, nucleation, and surface chemistry is not fully understood. Here we report on the kinetics of island growth for copper on ruthenium oxide. The small nucleation overpotential leads to enhanced lateral growth and the formation of hexagonal disk-shaped islands. The amorphous substrate allows the nuclei to achieve the thermodynamically favorable orientation, i.e., a 111 surface normal. Island growth follows power law kinetics in both lateral and vertical directions. At shorter times, the two growth exponents are equal to 1/2 whereas at longer times lateral growth slows down while vertical growth speeds up. We propose a growth mechanism, wherein the lateral growth of disk-shaped islands is initiated by attachment of Cu adatoms on the ruthenium oxide surface onto the island periphery while vertical growth is initiated by two-dimensional nucleation on the top terrace and followed by lateral step propagation. These results indicate three criteria for enhanced lateral growth in electrodeposition: (i) a substrate that leads to a small nucleation overpotential, (ii) fast adatom surface diffusion on substrate to promote lateral growth, and (iii) preferential anion adsorption to stabilize the basal plane.

Ordered arrangement of 9-aminoanthracene on Au(1 1 1) surfaces: A scanning tunneling microscopy study

We present a scanning tunneling microscopy (STM) investigation of 9-aminoanthracene (AA) on the reconstructed Au(1 1 1) surface. The bare Au(1 1 1) surface shows the herringbone reconstruction which is conserved upon deposition of the organic molecules. Most of the AA molecules are found to decorate the regions of fcc-stacking of the gold surface where a periodic linear arrangement is observed. The orientation of the long molecule axis of individual molecules is along the 〈 0 1 1 ¯ 〉 -directions of the Au substrate. In addition, for individual domains of the surface reconstruction, one of the three possible orientations is preferred. On substrate areas which exhibit a high step density, the steps are completely decorated by AA molecules. A detailed analysis of the STM images reveals that the molecules are located on top terrace levels. The fine structure of individual molecules on the terrace shows a clear dependence on the tunneling voltage and resembles the molecular orbitals of the free AA molecule.

Calcrete ‘fossilisation’ of alluvial fans in SE Spain: The roles of groundwater, pedogenic processes and fan dynamics in calcrete development

Dryland alluvial fans developed along the northern flanks of the Sierra Lisbona in the north-western Vera basin (Almería region, southeast Spain) display negligible amounts of incision despite distal base-level lowering of > 30 m by the modern drainage network. The fans are of early-middle Quaternary age and are graded to a coeval river terrace, now isolated from the modern drainage network. The negligible incision is due to the encasement and ‘fossilisation’ of the alluvial fans by calcrete. This paper explores the reasons for such fossilisation and assesses the relative importance of pedogenic and groundwater mechanisms for calcrete formation within an alluvial fan setting. A single fan was selected for detailed examination. The geomorphological and sedimentological features of the fan, its catchment area and their relationship to the distal river terrace were documented. Qualitative and semi-quantitative petrographic and scanning electron microscope analyses of calcrete samples collected from transects across the fan surface, and within its distal top river terrace surface, enabled the style, pattern and relative timing of calcrete development to be assessed. Calcrete fabrics comprised initial micritic grain-coating cements, pellets and glaebular carbonate nodules, with interstitial spaces infilled by equant sparite and microsparite mosaics. It is proposed that the early phases of calcrete development were dominated by pedogenic processes with increasing groundwater calcretisation over time. Point count data indicated increased quantities of interstitial sparite and microsparite cement within near-surface proximal fan calcretes and at depth across the fan, suggesting that groundwater processes played a more important role in calcrete formation in these locations. The contribution of groundwater to calcrete development can be best explained by the intrinsic funnelling of groundwater from the catchment through the proximal fan head area, a zone where the fan gravels are thinnest. Calcrete ‘fossilisation’ appears to have followed a reduction in the fan catchment area as a result of rockfalls and watershed stream capture, which reduced water and sediment supply to the fan and enabled surface stabilization and calcrete development to take place. The reduced sediment/water supply, combined with calcrete fossilisation, appears to have protected the alluvial fans from regional base-level lowering. The implications of these results for existing pedostratigraphic models of calcrete development in alluvial fans are subsequently explored.