Bedrock Surface Research Articles

Structural setup of Gara Marine area as deduced from 3D seismic interpretation and gravity-magnetic modelling, Gulf of Suez, Egypt

ABSTRACT An integrated interpretation utilising seismic data and inverse modelling is executed to more-reveal the subsurface complexity in Gara Marine area. The shallow features are identified by doing the seismic analyses on tops of Miocene formations. The deeper structures are deduced through constructing two gravity-magnetic models constrained by seismic and well data. The seismic results indicate that upper-Miocene rocks are characterised by a thick salt section (~2Km), which rarely cut by faults. South Gharib exhibits a structural closure at the centre due to a salt dome with dipping limbs towards the east and west. Belayim and Kareem are thinner in thickness, uniform distribution and steeply dipping to the southwest. Rudeis seems thicker westward, deposited in the downthrown side of a master block-fault. The uncertainty in seismic interpretation due to Miocene salts is estimated on tops of different horizons (±70 m static). The models indicate that Pre-Miocene sediments were deposited on a rough basement surface, with southwest regional dip-regime. The basement is deformed by the cross-faults (NW and NE) which vanish within Miocene section, with no evidences support presence of magmatic materials. The dip-reversal along flanks of Miocene salt structure, and limbs of pre-Miocene uplift, are the most potential traps.

Determination of The Site Amplification on Deep Soil Layer Using 1-D Site Response Analysis (Case study: Jakarta City, Indonesia)

The dynamic response of deep soil layers is used in the development of microzonation maps. The empirical correlation between standard penetratation blow count numbers and S-wave velocity was derived for practical purposes in site characterization based on local data in a case study in Jakarta, Indonesia. For estimating the intensity of potential earthquake shaking at the ground surface as a function of depth to the bedrock surface layer, 1-D site response analysis was carried out in 5745 simulations. The site amplification values were then evaluated by dividing the spectral acceleration (SA) at ground surface by the SA at rock outcrop. Plots of the SA amplification values at interested depth intervals of the bedrock surface layer were assigned. The results showed that the site amplification values estimated by considering the local depth of the bedrock surface layer were generally smaller than the SA amplification values from the Indonesian seismic building code SNI-1726-2012. Also, there appears to exist a tendency of lower levels of mean regression of amplification, in particular for the soft soil site class.

Landslide Hazard Zonation from a GIS Perspective and Urban Planning Solutions in Central –Division of Fiji Islands

As is a characteristic of rainfall-induced surficial landslides, the mass movement occurred where the landform or configuration of the bedrock surface facilitated the concentration of slope water. More than half of the landslides originated in slope depressions or drainage lines, with initial failure taking place on an upper slope, ridge or summit. The present study aims at assessing the rainfall intensity, site-soil-geology and seismicity of the central division of Viti Levu, Fiji Islands using the GIS and remote sensing techniques. Rainfall intensity, Site-soilgeology, geomorphology and SRTM DEM data were the main sources of layers used to carry out such analysis using the Frequency Ratio methodology and ArcGIS Multi-Criteria Analysis (MCA). The final result of Land Slide Potential Zone (LSPZ) indicates the ‘low’, ‘moderate’ and ‘high’ zones. The result presents a readable map for people to simply identify their state of vulnerability within the study area. Thus, this research will assist professionals such as urban planners, engineers and legislators to implement sustainable developments whilst reducing the risks of disasters.

Geology and lithological and structural conditions of brecciated rocks localization of the central part of the Ukrainian Shield (as an example of the Hruzske field of pipe-like bodies)

The main purpose of the article — is using the complex of structural-tectonic, geological-geophysical and lithologic-structural investigations, to describe in general the brecciated formations and pipe-like bodies of their localization as a precondition for solving the diamond-bearing problems of the Raihorodska suite (Paleocene) by the example of the Hruzske field of explosive structures in the central part of the Ukrainian Shield. Structural-tectonic, geophysical and morphostructural analyzes of the Precambrian basement surface have been used for diagnostics and outlining of local isometric pipe structures. Petrographic composition of the Hruzske field rocks was studied in core samples from wells No. 4087 (structure Hruzka-Pivdenna), 4040, 4043 and 4045 (structure Lisova-Skhidna), drilled by GPE-37 SE «Kirovheolohiia». Lithologic-stratigraphic and micropaleontological methods were used to determine the age of brecciated rocks. The morphology of bodies, some specific petrographic peculiarities of brecciated rocks of the Hruzske field are characterized by the complex of lithologic-structural features, as well as conclusions regarding their belonging to the endogenous (magmatogenic) group of diamond-bearing formations are made. Brecciated rocks of the Raihorodska suite are attributed to the clay-fine-clastic breccias, which are clearly confined to the lows in the Precambrian basement of the Shield central part and fill the depressions like the diatremes. Their explosive nature is substantiated; the Cretaceous-Palaeogene age of formation is confirmed; the rocks are compared with «sandy» tuffs of the Argyle lamproite pipe in Western Australia and volcanoclastic kimberlites of the Fort a la Corne area in Canada. It has been shown for the first time that the depressions in the Precambrian basement of the Ukrainian Shield central part are pipe-shaped bodies of the stockwork-maars type with a clearly expressed non-eroded craterous part. At a depth, they pass into the system of un-oriented joints, which played the role of incurrent canals for a mobile alkaline-ultrabasic magma, rich in volatile components. According to the structural-material characteristics, these specific brecciated formations of the Raihorodska suite are attributed to the products of rapid brecciation during the fluidizate-explosive processes of the maar volcanism development. Based on the performed researches it is possible to estimate the prospects of pipe-like bodies in relation to the search for industrial diamonds deposits on the Shield, to distinguish local structures, perspective on manifestations of explosive processes and related endogenous (kimberlite-lamproite) diamond content.

SISMOESTRATIGRAFIA DA FASE RIFTE DA PORÇÃO SUL DA BACIA DE CAMAMU

O trabalho consiste na interpretação regional de 91 linhas sismicas 2D pós-empilhadas do sul da Bacia de Camamu. A análise é auxiliada pela interpretação de perfis geofísicos de 21 poços e um cubo sísmico 3D. A metodologia do trabalho fez uso da estratigrafia de sequências aplicada às bacias rifte e contemplou a confecção de sismogramas sintéticos de dados de poços para correlação com as linhas sísmicas. Atributos sísmicos foram utilizados na identificação de terminações estratais, superfícies estratigráficas e feições estruturais, como falhas e superfície do embasamento. O mapeamento estratigráfico da supersequência rifte evidenciou discordâncias internas segmentando-a em sequências de terceira ordem associadas o caráter episódico do tectonismo sofrido pelas bacias durante o período de rifteamento. A análise de tratos de sistemas só foi possível de ser realizada na região proximal da Bacia devido ao grande número de poços disponíveis para amarração. Como resultados da interpretação foi possível confeccionar mapas de espessura sísmica das sequências e juntamente com a análise estrutural, compreender os principais sistemas sedimentares atuantes nos diferentes estágio de rifteamento da bacia. Foram mapeadas além das discordância que delimitam a fase rifte (DSR e DPR), quatro discordâncias internas (DR2, DR3, DR4 e DR5), totalizando cinco sequências deposicionais de terceira ordem (Rifte 1, 2 ,3,4 e 5). A separação em tratos tectônicos pela superfícies estratigráficas só foi possível na região proximal devido ao grande número de poços e à boa confiabilidade das amarrações. A ZTS favoreceu uma maior criação de espaço principalmente durante a deposição do primeiro rifte.

First data on cyanobacterial biodiversity in benthic biofilms during mass mortality of endemic sponges in Lake Baikal

This study investigates in detail species diversity of cyanobacteria which proliferated on the surface of diseased sponges and bedrock in Lake Baikal (Siberia, Russia) during the ecological crisis that has been progressing in the littoral zone since 2011 and caused numerous negative changes in biota. In 2014–2015, we examined samples from diseased and healthy sponges as well as rock substrates using Sanger sequencing and high-throughput 16S rRNA gene analysis. Based on morphology, we identified 15 cyanobacterial species in biofoulings. Tychonema sp., Symplocastrum sp., Pseudanabaena galeata and Tolypothrix distorta dominated on diseased sponges. In communities of healthy sponges, we detected mainly picocyanobacteria Synechococcus. Fascicles of Symplocastrum sp. and T. distorta dominated on bedrock substrates. Phylogenetically, most 16S rDNAs clustered among Tychonema, Pseudanabaena, Geitlerinema, Kamptonema, Leptolyngbya, and Tolypothrix spp. Cyanobacterial proliferation in the littoral zone of Lake Baikal should be considered as one of the most important negative ecological processes in recent times which requires an urgent study in order to understand the causes of its emergence and development. Mat-forming cyanobacteria Tychonema characterized by the highest tropism to sponges plays a special role in the disease and death of sponges.

Paleoenvironmental reconstructions and a sedimentological evidence of paleoseismic activity ca 9000 yr BP in Karelia, NW Russia, based on lake sediment studies on Mount Vottovaara

Karelia, like the entire Fennoscandian Shield, is a region with a low seismic activity. An example of the best-studied locality is a paleoseismic dislocation on Mount Vottovaara, which bears traces of disastrous Holocene geological events following the degradation of the last ice sheet. The evolution of the study area falls into three stages. At pre-Quaternary stage I, an uplifted block broken by numerous fractures and faults was formed. At glacial stage II, coarse clastic moraine was formed, the moving ice polished the crystalline basement surface and glacial scars were formed. At final deglaciation stages, the mountain top remained a nunatak. As Salpausselkä II marginal sediments retreated by about 70 km from the mountain, a postglacial stage in the region’s evolution, at which an earthquake occurred, began. It could have been triggered mainly by the consequences of the degradation of the Late Weischelian glaciations such as the rapid removal of the glacial load that contributed to the rejuvenation of various old faults. Changes in paleoecological conditions for the Mount Vottovaara area were reconstructed based on the results of lithological, palynological, diatom and radiocarbon studies of bottom sediments from a small lake on the mountain top. Vegetation dynamics from the Younger Dryas to the Subboreal period is presented. Small lake evolution stages were distinguished based on analysis of diatom complexes and the pollen and spores of aquatic and aquatic-subaquatic plants and Pediastrum algae. The data obtained show that minerogenic sediments were abruptly succeeded by organic in the late Preboreal-early Boreal period. The thickness of Boreal sediments and changes in the composition of diatom complexes and spore-and-pollen spectra suggest a depositional hiatus triggered by a strong earthquake which changed the water level of the pond and its basin structure. The earthquake is also indicated by numerous dismembered, displaced, thrown-away and shifted rock blocks and seismogravity downfalls. Deflation and other types of weathering are responsible for the formation of seide-shaped piles of blocks and boulders on the mountain top.

26Al/10Be ratios reveal the source of river sediments in the Kimberley, NW Australia

AbstractWe use cosmogenic 10Be and 26Al in both bedrock and fluvial sediments to investigate controls on erosion rates and sediment supply to river basins at the regional scale in the Kimberley, NW Australia. The area is characterised by lithologically controlled morphologies such as cuestas, isolated mesas and extensive plateaus made of slightly dipping, extensively jointed sandstones. All sampled bedrock surfaces at plateau tops, ridgelines, and in the broader floodplain of major rivers over the region show similar slow lowering rates between 0.17 and 4.88 m.Myr‐1, with a mean value of 1.0 ± 0.6 m.Myr‐1 (n=15), whilst two bedrock samples collected directly within river‐beds record rates that are one to two orders of magnitude higher (14.4 ± 1.5 and 20.9 ± 2.5 m.Myr‐1, respectively). Bedrock 26Al/10Be ratios are all compatible with simple, continuous sub‐aerial exposure histories. Modern river sediment yield lower 10Be and 26Al concentrations, apparent 10Be basin‐wide denudation rates ranging between 1.8 and 7.7 m.Myr‐1, with a median value of 2.6 m.Myr‐1, more than double the magnitude of bedrock erosion rates. 26Al/10Be ratios of the sediment samples are lower than those obtained for bedrock samples. We propose that these depleted 26Al/10Be ratios can largely be explained by the supply of sediment to river basins from the slab fragmentation and chemical weathering of channel gorge walls and plateau escarpments that result in diluting the cosmogenic nuclide concentration in river sediments measured at the basin outlets. The results of a mass‐balance model suggest that ~60–90% of river sediment in the Kimberley results from the breakdown and chemical weathering of retreating vertical sandstone rock‐walls in contrast to sediment generated by bedrock weathering and erosion on the plateau tops. This study emphasises the value of analysing two or more isotopes in basin‐scale studies using cosmogenic nuclides, especially in slowly eroding post‐orogenic settings. © 2019 John Wiley & Sons, Ltd.

Gravity and Magnetic Constraints on the Crustal Structure of the Ceará Plateau, Brazilian Equatorial Margin

The Brazilian Equatorial Margin is a transform passive margin with long fracture zones and several seamounts, including the largest one, the Ceara Plateau. This is a complex and poorly recognized area, with few available geophysical data. Gravity and magnetic data from the Equant I Project, seismic published lines and other previous studies are used to make several models and analyses. We recognized gravity and magnetic anomalies related to a denser, magnetized basement, internally comprised of two magmatic highs. Through gravity and magnetic forward modeling, we estimated the basement surface between 800 and 6000 m and the Mohorovicic discontinuity about 22-23 km below the Ceara Plateau. We also present and discuss the position of the continent-ocean boundary 40 km away from the continental shelf, placing the plateau at the oceanic crust domain, and the area of the transitional crust, with an extension of 40-50 km, formed during the rifting phase of the Atlantic Ocean opening.

Alluvial Morphodynamics of Bedrock Reaches Transporting Mixed‐Size Sand. Laboratory Experiments

AbstractResearch on the morphodynamics of bedrock rivers has primarily focused on bedrock incision, and little is known about the alluvial morphodynamics of rivers with exposed bedrock surfaces. More specifically, there is a lack of information on the morphodynamics of low slope bedrock reaches due to the recent recognition of such systems. Here, we present the results of laboratory experiments specifically designed to gain novel insight into flow resistances, flow hydrodynamics, and sediment transport processes in equilibrium partially exposed bedrock reaches transporting nonuniform sand as bed material in low slope areas. The experiments show that (1) downstream of a stable alluvial‐bedrock transition flow depth decreases in the streamwise direction, (2) bedform amplitude may decrease in the streamwise direction, and (3) stable patterns of downstream fining may form. Given the bedrock geometry, the water surface elevation at downstream boundary and the characteristics of the bedform regime in an alluvial channel subject to the same flow rate and sediment supply at equilibrium control bedform characteristics and sediment sorting patterns in the bedrock reach. When this distance is significantly smaller than the alluvial equilibrium flow depth or when the alluvial equilibrium bedform regime is close to the dune‐antidune transition, bedforms in the bedrock reach are closer to the dune‐antidune transition than at alluvial equilibrium with a consequent reduction in bedform amplitude. If the distance between the water level at the downstream boundary and the bedrock surface is close to the alluvial equilibrium flow depth and the alluvial equilibrium bedforms are well in the dune regime, a stable pattern of downstream fining can be expected. The comparisons between experimental and modeled sediment transport rates and equilibrium grain size distributions of the sediment further show that surface‐based bedload transport models derived for alluvial systems reasonably predict equilibrium sediment transport rates and bed surface size distributions in bedrock reaches if the presence of exposed bedrock is accounted for in terms of alluvial cover fraction.

Cosmogenic soil production rate calculator

To understand the rates at which soils form from bedrock, it is important to know the rates at which the bedrock surface lowers (the apparent erosion rate, which is assumed to be constant). Previous models that calculate apparent erosion rates using measured concentrations of cosmogenic radionuclides rely on the assumption that the bulk density of the soil which forms as a product of bedrock erosion either equals that of the bedrock itself or is constant with depth down the soil profile. This assumption fails to recognise that soils have significantly lower densities that might not be constant with depth. The model presented here allows for the calculation of isotopically-derived soil production rates, considering the bulk density profile of the soil overlying the bedrock surface. This calculator, which can be run both in MATLAB® and GNU Octave©, represents a novel and significant contribution to the derivation of soil production rates.

Postglacial erosion of bedrock surfaces and deglaciation timing: New insights from the Mont Blanc massif (western Alps)

Abstract Since the Last Glacial Maximum, ∼20 k.y. ago, Alpine glaciers have retreated and thinned. This transition exposed bare bedrock surfaces that could then be eroded by a combination of debuttressing or local frost cracking and weathering. Quantification of the respective contributions of these processes is necessary to understand the links between long-term climate and erosion in mountains. Here, we quantified the erosion histories of postglacial exposed bedrock in glacial valleys. Combining optically stimulated luminescence and terrestrial cosmogenic nuclide (TCN) surface exposure dating, we estimated the erosion rate of bedrock surfaces at time scales from 101 to 104 yr. Bedrock surfaces sampled from the flanks of the Mer de Glace (Mont Blanc massif, European Alps) revealed erosion rates that vary from 3.5 ± 1.2 ⋅ 10−3 mm/yr to 4.3 ± 0.6 mm/yr over ∼500 m of elevation, with a negative correlation between erosion rate and elevation. The observed spatial variation in erosion rates, and their high values, reflect morphometric (elevation and surface slope) and climatic (temperature and snow cover) controls. Furthermore, the derived erosion rates can be used to correct the timing of deglaciation based on TCN data, potentially suggesting very rapid ice thinning during the Gschnitz stadial.

Thickness of the divide and flank of the West Antarctic Ice Sheet through the last deglaciation

Abstract. We report cosmogenic-nuclide measurements from two isolated groups of nunataks in West Antarctica: the Pirrit Hills, located midway between the grounding line and the divide in the Weddell Sea sector, and the Whitmore Mountains, located along the Ross–Weddell divide. At the Pirrit Hills, evidence of glacial-stage ice cover extends ∼320 m above the present ice surface. Subsequent thinning mostly occurred after ∼14 kyr BP, and modern ice levels were established some time after ∼4 kyr BP. We infer that, like at other flank sites, these changes were primarily controlled by the position of the grounding line downstream. At the Whitmore Mountains, cosmogenic 14C concentrations in bedrock surfaces demonstrate that ice there was no more than ∼190 m thicker than present during the past ∼30 kyr. Combined with other constraints from West Antarctica, the 14C data imply that the divide was thicker than present for a period of less than ∼8 kyr within the past ∼15 kyr. These results are consistent with the hypothesis that the divide initially thickened due to the deglacial rise in snowfall and subsequently thinned in response to retreat of the ice-sheet margin. We use these data to evaluate several recently published ice-sheet models at the Pirrit Hills and Whitmore Mountains. Most of the models we consider do not match the observed timing and/or magnitude of thickness change at these sites. However, one model performs relatively well at both sites, which may, in part, be due to the fact that it was calibrated with geological observations of ice-thickness change from other sites in Antarctica.

Micromorphology and genesis of Quaternary stromatolitic tufa crust within the exposed Cretaceous Rayda Formation in Al-Jabal Al-Akhdar, Oman

This study aimed to improve the understanding of the causes of lamination in terrestrial tufa, using samples of terrestrial Quaternary stromatolitic tufa deposits occurring within narrow karst grooves carved along the bedding planes of the exposed Cretaceous Rayda Formation in Al-Jabal Al-Akhdar, Oman. These stromatolitic tufa deposits consist wholly of low-Mg calcite and are formed of vertically stacked laminated planar and digitate-like columnar structures. Lamination is exhibited as vertically stacked couplets of dark micritic and light sparitic laminae. Significant variations in the abundance and thickness of the micritic laminae, as well as progressive changes in the size and fabric of calcite in the sparitic laminae were recognised. Thorough microscopic and nanoscopic examinations revealed that these laminations initially developed as stacked, indistinct micritic laminae formed of calcitic microbialite that had initially precipitated within a biologically active microbial biofilm which thrived on the wet bedrock surface. This was followed by the growth of microglobules at certain horizons within the microbialitic laminae, forming incipient, discontinuous thin sparitic laminae consisting of laterally stacked biogenic skeletal disphenoidal calcite microcrystals. The latter were further growing via abiogenic syntaxial precipitation of calcite, forming coarsely crystalline dagger-shape bladed calcite crystals. These processes resulted in the formation of well-developed lamination. It is therefore suggested that the stromatolitic lamination has been generated through five consecutive stages of biogenic and abiogenic precipitation of calcite within initially developed microbial biofilms.Stable isotope values (δ13C and δ18O) suggest that these tufa deposits had precipitated from continuously renewed freshwater enriched with bicarbonate dissolved from the marine limestone bedrock during a cold pluvial period.

Linking bedrock discontinuities to glacial quarrying

AbstractQuarrying and abrasion are the two principal processes responsible for glacial erosion of bedrock. The morphologies of glacier hard beds depend on the relative effectiveness of these two processes, as abrasion tends to smooth bedrock surfaces and quarrying tends to roughen them. Here we analyze concentrations of bedrock discontinuities in the Tsanfleuron forefield, Switzerland, to help determine the geologic conditions that favor glacial quarrying over abrasion. Aerial discontinuity concentrations are measured from scaled drone-based photos where fractures and bedding planes in the bedrock are manually mapped. A Tukey honest significant difference test indicates that aerial concentration of bed-normal bedrock discontinuities is not significantly different between quarried and non-quarried areas of the forefield. Thus, an alternative explanation is needed to account for the spatial variability of quarried areas. To investigate the role that bed-parallel discontinuities might play in quarrying, we use a finite element model to simulate bed-normal fracture propagation within a stepped bed with different step heights. Results indicate that higher steps (larger spacing of bed-parallel discontinuities) propagate bed-normal fractures more readily than smaller steps. Thus, the spacing of bed-parallel discontinuities could exert strong control on quarrying by determining the rate that blocks can be loosened from the host rock.

Formation and evolution of an extensive blue ice moraine in central Transantarctic Mountains, Antarctica

AbstractMount Achernar moraine is a terrestrial sediment archive that preserves a record of ice-sheet dynamics and climate over multiple glacial cycles. Similar records exist in other blue ice moraines elsewhere on the continent, but an understanding of how these moraines form is limited. We propose a model to explain the formation of extensive, coherent blue ice moraine sequences based on the integration of ground-penetrating radar (GPR) data with ice velocity and surface exposure ages. GPR transects (100 and 25 MHz) both perpendicular and parallel to moraine ridges at Mount Achernar reveal an internal structure defined by alternating relatively clean ice and steeply dipping debris bands extending to depth, and where visible, to the underlying bedrock surface. Sediment is carried to the surface from depth along these debris bands, and sublimates out of the ice, accumulating over time (>300 ka). The internal pattern of dipping reflectors, combined with increasing surface exposure ages, suggest sequential exposure of the sediment where ice and debris accretes laterally to form the moraine. Subsurface structure varies across the moraine and can be linked to changes in basal entrainment conditions. We speculate that higher concentrations of debris may have been entrained in the ice during colder glacial periods or entrained more proximal to the moraine sequence.

Influence of climate on radial growth of scots pine (Pinus Sylvestris L.) In different habitats of Meshchera lowland

This paper is devoted to the patterns of radial growth of Scots pine (Pinus sylvestris L.) in various topoecological conditions of the Meshchera lowland (Ryazan region, the East European plain). The generalized tree-ring chronologies are constructed for 16 habitats differing in features of a relief of a day surface and a bedrock surface. Despite the relatively low-contrast relief of Meshchera, the average radial pine increment within the study area differs by 2.5 times (1.53.9 mm per year). The correlation and cluster analyses revealed that the key factor influencing the width of annual tree rings of pine is the amount of the available soil moisture. Its surplus (in wetlands) and deficiency (in conditions of sandy outliers) negatively affects the radial pine increment. It is established that in the waterlogged habitats positive correlation of the radial pine increment with temperature and negative with precipitation of autumn of the previous year is observed. The positive correlation of the radial increment with precipitation of autumn, May and with winter temperature is typical for a pine from arid habitats. The negative relationship between the pines growth and amount of precipitation and river discharge may occur with a lag of 14 years in conditions of wetlands. The positive relationship of the radial pine increment with the integral parameters of the current years moisture is more significant in conditions of relatively high hydrodynamics (for example, in conditions of sandy outliers).

Refinement of the Deep Structure and Tectonic Movement Kinematics in the Area of the 2001 Salsk Earthquake Based on New Geophysical Data

The article presents the results of a comprehensive study of seismotectonic, geological-geophysical, geodynamic, and seismological data in the region of the Salsk earthquake of May 22, 2001, M = 4.7, in the low-seismic region of the junction of the Scythian Plate and the East European Platform are presented. The parameters of this earthquake source associated with a deep tectonic structure identified by the microseismic sounding method (MSM) are estimated based on seismological and macroseismic data. The MSM section through the region of the Salsk earthquake was obtained by a submeridional profile about 65 km long across the strike of the Manych deflection zone. It was established that the hypocenter of the 2001 Salsk earthquake was confined to the Salsk fault zone included in the system of the Manych faults. This fault is expressed in a section in the form of a narrow subvertical strip with low seismic wave velocities traced from the Earth’s surface to a depth of more than 40 km. Immediately in the area of the earthquake below a depth of 10 km, the recognized fault zone has a local expansion at the point of transition through the plane when the sedimentary cover contacts with the Proterozoic basement. The assessment of the hypocentral position and source mechanism solution (a gently dipping thrust of the northern side at an angle of 25°–33°) correlates well here with the depth and inclination of the surface of the Proterozoic basement, the elements of which are also traced in the MSM section.

Fill and Spill Hillslope Runoff Representation With a Richards Equation‐Based Model

AbstractIntegrated surface‐subsurface hydrological models (ISSHMs) are well established numerical tools to investigate water flow and contaminant transport processes over a wide range of spatial and temporal scales. However, their ability to correctly reproduce the response of hydrological systems to natural and anthropogenic forcing depends largely on the accuracy of model parameterization, including the level of detail in the representation of the bedrock. This latter is typically incorporated in some way via the bottom boundary of the model domain. Issues of bedrock topography, variable soil depth, and the resulting hillslope storage distribution representation in ISSHMs are vitally important but to date have received little attention. A standard treatment of the bottom boundary, especially in large catchment and continental scale applications, is to model it as a flat or inclined (e.g., parallel to the surface) impermeable base (sometimes with some simple leakage term). This approach does not allow the model to correctly reproduce bedrock‐controlled threshold responses such as the fill and spill process, as observed across many hillslope and catchment scale field studies. It is still unclear whether Richards equation‐based numerical models are actually able to generate such responses. Here we use a Richards equation‐based model (CATHY) to simulate internal transient subsurface stormflow dynamics observed at the well‐characterized Panola experimental hillslope in Georgia (USA). Soil and bedrock properties were calibrated starting from values reported in previous studies at the site. Our simulation results show that the model was able to reproduce threshold mechanisms, which in turn affected both the integrated and distributed hydrologic responses of the Panola hillslope. We then developed a set of virtual experiments with modified boundary conditions and base topography at the soil‐bedrock interface to explore the bedrock boundary control on transient groundwater flow patterns. Our results show that accurate representation of the lower boundary is crucial for ISSHM simulations of hillslope‐scale storm runoff and for connectivity of transient groundwater. We summarize our findings with the development of a new bedrock topographic wetness index that takes into account the unsaturated infiltration dynamics. The index is able to help represent the spatial variability of water table response over the bedrock surface compared to standard surface topography‐based indices. This new index may be useful in larger‐scale ISSHM applications where an exact bedrock topography representation is not feasible or possible.

Holocene sedimentation in a blue hole surrounded by carbonate tidal flats in The Bahamas: Autogenic versus allogenic processes

The sediment in North Atlantic blue holes preserves paleoclimate records. However, accurate paleoclimate reconstructions require an improved understanding of allogenic versus autogenic processes controlling blue hole sedimentation. Here we provide a detailed case study of the Holocene stratigraphy within Freshwater River Blue Hole, which is currently surrounded by carbonate tidal flats in the northern Bahamas (Abaco Island). During the Holocene, concomitant coastal aquifer elevation and relative sea-level rise controlled internal blue hole depositional environments. The general Holocene facies succession observed is: (i) basal detrital and freshwater peat, (ii) palustrine to lacustrine marl, (iii) algal sapropel, and finally (iv) bedded carbonate mud. During the middle Holocene when groundwater levels were lower, small changes in accommodation space that were inherited from the bedrock surface below (<1 m) were able to promote significant lateral facies changes. Multiple cores are needed to characterize these lateral facies changes. Hydrographic characteristics of the coastal aquifer (e.g., vertical position, stratification, salinity) relative to the blue hole benthos exert a fundamental control on (a) benthic flora and meiofauna (e.g., charophytes, ostracodes, foraminifera, gastropods) and (b) organic matter production and preservation from pelagic productivity. Over the last 5000 years, water column stratification in Freshwater River Blue Hole was interrupted on millennial to sub-decadal timescales , which are potentially linked to changing aquifer recharge and rainfall. Lastly, historical intense hurricanes passing closely to the west of the site may have promoted deposition of coarse beds at the site. However, the lack of carbonate tidal flat microfaunal remains (foraminifera: Peneroplis) within these coarse intervals indicates that Freshwater River Blue Hole does not preserve a reliable record of hurricane-induced overwash deposition from the carbonate tidal flats during the last 2300 years.