Shallow Sedimentary Deposits Research Articles

CRONOLOGIA DE VERTENTES QUARTZÍTICAS EM MINAS GERAIS, BRASIL

Quartzitic mountain reliefs developed in a tropical savanna climate tend to have shallow sedimentary deposits. This is due to low level weathering processes of quartzite rocks and formation of colluvium ramps on the slopes. This study aims to determine the characteristics of colluvial materials, particularly grain size, and to analyze sediment chronology above the saprolite, seeking to identify accumulation rates and make correlations with the dynamics of the recent geomorphological evolution of the Canastra range in Minas Gerais, Brazil. Samples collected in different morphological positions were submitted to granulometric analysis and dated by Optically Stimulated Luminescence Single Aliquote Regeneration (OSL/SAR) and radiocarbon dating. Grain size analysis showed a predominance of sandy and gravel materials and demonstrated that the colluvial ramps in the study area contain materials derived from the decomposition of quartzite rocks, accumulated in shallow and sandy profiles. LOE/SAR dating indicated ages between the end of the Pleistocene, not exceeding 30,500 +/- 4,950 BP, to 500 years BP, whereas the 14C dating showed ages between 4460 +/- 30 BP and 1140 +/- 30 BP. These data indicate that the deposits from the study area are relatively recent, which demonstrate that variations in the weathering process and deposit formation have undergone recent changes, probably due to climatic fluctuations and changes in the regional hydrological regime. Keywords: Canastra Range, Paleogeomorphological Reconstruction, Weathering of Quartzites, OSL, Tropical Range

Integrated Offshore Seismic Survey Using an Unmanned Wave Glider

An autonomous surface vehicle, known as a wave glider, was used to record refracted and reflected signals from a seismic source penetrating the shallow subsurface. An integrated survey system consisting of a wave gilder and a human-operated source vessel was deployed. These survey systems are used to acquire wide-offset seismic survey data from specific areas, such as offshore structures. The wave gliders can collect seismic refraction and reflection data, which can be used to estimate subsurface information, e.g., acoustic wave velocity and subsurface structure. We processed raw data collected by a receiver equipped with the wave glider and used the relationship between travel time and offset distance to calculate the velocities of shallow sedimentary deposits and the acoustic basement. The velocities of the sedimentary deposits and basement were estimated to be 1557 and 3507 m/s, respectively. We then overlaid the velocities on subsurface data measured using a single-channel streamer. Our results indicate that unmanned equipment can be used for ocean exploration to aid offshore energy development.

Long‐Term Storage and Age‐Biased Export of Fluvial Organic Carbon: Field Evidence From West Iceland

AbstractTerrestrial organic carbon (OC) plays an important role in the carbon cycle, but questions remain regarding the controls and timescale(s) over which atmospheric CO2 remains sequestered as particulate OC (POC). Motivated by observations that terrestrial POC is physically stored within soils and other shallow sedimentary deposits, we examined the role that sediment storage plays in the terrestrial OC cycle. Specifically, we tested the hypothesis that sediment storage impacts the age of terrestrial POC. We focused on the Efri Haukadalsá River catchment in Iceland as it lacks ancient sedimentary bedrock that would otherwise bias radiocarbon‐based determinations of POC storage duration by supplying pre‐aged “petrogenic” POC.Our radiocarbon measurements of riverine suspended sediments and deposits implicated millennial‐scale storage times. Comparison between the sample types (suspended and deposits) suggested an age offset between transported (suspended sediments) and stored (deposits) POC at the time of sampling, which is predicted by theory for the sediment age distribution in floodplains. We also observed that POC in suspended sediments is younger than the predicted mean storage duration generated from independent geomorphological data, which suggested an additional role for OC cycling. Consistent with this, we observed interparticle heterogeneity in the composition of POC by imaging our samples at the microscale using X‐ray absorption spectroscopy. Specifically, we found that particles within individual samples differed in their sulfur oxidation state, which is indicative of multiple origins and/or diagenetic histories. Altogether, our results support recent coupled sediment storage and OC cycling models and indicate that the physical drivers of sediment storage are important factors controlling the cadence of carbon cycling.

Geochemical and palaeohydrological controls on the composition of shallow groundwater in the Netherlands

With the exception of the south of the country, the Netherlands has a strong bipartite hydrogeology: the Holocene part with a coastal dune belt and confining top layer of clay and peat further inland, and the Pleistocene, where thick phreatic aquifers dominate. This research aimed to ascertain the geochemical and palaeohydrological controls on the composition of shallow groundwater in 27 regions. Close to 6000 groundwater analyses were grouped and interpreted in terms of 1. salinity, 2. redox status, 3. acid/base and carbonate status and 4. natural nutrients NH4 and PO4. The a priori classification into geographical regions and geological formations revealed many statistically significant differences in medians, even for geologically or geographically related data groups. The compound-specific interpretation indicates that there are geogenically controlled, systematic differences in groundwater composition at the regional scale. The imprint of the geological sediments on the groundwater composition decreases in the order marine/estuarine via limnological, fluvial to aeolian. The imprints with respect to pH and carbonate status, natural nutrients and redox status are not necessarily interrelated. The vertical stratification in groundwater composition turns out to be often limited at the regional scale due to mutual occurrence of infiltrating and exfiltrating groundwater in regions and either the presence of a highly reactive Holocene, confining top layer or temporal changes in contamination. In the Holocene part, the salinity is controlled by the palaeoenvironmental conditions during the Holocene and by the recharge origin: the average Cl concentration decreases from estuarine via lagoonal to the former Zuider Sea (which was a bay). The most reduced states and also the highest nutrient concentrations and highest CO2 pressure are related to the presence of Holocene marine sediments in the confining top layer. Degradation of marine-derived organic matter as a nutrient source, is likely more intense in the Holocene deposits than that of peat and sedimentary remnants of terrestrial plants. A broad range in pH, carbonate status and redox status is encountered in the Pleistocene part. Here, the palaeohydrological evolution in terms of carbonate leaching together with the geological controls on the calcareous nature of the shallow sedimentary deposits cause regional differences in pH, calcite saturation and silicate weathering. One region with Late Pleistocene limnological deposits has deviating groundwater characteristics and appears more similar to the Holocene part of the Netherlands. Furthermore, reactive Fe is not abundant in all Pleistocene fluvial sediments nor is it maximally mobilised, as not all anoxic groundwater in these sediments is siderite-saturated. This leads to considerable intra- and inter-region variability.

Reaction capacity characterization of shallow sedimentary deposits in geologically different regions of the Netherlands

Quantitative insight into the reaction capacity of porous media is necessary to assess the buffering capacity of the subsurface against contaminant input via groundwater recharge. Here, reaction capacity is to be considered as a series of geochemical characteristics that control acid/base conditions, redox conditions and sorption intensity. Using existing geochemical analyses, a statistical regional assessment of the reaction capacity was performed for two geologically different areas in the Netherlands. The first area is dominated by Pleistocene aquifer sediments only, in the second area a heterogeneous Holocene confining layer is found on top of the Pleistocene aquifer sediments. Within both areas, two or more regions can be distinguished that have a distinctly different geological build-up of the shallow subsurface. The reactive compounds considered were pyrite, reactive Fe other than pyrite, sedimentary organic matter, carbonate and clay content. This characterization was complemented by the analysis of a dataset of samples newly collected, from two regions within the Pleistocene area, where the sedimentary facies of samples was additionally distinguished. The statistical assessment per area was executed at the levels of region, geological formation and lithology class. For both areas, significant differences in reaction capacities were observed between: 1. different lithology classes within a geological formation in a single region, 2. identical geological formations in different regions and 3. various geological formations within a single region. Here, the reaction capacity is not only controlled by lithostratigraphy, but also by post-depositional diagenesis and paleohydrology. Correlation coefficients among the reactive compounds were generally higher for sand than for clay, but insufficiently high to allow good estimation of reactive compounds from each other. For the sandy Pleistocene aquifer sediments, the content of reactive compounds was frequently observed to be below detection limits. From this, future characterization of sediment reaction capacity is best performed at the sublevel of lithology class, being the geochemically near-uniform unit identifiable for individual geological formations within geographic regions. Additional subdivision on facies provides particular insight in the spatial entity where relatively high reaction capacities may be encountered. To obtain quantitative insight into the reaction capacity of aquifer sediments, non-sandy minor subunits should be well characterised on their reaction capacity as well as their spatial occurrence in the geological formations. A straightforward approach is presented in which the regional statistics on geochemical reactivity become combined with a 3-dimensional geological voxel model. This results into 3-dimensional data fields on reactivity, which are suitable for, for example, groundwater transport modelling. The sedimentological architecture of the deposits becomes well maintained in the geochemical data field, which is an advantage in itself.

Seismic stratigraphy of the quaternary Yellow River delta, Bohai Sea, eastern China

The upper 40 m of stratigraphy of the Yellow River (Huang He) subaqueous delta has been well documented, but the nature of the underlying strata is currently unknown at high-resolution. To address this deficiency we used a Geopulse seismic system to image shallow sedimentary deposits up to 120 m deep on the Yellow River delta. High-resolution seismic reflection images were processed with a series of specific techniques (e.g. swelling attenuation, dynamic s/n filter; f-x deconvolution, predictive deconvolution dipscan stack), and used with borehole data to investigate the Quaternary offshore sequences in the Yellow River (Huang He) delta. Repetitive sequences were observed and interpreted as containing layers of transgressive and regressive deposits. Six seismic transgressive and regressive cycles are identified. Unit M6F–C6F correlates with a relative sea-level rise (173–157 ka) and fall (231–173 ka), while Unit M5F–C5F is associated with a relative sea-level rise (124–100 ka) and fall (157–124 ka). Unit M4F–C4F spans a period of sea-level fall at 100–87 ka, followed by a rise at 87–76 ka. Unit M3F–C3F is a transgressive–regressive cycle dated as 76–58 ka. Unit M2F–C2F correlates with relative sea level fall at 58.2–36 ka and subsequent rise at 36–22 ka. Unit M1F–C1F was deposited during relative sea level fall (22–18 ka), followed by a rise, especially since 8.5 ka.

Determination of elastic properties of shallow sedimentary deposits applying a spatial autocorrelation method

The capability of the spatial autocorrelation method for the determination of the elastic properties of shallow sedimentary deposits has been investigated at a test site in which the subsurface structure is known. The technique has been shown very innovative and convenient for this kind of study as no artificial sources are needed, and also overcomes the problems of soil invasive techniques. The method uses the three components of the seismic microtremor recorded simultaneously at different configuration arrays with the possibility to obtain dispersion curves of Rayleigh and Love waves. These curves together with the horizontal to vertical spectral ratio of the seismic microtremors can be used to calculate the S-wave velocities of the sediments. In order to test the goodness of this procedure a specific experiment was performed with real data. The underground model obtained from this methodology is compared with the geotechnical information available. Our results are in a good agreement with those models obtained previously by S-wave refraction experiments and borehole measurements.

Site Response and Attenuation in the Puget Lowland, Washington State

Simple spectral ratio (ssr) and horizontal-to-vertical (h/v) site- response estimates at 47 sites in the Puget Lowland of Washington State document significant attenuation of 1.5- to 20-Hz shear waves within sedimentary basins there. Amplitudes of the horizontal components of shear-wave arrivals from three local earthquakes were used to compute ssrs with respect to the average of two bedrock sites and h/v spectral ratios with respect to the vertical component of the shear-wave arrivals at each site. ssr site-response curves at thick basin sites show peak amplifications of 2 to 6 at frequencies of 3 to 6 Hz, and decreasing spectral amplification with increasing frequency above 6 Hz. ssrs at nonbasin sites show a variety of shapes and larger resonance peaks. We attribute the spectral decay at frequencies above the amplification peak at basin sites to attenuation within the basin strata. Computing the frequency-independent, depth-dependent attenuation factor ( Q s ,int ) from the ssr spectral decay between 2 and 20 Hz gives values of 5 to 40 for shallow sedimentary deposits and about 250 for the deepest sedimentary strata (7 km depth). h/v site responses show less spectral decay than the ssr responses but contain many of the same resonance peaks. We hypothesize that the h/v method yields a flatter response across the frequency spectrum than ssrs because the h/v reference signal (vertical component of the shear-wave arrivals) has undergone a degree of attenuation similar to the horizontal component recordings. Correcting the ssr site responses for attenuation within the basins by removing the spectral decay improves agreement between ssr and h/v estimates.

Observations of Basin Ground Motions from a Dense Seismic Array in San Jose, California

We installed a dense array of 41 digital seismographs in San Jose, California, to evaluate in detail the effects of a deep sedimentary basin and shallow sedimentary deposits on earthquake ground motions. This urban array is located near the eastern edge of the Santa Clara Valley and spans the Evergreen sedimentary basin identified by gravity data. Average station spacing is 1 km, with three stations initially spaced 110 m apart. Despite the high-noise urban environment, the stations of the array successfully triggered on and recorded small local earthquakes ( M 2.5–2.8 at 10–25 km distance) and larger regional events such as the M 5.0 Bolinas earthquake (90 km distance), M 4.6–5.6 earthquakes near Mammoth Lakes (270 km distance), M 4.9–5.6 events in western Nevada (420 km distance) and the M 7.1 Hector Mine earthquake (590 km distance). Maps of spectral ratios across the array show that the highest amplitudes in all frequency bands studied (0.125–8 Hz) are generally observed at stations farther from the eastern edge of the Santa Clara Valley. Larger spectral amplitudes are often observed above the western edge of the Evergreen Basin. Snapshots of the recorded wavefield crossing the array for regional events to the east reveal that large, low-frequency (0.125–0.5 Hz) arrivals after the S -wave travel from south to north across the array. A moving-window, cross-correlation analysis finds that these later arrivals are surface waves traveling from the south. The timing and propagation direction of these arrivals indicates that they were likely produced by scattering of incident S waves at the border of the Santa Clara Valley to the south of the array. It is remarkable that the largest low-frequency phases at many of the valley sites for regional events to the east are basin surface waves coming from a direction about 70 degrees different from that of the epicenters. Basin surface waves emanating from the eastern edge of the valley are also identified by the cross-correlation analysis.