Sedimentology

Interesting papers on ground penetrating radar related to sedimentology

Evert Slob

Deptartment of Geoscience & Engineering, Delft University of Technology,

P.O. Box 5048, 2600 GA Delft, The Netherlands

July 7, 2012

 

[1] Paleoenvironmental interpretation of proxy data derived from peatlands is largely based upon an evolutionary model for ombrotrophic bogs, in which peat accumulates in still environments. Reports on proxies obtained from minerotrophic fens, where hydrologic inputs are variable, are less common. In this study, a highland peatland in southern Brazil is presented through ground penetrating radar (GPR) and sedimentological, palynological and geochronologic data. The radar stratigraphic interpretation suggests a relatively complex history of erosion and deposition at the site since the beginning of Marine Isotope Stage 3 (MIS 3) interstadial period. In spite of this, radar stratigraphic and palynologic interpretations converge. Electromagnetic reflections tend to group in clusters that show lateral coherence and correlate with different sediment types, while pollen grains abound and are well preserved. As a result, the study of minerotrophic fens provides a source of proxies. suggesting that ombrotrophic bogs are not the only reliable source of data in wetlands for palynological analysis.

[2] West Greenland has been intensively studied to reconstruct and better understand past relative sea level changes and deglacial history. This study extends these efforts by linking sea level and deglacial history to the sedimentary infill successions of Kangerlussuaq Fjord and associated landward valleys. Based on published and new land- and sea-based geophysical data, radiocarbon dates and geological observations we have characterized the infill and reconstructed the sedimentation history during the Holocene. Based on a revised sea level curve and data presented in this paper we defined three depositional phases. Phase I (>7000 yr BP) is represented by dominant glaciomarine deposition associated with a tide-water glacier system. As the Greenland Ice Sheet (GIS) continued to retreat it became land based. During phase II (7000-1500 yr BP) two separate depocenters formed. Keglen delta depocenter arose from a temporary stabilization phase of the GIS and prograded rapidly over the glaciomarine deposits of Phase I. Further inland, proglacial lake formation and subsequent sedimentary infill associated with the ongoing GIS retreat is represents the second depocenter. The Watson River connected both depocenters by a flood plain which transferred sediment from the GIS to the Keglen delta. Ongoing sea level fall due to glacio-isostastic uplift combined with a gradually cooler and dryer climate resulted in terrace formation along the Watson River flood plain. Around 4000 yr BP, the GIS margin reached its most landward location and began to advance to its present location. The final phase (Phase III; < 1500 yr BP) is represented by a stabilized GIS position and a relative sea level rise which led to aggrading conditions near the present-day delta plain of Watson River. Simultaneously, subaqueous channels were formed at the delta front by hyperpycnal flows associated with jokulhlaup events.

[3] Ground-penetrating radar has not been applied widely to the recognition of ancient carbonate platform geometries. This article reports the results of an integrated study performed on an Upper Jurassic outcrop from the south-east Paris basin, where coral bioherms laterally change into prograding depositional sequences. Ground-penetrating radar profiles illustrate the different bedding planes and major erosional unconformities visible at outcrop. A ground-penetrating radar profile conducted at the base of the cliff displays a palaeotopographic surface on which the outcropping bioherms settled. The excellent penetration depths of the ground-penetrating radar (20 m with a monostatic 200 MHz antenna) images the carbonate platform geometries, ranging between outcrop workscale (a few metres) and seismic scale (several hundreds of metres). This study supports recent evidence of icehouse conditions and induced sea-level fluctuations controlling the Upper Jurassic carbonate production.

[4] Flights of Quaternary river terraces in south and east England have common characteristics involving low-gradient planed or irregular bedrock surfaces and single or multi-storey gravel deposits. Rather than depending on warm-cold or cold-warm transitions, it is suggested that bedrock planation, “working depths” of gravel and later-stage (relatively shallow) aggradations are all dominantly of cold-climate origin. Basal sediments show active incorporation of plucked and periglacially shattered materials, whilst super-incumbent units incorporating up-catchment and slope-derived materials demonstrate later cold-stage sediment influx and consequent cessation of active bedrock erosion. Channel activity effecting both planation and deposition are reviewed, together with the detailed sedimentology of gravelly sediments which show evidence of both autogenic processes (bar migration, channel switching and infilling, and truncation of upper sedimentation units), cold-climate indicators (turbation, ice-wedge casts, and frozen block transport), and (specifically for the last glacial-interglacial cycle) varying sediment flux as climates changed. Both interglacial and “transitional” activities are believed to be of lesser morphological significance, whilst prior uplift is taken as enabling rather than being a generator of terrace within the timescale of a glacial-interglacial cycle. Variations within cold-stage climates, varying sediment influx and channel-belt bedrock erosion are stressed as dominating mid-catchment and mid-latitude Quaternary terracing at the glacial-interglacial scale.

[5] The sedimentary architecture of an alpine alluvial fan with a surface of 300,000 m2 near Samedan, Switzerland, was three-dimensionally investigated using 9 km of ground penetrating radar sections with a penetration depth of 10 m. Radar facies patterns could be very well calibrated to sedimentology and dated horizons due to a 300 m long outcrop section at the foot of the fan. Six major reflectors have been identified and represent first order, fan-wide palaeosurfaces. They are made up of up to 20 cm thick fine-grained deposits partly with initial pedogenetic structures and wood remains which yielded ages between 5670 +/- 60 and 7515 +/- 65 a (14)C BP. Between these surfaces different depositional lobes and specific architectural elements like channels, levees or snouts of debris flows could be identified. All these data were geo-referenced to establish a complete quantitative 3-D time-stratigraphic framework. This allowed us to calculate deposited sediment volumes and sediment fluxes for different time slices between the dated palaeosurfaces. Sediment fluxes show an overall decline during the Holocene which we interpret: as the decling sediment production in the catchment area a function of the paraglacial cycle. Since the middle Atlantic period, the aggradation of the fan almost ceased and climate perturbations are no longer reflected in the sedimentary record since then. Within the aggradation period, distinct peaks of high sediment fluxes could be correlated with known periods of glacier retreats in the Swiss Alps, which points to a high sensitivity of the system to climate changes. The results of our study give valuable new insights into thresholds of activity and quiescence of an alpine catchment-fan system and its stage in the course of a paraglacial cycle. Although very important for the understanding of such dynamic systems under global warming scenarios and geo-risk assessment, such data sets are rarely available.

[6] GPR provides high resolution images of aeolian strata in frozen sand in the McMurdo Dry Valleys of Antarctica. The results have positive implications for potential GPR surveys of aeolian strata on Mars. Within the Lower Victoria Valley, seasonal changes in climate and a topographically constrained wind regime result in significant wind reversals. As a consequence, dunes show reversing crest-lines and flattened dune crests. Ground-penetrating radar (GPR) surveys of the dunes reveal sets of cross-strata and low-angle bounding surfaces produced by reversing winds. Summer sand transport appears to be dominant and this is attributed to the seasonal increase in solar radiation. Solar radiation which heats the valley floor melts ice cements making sand available for transport. At the same time, solar heating of the valley floor generates easterly winds that transport the sand, contributing to the resultant westward dune migration. The location of the dune field along the northern edge of the Lower Victoria Valley provides some shelter from the powerful foehn and katabatic winds that sweep down the valley. Topographic steering of the winds along the valley and drag against the valley wall has probably aided the formation, migration and preservation of the dune field. Optically-stimulated luminescence (OSL) ages from dune deposits range from 0 to 1.3 kyr showing that the dune field has been present for at least 1000yr. The OSL ages are used to calculate end-point migration rates of 0.05 to 1.3 m/yr, which are lower than migration rates reported from recent surveys of the Packard dunes and lower than similar-sized dunes in low-latitude deserts. The relatively low rates of migration are attributed to a combination of dune crest reversal under a bimodal wind regime and ice cement that reduces dune deflation and restricts sand entrainment.

[7] This paper presents similar to 30 km of ground penetrating radar (GPR) data from a mid-channel bar in the sixth largest river in the world, the Rio Parana, Argentina. GPR profiles, with depth of penetration up to 12 in below the bar surface, were collected from a sandy braid bar similar to 3 km long by similar to 1 km wide on a grid with a 200 to 400 in spacing. Two facies were found to dominate the sedimentary architecture. The principal facies (similar to 83%, of total facies) comprises trough and planar cross-strata related to the migration of dunes, with the thickness of the cross-strata decreasing towards the bar surface. The second significant facies (similar to 15%) is high-angle (generally 10-20 degrees) strata that typically form by accretion at the bar margins or bartail. Clay drapes (< 2%) and cross-bar channels (< 1 %) are shown to constitute only a minor part of the deposits. The Rio Parana GPR surveys are compared with other GPR studies of sandy braid bars of different sizes from the South Saskatchewan, Wisconsin, and Jamuna rivers. The dominance of dune deposits is ubiquitous to all rivers, with each also possessing a significant proportion of large-scale high-angle strata. However, two differences were found to exist between the deposits of these rivers: (1) the compound-bar deposits of smaller rivers contained greater proportions of the fills of cross-bar channels, which suggests a potential role for discharge as a factor in shaping the alluvial architecture through its impact on the frequency of sediment reworking over the bar tops, and, (2) the thickness of large-scale, high-angle sets decreases with the age of the bar, which suggests that the deposits of older bars may provide more useful geometrical analogues for interpreting ancient successions, than smaller transient, or recent, bar forms that have undergone only limited modification.

[8] The Tortonian carbonate ramp of Menorca was previously studied on the basis of outcrops along sea-cliff outcrops. These sea cliffs, in combination with inland water wells, are the basis for a facies model for the reconstruction of the internal architecture and for characterizing the internal heterogeneities of this carbonate platform. However, any such three-dimensional reconstruction is generally limited by the given geometrical arrangement of the two-dimensional outcrops and the uncertainties of correlation with the one-dimensional wells. Here, ground-penetrating radar (GPR) has been employed in order to test and refine the depositional model. Although GPR is well known for being an excellent tool for high-resolution underground studies of sedimentary systems, the application for studying carbonate rocks is still far from routine. The reason for this discrepancy is two-fold: the minor mineralogical contrast between lithologies in carbonate rocks results in subtle reflections, and, even more important, the porosity structure in carbonates is thoroughly and repeatedly changed during diagenesis, commonly across the different facies, leading to problems in predictability of the petrophysical properties. The study of the Menorcan carbonate ramp with large distance-deep penetration GPR sections demonstrates that in spite of these difficulties, GPR is a valuable tool for extrapolating information from outcrops and wells. It is useful for characterizing heterogeneities larger than outcrop scale.

[9] Investigation of a group of landforms and their underlying deposits on the eastern margin of the Fenland in East Anglia has demonstrated that they represent a series of glaciofluvial delta-fan and related sediments. Section logging, borehole records and previous descriptions combine to indicate that the sediments were deposited in ice-marginal deltaic settings in an ice-marginal lake. The internal structure and form of the fan-like deltas has been demonstrated using extensive ground penetrating radar investigation. The lake formed by the ice damming westward-aligned river valleys. Together, this evidence confirms historical descriptions of a glaciation of the Fenland, and clarifies the interpretation of gravels of the eastern Fenland margin. Recent reinterpretations of the latter as of fluvial rather than glacial meltwater origin are shown to be incorrect. It is concluded, on the basis of regional correlation, supported by optically stimulated luminescence dating, that the glaciation occurred at c. 160 kyr, i.e. in the Wolstonian (=Saalian) Stage (broadly equivalent to MIS ?11b-6). Comparison with The Netherlands’ sequence shows a similarity of glacial marginal morphology, and the dates confirm the time equivalence with that during the late Saalian Drenthe Substage, Amersfoort ice-pushed ridge complex. The implications include that the c. 200 kyr interval, between the Hoxnian (Holsteinian) temperate Stage and the Wolstonian glaciation, was a period during which fluvial and periglacial activity modified the landscape under cold climates, with organic sediments laid down during warmer events. Palaeolithic humans were periodically present during this interval, their artefacts having been reworked by the subsequent glaciation.

[10] A distinct lens of marine sand, up to 90 cm thick, confined vertically by peat, is found in the upper fill of a closed freshwater back-barrier lagoon on the southeast Australian coast. Coring of the deposit suggests it extends continuously up to 600 m inland and tapers landward rising to similar to 1.6 m above principle datum. In places the sand is overlain by accumulations of organic-rich silt that contain charophytes, indicating re-establishment of lagoon conditions. Hypotheses considered for the deposition of the sandsheet are higher Holocene sea level, storms and tsunami. Ground penetrating radar transects of the seaward dune system suggest a penecontemporaneous erosional contact between a series of truncated pre-event dunes and several small overlying post-event dunes. Dating the sandsheet was problematic but it is confined to the last 800 years. The young age combined with a lack of associated beach deposits and evidence of wave scouring suggest that a higher sealevel hypothesis is unlikely. This sand lens is attributed to a large-scale washover event from the southeast. Based on comparisons with modern storm deposits from the same coast and sedimentological diagnostic criteria derived from studies of modern storm- and tsunami-deposited sandsheets, it is concluded that this sand deposit is the product of a short-lived, large-scale overwash event attributed to a late-Holocene tsunami.

[11] An understanding of the heterogeneity of quaternary gravelly deposits is required to predict flow and contaminant transfer through these formations. In such deposits, preferential flow paths can lead to contamination at depths greater than predicted under the assumption of a homogeneous medium. The difficulties in characterizing their complex structure with conventional methods represent an obstacle for this prediction. In this study, we developed an approach relying on the use of ground penetrating radar ( GPR) for the detection of sedimentary depositional units. A genetic interpretation of the radar stratigraphy allowed us to construct a distribution model of lithofacies. The study was conducted on glaciofluvial deposits underlying a stormwater infiltration basin. Two main system tracts were characterized: a top stratum ( 50 - 80 cm deep) corresponding to massive gravel and open-framework gravel, and a base stratum corresponding to trough-fill structures with associated sandy, open-framework, massive, and matrix-rich gravelly lithofacies. The knowledge of the hydraulic properties linked to each lithofacies led us to propose a hydrostratigraphic model. Based on this model, we formulated a hypothesis about the hydraulic behavior of the deposit during stormwater infiltration. Open-framework gravels can act, during complete saturation, as preferential flow paths, and capillary barrier effects may occur under variably saturated conditions. These hypotheses were tested by measuring water content variations ( using time domain reflectometry probes) at three depths ( 0, -0.5, and -1.15 m). Experimental data show infiltration behavior that can be explained by a capillary barrier effect between the two lower probes. These results suggest that our hypothesis about hydraulic behavior is reasonable.

[12] Regional-scale washover deposits along the Florida Gulf and Atlantic coasts induced by multiple hurricanes in 2004 and 2005 were studied through coring, trenching, ground-penetrating radar imaging, aerial photography, and prestorm and poststorm beach-profile surveys. Erosional and depositional characteristics in different barrier-island sub-environments, including dune field, interior wetland and back-barrier bay were examined. Over the eroded dune fields, the washover deposits are characterized by an extensive horizontal basal erosional surface truncating the old dune deposits and horizontal to slightly landward-dipping stratification. Over the marshes in the barrier-island interior, the washover deposits are characterized by steep tabular bedding, with no erosion at the bottom. Overwash into the back-barrier bay produced the thickest deposits characterized by steep, prograding sigmoidal bedding. No significant erosional feature was observed at the bottom. Washover deposits within the dense interior mangrove swamp demonstrate both normal and reversed graded bedding. The washover deposits caused by hurricanes Frances (2004) and Jeanne (2004) along the southern Florida Atlantic coast barrier islands are substantially different from those along the northern Florida barrier islands caused by Ivan (2004) and Dennis (2005) in terms of regional extension, erosional features and sedimentary structures. These differences are controlled by different overall barrier-island morphology, vegetation type and density, and sediment properties. The homogeneity of sediment along the northern Florida coast makes distinguishing between washover deposits from Ivan and Dennis difficult. In contrast, along the Atlantic coast barrier islands, the two overwash events, as demonstrated by two phases of graded bedding of the bimodal sediments, are easily distinguishable.

[13] We reconstruct postglacial lake-level history within the Lake Michigan basin using soil stratigraphy, ground-penetrating radar (GPR), sedimentology and (14)C data from the Silver Lake basin, which lies adjacent to Lake Michigan. Stratigraphy in nine vibracores recovered from the floor of Silver Lake appears to reflect fluctuation of water levels in the Lake Michigan basin. Aeolian activity within the study area from 3,000 years (cal yr. B.P.) to the present was inferred from analysis of buried soils, an aerial photograph sequence, and GPR. Sediments in and around Silver Lake appear to contain a paleoenvironmental record that spans the entire post-glacial history of the Lake Michigan basin. We suggest that (1) a pre-Nipissing rather than a Nipissing barrier separated Silver Lake basin from the Lake Michigan basin, (2) that the Nipissing transgression elevated the water table in the Silver Lake basin about 6,500 cal yr. B.P., resulting in re-establishment of a lake within the basin, and (3) that recent dune migration into Silver Lake is associated with levels of Lake Michigan.

[14] The internal structure of a modern baymouth barrier is analyzed in detail, including its historical evolution and the factors controlling it. Historical evolution was reconstructed using available aerial photographs from 1956 to 2003. Detailed geophysical data were acquired using ground-penetrating radar (GPR) and analyzed along with airphotos; to reconstruct the recent evolution of a barrier located in northwestern Spain. The GPR data reveal the internal architecture of the barrier, which consists of two main radar facies: (1) washover deposits associated with landward barrier migration and vertical aggradation, and (2) beach deposits associated with seaward barrier progradation. The reconstruction of the stratigraphic sequence indicates the occurrence of two erosive periods followed by an accretionary period. The erosion along the barrier is outlined by the presence of three well-defined bounding surfaces. The stratigraphic record of the subsequent recovery phase depends on the impact level of the erosive phase, which is controlled by the presence of ephemeral inlets along the barrier. This alternating behavior implies a discontinuous shoreline trajectory in spite of a net landward migration. This trend is consistent with regional sea-level rise albeit being intensified locally by the interplay of sediment starvation and storm regime. Landward barrier retreat occurs by overwash and inlet breaching, which permits barrier preservation of volume due to a continuous constructive-destructive cycle. The results acquired from this example expand our understanding of the internal structure and evolutionary trends of transgressive sand barriers.

[15] This study explores the link between spatial variability within fluvial sedimentary strata and river channel geometry. This link is then used to determine the macrodispersion coefficients for solute transport in groundwater flow in river deposits. In doing that we combine concepts from sedimentology, geostatistics, and the stochastic-Lagrangian theory of subsurface transport. It is proposed to analyze the spatial variability of river sediments in terms of transition probabilities. The transition probabilities can be determined from the averages and variances of the lengths of stratasets and their volumetric fractions using concepts developed by Carle and Fogg ( 1996) and Ritzi ( 2000). Strataset length scales are shown to correlate well with the geometry of the river channel and its bed forms and can be determined indirectly from aerial photogrammetry, and, more directly, through surface ground penetrating radar surveys. Stratal dimensions can also be related, using Lagrangian transport analysis, to macrodispersivity of subsurface transport. This allows relationships between the geometry of river channels and subsurface transport parameters of associated fluvial deposits. For demonstration the longitudinal macrodispersivity in a compound bar deposit in the gravelly, braided Sagavanirktok River was investigated.

[16] Ground penetrating radar (GPR) surveys of unit and compound braid bars in the sandy South Saskatchewan River, Canada, are used to test the influential facies model for sandy braided alluvium presented by Cant & Walker (1978). Four main radar facies are identified: (1) high-angle (up to angle-of-repose) inclined reflections, interpreted as having formed at the margins of migrating bars; (2) discontinuous undular and/or trough-shaped reflections, interpreted as cross-strata associated with the migration of sinuous-crested dunes; (3) low-angle (< 6 degrees) reflections, interpreted as formed by low-amplitude dunes or unit bars as they migrate onto bar surfaces; and (4) reflections of variable dip bounded by a concave reflection, interpreted as being formed by the filling of channel scours, cross-bar channels or depressions on the bar surface. The predominant vertical arrangement of facies is discontinuous trough-shaped reflections at the channel base overlain by discontinuous undular reflections, overlain by low-angle reflections that dominate the deposits near the bar surface. High-angle inclined reflections are only found near the surface of unit bars, and are of relatively small-scale (< 0.5 m), but can be found at a greater range of depths within compound bars. The GPR data show that a high spatial variability exists in the distribution of facies between different compound bars, with facies variability within a single bar being as pronounced as that between bars. Compound bars evolve as an amalgamation of unit bars and other compound bars, and comprise a facies distribution that is representative of the main bar types in the South Saskatchewan River. The GPR data are compared with the original model of Cant & Walker (1978) and reveal a much greater variability in the scale, proportion and distribution of facies than that presented by Cant & Walker (1978). Most notably, high-angle inclined strata are over-represented in the model of Cant and Walker, with many bars being dominated by the deposits of low- and high-amplitude dunes. It is suggested that further GPR studies from a range of braided river types are required to properly quantify the full range of deposits. Only by moving away from traditional, highly generalized facies models can a greater understanding of braided river deposits and their controls be established.

[17] Recent Gulf of Mexico shoreline studies interpret middle to late Holocene sea level as failing from a level above present elevation or stable at present elevation; however, the architecture of Morgan Peninsula, Alabama, does not support this. Morgan Peninsula is a beach-ridge strandplain composed of two obliquely aligned Holocene beach-ridge sets. Ground-penetrating radar profiles discriminate between parallel, even to wavy reflectors of the eolian dune environment and the underlying seaward-dipping, complex sigmoidal-oblique reflectors of the foreshore and upper-shoreface environment. The contact between foreshore and eolian facies in beach ridges can be used as a sea-level indicator. The average elevation of this contact in Morgan Peninsula rises throughout shoreline accretion, which occurred throughout the last 5.4 ka, suggesting that there was continual sea-level rise during this time. Morgan Peninsula is a useful modern analog to ancient shoreface-shelf parasequences and demonstrates the significant internal complexities that can exist in these deposits. Erosional discontinuities imaged in the Holocene foreshore-upper shoreface environment are laterally continuous, extend to elevations above mean sea level, and correlate to beach ridges and the transition between beach-ridge sets. An increase in the wave regime or fluctuations in sediment supply appear to be likely mechanisms for forming erosional discontinuities below beach ridges. The erosional surface separating beach-ridge sets may have formed by increased storm activity and associated barrier breaching, or a reconfiguration of the Mobile Bay tidal-delta complex superimposed on a gradual rate of sea-level rise. This boundary is recognized by a change in beach-ridge orientation at the surface and an increase in the aggradational component of shoreline accretion.

[18] The southern part of the Hvidbjerg coastal dunefield and the adjacent part of the Lodbjerg coastal dunefield system in northwestern Denmark has been mapped using ground-penetrating radar. Coastal cliff exposures and C-14 datings indicate that the presently stabilized dunefield is underlain by a 10-15 m thick succession of Holocene aeolian sand deposits and interbedded peaty palaeosols. This succession is superimposed on a Weichselian till towards the south and a mid Holocene beach ridge and swash bar complex towards the north. The peaty palaeosols represent preceding periods of dunefield stabilization. These palacosols reveal ages about AD 1100-1200, 700 BC and 2100 BC and act as marker horizons in the study area. More than 16 km of high-resolution GPR data covering an area of 6 km2 were collected in the near-coastal part of the dunefields and enabled a correlation of these marker horizons across the dunefields. These data allowed the elaboration of topographic subsurface and isopach maps, and enabled an interpretation of the accumulation history of the aeolian system since about 2200 BC. This accumulation history tells of alternating periods with a stabilization of the dunefields and periods with high sand accumulation. Basically a perched ground water level and the morphology of the previous stabilized dunefield made it possible to generate an accumulation space large enough to preserve the sand during periods with stronger winds and marked aeolian activity. This 3D mapping of the stabilized dunefield subsurfaces has made it possible to unveil the development of the past aeolian landscape. (c) 2005 Elsevier B.V All rights reserved.

[19] One hundred years ago Georg Popp became the first scientist to present in court a case where the geological makeup of soils was used to secure a criminal conviction. Subsequently there have been significant advances in the theory and practice of forensic geoscience: many of them subsequent to the seminal publication of “Forensic Geology” by Murray and Tedrow [Murray, R., Tedrow, J.C.F. 1975 (republished 1986). Forensic Geology: Earth Sciences and Criminal Investigation. Rutgers University Press, New York, 240 pp.]. Our review places historical development in the modem context of how the allied disciplines of geology (mineralogy, sedimentology, microscopy), geophysics, soil science, microbiology, anthropology and geomorphology have been used as tool to aid forensic (domestic, serious, terrorist and international) crime investigations. The latter half of this paper uses the concept of scales of investigation, from large-scale landforms through to microscopic particles as a method of categorising the large number of geoscience applications to criminal investigation. Forensic geoscience has traditionally used established non-forensic techniques: 100 years after Popp’s seminal work, research into forensic geoscience is beginning to lead, as opposed to follow other scientific disciplines.

[20] Washovers, dune scarps and flattened beach profiles with concentrations of coarse-grained sediment or heavy minerals are the diagnostic geological signatures of large storms on barriers today. It is clear that storms are a major force driving transgressive barriers onshore, but what is not as well understood is the role these powerful erosive events play in the evolution of prograding barriers. Application of ground-penetrating radar (GPR) and a combination of coring techniques have significantly improved our ability to image barrier architecture. Results of these studies reveal a more complex evolution than previously recognized. It is now possible to precisely locate and map storm deposits within prograded barrier lithosomes. A comprehensive study of northern Castle Neck, Massachusetts was performed using 15 kin of GPR surveys, a 120-m-long seismic line, 11 cores, and several radiocarbon dates. Storm-related layers are the most prominent horizons contained in the barrier stratigaphy. The geometry and sedimentology of these layers closely resembles those of a present-day post-storm beach. Twenty closely spaced, curvilinear heavy mineral layers imaged in the landward portion of the barrier suggest that the Castle Neck barrier was heavily influenced by storms during its initial phase of progradation beginning similar to 4000 years BP. Approximately 1800 years BP, two intense storms impacted the coast depositing two extensive coarse-grained units. These layers mimic the flat-lying sand and gravel deposits that occur in front of a nearby eroding till outcrop following major storms. The great number of storm deposits in the early history of Castle Neck is related to either a period of greater storm activity and/or a slow rate of barrier progradation. The occurrence and preservation of these earlier storm layers are likely a product of the exposure of nearby drumlins resulting in greater availability of iron oxide and ferromagnesian sands. The supply of heavy-mineral sands has gradually diminished as the barrier prograded and the proximal drumlin source was buried by beach and dune sands.

[21] Ground-penetrating radar (GPR, also referred to as ground-probing radar, surface-penetrating radar, subsurface radar, georadar or impulse radar) is a noninvasive geophysical technique that detects electrical discontinuities in the shallow subsurface (< 50 in). It does this by generation, transmission, propagation, reflection and reception of discrete pulses of high-frequency (MHz) electromagnetic energy. During the 1980s radar systems became commercially available, but it was not until the mid-1990s that sedimentary geologists and others began to widely exploit the technique. During the last decade numerous sedimentological studies have used GPR to reconstruct past depositional environments and the nature of sedimentary processes in a variety of environmental settings; to aid hydrogeological investigations, including groundwater reservoir characterisation, and to assist in hydrocarbon reservoir analogue studies. This is because in correctly processed radar profiles, and at the resolution of a survey, primary reflections usually parallel primary depositional structure. Despite the wide use of GPR, a number of fundamental problems remain in its application to sedimentary research. In particular, there are a wide range of approaches to the processing of radar data and interpretation techniques used on the final subsurface images vary widely, with little consensus over a common methodology. This review attempts to illustrate that methods for the collection, processing and interpretation of radar data are intimately linked and that thorough understanding of the nature, limitations and implications of each step is required if realistic sedimentological data are to be generated. In order to extract the maximum amount of meaningful information, the user must understand the scientific principles that underlie the technique, the effects of the data collection regime employed, the implications of the technique’s finite resolution and depth of penetration, the nature and causes of reflections unrelated to primary sedimentary structure, and the appropriateness of each processing step with respect to the overall aim of the study. Following suitable processing, a radar stratigraphy approach to reflection profile interpretation should be adopted. New or modified terminologies and techniques to define a radar stratigraphy are also recommended, in order to make the interpretation process more transparent and to avoid confusion with related methodologies such as seismic stratigraphy and sequence stratigraphy. The full potential of GPR in sedimentary research will only be realised if more thorough and systematic approaches to data collection, processing and interpretation are adopted.

[22] The distinction of cheniers from other types of beach ridge can often be problematic. The stratigraphy, sedimentology and geomorphological development of sand- and gravel-rich beach ridges at three sites on the northern Essex coast, England were determined using ground-penetrating radar (GPR), ground-truthing trenches and auger holes, and historical-aerial photograph analysis. The 900 MHz GPR system used achieved a maximum vertical resolution between 0.02 and 0.06 m. There was good correspondence between the radar stratigraphy obtained from time-migrated radar reflection profiles and the nature and form of bounding surfaces, sets of lamination, beds and bedsets observed in trenches. Data for one of the study sites (Colne Point) illustrate a complex beach-ridge stratigraphy and sequence of development that confirms they are not true cheniers. Instead, the ridges form part of both retrogradational and progradational barrier-spit sequences. Chenier designation at the other two study sites (Foulton Hall and Stone Point) is more straightforward, as the beach ridges lie at the junction between actively eroding mudflat and saltmarsh. However, despite the distinction between the barrier-spit beach ridges and the cheniers, the same types of deposit are recognised in both, indicating similar formative processes. Washover-sheet deposits consist of low-angle (< 5degrees), sub-parallel, landward-dipping stratification, which is concordant with bounding surfaces above and below. Washover sheets develop when high-wave energy/storm-related overwash moves landward onto unflooded marsh or lagoonal surfaces. Washover-delta deposits are characterised by high-angle cross-stratification (up to 28degrees) that downlaps onto the underlying marsh or lagoonal surface. Washover deltas develop when overwash enters a significant body of standing water to landward, such as a flooded marsh or lagoon. Alternations between washover-sheet and washover-delta development are seen in many instances, but their time scale is unclear due to paucity of detailed information regarding overwash sedimentation rates, or chenier and berm-ridge migration rates. Similarity of the internal structure in both the cheniers and barrier-spit beach ridges confirms that this criterion alone cannot be used to distinguish different types of beach-ridge deposit. Detailed stratigraphic data, preferably complemented by direct evidence for the geomorphological context in which the ridge developed, are also required.

[23] Ground-penetrating radar (GPR) transects and sediment cores have been used to examine the basement morphology, stratigraphy, and environmental history of maritime ponds along the peninsular coast of Maine. Silver Lake, Lily Pond, and North Pond are shallow (< 3 m) water bodies bordered by steep bedrock ridges in the north, east, and west, and sandy barriers to the south. The bedrock basins of the ponds are formed in metasedimentary rocks surrounded by resistant pegmatitic intrusions. A dense network of GPR traverses obtained over the ice-covered Silver Lake reveals a series of prominent wavy-parallel and basin-fill reflector geometries terminating against the bedrock or grading into the barrier sediments and interpreted as organic lake-bottom facies. The transparent units represent sand-rich horizons, mostly eolian in origin. Convex-up structures found both on the surface and within the basin-fill sequence are interpreted as preserved parts of coastal dunes. The present study indicates that freshwater conditions prevailed since at least 4.6 ka, with an initial sedimentation rate of 1.7 mm/yr. The position of this unit below the contemporary sea level suggests presence of a welded barrier by that time. Radar profiles taken along the shores of Lily Pond, a small water body behind the Sand Dune Barrier, indicate a significantly larger areal extent of the pond in the past. A succession of organic deposits overlying a Pleistocene glaciomarine unit indicates progressive inundation of the paleo-lagoon by rising sea level. Saltwater peat seaward of Lily Pond was buried by washover sands, about 1.2 ka, and a narrow pond existed here prior to dredging and artificial infilling of its eastern part in the 1950s-60s. The organic and eolian units are absent in the North Pond, where sedimentary fill consists of glaciomarine clay overlain by marine sands. A proposed three-stage model of pond evolution along an embayed coastline consists of: (1) organic accumulation in an upland depression during lower sea level; (2) predominantly washover or tidal deposition in a lagoon (Stage 2a) or blocked coastal pond (Stage 2b) during initial transgression, and (3) mainly eolian and organic deposition behind a prograded or aggraded barrier. Future accelerated rise in relative sea level and inadequate sediment supply will cause many back-barrier ponds to reenter Stage 2 of the proposed model.

[24] We present a three-dimensional model of heterogeneous modern channel bend deposits developed through purely structure-imitating interpolation (kriging) of hydraulic properties. This model, augmented with ground-penetrating radar data and directional variograms, agrees with detailed observations in similar modern environments and leads to a process-based interpretation of the presented hydraulic conductivity structure. Integration of all available information permitted delineation and characterization of the modern streambed as a distinct hydrostratigraphic unit without coring or outcrop studies. Our results imply that the modern streambed is commonly oversimplified in available analytical and numerical models of groundwater-surface water interactions where it is assumed to be homogeneous and isotropic and characterized by a constant width and thickness. This three-dimensional approach that integrates concepts and principles developed in sedimentology, hydrogeology, geophysics, and geostatistics has potential implications on model development of stream-aquifer systems.

[25] Architecture of recent channel-belt deposits of the Niobrara River, northeast Nebraska, USA, records the response of a sandy braided river to rapid base-level rise. Up to 3 m of aggradation has occurred within the lower 14 km of the Niobrara River since the mid-1950s as a result of base-level rise at the confluence of the Niobrara and Missouri Rivers. Aerial photographs and channel surveys indicate that the lower Niobrara has evolved from a relatively deep, stable channel with large, bank-attached braid bars to a relatively shallow, aggrading channel with braid bars and smaller secondary channels. Architecture of channel-belt deposits associated with the recent aggradation has been defined using ground-penetrating radar (GPR) and vibracores. The channel-belt deposits exhibit a series of amalgamated channel fills and braid bar complexes (i.e., macroforms). Radar facies identified in the GPR data represent architectural elements of the braid bar complexes, large and small bedforms [two-dimensional (2-D) and three-dimensional (3-D) dunes], and channels. Individual braid bars appear to consist of basal high-flow and upper low-flow components. Preservation of the complete, high-flow bar geometry is generally incomplete due to frequent migration of smaller scale, secondary channels within the channel belt (i.e., braided channel network) at low discharges. The large-scale stratification of the braid bar deposits is dominated by cross-channel and upstream accretion. Elements of downstream accretion are also recognized. These accretion geometries have not been documented previously in similar sandy braided rivers. Braid bar deposits with low-flow modification (e.g., incision by secondary channels) are recognized in the deeper portions of the deposits imaged by GPR. Preservation of braid bars, with both high- and low-flow components, is a result of the rapid baselevel fise and channel-bed aggradation experienced by the Niobrara River over the past 45 years. Recent avulsion of the river channel allowed preservation of the upper, low-flow component of the braid bar deposits (i.e., bar-top sequences). The relative abundance and stratigraphic position of the amalgamated channels and braid bar complexes within the channel-belt deposits constitute a “signature” of the recent base-level rise.

[26] The stratigraphy and landscape evolution of the Lodbjerg coastal dune system record the interplay of environmental and cultural changes since the Late Neolithic. The modern dunefield forms part of a 40 km long belt of dunes and aeolian sand-plains that stretches along the west coast of Thy, NW jutland. The dunefield, which is now stabilized, forms the upper part of a 15-30 m thick aeolian succession. The aeolian deposits drape a glacial landscape or Middle Holocene lake sediments. The aeolian deposits were studied in coastal cliff exposures and their large-scale stratigraphy was examined by ground-penetrating radar mapping. The contact between the aeolian and underlying sediments is a well-developed peaty palaeosol, the top of which yields dates between 2300 BC and 600 BC. Four main aeolian units are distinguished, but there is some lateral stratigraphic variation in relation to underlying topography. The three lower aeolian units are separated by peaty palaeosols and primarily developed as 1-4 m thick sand-plain deposits; these are interpreted as trailing edge deposits of parabolic dunes that moved inland episodically. Local occurrence of large-scale cross-stratification may record the head section of a migrating parabolic dune. The upper unit is dominated by large-scale cross-stratification of various types and records cliff-top dune deposition. The nature of the aeolian succession indicates that the aeolian landscape was characterized by alternating phases of activity and stabilization. Most sand transported inland was apparently preserved. Combined evidence from luminescence dating of aeolian sand and radiocarbon dating of palaeosols indicates that phases of aeolian sand movement were initiated at about 2200 BC, 700 BC and AD 1100. Episodes of inland sand movement were apparently initiated during marked climate shifts towards cooler, wetter and more stormy conditions; these episodes are thought to record increased coastal erosion and strong-wind reworking of beach and foredune sediments. The intensity, duration and areal importance of these sand-drift events increased with time, probably reflecting the increasing anthropogenic pressure on the landscape. The formation of the cliff-top dunes after AD 1800 records the modern retreat of the coastal cliffs.

[27] Fluvial landforms and deposits provide one of the most readily studied Quaternary continental records, and alluvial strata represent an important component in most ancient continental interior and continental margin successions. Moreover, studies of the long-term dynamics of fluvial systems and their responses to external or ‘allogenic’ controls, can play important roles in research concerning both global change and sequence-stratigraphy, as well as in studies of the dynamic interactions between tectonic activity and surface processes. These themes were energized in the final decades of the twentieth century, and may become increasingly important in the first decades of this millennium. This review paper provides a historical perspective on the development of ideas in the fields of geomorphology/Quaternary geology vs. sedimentary geology, and then summarizes key processes that operate to produce alluvial stratigraphic records over time-scales of 10(3)-10(6) years. Of particular interest are changes in discharge regimes, sediment supply and sediment storage en route from source terrains to sedimentary basins, as well as changes in sea-level and the concept of accommodation. Late quaternary stratigraphic records from the Loire (France), Mississippi (USA), Colorado (Texas, USA) and Rhine-Meuse (The Netherlands) Rivers are used to illustrate the influences of climate change on continental interior rivers, as well as the influence of interacting climate and sea-level change on continental margin systems. The paper concludes with a look forward to a bright future for studies of fluvial response to climate and sea-level change. At present, empirical field-based research on fluvial response to climate and sea-level change lags behind: (a) the global change community’s understanding of the magnitude and frequency of climate and sea-level change; (b) the sequence-stratigraphic community’s desire to interpret climate and, especially, sea-level change as forcing mechanisms; and (c) the modelling community’s ability to generate numerical and physical models of surface processes and their stratigraphic results. A major challenge for the future is to catch up, which will require the development of more detailed and sophisticated Quaternary stratigraphic, sedimentological and geochronological frameworks in a variety of continental interior and continental margin settings. There is a particular need for studies that seek to document fluvial responses to allogenic forcing over both shorter (10(2)-10(3) years) and longer (10(4)-10(6) years) time-scales than has commonly been the case to date, as well as in larger river systems, from source to sink. Studies of Quaternary systems in depositional basin settings are especially critical because they can provide realistic analogues for interpretation of the pre-Quaternary rock record.

[28] A considerable part of today’s drinking water supplies in Europe and North America rely on clean groundwater from gravelly Valley aquifers of Quatemary age. The sedimentary architecture, the distribution of lithofacies and of architectural elements in such heterogeneous deposits are of fundamental importance for the analysis of groundwater flow and contaminant transport. As the aquifers are not directly accessible for observation, representative outcrop analogues were used to study the sedimentology on a local scale. Conventional sedimentological classification schemes were adapted for the purpose of hydrogeological evaluations. Measurements of hydraulic properties were then used to define a set of 5 hydrofacies from 23 possible sediment lithofacies. A digital-photographic mapping procedure was developed to allow fast data acquisition in the field. The sedimentologically interpreted outcrops were stored in a GIS style database and thus allow the output for further sedimentological or hydrogeological analysis.

[29] The ground-penetrating radar (GPR) is a good candidate for the exploration of the Martian sub-surface because it is smaller and lighter than seismic instruments, and, due to the lack of water in the Martian rocks, has great penetration capability. The modelling of the GPR signal response has been performed by computing the dielectric properties of each simulated layer as a linear function of porosity, known values of the solids, and the nature of the material filling the voids (ice water, carbon dioxide ice, gas, liquid water). The synthetic response was computed by reflecting raytracing at various peak frequencies. The complex results show that reflections are due to variations electromagnetic waves provide a picture of the geometries of the layers of the subsurface and give clues on the nature of rocks. Permafrost and liquid water can be investigated, chiefly their seasonal changes can be analysed by means of repeated profiles. The use of the GPR would be a major breakthrough in the reconstruction of the past geological history of the planet.

[30] The Kronhede sandur forms part of a ca. 1500 km2 Late Pleistocene outwash system located in western Jylland, Denmark. A sedimentological study was carried out in the proximal part of the sandur. The sediments were deposited during progradation of the glacier as shown by a general coarsening-upward lithology, terminating with a jokulhlaup episode characterized by large compound dune migration and slack-water draping. Mapping of a more than 200 m long well exposed pitwall and ground-penetrating radar measurements in a 50 X 200 m grid along the pitwall made it possible to outline the three-dimensional geometry of the jokulhlaup deposit, forming the top part of the succession. The paper describes the sedimentology of the sandur deposits and the application of the ground-penetrating radar technique to sedimentary architecture studies.

 

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