Gravel Bodies Research Articles

Paleosubsidence and active subsidence due to evaporite dissolution in the Zaragoza area (Huerva River valley, NE Spain): processes, spatial distribution and protection measures for transport routes

The lowest 17-km long reach of the Huerva River valley, down to its confluence with the Ebro River in Zaragoza city, flows across salt-bearing evaporites of the Ebro Tertiary Basin (NE Spain). Upstream, the horizontally lying Miocene evaporites are interfingered with non-soluble distal alluvial fan facies (shales and sandstones). The proportion of soluble facies in the Huerva River valley increases in a downstream direction towards the basin depocenter. On the basis of the type and magnitude of the paleosubsidence features, the valley has been divided into four reaches. Along reach I, undeformed terrace deposits less than 4 m thick rest on insoluble detrital bedrock. In reaches II and III, dissolution at the alluvium–bedrock boundary has generated local thickening, deformation and paleocollapse structures, which only affect the alluvial mantle. In reach IV, terrace deposits thicken to over 60 m resulting from a large-scale synsedimentary subsidence. In this sector, subsidence locally affects to both the alluvium and the underlying bedrock. This indicates that dissolution acts at the rockhead beneath the alluvial cover (alluvial karst) and within the evaporitic substratum (interstratal karst). The development of an intraevaporitic karst in reach IV is attributed to gypsum and salt dissolution. Irregular terrace gravel bodies (gravel pockets) embedded in a fine-grained matrix associated with paleocollapse structures have been interpreted as liquefaction–fluidization structures resulting from ground acceleration and suction induced by catastrophic collapses. Subsidence is currently active in the region affecting areas with a thin alluvial cover in reaches III and IV. The low subsidence activity in most of Zaragoza city is explained by the presence of thickened (around 50 m) and indurated alluvial deposits. In the surrounding area, numerous buildings in Cadrete and Santa Fe villages have been severely damaged by subsidence. Natural and human-induced subsidence favours the development of slope movements in the gypsum scarp overlooking Cadrete village. Several transport routes including the Imperial Canal (irrigation canal) and the recently completed Madrid–Barcelona high-speed railway are affected by human-induced sinkholes. The paleocollapse structures exposed in the trenches of this railway and a ring road under construction point to hazardous locations underlain by cavities and collapse structures where special protection measures should be applied. Rigid structures are recommended beneath the high-speed railway with sufficient strength to span the larger sinkholes with no deformation. Electronic monitoring devices linked to a warning system can detect subtle subsidence-induced deformations in carriageways or railways. This research demonstrates that the study of the paleokarst helps to understand the processes involved in the present-day subsidence phenomena and their general spatial distribution.

Three-dimensional GPR analysis of various Quaternary gravel-bed braided river deposits (southwestern Germany)

Abstract Closely spaced (1 m) ground penetrating radar (GPR) profiles were used for a three-dimensional characterization and comparison of glaciofluvial gravel-bed deposits in palaeodischarge zones of the Würmian Rhine glacier (southwestern Germany). Previous sedimentological outcrop investigations revealed three regionally reoccurring architectural styles of gravel bodies. For each of these styles a three-dimensional GPR dataset has been acquired in active gravel pits in order to calibrate the radar profiles with outcrop walls and to analyse, in three dimensions, the depositional elements and their stacking pattern in the subsurface. The GPR data were interpreted by mapping reflection terminations in order to delineate genetically related units. In particular, radar facies types and radar sequence boundaries were used to define and map depositional elements. Both accretionary and cut-and-fill depositional elements could be identified. Accretionary elements are characterized by horizontally to low-angle inclined (1–3°) and moderately continuous reflections (5–30 m) terminating on flat sequence boundaries; they represent the deposits of gravel sheets and traction carpets. In contrast, cut-and-fill elements are characterized by low to steeply inclined (3–25°), often discontinuous reflections terminating on concave to trough-shaped lower truncation boundaries; these are interpreted as scour-pool fills and small dissection elements (e.g. chutes and lobes). The three basic architectural styles of gravel bodies can be distinguished on the basis of the size and proportion of cut-and-fill elements mapped within the radar images. One type of gravel body is composed of an amalgamation of large cut-and-fill elements whereas the other two types are dominated by accretionary elements and differ by the proportion of smaller cut-and-fill elements. The results show that GPR is an adequate technique to illuminate the sedimentary architecture of the various types of gravel bodies. GPR data allow detailed three-dimensional reconstruction of depositional elements and their stacking pattern in the subsurface.

Heterogeneity patterns of Quaternary glaciofluvial gravel bodies (SW-Germany): application to hydrogeology

The architecture of sedimentary bodies determines the heterogeneity of many aquifers. Thus, for environmental risk-assessment and quantification of clean-up efficiencies, determination of the spatial distribution of hydraulic properties is required. In this study, outcrop analogues of glaciofluvial gravel-bed deposits are used for a process-based analysis of sedimentary heterogeneities, which in turn are transformed into hydraulic parameters. Three scales of heterogeneities are distinguished: (a) the lithofacies-scale is formed by unique transport and depositional processes that form the fundamental building blocks (hydrofacies types), (b) the depositional element-scale shows distinct geometrical characteristics (internal structure and bounding surfaces) and determines the local distribution of lithofacies and, hence, the correlation structure of permeabilities, (c) the architectural-scale of gravel bodies is formed by the stacking of depositional elements, which is controlled by the dynamics of aggrading paleofluvial systems. Comparison of numerous gravel pits in SW-Germany revealed three major architectural patterns of glaciofluvial gravel bodies that can be distinguished statistically by the preservation of depositional elements as well as the frequency of lithofacies types. The facies analysis results in three conceptual facies models of proglacial river systems, which are regionally classified as ‘main-’, ‘intermediate-’ and ‘minor’ discharge types. Based on calculated and laboratory measured hydraulic properties of the various lithofacies types detailed outcrop wall maps were digitized into polygons of defined hydrofacies types by using the GIS system Arc Info. The two-dimensional fields of hydraulic properties were then transferred into a numerical model of groundwater flow. Modeling results clearly illustrate that the three heterogeneity patterns of gravel bodies also have distinguishable hydraulic response characteristics.

Quaternary geology of eastern Germany (Saxony, Saxon–Anhalt, South Brandenburg, Thüringia), type area of the Elsterian and Saalian Stages in Europe

The characteristic feature of the Quaternary geology of eastern Germany (location map, Fig. 1) is a sequence of more than 50 horizons and complexes of glacial (laminated clays, tills, glaciofluvial sediments) and periglacial (river gravels, gelifluction sheets, alluvium, loess) facies. In the period between the younger Tertiary and the first Elsterian continental glaciation, at least four gravel terraces were built up under cold climate conditions. The youngest of these gravel bodies lacks Scandinavian rocks and is interbedded with Elsterian glaciolacustrine deposits. Pre-Elsterian temperate periods have been recorded at a number of places (i.e. Zeuchfeld, Untermassfeld, Voigtstedt, Süssenborn). Two major ice advances with additional minor oscillations have been identified, of Elsterian and Saalian ages. Complete Eemian and Holsteinian sequences are represented showing vegetational development through each interglacial cycle (cool, temperate–warm, cool). The Weichselian glacial Stage in eastern Germany is represented by thick periglacial sedimentation, including gravels, gelifluction sheets, alluvium, debris covers and loess. Within these sediments is a wide variety of glaciotectonic structures and periglacial soil structures, including classical involutions and ice-wedge casts. An outline of the present state of knowledge of the Quaternary of eastern Germany is given in Tables 1–3 while Table 4 presents a stratigraphical comparison with the neighbouring regions and countries. The palaeogeography of the region during the periods of the European continental glaciation is depicted in Fig. 2.

Upper Cenozoic fluvial history in the Bohemian Massif

Abstract Two periods of fluvial development are identified in the Bohemian Massif (BM) uplands during the Upper Cenozoic. The first, Lower Miocene period is characterised by large unterraced gravel bodies, up to 100 m thick, filling overdeepened valleys or forming nowadays highly fragmented spreads upon plateaux and interfluves. For the second, Upper Pliocene–Holocene period a diverse style of deposition is typical. The sedimentation is discontinuous and all types of continental sediments show repeated breaks in deposition. This type of deposition is believed to be driven by the very essence of this time span, i.e. climatically controlled alternation of erosion and accumulation phases. Because the uplands of the BM remained ice-free, and higher relief ensured repeated down-cutting whenever the environment permitted, the river valleys preserve complete records of the entire younger period. The progressive series of incisions, each of which is followed by an aggradation phase, resulted in the formation of stairway-like terrace sequences starting some 120–150 m above the rivers. The onset of a new style of fluvial deposition, together with the origin of a new drainage pattern, is dated approximately to the Gauss/Matuyama palaeomagnetic reversal. This corresponds in Central Europe with the start of deposition of loess and formation of complex, “Quaternary type” sequences of slope sediments in uplands and can be taken to indicate a new geological era. These changes may be, therefore, taken as support for the lowering of the Quaternary/Tertiary Erathem boundary.

A gravel lobe deposit in the prodelta of the Doumsan fan delta (Miocene), SE Korea

Abstract The Doumsan fan-delta system in the Pohang Basin (Miocene), SE Korea comprises tripartite components of Gilbert-type topset, foreset, and bottomset environments with an extended prodelta in the deeper part (a few hundred metres deep). The present study documents sedimentologic features and origin of a gravel body (here interpreted as a gravel lobe) formed in the prodelta region of the Doumsan fan delta. The gravel lobe lies on sandy mud deposits and is capped by a thick massive sand bed. It extends for more than 1.5 km with a height of up to 30 m and shows a narrow tongue-like geometry. Eight sedimentary facies have been distinguished to describe characteristic features of the gravel lobe and the associated deposits. Of these, three types of gravelly sedimentary facies are important with regard to volumetric contribution and depositional processes: (1) crudely stratified pebble-grade conglomerate; (2) disorganized, clast-rich pebble(-to-cobble)-grade conglomerate; and (3) matrix-rich, bimodal cobble-grade conglomerate. The former two types dominate the central part of the lobe where they are not accompanied by sand beds, whereas the latter, as subordinate units, is prevalent in the fringe which otherwise is dominated by thick sandy mud deposits. The stacked successions of crudely stratified pebble-grade conglomerate are representative of the active aggradational phases of the gravel lobe, whereas the occurrence of channels within the lobe reflects that the gravel lobe prograded under the influence of subaqueous channel systems. The gravel lobe resulted from catastrophic disturbance (slumping) on the foreset region that further caused the development of channel systems, promoting efficient transport of gravelly sediments. This type of deposit may represent an important additional category of low-efficiency subaqueous fans.

Towards realistic aquifer models: three-dimensional georadar surveys of Quaternary gravel deltas (Singen Basin, SW Germany)

Abstract Understanding sedimentary heterogeneities has nowadays become crucial for sophisticated aquifer management. The methods used so far for aquifer characterization (e.g. pump test, coring analysis) cannot provide the needed resolution for a sufficient prediction of the subsurface properties (e.g. Sudicky, 1986 ). This calls for the application of the `outcrop analogue-approach' to aquifers (e.g. Macfarlan et al., 1994 ; Aigner, 1995 ). In this project we used georadar (ground-penetrating radar or GPR) to acquire the subsurface information for high-resolution three-dimensional images and radar facies mapping of glaciolacustrine delta sediments in the Singen Basin (SW Germany). A three-dimensional reconstruction of sedimentary heterogeneities for a detailed aquifer characterisation becomes possible by combining GPR profiling and outcrop information. A case study is presented, showing a complex, three-dimensional architecture of a Quaternary gravel body, consisting of the following three types of architectural elements: (1) glaciolacustrine delta foresets, (2) erosional surfaces with scour pool fills, and (3) horizontal glaciofluvial gravel sheets. Although this seems a relatively simple architectural inventory, the three-dimensional georadar profiling shows considerable internal heterogeneities. The radar data are interpreted following the seismic stratigraphic approach, i.e. the delineation of radar facies types and of radar sequences. The three-dimensional heterogeneities are visualized by means of time slices, 3-D block or chair diagrams, or series of radar facies maps.

Clast assembling, bed-forms and structure in gravel beaches

AbstractBoth textural maturity and structure acquired by gravels on beaches are largely a response to the interaction between beach surface and wave-backwash energy. The turbulent energy driving the processes of particle rejection andselection at the surface of growing gravel sheets is partly controlled by the grain size of the sheet itself, so the process is to a large extent self-regulating. Beach surfaces are seen to comprise many discrete sheets of gravel competing for growth at their boundaries, but each characterised by a uniform or uniformly gradational texture.There are two main types of gravel sheet: (1) selection pavements which occur on low beach slopes, showing little areal grain-size or grain-shape variation, and (2) turbulence shadows which occur on steeper slopes and produce assemblages of clasts which may show perfect lateral shape and size gradation.The clasts which make up these various gravel sheets are termed ‘clast assemblages’, and such assemblages are the fundamental units from which beaches are constructed. Clast assemblages are classified in terms of their textural maturity—the degree to which they exhibit uniformity in clast size and shape. In beach sections they are, either singly or in combination, bounded by planes of discontinuity (bedding planes) to form beds.Repeated combinations of either clast assemblages or beds in a genetic association are regarded as sedimentary structures, many of which are diagnostic of the gravel beach environment. Growth of beaches involves the stacking of sedimentary structures, and four growth patterns have been identified. The beach structure is, therefore, classified in a hierarchy comprising clast assemblage, bed, structures and growth form, and it is the growth pattern which may be related to tidal range. P.ecognition of the processes which generate beach gravels through the structure of the gravels permits an analysis of the internal structure of major gravel bodies such as barrier beaches. It adds another set of criteria which may further lead to an understanding of the processes responsible for the generation and evolution of these large gravelforms.Three types of gravel lithosomes have been identified. (1) Regressive barrier bars which form a series of gravel ridges separated by lagoonal deposits. Barriers are built initially by swash bars which grow in size and coarsen through time to become wave-resistant forms. They form as a response to times when the sediment, unable to be evenly distributed and sorted on the beach surface, forms a discrete bar seaward of the active beach. This is the result of a punctuated or continuously high sediment supply compared with the wave energy available to disperse the sediment, or a falling sea level which shifts the locus of sediment accretion. (2) In contrast, regressive (prograding) gravel sheets form as a response to a continuous supply of sediment to the beach surface, allowing it to build seaward by the uniform accretion of sediment which is sorted and retained on its surface. (3) Gravel sheets produced in transgression are characterised by an abundance of spherical clasts and are often overlapped by the sand beaches which occur seaward of them.

Assessment of offshore sand and gravel bodies for dredging

Abstract Reliable assessment of offshore sand and gravel resources permits efficient dredging, the maintenance of cargo quality control and the effective mitigation of environmental impacts. Site investigation should be based on the interpretation and correlation of high resolution seismic profiling and CPT/sampling data. A preliminary interpretation of the seismic data reveals the geological setting of the sand bodies and leads to the selection of appropriate sampling methods and the recognition of key sampling positions. Geologically complex sand bodies demand phased data acquisition to delineate geometry, physical properties and compositional variability. The alternative approach, of grid-based sampling using a predetermined sampling density, is costly at best and probably misleading. A three-dimensional model is created from the integration of acquired data and a resource volume calculated. Dredging constraints and overflow losses are applied to the model resulting in the determination of a reserve volume and critical dredging parameters. It is advisable to carry out a wide-ranging testing programme on the recovered samples to ensure compliance with relevant standards or requirements. The potential penalties for superficial site investigation include delay, unpredictable cargo quality and unforeseen environmental problems.

Sedimentary facies in an extensional and deep-lacustrine depositional system: the Pliocene Tiberino Basin, Central Italy

The Tiberino Basin was formed during Plio-Pleistocene extensional tectonic activity in Central Italy. The Pliocene sediments that fill this structure consist predominately of fine-grained clastic lithofacies deposited in a lacustrine system; four facies associations have been distinguished. Facies association A was deposited in a deep-offshore lacustrine environment and consists of massive, laminated, bluish-gray marly clays that lack benthonic fauna but have preserved organic matter (leaves, wood). This facies association is the main lacustrine deposit, whereas the other three represent marginal facies. Facies association B is interpreted as a delta system deposit; it has Gilbert-type sandy gravel bodies with foresets up to 50 m high and prodelta bodies consisting of laminated to massive marly clays that alternate with sandy and gravelly mud strata. Association C corresponds to a coastal environment, where two sub-environments are recognized: (1) a wave-dominated coastline characterized by interbedded muddy and sandy strata with sedimentary structures attributed to hummocky cross-stratification; and (2) clayey silts and lignites interpreted as a coastal wetland; a spectacular fossil forest is preserved in situ within this unit. Association D, formed on the distal part of a muddy alluvial fan, consists of sheet-flood clayey sandy silts, that alternate with incipient paleosols; rare, ribbon-channel fills are also present. Paleoenvironmental reconstruction indicates that the Tiberino Basin was occupied by a narrow, deep meromictic lake during the Pliocene. The size, shape, depth and sub-environment distribution of this lake were controlled by tectonic phenomena, such as the high subsidence rate, which caused the lake to be large and deep. Only in the few coastal areas that had high sedimentary input were wetland or alluvial fan deposits formed.

Abstract :Holocene Analogs for Forced Regression Sand and Gravel Bodies

Abstract :Holocene Analogs for Forced Regression Sand and Gravel Bodies

The depositional history of the bloomington complex, an ice-contact deposit in the Oak Ridges Moraine, Southern Ontario, Canada

The Oak Ridges Moraine is a major physiographic feature of south-central Ontario, Canada, that is made up of a number of distinct depositional elements. One of these elements, the Bloomington complex, is a body of sand, gravel and diamict that was deposited largely within an east-west trending interlobate lake basin that formed as the continental glacier split into two lobes: a northern lobe termed the Simcoe lobe and a southern lobe termed the Ontario lobe. The oldest exposed sediment in the Bloomington complex is a sequence of stratified sand and gravel that is interpreted to have been deposited as coalesced, subaqueous outwash fans that developed where several glacial meltwater conduits emptied into the proglacial lake. These deposits are locally sharply incised by stratified sand and gravel that is interpreted as a glaciofluvial deposit, laid down when the lake basin drained as ice margin retreat caused outlets to open along the Niagara Escarpment, 60 km to the west. The fan complex and glaciofluvial deposits are overlain, in turn, by stratified sand, silt and clay that was deposited within the lake basin after the ice margins readvanced, closing the drainage outlets and allowing the lake to rise. The glaciolacustrine deposits pass gradationally upward into a heterogeneous deposit of sand, silt, clay and diamict that is interpreted as an ice-marginal deposit of the last stages of closing of the lake basin as the northern and southern ice lobes converged. Finally, a cap of diamict covers the area, deposited as glacial ice, advancing from the south, over-rode the study area. The complete package of sediment that makes up the Bloomington complex illustrates the effects of fluctuating ice margins on proglacial deposition in a glaciolacustrine setting.

Genetically related alluvial deposits across active fault zones: an example of alluvial fan-terrace correlation from the upper Quaternary of the southern Po Basin, Italy

Integrated studies of alluvial fan systems and related intramontane valley deposits provide potentially useful tools for understanding the character of alluvial sedimentation and its control in fault-controlled piedmont zones. An example is reported from the uppermost Quaternary alluvial deposits of the northern Apennines and the southern margin of the Po Basin. A distinctive cyclicity characterizes both the outcropping and the buried units in the study area. In intramontane valleys individual cycles consist of regular alternations of sections displaying high and low terrace preservation. In the subsurface, a rhythmic alternation of coarse-grained bodies and laterally extensive pelitic horizons occurs at various scales and represents the basic cyclic motif of the depositional system. The correlation between the terrace sequences and the depositional cycles in the subsurface allows the differentiation of distinct sequences with a characteristic internal architecture. In the lower part of sequences, poorly preserved terrace deposits are laterally correlatable with alluvial fan gravel bodies, whereas highly preserved terrace deposits are correlated laterally with predominantly floodplain sediments in the upper part of sequences. Reliable correlation lines have been constructed using 14C dating. Thrust tectonics and climate exerted a major control on sequence development. Deep valley excavation, with subsequent low terrace preservation, occurred in response to relief rejuvenation during successive thrusting events. On the other hand, decreasing erosion related to periods of minor tectonic activity led to terrace preservation in the intramontane valleys. The coeval episodes of alluvial fan growth and abandonment, recorded at the basin margin, are interpreted to reflect changes in the efficiency of the drainage system, due to variations of tectonic uplift in the adjacent areas. Alternating phases of deglaciation and renewed glaciation, however, are inferred to have played a major role in controlling the amount and type of sediment delivered to the alluvial fan complexes.

Die glazifluvialen Sedimente im unteren Günztal (Bayerisch Schwaben/ Deutschland) nach morpho- und pedostratigraphischen Befunden sowie TL-Daten

Abstract. Until now in the lower valley of the river "Günz" the glaciofluvial gravel deposits were interpreted as an accumulation of the same glaciation (Riss). But the result of the stratigraphic investigation of loess cover sediments and glaciofluvial gravel layers - in connection with TL (Thermoluminescence)-datings - show that a mindelian gravel body lies beside the gravel deposits of the rissian upper high-terrace. The comparison of both subfaces of the gravel layers demonstrate that the older mindelian gravel deposits can be found on a higher level. Moreover, it is possible to connect the subface of the mindelian gravel with the subface of the "Grönenbach-Schwaighausener" gravel (it is the mindelian gravel of A. PENCK'S classical subdivision of the Ice Age with four glaciations in the area of Memmingen) in the south of the "Iller-Lech”-region. Furthermore, the stratigraphie division of the aeolian cover sediments at the localities of "Autenried" and "Günzburg" indicates the different ages of both gravel terraces (rissian and mindelian age). In that case the development of both gravel terraces do not belong to the same glaciation; they represent the main gravel deposits of two different glaciation periods.

Interpretation of the Sedimentology and Morphology of Perched Glaciolacustrine Deltas on the Flanks of the Lake Superior Basin, Thunder Bay, Ontario

Elevated shoreline features found around the north shore of Lake Superior are normally attributed to one of the several post-glacial lakes which successively occupied the basin prior to the present level. Three deltaic features perched on the flanks of Mt. Baldy, Thunder Bay, Ontario, are found to be at higher elevations than inferred post-glacial lake-levels in the area. Based on analysis of lithofacies and depositional environments they are found to be glaciolacustrine deltaic bodies, in part of Gilbertian type. Paleocurrent directions are contrary to those expected for deltas prograding into a Superior lake margin. Dropstone units indicate icebergs were actively involved in deposition. Some prodeltaic deposits are extensively wave reworked and there is evidence that the lake into which these deltas formed fluctuated in level and drained suddenly. The largest and best preserved delta shows evidence of ice push along its outer margin. It is proposed that a proglacial lake, here named Lake Baldy, persisted for a short period of time between the Mackenzie interlobate moraine and the southeastward wasting margin of the Superior lobe. A series of ice-contact deltas in the valley of the Current River suggest an interpretation of the stages of Lake Baldy can be made. The presence of Lake Baldy introduces a new detail in the sequence of regional deglaciation events and their relation to the proposed Marquette Readvance (9,900 yrs. BP). The study demonstrates the necessity of thorough geomorphic and sedimentological analysis of such sand and gravel bodies, which might otherwise be interpreted as related to the post-glacial lake-level history of Lake Superior.

The effects of fluctuating base level on the structure of alluvial fan and associated fan delta deposits: an example from the Tertiary of the Betic Cordillera, Spain

ABSTRACTThe Villanueva Complex is a lacustrine alluvial fan ‐ fan delta system which accumulated on the northern margin of the Guadix Basin during the Pliocene. Five transitional zones can be distinguished from proximal to distal areas: proximal alluvial fan, mid‐fan, fan fringe (transition zone), proximal fan delta and distal fan delta (lacustrine).This paper focuses on sedimentation in the fan fringe and the proximal fan delta areas, where the effects of fluctuations in the base level (lacustrine level) are more easily observed. The stratigraphical succession is here characterized by an alternation of fine lacustrine sediments (mudstones and siltstones) and gravels. The gravels appear as isolated channels, stacked channels, lenses and sheets.The isolated channels, which have a V‐shaped profile and better developed wings towards the top, appear in the fan fringe zone and in the fan delta. The stacked channels originate in the fan fringe zone and evolve laterally and distally to isolated channels. The lenticular gravel bodies (lenses) are well represented throughout the fan delta and present small channels at the base, indicating a radial flow pattern. Finally, the sheets are characteristic of the fan fringe zone. The first episodes of channel incision occurred at lowstand lake level. The channels and lenses developed in rising lake level conditions, and sheet deposit took place at highstand lake level. Although the final form of the lithosome was heavily controlled by the fluctuations in lake level, it depended on the relative proportions of gravel and silt sedimentation. The gravel‐siltstone interface therefore represents an equilibrium surface between the proportions of sedimentation of these lithologies.The bodies mentioned above are organized in coarsening and thickening upwards sequences tens of metres thick, in which a distal to proximal evolution can be observed from isolated channels to lenses or from stacked channels to sheets. The building and thickness of these sequences were controlled by fluctuations in the lake level. An initial fall in base level caused lengthening of the channels and entrenchment from distal to proximal areas, and a continuous increase in supply due to erosion in the drainage basin installed on the subaerial fan. When the lake level rose, lobes were formed at the channel endings and overbank processes were made possible, thus generating radial channels, whose levees were formed by amalgamation with the levees of the main lens channels. At the same time, sheets were formed in proximal areas, where the available amount of sediment was greater.

Glacial geology of Grand Manan Island, New Brunswick: Discussion

Robert Legget's review of and contribution to knowledge of the glacial geology of Grand Manan Island bring forward a number of pieces of new information and present several other aspects that make his paper a most welcomed and interesting note. My main concern is that there are sections in the paper that leave me wanting further observations. Perhaps Mr. Legget could be encouraged to have a second look at some few localities and phenomena on future visits to the island. Aside from the steep rock faces ranging up to 120 high (p. 441, col. I), I am obliged to note that the author has referred to a large number of localities not shown on the only map. The author could have been influenced to use a more detailed map at larger scale so that all pertinent sites might be located more readily. Of less frivolous concern is the matter of the origin of the flocks of sheep (p. 446 et seq .). To this reader, the most significant fact presented in the description and in the illustrations of this phenomenon is that some of the larger boulders on the landward side of the flock are embedded in gravel and sand. The body of gravel and sand has a flat upper surface (e.g., Figs. 9b, 10) reminiscent of raised wave-washed surfaces on the New Brunswick mainland. The material appears to be very similar in nature to the material overlying till shown in Fig. 5, except perhaps for the maximum size of boulders. Because the flocks of sheep and the associated bouldery gravel and sand apparently lie not much more than 4 m above present sea level (my estimate from Fig. 9 a and b), and because marine fossils have been collected from a similar level (4 m above HWOST, p. 451, col. l ) , would it not be possible to suggest, as a working hypothesis, that the bouldery gravel of Figs. 5

Quaternary piedmont plains on weakly resistant rocks in the Lower Saxonian mountains (W. Germany)

Summary The two piedmont plain levels of the investigated area are of Old Quaternary respective Middle to Young Quaternary age. Most of the proximal parts of these Quaternary piedmont plains are erosional piedmont plains on weakly resistant rocks of the Keuper and Lias. The central and distal parts of the piedmont plains are underlain by a variably thick cover of Middle to Young Quaternary sediments. Therefore the piedmont plains are partly a genuine erosional feature, the result of a combination of erosion and accumulation forms, or a genuine feature of accumulation. The loose Quaternary sediments underlying the piedmont plains consist mainly of loess which very often overlies solifluction layers and colluvial sediments, and which partly lies on the gravel body of the Middle Terrace, and, marginally on the gravel body of the Lower Terrace. Consequently, piedmont plains that appear morphographically uniform, have been formed here polygenetically and in several phases. The Young Wuermian phase, for example, resulted in a predominantly eolian sedimentary infilling of the dissected parts of the older piedmont plains so that their surfaces partly exceeded that of the original erosion piedmont plains; by the accumulation of very thick loess layers this sedimentation partly lead to an enlargement of the plains at their eastern margins. Especially in the lower and middle sections the thick loesses are interspersed with colluvial debris layers and sporadical debris stones thus proving a frequent interruption of the eolian sedimentation by extensive wash processes. These wash processes brought about a planation of an originally pronounced eolian micro relief; it obviously caused the wide distribution of almost level summit areas that are especially characteristic of the pure accumulation piedmont plains. In spite of the temporary predominance of accumulation processes in the course of the Quaternary formation of the piedmont plains the existance of two piedmont levels verifies the denudational reduction of the surface which was favoured by the minor resistance of the predominant clay and silt rocks. A favouring of the denudational reduction by the easy weathering readiness of these rocks becomes obvious at the outcrop of the occasionally embedded sandstone layers of the Lias which have been formed to small hogbacks out of a piedmont plain during the Middle and Young Quaternary degradation of the piedmont plains. On the margins of the basin the reduction of the surface was essentially influenced by the surrounding hogbacks and other mountains: on the one hand there was a (relatively small) reduction and widening of the piedmont plains by predominant formation of the slope feet which lead to a greater inclination of the back slopes of the hogbacks; on the other hand near the mouth of the valleys into the basin rock fans were formed. These elevated parts of the proximal parts of the piedmont plains the reduction of which lagged behind that of the adjacent parts of the piedmont plain were the initiating centres of the denudational wash of the piedmont plains. Besides this, they determined the position of the valleys of the central parts of the basin, the initial troughs of which always are situated in the contact area of two rock fans. This pattern therefore proves the intense connection of the piedmont plains to the adjacent slopes and valleys of the hogbacks and mountains that are surrounding the basin. The reduction phases of the local base levels and the piedmont plains correspond to an inset of differently old fans. By this the higher portions of the fans are often starting points of younger accumulation piedmont plains. The piedmont plains of the investigated area exhibit regional differences as to their extension and dissection which are in accordance with the different distance of the base level of both areas.

A simulation model of alluvial stratigraphy

ABSTRACTThe quantitative model presented simulates the development of a two‐dimensional alluvial sedimentary succession beneath a floodplain traversed by a single major river. Several inter‐related effects which influence the distribution of channel‐belt sand and gravel bodies within overbank fines are accounted for. These are (a) laterally variable aggradation, (b) compaction of fine sediment, (c) tectonic movement at floodplain margins, and (d) channel avulsion. Selected experiments with the model show how the interconnectedness and areal density of channel‐belt deposits decrease with increasing floodplain width/channel‐belt size, mean avulsion period, and channel‐belt aggradation rate. Separation of stream patterns based on interconnectedness and channel deposit density is difficult. Tectonic movements do not have a significant influence upon the successions unless a preferred direction of tilting is maintained (half‐graben). Then channel‐belt deposits showing offlap tendencies tend to cluster adjacent to the active floodplain margin, leaving dominantly fine‐grained alluvium to accumulate on the inactive side. Individual channel‐belt deposits thicken during aggradation, although a self‐regulating limit to such thickening is likely to operate. ‘Multistorey’features resulting from aggradation may be difficult to tell apart from those arising through superposition of distinct channel‐belt deposits of avulsive origin.

COMBINED GEOELECTRICAL AND DRILL‐HOLE INVESTIGATIONS FOR DETECTING FRESH‐WATER AQUIFERS IN NORTHWESTERN MISSOURI

A geoelectrical investigation was conducted in an area of northwestern Missouri which was previously explored by drill holes. The purpose of the survey was to evaluate the effectiveness of the method in locating potable groundwater. It was found that resistivity depth soundings using the Schlumberger arrangement can partially localize and determine the thickness and depth of both near‐surface and basal fresh‐water‐bearing gravel bodies in glacial deposits. The basal gravel is usually confined to preglacial stream channels and has a particular importance as a fresh‐water aquifer. Only in a few cases does the resistivity increase at the depth of the bedrock. More often the depth soundings show an appreciable increase of resistivity at a depth well within the bedrock. High permeability and porosity at a lower water conductivity in the gravel is compensated by lower permeability and porosity at a higher water conductivity in the bedrock. This can explain why the formation resistivities found from depth soundings are basically the same. An increase of resistivity in the deeper bedrock occurs due to a tightening of fissures containing salt water. In one particular well, which penetrates a basal gravel, it can be shown that the salt content of the well water originates from salt water encroachment, most likely from the bedrock.