Formation Of Push Moraines Research Articles

The interplay between deformation and deposition in a Pleistocene push moraine: New insight from structural interpretation and area-depth-strain analysis of the growth strata

There are many examples suggesting that push moraine formation requires building up of the proglacial wedge and then post-depositional folding and/or faulting. Only a few studies have been carried out on syntectonic deposition and erosion within proglacial sediment wedges; however, the resulting growth strata were determined based on the high variability in thickness and occurrence of erosional unconformities across the deformed sequence. This study shows how structural interpretation and the area-depth-strain (ADS) balancing method of the growth strata can be applied to better understand the interplay between deformation and deposition within an evolving push moraine primarily composed of a folded sequence of a glaciofluvial fan. This approach, together with analysis of the onlap/overlap relationships, allowed us to determine the relative rate between deposition and deformation. Moreover it provides an insight into the topography of the push moraine that may be inherited not only from the geometry of deformation structures but also from depositional processes controlling the spatial distribution and thickness variation of the growth strata. This study also gives an opportunity to reconstruct the ranges of probable parameters to quantitatively describe the heavily eroded push moraine in terms of the primary topography, height and relative time frame within which it formed. In addition, the probable thermo-hydro-mechanical properties of the deformed strata are interpreted in terms of drained/undrained conditions of volume strain evidenced by structural data and possibilities of lateral flow from beneath synclines into anticlines indicated by ADS-based balancing.

Development of push moraines in deeply frozen sediment adjacent to a cold‐based glacier in the McMurdo Dry Valleys, Antarctica

AbstractPrevious studies of push moraines have concluded that structural architecture is controlled by a combination of stress exerted by flowing ice and the rheology of the deforming sediment. However, the relationship between deformation processes and the thermal/hydrologic conditions within the sediment remains ambiguous. Using a combination of surface mapping, logging exposures and ground penetrating radar we examine the relationships between surface morphology, material properties and development of deformation structures in deeply frozen sediment that has been deformed by Joyce Glacier, a cold‐based glacier in the McMurdo Dry Valleys. The structural architecture of the push moraines is characterized by a combination of brittle and ductile deformation structures that have produced a structurally complex pattern of thrust faults, low angle listric thrusts and recumbent folds that extend 400 m beyond the glacier margin. Deformation is ductile where the ice concentration exceeds c. 65% volume and predominantly brittle where the ice concentration is less than c. 65%. The change in rheology reflects transition in behaviour from the material having a predominantly frictional character when the ice is limited to pore spaces to a non‐frictional character in which strength is primarily determined by the cohesive strength of the ice. This work shows that glaciotectonic deformation can occur in deeply frozen permafrost where there is no liquid pore water. We conclude that the presence of liquid porewater is not a necessary condition for the development of glaciotectonic deformation or for the formation of push moraines. © 2019 John Wiley & Sons, Ltd.

Younger Dryas glaciomarine sedimentation, push-moraine formation and ice-margin behavior in the Middle Swedish end-moraine zone west of Billingen, central Sweden

New highway exposures and drilling reveal the stratigraphy and structure of the Middle Swedish end-moraine zone west of Billingen, Sweden. The material in the end moraines is primarily glaciomarine clay of Younger Dryas age that was deposited as varved clay in front of the retreating glacier and then pushed glaciotectonically to form push moraines during minor ice-margin oscillations during overall retreat during the Younger Dryas cold event. The moraines are composed of deformed and remobilized clay with some clayey diamicton and penecontemporaneously deposited and deformed sand. Between the moraines lie ‘intermoraine flats,’ composed of undeformed varved clay of Younger Dryas age and surface sands of Younger Dryas to early Holocene age. Based on estimations of moraine volume, sedimentation rate and ice-margin retreat rates, we calculate the overall ice-margin retreat and end-moraine construction to span 350–800 years within the Younger Dryas. Because the number of moraines in the Middle Swedish end-moraine zone varies across Sweden, we regard the individual oscillations west of Billingen to be driven by local physical and glaciologic factors rather than ice-sheet wide climate drivers. The study area is also the location of the early and final drainages of the Baltic Ice Lake. The final drainage of the Baltic Ice Lake took place several decades after the youngest moraine was formed. We consider it likely that the earlier, Allerød drainage of the Baltic Ice Lake (BIL) also took place at Billingen, despite the lack of clear local stratigraphic evidence. However, based on our model, a retreat driven solely by climate would not have exposed the outlet at Billingen, and we propose a dynamic break-up of the ice-margin likely centered on Valle Härad that was driven by the head difference between the BIL and the sea.

Response of salt structures to ice-sheet loading: implications for ice-marginal and subglacial processes

During the past decades the effect of glacioisostatic adjustment has received much attention. However, the response of salt structures to ice-sheet loading and unloading is poorly understood. Our study aims to test conceptual models of the interaction between ice-sheet loading and salt structures by finite-element modelling. The results are discussed with regard to their implications for ice-marginal and subglacial processes. Our models consist of 2D plane-strain cross-sections, which represent simplified geological cross-sections from the Central European Basin System. The model layers represent (i) sedimentary rocks of elastoplastic rheology, (ii) a viscoelastic diapir and layer of salt and (iii) an elastoplastic basement. On top of the model, a temporarily variable pressure simulates the advance and retreat of an ice sheet. The durations of the individual loading phases were defined to resemble the durations of the Pleistocene ice advances in northern central Europe. The geometry and rheology of the model layers and the magnitude, spatial distribution and timing of ice-sheet loading were systematically varied to detect the controlling factors. All simulations indicate that salt structures respond to ice-sheet loading. An ice advance towards the diapir causes salt flow from the source layer below the ice sheet towards the diapir, resulting in an uplift of up to +4 m. The diapir continues to rise as long as the load is applied to the source layer but not to the crest of the diapir. When the diapir is transgressed by the ice sheet the diapir is pushed down (up to −36 m) as long as load is applied to the crest of the diapir. During and after ice unloading large parts of the displacement are compensated by a reversal of the salt flow. Plastic deformation of the overburden is restricted to the area immediately above the salt diapir. The displacements after unloading range between −3.1 and +2.7 m. Larger displacements are observed in models with deep-rooted diapirs, thicker ice sheets, longer duration of the loading phase, thicker salt source layers and lower viscosity of the salt. The rise or fall of diapirs triggered or amplified by ice-sheet loading are likely to affect glacigenic deformation, erosion and deposition above the diapir and within the rim synclines. Ice-load induced uplift in front of an ice sheet will provide favourable conditions for the formation of push moraines, for example by creating a topographic obstacle and inclining potential detachments. Subglacial subsidence of salt structures will enhance erosion by providing a preferential drainage pathway and fracturing of the overburden of the salt structure and thereby contribute to the incision of tunnel valleys. However, the resulting displacements are probably too low to have a marked effect on the advance or retreat pattern of the ice sheets.

Influence of palaeokarst morphology on the formation of ice-pushed ridges: a case study from Rejowiec, eastern Poland

This article describes the well-exposed landform/sediment assemblage of a Pliocene palaeouvala and Pleistocene (Saalian) ice-pushed ridge in a large quarry in Upper Cretaceous marls near Rejowiec, eastern Poland. The site provides a rare opportunity to study the development of glacial depositional and deformational processes on morphologically and structurally variable bedrock. The internal structures of both chronostratigraphic units which comprise this assemblage provide evidence of two principal stages of deformation associated with an ice advance on a karstified foreland, each stage preceded by stabilization of the ice mass. The first deformational stage was characterized by the development of imbricate structures in outwash sediments (the initial ice-pushed ridge) and listric faults at the contact between the bedrock and palaeouvala fills (caused by an increasing ice load). Shallow subglacial folding of the proximal fan deposits, and a listric thrust which limited the extent of the ice-pushed ridge under development occurred in the second deformational stage. The data assembled indicate that the formation of push moraines (in a general sense) during ice-sheet advance is controlled by the substratum undulations. Closed depressions with loose fills in the foreland of advancing ice sheets can control the deformational style, geometry and extent (both lateral and vertical) of the push moraines.

Submarine push moraine formation during the early Fennoscandian Ice Sheet deglaciation

The glacial history in the topographically confined paleo-ice stream drainage route of Vestfjorden, North Norway, was analysed based on bathymetric data, high-resolution seismology and 14C AMS-dated gravity cores. The inner part of the fjord is characterised by axial-parallel mega-scale lineations whereas the outer part is dominated by two marginal morainal bank systems. The Værøy (inner) and Røst (outer) marginal moraine systems comprise several transverse, zigzag-shaped ridges. Seismic records show thrusted and folded sediment blocks within the ridges. The landforms are inferred to reflect basal processes and the transition from warm-based (inner fjord) to cold-based (outer fjord) conditions, i.e. fast flow followed by basal freeze-on, sediment deformation and morainal bank formation. The moraines formed during the final part of two paleo-ice sheet re-advances. 14C AMS dating indicates a maximum age of 13.7 14C ka BP (16.2 cal ka BP) for the Røst system whereas the Værøy system is inferred to be slightly older than 12.5 14C ka BP (14.5 cal ka BP). This demonstrates that the northern part of the Fennoscandian Ice Sheet behaved in a much more dynamic way during the early deglaciation than previously assumed.

The morphology, structural evolution and significance of push moraines

Push moraines (glaciotectonic ice-marginal moraines) have a restricted distribution at modern glacier margins and consequently are of potential value in reconstructing Pleistocene ice sheets, providing data both on former glaciodynamics and on the palaeoenvironment of the glacial foreland. To the wider earth science community, push moraines are of interest as analogues for thin-skin tectonics within orogenic belts. This paper reviews the morphology, structural geology, formation and significance of push moraines. The morphological and structural characteristics of small, seasonal, push moraines through to large, multi-crested, examples produced by sustained glacier advances are reported, before the primary controls on push moraine formation are examined. These controls include the nature of the applied glacial stress field, the presence and properties of décollement horizons, and the shear strength and rheology of the glacial foreland. A conceptual model of push moraine formation is introduced, in which the range of observed morphological and structural forms are viewed within a matrix defined by the main variables which control their formation. The absence of consensus over which of these variables is of greatest importance currently limits the significance of push moraines in palaeoglaciological research. As a consequence, this review emphasises the need for future research in order to realise the true potential of push moraine in the reconstruction of Pleistocene environments.

Ice-marginal characteristics of Fridtjovbreen (Svalbard) during its recent surge

Fridtjovbreen is an actively surging tidewater glacier in western Spitsbergen, Svalbard. This paper presents observations made at the western flank of the glacier in July 1997, when the glacier front was still advancing rapidly and was characterised by steep lateral ice cliffs. The geomorphological consequences of the surge include the formation of push moraines from toppled ice blocks along this ice cliff and the development of debris-rich thrusts. There is also evidence of glaciotectonic deformation within deformed debris-rich thrusts. Thrusts are restricted to the lateral margins and are not seen in the terminal calving cliff. On its western flank, Fridtjovbreen is over-riding Sagabreen, a small-tributary glacier. Large facies variations in this area are a result of recycling by Fridtjovbreen of material originally transported as medial moraines on Sagabreen. There are few observations of landform development at actively surging glaciers in Svalbard. The Fridtjovbreen descriptions provide documentation for post-surge landform development. More observations are required at the margins of actively surging glaciers to determine how representative Fridtjovbreen is of a surge event.