Glacitectonic Deformation Research Articles

New insights into the structural development and shortening of the southern Jasmund Glacitectonic Complex (Rügen, Germany) based on balanced cross sections

Late Pleistocene glacitectonism at the southern Scandinavian Ice Sheet margin caused folding and thrusting of Upper Cretaceous chalk layers and Pleistocene glacial deposits in parts of the southwestern Baltic Sea area in Europe. Beside Møns Klint (SE Denmark), the Jasmund Glacitectonic Complex (JGC) on Rügen Island (NE Germany) is a similar striking example of glacitectonic deformation creating large composite ridges. In spite of a long research history and new results from modern datasets, the structural development of the JGC is still poorly understood, especially the detailed evolution of the southern JGC and its relationship to the northern JGC remain enigmatic. In this contribution, we demonstrate how the understanding of the JGC benefits from the application of established structural geological methods comprehending the formation of fold-and-thrust belts. The methods include cross-section balancing of the eastern coast (southern JGC) and quantification of the amount of folding and faulting. The proposed geometric model shows the current fold-and-thrust stack of glacially deformed sedimentary strata ca. 5720 m in length evolved by shortening from the original length (11,230 m) by 5510 m (49.1%). We present a spatial and temporal development of fault-related folding with a transition from detachment folds through fault-propagation folds to fault-bend folds. Together with morphological information from a digital elevation model, the thrust faults mapped in the cliff section are mainly inclined towards the S to SW and imply that a local glacier push occurred from the south. These results highlight the complexity and individual architecture of the JGC when compared to other Pleistocene and modern glacitectonic complexes. Resolving its structural development provides new insight into the deformation history and shortening of this spectacular glacitectonic complex lying in the southwestern Baltic Sea region.

Testing a proposed new chronology for the Jasmund Glacitectonic Complex (SW Baltic Sea): No indication of incipient deformation during MIS 3

Weichselian advances of the Scandinavian Ice Sheet have generated several glacitectonically deformed structures in the southwestern Baltic Sea area. One example is the 100 km 2 large Jasmund Glacitectonic Complex (JGC), which was formed proglacially and consists of two subparallel-orientated sets of composite ridges that represent a northern and southern structural complex. The two-part morphological structure of the JGC suggests a formation by two ice advances, one approaching from NE and one from SE direction. So far, this divided structure has been assumed to have been formed by short-time ice-front oscillations during an MIS 2 ice advance. However, based on their recently published ice dynamic model for MIS 3 and the available age data from Jasmund, lüthgens et al. (2020) propose a chronological reinterpretation of the JGC development, according to which two distinct ice advances during early and late MIS 3 formed the JGC. In order to test this novel stratigraphical model for the JGC formation, five OSL samples were taken from fluvial and lacustrine deposits at a key section near Glowe (NW Jasmund). The investigated succession is divided into pre-kinematic sediments, deposited before the glacitectonic deformation, and post-kinematic sediments, deposited after the deformation. Our results show that the youngest dated pre-tectonic sediment has a burial age between ∼40 and 34 ka, which rules out a glacitectonic deformation during an early MIS 3 ice advance (∼60–50 ka). In addition, by reviewing the existing age data set, a development of the JGC during an early and late MIS 3 advance of the SIS must be rejected. Instead, our data confirm the genesis of the JGC during MIS 2.

The multistage structural development of the Upper Weichselian Jasmund Glacitectonic Complex (Rügen, NE Germany)

Glacitectonic deformation in the Quaternary caused the tectonic framework of large-scale folds and displaced thrust sheets of Maastrichtian (Upper Cretaceous) chalk and Pleistocene glacial deposits in the southwestern Baltic Sea area. A wide spectrum of methods has been compiled to unravel the structural evolution of the Jasmund Glacitectonic Complex. The analyses of digital elevation models (DEM) suggest a division into two structural sub-complexes – a northern part with morphological ridges striking NW–SE and a southern part with SW–NE trending ridges. Geological cross sections from the eastern coast (southern sub-complex) were constructed and restored using the software Move™ and the complementary module 2D Kinematic Modelling™. The final geometric model of the southern sub-complex shows a small-scale fold-and-thrust belt. It includes three different orders of architectural surfaces (see PEDERSEN, 2014): erosional surfaces and the decollement (1st order), thrust faults (2nd order), and beds outlining hanging-wall anticlines as well as footwall synclines (3rd order). Thrust faults of the southern structural sub-complex are mainly inclined towards south, which indicates a local glacier push from the S/SE. The glacitectonic structures have a surface expression in form of sub-parallel ridges and elongated valleys in between. Geomorphological mapping and detailed landform analyses together with the structural investigations provide an insight into the chronology of sub-complexes formation. The northern part of the glacitectonic complex is suggested to have been formed before the southern one, considering the partly truncated northerly ridges and their superimposition by the southern sub-complex. Although there is a high number of scientific publications on the glacitectonic evolution of Jasmund, these presented models often lack a consistent theory for the development integrating all parts of the 100 km2 large complex. Therefore, the combination of all results leads to a more self-consistent genetic model for the entire Jasmund Glacitectonic Complex.

Synsedimentary glacitectonic deformation within a glacilacustrine-esker sequence, Teesdale, Northern England

In glacial sedimentology there has been a recent improvement in the understanding of both progressive and polyphase deformation of glacigenic sequences, and the role played by water during these complex deformation histories. However, the processes occurring during the detachment and transport of sediment blocks during ice-marginal glaciotectonic thrusting remain poorly understood. This lack of understanding is addressed in detail through a macro- and microscale study of the deformation structures in the glacigenic sequence exposed at Hayberries, Teesdale, County Durham (UK), where esker sands and gravels and associated tills truncate and overlie a sequence of rhythmically bedded glacilacustrine sands, silts and clays. Thrusts within the glacilacustrine and glacifluvial sediments appear to be relatively sharp, planar structures. However, orientated thin sections reveal that these bedding-parallel detachments are marked by a thin layer of massive to foliated sand. The geometry of both meso and small-scale folds and sense of displacement on the thrusts is consistent with both brittle and ductile structures having formed in response to ice-push from the N/NW. Detailed analysis of the thin sections reveals that initial folding and thrusting was followed by the liquefaction and injection of a massive, matrix poor sand along the propagating thrust. Evidence for liquefaction and injection (sand-filled veins) increases towards the NW consistent with fluid flow and sediment injection accompanying SE-directed ice-push. These results suggest that the introduction of pressurised meltwater and sediment along the thrusts during deformation may facilitate decoupling and displacement along these detachments by thrust gliding.

Coastal cliff at Lenzer Bach on Jasmund Peninsula, Rügen Island (Pleistocene Stripe 4): reconstructed history of glacitectonic deformation based on fold geometry and microstructural mapping

Abstract. A thrust-bound footwall syncline located within the proximal part of the southern Jasmund Glacitectonic Complex is investigated, exploring the spatio-temporal relationship between glacitectonic macro- and microstructures. Orientation and geometry of macroscale folds and thrust faults reveal a two-phased deformation history recorded by the sedimentary sequence. The deformation is a result of glacitectonic imbrication and subsequent ice flow across Jasmund Peninsula during the late Weichselian. Clast microfabrics preserved within the folded glacial diamicts reveal that till-internal deformation is mainly related to subglacial shearing within the glacier bed, which predates large-scale imbrication and folding.

A tale of two rift shoulders, and two ice masses: the Cryogenian glaciated margin of Death Valley, California

Abstract The Death Valley area of California, USA, exposes an outstanding record of a Neoproterozoic (Cryogenian) glaciated margin: the Kingston Peak Formation. Despite the quality of the exposure, however, the outcrops of glaciogenic strata are fragmentary, forming isolated, laterally offset outcrop belts at the western extremity of the Basin and Range province. Excellent evidence for glacially modulated sedimentation includes (1) ice-rafted dropstones in most ranges, (2) thick diamictites bearing a variety of exotic (extrabasinal) clasts, (3) striated clasts and (4) local occurrences of glacitectonic deformation structures at the basin margins. In tandem with this, there is a distinct signature of slope collapse processes in many ranges, including (1) up to kilometre-scale olistoliths, (2) extensional growth fault arrays, (3) dramatic proximal-distal thickness changes and (4) basalt occurrences. New sedimentological observations reinforce long-held views of rifting superimposed on glaciation (or vice versa), with both processes contributing to a complex record whereby rift and glacial processes vie for stratigraphic supremacy. We consider that a mechanism of diamictite accumulation in a series of rift-shoulder minibasins produced greatly contrasting successions across the Death Valley area, under the incontrovertible influence of hinterland ice sheets.

Geomorphological Mapping and Spatial Analyses of an Upper Weichselian Glacitectonic Complex Based on LiDAR Data, Jasmund Peninsula (NE Rügen), Germany

Glacitectonic deformation in the Upper Weichselian led to the tectonic framework of large-scale folds and displaced thrust sheets of Maastrichtian (Upper Cretaceous) chalk and Pleistocene glacial deposits in the southwestern Baltic Sea region. They form surface expressions of sub-parallel ridges and elongated valleys in between and on the Jasmund Peninsula. Geomorphological mapping and detailed landform analyses give another insight into the arrangement and the formation history of these proglacial surface structures. Light detection and ranging (LiDAR) digital elevation models (DEM) analysis techniques were applied to a proglacial rather than a subglacial environment. Results suggest a division into a northern part with morphological ridges striking NW–SE and a southern part with SW–NE trending ridges. The observation of partly truncated northerly ridges and their superimposition by the southern sub-complex suggest that the northern part was generated earlier than the southern part. The applied spatial analyses tools were used to develop a new, self-consistent genetic model integrating all parts of the 100 km2 large Jasmund Glacitectonic Complex. Results suggest a more consistent terminology for the tectonic setting and a revised genetic model for Jasmund, including three evolutional stages that are characterized by different ice flow patterns.

Large-scale glacitectonic deformation in response to active ice sheet retreat across Dogger Bank (southern central North Sea) during the Last Glacial Maximum

High resolution seismic data from the Dogger Bank in the central southern North Sea has revealed that the Dogger Bank Formation records a complex history of sedimentation and penecontemporaneous, large-scale, ice-marginal to proglacial glacitectonic deformation. These processes led to the development of a large thrust-block moraine complex which is buried beneath a thin sequence of Holocene sediments. This buried glacitectonic landsystem comprises a series of elongate, arcuate moraine ridges (200 m up to > 15 km across; over 40–50 km long) separated by low-lying ice marginal to proglacial sedimentary basins and/or meltwater channels, preserving the shape of the margin of this former ice sheet. The moraines are composed of highly deformed (folded and thrust) Dogger Bank Formation with the lower boundary of the deformed sequence (up to 40–50 m thick) being marked by a laterally extensive décollement. The ice-distal parts of the thrust moraine complex are interpreted as a “forward” propagating imbricate thrust stack developed in response to S/SE-directed ice-push. The more complex folding and thrusting within the more ice-proximal parts of the thrust-block moraines record the accretion of thrust slices of highly deformed sediment as the ice repeatedly reoccupied this ice marginal position. Consequently, the internal structure of the Dogger Bank thrust-moraine complexes can be directly related to ice sheet dynamics, recording the former positions of a highly dynamic, oscillating Weichselian ice sheet margin as it retreated northwards at the end of the Last Glacial Maximum.

Glacitectonism, subglacial and glacilacustrine processes during a Neoproterozoic panglaciation, north‐east Svalbard

AbstractPalaeoenvironmental reconstruction of Neoproterozoic successions has been the subject of long‐standing debate, particularly concerning the interpretation of diamictites. The Wilsonbreen Formation of north‐east Svalbard is a 130 to 180 m thick diamictite‐dominated glacigenic succession deposited during a late Cryogenian (Marinoan) glaciation. Previous research has highlighted a complex sedimentary architecture with evidence of subaqueous, subglacial and non‐glacial conditions. This study combines well‐established sedimentological techniques with the first sedimentological application of the anisotropy of magnetic susceptibility technique in Neoproterozoic glacial sediments, to investigate the origin and palaeoenvironmental significance of glacigenic sediments within the Wilsonbreen Formation. A range of lithofacies occurs within the succession, dominated by massive diamictites, sandstones and conglomerates. Some of these facies display evidence of primary deformation and can be grouped into a Deformed Facies Association; these are interpreted to have been formed through glacitectonic deformation in a subglacial environment. Fabric investigation reveals that this deformation was associated with glacier flow towards the north. In addition, an Undeformed Facies Association records deposition in ice‐proximal and ice‐distal subaqueous environments. Taken together with intervening non‐glacial facies, the glacigenic sediments record a series of advance–retreat cycles, with ice flow involving sliding and sediment shearing below wet‐based ice.

Aligned glacitectonic rafts on the floor of the central Barents Sea

Numerous mounds are found aligned in a SW-trending chain on the floor of the central Barents Sea and are interpreted as glacial rafts. Glacitectonic megablocks and rafts occur widely in glaciated lowlands, as shown by many terrestrial examples of flat-topped allogenic pieces of sedimentary rock or till (Ringberg et al. 1984; Ruszczynska-Szenajch 1987; Aber 1988). In the Barents Sea, Cenozoic–Mesozoic subcropping sedimentary bedrock appears particularly susceptible to glacitectonic deformation and erosion, as shown by the occurrence of megablocks and rafts within Pleistocene glacial sediments (Saettem 1994; Andreassen et al. 2004). Terrestrial and marine examples both show that megablocks and rafts tend to be found within till rather than at its surface and such features are, therefore, rare as seafloor landforms. About 1300 flat-topped mounds are present on the seafloor in the Bear Island Trough in the central Barents Sea, at water depths of c. 350 m (Fig. 1). The most pronounced mounds, with heights up to 28 m above the surrounding seafloor, occur aligned in a chain extending for at least 50 km. Mounds tend to be elongate with their preferred orientation varying across the area. Overall, the chain of mounds trends NE–SW, roughly parallel to …

Drumlin sedimentology in a hard‐bed, lowland setting, Connemara, western Ireland: implications for subglacial bedform generation in areas of sparse till cover

ABSTRACTCores of coastal drumlins in Connemara contain stratified diamictons that interdigitate with gravelly clinoforms and finer grained rhythmites. The diamictons are interpreted as subaqueous mud apron deposits delivered by subglacial till advection to continuously failing subaqueous ice‐contact fans, whose strata were being syn‐depositionally over‐steepened by glacitectonic deformation. The localized nature of the stratified sediments reflects the emergence of subglacial deforming tills and meltwater deposits in a glacilacustrine environment to produce interdigitated mass flow diamictons and grounding line fans/wedges. These depo‐centres became glacitectonized and subglacially streamlined during glacier overriding and hence regional drumlin sedimentology reflects the varying degrees of inheritance of pre‐existing glacigenic deposits and suggests that drumlin production relates more to the position of localized sediment accumulations at the glacier bed than full‐depth till deformation processes (e.g. instability mechanisms) within the same drumlin field. Till cored drumlins give way down ice to stratified cored drumlins with till caps and then to stratified drumlins. This zonation is compatible with the increased lateral variability in drumlin composition that would arise from the occurrence of linear assemblages of glacifluvial (esker) and subaqueous (grounding line) sediments in an otherwise marginal‐thickening till sheet.

The Middle Pleistocene tunnel valley at Schöningen as a Paleolithic archive

Schöningen represents one of the key sites for Lower Paleolithic archaeology in central Europe, where a Middle to Late Pleistocene sedimentary succession, locally up to 45 m thick, has been preserved in an Elsterian tunnel valley. After deglaciation, the tunnel valley remained underfilled and provided the accommodation space for Holsteinian interglacial deposition and also kept the artifact-bearing strata below base level for subsequent erosion. The Holsteinian (MIS 9) succession consists of laterally and vertically stacked lacustrine delta systems, which were controlled by repeated lake-level changes. In the face of changing climatic and environmental conditions the long-lived interglacial lake provided an attractive site for animals and early humans. Artifacts were deposited on the subaerial delta plain and became embedded during lake-level rise. Although the area was considerably affected by erosion and glacitectonic deformation during the subsequent Saalian glaciation, the artifact-bearing Holsteinian strata were preserved in the deeper part of the tunnel valley.Tunnel valleys should be regarded as potential archives for interglacial deposits, which may contain important Paleolithic sites. Tunnel valleys may provide accommodation space and also have a high preservation potential. Interglacial lakes situated within underfilled tunnel valleys represented attractive sites for animals and early human hunter-gatherers.

Osady paleogenu i neogenu w wielkoskalowych strukturach glacitektonicznych Wzgórz Dylewskich

PALEOGENE AND NEOGENE SEDIMENTS IN MEGA-SCALE GLACIOTECTONIC STRUCTURES OF THE DYLEWSKIE HILLS Abstract. The Wysoka Wieś borehole located in the center of Dylewskie Hills (NE Poland) reached a depth of almost 490 metres, with more than 400 metres of the Paleogene and the Neogene deposits. Palynological studies were designed to determine whether this huge Paleogene and Neogene sedimentary complex is located in situ or it has been glacitectonically redeposited. Forty-seven samples were analysed for all palynological matter components, i.e. palynomorphs (sporomorphs, phytoplankton, zoomorphs), palynoclasts (phytoclasts) and inorganic debris. Among the palynomorphs, 13 assemblages have been distinguished. Describing these assemblages from the bottom to the top of the section, dual or triple repetition of the same palynomorph zones indicates a strong glacitectonic deformation of the sediments. Palynological data evidence several disengagement zones and displacement of rock packages. The lowest sedimentary complex occurs in the primary position and it contains the pollen and phytoplankton assemblage D3–D4 of the Middle Paleocene to the Upper Paleocene (terrestrial facies), and the Lower Eocene assemblage D8. At a depth of ca . 455m there is the first discontinuity zone, along which the over 100-metres thick package of Middle Miocene sediments, containing pollen zones V, VI, VIII, IX and XI, became intruded/ pressed into the Paleogene deposits. The next disengagement zone occurs at a depth of ca . 339 m, where the Miocene sediments are overlain by a about 60-metres thick package of Paleogene deposits dated with the stratigraphically younger pollen and phytoplankton zones D12 of the Upper Eocene and Lower Oligocene (terrestrial facies). These Paleogene deposits were intruded by the Miocene series, a few metres thick, representing pollen zone IX. Then, at a depth of 240 m, occurs the Middle Miocene sedimentary complex, ca . 150m thick, with pollen zones V, VII and IX. The complicated glacitectonic processes caused such mosaic succession, reflecting the slice structure of glaciotectonic thrust moraines. Palynological studies provided unique data to define the glaciotectonic disengagement zones. The absence of Quaternary deposits in the internal moraine structure indicates the Early Pleistocene age of deformation.

Glacitectonic deformation in the Chuos Formation of northern Namibia: implications for Neoproterozoic ice dynamics

Abstract The Chuos Formation is a diamictite-dominated succession of Cryogenian age, variously interpreted as the product of glaciomarine deposition, glacially related mass movement, or rift-related sediment remobilisation in a non-glacial environment. These interpretations have wide ranging implications for the extent of ice cover during the supposedly pan-global Neoproterozoic icehouse. In the Otavi Mountainland, northern Namibia, detailed analysis of soft-sediment deformation structures on the macro- and micro-scale support glacitectonic derivation in response to overriding ice from the south/south-east. Overall, the upward increase in strain intensity, predominance of ductile deformation features (e.g. asymmetric folds, rotational turbates and necking structures, clast boudinage, unistrial plasmic fabrics) and pervasive glacitectonic lamination support subglacial deformation under high and sustained porewater pressures. In contrast, soft-sediment structures indicative of mass movements, including flow noses, tile structures, and basal shear zones, are not present. The close association of subglacial deformation, abundant ice-rafted debris and ice-contact fan deposits indicate subaqueous deposition in an ice-proximal setting, subject to secondary subglacial deformation during oscillation of the ice margin. These structures thus reveal evidence of dynamic grounded ice sheets in the Neoproterozoic, demonstrating their key palaeoclimatic significance within ancient sedimentary successions.

The glacial sedimentology and geomorphological evolution of an outwash head/moraine-dammed lake, South Island, New Zealand

Extensive exposures through the glacial landforms around southern Lake Pukaki, New Zealand, comprise seven lithofacies (LFs 1–7). LFs 1–3 are grouped together as LFA 1 (Pukaki Member) and record pulsed subaqueous grounding line fan progradation, cohesionless debris flows, underflow activity and rhythmite deposition by suspension settling, iceberg rafting of dropstones, and pulsed traction current activity. Localized disturbance of these deposits by glacitectonic deformation and multi-generational hydrofracture fills records minor readvances by the glacier snout and the emplacement of a glacitectonite (LF 4) derived from cannibalization of glacilacustrine sediments. LFs 4–6 are grouped together as LFA 2 (Twizel Member) and record direct glacigenic deposition of glacitectonite (LF 4), subglacial traction till (LF 5) and supraglacially dumped boulder rubble (LF 6). Stratigraphic relationships between LFA 1 with LFA 2 record the oscillatory behaviour of the former Tasman Glacier snout when it formed a calving margin in a proglacial and locally supraglacial lake dammed by a glacitectonically disturbed outwash head and lateral moraine ridges. This is entirely consistent with the landform–sediment record of its coeval terrestrial margins, where flutings and push moraines are diagnostic of active temperate glacier recession from a glacially overridden outwash head, the latter being recorded by the vertically stacked sub-horizontally bedded and coarse-grained gravels of LF 7 (Waitaki Member). Previous proposals that late Pleistocene lake damming was initiated by an ice-cored moraine arc appear unfounded, because the glacilacustrine deposits only lie above the altitude of the outwash head/lateral moraine arc in locations where they have been glacitectonically compressed. Alternatively, it is proposed that the overdeepened subglacial topography was produced by the construction of an outwash head, leading to a glacilacustrine sediment sink which operates at times when the expanded Tasman Glacier actively retreats from the outwash head apex. The changing landsystem imprint related to the shrinkage of the Tasman Glacier records spatio-temporal landsystem change, involving evolution from a coupled landsystem to a moraine-dammed to an uncoupled landsystem.

Periglacial disruption and subsequent glacitectonic deformation of bedrock: an example from Anglesey, North Wales, UK

Abstract The deformed metasedimentary bedrock and overlying diamictons in western Anglesey, NW Wales, record evidence of glacier-permafrost interactions during the Late Devensian (Weichselian). The locally highly brecciated New Harbour Group bedrock is directly overlain by a bedrock-rich diamicton which preserves evidence of having undergone both periglacial (brecciation, hydrofracturing) and glacitectonic deformation (thrusting, folding), and is therefore interpreted as periglacial head deposit. The diamicton locally posses a well-developed clast macrofabric which preserves the orientation of the pre-existing tectonic structures within underlying metasedimentary rocks. Both the diamicton and New Harbour Group were variably reworked during the deposition of the later Irish Sea diamicton, resulting in the detachment of bedrock rafts and formation of a pervasively deformed glacitectonite. These structural and stratigraphic relationships are used to demonstrate that a potentially extensive layer of permafrost developed across the island before it was overridden by the Irish Sea Ice Stream. These findings have important implications for the glacial history of Anglesey, indicating that the island remained relatively ice-free prior to its inundation by ice flowing southwards down the Irish Sea Basin. Palynological data obtained from the diamictons across Anglesey clearly demonstrates that they have an Irish Sea provenance. Importantly no Lower Palaeozoic palynomorphs were identified, indicating that it is unlikely that Anglesey was overridden by ice emanating from the Snowdon ice cap developed on the adjacent Welsh mainland. Permafrost was once again re-established across Anglesey after the Irish Sea Ice Stream had retreated, resulting in the formation of involutions which deform both the lower bedrock-rich and overlying Irish Sea diamictons.

Sand intraclasts as evidence of subglacial deformation of Middle Pleistocene permafrost, North Norfolk, UK

Sand intraclasts are common within the Bacton Green Till Member, a glacitectonic mélange subjected to polyphase deformation during the Middle Pleistocene in North Norfolk, UK. The intraclasts range from a few tens of centimetres to >10 m in length and have sharp contacts with the surrounding till. Sand within the intraclasts is unconsolidated and contains well-preserved primary stratification. The wrapping of glacitectonic foliation around the intraclasts and the development of folds relating to mechanical instabilities indicate that the intraclasts acted as competent masses within a more easily deformable fine-grained till that accommodated the majority of the strain. Sharp contacts and distinctive heavy-mineral assemblages indicate little intermixing between the sand and till. Five hypotheses about the entrainment and evolution of the intraclasts are tested against sedimentological, structural and mineralogical observations. The most reasonable hypothesis attributes the intraclasts to glacitectonic deformation of “warm” permafrost. Initial ice advance caused large-scale thrusting of proglacial permafrost that led to the stacking of pre-glacial and ice-marginal sediments that were subsequently deformed sub-marginally to generate the intraclasts. Preservation of primary stratification within the intraclasts is attributed to deformation at temperatures slightly below the pressure-melting point, when pore ice cemented the intraclasts as rigid bodies. At the same time deformation was concentrated into the surrounding finer-grained till because of its significant liquid water content and ductile rheology. It is concluded that the intraclasts provide a criterion to identify past glacier–permafrost interactions and a potential means of differentiating between subglacial deformation under unfrozen and partially-frozen conditions.

The stratigraphy, timing and climatic implications of glaciolacustrine deposits in the middle Rakaia Valley, South Island, New Zealand

Stratigraphic interpretations and infrared stimulated luminescence (IRSL) ages document the glacial history of the Middle Rakaia Gorge section of the Rakaia Valley, Canterbury, New Zealand, during the last two glacial cycles. Sheets of glaciolacustrine sediments several tens of metres in thickness can be traced at least 10 km upstream of the Rakaia Gorge. The stratigraphic package is capped by outwash gravels associated with the last glacial maximum (LGM) advances. The dominant inferred sedimentary environments in the sequence are 1) pro-glacial and paraglacial lake beds, 2) sub-aqueous ice-contact fans, 3) sub-aqueous mass flow deposits 4) supraglacial dump material and 5) outwash gravels. Syndepositional deformation, associated with glacitectonic deformation, is common. The stratigraphy records glacier margin oscillations, including six significant advances. These occurred in early MIS 6, mid-MIS 6, MIS 5b (c100–90 ka), MIS 5a/4 (c 80 ka), mid-MIS 3 (c 48 ka), and late MIS 3 (c 40ka). All the post-MIS 6 advances can be corroborated from other sites in New Zealand and the timings appear to coincide with both Southern Hemisphere insolation minima and maxima, suggesting variable combinations of climatic forcing in New Zealand glaciation.

The glacial sequence at Killiney, SE Ireland: terrestrial deglaciation and polyphase glacitectonic deformation

Depositional conditions of a complexly deformed glacigenic sequence at Killiney Bay, on the west-central margin of the Irish Sea are reconstructed. Deformation geometries provide conclusive evidence for polyphase glacitectonic deformation generated by terrestrial ice sheets. They are dominated by pure shear, simple shear and compressional deformation styles with abundant evidence for glacitectonic induced fracturing and hydrofracturing. There is no evidence of subaqueous sediment flow-induced deformation and impact deformation (iceberg scouring, rained out clasts). Glacigenic diamicton facies associations (FA's) are interpreted as glacitectonically stacked diachroneous units younging laterally away from palaeo-ice margins. Simultaneous deposition of proximal overconsolidated subglacial tills and distal low viscosity flowtills occurred, with the latter being deformed during re-advances. Structural chronology of deformation styles demonstrates polyphase deformation of tills, where glacial overriding produced overconsolidated and polyphase deformed subglacial tills from sediments initially deposited as low viscosity flowtills. It is proposed that relatively late during deglaciation, after retreat of a grounded Irish Sea Ice Sheet, Midland Ice invaded the area and deformed and reworked recessional Irish Sea Ice sequences. Deformation of these recessional sequences led to a heterogeneous stack of polyphase deformed glacigenic sediments with abundant hydrofracturing. The Killiney Bay sequence and other sequences in the Southern Irish Sea Basin demonstrate that MIS 2 deglaciation took place under terrestrial conditions with lower relative sea levels than present.

Glacitectonic Deformation in Sediment and Bedrock, Hat Creek, British Columbia

A variety of deformation structures attributed to glacial overriding occur in rock and sediment of an intermontane valley at Hat Creek, British Columbia. Sediments exposed in vertical outcrops along Hat Creek, display contrasting styles of deformation involving fluidization, as well as brittle and ductile deformations that appear to have been formed concurrently. Typical structures include: joints, faults, infillings, and clastic dikes comprising; fluid-escape structures, glacigenic injections, as well as fluidal and viscous hydraulic expulsions. A model is presented for the glacitectonic formation of hydraulic expulsions during compression of underlying partially saturated unfrozen sediments. Bedrock exposed in excavations at higher elevations displays joints, faults and wedge fillings possibly associated with subglacial freezing during glacial advance. Orientation of the structures are correlative with directions of glacier flow as inferred from fabric, striae and geomorphology. The structures are believed to have been the product of several interrelated factors, including: glacial dynamics, engineering properties of the glacier bed material, subglacial relief, and the variation between coalescing glaciers. Correlation with directions of glacial movement, association with glacial faciès and infilling by glacial sediments, are conditions considered to be glacigenic signatures useful in differentiation of glacial from nonglacial (e.g. earthquake) origins for similar structures elsewhere.