Ice-flow Phases Research Articles

Deglaciation of Nova Scotia: Stratigraphy and chronology of lake sediment cores and buried organic sections

The deglaciation of Nova Scotia is reconstructed using the AMS-dated chronology of lake sediments and buried organic sections exposed in the basins of former glacial lakes. Ice cleared out of the Bay of Fundy around 13.5 ka, punctuated by a brief read- vance ca. 13-12.5 ka (Ice Flow Phase 4). Glacial Lake Shubenacadie (1) formed in central Nova Scotia, impounded by a lobe of ice covering the northern Bay of Fundy outlet. Drainage was re-routed to the Atlantic Ocean until the Fundy outlet became ice free after 12 ka. When this lake drained, bogs and fens formed on the lake plain during climatic warming. Organic sediment (gyttja) began to accumulate in lake basins throughout Nova Scotia. Glacierization during the Younger Dryas period (ca. 10.8 ka) resulted in the inundation of lakes and lake plains with mineral sediment. The nature and intensity of this mineral sediment flux or "oscillation" varies from south to northern regions. Southern lakes simply record changes in total organic content whereas northern lakes, where most buried peat sections are found, feature a thick inorganic sediment layer. Glacial ice or permanent snow cover and seasonal melting are essential in the formation of this mineral sediment layer; both to provide the water source for erosion, and to prevent plant re- colonization and landscape stabilization. Some northern lakes do not appear to record the Younger Dryas event, with organic accumulation starting around 10 ka. During the Younger Dryas, fine and coarse-grained deposits were deposited in Glacial Lake Shubenacadie (2) and other lowland areas at elevations similar to former (12 ka) lake levels, impounded by re-invigorated residual ice caps and permanent snow/aufeis.

An evolutionary model of glacial dispersal and till genesis in Maritime Canada

Abstract Glacier process models of till genesis cannot fully explain the properties of tills in Maritime Canada. A succession of local ice caps, called the Appalachian Ice Complex, developed during the last glaciation and were drained by ice streams into the submarine channels bordering the region. The migration of these centres produced areas with widely differing flow patterns, landform assemblages and deposits. Early regional phases of ice flow were characterized by wide, rapidly-flowing ice-streams that formed thick exotic, silty tills. In later phases, ice divides developed over highland areas underlain by metamorphic and igneous rocks, forming stony local tills. Terrain zones characterized by distinct transport histories and depositional sequences were produced by the interplay and migration of regional ice sheets and local ice caps. The interaction of local glaciers and previously deposited tills formed hybrid tills through two reworking processes: inheritance and overprinting. Inheritance is incorporation of till components and/or fabric into a younger till by erosion and entrainment of material from an older till. Overprinting is the injection or imprint of matrix, clasts or fabric on older tills by overriding ice. Glacial dispersal of rocks, minerals and trace elements in this complex glaciated terrain is controlled by the location of former ice divides. Simple unidirectional trains are preserved in relict terrains under divides. In ice marginal areas, reworking processes result in complex dispersal fans produced by smearing and widening of previously formed trains and fans. These dispersal fans can be modelled by vector addition of discrete flow events within each dispersal zone. A simple empirical model of glacier dispersal is presented using exponential decay and uptake algorithms, and incorporating the reworking of older till material.

Evidence for Heinrich event 1 in the British Isles

In the north Irish Sea basin (ISB), sedimentary successions constrained by AMS 14C dates obtained from marine microfaunas record three major palaeoenvironmental shifts during the last deglacial cycle. (i) Marine muds (Cooley Point Interstadial) dated to between 16.7 and 14.7 14C kyr BP record a major deglaciation of the ISB following the Late Glacial Maximum (LGM). (ii) Terminal outwash and ice-contact landforms (Killard Point Stadial) were deposited during an extensive ice readvance, which occurred after 14.7 14C kyr BP and reached a maximum extent at ca.14 14C kyr BP. At this time the lowlands surrounding the north ISB were drumlinised. Coeval flowlines reconstructed from these bedforms end at prominent moraines (Killard Point, Bride, St Bees) and indicate contemporaneity of drumlinisation from separate ice dispersal centres, substrate erosion by fast ice flow, and subglacial sediment transfer to ice-sheet margins. In north central Ireland bed reorganisation associated with this fast ice-flow phase involved overprinting and drumlinisation of earlier transverse ridges (Rogen-type moraines) by headward erosion along ice streams that exited through tidewater ice margins. This is the first direct terrestrial evidence that the British Ice Sheet (BIS) participated in Heinrich event 1 (H1). (iii) Regional mud drapes, directly overlying drumlins, record high relative sea-level (RSL) with stagnation zone retreat after 13.7 14C kyr BP (Rough Island Interstadial). Elsewhere in lowland areas of northern Britain ice-marginal sediments and morainic belts record millennial-scale oscillations of the BIS, which post-date the LGM advance on to the continental shelf, and pre-date the Loch Lomond Stadial (Younger Dryas) advance in the highlands of western Scotland (ca. 11–10 14C kyr BP). In western, northwestern and northern Ireland, Killard Point Stadial (H1) ice limits are reconstructed from ice-flow lines that are coeval with those in the north ISB and end at prominent moraines. On the Scottish continental shelf possible H1-age ice limits are reconstructed from dated marine muds and associated ice marginal moraines. It is argued that the last major offshore ice expansion from the Scottish mountains post-dated ca. 15 14C kyr BP and is therefore part of the H1 event. In eastern England the stratigraphic significance of the Dimlington silts is re-evaluated because evidence shows that there was only one major ice oscillation post-dating ca.18 14C kyr BP in these lowlands. In a wider context the sequence of deglacial events in the ISB (widespread deglaciation of southern part of the BIS → major readvance during H1 → ice sheet collapse) is similar to records of ice sheet variability from the southern margins of the Laurentide Ice Sheet (LIS). Well-dated ice-marginal records, however, show that during the Killard Point readvance the BIS was at its maximum position when retreat of the LIS was well underway. This phasing relationship supports the idea that the BIS readvance was a response to North Atlantic cooling induced by collapse of the LIS. © 1998 John Wiley & Sons, Ltd.

Dating and rhythmicity from the last deglacial cycle in the British Isles

In northern Britain glaciotectonically thickened sediment piles represent the sediment flux transferred to ice margins by phases of fast ice flow and drumlin streamlining during the last deglacial cycle. Radiocarbon dating of marine faunas within deglacial muds age-constrain phases of drumlinisation (shaping) to between 19 and 14.5 ka bp . Individual phases of drumlinization can be correlated with millennial time scale discharge events documented from the circum-North Atlantic which are probably climate-driven. The overall deglacial cycle records at least five major millennial time-scale ice-margin oscillations and phases of sediment sequence rhythmicity.

Ice-flow patterns and glacial transport in the eastern Hudson Bay region: implications for the late Quaternary dynamics of the Laurentide Ice Sheet

Recent field surveys in the eastern Hudson Bay region have led to the discovery of regional ice-flow sequences that require a significant reassessment of the late Quaternary dynamics of the Laurentide Ice Sheet. Two regional ice-flow phases can be recognized from till compositional data and from crosscutting relationships observed on striated bedrock surfaces: the oldest is directed toward the northwest and north-northwest, while the youngest is directed toward the west and includes a late-glacial deflection toward the southwest. The wide regional distribution of striae formed during the early northwestward glacial movement together with the recognition of palimpsest glacial dispersal trains associated with this phase suggest that it was a long-lived, time-transgressive regional event. The ensuing glacial movement is a regionally dominant westward ice-flow phase during which several large glacial dispersal trains were formed downglacier from distinctive bedrock sources. The largest of these trains extends westward over a distance of 120 km from Lac à l'Eau Claire to Hudson Bay. Regional glacial transport data as well as glacial and deglacial landforms indicate that this was a long-lived glacial phase, likely lasting throughout the Late Wisconsinan glacial maximum and until déglaciation about 8000 BP. The erosional and depositional record of the northwestward ice-flow event is quite comparable to that of the ensuing glacial phase, and it is thus thought to represent the Early Wisconsinan glacial maximum. In view of the large regional extent of the northwestward ice-flow phase, it must postdate the early buildup of the ice sheet. Along the southeastern Hudson Bay coast, the Late Wisconsinan westward glacial movement was followed by a southwestward deflection that was likely caused by glacial streaming prior to 8000 BP in James Bay, in response to calving and surging into Glacial Lake Ojibway.

Mega‐scale glacial lineations and cross‐cutting ice‐flow landforms

AbstractLandsat images reveal a previously unsuspected large‐scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow. Such a glacial grain can be regarded as a landform assemblage comprised of a number of components. Drumlins and megaflutes form part of the pattern, but in addition there are two previously undocumented ice‐moulded landform elements: streamlined lineations of much greater proportions, referred to as mega‐scale glacial lineations, and a distinctive cross‐cutting topology within the grain.The ice‐moulded landform assemblage is described and illustrated with reference to examples from Canada. Possible modes of genesis of such landforms are discussed and their glaciological implications outlined. The discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment, and demands a radical re‐interpretation of ice sheet dynamics.

Secondary copper iodide in till in the Löytösuo area, Ylikiiminki, Finland

Till geochemical studies were carried out in the Ylikiiminki area in northern Finland while prospecting for tungsten and sulphide ores. The area forms part of the Proterozoic schist belt of northern Ostrobothnia. Lodgement till was deposited on the bedrock surface in an earlier ice flow phase, followed during the later deglaciation by looser till types overlaid in succession by beach gravels and sands. A special postglacial type of Cu anomaly was found at Löytösuo, the nature of which was studied using partial extractability analyses and mineralogical investigations. Copper was found to be present in the form of both primary and secondary sulphides and CuI (marshite) was found as a rare mode of occurrence. The source of the I is possibly related to enrichment in algae in the Litorina phase of the Baltic Sea; in general the more iodine-rich soils occur within the sphere of this brackish water stage.

Variation in drumlin orientation, form and stratigraphy relating to successive ice flows in southern and central Nova Scotia

A strong correspondence exists between striation and drumlin long-axes trends in Nova Scotia. The variation in striation trends is largely due to successive ice flow events which inscribed many outcrops with sets of superimposed striations that can be correlated over broad areas of Nova Scotia. A type of drumlin in the Nova Scotia fields is the “lobate” or “reoriented” drumlin which is a form with two principal axes parallel to ice movements established by striations and dispersal of distinctive erratics. The internal structure of Nova Scotia drumlins is generally a layer cake of till sheets. The most common situation is a grey “core” till formed by a southeastward ice flow across grey metasedimentary bedrock and a distinctly red “mantle” till formed by phases of southward to southwestward ice flow. Drumlins in the Yarmouth field of southern Nova Scotia exhibit the greatest number of stacked units capped by a coarse-textured till that represents deposition by a late stage ice mass centered inland. Nova Scotia drumlins appear to have a multi-phase origin. The first and major drumlin-forming ice flow phase was southeastward. Nucleating agents for initiation of these drumlins may have been preexisting tills or waterlain deposits. Individual and lobate drumlin axes imply a late, southward drumlin-forming ice flow phase parallel to the inferred ice flow trend for the mantle till units based on clast provenance. Some of these drumlins nucleated from preexisting southeast-trending forms.