Interactions of Arctic and Atlantic water-masses and associated environmental changes during the last millennium, Hornsund (SW Svalbard)

The fjords of southwestern Spitsbergen (European Arctic) are a climatically sensitive area neighbouring the mixing zone of warm northward-flowing Atlantic water-masses and cold Arctic Water. Owing to reasonably high accumulation rates, these settings are especially suitable for providing high-resolution sedimentary records of regional hydrological and environmental changes. A sediment core spanning the last millennium was retrieved from the outer Hornsund fjord basin, ¹⁴C dated and analysed for sediment grain size, ice-rafted debris (IRD), the distribution of benthic foraminifera and their oxygen and carbon stable isotope composition. The record of sub-centennial resolution reveals three distinctive periods: the Medieval Warm Period, the Little Ice Age (∼AD 1600–1900) and 20th-century warming. The marine record obtained is well correlated with regional high-resolution ice-core records as well as with atmospheric palaeotemperature reconstructions and sea-ice data. The colder periods stay in phase with the greater influence of less saline, cold Arctic Water indicated by subtle changes in benthic foraminifera assemblages and the δ¹⁸O signal, which is dominated by changes in salinity. The IRD record clearly indicates that tidewater glaciers were present in SW Spitsbergen throughout the last millennium, and most actively from the late 16th century until the end of the 19th century.     

Exhumed rotational slides and scar infill features in a Cretaceous delta front, eastern Spitsbergen

Coastal cliff exposures in the Helvetiafjellet Fm. deltaic succession near Kvalvågen on eastern Spitsbergen show spectacular evidence of syndepositional, gravity-driven deformation and resedimentation associated with delta-front instability, most probably triggered by earthquakes. This short article summarizes the results of a recent study of these features, with emphasis on the actual sequence of processes recorded in the outcrop. Detailed analysis of the structural style of the delta-front collapse and the associated sedimentation processes is presented elsewhere (Nemec et al. 1988).     

Regressive and transgressive sequences in a raised Holocene gravelly beach, southwestern Crete

Both a modern and a raised, late Holocene gravelly beach have been studied along the cliffed coast near the village of Chora Sfakion in southwestern Crete, Greece. The Holocene beach shows the record of regional, well-documented relative sea-level changes, in the form of regressive and transgressive sequences. The texture, fabric and depositional architecture of these sequences are described and compared with those of the modern, microtidal beach there. The depositional history of the raised beach is discussed in terms of the sediment transport processes and relative sea-level changes. The regressive sequences of both the modern and the Holocene beach resemble those of the mesotidal beaches in the Sker district of southwest Wales, described by Bluck (1967). The transgressive sequence of the Holocene beach resembles the mesotidal beach sequence near Newton (southwest Wales), recognized by Bluck as another type of progradational (regressive) beach. Therefore, transgressive beach sequences in the stratigraphic record might easily be overlooked or misinterpreted as regressive. The stratigraphic depositional patterns of gravelly beach systems are, apparently, still poorly understood.     

Pleistocene explosions and pyroclastic currents in west-central Anatolia

The discovery of unexpectedly young tephra, with K-Ar dates of 1.50 ± 0.18 and 1.38 ± 0.13 Ma BP, in the Lake Eirdir area sheds new light on the time-span and magnitude of explosive volcanism in west-central Anatolia and on the physical behaviour of pyroclastic currents. The Gölcük maar near Isparta, widely thought to have ceased its activity in Pliocene time, was apparently still erupting with strong explosions in the Early Pleistocene. The Eirdir tephra, 34 km NE from the volcano, indicate deposition from powerful pyroclastic surges directed by the topography. The depositing currents were fully turbulent and carried unusually large lithic clasts, up to 2–3 cm in diameter, in suspension, which is incompatible with the existing physical model for pyroclastic currents. Based on the tephra characteristics, a conceptual model of the pyroclastic current's segmentation, or lateral dynamic partition, is suggested and used further to explain the current's specific response to an encountered topographic barrier. The widely held concept of a pyroclastic current's vertical separation, or sharp rheological decoupling, is thought to describe a common secondary phenomenon.     

Texture and structure of resedimented conglomerates: examples from Książ Formation (Famennian—Tournaisian), southwestern Poland

The Famennian-Tournaisian conglomerates and sandstones of the Ksiaz Formation are interpreted as marine resedimented deposits. Matrix- and clast-supported conglomerate beds are equally common, and two textural sequences (motifs) have been recognized: (I) matrix-rich conglomerate → pebbly sandstone → sandstone, and (II) clast-supported conglomerate → sandstone. Variation in clast type partly controls motif type, and therefore, to some extent, matrix percentage in the conglomerates generally. Grading is extremely common in both clast- and matrix-supported conglomerates: inverse (19%), inverse-to-normal (14%) and normal (26%). The studied succession, itself part of a 4 km thick, fan delta, basin-fill sequence, is organized into large (110–150 m) and small-scale (5–30 m) sequences, both of which show (1) upward coarsening and thickening, (2) upward trend of sandstones and pebbly sandstone → matrix-rich conglomerates → clast-supported conglomerates and (3) a less clear upward tendency of massive and normally graded beds → inversely graded beds. Variation in matrix percentage in beds is therefore also partly controlled by fan processes, during the progradation of fan bodies and lobes. It is predicted that individual resedimented conglomerate beds or motifs show general downfan trends in thickness, texture and structure opposite to those evident in the vertical sequences.     

The Kregnes moraine in Gauldalen, west-central Norway: anatomy of a Younger Dryas proglacial delta in a palaeofjord basin*

The Kregnes “moraine” ridge in Gauldalen, a north-trending valley south of Trondheim, is a Gilbert-type delta formed at a Younger Dryas glacier terminus. The gravelly delta consists of a north-dipping foreset, 150 m thick, comprised of turbidites, debrisflow beds and debrisfall deposits. The bottomset consists of turbiditic sand and mud layers. The topset, 2-3 m thick, is a braided-river alluvium with local beach deposits, matching the marine limit of 175 m a.s.l. The fjord-wide delta front had an extent of 3 km and prograded over a distance of 1.5 km, in probably less than 100 years, with the delta toe climbing by 50 m against the basin's rapidly aggrading muddy floor. The delta advanced through the alternating episodes of its toe aggradation and progradation, related to the increases and decreases of the delta-slope gradient. Slope steepening led to intense sediment sloughing by chutes and occasional large-scale failures. The fjord's wave fetch was low and the wave base no deeper than 1.5-2 m, but strong storm waves occasionally reworked the delta front to a depth of 6 m. Glacitectonic deformation was limited to the system's upfjord end. Allostratigraphic analysis suggests that the proglacial system commenced its development as an ice-contact submarine fan that was deformed, quickly aggraded to the sea surface and turned into an ice-contact delta, which further evolved into the large glaciofluvial delta. The Kregnes ridge represents an episode of the ice-front re-advance due to climatic deterioration and is tentatively correlated with the Hoklingen substage.     

Postglacial colluvium in western Norway: depositional processes, facies and palaeoclimatic record

The postglacial Quaternary colluvial systems in western Norway are arrays of steep fans, often coalescing into aprons, developed along the slopes of valley sides and fjord margins. The coarse debris, derived from weathered gneissic bedrock and its glacial-till mantle, varies from highly immature to mature. The depositional processes are mainly avalanches, ranging from rockfalls and debrisflows to snowflows, but include also waterflow and debris creep. The mechanics and sedimentary products of these processes are discussed, with special emphasis on snow avalanches, whose role as an agent of debris transport is little-known to sedimentologists. The subsequent analysis of sedimentary successions is focused on colluvial-fan deltas, which are very specific, yet little-studied, coastal depositional systems. The stratigraphic variation and depositional architecture of the colluvial facies assemblages, constrained by abundant radiometric dates, are used to decipher the signal of regional climatic changes from the sedimentary record. The stratigraphic data from two dozen local colluvial successions are compiled and further compared with other types of regional palaeoclimatic proxy record. The analysis suggests that the colluvial systems, although dependent upon local geomorphic conditions, have acted as highly sensitive recorders of regional climatic changes. The study as a whole demonstrates that colluvial depositional systems are an interesting and important frontier of clastic sedimentology.