Wojciech Nemec

Large floating clasts in turbidites: a mechanism for their emplacement

Abstract The transportation mode for large, isolated clasts “floating” in turbidites is a problem that has been difficult to resolve. New observations from experimental, high-density turbidity currents indicate that large, outsized clasts can be transported along a rheological interface which develops within the flow. The clasts “glide” along the top of an underlying, pseudolaminar inertia-flow layer, partly submerged in it, and are driven by the downflow component of turbulent shear-stresses transmitted from the overlying, faster-moving turbulent layer. As the inertia-flow layer freezes and a new one forms, or as the layer thickens, the gliding clast may be forced to a progressively higher level within the flow. With deceleration, the inertia-flow phase of the flow freezes entirely and the large clast is then trapped “suspended” above the base of the resulting turbidite. Both a(p)a(i) and a(t)b(i) orientations of the floating clasts are observed. It is suggested that, hypothetically, a similar mechanism for megaclast emplacement may also operate in other types of viscous sediment flows, subaqueous or subaerial, wherever there is a distinct rheological interface developed within the flow.

Sedimentary Architecture and Dynamic Stratigraphy of a Marine Ice-Contact System

ABSTRACT The Mona moraine in southern Norway is a sedimentary ridge, 3.5 km long, formed at the mouth of a shelf trough during a late Younger Dryas readvance of the Scandinavian ice sheet. The moraine architecture, sedimentation processes, and record of ice-front behavior have been reconstructed in a high-resolution allostratigraphic framework on the basis of ground-penetrating radar profiles and sedimentologic analysis of outcrop sections. The shelf trough was draining ice from adjacent platforms, which resulted in ice-front protrusion with a moraine offset relative to the coeval swarm of small moraines formed in the shallow-water zone. The grounding-line system at Mona commenced its development as an ice-contact submarine fan, while the glacier front was thinning and grinding to a halt on a bedrock threshold. The ice flux declined, but fluctuated, and the subglacial sediment supply by meltwater prevailed. The ice-front stillstand allowed the submarine fan to aggrade to the sea level and turn into a Gilbert-type delta with a short (160 m) distributary plain. The delta foreset, 50-60 m thick, consists of gravelly massflow deposits, and its sandy turbiditic toe has been extended on the thresholds steep lee slope to a distance of at least 2 km. The delta was abandoned by rapid calving of the ice front, and the ice-contact slope was covered with marine mud and resedimentation products. A radiocarbon date from the base of this thin cover indicates ice retreat at around 10,100 yr BP. The moraine was surrounded by the sea and subject to reworking by waves, tides, and longshore currents. A thick package of regressive foreshore deposits with downstepping wave-cut terraces on the moraine slopes reflects gradual emergence due to regional glacioisostatic uplift. The study demonstrates that the facies architecture of marine moraines bears an important record of glacier dynamics and ice-front behavior.

The shape of the rose

Abstract The vast majority of rose diagrams that appear in the sedimentological literature are constructed incorrectly, with the use of a linear frequency scale. This leads to gross distortions of the shape of the rose. Correct construction of a rose diagram requires non-linear frequency scale to be used. The correct lengths (radii) of the diagrams sectors can be derived numerically, or determined more rapidly by plotting the frequencies directly on an appropriate net.

Modes of sediment delivery to the grounding line of a fast-flowing tidewater glacier: implications for ice-margin conditions and glacier dynamics

Abstract The study focuses on the terminal moraine of a fast-flowing, temperate tidewater glacier that protruded in Oslofjorden trough, southern Norway, during one of the re-advances of the receding Fennoscandian Ice Sheet in the Younger Dryas time. Allostratigraphic mapping is used to reconstruct the moraines morphodynamic development, showing how information on the dynamics of ancient glaciers can be derived from their grounding-line deposits. The Storsand moraine commenced its development in the latest phases of ice-margin advance and continued to grow during the stillstand phase, as long as the ice flux persisted. The thick moraine (>100 m) formed in a few decades, to be rapidly abandoned and later emerged by regional uplift. The study concludes that: (a) both meltwater and ice flow invariably supply sediment to the grounding line, and it is the varied preservation potential of ice-derived diamicton that results in misleading differences between moraines; (b) the glacier-front kinematics is asymmetrical with slow advances and rapid retreats; (c) no moraines can form during glacier retreat; (d) the front of an outlet glacier may stabilize while the adjacent ice margin is oscillating or virtually retreating; and (e) marine moraines are an important source of information about ancient ice margins and glacier dynamics.

Latest evidence of Palaeoamasia (Mammalia, Embrithopoda) in Turkish Anatolia

ABSTRACT Geological investigation of the Boyabat Basin in north-central Anatolia, Turkey, yielded palaeoamasid (Embrithopoda, Mammalia) gnathodental fossil specimens from two localities dated to the Eocene—Oligocene transition. These specimens include an upper incisor (BOY-1) and maxillary fragment preserving M2–M3 (BOY-2), and are geologically the youngest embrithopods known from Eurasia. The maxillary specimen is taxonomically more useful and, among embrithopods preserving the same molars, is most similar to the late Paleocene—middle Eocene Turkish palaeoamasid Palaeoamasia kansui. However, the new specimens are cautiously classified as Palaeoamasia. sp. nov., based on distinctions between BOY-2 and P. kansui in M3 morphology. Features distinguishing the M3 in BOY-2 are intermediate between those of P. kansui and the more derived Afro-Arabian arsinoitheriid embrithopods Arsinoitherium spp. and Namatherium blackerowense, although the new Turkish specimens are stratigraphically too young to be ancestral to these middle Eocene—late Oligocene arsinoitheriids. Salient differences in dental and gnathic morphology between contemporaneous Eurasian and Afro-Arabian embrithopods indicate long, separate phylogenetic trajectories for these taxa, supporting the view that they are divisible at the family level into Palaeoamasidae and Arsinoitheriidae, respectively. Improved documentation of the lengthy paleogeographic separation of palaeoamasids and arsinoitheriids confirms that Afro-Arabia was sufficiently isolated from Eurasia during the Paleogene to limit embrithopods to rare sweepstakes dispersal between these landmasses.

The kinematics of ancient tidewater ice margins: criteria for recognition from grounding-line moraines

Abstract Modern studies indicate that the kinematic behaviour of tidewater glaciers is a crucial part of ice-sheet dynamics. A similar relationship may be expected for ancient ice sheets, but can the kinematics of ancient tidewater ice margins be recognized? The paper addresses this methodological issue by pointing to the high-resolution stratigraphic record of marine moraines, thus far little explored. On the basis of a series of case studies, a range of field criteria are proposed for the recognition of short-term grounding-line movement and possible oscillations in moraine outcrop sections. The method combines allostratigraphic mapping and architectural facies analysis of the moraine sedimentary units formed during glacier advance, subaqueous stillstand and eventual subaerial stillstand, with recognition of the successive ice-contact surfaces. The stacking architecture of the sedimentary units and their ice-contact bounding surfaces reveal the time–distance trajectory of the grounding-line positions. It is also possible to recognize changes in the mode and rate of subglacial sediment delivery, as well as fluctuations in the ice flux and meltwater discharge. This methodology invites detailed studies of marine moraines. Systematic case studies on a regional scale may provide new information on the behaviour of tidewater ice margins and lead to unprecedented insights into the dynamics of ancient ice sheets.