Contrasting morphodynamics in alluvial fans and fan deltas: effect of the downstream boundary

Alluvial fans and fan deltas can, in principle, have exactly the same upstream conditions, but fan deltas by definition have ponding water at their downstream boundary. This ponding creates effects on the autogenic behaviour of fan deltas, such as backwater adaptation, mouth bars and backward sedimentation, whereas alluvial fans may lack these effects. Hence the present authors hypothesize that morphodynamics on alluvial fans are determined primarily by upstream boundary conditions, whereas morphodynamics on fan deltas are determined by both the upstream and the downstream boundary condition and changes therein. To isolate the effects of the upstream and downstream boundaries, five new alluvial fan experiments are compared with the details of three fan deltas published earlier that were formed under very similar and simple conditions. Similar to the fan deltas, the alluvial fans build up by sheet flow, whilst quasi‐regular periods of incision cause temporary channelized flow. Incision is followed by channel backfilling, after which the fan returns to sheet flow. The channelization and backfilling in alluvial fans is markedly less pronounced and more prone to autogenic disturbance than in fan deltas. The difference is caused by morphodynamics at the downstream boundary. In a fan delta, the flow expansion of the channel causes deposition of all the sediment, which forms a mouth bar and causes strong backfilling. In an alluvial fan, on the other hand, the slope break at the fan perimeter causes some deposition, but transport is not reduced to zero. Consequently, the backfilling in alluvial fans is less pronounced than in fan deltas. Other published experiments support this trend: removal of the mouth bar by a river leads to permanent channelization, whilst pronounced mouth‐bar formation in highly channelized deltas promotes backward sedimentation. The experimental results for this study predict that, when alluvial fans prograde into lakes or deep rivers, they transition to fan deltas with increasingly deeper channels and thicker backfill deposits.     

Full-glacial — late-glacial palaeoclimate of the Southern Andes: evidence from pollen,beetle and glacial records

Palaeoecological studies carried out in the Chilean Lake District and Chilotan Archipelago (41°–43°S) record full-glacial and late-glacial pollen assemblages beginning just after 21000 and beetle assemblages after 18000, both sets extending until 10000 ¹⁴C yr BP. Pollen records indicate that Subantarctic Parkland, the vegetation of the early millennia of record, changed after about 14000 yr BP to become open woodland and later North Patagonian Evergreen Forest. Assemblages of plants and beetles, responding more or less in unison to a strong rise in temperature (≥ 6°C), behaved in accord at around 14000 until 13000–12500 yr BP, the beetle fauna displaying a marked increase in obligate forest types. During full-glacial conditions (17400–16100 and 15300 and 14400 yr BP) and in the late-glacial interval (after about 13000 yr BP), however, climate evidently coerced populations dissimilarly, the pollen sequence showing an increase in plant taxa indicative of colder climate, whereas the beetle fauna underwent little or no variation. Contrasting climate modes implied by plants and beetles may be attributed to differential responses to apparent low-order temperature changes (≤ 2–3°C).     

Occurrence of polar seabirds at sea in relation to prey concentrations and oceanographic factors

The distribution and abundance of breeding and foraging seabirds is usually a reflection of the availability of prey in the marine ecosystems on which the birds depend. At the scale of hundreds of km, differences in marine communities may be reflected in variation in the species composition of resident seabirds. At small scales, environmental features such as fronts or ice can influence where birds will aggregate to forage. Features at which prey become concentrated near the surface are of particular importance to bird species dependent upon small planktonic organisms. Concentrations of foraging seabirds frequently indicate the presence of such features, or of areas of unusually high prey biomass. The absence of birds does not mean that concentrations of potential prey are absent. "Assembly rules," by which one might predict aspects of the marine environment that birds should use in selecting foraging areas are proposed.     

Lithofacies and sequence development on an Upper Devonian mixed carbonate-siliciclastic fore-reef slope, Canning Basin, Western Australia

Detailed sedimentological and stratigraphical analysis coupled with conodont biostratigraphy of a fore-reef slope succession in the Napier Range (Napier Formation) is used to develop a depositional model and relative sea-level history for late Frasnian to late Famennian reef evolution in the Canning Basin of north-western Australia. Changes in sedimentary style on the slope, reflecting differing rates of carbonate production on the platform, are linked to third- and higher order relative sea-level fluctuations. Overlapping slope aprons accumulated along the base of a steep-walled platform margin. Coarse carbonate debris was deposited adjacent to the margin as talus breccias (via rockfall) and debris-flow breccias. Depositional slopes up to 45°, and locally steeper, are demonstrated using rotated geopetal cavity fills. The predominance of channel-filling lithofacies throughout the slope succession indicates the highly channelized nature of the aprons. The middle slope is dominated by sandy oolitic-peloidal turbiditic grainstones interpreted as sediment exported from an active platform. The turbidites and associated debris-flow breccias contrast with condensed carbonate intervals and deep-water, non-fenestral stromatolites that record times of very low platform production. Lower slope turbidites and associated intraclastic breccias indicate widespread redeposition of sediment eroded from lithified and semi-lithified limestones higher up the slope. Several third-order sequences are recognized in the fore-reef succession and these are composed primarily of transgressive and highstand deposits. Carbonate production was severely restricted in the early Famennian coinciding with development of onlapping siliciclastic aprons during a relative sea-level lowstand. Evidence for a subaerial exposure event is also preserved within the siliciclastic strata. Controls on sequence development are difficult to constrain. Although two sequence boundaries can be correlated with falls on the global sea-level curve, the reef complexes evolved in an active extensional regime and it is highly likely that tectonism, in conjunction with eustasy, controlled accommodation on the platform and therefore carbonate productivity.     

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.     

IMPLICATIONS FROM CHEMICAL,STRUCTURAL AND MINERALOGICAL STUDIES OF MAGNETIC MICROSPHERULES FROM AROUND THE LOWER YOUNGER DRYAS BOUNDARY (NEW MEXICO,USA)

Hollow magnetic microspherules from along the lower Younger Dryas boundary (c. 12.9 ka bp ) in New Mexico (USA) were studied using scanning electron microscopy, electron probe microanalysis, X‐ray diffraction, and laser‐ablation inductively coupled‐plasma mass spectrometry methods. The shell of the microspherules (10–15% of the spherule's diameter) displays dendritic surface textures, which are likely due to quenching during rapid cooling of molten material. Structurally, multiple single‐magnetite crystals attached together form the bulk of the microspherules. Iron dominates the microspherules’ composition (～90% FeO_tot), Mn is the second most abundant element (up to 0.4% MnO), Al is detected in low concentrations (<0.30% of Al₂O₃). Among the trace elements, the rare earth elements display slightly fractionated patterns with concentrations of 0.1–1.0× CI chondrite. The microspherules contain elevated concentrations of Ni relative to detrital magnetite (up to 435 ppm) and very low concentrations of Ti (down to 5 ppm). Chemical, structural and mineralogical features of the microspherules do not contradict the existing models of the formation during ablation while a meteoroid goes through the Earth's atmosphere. Elevated concentrations of the magnetic microspherules in sediments can be a stratigraphic marker for the lower Younger Dryas boundary in North America.     

URALIAN AND PERMIAN OF THE URALS

The question of the stratigraphy of the Uralian and Permian has received in recent years much attention on the part of specialists from different continents of the Globe. This subject is also a sore and much discussed question for Russian geologists. (?)eologists of the country where these de(?)     

VISCOELASTIC SEISMIC RESPONSES OF 2D RESERVOIR MODELS1

The use of relaxation mechanisms has recently made it possible to simulate viscoelastic (Q) effects accurately in time-domain numerical computations of seismic responses. As a result, seismograms may now be synthesized for models with arbitrary spatial variations in compressional- and shear-wave quality factors (Q₉, and Q_s, as well as in density (ρ) and compressional- and shear-wave velocities (V_p, and V_s). Reflections produced by Q contrasts alone may have amplitudes as large as those produced by velocity contrasts. Q effects, including their interaction with V_p, V_s and p, contribute significantly to the seismic response of reservoirs. For band-limited data at typical seismic frequencies, the effects of Q on reflectivity and attenuation are more visible than those on dispersion. Synthetic examples include practical applications to reservoir exploration, evaluation and monitoring. Q effects are clearly visible in both surface and offset vertical seismic profile data. Thus, AVO analyses that neglect Q may produce erroneous conclusions.     

Insolation and glacials

Climatic changes result from variables in planetary orbits which modulate solar energy emission and change seasonal and latitudinal distribution of heat received by the Earth. Small insolation changes are multiplied by the albedo effect of the winter snow fields of the Northern Hemisphere, by ocean-atmosphere feedbacks, and, probably, by the stratospheric ozone layer. The role of volcanic explosions and other aperiodic phenomena is secondary. The immediate climate response to insolation trends permits astronomic dating of Pleistocene events. A new glacial insolation regime, expected to last 8000 years, began just recently. Mean global temperatures may eventually drop about 1^oC in the next hundred years. A refinement of the Milankovitch theory in terms of the lunar orbit and more data on solar periodicities are needed for reliable long range predictions.     

Water escape structures in the context of a depositional model of a mass flow dominated conglomeratic fan-delta (Abrioja Formation, Pliocene, Almeria Basin, SE Spain)

ABSTRACT Pebbly sediments of the shallow marine Abrioja fan-delta show pockets (bowl-shaped structures, partly filled with pebbles) and pillars (elongate structures, filled with sand and pebbles). These structures are most abundant in pebbly sediments deposited on a steep slope ( ca. 25°-10°) and are absent in conglomerates deposited on a slope of ca . 6° and less, although they are present in the pelitic top of these beds.
The pocket and pillar structures are interpreted as fluid escape structures originating from local liquefaction and fluidization, processes which are favoured by rapid deposition, rapid sediment accumulation, the presence of less permeable layers and an immature sediment texture.
These conditions are met in conglomeratic fan-deltas, which have steep slopes with immature sediments. It is concluded that the presence of fluid escape structures in conglomeratic sediments may indicate a steep depositional slope.