Marta Pérez-Arlucea

Fine-Grained Splay Deposition in the Avulsion Belt of the Lower Saskatchewan River, Canada

ABSTRACT In or about 1883, a northward avulsion of the meandering lower Saskatchewan River at the Cumberland Marshes initiated a broad belt of alluvial sedimentation that has continued to evolve to the present, and now covers some 500 km2 The avulsion belt comprises a complex alluvial terrain dominated by individual and coalesced splays and associated wetlands connected by networks of active and abandoned channels of various sizes. The Windy Lake splay, a predominantly fine-grained feature typical of large parts of the avulsion belt, was examined to investigate the relationships between splay evolution and facies development. The splay, now largely inactive, mostly formed over a 35-year period, depositing nearly 2 m of sediment over a 7.2 km2 area. The splay surface is ch racterized by stable, well-defined anastomosed distributary channels that separate interchannel wetland basins. Heights of channel levees diminish downstream, and the levees merge with channel-mouth bars whose progradation forms the locus of lakeward channel extension and the base for subsequent subaerial levee deposition. The anastomosed channel pattern was created when the mouth bars of separate distributary channels merged during progradation. Vibracore and soil-probe profiles show the splay to be dominated by upward-coarsening lacustrine-prodeltaic mouth-bar facies sequences capped by levees and organic-rich interchannel wetland deposits. Channels, now containing sand only sporadically in narrowed or slightly migrated reaches, will eventually fill with fine sediment when abandonment s complete. The Windy Lake splay is typical of stage III splays (Smith et al. 1989), whose deposits and associated lacustrine fills form at least half of the avulsion belt. Thin, widespread, predominantly upward-coarsening sequences capped by either nondepositional horizons or true overbank fines may be characteristic of many avulsive suites in the ancient alluvial record.

Normal faulting and crustal deformation, Alkyonides Gulf and Perachora peninsula, eastern Gulf of Corinth rift, Greece

Geophysical, structural, geochronological and geomorphological data indicate that the Psatha, East Alkyonides, Skinos and Pisia faults are Holocene-active structures whereas the status of the West Alkyonides, Strava, Perachora and Loutraki faults is less certain. We see no evidence for significant lateral surface fault growth. New data for late Pleistocene footwall uplift of the Psatha fault are comparable with previously estimated Holocene rates. Pre-Holocene stratigraphic sequences in the Alkyonides Gulf allow calculation of vertical displacement on the Skinos fault of 1.42–1.60 km over a period of >0.6 Ma. Previous palaeoseismological studies indicate comparable displacement rates extrapolated to 0.61–2.20 Ma, whereas extrapolation of previous geodetic data indicate a range of 0.17–0.46 Ma. The latter is too short given the evidence of the stratigraphic record, signifying either that these data may not be representative of longer-term rates, or that significant deformation has taken place elsewhere, for example, on offshore antithetic faults. A case is established for uniform late Quaternary (post-MIS 7) uplift of the Perachora peninsula at rates of c. 0.2–0.3 mm a−1. The lack of regional tilting over Perachora–Corinth–Isthmia is in marked contrast to the situation in the Alkyonides–Megara basins to the east.

Early Permian silt-bed fluvial sedimentation in the Orogrande basin of the Ancestral Rocky Mountains, New Mexico, USA

Abstract The Lower Permian (Wolfcampian) Abo Formation of south-central New Mexico was deposited by a silt-dominated fluvial system along the western half of the Orogrande basin a few degrees north of the equator in western Pangaea. Fluvial channel deposits consist primarily of: (1) inclined siltstone stratasets up to 4.5 m thick and 25 m wide interpreted as point bar deposits, and (2) symmetrically infilled siltstone stratasets up to 2.4 m thick and 14 m long that may represent avulsion crevasse channels. Both types of channels are dominated by climbing ripple cross-laminae and plane bed laminae, but trough cross-beds are also present, as are several types of soft-sediment deformation structures and desiccation cracks. Red silty mudstones interpreted as floodplain deposits comprise up to 70% of the formation and are interbedded with thin ( Common pedogenic features in floodplain strata and on the tops of some fluvial channel beds include root traces, peds, and calcic nodules/tubules, whereas gypsum, gley colour mottling, and translocated clay and/or iron oxides are rare. Also present is an ichnofauna dominated by opportunistic arthropods that colonized the moist upper surfaces of recently deposited crevasse-splay/levee beds and the uppermost surfaces of fluvial channels following avulsion. The relative abundance of channel and crevasse-splay/levee deposits increases eastward in response to moderate asymmetrical subsidence of the basin, whereas calcic paleosols are more numerous on the more slowly subsiding western margin of the basin. A semi-arid to sub-humid palaeoclimate with seasonal precipitation favoured relatively deep rivers whose point bars were seasonally exposed, the formation of calcic and vertic soils, a sparse ichnofauna and flora, periodic desiccation of small carbonate lakes, and perhaps reworking of loess from the floodplain and/or upstream catchments.

Sedimentology, Paleontology, and Sequence Stratigraphy of Early Permian Estuarine Deposits, South-Central New Mexico, USA

Abstract The Lower Permian (Wolfcampian) Hueco Formation in the Dona Ana Mountains of south-central New Mexico contains a 30-m-thick, mixed siliciclastic-carbonate succession exposed for up to 1 km perpendicular to regional paleoslope. The succession was deposited in shallow-marine, wave- and tide-dominated estuarine, and fluvial environments, and is arranged into three fourth-order sequences. The lower two sequence boundaries incise marine fossiliferous packstone and offshore to lower-shoreface mudstone-siltstone and are overlain by fluvial sediment comprising the lowstand systems tract. Fluvial-channel sedimentation above the lower two sequence boundaries changed from vertical to lateral accretion in response to decreasing gradient in the incised valley. The transgressive systems tract of the lower two sequences begins with a tidal ravinement surface, which is mantled locally by a pebble–cobble lag of rip-up clasts and fossils. The tidal ravinement surface cuts channels up to 4 m deep, locally removing transgressive-estuarine and some lowstand-fluvial sediment. Symmetrically rippled sandstone-siltstone deposited near the mouth of the estuary overlies the tidal ravinement surface and is overlain and locally truncated by a wave ravinement surface. In sequence 2, the highstand systems tract consists of a progradational package of heterolithic mudstone–siltstone–sandstone and bivalve packstone deposited in the central basin of the estuary; and sandstone with wood and bivalves, and carbonaceous mudstone and sandstone deposited in bayhead deltas and marshes. Sequence boundary 3 is characterized by a restricted-marine ostracode packstone sharply overlying a marsh mudstone with a vertic Calcisol and burrows of the Glossifungites Ichnofacies, and was produced on an interfluve. Sequence 3 is not present in the northern exposures, having been removed by erosion associated with sequence boundary 4. The temporal scale (105 yrs) of the fourth-order sequences implies a glacial-eustatic origin. Interbeds of shallow-marine fossiliferous packstone and mudstone–siltstone in the lower part of the succession resemble fifth-order sequences in the Midcontinent, but also may represent parasequences or autocycles.

The TESS-1 Suction Corer: A New Device to Extract Wet, Uncompacted Sediments

Conventional coring can be an expensive and cumbersome technique in uncompacted, waterlogged sediments, especially when large numbers of samples are required rapidly and when vehicular access is restricted. The existing Van der Staay suction corer (Van de Meene et al. 1979) solves economic and other problems (chiefly logistical and environmental issues), but sample extraction has to be done in the field. The original purposes of the Van der Staay suction corer were to enable rapid drilling through sand and allow field sampling. It was not designed to recover intact samples for subsequent laboratory analysis. Cores from the new TESS-1 suction corer, however, are encapsulated in the field. The TESS-1 therefore combines the advantages of the original corer with the possibility of taking samples for laboratory analysis. Samples collected using the TESS-1 suction corer show little deformation, allow coring of soft uncompacted mud and retention of sedimentary structures. Problems due to sediment fluidization and movement inside the core pipe are usually minimal. The TESS-1 suction corer can be used in both wetland areas (beach, tidal flats, and marshes) and in shallow subaqueous environments (lakes, rivers, and nearshore subtidal areas).

Late Neogene rift-basin evolution and its relation to normal fault history and climate change along the southwestern margin of the Gerania Range, central Greece

ABSTRACTThe south- to southwest-dipping North and South Saros normal faults occupy the southeastern margin of the Gerania Range, a major massif of Mesozoic limestone and o phi-o lite in the eastern Gulf of Corinth Rift, cen-tral Greece. The history of the Saros faults is constrained by adjacent basin-fi ll sediments and modern drainage patterns. Older basin fi ll (Pliocene–lower Pleistocene?), which consists of terrestrial fi ne sands, carbona-ceous muds, marls, and micrite limestones, is uplifted on the South Saros fault block and onlaps the North Saros fault footwall block on its northern side to near its crest, suggest-ing that it predates major fault activity. In contrast, younger basin fi ll (middle Pleisto-cene?) was deposited and locally deformed during fault activity in a main basin south of the faults and in a perched basin on the South Saros fault block. Main-basin deposits consist of alluvial-fan, fan-delta, and shallow-marine sediments up to 50 m thick, whereas perched-basin fanglomerates are 5–15 m thick. Fault-scarp retreat and sediment onlap indicate that the faults became inactive late in the history of deposition of the younger basin fi ll. The perched and main basins are currently undergoing incision, allowing mod-ern sediment to bypass the basins and to be deposited on modern fan deltas and beaches along the Gulf of Megara coast. The absence of knickpoints and upstream deepening of the canyons suggest that a climatically driven increase in catchment runoff was responsible for basin incision, although uplift and sea-level change may have contributed as well.The North and South Saros faults each consist of three segments characterized by different strikes. The major drainages cross the Saros faults at or near segment boundar-ies, suggesting that the drainages may have initially fl owed between the tips of isolated fault segments. As the segments became linked, the antecedent drainages maintained their course and cut deep gorges. At the scale of the entire eastern Gulf of Corinth region, however, older unlinked faults were aban-doned, and new faults were created in a gen-eral northward and basinward direction; the origin of this process remains controversial.INTRODUCTION

Low-level gamma spectrometry for pollution assessment in San Simón Bay (Vigo, Spain)*

A gamma spectrometer with HPGe detector of 50% relative efficiency and 1 cps total background has been dedicated to the measurement of an intertidal sediment core from a coastal environment at the Ria de Vigo (Spain). The area is affected by lead pollution and the source identification needs of a precise dating of the sediment core. Such a precise dating requires the measurement not only of the radionuclides directly involved in time calculation, as 210 Pb and 226 Ra, but also of ancillary radionuclides which inform about the dating model to apply and about the validity of its time estimation. Gamma spectrometry with Ge detectors performs a simultaneous measurement of the full content in γ-emitters of the sample. However, its use is limited by its high spectral background. We present the characteristics of our low- level background gamma spectrometer and also of Galea, the computing tool for the expert analysis of natural radionuclides. Both make possible to get the proper experimental results to reach a suitable dating. The results allowed us to detect a change in the sedimentation dynamics in the area under study, to verify the impact of lead pollution in the 210 Pb level, to obtain a sedimentation rate by using the CF:CS model with a suitable correction factor and, finally, to validate the sediment dating.