Tidal influence in Cretaceous fluvial strata from Utah, USA: a key to sequence stratigraphic interpretation

Detailed models already exist that outline physical and temporal relationships in marine and marginal marine strata. Such models are still in their infancy in alluvial deposits. Recognition of tidal and estuarine influence in fluvial strata is critical to the development of high resolution sequence stratigraphic correlations between marine and non-marine strata. Strata that have previously been interpreted as low energy meandering river deposits contain sedimentary and biogenic structures that suggest a tidal influence. These structures include sigmoidal bedding, paired mud/silt drapes, wavy and lenticular bedding, shrinkage cracks, multiple reactivation surfaces, inclined heterolithic strata, complex compound cross-beds, bidirectional cross-beds, and trace fossils including Teredolites, Arenicolites and Skolithos. Although none of these structures is unique to tidal processes, the preponderance of data suggests that fluvial systems have been affected by tidal processes well inland of coeval shoreline deposits. These deposits rarely form a significant proportion of a depositional sequence; however, their occurrence allows time significant surfaces to be extended for tens or even hundreds of kilometres inland from coeval shoreline deposits. In Turonian through Campanian strata exposed in the Kaiparowits Plateau of southern Utah, tidally influenced facies are recognized within at least two distinct stratigraphic levels that were deposited during periods of relatively rapid base level rise. These strata form part of an alluvial transgressive systems tract. Landward of each of the marine transgressive maxima, tidal facies are present in fluvial channels that are completely encased in non-marine strata at distances up to 65 km inland from a coeval palaeoshoreline. Our work suggests that such deposits may have gone unrecognized in the past, but they form a significant component of alluvial strata in many depositional sequences. Although these tidally influenced fluvial deposits may be difficult to recognize, they are temporally equivalent to marine maximum flooding surfaces and provide a chronostratigraphic correlation between alluvial and nearshore marine deposits.     

High-resolution sequence stratigraphy of a complex, incised valley succession, Cobequid Bay — Salmon River estuary, Bay of Fundy, Canada

The post-glacial succession in the Cobequid Bay — Salmon River incised valley contains two sequences, the upper one incomplete. The lower sequence contains only highstand system tracts (HST) deposits which accumulated under microtidal, glacio-marine deltaic conditions. The upper sequence contains two, retrogradationally stacked parasequences. The lower one accumulated in a wave-dominated estuarine environment under micro-mesotidal conditions. It belongs to the lowstand system tract (LST) or early transgressive system tract (TST) depending on the timing and location of the lowstand shoreline, and contains a gravel barrier that has been overstepped and preserved with little modification. The upper parasequence accumulated in the modern, macrotidal estuary, and is assignable to the late TST. Recent, net progradation of the fringing marshes indicates that a new HST has begun. The sequence boundary separating the two sequences was formed by fluvial incision, and perhaps also by subtidal erosion during the relative sea level fall. Additional local erosion by waves and tidal currents occurred during the transgression. The base of the macrotidal sands is a prominent tidal ravinement surface which forms the flooding surface between the backstepping estuarine parasequences. Because fluvial deposition continued throughout the transgression, the fluvial-estuarine contact is diachronous and cannot be used as the transgressive surface. The maximum flooding surface will be difficult to locate in the macrotidal sands, but is more easily identified in the fringing muddy sediments. These observations indicate that: (1) large incised valleys may contain a compound fill that consists of more than one sequence; (2) relative sea level changes determine the stratal stacking patterns, but local environmental factors control the nature of the facies and surfaces; (3) these surfaces may have complex origins, and commonly become amalgamated; (4) designation of the transgressive surface (and thus the LST) is particularly difficult as many of the prominent surfaces in the valley fill are diachronous facies boundaries; and (5) the transgression of complex topography may cause geologically instantaneous changes in tidal range, due to resonance under particular geographical configurations.     

The pattern of grainfall deposition in the lee of aeolian dunes

ABSTRACT
A simple model for the deposition pattern in the lee of aeolian dunes is presented that relies heavily upon a recently developed understanding of aeolian saltation. Grainfall deposition at any position on the lee face is the result of all saltation trajectories that leave any point on the surface of the dune upwind of the brink with sufficient initial velocity to travel the intervening distance. The deposition rate at any position on the lee slope is obtained by integrating over all combinations of initial position and required velocity, the velocity being weighted by its probability density.
The resulting calculated total deposition rate patterns show distinct maxima on the order of one to a few decimetres from the brink, beyond which deposition rates fall off roughly exponentially. An important length scale emerges that characterizes this decay with distance from the brink, the length increasing with wind velocity, and decreasing with grain diameter. It is shown that this length scale is on the order of one metre for typical grain size and wind conditions. That this is typically smaller than the length of the lee slope is what gives rise to the oversteepening and eventual avalanching of the lee sides of aeolian dunes. The position of a pivot point on the lee slope may be predicted, separating source regions from accumulation regions for grainflow avalanche deposits.
The calculated patterns provide not only a means for quantitative interpretation of active and fossil dune grainfall deposits, but they provide the initial geometry for grainflow avalanches. The initial failures should coincide with the steepest gradient in grainfall deposition, slightly downslope from the grainfall maximum.     

Kettle holes - stratigraphical archives for Weichselian geology and palaeoenvironment in northernmost Sweden

An active search for pre-Holocene organic deposits in stratigraphically well-defined positions has proved to be a useful tool for reconstruction of glacial and non-glacial Weichselian development in northernmost Sweden. The investigations have been concentrated to kettle holes connected to eskers belonging to a morphologically prominent northwesterly glacial system characterized by extensive drumlinization. Organic deposits, radiocarbon-dated as older than the Holocene, have been found at some 25 localities. Some sites display two organic-bearing sequences separated by a till bed. The pollen flora in the lowest organic bed reflects an interstadial vegetational development: Arctic shrub and herb tundra-subarctic light birch forestherb tundra with ericaceous shrubs and dwarf birch. This interstadial is correlated with Peräpohjola in Finland. During the younger Tärendö Interstadial the vegetational development was similar, but a more continental climate is indicated hy high values of steppe plants, e.g. Artemisia and Chenopodiaceae. The Tärendö Interstadial, correlated with Odderade, wascharacterized by open, unstable soils due to periglacial processes such as cryoturbation and eolian activities. The Weichselian glaciation is divided into three glacial stades separated by ice-free interstadials in the area studied. Except for a zone with S-SW drumlinization the impact of the two later ice sheets is insignificant.     

MOSCOW ON THE RISE: FROM PRIMATE CITY TO MEGAREGION*

In this article I examine Moscow's role in the political‐economic space of the Russian Federation. A broad range of data supports the thesis that the capital has become a primate city, one that serves no longer as the command center of a closed system but as the primary node of interconnection between Russia and the rest of the world. The effort to create a larger, polycentric “New Moscow” next to the ancient capital is marked by a tightening of central control, in contrast to governance regimes of European megaregions. Nevertheless, expansion of the capital region very likely will further boost Moscow's dominance over the country.     

Teaching generic GIS using commercial software

Abstract

The debate between training and education in GIS in a university or college setting and whether or not commercial systems should be used in such institutions may be far from resolution, but an indication of the level of interest shown in hands-on, computer-based GIS education using a commercial system is the appearance and growth of the Institute for GIS in Education. The Curriculum Development Toolkit produced by the Institute has involved academic input and evaluation, and contains both generic GIS background information and system-specific computer-based learning materials, used in over 200 academic institutions world-wide.     

Younger Dryas glacial advance in the southern Gulf of St. Lawrence,Canada: analogue for ice inception?

In the summer of 1999, the Maritimes and Northeast Pipeline Company excavated a 3-m-deep trench across northern Nova Scotia exposing a continuous transect of surficial deposits along a 237-km corridor. A Lateglacial palaeosol with preserved A horizon (peat and wood) buried under 2-10 m of surface till consisted mainly of herbaceous plant material with few large wood fragments. Large pieces of wood from two sites yielded conventional radiocarbon ages of 10.9 ¹⁴C kyr BP (GSC-6435) and 10.8 ¹⁴C kyr BP (GSC-6419). Previous to these finds, only a few localities were known to reveal till overlying peat, so the extent of Younger Dryas (YD) glaciers could not be clearly established. Glacial flow lines indicated by fabric and fluting of the YD surface till sheet in northern Nova Scotia and ice-marginal deposits imply an ice cap centred over eastern P.E.I. and the southern Gulf of St. Lawrence. This glacier also dammed a series of glacial lakes against the highland-rimmed west coast of Cape Breton Island. Glaciers developed and advanced during the YD in the uplands and offshore shelf areas from small remnants of Late Wisconsinan ice. Renewed ice growth was enabled by increased precipitation and local cooling in the Gulf of St. Lawrence due to deflection of the jet stream and expanded sea-ice cover in the North Atlantic. The YD may provide an analogy to glacier development in Maritime Canada during the interglacial/glacial transition.