Autogenic Processes Research Articles

Allogenic versus Autogenic Processes in the Genesis of Middle Ordovician Brachiopod-Rich Shell Beds, Verulam Formation, Ontario

Ordovician brachiopod-rich shell beds have been regarded as the products of autogenic ecologic succession involving gradual accumulation of shells at the sediment surface, culminating in a climax community dominated by epifaunal, attached atrypid or rhynchonellid brachiopods. To evaluate this model, we examined brachiopod-rich shell beds from the Middle Ordovician Verulam Formation of southern Ontario. Polished slabs, acetate peels, and thin sections revealed several features indicative of storm-influenced deposition: normal grading, rip-up clasts, spar-filled shelter porosity beneath bioclasts, and mud-filled shells that had been exhumed from muddy substrates. Complex shell beds with internal discontinuity surfaces are common. Syndepositional reworking during storm events is also supported by predominantly convex-up attitudes of concavo-convex shells. Many beds are directly overlain by sparsely fossiliferous to barren, laminated shales and mudstones that are interpreted as mud tempestites deposited during mud-blanketing events. Cluster analysis of faunal abundance data from the tops of shell beds defined two intergrading assemblages -strophomenid-dominated and atrypid-dominated assemblages. These assemblages represent stages in a sequence of storm-generated, allogenic taphonomic feedback. Strophomenid-rich assemblages formed by the concentration of epifaunal free-lying brachiopods into shell pavements by storm waves and currents. If not buried quickly by mud tempestites, shell pavements were colonized by atrypid brachiopods to produce mixed strophomenid-atrypid assemblages and, eventually, atrypid-dominated assemblages. The complex, storm-influenced histories of the Verulam shell beds affirm the importance of allogenic pathways in small-scale successional changes and suggest that the autogenic model may not be widely applicable.

Peat accumulation and succession following permafrost thaw in the boreal peatlands of Manitoba, Canada

Permafrost thaw in boreal peatlands may be widespread as climate warms over the next century. Thaw will lead to dramatic changes in vegetation, and an understanding of the patterns and rates of succession following thaw is important for assessing future ecosystem changes, such as carbon accumulation and the likelihood of permafrost formation during climate warming. This paper describes the autogenic processes of peat accumulation and succession from recently thawed, aquatic Sphagnum communities to lawn and hummock Sphagnum communities favorable for permafrost formation. Peat cores were used to construct bulk density profiles in aquatic, lawn, and hummock habitats. Soil bulk density increased over time from the edges to the centers of thawed bogs. These changes corresponded to changes from aquatic to lawn/hummock communities. Macrofossils from 210Pb-dated peat cores were used to document historical changes to hummock communities. Succession to hummock communities capable of forming permafrost was less than 80 years. Picea mariana seedlings colonized emergent Sphagnum hummocks in collapse scars, which may lead to permafrost formation. However, permafrost formation in today’s discontinuous permafrost landscape is unlikely during future climate warming because of limited tree establishment and shifting mean annual temperature isotherms.

Controls on cyclostratigraphy of Lower Cretaceous carbonates and evaporites, Cupido and Coahuila platforms, northeastern Mexico

The Lower Cretaceous Cupido (Barremian-Aptian) and Coahuila (Albian) carbonate platforms of northeastern Mexico exhibit thick successions of meter-scale cycles deposited in three unique paleoenvironmental settings. (1) The Cupido shelf lagoon is composed of peritidal carbonate cycles deposited in the protected lee of a reef-rimmed to barrier-shoal margin. (2) The restricted Coahuila ramp interior consists of alternations of subtidal evaporites and peritidal carbonates. (3) The deep-water parts of both the Cupido and Coahuila platforms are composed of foraminiferal wackestones and lime mudstones interspersed with firmgrounds and hardgrounds in a cyclic arrangement. Vertical successions of meter-scale evaporitic cycles and peritidal cycles exhibit systematic stacking patterns that build into intermediate-scale high-frequency sequences (tens to hundreds of meters thick), and large-scale composite sequences (hundreds of meters thick) that can be correlated across the Cupido and Coahuila platforms. These large-scale stacking patterns are interpreted to reflect longterm accommodation events and, when combined with the scale-independent architecture of all genetic units, permit the inference that all three meter-scale cycle types on the Cupido and Coahuila platforms are also governed by relative sea-level change. The composition, thickness, and number of meter-scale cycles within individual high-frequency sequences can be highly variable across the Cupido and Coahuila platforms, however, even though the overall upward-shallowing patterns are evident. The lateral complexity of cycle architecture and distribution is interpreted to be a natural response to fluctuations in regional climate interacting with autogenic processes such as variations in carbonate production and dispersal, intensity and frequency of tropical storms and monsoons, thermohaline circulation patterns, and ambient ocean chemistry and temperature. These interacting processes created laterally variable physiographic and oceanographic conditions across the Cupido and Coahuila platforms, complicating the sedimentary record generated by the composite sea-level signal. Well-documented evidence from Barremian-Cenomanian pelagic cycles throughout the Tethyan seaway strongly indicates that Milankovitch-driven climatic changes operated during the Early Cretaceous. Contemporaneous shallow-marine cyclicity in several locations suggests that these climatic changes may have had globally widespread effects. In an effort to link the shallow-water and deep-water realms, we propose a model whereby Milankovitch-driven global climatic changes generated low-amplitude, high-frequency eustatic fluctuations through some combination of thermal expansion and contraction of ocean water, waxing and waning of small ice caps and alpine glaciers, and changes in the storage capacity of aquifers and lakes to produce meter-scale cycles across Lower Cretaceous shallow-marine platforms.

Distinguishing the roles of autogenic versus allogenic processes in cyclic sedimentation, Cisco Group (Virgilian and Wolfcampian), north-central Texas

Meter-scale transgressive-regressive cycles of the subsurface Cisco Group are composed of marine and nonmarine carbonate and siliciclastic rocks deposited on the Eastern shelf of the Midland basin during Late Pennsylvanian and Early Permian time. Five cycle types are characterized by thickness, magnitude, order, and principal lithofacies. Cycle magnitude is defined as the maximum facies shift in a cycle, indicating extent of shoreline migration. Cisco cycles belong to three orders—minor, intermediate, and major—and they are superimposed and form a stratigraphic hierarchy. Each order of cycles has a distinct range of thickness and possibly duration. A cycle is also divided into a lower sand-poor interval, during which coarse siliciclastic supply at the depositional site was diminishing, and an upper sand-rich interval, during which coarse siliciclastic supply was high. Regional thickness and lithofacies variations of sand-rich intervals indicate that progradational infilling at a depositional site lagged marine regression, suggesting a delay in sediment supply from the upland source relative to the time of base-level fall. Regional systematic variations in cycle abundance, continuity, and characteristics along depositional dip and strike record the interplay among regional topography, pattern of siliciclastic supply, and shelf subsidence, which controlled distribution of depocenters and bypass zones and, thus, stratigraphic completeness and resolution. Regional persistence of cycles suggests a eustatic control on regional, ordered transgressive-regressive events. In contrast, local variations of cycle characters suggest controls by local topography and depositional dynamics, which determined depositional loci, differential compaction, and erosion. A predominantly autocyclic Cisco record in the upper platform does not imply the absence of allogenic processes. An allocyclic Cisco record in the lower platform contains abundant autocyclic imprints, because allogenic controls on cyclic sedimentation were accomplished through local autogenic processes. Distinguishing the roles of autogenic versus allogenic processes in cyclic sedimentation is an important step in establishing a high-resolution (meter-scale) chronostratigraphy of any sedimentary record 1 .

Structure and Causes of Vegetation Change in State and Transition Model Applications

State and transition (ST) descriptions of rangeland vegetation dynamics provide information on current perceptions of explicit causes of change in dominant vegetation. Structural attributes of ST applications allow an evaluation of the complexity of the ST model and comparisons with the organization of the traditional succession-retrogression model of secondary succession. An analysis of 29 applications of the ST model revealed consistent trends. The number of transitions connecting states showed a less-than-expected increase with the size of the application. This is probably associated with limitations to interpret complex relationships and a need to produce relatively simple applications. Larger applications exhibited a shift towards stable states with pivotal positions within structures less connected (ie., with fewer transitions) than expected by chance for a given number of states. Thus, some stable states assume key intermediary roles as the number of states considered increases. It is debatable whether this is a property of larger systems or an effect of modeling bias. The analysis of causes of vegetation change confirmed current perceptions about the importance of man-related sources of disturbance. Grazing, fire, and control of woody plant species are visualized as the most relevant man-related agents of change. Some ST applications retain autogenic behaviors embedded in transitions in spite of the event-driven nature of the approach. However, the ST model removes autogenic processes from their central role as general causes for vegetation change. This approach is theoretically very limited because no general properties or attributes of the components (e.g., plant species assemblages, individual species) or processes (e.g., growth, reproduction, mineralization) of the system are used in any comprehensive way to generate predictive rules of wider than local relevance. Alternative approaches are suggested that would allow ecological generalizations and comparisons across systems.

The palaeoecology of a treed bog in western boreal Canada: a study based on microfossils, macrofossils and physico-chemical properties

The developmental histories of a wetland and the surrounding upland vegetation in the southern boreal region of western Canada are traced through microfossil, macrofossil, physico-chemical and radiometric analyses of a 425 cm deep core consisting of sandy gyttja and peat. Microfossil analysis includes algal, fungal and zoological ‘Types’ which are illustrated and described, with notes on their occurrence as observed in several peat sequences of western boreal Canada. Local wetland development started in the central area of a 65 ha closed depression at c. 5000 BP, due to a rise in the groundwater table related to a gradual increase in effective precipitation in the region. Initial stages were characterised by Typha-dominated marsh communities that suggest marked seasonal fluctuations in local water levels. The accumulation of organic matter could not keep pace with the ever-rising groundwater table, and a shallow eutrophic pond with aquatic macrophytes developed locally. The body of open water was surrounded by marsh vegetation that was gradually being displaced towards the margins of the depression. At 4330 BP moist conditions became prevalent throughout the growing season. A brown-moss dominated treed rich fen replaced the shore marsh community and gradually began to encroach on the small pond. The basin completely terrestrialised at 3260 BP. Autogenic processes caused by peat accumulation resulted in a transition towards a Sphagnum-dominated treed poor fen. At 2790 BP a treed bog became established, firstly dominated by Sphagnum angustifolium and S. magellanicum indicating moist local conditions. After 1870 BP S. fuscum became dominant, indicating drier conditions locally. The upland vegetation just before local wetland formation was characterised by Picea in relatively wet depressions and by Populus in drier areas. Some grassland patches were present, most probably on the more southerly exposed slopes of the hills. Picea was locally displaced by wetland vegetation at c. 5000 BP, whereas deciduous forest and some grassland stands continued to dominate the surrounding hills. The coniferous taxa Pinus and Picea began to invade the uplands after 4330 BP, contemporaneously with the development of the local peatland. This marks the time at which the modern southern boreal ecosystem dominated by peatlands in lowlands and mixed wood forest in uplands became established in the area.

Temporal scale and the accumulation of peat in a Sphagnum bog

We examined short-term (decadal) and long-term (millenial) processes of peat accumulation, and the links between them, in a Sphagnum bog in continental Canada. A previously published model of bog growth was fitted to age profiles of the oxic acrotelm (surface, < 60 cm thick) and the underlying, anoxic catotelm (210 cm thick). Approximately 5300 years of accumulation were represented in a radiocarbon-dated core that extended to the base of the deepest part of the peat deposit. The model estimated that the overall rate at which material entered long-term storage in the catotelm was 60–66 g∙m−2∙a−1. Although the decay rate coefficient was near zero, the bog stopped accumulating peat within the past 1500 years, resulting from either a decrease in the rate of transfer of material from the acrotelm or an increase in the rate of decay of material at the top of the catotelm. The model of bog growth estimated recent inputs to the acrotelm (90–930 g∙m−2∙a−1) that were twofold higher than published field measurements of aboveground productivity, and decay rate coefficients (0.005–0.040 a−1) that were 10-fold lower than published litter-bag measurements of mass loss. The pattern of mass loss over time, approximated from nitrogen concentration data, deviated from the pattern predicted by exponential models of decay. Calculations of the balance between additions to and losses from the acrotelm suggest that the amount of material transferred to the underlying catotelm differs among microhabitats. Such spatial variability in short-term processes is incompatible with long-term processes determining the position of the acrotelm–catotelm boundary. We discuss the applicability of the model to continental peatlands and suggest ways to improve modelling of short-term autogenic processes. Keywords: peatland, peat accumulation, organic matter, decomposition, age profiles, mathematical models.

Cycle Symmetry and its Causes, Cisco Group (Virgilian and Wolfcampian), Texas

ABSTRACT 181 transgressive-regressive cycles composed of nonmarine and marine carbonate and siliciclastic rocks of the Cisco Group on the Eastern Shelf, Texas, display complex characteristics at both hemicycle and full-cycle scales. They are delineated on the basis of successive changes of depositional environments, stratal boundary relations, and stratigraphic position. Transgressive and regressive stratigraphic environment gradients are defined as the magnitude of environmental shift divided by thickness for each hemicycle. They indicate the rates of lateral environmental shifts during transgression and regression. Cycle symmetry index is defined as the ratio between transgressive and regressive stratigraphic environment gradients. It provides a measure of stratigraphic response to controlli g processes. Most Cisco cycles are asymmetrical with steep transgressive and shallow regressive stratigraphic environment gradients, indicating cyclic but asymmetrical variations of major allogenic processes over a cycle interval. These systematic variations are delineated from climate-sensitive rock types, stratal thickness, and sedimentary features. The relative dominance of processes controlling accommodation (i.e., glacio-eustasy and basement subsidence) versus those controlling sedimentation (i.e., climate, upland sediment yield, and sediment supply at the depositional site) determines cycle symmetry. A formulated relation between cycle symmetry index (i.e., stratigraphic response) and rates of these processes demonstrates that transgressive sedimentation is controlled by sediment supply, whe eas regressive sedimentation is controlled by accommodation. This relation is potentially useful in quantitatively predicting stratigraphic responses to specific allogenic processes in cyclic sedimentation. Five Cisco cycle types defined by the type of component lithofacies display the stratigraphic response mainly to noncyclic allogenic and autogenic processes. The cycle types have varying magnitude, thickness , and symmetry. They also vary in lateral extent and in abundance. A process-response model of cyclic sedimentation of the Cisco Group on the Eastern Shelf is established. It emphasizes the interplay between autogenic and allogenic processes at the sub-cycle scale. Understanding interactions among glacio-eustasy, climate, shelf subsidence, sediment supply, and depositional dynamics during various stages of transgression and regression is central to a clearer comprehension of the observed variations in cycle characteristics.

Whither stratigraphy?

There have been three revolutions in sedimentary geology. The first two began in the 1960s, consisting of the development of process-response sedimentary models and the application of plate-tectonic concepts to large-scale aspects of basin analysis. The third revolution, that of sequence stratigraphy, began in the late 1970s and helped to draw together the main results of the first two: the knowledge of autogenic processes learned through facies analysis, and the understanding of tectonism implicit in the unravelling of regional plate kinematics. Developments in the use of seismic-reflection data and the evaluation of a hypothesis of global eustasy provided considerable stimulation for stratigraphic research. Current developments in the field of sequence stratigraphy are focusing on three areas. (1) Elaboration of the sequence-architecture models for various configurations of depositional environment and sea-level history. (2) Exploration of various mechanisms for sequence generation, especially tectonism and orbital forcing. (3) Attempts to improve the level of precision in stratigraphic correlation and to refine the geological time scale, as a means to test the model of global eustasy. The growth in the power of computers and our knowledge of physical and chemical processes has led to the evolution of an entirely new way of evaluating earth history, termed quantitative dynamic stratigraphy. Mathematical modelling and numerical simulation of complex earth processes are now possible, and require the collection and integration of a wide array of quantitative and qualitative data sets. Applications include the study of the geodynamic evolution of sedimentary basins, modelling of stratigraphic sequences and global climates, studies of Milankovitch cycles (cyclostratigraphy) and simulation of fluid flow through porous media. The Global Sedimentary Geology Program has brought many of these areas of study together in multidisciplinary, global-scale studies of the sedimentary history of the earth. The results of these studies have wide application to many problems of importance to the human condition, including the past history of global climate change and other environmental concerns. The study of stratigraphy is at the centre of the new view of the earth, termed earth-systems science, which views earth as an ‘organic’ interaction between the lithosphere, biosphere, hydrosphere, and atmosphere.

Postglacial development of a Maine bog and paleoenvironmental implications

Stratigraphic data on gross peat components, plant macrofossils, pollen, ash after ignition, and radiocarbon ages are used to reconstruct the developmental history of the north unit of Caribou Bog in central Maine, which presently consists of a sparsely wooded, ombrotrophic (raised), center surrounded by extensive wooded fen. A lake existed from 12 500 to 8500 BP, when terrestrialization led to peatland initiation throughout the basin within ~500 years. Near the center of the present peatland, limnic sedge fen developed to sedge fen at 8000 BP, and to Larix laricina wooded fen at 7000 BP. At 5500 BP the wooded fen became Sphagnum dominated and sparsely wooded, a condition that has persisted at the peatland center. Only two early transitions are clearly related to climate forcing. An abrupt shift from silt–clay to organic lake sediment ~10 400 BP is typical of basins throughout the region. This shift correlates with rapid changes in local and regional vegetation, and likely reflects increased biological productivity caused by a rapid warming of climate. Peatland initiation 8500–8000 BP coincides with a drop of water level, probably associated with a rapid shift to drier climate. We estimate that the water level around the peatland was at least 0.4 m above the present level at and before 8500 BP, dropped at least 1.7 m (min. 1.4 m with correction for peat compaction) in the next 500 years, and rose to the present level around 7000 BP or later. Major hydroseral changes after 8000 BP are asynchronous at several coring sites and are not consistently correlated with regional paleoclimatic events. These changes were probably induced by autogenic hydrological processes associated with peat accumulation. Key words: wetland history, paleoclimate and paleohydrology, pollen, plant macrofossils, peat stratigraphy, Maine.

The disturbance of forest ecosystems: the ecological basis for conservative management

The extensive literature on natural disturbance in forests is reviewed in terms of the hypotheses: (1) that disturbance is a major force moulding the development, structure and function of forests; and (b) that management of forests for all their benefits can be controlled so that the effects can be contained within those which result from natural disturbance. The causal factors of natural disturbance are both endogenous and exogenous; there are major difficulties in the formal characterization of disturbance and of recovery after disturbance. As to the latter, the acceptance of classical generalizations of the nature of succession has led to particular difficulties in the assessment and interpretation of recovery. Tree fall, which creates gaps, is fundamental to the development of many forests, and has been most intensively studied in tropical forests of Central America and the Amazon and in temperate forests of North America. Tree fall is part of autogenic change; mechanisms of gap-filling and subsequent growth and species composition vary widely with forest type and geography. Disturbance by wind is particularly difficult to characterize. Wind varies along a continuum; the blow-down of an individual tree may be mostly due to autogenic processes of ageing and decay, whereas catastrophic hurricanes and cyclones may be defined as wholly exogenous. Nevertheless, the resilience in terms of species diversity of tropical forests following catastrophic disturbance by hurricane is remarkable. A number of studies support the view that the tropical forest in hurricane-prone areas is not a stable steady-state ecosystem but rather that heterogeneity is maintained by catastrophe. The ability to regenerate by suckers and the coincidence of regenerative space and gregarious flowering are important components of the response of rainforest following disturbance. For much of the world, ‘fire is the dominant fact of forest history’. As examples, fire and its effects are reviewed for the northern boreal forests, oak-pine forests and north-western sub-alpine forests of North America. The effect of fire on species composition varies with intensity and frequency. That, together with the popular view of fire as unnatural and therefore unacceptable, places great demands on management of forests for all of their benefits, including national parks and reserves. These difficulties also affect management of other ecosystems, such as Mediterranean-type shrublands and heathlands where species diversity, productivity and cycles of regeneration and degradation are governed by fire as a natural disturbance. Shifting agriculture is a traditional form of agriculture used by at least 240 million people in the humid tropics. Shifting agriculture, together with wind, lightning and fire, is an exogenous disturbance which has little effect on soil fertility and on structure and composition of the rainforest which re-establishes after abandonment. As the intensity of disturbance of rainforest increases, resilience of the forest decreases and the current problems of extensive clearing for improved pasture and of uncontrolled logging are resulting in degraded ecosystems. Regeneration follows the often extensive death of trees caused by outbreaks of insects in many coniferous forests of northern America. This disturbance by herbivory halts increasing stagnation (as measured by decreasing rates of ecosystem production and nutrient cycling) and reinitiates succession. Other disturbances to forests occur through damage from ice-storms, snow avalanches, erosional and earthquake landslides, and volcanic activity; the development of Nothofagus forests in Chile and New Zealand is determined by such catastrophic mass movements. An extensive literature supports the hypothesis that natural disturbance is fundamental to the development of structure and function of forest ecosystems. It follows that our management of natural forest should be based on an ecological understanding of the processes of natural disturbance. Whether or not we want to do this, and the extent to which we want to derive all of the benefits from the forest, including timber, depends on social attitudes. Whereas humanism may treat conservation as the wise husbanding of forests in the interests of social traditions and harmony, animism may give nature unalienable rights. The conclusion from this review is that the ecological framework of natural disturbance and the knowledge of its component processes and effects provides the basis on which we can manage our forests as a renewable resource which can be utilized so that the forests ‘retain their diversity and richness for mankind's continuing benefit’. Nowhere is this management more desperately needed than for the protection of the world's tropical forests, its peoples and their cultures.

The Euramerican Coal Province: controls on Late Paleozoic peat accumulation

The ancestral, tropical peats and associated strata of the Euramerican Coal Province record the interwoven effects of climate, tectonism and eustasy during the Late Paleozoic. Many peats formed in foreland and intermontane basins associated with equatorial orogenic belts along the borderlands of Euramerica and Gondwana, while other peats formed in adjacent cratonic basins. The resolution of controls on Euramerican peat formation requires testing against a well-constrained temporal framework. The Late Paleozoic “coal age” followed the evolution of vascular plants in the late Silurian and was associated with assemblage of the Pangean supercontinent. Devonian and Early Carboniferous peats developed locally. The major phase of peat formation commenced in the Namurian A and persisted to the end of the Stephanian, spanning 10–30 Ma in different regions of Euramerica. It is ascribed in part to the residence of northward-drifting Euramerica within the equatorial rainy belt. Within individual depocenters, basin hydrology was suitable for peatland formation during a period (10 6–10 7 years) represented by the stratigraphic distribution of coal measures and termed here the basin-fill “coal window”. Repeated successions (cyclothems) of coal, siliciclastic and carbonate strata punctuate the coal window and typically represent 10 4–10 5 years. These stratal successions are widely ascribed to orbitally driven glacioeustasy, although cyclothems that lack marine strata may represent associated climatic shifts. Cyclothem groupings (mesothems) of 10 6 years duration are recognised locally, and may have eustatic or tectonic causes. Cyclothems produced by autocyclic events such as delta switching should have durations in the order of 10 3 years. The coal beds themselves represent autogenic processes of peat formation over periods of 10 3–10 4 years, constrained by longer term allogenic effects. Recorded in the coal bed are tolerable allogenic events such as fires and floods caused by short-term climatic excursions. The termination of most mires probably occurred when single or combined allogenic effects surpassed the inherent ability of the ecosystem to adapt.

Thin eolianites interbedded within a fluvial and marine succession: early proterozoic whitworth formation, mount isa inlier, australia

The early Proterozoic Whitworth Formation in the Mount Isa Inlier, Australia contains the oldest known, thinly bedded eolian dune and amalgamated interdune deposits, and records the complex interplay of eolian, fluvial and marginal-marine depositional processes. The Whitworth Formation accumulated during the waning phases of rifting in a north-south trending trough that began between 1808 and 1790 Ma and ended before 1740 Ma. Three eolian facies are recognized: (1) cosets of cross strata, (2) sets of horizontal to low-angle strata, and (3) isolated sets of cross strata. Facies El records preservation of dunes with low angles of climb without intervening interdune deposits. Sets of facies E2 are interpreted as either sand sheet, preserved dune plinths or amalgamated interdune deposits. Facies E3 is encased in facies E2 and consists of lenses of cross strata with irregular tops. This association of facies is comparable to recent gypsum dune and interdune deposits at White Sands, New Mexico. Facies E3 reflects limited dune preservation during migration and accumulation at or near the groundwater table. Vertical change from amalgamated interdune deposits to dune without interdune deposits records dunefield initiation or lateral migration from a peripheral to a more centralized position within the dunefield. Increased sedimentation rates promoted preservation of dune deposits represented by facies E1. Decreased sedimentation rate and possible limited sediment bypassing resulted in amalgamation of interdune deposits on the dunefield periphery. Eolian facies E1 and E2 are interbedded with ephemeral-river deposits. Three ephemeral-river facies are recognized: (1) cosets of horizontal strata; 2) cosets of horizontal strata and overlying trough cross beds; and (3) cosets of medium-scale trough cross beds. Ephemeral-river facies F3 often are preserved in channels incised into eolian facies E3. Associated marginal-marine facies consist of: (1) thin-bedded arenites of intertidal to supratidal origin, and (2) trough and tabular planar cross-bed sets and cosets that resulted from migration of subaqueous dunes within a tide-dominated subtidal setting. Interbedded fluvial and eolian deposits reflect either autogenic processes, such as reworking of fluvial sands by eolian processes, or incursion of a fluvial system into a dunefield related to allogenic processes such as climatic change. Dune deposits in the Whitworth Formation probably developed under arid to semi-arid conditions as indicated by extensive ephemeral-river deposits, presence of evaporite pseudomorphs, and absence of wet interdune deposits. Interbedded eolian, fluvial, and marginal-marine deposits resulted from relative sea-level fluctuations related to a combination of eustasy and intrabasinal tectonics.

A 12 000 year record of vegetation change and soil development from Wien Lake, central Alaska

Pollen, plant-macrofossil, macroscopic-charcoal, and geochemical analyses of a sediment core from Wien Lake provide new information on the late Quaternary environmental history of central Alaska. Shrub tundra dominated by Betula glandulosa occupied the area 12 000 – 10 500 BP. Low plant cover and intensive soil erosion of the tundra landscape are indicated by low pollen-accumulation rates, high sediment inorganic content, and high allogenic elemental concentrations. Around 10 500 BP, Populus and Salix invaded the shrub tundra and open ground to form dense stands within the lake catchment. The marked increases in sediment organic content and authigenic concentrations of Fe, Mn, and Al during the period of Populus–Salix dominance suggest humic buildup and stabilization of the catchment soils. These soil changes in turn may have contributed to the demise of Populus–Salix communities 9500 BP. Fossil seeds indicate that Betula papyrifera arrived 9500 BP, rather than in the middle to late Holocene as suggested by previous palynological studies. Picea glauca codominated open woodlands with B. papyrifera 9500 – 7500 BP. The decline of Picea glauca 7500 BP was probably due to an episode of climatic cooling rather than autogenic processes resulting in waterlogged soils. Alnus arrived in the region 7500 BP. After 6500 BP, modern boreal forest dynamics are indicated by the dominance of Picea mariana, fluctuations of Picea glauca, and frequent occurrence of local fires. Key words: late Quaternary paleoecology, Alaska, vegetation and soil history, pollen, plant-macrofossils, geochemistry.

Role of Ecosystem Development and Climate Change in Bog Formation in Central Sweden

Bog development, in terms of the rates of horizontal and vertical accumulation of peat and the timing of landform development of open—water pools, was examined on two concentric raised bogs in central Sweden. The results are compared with three models (allogenic, autogenic, and neutral models) of bog development in order to evaluate the relative contribution of environmental factors vs. ecosystem control of developmental processes. Both mires began to grow on discrete upland sites °6000 BP and then spread concentrically and accumulated peat vertically at approximately continuous rates to the present. Radiocarbon dates from pool sediments are progressively younger from the center to the margin of the mire, suggesting that pool formation is triggered autogenically by changes in hydrology. The results conform to hydrological models of mire formation based on groundwater mound equations and suggest that autogenic processes exert a major control over bog expansion, landform development, and the formation of conspicuous features in the stratigraphic record.

Gap Characteristics and Vegetation Response in Coniferous Forests of the Pacific Northwest

The role of gaps of various sizes and origins in the regeneration of Nothofagus dombeyi varies markedly along the transandean gradient. This variation chiefly results from site-to-site differences in associated tree species and understory bamboos, as well as from differences in the abiotic environment. Other wide-ranging tree species, such as Pseudotsuga menziesii in the Pacific Northwest (Spies and Franklin 1989), are also known to respond differently to gaps in different forest types. In assessing the response of a given species to gaps, ecologists should consider that the response may vary geographically. Gap response may be predicated on the particular physical and biotic characteristics of the study site and might well be different in other parts of the species range. In South American Nothofagus forests, gap-creating disturbances of various types clearly have a major influence on forest structure and composition, and knowledge of disturbance regimes is essential to explaining vegetation patterns. Despite the striking influence of coarse-scale allogenic disturbance on the composition of these forests, the influence of fine-scale autogenic processes must also be considered. For example, microsite modification by both understory bamboos (Veblen 1982) and canopy trees (Veblen et al. 1979) may influence the response patterns of Nothofagus spp. to gaps. The recent emphasis in forest ecology on the pervasive influences of allogenic disturbance should not divert attention from autogenic processes, since most vegetation change reflects both types of processes.

Recognition of Cycle Mechanisms from Study of Single Section: Lessons Learned in Madera Formation, Sandia Mountains, New Mexico: ABSTRACT

Eustasy, tectonics, and autogenic shoreline-related processes contribute to generation of transgressive-regressive (T-R) depositional cycles. Understanding the origin of a transgressive surface or a progradational sequence is critical for reconstructing paleogeography and predicting facies geometries. Knowledge of lateral extent and character of a cycle package provides the least ambiguous interpretation of the mechanism for changing shoreline position. In cases of poor exposure, low density of well data, or structural complexity, such interpretations must come from examination of single sections. This has been attempted during study of 300 m of cyclic carbonate/clastic sediments of the Pennsylvanian Madera Formation. There are 27 complete T-R cycles in the section, 15 in the lowest 120 m. The implied high frequency of the cycles (fourth order) implies that tectonism is less important than eustasy or autogenic processes in generating the cycles. Upward-coarsening delta-progradation sequences are rare. Most regressive sequences feature placement of delta-plain clastics on subtidal limestone without intervening delta-front or prodelta clastics. The uppermost meter of limestone shows evidence of abrupt shoaling; regressive grainstones underwent pervasive early cementation by meteoric waters. These relationships require that sea level fall. Most transgressive sequences exhibit ravinement surfaces with phosphate-bearing lags that truncate delta-plain facies and pass upward throughmore » thin transgressive grainstones to offshore lime mudstones. These may be of autogenic or eustatic origin. Few transgressive sequences show transitions from fluvial to shoreface clastics; transgression without diminished clastic input and accommodation of transgressive clastics would be favored by sea level rise. Most regressions appear to be eustatic, whereas the origin of most transgressions is ambiguous.« less

Historical influence of man on the riparian dynamics of a fluvial landscape

Man's influence, over the last three centuries, has gradually influenced the dynamics of forest cover along the valley of the Garonne, a seventh order river in Southern France. The vegetation cover of the floodplain depends on topographical levels which govern the frequency and duration of submergence during flooding. Along the valley, forest patches vary from a continuous ribbon of riparian wood along the river to a mosaic of groves towards the upland terraces. In the floodplain, the forest dynamics are influenced by floods, appear to be reversible, and are subject to dominant allogenic processes. On the contrary, forest dynamics on the terraces, which are not influenced by floods, are irreversible and subjected to dominant autogenic processes. Since the end of the 17th century, the structure of riparian woods has been modified by navigation and agriculture leading to a fragmentation of forest cover in the floodplain. Modern agriculture and urbanization have accentuated these tendencies by modifying the hydrologic regime of the river. These historical changes result in a fragmentation of forest cover and a substitution of species in the riparian zone, the forest dynamics being still reversible in the floodplain.

Mid-holocene adaptations and later Mesolithic forest disturbance in Northern England

The Holocene period saw the gradual replacement of tundra vegetation by, first, pine—hazel woodland and then by thermophilous deciduous forest. Within these environments the Mesolithic cultures of the British Isles developed their foraging economy. In the Later Mesolithic ( c 8500-5300 bp) we find that palaeoecological investigations provide evidence of disturbance of the forest vegetation, usually associated with the presence of fire. Although there is no direct evidence of human involvement in the cycle of disturbance and regeneration, it seems more probable to invoke the activity of human subsistence-related activities than natural autogenic processes. We suggest that in both uplands and lowlands, population pressure in the Later Mesolithic forced the adoption of more intensive use of game and vegetable resources, particularly deer and hazel nuts. Eventually, the adoption of cultivated cereals and probably of domestic animals can be seen as a shift in economy well within the scale of previous changes in subsistence strategy; there was no need for a change in social—ecological structures. Thus, the historic British tradition of reform rather than revolution seems to have had quite an early start.

Punctuated aggradational cycles: Implications for stratigraphic analysis

Application of the hypothesis of punctuated aggradational cycles (PACs), a model of episodic stratigraphic accumulation, has yielded results fundamentally in conflict with those predicted and generated by gradualistic models. The assumption that stratigraphic accumulation is controlled principally by rapid small‐scale base level fluctuations in the Milankovitch band leads to the adoption of the PAC, a thin time‐stratigraphic unit, as the fundamental unit of stratigraphic description and interpretation; the thicker generally diachronous formation is inadequate for stratigraphic analysis because it is an artifact of stratigraphic gradualism. Correlation of stratigraphic sections is accomplished by matching patterns of facies change, particularly the abrupt changes at the synchronous surfaces that mark PAC boundaries. This kind of physical event correlation is far more accurate than traditional biostratigraphic methods. Focus on synchronous PAC boundaries separating environmentally disjunct facies reveals discontinuities that preclude application of Walther's law across those surfaces. When viewed from this episodic perspective, the Manlius Formation of the Helderberg Group in New York state becomes a highly ordered set of time‐stratigraphic cycles rather than a disordered mosaic of facies representing short‐term autogenic processes in a single nearshore environment. Not only was Manlius deposition episodic with no gradual upward deepening as is predicted by stratigraphic gradualism, but also the Manlius‐Coeymans formational boundary is interpreted as a small‐scale unconformity rather than a paleoenvironmental boundary created by gradual transgression. Each of these interpretations is the direct result of assuming and applying a model of episodic stratigraphic accumulation.