J Alexander

Ground penetrating radar: application to sandbody geometry and heterogeneity studies

Abstract Ground penetrating radar (GPR) offers a high-resolution, shallow subsurface profiling technique for use in sedimentological and reservoir analogue studies. GPR is similar to seismic reflection profiling but uses electromagnetic radiation in the 50 to 500 MHz frequency range (in geological applications). By using these relatively high frequencies, high resolution data can be obtained. Short duration pulses of electromagnetic energy are transmitted into the ground, reflected from interfaces across which there are abrupt changes in dielectric properties, and are detected by a receiver. These received signals are displayed in nanoseconds two-way time and may be recorded digitally allowing subsequent processing. Some 1000 m of 2D GPR profiles were collected from a modern point bar on the Madison River, Montana USA and have been interpreted using an approach similar to seismic stratigraphic analysis. This has allowed identification of a number of radar sequences and radar facies. Radar sequence boundaries are identified by reflector terminations (onlap, downlap, toplap and erosional truncation) and represent episodes of erosion during the development of the point bar. In contrast, radar sequences and their component radar facies record phases of accretion of the point bar. Each radar sequence is linked to a discrete accretionary unit that can be mapped on the surface of the point bar. Mapping of the radar sequences and radar facies has allowed quantification of their 3D geometry.

Observations on Experimental, Nonchannelized, High-Concentration Turbidity Currents and Variations in Deposits Around Obstacles

ABSTRACT The influence of sea-floor topography on the deposits and flow paths of high-density, coarse-grained turbidity currents was investigated in experiments that concentrated on simple, wedge-shaped obstacles, representing tilted fault blocks, with heights of the same order as the thickness of the flows on a flat tank floor. Particle size, initial particle concentration, and tank floor configuration were varied independently, and the flow paths, sediment distribution, and surface features were recorded. These small topographic features significantly alter sediment distribution. Stationary mixing vortices (standing billows) formed near the obstacles, and the variations in flow thickness and velocity around the obstacle caused local, rapid deposition and resulted in very abrupt thickness var ations across the tank floor. Although the amplitude of thickness variations around obstacles is dependent on grain size and concentration (because of differences in settling velocity), the position of thickness variations is controlled strongly by the obstacle and flow characteristics and is largely independent of grain size.

Holocene Meander-Belt Evolution in an Active Extensional Basin, Southwestern Montana

ABSTRACT Lateral migration fluvial meander belts preserves wider than normal coarse-sediment bodies, asymmetric meander belts, that have a complex architecture recording the history of channel movement. Detailed investigations of the morphology and deposits of asymmetric meander belts of the Madison River and its South Fork, southwestern Montana (using topographic mapping, ground-penetrating radar profiling, coring, sonar, and radiocarbon dating) have been undertaken in an attempt to show how they are influenced by tectonic tilting and faulting, base-level change, catchment evolution, and climate change. The asymmetrical form of the meander belts resulted from a regional tectonically induced lateral bias to channel position over at least the last 10 ky. Radiocarbon ages from abandoned channel ills increase progressively up-tilt away from the active meander belts. The South Fork meander loops decreased in wavelength and channel width through the Holocene, but no systematic changes in channel size have been recognized in the Madison meander belt. These results indicate that the changes in discharge patterns of the two rivers differed, and this difference may be related to local climate change and drainage evolution in their catchments. Episodes of incision during meander-belt evolution have isolated abandoned meander loops with restricted age ranges on terraces above the modern flood plains. The terraces are extensive on the up-tilt side of both meander belts and on the footwalls of intrabasinal faults, with only small terrace remnants on the down-tilt side of the meander belts The distribution of terraces and large abandoned meander loops in the South Fork shows that channel-belt asymmetry did not develop by progressive lateral migration of the meander-belt axis, as previously suggested, but rather that the flood plain episodically decreased in width, with the down-tilt margin fixed in space. Some of the Madison River terraces, however, do show evidence of down-tilt migration of the river. Preservation of diatomite in downstream parts of both meander belts suggests periodic transgression by an ancestral Lake Hebgen. Changing lake shore position may explain some of the episodes of incision, and is probably associated with large seismic events and surface deformation.

Abrupt change in slope causes variation in the deposit thickness of concentrated particle-driven density currents

Abstract Abrupt changes in slope occur frequently in some marine environments. Laterally confined laboratory experiments show that when dense surge-like particulate flows travel over abrupt slope reductions their deposits may have an asymmetric bell-shaped thickening near the break of slope. This thickening is herein called a slope-break deposit and is related to the change in kinetic energy of the flow. Slope-break deposits are particularly pronounced under relatively high-velocity flows. In our experiments these are flows with high particle concentrations ( c =5–10% by volume silicon carbide) moving down slopes steeper than 3°. The experiments were designed such that currents generated by a lock-exchange mechanism flowed down a slope (variable slope angle) onto a flat surface. The bodies of the flows became thicker and slower just downstream of the slope break. Because the flow body slows very rapidly, particles are dumped forming the slope-break deposit. The peak in sediment thickness of the slope-break deposit is downstream of the break in slope because falling particles continue to move forward in the flow during settling. The slope-break deposits may be several tens of percent thicker than deposits from an equivalent flow in which no change in slope occurred. The parameters of the slope-break deposit correlate with slope angle change.

Facies model for fluvial systems in the seasonal tropics and subtropics

Facies models that summarize the deposits of fluvial systems are well established for humid climate settings and for desert environments, but the deposits of rivers in the sub-humid and semiarid seasonal tropics have been largely ignored. Our observations and data from modern streams and recent deposits in northeastern Australia show how such rivers, with extremely variable discharge, have distinctive deposit characteristics that are substantially different from conventional fluvial facies models. These properties include (1) erosionally based channel-fill lithosomes that exhibit complex lateral facies changes, with (2) abundant, pedogenically modified mud partings, (3) complex internal architecture that may lack the macroform elements typical of other fluvial sediment bodies, (4) an abundance of sedimentary structures formed under high flow stage, and (5) an abundance of in situ trees that colonize channel floors and are adapted to inundation by fast-flowing water. We illustrate examples of this fluvial style from the rock record, and set out a new facies model. The recognition of such a distinctive fluvial character, and of changes in character through vertical successions, will aid paleoclimate and reservoir analysis.

Preservation of in situ, arborescent vegetation and fluvial bar construction in the Burdekin River of north Queensland, Australia

Abstract In sub-humid parts of north Queensland, NE Australia, certain types of trees are well adapted to living in river bed habitats. The bed of the tropical, variable-discharge, upper Burdekin River hosts a community dominated by the paperbark Melaleuca argentea. Trees grow preferentially in flow-parallel, linear groves, and engineer their own environment by deflecting currents, building sand and gravel bars and stabising banks. This is the first study to document in-channel bar development resulting from vegetation growth, rather than the reverse which has been inferred by previous workers. In the Burdekin River study site, individual Melaleuca range from seedlings to mature trees over 100 years old. These trees survive regular, partial to total submergence and impact damage during wet season runoff events (often reaching over 20,000 m3 s−1 at peak discharge) partly by adopting structural and growth modifications. These modifications include a reclined, downstream-trailing habit, multiple-stemmed form, modified crown with weeping foliage, development of thick, spongy bark, root regeneration and group strategies, notably development of flow-parallel, linear groves. Following death, in situ remains of trees are preserved within the mainly coarse sand to gravel channel fill, either as reclined stems/trunks stripped of branches and foliage or as more upright trunks snapped at a height of typically 1–2 m above base, both with roots. The morphological adapations and styles of preservations of in situ vegetation within the Burdekin River are considered distinctive of variable-discharge rivers. and may be useful in the identification of facies formed in such environments in the rock record, particularly when associated with bar development.

Variations in character and preservation potential of vegetation-induced obstacle marks in the variable discharge Burdekin River of north Queensland, Australia

Abstract Vegetation-induced obstacle marks are described from Dalrymple Bend in the Burdekin River of Queensland, Australia, and their preservation potential is discussed. The bend is divided into a sand- and gravel-covered lower bar, and a vegetated upper bar. Obstacle marks, comprising an erosional scour and a depositional sediment tail, are recognised on the lower bar and the lower margin of the upper bar. Two types of obstacle marks are dominant in the lower part of the lower bar; sediment tail-dominated obstacle marks associated with inclined trees of Melaleuca argentea (Silver-leaved Paperbark), and comparatively small obstacle marks around stranded driftwood. Obstacle marks in the upper part of the lower bar are formed around grass clumps armoured with mud and gravel. At the lower margin of the upper bar, scours and gravelly sediment tails are recognised around mature tall trees. In these cases, the scour is >4 m in width, and covered with alternating beds of fine sand and mud. Among the obstacle marks described, sediment tail-dominated obstacle marks around groves of M. argentea have the highest preservation potential. Obstacle marks in the lower margin of the upper bar are also considered to have high preservation potential. Obstacle marks around stranded driftwood have the lowest preservation potential. Obstacle marks around grass clumps survive more than 1 year, but their long-term preservation potential is nonetheless low. Implications for the stratigraphic record are also discussed.

Experimental quasi-steady density currents

Abstract Hyperpycnal flows may be important mechanisms for transporting particles from continents to oceans given their frequency of generation at many modern river mouths. Their deposits are probably very common in the rock record, although they are not frequently recorded in the literature. These currents may last longer and be steadier than surge-like turbidity flows and consequently their deposits are likely to be significantly different. In this paper simple laboratory experiments are used to investigate how flows and deposits may vary. The thickness of the experimental flows near the flow front varies with distance from the source depending on slope, effluent discharge, grain size and sorting. Runout distance varies with the proportion of fine-grained sediment rather than with mean grain size. Flow front velocity varies with effluent discharge and sediment concentration and in a complex manner with mean grain size and sorting. The variation of deposit mass with runout distance depends on proximal slope angle and the maximum mass per unit area (approximating to bed thickness) is inversely dependent on slope angle. Variation in initial suspended particle concentration results in deposits of similar shape but different volume. Although the total mass of the deposit depends on the discharge the maximum mass per unit area (approximating to maximum bed thickness) does not. The distance over which particles are deposited increases and the deposit flattens as particles are deposited progressively further downstream at higher discharges, perhaps relating to more efficient turbulent suspension at higher Re (more complete energy cascade), in addition to longer horizontal component of settling trajectory at higher velocity. The amplitude of mass variation depends on the grain size. As with laboratory surges [Gladstone et al., Sedimentology (1998) 833–844], the finer the sediment the more efficient is the sediment transport.

Fossil trees in ancient fluvial channel deposits: Evidence of seasonal and longer-term climatic variability

It has been established that large numbers of certain trees can survive in the beds of rivers of northeastern Australia where a strongly seasonal distribution of precipitation causes extreme variations in flow on both a yearly and longer-term basis. In these rivers, minimal flow occurs throughout much of any year and for periods of up to several years, allowing the trees to become established and to adapt their form in order to facilitate their survival in environments that experience periodic inundation by fast-flowing, debris-laden water. Such trees (notably paperbark trees of the angiosperm genus Melaleuca) adopt a reclined to prostrate, downstream-trailing habit, have a multiple-stemmed form, modified crown with weeping foliage, development of thick, spongy bark, anchoring of roots into firm to lithified substrates beneath the channel floor, root regeneration, and develop in flow-parallel, linear groves. Individuals from within flow-parallel, linear groves are preserved in situ within the alluvial deposit of the river following burial and death. Four examples of in situ tree fossils within alluvial channel deposits in the Permian of eastern Australia demonstrate that specialised riverbed plant communities also existed at times in the geological past. These examples, from the Lower Permian Carmila Beds, Upper Permian Moranbah Coal Measures and Baralaba Coal Measures of central Queensland and the Upper Permian Newcastle Coal Measures of central New South Wales, show several of the characteristics of trees described from modern rivers in northeastern Australia, including preservation in closely-spaced groups. These properties, together with independent sedimentological evidence, suggest that the Permian trees were adapted to an environment affected by highly variable runoff, albeit in a more temperate climatic situation than the modem Australian examples. It is proposed that occurrences of fossil trees preserved in situ within alluvial channel deposits may be diagnostic of environments controlled by seasonal and longer-term variability in fluvial runoff, and hence may have value in interpreting aspects of palaeoclimate from ancient alluvial successions

Plant-material deposition in the tropical Burdekin River, Australia: Implications for ancient fluvial sediments

Abstract In the deposits of the Burdekin River (north Queensland, Australia), plant material (character and directional fabric) together with the sedimentary facies character are generally diagnostic of depositional environment. Plant material is selectively entrained, transported, and deposited in all flow conditions ranging from dry season minimal flow (