Kathryn J. Amos

Actualistic Studies of the Spatial and Temporal Distribution of Terrestrial and Aquatic Organism Traces in Continental Environments to Differentiate Lacustrine from Fluvial, Eolian, and Marine Deposits in the Geologic Record

Actualistic studies of modern continental environments and the spatial and temporal distribution of terrestrial and aquatic organisms are summarized and synthesized to understand how to better interpret the significance of trace fossils to differentiate lacustrine from fluvial, eolian, and marine deposits in the geologic record. The purpose of this approach is to develop an understanding of the physicochemical factors that control the occurrence, diversity, abundance, and tiering of organism behavior and parallels what is known for benthic and other trace-making organisms in marine environments. The distribution of traces observed in Lake Tanganyika and Lake Eyre, an overfilled lake in a tropical rift basin setting and an underfilled lake in an arid midlatitude ephemeral playa setting, respectively, are described, synthesized, and compared with the Mermia, Coprinisphaera, Termitichnus, Skolithos, and Scoyenia ichnofacies models proposed for continental environments. The comparisons show that all the models are inappropriate for the fluvial-lacustrine settings of Lake Tanganyika and Lake Eyre because the models do not support the environmental uniqueness or distinctive collection of traces across these environments, nor do they provide sufficient interpretive power. The multiple ichnocoenoses for each subenvironment observed in the balanced-filled and underfilled lacustrine systems more accurately record the environmental uniqueness and distinctive collection of traces found in each environment. Ichnocoenoses are better suited for continental depositional systems and their environments because they reflect the nature of processes and distribution of life in continental settings, which are inherently heterogeneous spatially and temporally. Ichnocoenoses also provide sufficient interpretive power for trace-fossil associations formed under different physicochemical conditions for each type of environment. General trends in trace-fossil diversity, abundance, distribution, and tiering are predicted for alluvial (fluvial), lacustrine, and eolian environments so that new models based on the distribution of ichnocoenoses and their sedimentary and pedogenic characteristics from outcrop and core can be constructed.

Sedimentological interpretation of an Ediacaran delta: Bonney Sandstone, South Australia

ABSTRACT The type section of the late Ediacaran (ca 565 Ma) Bonney Sandstone in South Australia provides an opportunity to interpret a succession of Precambrian clastic sediments using physical sedimentary structures, lithologies and stacking patterns. Facies models, sequence stratigraphic analysis, and process-based architectural classification of depositional elements were used to interpret depositional environments for a series of disconformity-bounded intervals. This study is the first detailed published work on the Bonney Sandstone, and provides additional context for other Wilpena Group sediments, including the overlying Rawnsley Quartzite and its early metazoan fossils. Results show that the ∼300 m-thick section studied here shows a progressive change from shallow marine to fluvially dominated sediments, having been deposited in storm-dominated shelf and lower shoreface environments, lower in the section, and consisting primarily of stacked channel sands, in a proximal deltaic environment near the top. Based on the degree of influence of wave, tidal or fluvial depositional processes, shallow marine sediments can be classified into beach, mouth bar, delta lobe and channel depositional elements, which can be used to assist in predicting sandbody geometries when only limited information is available. Sediments are contained within a hierarchical series of regressive, coarsening-upward sequences, which are in turn part of a larger basin-scale sequence that likely reflects normal regression and filling of accommodation throughout a highstand systems tract. Paleogeographic reconstructions suggest the area was part of a fluvially dominated clastic shoreline; this is consistent with previous reconstructions that indicate the area was on the western edge of the basin adjacent to the landward Gawler Craton. This research fills in a knowledge gap in the depositional history of a prominent unit in the Adelaide Rift Complex and is a case study in the interpretation of ancient deposits that are limited in extent or lacking diagnostic features.

Fluvial reservoirs in dryland endorheic basins: the Lake Eyre Basin as a world-class modern analogue

Intracratonic dryland basins have been common throughout geological time and significant hydrocarbon reservoirs are contained in these basins. Based on a literature compilation of fluvial dryland reservoirs, the authors demonstrate the need for new modern analogue data from dryland fluvial systems, and present new field data from the Neales River, in the Lake Eyre catchment. The selected study reach has a complex planform, with a downstream transition from single channel to anabranching. Results of the observations of the channel bed grain size, the geomorphology and the channel geometry (width and depth) allow the authors to infer that the channel bed grain size is more strongly related to planform geomorphology than the channel geometry (width to depth ratios). Based on the grain size and channel geometry data the authors present, the authors conclude that the planform geomorphology exerts a greater control on channel bed material size than channel geometry. This interpretation is based on the analysis of satellite imagery, topographic survey data and grain size descriptions. In this paper, the authors provide channel geometry data and grain size data that will improve understanding of dryland fluvial sedimentology. The authors hope this contributes to enhancing hydrocarbon exploration and production in petroleum reservoirs developed in dryland fluvial settings.

An outcrop analogue for deepwater salt withdrawal mini-basins: lateral and vertical variations in basin-fill

Mini-basins are well known targets for petroleum exploration as they can contain significant hydrocarbon reserves, such as in the Gulf of Mexico. Though mini-basins have been studied before, their reservoir rock distributions remain poorly predictable. This is especially the case where mini-basins are near salt-diapirs. The Donkey Bore Syncline in the Flinders Ranges, South Australia, presents an excellently exposed deepwater mini-basin reservoir analogue. Detailed outcrop study, including vertical and lateral logged sections presented here, shed considerable light on the depositional system, especially with regards to depositional style and stages of basin-fill. Based on the observed facies and deposit geometries, the authors have been able to detect the change from a carbonate to a siliciclastic deposition system and interpret mini-basin-fill initiation and progradation processes. Sandy gravity-flow deposit lobe areas are identified, enclosed within shales. Along with paleocurrent data, these enable the description and interpretation of basin-fill stages and the lateral character of reservoir-analogue depositional tracts. This will be useful for better interpretation of mini-basin turbidite sequences and will help reduce uncertainties in subsurface exploration. This may also improve prediction of basin-fill where well spacing is inadequate.