Rachel A. Nanson

Dynamic spatial and temporal prediction of changes in depositional processes on clastic shorelines: Toward improved subsurface uncertainty reduction and management

Existing classification schemes and models for clastic coastal depositional systems do not consider the potential amplifying or moderating effects of coastal morphology on depositional processes and do not provide a mechanism for the dynamic prediction of changes in coastal depositional style. A new process-based classification scheme based on the relative importance of primary, secondary, and tertiary processes is presented. This scheme permits a semiquantitative classification of clastic coastal depositional systems. In addition, it provides the basis for new models for clastic shorelines that convolve the effects of basin shape, coastal morphology, accommodation space, sediment supply, shoreline trajectory, and shelf width parameters on depositional processes. The end result is a marked improvement in the predictive capabilities of models. The models can describe and predict the likelihood of primary, secondary, and tertiary depositional processes acting in shoreline depositional environments via either a matrix or a decision tree approach. They are also dynamic in nature and can be applied to predict along-strike, updip, and downdip, or vertical changes in the dominance of depositional processes acting at any given location through geologic time. The key implications of these models are that given sets of known parameters, dominant and subordinate depositional processes or ranges of potential dominant and subordinate depositional processes acting at a coastline can be predicted. This provides an auditable methodology for determining reservoir modeling scenarios and reducing and managing the uncertainties in predictions of changes in clastic coastal depositional processes through time and space.

Evolution and architectural styles of a forced-regressive Holocene delta and megafan, Mitchell River, Gulf of Carpentaria, Australia

Abstract The monsoonal Mitchell River delta and megafan shows minimal anthropogenic disturbance and thus provides a unique opportunity to examine the autogenic and allogenic controls on the evolution of deltas and fluvial megafans. Detailed mapping, vibracoring, trenching and topographic surveying were used to characterize the palaeodistributary channel belts on the megafan and the depositional elements that comprise the delta. Chronological analyses of these data facilitated the reconstruction of the megafan and delta evolution and enabled the identification of discrete periods of delta progradation within the last 6000 years. These results indicate that sediment distribution is controlled primarily by two types of avulsion: (1) delta avulsions, which are frequent (>16/1000 years), typically backwater-mediated and associated with local shifts in sedimentation loci; and (2) megafan avulsions, which are less frequent (>3/1000 years), but which are also associated with more significant shifts in depositional loci. These links between megafan and delta processes and geomorphology in the Mitchell River region were integrated to develop a new model of channel belt facies associated with fluvial (F), fluvial backwater-affected (FBW), fluvial-dominated, tide-influenced (Ft) and tide-dominated, fluvial-influenced (Tf) channels. This model enables improved predictions of channel belt composition in modern and ancient marginal-marine systems by providing sedimentological and ichnological criteria for distinguishing between channel types. Supplementary material: Mitchell River region optically stimulated luminescence (OSL) dating methodology and results are available at https://doi.org/10.6084/m9.figshare.c.3280949

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.

Geometric attribute and shape characterization of modern depositional elements

The recent rise in the quality and availability of remote sensing data has benefitted the geoscience community by allowing high resolution studies of the geometry of modern clastic depositional elements, which are analogous to the elements that control fluid flow in subsurface reservoirs. Established methods used to describe the geometry of these features have been predominantly subjective. We present a new objective technique to automate the characterization of centerline attributes (CA) of mapped depositional elements. This technique measures key parameters at a defined sampling interval along a calculated centerline for each input shape, which are automatically analyzed to define geometric shape, length, width, sinuosity, adjacency and centerline deviation. To demonstrate the applicability of the method to a range of depositional environments, mapped sandbodies from two contrasting modern systems were analyzed: (1) the 520km2 mixed-process Mitchell River Delta, Gulf of Carpentaria, Australia; (2) a 1200km reach of the anabranching Congo River, Democratic Republic of the Congo.1696 Wave- and fluvial-derived elements from the Mitchell Delta were analyzed using our CA method and the conventional minimum bounding box (MBB) approach. The MBB results defined the regression slopes as 1.25-4.47 times wider and 0.31-0.97 times shorter than their CA values. Results applied to 2221 mid-channel bar elements in the Congo River showed similar CA and MBB relationships, with linear regression slopes of a MBB as 1.06 times wider and 0.97 times shorter. The inconsistency in the comparative MBB and CA results for these two datasets is attributed to the very different geometries of the sandbodies in these contrasting depositional environments. This suggests that caution should be exercised when applying current methods. A major benefit of the proposed CA method is that it allows quantitative study at scales and levels of detail typically not practical using manual solutions. A new methodology for the quantitative geometric description of modern elements.Automated and objective description of geometric attributes and shapes.Planform adjacency of sandbody elements for analyzing heterogeneity.Significant improvements have been demonstrated over standard GIS tools.

Detailed mapping, three-dimensional modelling and upscaling of a mixed-influence delta system, Mitchell River delta, Gulf of Carpentaria, Australia

Abstract Data from satellite imagery, field measurements and analogues were used to construct a three-dimensional (3D) geocellular facies model of the Mitchell River Delta, Australia; a modern mixed-influence delta system. Detailed mapping identified 16 different facies elements and classified the delta as tide dominated, fluvially influenced and wave affected. The 3D model was subjected to varying degrees of upscaling of the horizontal and vertical dimensions and allowed comparison of volume and connectivity changes throughout. The upscaling process, to coarser grid cells up to 100 m horizontally and 4 m vertically, created false compartmentalization of facies bodies and significant changes in facies bulk volumes. The vertically upscaled models produced greater changes when compared to the horizontally upscaled models. Key changes in reservoir facies connectivity and bulk volume due to upscaling are associated with the facies architecture, including the elongate and thin morphology of beach ridge and channel facies in this mixed-influence delta system. Recognition of the defining reservoir features and incorporation into reservoir modelling methodology can improve volumetric estimation and allow for better predictions of reservoir connectivity in ancient delta systems.

Holocene sea-level change and coastal landscape evolution in the southern Gulf of Carpentaria, Australia:

A revised Holocene sea-level history for the southern Gulf of Carpentaria is presented based on new data from the South Wellesley Archipelago and age recalibration of previous research. Results confirm that rising sea levels during the most recent post-glacial marine transgression breached the Arafura Sill ca. 11,700 cal. yr BP. Sea levels continued to rise to ca. –30 m by 10,000 cal. yr BP, leading to full marine conditions. By 7700 cal. yr BP, sea-level reached present mean sea-level (PMSL) and continued to rise to an elevation of between 1.5 m and 2 m above PMSL. Sea level remained ca. + 1.5 between 7000 and 4000 cal. yr BP, followed by rapid regression to within ± 0.5 m of PMSL by ca. 3500 cal. yr BP. When placed into a wider regional context results from this study show that coastal landscape evolution in the tropical north of Australia was not only dependent on sea-level change but also show a direct correlation with Holocene climate variability. Specifically, the formation and preservation of beach-rock deposits, intertidal successions, beach and chenier ridge systems hold valuable sea-level and Holocene climate proxies that can contribute to the growing research into lower latitude Holocene sea-level and climate histories.