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‘Wind tide’ deposits in a playa lake

Written by Kathryn Amos on . Posted in Research highlights

Here is something a little out of the ordinary; classic tidal indicator sedimentary structures that were observed at the shoreline of Lake Eyre, a playa in semi-arid central Australia. They include mud draped foresets, with lateral change from muddier to sandier bundles, paired mud drapes, reactivation surfaces, flaser, wavy and lenticular bedding, and sand- and mud-filled dessication cracks. 116 Such features are commonly formed by tidal currents in marginal marine environments, related to astronomical forcing by the sun and moon. This isn't possible in a shallow ephemeral lake! 107 These features in Lake Eyre are interpreted to result from tides formed as a result of daily fluctuations in wind direction and velocity, along with weekly to monthly variations in discharge from the river. It is possible that interpretations of such structures in ancient successions without other lines of evidence for a marine environment may instead be continental. If you're keen to read more, this work was published by Ainsworth et al in Geology in 2012 (v40 n7 p607-610). Screenshot_20160729-181024

Unusual silty turbidites

Written by Kathryn Amos on . Posted in Research highlights

A new paper in Sedimentary Geology presents some great research describing interesting proximal silt-rich turbidites from the Secretary Basin, New Zealand. They're unusual, because silty turbidites are usually interpreted as distal marine deposits. This paper presents multibeam bathymetry and shallow sediment core data to present a suite of late Holocene proximal sandy-silt and silty-sand turbidites that contain negligible clay. The authors suggest that these deposits represent a previously undescribed suite of proximal continental slope deposits, based on their coarseness, sand-silt character, wide variety of bed types and inferred formative flow types. A range of non-cohesive flow processes are interpreted, with deposition from multi-phase and mixed-mode (ie both turbulent and laminar) flow. Flow partitioning is interpreted to most likely be the result of a variety of up-dip partial flow transformtations. This paper is a "must read" if you're interested in gravity flow processes and deposits!

Schematic diagrams of turbidity current processes. Figure taken from Strachan et al paper, link below.

Schematic diagrams of turbidity current processes. Figure taken from Strachan et al paper, link below.



http://www.sciencedirect.com/science/article/pii/S0037073816301506

AESC Day 1, excellent sedi presentations

Written by Kathryn Amos on . Posted in Meetings, Research highlights

The Australian Earth Sciences Convention is off to a great start today with some excellent sedimentology presentations. Highlights included the Reservoir Analogues session talks, including an interpretation of the Durham Downs gas field fluvial reservoirs, a modern study investigating why rivers avulse, a detailed outcrop investigation of fluvial-deltaic and salt-withdrawal minibasin deposits in the Flinders Ranges, and an investigation of the influence of organic matter on dynamics of clay sediment gravity flows. Another highlight was the full day of presentations on the scientific results of the International Ocean Discovery Program. We're looking forward to the next 3 days of the conference!

Melissa Craig presenting on the influence of organic matter on sediment gravity flows

Melissa Craig presenting on the influence of organic matter on sediment gravity flows

Dryland Analogues and the Lake Eyre Basin

Written by Kathryn Amos on . Posted in Research highlights

By Gresley Wakelin-King and Kathryn Amos.

Dryland environments form an important part of the rock record, yet there is a lot that we don’t understand about the sedimentology and geomorphology of these systems. Studies of more modern drylands will help us to better interpret the deposits from similarly arid in the ancient record. The rock record around the world includes many fluvial deposits laid down in dryland basins within stable continental interiors (intracratonic), some of which are significant hydrocarbon reservoirs. Within a hydrocarbon province, the spatial arrangement and connectivity of permeable rocks is an important influence on resource value. In clastic sediments, sands and muds are a key factor in permeability, and understanding sediment distribution patterns is important for predicting reservoir, seal, migration pathway and production properties.

An effective way to study modern analogues is to combine geomorphology and sedimentology: the sediment content in, and spatial relationships between, landform elements not only maps lithology distributions, it also demonstrates the agents and processes that link landforms in causally-related systems which govern sediment distribution. These processes operate across scales from macro (basin scale) to micro (sedimentary structures and grain scale) and can be used to inform facies models, can be set within a process-based and sequence stratigraphic hierarchy.

fluvial-aeolian deposits Dryland rivers are more diverse than has been recognised in the past, and the Australian drylands’ history of tectonic stability under a variety of climates has created landforms that are in some ways dissimilar to other drylands of the world. The present surface of Australia’s Lake Eyre Basin contains a range of varied depositional environments, which are a presently under-utilised modern analogue for ancient fluvial systems. Its remote location and harsh climate means that its rivers are considerably under-researched, with great potential for future investigations.

The Lake Eyre Basin (LEB) occupies more than one seventh of the Australian continent, and is a wide and shallow inland-draining (endorheic) basin. The depocentre is the large playa lake Kati Thanda – Lake Eyre. Potential annual evaporation greatly exceeds annual rainfall throughout the basin, and so its rivers are dryland systems. The rivers of the Channel Country in the eastern LEB are within reach of monsoonal rains, and tropical depressions sometimes bring heavy rain to central Australia. The present surface of the LEB consists of sandy and/or muddy depositional lowlands (river valleys, lakes, plains and sandy dunefields) fringed by and separated by non-depositional or erosional uplands (rocky uplands, alluvial fans, desert pavements, residual red-earth or black-soil plains). Modern sediments are in onlap, downlap and basin-fill relationshops with the underlaying formations. It is important to note that the floor of the playa Kati Thanda – Lake Eyre is not the only base level of relevance to LEB rivers. Slow uplift in the Innamincka Dome constrains Cooper Creek into a narrow valley, with flow and sediment impounded upvalley of the dome; similar local uplift controls a local base level and acts as a barrier to downvalley sediment transport on the Diamantina and Georgina Rivers.

Lake Eyre drainage basin An example of an under-studied fluvial process and related deposits is that of arid-zone river floodouts, which are a common feature of the Australian dryland landscape. The description and interpretation of fluvial networks with downstream-radiating channels that decrease in size and sand:mud rations downstream have been the topic of some debate in the published literature of late. The LEB has a diversity of fluvial systems that fit this general description. In geomorphic terms, they are either floodouts, or drainage networks on low-angle alluvial fans. As sedimentary packages, some are depositing valley-or basin-fill, and some are downlapping from a locally elevated base-level onto a lower-elevation surface.

Floodouts are a landform in which a downstream decrease in sediment transport capability leads to declining channel capacity and an increasing proportion of overbank flow. Eventually the channel ceases to exist, and floods travel as unchannelised shallow flow across river reaches that are effectively 100% floodplain. The Finke River Floodout is the terminus of the Finke River, a river with a sandy bedload and single-thread pattern. It is semi-confined and partially sinuous within the valley of the larger palaeo-Finke River.

Finke River Floodout As the Finke River approaches the depositional lowlands of the Simpson Desert, its valley widens and the river channel widens, shallows, and becomes multi-thread and unstable (Figure A). Channel width decreases with distance downstream, and floodplains are more often inundated than they are upstream. The valley margin widens abruptly (B) to form a non-lacustrine topographic basin ~ 20km wide, and channel size diminishes rapidly through a series of bifurcations (C, D). The channel terminates ~ 8.5km downstream from its entry into the basin. Channel sediments are deposited in flat-topped 2D dunes and low-flow regime planar beds in shallow cut-and-fill structures.

Bifurcations have a major and a minor arm; the major arm propagating flow and sediment downstream into the floodout and leads to the next bifurcation. Lobes of sediment are deposited adjacent to and downstream of the ends of the bifurcated channels (C). At present, four bifurcated channels are actively carrying flow (C), three of which are delivering water to low-elevation swampy areas, and one is currently depositing sandy sediments (D). The channels here are therefore not distributary in terms of sediment load. Downvalley, unconfined flow travels ~ 20k m across floodplain. In some places, short discontinuous channels are formed before these floodout again. No subsurface investigations have been conducted here to date; surface geomorphology indicates that preserved in the rock record these sediments would present as weakly stratified or massive bioturbated silty sands with organic-rich horizons, common rhizomorphs, cut-and-fill structures, and sparse Aeolian layers.



*This text and figures have been taken from a recent publication by Wakelin-King and Amos (2016) A time-slice of the Lake Eyre Basin: Sand/mud depositional geometries in a diverse lowstand endorheic drylands setting. Eastern Australian basins Symposium: Publication of Proceedings, 97-113.
*See Wakelin-King and Amos (2016) for a full reference list. If you found this interesting, here are a couple of key references:
- Nanson, G.C., Tooth, S. and Knighton, A.D., 2002. A global perspective on dryland rivers: perceptions, misconceptions, and distinctions. In: Bull, L.J. and Kirkby, M.J. (eds.), Dryland Rivers: Hydrology and Geomorphology of Semi-Arid Channels. John Wiley and Sons, Chichester; pp. 17-54.
- Tooth, S., 2000a. Process, form and change in dryland rivers; a review of recent research. Earth-Science Reviews 51: 67-107.

Solving a tuff problem: a practical approach to correlating nonmarine strata

Written by Kathryn Amos on . Posted in Research highlights

By Carmine Wainman, PhD Candidate, Australian School of Petroleum, University of Adelaide.

Correlating nonmarine strata is a major challenge for any academic or industry geologist. The idiom “there are as many interpretations as there are geologists” usually pops into one’s mind when geomodels of fluvio-lacustrine strata are presented in journal articles or in meetings discussing drilling proposals.

Fluvio-lacustrine lithofacies elements are more often than not heterogeneous and laterally discontinuous at the decimetre scale. One only has to go to an outcrop to see what has to be contended with in the subsurface (and worse still if you only have well logs and cuttings as your dataset!). 3D Seismic has substantially improved subsurface mapping of these systems in the past decade, but one thing remains to be resolved – defining meaningful chronostratigraphic surfaces for basin-wide correlation.

Slide1 Without widespread marker beds, marine influences or a biostratigraphic scheme tied to the international geologic timescale, stratigraphic models of fluvial systems are open to interpretation and can go wrong very quickly at scales larger than the average size of a typical gas field (~10km2). Geomodels and well plans may look spectacular on a computer screen, but if a chronostratigraphic surface is mis-picked, it could have expensive implications for the prediction (and drilling) of reservoir-seal pairs.

One solution is to radiometrically date a series of coeval volcanic ash-fall tuff beds across a basin. Dating zircons within these tuff beds using the high precision CA-TIMS technique (short for chemical abrasion thermal ionization mass spectrometry) from the ratio of uranium and lead hosted within each crystal allows radiometric ages to be determined within an error margin of “40kyr”. Correlating by radiometric ages within a defined error margin over large distances brings certainty to nonmarine correlation – rocks which have been dated at a specific depth in any the well are time equivalent, or they are not! Slide2 This technique has been applied for the Jurassic Walloon Coal Measures of the Surat and Clarence-Moreton Basins of Australia, infamous for its thin, discontinuous coal beds. Radiometric ages have not only revealed the Walloon Coal Measures to be younger (Late as opposed to Middle Jurassic) than anticipated from palynology, but has revealed diachroneity in the lithostratigraphic base of this stratigraphic unit between the two basins – a whole 2 stages of Jurassic (Bathonian and Oxfordian). The ability to define chronostratigraphic surfaces over 100‘s of km has revealed thinning of this stratigraphic interval from east to west, but also the role syntectonism played in sediment dispersion and the continual movement of peat mires across these basins on timescales of less than 1 million years. The same dates will enable correlation to be undertaken on regional scales for precise palaeogeographic reconstructions, important for resource prediction. Although the high abundance of volcanic air-fall tuffs (10+) in the Walloon Coal Measures is more of an exception than the norm, if fluvial sandstones comprise a volcaniclastic component and are thousands of kilometers from known sources of volcanism, they too contain zircon that can be dated using CA-TIMS to determine the maximum age of deposition for these rocks (and subsequently correlated). This technique has the ability to save time, effort and the ambiguity associated with exploring nonmarine strata.