Archive for May, 2016

Research Story: Sedimentology in the Flinders Ranges of South Australia

Written by Kathryn Amos on . Posted in Research Stories

By John Counts, PhD Candidate, Australian School of Petroleum, University of Adelaide.

My research at the University of Adelaide uses different aspects of sedimentology to reconstruct the paleogeography of South Australia during the Ediacaran Period, just before the first appearance of multicellular life. By looking the distribution of sedimentary features across a large area, we can get a good idea of the physical processes operating at the time of deposition. These could include waves, tides, fluvial processes in rivers, windblown dunes, soil formation, etc.- all of which reveal important information about the ancient landscape. By combining this data with sediment provenance from detrital zircons, we can start to put together a complete picture of the sedimentary system that existed 560 million years ago.

Hiking through an Ediacaran salt diapir

Hiking through an Ediacaran salt diapir, Wirrealpa Station

My work focuses specifically on the Bonney Sandstone, a formation exposed in the Flinders Ranges north of Adelaide. The Flinders were part of a basin that was on the edge of the Australian subcontinent during the Precambrian and Cambrian. Most of the basin fill has been uplifted and is now well-exposed, forming one of the most complete Neoproterozoic successions in the world. Despite this, relatively little work has been done on the detailed sedimentology of individual units. My work attempts to improve our understanding of a key interval in the basin, and to use the detailed study of outcrops to infer properties of similar deposits in the subsurface.

Asymmetric ripples in the Bonney Sandstone

Asymmetric ripples in the Bonney Sandstone

Results thus far show that the Bonney Sandstone is primarily composed of fluvial-deltaic and shallow marine sands and shales, contained within a hierarchical series of coarsening-upward parasequences. These parasequences shallow-upward as deltaic sediments prograde outward into the basin, and depositional environments also become progressively shallower throughout the formation as accommodation decreases. Zircon data indicate that sands are transported from distant cratonic provinces, allowing the reconstruction of large-scale sediment transport patterns across the continent. Salt diapirs were also active during deposition, in some cases penetrating the surface and bringing up clasts of various lithologies from deeper in the basin.

Camping out for field work in the northern Flinders Ranges

Camping out for field work in the northern Flinders Ranges

One of the challenges of working with many Precambrian sediments is the lack of diagnostic trace or body fossils that can pinpoint the environmental setting in otherwise ambiguous sediments. Much of the Bonney is composed of interbedded sands and shales that might be formed in a range of environments; only through careful study of the suite of sedimentary lithofacies present can the environment be determined with some degree of confidence. Fieldwork can also be challenging; most of the Flinders Ranges is remote. The landscape is arid, mountainous, and rugged; mapping and measuring sections usually involves many days self-sufficient camping and many kilometres of hiking each day. Despite these difficulties, my PhD work has been an extremely rewarding experience. Three years ago, I left my job in the industry to get back to my roots as a geologist. I have found that Australia contains excellent geologic features, and in many cases, one can be the first geologist to examine many beautiful outcrops in detail. There are definitely ample opportunities for much more high-impact research to be done here in the future.

Gigapan image of a well exposed section of the Pound Subgroup in the central Flinders Ranges. Click on the image to zoom from outcrop scale to decimetre scale!

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.

New app for the journal Sedimentology

Written by Kathryn Amos on . Posted in Uncategorized

The International Association of Sedimentologists (IAS) have launched a new app for the journal Sedimentology, available free for iphones and ipads. If you're a subscriber, you can now read the journal via the app - you can link the app to an institutional subscription as well as a personal one. Lets hope they're working on bringing one out for android soon too!Capture2

Love this recent ‘President’s comments’ from SEPM’s ‘The Sedimentary Record’ on diversity

Written by Kathryn Amos on . Posted in Uncategorized

Worth a read, and thinking about what we can all do to increase diversity when nominating deserving people for professional society awards.Capture