Quantifying lateral and distal variability within hybrid beds, case studies from Central and Northern Italy

Brooks, Dr Hannah1, Steel,Dr Elisabeth2

1 School of Earth Sciences, 253-283 Elgin St, University of Melbourne, Carlton Victoria, Australia, 2 Department of geological sciences and geological engineering, Bruce Wing/Miller Hall, 36 Union Street, Queen’s University, Kingston, Ontario, Canada.

Hybrid beds or linked debrites are deposits that form under bi- or tri-partite flow conditions, involving both turbulent and laminar flow conditions. Often, hybrid beds occur with distal or lateral flow transformation following significant entrainment of a muddy substrate and/or declining turbulent energy. Hybrid beds have been noted to make up significant proportions of deposits within basin floor setting worldwide, most commonly within the distal fringes of lobe systems. The study examines dip and lateral variations in facies and architecture in hybrid beds using detailed facies analysis of selected sections within the Castagnola, Marnoso-Arenacea, and Gottero Formations, deposited within three different basins within central and northern Italy. Sections within these systems were selected where hybrid beds could be traced out laterally or down-dip for several metres to several kilometres. In total 407 samples were taken for laser-diffraction grain-size analysis. Samples were selected through beds at 20 cm intervals and across/ down-dip at 10’s of metre to several kilometre intervals. Layers within beds were classified into facies divisions which were ran through ‘EMMA’ End Member Modelling Analysis, this splits beds into an optimum number of end members that when combined create the trends found within the dataset. This method is utilized to establish patterns of changes within beds laterally and down-dip which are otherwise difficult or too complex to be quantified from field data alone.

This detailed study of facies, architectural and grain-size changes within the targeted deposits will help to establish how flow processes varied as flows spread laterally and moved downstream. Through quantifying the amount of mud within the matrix and clasts at any one time within the flow it is possible to interpret how and when turbidites and hybrid beds erode and incorporate sediment from underlying substrate. Initial results from EMMA indicate that using three end members for each formation is an optimum number for observing basinal trends within beds. These end members constitute a sand-rich unimodal end member, a silt-rich unimodal end member, and a bimodal poorly sorted end member, interpreted to represent a high-density turbidite, a low-density turbidite and a debrite respectively. Recognised trends within the data include a decrease laterally in the sand-rich unimodal end member within the Marnoso-Arenacea section, interpreted as an increased distance from the flow input or the presence of a lateral basin slope. Through application of this methodology in basins with well-established bed correlation it is possible to provide novel understanding that will significantly augment traditional field techniques. It is evident that laser diffraction grain-size analysis and EMMA are vital tools in furthering our understand of process sedimentology and should be applied more widely.


Biography

PhD in Sedimentology, University of Leeds, UK, studying Permian deepwater sediments in the Karoo Basin, South Africa. Undertook postdoc in Chiba University, Japan studying Pliocene deepwater outcrops, and Queens University studying hybrid beds in central and northern Italy. Now, postdoc at University of Melbourne studying Neoproterozoic rocks in Flinders Ranges.

Correlation of stratigraphic sequences to evaluate downstream transitions within the Wonoka canyon at Umberatana syncline, South Australia

Giles, Sarah M.1, Christie-Blick, Nicholas 1

1Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA

Mid-Ediacaran (~580 Ma) paleocanyons as much as 1 km deep in the Wonoka Formation of South Australia are associated with the largest carbon-isotope excursion in Earth history, the Shuram anomaly. Widely interpreted as submarine, the canyons are thought by many to be comparable to those found at modern continental margins. New data from the northern Flinders Ranges reinforce an alternative hypothesis: that the Wonoka canyons were subaerially incised as a result of evaporative drawdown in a temporarily isolated marine embayment at the onset of the Gaskiers glaciation. Critical supporting evidence has emerged in the canyon-fill at Umberatana syncline, where four oblique cross-sections interpreted as a single sinuous canyon are being independently analyzed. The three incisions studied so far are characterized at the base by ~ 30-m-thick conglomerate-based cycles that are bounded by laterally persistent erosion surfaces and fine upwards into sandstone, siltstone, and minor carbonate (canyon-marginal tongues). Nine such cycles are confidently correlated between the first two incisions (Fortress Hill and Mt. Curtis), and at least plausibly related to seven cycles identified in the third incision (Muccabaloona south) on the basis of 168 measured sections and high-resolution physical stratigraphic mapping. Our correlation of cycles is based on similarities in facies stacking and stratigraphic position. Greater variability in cycle thickness at Muccabaloona south is attributed to facies changes within some cycles in the direction of sediment transport from channelized boulder conglomerate to pervasively rippled sheet-like sandstone and siltstone event layers, as well as a northward increase in structural complexity. The local erosional relief at cycle boundaries is comparable across the three outcrops: 3-6 m at Fortress Hill, 5-10 m at Mt. Curtis, and 3-7 m at Muccabaloona south. The observed stratigraphic organization and details of the facies lead us to interpret the diffusely stratified, channelized conglomerates as fluvial, and the prevalently rippled and laminated tabular sandstones/siltstones as deltaic. The lack of dish structures and nested channels in the sandstone facies, as well as the apparent absence of classical turbidites interstratified with hemipelagic mudstone further supports our fluvial-deltaic interpretation rather than a deep marine interpretation. An upward transition to primarily siltstone event layers interstratified with cm-scale carbonate couplets and monolithologic breccias is interpreted to represent canyon drowning. New field work is planned at the fourth incision (Muccabaloona north) to further test our model. We expect fewer conglomerate-based cycles, and additional fining overall. The correlation of stratigraphic cycles across all four incisions will allow us to evaluate the length scale of downstream facies transitions, which we expect to be much more abrupt in the subaerial incision model than the deep marine canyon interpretation.


Biography

Sarah Giles is a Ph.D. student at Columbia University in New York City, USA. Sarah’s Ph.D. research integrates geologic mapping, sedimentology, stratigraphy, isotope geochemistry, and geochronology to evaluate the origins, timing, and stratigraphic context of the mid-Ediacaran Shuram carbon isotope excursion in South Australia and eastern California.

Preservation of ancient eolian landscapes beneath flood basalt: an example from the Officer Basin, Western Australia

Haines, Peter1

1Geological Survey of Western Australia, Perth, Australia

Eolian landscapes should be common on planets with an atmosphere, and were presumably more widespread on Earth before the evolution of land plants. However, preservation of intact ancient eolian landscapes from this time period are rare. The Ediacaran to middle Cambrian succession of the Western Australian (WA) Officer Basin is dominated by eolian sandstone with interbedded alluvial fan, fluvial and playa deposits. Outcrops of this succession near the western end of the basin (McFadden Formation) include eolian foresets on a massive scale, possibly exceeding 60 m in height. Farther east, the broadly equivalent concealed Lungkarta Formation displays steeply-dipping single foresets up the 30 m in thickness in drillcore, and other sedimentary features confirming an eolian mode of deposition. In the central Officer Basin in eastern WA this eolian sandstone succession is overlain by the basaltic Table Hill Volcanics (THV), a component of the widespread c. 511 Ma (middle Cambrian) Kalkarindji Large Igneous Province of flood basalts and associated intrusions. Although rarely exposed, the distribution of the flat-lying THV can be mapped from drillhole intersections and aeromagnetic datasets. In one area the aeromagnetic patterns indicate that the basalt flowed over and entombed an active dune field. Near the southern margin of the flood basalt the flows thinned to be thinner than the height of the dune crests, allowing the dunes (relatively non-magnetic) to be clearly distinguished from the interdune corridors (filled with magnetic basalt). The resulting aeromagnetic patterns, enhanced by viewing the first vertical derivative of the total field data, indicate south-southwest trending compound linear dune ridges, each separated by parallel interdune corridors. The parallel basalt flows terminate southward along an irregular north-northwest trending boundary that was likely controlled by an inflection in the paleoslope. Details of dune morphologies indicate east-northeast directed prevailing winds, somewhat oblique to the east-southeast migration direction the overall composite linear dune crests. Modern analogues can be found in extremely arid vegetation-free dune areas such as the Rub’ Al-Khali sand sea of southern Saudi Arabia. A particularly good match is found in the east of this extensive sand sea (near 21ºN, 54ºE). This area likewise shows compound linear dune ridges moving obliquely to the prevailing wind direction indicated by the orientation of smaller scale dune components, although the interdune corridors are broader than the Officer Basin example. Coincidently, the ancient Officer Basin dune crest spacing (1 – 1.5 km) is similar to that of the modern vegetation-stabilised longitudinal dunes of the Great Victoria Desert in the same area today, although dune type is different and inferred crest heights are significantly greater in the ancient deposit. Apart from revealing the morphology of an ancient eolian landscape, the relationship with the dated basalt can now be used to infer a precise date for the top of this previously poorly-dated succession, as the dunes were apparently active at the time of entombment in the middle Cambrian. Similar burial of eolian and fluvial landscapes are suggested by aeromagnetic patterns elsewhere in the Kalkarindji Large Igneous Province.


Biography

Peter Haines has Honours and PhD degrees from the University of Adelaide. He has worked for the Northern Territory Geological Survey and Universities of South Australia and Tasmania. He is currently with the Geological Survey of Western Australia where he works on Neoproterozoic and Paleozoic basins throughout the state.

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