3D Inversion Modelling of Newly Acquired Full Spectrum FALCON® Airborne Gravity Data Over the Otway Basin, Victoria

Geological Society of Australia Victoria Division General Meeting Thursday 26th August at 6 p.m. Due to ongoing Covid restrictions this will now be a Zoom meeting.

When: Aug 26, 2021 06:00 PM Canberra, Melbourne, Sydney

3D inversion modelling of newly acquired Full Spectrum FALCON® airborne gravity data over the Otway Basin, Victoria

Dr Mark McLean | Geological Survey of Victoria

A new airborne Full Spectrum Gravity and magnetic survey was undertaken over the Otway Basin as part of the Geological Survey of Victoria’s (GSV) Victorian Gas Program (VGP) using the FALCON® airborne data acquisition system. A total of 31042 line km of gravity, gravity gradiometry (Full Spectrum), magnetic and laser scanner data were acquired along 500 m spaced lines in a NW-SE orientation. This survey is the first publicly available Full Spectrum Falcon survey and is intended to capture the full spectrum of wavelengths by conforming the short wavelengths from the gravity gradiometry with the longer wavelengths obtained from concurrently acquired conventional gravity.

Forward and inversion modelling results in the Otway Basin model suggest that significant gravity anomalies are caused not only by the thickness of basin fill, but also by structures and geological diversity contained within the Proterozoic-Early Palaeozoic basement crust that hosts the basin, and by the shape of the Moho at the base of the crust. The Moho depth shallows from the continental interior into the Otway Basin region and beyond the southern coastline, and this change has a very significant influence on the gravity response. Previous models of Victoria’s crustal and Moho architecture constrained by surface geological mapping, seismic reflection data, aeromagnetic data, ground gravity, and petrophysical properties measured for a variety of local basement rocks were used as a starting point to build structures and attributed rock volumes within the basement under the Otway Basin, and test and refine these models iteratively. This study showcases a workflow that allowed for full-crust- scale variations to be taken into account in the modelling of a sedimentary basin. This is particularly important on continental margins where the crust changes dramatically in thickness across the width of a single basin. The study has improved the understanding of the basement structures below the Otway Basin and, with the geological complexity of the basement accounted for, also allowed for refinement of the geometry of the top of basement interface. Specifically, inversion results allowed the shape of the Otway Basin rift to be mapped in greater detail, revealing new potential exploration opportunities such as indications that the Portland Trough in the southwestern region of the onshore Otway Basin is significantly deeper than previously thought.

Link to the report: http://earthresources.efirst.com.au/product.asp?pID=1246&cID=68&c=193225

About the Presenter

Mark completed Arts/Science and Master of Science degrees at Monash University and then completed a PhD at The University of Melbourne in 2008 which involved acquisition, interpretation and modelling of an airborne geophysical survey over the Lambert Rift region in East Antarctica. Since then, Mark has worked at the Geological Survey of Victoria building regional 3D framework and rock property models using geological and geophysical datasets. Mark's time is now split between the GSV, and The University of Melbourne where he lectures in Applied Geophysics.

Vote Now for the State Fossil Emblem for Victoria

Put your vote in to select a state fossil emblem for Victoria, with eight locally discovered fossils to choose from.

Museums Victoria has launched a public voting campaign to select an official state fossil emblem.

The fossil specimens will be on display in 600 Million Years in Melbourne Museum’s Science & Life Gallery when Melbourne Museum reopens.

How to vote

Public voting is now open https://museumsvictoria.com.au/melbournemuseum/state-fossil-emblem/ and closes at 5pm Monday 4 October 2021. Following the public vote, the winner will be submitted to Government for official endorsement after which our new fossil emblem will be announced.

Some recent media:

The Age: Can you dig it? Vote for Victoria's first fossil emblem 18/Aug/21 https://www.theage.com.au/national/victoria/can-you-dig-it-vote-for-victorias- first-fossil-emblem-20210818-p58jsk.html

Vote for Bayside’s Pelagornis to be Victoria’s fossil emblem 19/Aug/21 https://www.bayside.vic.gov.au/news/vote-baysides-pelagornis-be-victorias- fossil-emblem#

Should a 'mysterious marsupial' or a giant, round-headed amphibian be Victoria's state fossil? 18/Aug/21 https://www.abc.net.au/news/2021-08-18/vote-for-victorias-fossil- emblem/100377548

If you'd like to campaign for your favourite fossil consider posting an article in our next September issue of 'The Victorian Geologist'. Send submissions to [email protected] by 5th September.

I spy with my little eye

Arthur Day

Spring is a great time to visit some of the spectacular geological sites right on Melbourne’s doorstep. This month we travel up the Calder Freeway to a small national park that lies right in the shadow of Tullamarine Airport. The Organ Pipes National Park is largely under the tourism radar but it hosts the most spectacular and easily accessible columnar basalt outcrop in Victoria. The park’s highlights are easily accessed around a three kilometre circuit track paved with asphalt. Such field conditions should only be endured by the most intrepid of geologists!

Weird Nature: the Organ Pipes National Park

This tiny national park, the closest to Melbourne, is named after a 20 metre high wall of million-year-old basalt columns that look strikingly like organ pipes as they tower above Jacksons Creek. They make a spectacular sight about 700 metres along the track from the visitor centre.

Source*: https://www.publicdomainpictures.net/pictures/30000/velka/volcanic- formation.jpg

The Organ Pipes lava flow is one of many thousands that make up the Quaternary Newer Volcanics. The Newer Volcanics extend westwards from Melbourne almost to the South Australian border. They comprise one of the world’s largest continental volcanic provinces. As a result of the eruptions, old river beds and creek lines were filled with molten lava which completely covered the landscape to form a plain. The Organ Pipes flow may have erupted from the Mount Holden volcano to the north, or others in that vicinity. Big basalt columns form when a thick sheet-like body of molten lava cools uniformly. Once solidified, the basalt is still extremely hot. Further cooling then causes vertical shrinkage cracks at right angles to the cooling surfaces, leading to evenly shaped vertically standing columns. Many of these can have near-perfect hexagonal shapes.

A short walk upstream from the organ pipes leads to the ‘Rosetta Stone’, a prominent outcrop of outwardly-radiating basalt columns that look like the spokes of a wheel. These highly unusual radial cooling structures are believed to form inside lava flows when a pocket or tube of still-molten lava slowly freezes whilst confined within the chilled margins of a stalled flow. Subsequent erosion has revealed the interior of this flow.

Source*: https://www.publicdomainpictures.net/pictures/30000/velka/rosetta- stone.jpg

Further on is the ‘Tessellated Pavement’, a distinctive tile-like erosion surface cutting across basalt columns. This park also has fossils, including graptolites, in the Silurian basement. So there is a lot to see and, if you have a thing for big aeroplanes, everything is located very close to the end of a major airport runway.

Access: from Melbourne CBD, drive 27 kilometres up the Calder Freeway (M79) and, just before the Calder Park Motorsport Complex, turn right down Organ Pipes Road.

If you have a favourite geological location to share in this Newsletter, then write it up in 500 words or less and send it to the editor at [email protected].

* The photographer, Robert Lynch, has released these images under a Creative Commons Public Domain licence. They can be used for personal and commercial purposes.

I spy with my little eye**

If you have a favourite geological location to share in this Newsletter, then write it up in 500 words or less and send it to the editor at [email protected].

**This story was posted in the July 2021 TVG issue but was not readable. The editor apologises to John for a poor choice of font colour.

The You Yangs batholith

Prof. Emeritus John D. Clemens

Image 1: The You Yangs, showing one of the two prominent, vertical sets of joints. The interplay between the slope of the hillsides and the joint direction gives the illusion of layering in the granite.

The You Yangs are a prominent range of granitic hills rising to about 300 m and situated 50 km WSW of the city of Melbourne. To get there, take the M1 (Princes Freeway) out of Melbourne, toward Geelong, turn off toward Little River, and follow the signs for You Yangs Regional Park.

The hills rise abruptly above the surrounding plains, which were formed mainly by Pliocene to Holocene mafic lava flows of Newer Volcanics. Flinders Peak (319 m ASL) is the highest point in the range and forms the central feature. The peak and surrounding lands were home to the Yawangi clan of the Wadawarrung people, and the name is most probably derived from the ‘wurdi youang’ – ‘big mountain in the middle of the plain'.

Image 2: Two inclusions in the granite – a rounded magmatic enclave that has evidently incorporated crystals from the host granite and a small, angular xenolith derived from metasedimentary rocks beneath the granite.

The granitic rocks here are I-type biotite monzogranites, commonly with large phenocrysts of K-feldspar, and the magmas were emplaced at 365 Ma in the Late Devonian. In some localities, there are many fine-grained, rounded, lobate or disc-shaped enclaves of mingled hornblende-bearing, quartz monzodiorite and tonalite. You can see these, some quite large, as you walk up to the summit. There are also less common, angular xenoliths of pelitic hornfels, derived from the underlying Ordovician metaturbidites.

Image 3: Foliation in the granite, with euhedral K-feldspar phenocrysts and the long axes of magmatic enclaves aligned. This is interpreted as having been produced through local shearing during magma flow.

For more information see the following recent publication:

Clemens, J. D. (2019). The You Yangs batholith in southeastern Australia, the sources of its magmas and inferences for local crustal architecture. Australian Journal of Earth Sciences, 66(2), 247- 264. https://doi.org/10.1080/08120099.2019.1525430

Dept of Earth Sciences University of Stellenbosch

Darebin Sites of Significance

Field Trip

Despite having to reschedule due to a COVID-19 lockdown the field trip to a few of the documented sites of Geological Significance occurred in sunny, if cool weather on Sunday 11 July. This was a trial event to see how feasible these types of geoheritage sites can be repurposed for geotourism. The group was small, but we all had a great time looking at the geology!

The four sites chosen were from a consultancy undertaken in 2010 as part of the Darebin Heritage study, some details of which can be found at https://www.darebin.vic.gov.au/Waste-and-environment/Sustainable- Darebin/Conservation-and-wildlife/Natural-heritage. There are 21 known extant sites in the City of Darebin and 2 destroyed sites. There are other sites on the boundaries, especially along Merri Creek and information on these can be found at https://www.mcmc. org.au/about-merri-creek/geology- geomorph ology/significant-sites.

Four sites were visited; three are volcanic basalt sites and one is an exposure of the Silurian sandstone and mudstone sediments. The four sites are

Mt Cooper (Bundoora Park) (ML 379): an 8Ma old eruption point that is the highest point in inner Metropolitan Melbourne (ASL 137.3m).

Darebin Boulevard Lava Flows (Reservoir )(ML 389): Edge of lava flow along creek which is a non vesicular and rather blocky style of flow. Viscous flow characteristics can be seen. This site demonstrates some of the principal features of a volcanic landscape.

Darebin Parklands Basalt quarry face (Alphington) (ML 390): Basalt quarry face with excellent exposure of single vesicular basalt flow showing flaggy joint pattern. Weathering features. This site demonstrates some of the principal features of a volcanic landscape.

Darebin Parklands Silurian outcrop and contact with overlying basalt (Alphington) (ML 391): Several outcrops of the steeply dipping Silurian Melbourne Formation occur where the creek has cut through the basalt flow. At the small bridge the sediments can be clearly seen in the bed of the creek. The contact with the overlying basalt is seen in a couple of the exposures.

It was a great fun day and we all enjoyed discussing the geological features and continued discussing these at the pub afterwards. A great social occasion mixing people and rocks!

It takes a bit of organising this type of activity but it is certainly worthwhile and we should try and do it again in other areas of Melbourne, GSA(V) has an extensive geological data base of sites that can be used to identify other suitable groups of sites. The choice of 4 sites appropriate; too many sites in one day is not a good idea as this needs to be a fun activity. Once we get out of this cyclic lockdown situation it is worth looking at in more detail.

Susan White

Forthcoming events

Unless otherwise noted the remaining 2021 talks will all be via Zoom. If you would like a link to a talk that you have missed in Victoria or around the country please contact the Secretary.

September 30th 2021: The Deep Roots of Geology: Tectonic History of Australia and its Margins expressed as Mantle Anisotropy Dr Caroline Eakin, ANU Research School of Earth Science, more details https://www.gsavic.org/monthly-meetings.html 6.00 pm. Register https://zoom.us/meeting/register/tJMtfuGhpzMqEtz0InmPV7mADxyDZ56dV_N7 . After registering, you will receive a confirmation email containing information about joining the meeting.

October 28th 2021: A Global Tour of Mud Volcanoes Dr Mark Tingay, University of Adelaide

If you have not renewed your membership yet for this year there's still time – lapsed members can also re-activate their memberships too.

Being a GSA member can give you significant discounts for GSA events, such as registering for the SGGMP Biennial Conference 2021: Earth, Sea and Sky.

Other Member Benefits

GSA presentations, workshops, fieldtrips and GESSS events plus reduced prices for GSA publications • Access to Australian Journal of Earth Sciences (AJES) or Alcheringa, plus online access to Geodinamica Acta and International Geology Review (if you receive AJES) • The Australian Geologist, featuring articles, society updates, geoscience news, letters, book reviews and much more. • Enjoying camaraderie and networking with fellow geoscientists at Division meetings, talks and activities

Payment methods

• Renew through the GSA website

1: Click here to sign into the GSA website; 2: Click on your name at the top of the homepage; 3: Click on the orange Renew Now button. This will allow you to pay for your renewal.

Paying via credit card over the phone to the GSA Business Office on (02) 9290 2194 • Mailing a cheque or a renewal form to the GSA Business Office (a self-addressed envelope and renewal form were enclosed with renewal notices) • Paying via Electronic Funds Transfers (make sure to quote your name and member no.) Account Name: Geological Society of Australia Inc. (BSB: 082067 Account Number: 52-507-4491)

If you need assistance renewing or re-activating your membership and/or want to change an aspect of your membership, please email [email protected].

Student Scholarships

The GSAV are pleased to offer scholarships for honours and postgraduate students in geological sciences for assistance with travel costs associated with attending conferences (fieldwork excluded). The number and value of the scholarships awarded each year is made at the discretion of the GSA Victoria committee. Up to $500 for travel within Australia and between Australia and New Zealand and $700 for travel elsewhere is available, paid half before and half after the conference. More information, including the eligibility criteria and application form, is available at http://www.gsavic.org/scholarship.html.

Contributions to The Victorian Geologist

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Newsletter deadline: First Friday of the month, except for December and January.

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About the GSA Victoria Division

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