Lune River Tree Fern Fossicking- A Jurassic Discovery in the Far South

Fossicking in the Field:

It’s not every day that we step out of our front door, into the bush, and ‘discover’ a new piece in the scientific jigsaw of understanding. Such was the experience for Boris and myself, when one day, back in 2002, we wandered into our local gemfield south of Lune River, along the Leprena Track, which had recently been cleared of forest.

Though enthusiasm and the hope of a great find always accompanies the fossicker, we had no inkling that something unusual was to happen. This particular area, located between the adjoining Southern Ranges in the South West National Park (WHA) and the Southern Ocean itself, is one hundred kilometres south of Hobart and reflects a rich variety of geological earth history. Past glaciers have scoured the area, caves have formed vast, labyrinthine underground networks in the surrounding hills, with limestone found in high outcrops and occasional deposits of shell fossils. Indeed, five earth-ages occur in the surrounding country – Triassic, Permian, Jurassic, Ordovician, Pre-Cambrian.

Relevant to this story is the narrow but extensive North/South-oriented deposit of basalt, the result of magma (molten lava) that erupted through the earth’s surface, resulting in the formation and preservation of colourful agates, quartz crystal, jasper, petrified wood and rare fossil fern. This gem and fossil abundance was formally acknowledged by the creation of two Public Fossicking Areas (PFA) at Lune River, zoned within State Forest. In such areas, people can fossick and hand-dig, without the need for a Prospector’s Licence.

Earlier Aboriginal people of Llylequonny tribe would have most certainly made use of the hard chalcedony and enjoyed the beauty of these gemstones. Enthusiastic collectors now come to the area, in the hopes of finding their own stones and fossils. The gemstones that formed in that cooling magma, along with the trees and ferns that were buried, and later fossilised, were broken up over time and settled into gravel layers, buried beneath soils and clay. The earliest conjecture as to the age of this ‘deposit’ ran from Tertiary through to Jurassic.

Back to that particular day, when out for a morning’s fossick, we found ourselves, early sun on our backs, walking up one of the access tracks created during logging. We had already checked most of the eastern slopes over some months, noticing a distinct tendency for fossil wood occurring at about the same altitude. As we climbed higher, nearing the identified ‘zone’ of woods, our attention was caught by some unusually light-grey coloured segments of fossilised limbs, between 10-50mm long and around 13mm diameter. Also scattered here and there, exposed by winter rains, were some fragments of fossil fern leaves. To find a petrified limb is rare enough, but to come across a number of them over a few square metres, is an indicated something unusual.

We returned to the area over a number of days, carefully scanning the ground, trying to pin-point where they had come from. Our efforts finally drew us to one spot, and we began to dig. This is when fossicking starts to get really interesting. Over some hours, our digging revealed an upright, 30cm ’round’ of fossil wood, which gave the distinct impression of being ‘in situ’, that is, found in the position in which it actually grew.

Finding fossils ‘in situ’ had not been reported at Lune River, so we were very excited at such a possibility. Over many months of working the site, more of the petrified wood was revealed and, until we knew more, all excavations had to be carefully re-covered at the end of each session of digging. We were hoping to discover if our first impressions of an ‘in situ’ tree were accurate or perhaps it was a large, isolated block of Jurassic sediments. Hoping to establish the context of what we were revealing, the proliferation over time, of fossil limbs, fern leaves and stems of understorey plants told us that this was a significant site.

Finally we decided it was time to involve others in this discovery, and initially the University was contacted. What followed over the next few months, were a series of meetings, involving various government departments and authorities, with the aim to further explore the site. It was finally agreed to excavate the site, using a small excavator and operator provided by Forestry Tasmania over a two day period, under the direction of a student from UTAS, who would write her thesis on the dig and the resulting research.

On the first day of the excavation, a number of people gathered at the site, keen to participate. Once underway, the excavation work itself was slow and methodical, as science dictates. Much of the machine-work involved the removal of soil ‘over-burden’, and when more delicacy was required, people moved in with hand tools. What was eventually revealed was a fallen fossilized tree, lying in a NE angle down the slope and what Boris and I had first thought to be a tree trunk, turned out to be a cross-section of a large root. Amongst these root structures were deposits of volcanic/sandstone sediments, with pockets of fern and plant fossils – leaves, stems etc. From these volcanic sediments, it was hoped that zircons could be found, to accurately date the gemfield*. Many samples were collected and carefully catalogued and photographed. Later, back in the UTAS laboratory, zircons were indeed found and tested and were able to establish an age of 182 million years old, establishing the ‘field’ as Early Jurassic in age.

The excavation study resulted in an Honours paper**, science was given more to digest and Boris and I were personally rewarded just by being involved, from beginning to end..If such things ever end!

This tree, with its palaeobotanical skirt of fossil ‘memorabilia’, is now reburied and the area has received special status as a Fossil Site, and, for its greater protection, is no longer included in the surrounding Public Fossicking Area.

The site is listed with the Australian GeoHeritage Data Base and it is hoped that further research will one day be possible, to reveal more of this Early Jurassic earth history. Much will continue to lay hidden, unknown, till the next person, eyes open and curious, will notice something…..there, on the ground….and another discovery happens!

 

*Bromfield, K., Burrett, C.F., Leslie, R.A. and Meffre, S. (2007) –
‘Jurassic volcaniclastic – basaltic andesite – dolerite sequence in
Tasmania: new age constraints for fossil plants from Lune River’. Australian
Journal of Earth Sciences, 54:7, 965 – 974.
** Bromfield, K. (2004) – Palaeoenvironmental Reconstruction of Jurassic
Lune River. Unpublished B.Sc. Honours thesis. Department Of Earth Sciences,
University of Tasmania.

Images courtesy of Lunaris Gemstones, Lune River:
Lunaris Gemstones,Tasmanian Fossils,Tasmanian Minerals,Crocoite,Stichtite