High Precision Zircon Dating of Tuffs in the Sydney-Gunnedah Basin

Dr Kevin Ruming

TUFFS

NOBBYS TUFF

TUFFS

CHARBON COLLIERY BOX CUT

CA-IDTIMS Chemical Abrasion Isotope Dilution Thermal

Ionisation Mass Spectrometry Process • Extract zircons • Select primary magmatic zircons • Zircons are individually thermally annealed and

chemically abraded • U - Pb isotope concentrations • Age of each zircon calculated, and a weighted mean

age for each tuff sample is determined

The Project

Progress – Overall Project • 526 Samples collected from

Australia • 87 Dates, 154 Samples in progress

or with preliminary dates

The Project – NSW NSW Project • Enhanced correlation within &

between coal basins • High resolution estimates of

sedimentation rates • Modelling depositional evolution of

NSW coal basins Progress – NSW Project • 391 Samples collected from NSW to

date • 59 Dates • 100 Samples in progress or with

preliminary dates • 38 Samples had no zircons

Correlations Ulan Seam F Ply

Regional Chronostratigraphic Correlations - Ulan Seam F Ply & Fairford

WESTERN COALFIELD Ulan Coal F Ply 257.24 ± 0.14 257.46 ± 0.22 (Prelim)

HUNTER COALFIELD Fairford Claystone 257.04 ± 0.15 257.43 ± 0.09 Mount Arthur Amoco Wybong 1

Correlations

Chronostratigraphic v’s Lithostratigraphic Units

SCALE

Ma GROUP AGE AGE GROUP

252.50252.75253.00254.50254.75 254.82 ± 0.04 254.88 ± 0.19 (P)255.00255.25 255.26 ± 0.14255.50 255.37 ± 0.17255.75256.00256.25

NEWCASTLE COALFIELD

Malabar Formation to Milbrodale Fm

Althorpe FormationMount Leonard Fm

SUBGROUP/FORMATION SUBGROUP/FORMATION

Narrabeen Group

Lambton Subgroup

Nobbys Tuff

Moon Island Beach Adamstown Subgroup

Newcastle Coal

Measures

Waratah Sandstone / Dempsey Formation

Moon Island Beach to Adamstown Subgroup

Tomago Coal Measures

Nobbys TuffWatts Sandstone / Denman Formation

Wittingham Coal

Measures

HUNTER COALFIELD

Narrabeen Group

Newcastle Coal

Measures

Hunter Coalfield Gunnedah Basin

Ulan Seam – Western Coalfield

Depositional Rate: C Marker to F Ply = 6.94m/my (On compacted sediment, does not account for post-depositional compaction)

ULAN COAL CA-IDTIMS DATES B Parting: 255.97 ± 0.14 C Ply: 256.19 ± 0.05, 256.37 ± 0.21 E31 Ply: 258.58 ± 0.24 (Prelim. Date) F Ply: 257.24 ± 0.14, 257.46 ± 0.22 (Prelim) Tuffaceous Units below F Ply: 257.17 ± 0.09, 257.84 ± 0.16, 258.77 ± 0.22

Biostratigraphy • Palynological

samples are being taken in conjunction with CA-IDTIMS samples (where possible)

• Calibrating endemic Australian biozonation against the established global timescale

Finding the P-T Boundary – Southern Coalfield

P-T Boundary = Historically been placed above the Bulli Coal

Finding the P-T Boundary • P-T Boundary = Between 251.941 ±

0.037 and 251.880 ± 0.031 • Bulli Coal = 252.60 ± 0.04Ma • Gap of ~700,000 Years

Future Work • Redefining stratigraphic nomenclature of major NSW basins/coalfields • Possible redefining type holes/sections • Updating Departmental publications

Basin Scale Fence Diagram

Acknowledgements

• Bob Nicoll, John Laurie – Geoscience Australia • Ian Metcalfe - University of New England • Erin Holmes - GSNSW • Research School of Earth Sciences – ANU • Isotope Geology Laboratory – BSU