Depositional influences on Re-Os systematics of Late Cretaceous–Eocene fluvio-deltaic coals and coaly mudstones, Taranaki Basin, New Zealand

Abstract

The factors controlling Re-Os systematics and potential for geochronology in organic-rich sedimentary rocks deposited in fully terrestrial to paralic environments are not well understood. Here we present Re-Os, bulk pyrolysis and sulfur data for coals and coaly mudstones from the Late Cretaceous Rakopi and North Cape, Paleocene Farewell and Eocene Mangahewa formations, Taranaki Basin, New Zealand, to investigate a range of depositional controls on the behaviour of Re and Os in coaly rocks. These rocks were deposited in various fluvial, estuarine and coastal plain environments, and exhibit varying degrees of marine influence, as indicated by total sulfur content, presence of dinoflagellate cysts, and other parameters. The Taranaki coaly rocks have low Re (0.1–1.3 ppb) and Os (14.2–66.2 ppt) concentrations, even for strongly marine-influenced, high‑sulfur samples. These low concentrations are similar to those reported for entirely terrestrial coals, but are up to two orders of magnitude lower than in marine-influenced coals from the Carboniferous Matewan coal seam, USA. Unlike the Taranaki coaly rocks and other coals analysed for Re and Os, the Matewan coal seam is directly overlain by a fully marine shale. This suggests that such juxtaposition of depositional environments may be required for enhanced Re and Os enrichment in coals, for example, through drowning of the precursor peat mires by Re- and Os-rich seawater during the deposition of the overlying marine shale. The initial 187Os/188Os (Osi) compositions of the Taranaki coaly rocks show significant variation. Samples from the Rakopi Formation exhibit radiogenic Osi values (0.8–1.2), which is expected for coals deposited in fully terrestrial settings and which source Os from weathering of surrounding upper continental crust. In contrast, samples from the progressively younger North Cape, Farewell and Mangahewa formations exhibit significantly less radiogenic Osi values (0.3–0.5). We attribute this to variable levels of marine influence from moderately radiogenic contemporaneous seawater and a change in sediment source composition following eruption and weathering of nearby subaerial volcanoes between 78 and 72 Ma, after deposition of the Rakopi Formation. The 187Re/188Os vs 187Os/188Os relationship for coaly rocks from the Farewell Formation exhibits significant scatter without any linear trend, precluding Re-Os geochronology. This scatter appears to have resulted from a combination of heterogeneous Osi (0.1 units) and limited variations in 187Re/188Os (145 units). Improved Re-Os isochroneity is noted in strongly marine-influenced coaly rocks from the Mangahewa Formation, with the 187Re/188Os vs 187Os/188Os relationship yielding an isochron age of 28 ± 16 Ma. Although this age is imprecise, it is within uncertainty of the estimated biostratigraphic age (37 ± 1 Ma) of the rocks.