The mid‐Palaeozoic turbiditic Mathinna Group, northeast Tasmania

Abstract

Palaeocurrents, sandstone compositions and sedimentologic profiles in the turbiditic Mathinna Group of northeastern Tasmania have enabled the geometry and dynamics of the Late Silurian‐Early Devonian part of the depositional basin to be defined. The oldest rocks in the Mathinna Group are Early Ordovician or older, and comprise a thick‐ to thin‐bedded turbiditic arenite succession ∼ 1 km thick (Stony Head Sandstone) overlain by a 1–2 km thick pelite (Turquoise Bluff Slate) containing the only known Ordovician fossil in the succession — an Early Ordovician graptolite. This pelite succession passes conformably upwards by influx of gradually increasing proportions of siltstone and fine‐grained sandstone into a thin‐ to thick‐bedded interval of classical turbidites at least several kilometres thick containing Early Devonian fossils in its upper part. The lower part of this upper turbiditic succession (Bellingham Formation) contains the oldest palaeocurrent pattern measured and indicates lobes on a submarine fan system prograding to the east‐northeast. Dominant sandstone composition is quartzose sublitharenite, consistent with derivation from stable platform areas inferred to have lain to the southwest. The age of this fan system is uncertain, but could be Late Silurian. The upper part of the Mathinna Group is a several kilometre thick sand‐rich succession of sublitharenite and litharenite with variable feldspar content entering a north‐northwest‐elongate basin. Measurement of several hundred cross‐laminations and sole marks has enabled recognition of two large fan systems with mutually opposed palaeocurrent directions parallel to the basin axis. These strata contain a minor, but significant, proportion of feldspar which is not present in the older east‐northeast‐flowing turbidites. The south‐southeast‐prograding fan system has twice the feldspar content of the north‐northwest‐directed system, reflecting probable derivation of the former from the wide area of Late Silurian‐lowermost Devonian silicic igneous rocks in southeastern Australia. Graptolites and fragmentary plant remains indicate that this basin geometry is, at least in part, Early Devonian. Comparison of palaeocurrent directions from cross‐laminations in Bouma C divisions of both A/BC(D)E and C(D)E graded beds with directions from sole marks (predominantly flute and groove casts) on beds in the same outcrops, shows that Bouma C cross‐laminations can give the same vector‐mean azimuth, within statistical error, but with a higher variance, as the sole marks. When the palaeocurrent patterns are considered at the highest summary levels, simple and consistent palaeodrainage patterns emerge, including coincidence of the probable Early Devonian basin axis with the regional fold trend.