The Effects of Palaeoenvironment on Eruption Styles and Deposit Characteristics: Assessing Differences Between the Subaerial and Subaqueous Parts of the Late Devonian Boyd Volcanic Complex, NSW, Australia, and the Causes

Abstract:

The Late Devonian Boyd Volcanic Complex, south coast, New South Wales, Australia, is unique in preserving the deposits from both subaerial and subaqueous eruptions from the same volcanic system in well-exposed coastal outcrops. Both successions consist predominantly of rhyolites, basalt and rare andesite, consistent with a continental rift setting, probably behind a developing continental margin arc, located in northeastern NSW. The subaerial succession is dominated by magmatic and lesser phreatomagmatic ignimbrites, some rhyolite lavas, in places extensive basalt lava successions, and locally phreatomagmatic tuff cone deposits, all variably interbedded with fluvial-alluvial conglomerates, sandstones and mudstones , and lacustrine mudstones.

In contrast, the subaqueous succession, known as the Bunga Beds, is marked by a facies association of below wave base, anoxic, pyritic black mudstones, turbidites, debris flow and slump deposits, syn-depositional to late intrusive to extrusive rhyolites, abundant rhyolite hyaloclastite and peperite breccias, and localised, small syn-depositional basalt intrusions and associated hyaloclastite and peperite breccias. Pyroclastic deposits are minor, confined to two local pumice-tuff cone successions. This association together with rare plant and fish fossil fragments indicate a deep, fresh or brackish water environment.

The minor explosive activity in the Bunga Beds could be due to: 1) Low magmatic volatile content; 2) Low magma uprise velocity and decompression rate; 3) Hydrostatic pressure in deep water exceeding the magma H₂O volatile saturation pressure (< 100-200 MPa?), and critical point of water (22MPa) so accounting for the almost non-vesicular nature of the volcanics. 4) The bulk modulus of water (~2,300 MPa), suppressed explosive intensity of magmatic and phreatomagmatic explosive activity during pumice-tuff cone formation in shallower water. However, minor hornblende in some deposits indicate the magmas were hydrous, suggesting that factors 3) and 4) were more significant. Water depth may have been ~1,000m =/-.

The paleoenvironment is interpreted to have been a deep, volcanically active, trans-tensional, land-locked, graben lacustrine basin. Analogues include Lake Tanganyika, in the east Africa rift zone, which is up to 1,500m deep.