Terminology and Stratigraphy

This section gives basic definitions of the terms used in this thesis, more detailed explanations of the volcanological terms can be found in Cas and Wright (1987) or Bates and Jackson (1984).

The term `volcaniclastic' is a non-genetic term applied to any fragmental aggregate of volcanic parentage, regardless of actual origin. Pyroclastic material is formed solely by explosive volcanism and is deposited by processes directly related to this volcanism. The term `epiclastic' refers to mechanically deposited sediments (gravel, sand, mud etc) consisting of weathered products of older rocks.

Magmatic volcanism is driven entirely by magmatic processes such as degassing of magma or the force of rising magma. On Banks Peninsula, hawaiian type eruptions were predominant, generally forming lava flows and spatter deposits, while the less common and lower volume strombolian eruptions formed scoria cones.

Phreatomagmatic eruptions involve the interaction of magma and either surface or ground water; interaction with ground water forming broad, low relief volcanic craters or maars. Phreatomagmatic volcanism is sometimes inferred to be synonymous with surtseyan volcanism, however this is incorrect as the term `surtseyan' only refers to emergent, basaltic deposits, erupted into large bodies of water, in a similar fashion to the tuff coneat Surtsey, Iceland. For further discussion on this subject, refer to section 7.1.1 (chapter 7).

Tuff cones are generally formed by the interaction of magma with surface water and form deposits with a high relief and bedding dipping at approximately 20° away from the cone. These deposits are formed by poorly inflated, water saturated (due to condensed steam) surge clouds and cover a lesser area than those formed by drier surges. Tuff rings are the result of highly inflated, dry surge clouds, generally formed by the interaction of magma and ground water. Tuff rings cover a greater area and generally have beds dipping 10° to 12° from the source crater (Wohletz and Sheridan 1983).

Eruptions driven entirely by steam and involving no juvenile (molten magma) material are termed `phreatic'. These generally occur when superheated steam nears the surface and hydrostatic load becomes greater than the confining pressure of the surrounding rocks, causing an explosion. Phreatic eruptions also occur when hot pyroclastic are deposited over bodies of water or water saturated material, which may then flash to steam and cause an explosion. Deposits formed by phreatic eruptions are termed secondary deposits, as they contain no juvenile material.

A base or pyroclastic surge is a turbulent, low particle concentration flow of pyroclastic material that is generally associated with phreatomagmatic volcanism. For a more detailed description, see section 7.1.2 (chapter 7).

Sideromelane is a light brown, unaltered basaltic volcanic glass, while tachylite refers to black (due to quenched crystallites of pyroxene; Kawachi et al 1983), basaltic volcanic glass (not easily identified in outcrops). Volcanic glass is formed by the chilling or quenching of molten magma. Basaltic glasses, especially sideromelane, are susceptible to alteration by hydration and ion exchange (weathering processes) to form palagonite. Palagonite is yellow-brown, isotropic and either finely fibrous or gel-like. Palagonite often acts to cement deposits in which it is found (Cas and Wright 1987).

The term "facies" refers to a body or unit of rock with a unique definable character that distinguishes it from other facies or intervals of rock or sediment. The character may be defined by fossil assemblage (biofacies), lithology (lithofacies) or any feature of the unit (chemistry, petrography, depositional environment, stratigraphy etc).

The table 3.1 gives the definitions used to describe the grainsizes of the various deposits studied, and table 3.2 gives the definitions of bedding thickness descriptions used in this thesis.

Clast Size/mm

Non-Volcanic

Pyroclastic

256

0.5

0.25

Boulder

Cobble

Pebble

Granule

Very Coarse Sand

Coarse Sand

Medium Sand

Very Fine Sand

Silt

Clay

Coarse Block

Fine Block

Coarse Lapillus

Medium Lapillus

Fine Lapillus

Coarse Ash

Medium Ash

Fine Ash

Table 3.1 Granulometric classification of pyroclasts (after Chough and Sohn 1990)

Thickness

Definition

<1cm

1-3cm

3-10cm

10-30cm

30-100cm

>100cm

Lamina

Very Thin Bed

Thin Bed

Medium Bed

Thick Bed

Very Thick Bed

The study and description of the volcaniclastic rocks in the Orton Bradley Formation began with the measurement and description of the outcrops. This focused on the larger scale bedding features and allowed the deposits to be divided into units, representing different eruptive phases. The detailed measurement of small sections of some of the thinly bedded units, was carried out to provide a more detailed characterisation of the unit. No outcrop exposes the entire thickness of the unit, so the thickness is estimated using geomorphological techniques such as change in slope and the position of outcrops from lava flows.

Since this thesis is a study of volcaniclastic rocks within the Orton Bradley, it is necessary to characterise the rocks that make up both the Mt. Bradley Volcaniclastic Member and the newly defined Tablelands Volcaniclastic Member. This is done by measuring and characterising the major outcrops of the units, as well as strategically placed minor outcrops. The positions of each of the locations described is shown in Fig 3.3, which also give a rough approximation of the outcrops extent. Each outcrop was logged on a metre scale, and was broken down into units. Each unit is identified by a number corresponding to the outcrop location and a letter corresponding to the position of the unit within the section (A corresponds to the lowest exposed unit).

In the later section on lithofacies analysis (section 6.5), some of the units can be characterised by a single lithofacies, while other, generally thinly bedded units, may be made up of several lithofacies with no clear divisions, indicating that several depositional processes have occurred within the surge or surges of a single unit. Generally only the major outcrops are divided into units, as the minor outcrops often only consist of a single unit. These minor outcrops are nevertheless described in the text as they provide some indication of the variation of the deposits with distance from source (the outcrops describe in order of increasing distance from source).

Correlation between locations proved difficult due to the lack of exposure, as even at major outcrops up 50% of the vertical thickness is commonly obscured. Some obvious marker beds are present, particularly in the outcrops at locations 1 and 2, but the thicknesses of the associated units do not correlate very well, indicating that the range of the base surges was quite variable.

The two eruptive vents marked in figure 3.3 by the massive volcanic breccia deposits are the only known vents in the area which have erupted volcaniclastic material. Other vents may have been buried by later units or eroded away or covered by vegetation or valley fill. However from the evidence gathered, particular flow directions, it appears that the two identified vents were most likely to have been the source of both the Mt. Bradley Volcaniclastic Member and the Tablelands Volcaniclastic Member, with the bulk of the deposits (all of the Mt. Bradley Volcaniclastic Member and the eastern deposits of the Tablelands Volcaniclastic Member) having been erupted from the vent on the northern flanks of Mt. Bradley.