Black Point

This sequence provides a unique area in which the erosion of Lyttelton Volcano and the formation of Lyttelton Harbour can be analysed. Deposits display features common to facies associated with a composite volcano’s ring plain (Mathisen and McPherson, 1991), with frequent lateral facies variation in localised areas. The following provides a synthesis of the Black Point sequence.

Two epiclastic units occur in the sequence at Black Point. The first is interbedded with the Church Basalts, covered by the Kaioruru Hawaiite and Stoddart Basalt, and the second is the main sequence overlying Allandale rhyolite and interbedded with Kaioruru Hawaiite and capped by Stoddart Basalt.

The eastern shore platform indicates epiclastic deposits were being deposited throughout eruption of the Church Basalts (7.8 – 7.3 Ma). Lava flows dip to the north (into the present Church Bay), indicating a source direction to the south. Flow indicators in the epiclastic unit also align to the south, with clast types reflecting erosion to the underlying Allandale Rhyolite, Lyttelton Volcanics and Mt Herbert Volcanic Group. The contact between this lower sequence and the Kaioruru Hawaiite is highly eroded, with Kaioruru Hawaiites unconformably overlying the epiclastic deposits exposed in the cliffs above the shore platform before dropping into an eroded channel within the Church Basalts infilled with epiclastic deposits and then Kaioruru Hawaiite. Kaioruru Hawaiites descend in height to the west, and are then conformably overlain by the Stoddart Basalt. Stoddart Basalt that

CHAPTER 3:EPICLASTIC DEPOSITS ON THE SOUTH-EASTERN SIDE OF LYTTELTON HARBOUR 106

reached low levels within the proto-Lyttelton Harbour interacted with water, fragmenting producing the hyaloclastic deposits on the northern end of Black Point.

The main epiclastic sequence of Black Point overlies a paleo-high of Allandale Rhyolite, on the southeast of the present day harbour. This paleo-high was progressively covered as Lyttelton Volcano developed, then became exposed as Lyttelton Volcano underwent extensive erosion, degrading to a state similar in topography to the present day. A hypothesised cross section through Black Point (Figure 3.2) indicates the relationship between the descending rhyolite high and the onlapping Kaioruru Hawaiite, infilling and overlying epiclastic sequence and the overlying Stoddart Basalts. The contact between Kaioruru Hawaiite and the Allandale Rhyolite is based on limited exposure, but is assumed to be highly irregular. Also highlighted in the cross section is the necessary removal of Lyttelton Volcanic Group material prior to the deposition of the epiclastic sequence.

Kaioruru Hawaiites stratigraphically overlie the rhyolite high to the west, although the epiclastic sequence directly overlies rhyolite to the east (uphill) and may underlie Kaioruru Hawaiite as a wedge, thinning from the northeast to southwest. Kaioruru Hawaiites erupted at 6.85 Ma, from a source vent hypothesised to the east, now covered by Stoddart Basalt flows. These lavas infilled the irregular topography of the eroded rhyolite and underlying Church Basalt and epiclastic sequence.

Lower tuffaceous sandstone and mudstone beds reflect a low energy depositional environment, isolated from the main epiclastic sequence, although stratigraphic relationships indicate it later became covered. This depositional low point existed due to the Allandale Rhyolite descending to the west, with Kaioruru Hawaiite lavas covering this highly irregular surface producing further localised depressions. This resulted in a low lying depression, in which tuffaceous sediments accumulated. The lack of fossil remnants within this sequence indicates an environment not conducive to biologic activity, through rapid sedimentation or anaerobic conditions.

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The main epiclastic sequence represents a series of debris flows following a valley system depositing as an alluvial fan system, controlled by eroding basement lithologies (Allandale Rhyolite and Lyttelton Volcanic Group lavas) and the Mt Herbert Volcanic Group.

Sandstones and mudstone layers are indicative of paleo-depressions on the alluvial plain, while smaller tuffaceous sandstone lenses are low energy deposits in small meander bends, deposited during fluvial dominated periods. Channel orientations throughout the conglomerate sequence indicate the axis of predominant drainage had limited migration overtime, primarily draining between the NWW and NNE, into a proto-Lyttelton Harbour, between ~7.8 to 6.8Ma, based on stratigraphic relationships. This fan was fed with debris from the exposed Allandale Rhyolite, Lyttelton Volcanic Group, Mt Herbert Volcanic Group lavas and tuffaceous units, and towards the top of the sequence with primary volcanic fragments of the Stoddart Basalt.

Epiclastics were preferentially deposited in this area due to the control of the Allandale Rhyolite basement high, Lyttelton Volcanics and Mt Herbert Volcanic Group, with the overall paleo-valley system later being infilled by the Diamond Harbour Group lava flows.

An important aspect of the main epiclastic sequence is the exclusion of the Church Basalts and Kaioruru Hawaiites prior to the Stoddart Basalt lava flows. The Church Basalt were small localised eruptions on the eroding Lyttelton Volcano (Sewell, 1988). In examining the location of the Church Basalts, they are confined to the north of the Allandale Rhyolite high, limited to the low lying regions as the top of the conglomerate was at significant elevation, and the morphology of the fan surface restricted flows to the topographic lows to the side of the fan surface. It is hypothesised that the Church Basalt in this region were being erupted contemporaneously with the early stages of deposition of the conglomerate, exposed in the eastern shore platform.

The main sequence of Black Point however overlies middle to upper lava flows of the Kaioruru Hawaiite on the western shore platform. This indicates the main sequence of epiclastics were deposited contemporaneously throughout the eruption of the Kaioruru

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Hawaiite, as the contact between Kaioruru Hawaiite and the overlying Stoddart Basalt is marked by a thin red epiclastic horizon on the shore platform. Kaioruru Hawaiites overlying the main epiclastics sequence are highly irregular, defining the upper surfaces of epiclastic. Where this contact is observable the Church Basalt lavas descend rapidly over the conglomerate, supportive of the lobate morphology of the alluvial fan and individual debris flows. The Kaioruru Hawaiite follow a similar deposition regime as the Church Basalt, but had a larger alluvial fan surface to erupt over, due to eruptive centre location, suggested to be covered by the Stoddart Basalt lava flows of the Diamond Harbour dip slope.

The Stoddart Basalt erupted during the last stages of the main epiclastic sequence, highlighted by the red, pyroclastic rich horizon at the top of the sequence. Stoddart Basalt flows are found stratigraphically lower on the eastern side of Black Point (Figure 3.2), suggesting that as these lavas flowed from the east, they progressively infilled the lower regions of the proto-Lyttelton Harbour, and resulted in the thickest Stoddart Basalts being focussed to the north-eastern side of the Peninsula.. These lavas had limited coverage over the rhyolite high, supporting the concept that Stoddart Basalts were channelized by the underlying lithologies. Initial flows at low levels interacted with water, reacting explosively forming hyaloclastic breccia. On the north-western end of Black Point flows drop steeply over the lobate edge of underlying lithologies, producing the lavas that cap Black Point and the steeply dipping lavas of the shore platform and cliffs.

The following provides a summary of deposition and emplacement for the Black Point epiclastic sequence.

1. Extensive erosion to Lyttelton Volcano, uncovering basement Allandale Rhyolite, and removal of Lyttelton Volcanic Group flows.

2. Eruption and deposition of the interbedded epiclastic sequence and the Church Basalt at the low lying edges of the alluvial fan.

CHAPTER 3:EPICLASTIC DEPOSITS ON THE SOUTH-EASTERN SIDE OF LYTTELTON HARBOUR 109

3. Eruption of the Kaioruru Hawaiite over the low lying areas of Allandale Rhyolite and Church Basalt. Contemporaneous deposition of the epiclastic sequence, beginning with the basal, rapidly depositing tuffaceous sandstone and mudstone.

At higher levels an alluvial fan system began to develop in this region, fed from the larger paleo-valley to the east.

4. Continued eruption of Kaioruru Hawaiites at lower areas of the alluvial fan complex, mantling the channelized fan surface.

5. Eruption of the Stoddart Basalts, producing a thick series of lava flows, sourced in upper Purau Valley, which further mantled the fan surface, invading Lyttelton Harbour.