wave-cut surface at 7m a.s.l.and the yellow and pink dashed lines represent the Kawakawa and Rangitawa Tephras, respectively.
Figure 5.2: Stratigraphic column of the North Red Bluff Quaternary Sequence (note: paleosol refers to a buried soil horizon).
quiet lagoonal environment. The Titirangi Sand exposed in the Red Bluff area differs from these other exposures. Its faunal content is impoverished when compared to the assemblages at the other exposures, and this is interpreted to have been due to it being deposited in an open sandy coastline environment (A. G. Beu, pers. comm.). A full list of fauna within the Sand at this site is presented in Table 5.3.
The Titirangi Sand is overlain by organic-rich silt, interpreted as a paleosol, which is in turn overlain by several metres of finely laminated current-bedded beach sands. Within this unit is a conspicuous firm layer, which varies in thickness from 20cm to 75cm. This unit is composed of calcareous silt, and is interpreted to represent the development of a small lagoon or estuary at the site, which was subsequently overwhelmed by more sand. The remainder of the units within the section are all aeolian sands, silty sands and buried soil horizons. Both Chatham Island rhyolitic tephra marker beds are present within the sequence.
The sequence preserves a record of repeated cycles of sand incursion, occurring as the result of landward transgression during interglacial sea level rise, followed by periods of landscape stability, soil and vegetation development. This is reflected in both the sedimentology and the mineralogy (Figure 5.3) of the sand units (the relevant raw data can be found in Appendices 10 and 11). The units have been deposited in a pattern, from clean, well sorted sands followed by less well sorted, silt-rich sands with increased evidence of soil formation. The well sorted sands are interpreted as representing high sea level stands during interglacial periods. They are dominated by calcareous fragments derived from erosion of limestone, with varying amounts of ferromagnesian minerals, dominantly pyroxenes, and limited amounts of quartz and feldspar. This mineralogy reflects increased erosion of limestone exposures by wave action during high sea level stands. Limestone is the dominant pre-Quaternary lithology in the central region of Chatham Island. These interglacial sand units are followed by finer, less well sorted aeolian sands, which, mineralogically, are dominated by quartz and to a lesser extent feldspar with minor amounts of magnetite. These units are interpreted to represent glacial periods where the bulk of the sediment was derived from areas of the continental shelf exposed during low sea level stands. Thus the sediment is quartzose, and the calcareous input is minimal (Figure 5.3).
The deposition of these units takes place during the early stages of sea level fall. When the continental shelf reverts to terrestrial conditions vegetation becomes established and soil development occurs. This is reflected in the silt deposits and paleosols adjacent to the Rangitawa Tephra, which was erupted during the latter stages of the glacial period of OIS 10. The Tephra deposit overlies a unit of silty sand and a paleosol. This soil redeveloped into the Tephra deposit after it fell. The soil is then overlain by a unit of calcareous interglacial sands. The 27.1 ka Kawakawa Tephra, deposited during the last glacial period, during OIS 2, also occurs in proximity to a buried soil horizon.
Table 5.3: List of macrofossils preserved within the exposure of Titirangi Sand in the North Red Bluff Quaternary sequence. Identifications by A.G. Beu. Bivalvia:
Paphies porrecta (Marwick), abundant Tawera marthae Marwick, common Ruditapes largillierti (Philippi), common
Glycymeris (Glycymerula) modesta (Angas), 2 whole valves, abraded. Gastropoda:
(All rare)
Axymene aucklandicus (Hutton) Xymene drewi (Hutton), Cominella sp., one juvenile, Buccinulum sp.,
?Paratrophon or similar muricid,
The units present within the sequence have been tentatively assigned to the now well known chronology of Pleistocene climate cycles based on the marine oxygen isotope curve of Shackleton et al. (1990). My correlations in Figure 5.3 and 5.4, are largely underpinned by the position of the units relative to the two rhyolitic tephra marker beds. It becomes obvious that a considerable number of cycles are missing between the Titirangi Sand at the base of the sequence, and the overlying units. Other older units that once existed in this stratigraphic position must have been removed by erosion during a high sea level stand. Proxy sea-level curves based on isotope and other data suggest that OIS 11, the interglacial period that preceded the fall of the Rangitawa Tephra, was accompanied by the greatest sea level rise (13 – 20 m above
Figure 5.3:
Grap
h illust
ra
ting the mineralogi
cal and grain size co mpone nts of the units withi n the North
Red Bluff Quaterna
ry se que nce. (note: the dat a for e ach uni t within the column wa s o btained from one cha nnel sampl e colle cted as to be repre se ntative of that
unit. Also, grain si
ze data
from the two
tephra ha s b een in clud ed,
but not mine
ralogi cal d ata ).Lithologi es expl ained i n Figure 5.2, QF = qu artz an d feldspar, pxn = p yroxene.
Figure 5.4:
Correlatio
n o
f units within the No
rth Re
d Bluff Quaternary S
equ
en
ce with OIS stages a
nd eq uival ent ch an ges in sea le vel.
present) out of the last ten glacial/interglacial cycles (Hearty et al. 1997, 2007). It was most probably responsible for the erosional removal of those units that may have previously existed above the Titirangi Sand. The paleosol overlying the Titirangi Sand (at 18.05 m – 18.20 m) must have formed following this erosional period. As mentioned above, tectonic uplift is not as prominent in the Chathams region as it is on the mainland, thus deposits, particularly in coastal areas, are not so well protected from erosion by marine incursion during subsequent high sea level stands.
The late Castlecliffian and Haweran portion of the North Red Bluff sequence of strata is present in the wider Red Bluff region, draped over older Paleocene/Eocene volcanics. The wave-cut surface overlain by the basal lag and Titirangi Sand is only exposed in the North Red Bluff cove area. Elsewhere, to the south, the units contain increased amounts of silt-sized material, but a similar pattern of relatively sand-rich deposits followed by relatively silt-rich deposits persists. The Rangitawa Tephra is well preserved along this length of the coast.
5.3.2 Mairangi Sequence
A second significant exposure of pre-Last Interglacial (Castlecliffian – Haweran) deposits is exposed in the Cape Young/Mairangi area (Plate 5.6. and Figure 5.5). The sequence is also composed of terrestrial sediments and at its maximum is ten metres thick. It drapes over and around the older volcanics at Cape Young, Momoe-a-toa and Mairangi volcano and over the Late Cretaceous/Paleocene Tutuiri Formation in the Takapu - Tioriori area.
The sequence is dominated by silt, silty sand and sand units of the Maipito and Wharekauri Formations, with numerous inter-bedded peat beds and buried soils. The two rhyolitic tephra maker beds are also present within the sequence. Where the sequence overlies competent lithologies, (mostly basalt pillow lavas) a planar, wave cut surface is obvious (see Plate 5.6.b), marking the disconformity between the older submarine volcanics and the young Quaternary terrestrial sequence. However, where the underlying lithology is dominated by palagonitic ash and tuff beds, the disconformity is not so obvious.
a)
Plate 5.6: a) View of the Mairangi - Cape Young area, where late Castlecliffian –