ASPECTS OF THE GEOLOGY OF UPPERMOST JURASSIC STRATA AT PORT WAIKATO. by J.A. Kenny, T.C. Leonard, G.P. O'Leary and S.S. Palmer INTRODUCTION

Transcription

1 TANE 23, 1977 ASPECTS OF THE GEOLOGY OF UPPERMOST JURASSIC STRATA AT PORT WAIKATO by J.A. Kenny, T.C. Leonard, G.P. O'Leary and S.S. Palmer Department of Geology, University of Auckland, Private Bag, Auckland INTRODUCTION During the Rangitata Orogeny, Jurassic sediments at Port Waikato, which belong to the Murihiku Supergroup of the New Zealand Geosyncline, were folded into a major regional syncline, with a smaller, subparallel anticlinal structure on the western flank. Until recently, both folds were regarded as plunging gently south-south-east (e.g. Purser 1961; page 21), but further studies now suggest that the anticline plunges north as it approaches the transverse Waikato Fault. The area of interest lies near the mouth of the Waikato River, between South Head and Maretae Stream. The northern boundary is formed by the east-northeast-trending Waikato Fault which truncates the Jurassic structures. The southerly extent of our investigation so far is the Waikawau Fault (Purser 1961), south of the Huriwai Stream, which trends north-north-east. Grid references are based on the national metric grid of the 1:50,000 topographical map series (NZMS 260), sheet R13. Recognition of bedding planes is hampered by extensive jointing and fracturing of the rocks. Fossils are plentiful, but in many areas have been distorted by movements within the rock. The following observations suggest ammendments to previous interpretations of the gross structure of the area. THE ANTICLINE AND ITS PLUNGE Purser (1961, page 21) states that "uppermost Ohauan and Puaroan A and B sediments form the visible core of the Structure [of the anticline]". His maps indicate that he believed the southward plunge of the anticline continued north of the Huriwai Stream. Consequently, he indicates on his Map Number 3 probable Ohauan rocks outcropping in the crest of the anticline, between Maretae Valley and South Head. Measurements taken on the eastern limb of the fold in the Huriwai Stream at Rl 3/ are approximately 148 /22 E, while those on the west are approximately 158 /32 W. At the fold crest the anticline appears to be plunging only two degrees north-north-west. During the present examination of outcrops in the area, the bivalve Buchia plicata was found at localities R13/f6, R13/f7 and R13/f6920, and its occurrence indicates that these localities are situated in the Puaroan B Zone, and thus it would appear that the anticline is not plunging 129

2 Fig. 1. Study area at Port Waikato showing structure and biostratigraphic zones in Upper Jurassic rocks in the Maretae and Huriwai Valleys. 130

3 south in this area. Strike readings of the Jurassic strata along the Huriwai Stream confirm this deduction. (Puaroan Ai is shown on the map from measurements taken in the Huriwai Stream, however no fossil evidence has been found in this area.) Evidence of a regionally significant plunge on the anticline and syncline is lacking at present. The slightly northerly plunge noted by us may be a purely local phenomenon, perhaps caused by a downdrag at the Waikato Fault, which has a northerly downthrow of 2.7km (Hochstein and Nunns 1976). ZONES IN THE UPPER JURASSIC While working in the area the Jurassic rocks of the Maretae and Huriwai Valleys were mapped. Purser's Puaroan A Zone has been divided, according to fossil assemblages, into a or Puaroan Ai Zone, and a Buchia hochstetteri or Puaroan A 2 Zone. is the tentative marker fossil for Puaroan Ai, and is differentiated from Buchia hochstetteri by the presence of radial as well as concentric ribbing. The Buchia hochstetteri horizon is ten to thirteen metres thick, with its outcrop pattern in the area studied shown in Fig. 1. The delineation of the boundary between the zones is uncertain. The youngest of the Puaroan Zones, Puaroan B, is recognised by the occurrence of Buchia plicata. Variation in the typical siltstone lithology of the Puaroan is so slight and so localised that lithostratigraphic units paralleling the biostratigraphic units are not recognised. THE MARETAE VALLEY The Maretae Valley seems to have resulted from strong structural control. Purser (1961, page 24) states that the Huriwai Fault "appears to decrease in throw north of Huriwai Stream and cannot be traced north of Tetehe Trig." The authors have mapped the Huriwai Fault passing 100 metres east of Tetehe Trig and believe that, instead of dying out, it continues north to intersect the Waikato Fault near the mouth of the Maretae Stream. The postulated extension of the Huriwai Fault gains support from the asymmetric shape of the Maretae Valley. On reaching the fault the Maretae Stream changes from a westerly direction to follow the fault trace north. It has eroded a valley along the fault. The trace of the Huriwai Fault, as inferred from the Maretae Valley, runs subparallel to the topographic contours, suggesting that it dips steeply to the east. FOSSIL LIST The following table lists those fossils at the numbered localities in Fig. 1 which were important in the determination of the position of the new Puaroan 131

4 A 2 Zone between the Puaroan Aj and Puaroan B Zones. (All fossils were found in grey or brown siltstone.) Locality (metric grid) R13/f3 (R13/636214) (lower Maretae Stream) R13/f4 (R13/637207) (lower Maretae Stream) R13/f5 (R13/636206) (mid Maretae Stream) R13/f6643 (R13/ (roadside, mid Maretae Valley) R13/f6647 (R13/642195) (upper Maretae Stream) R13/f6 (R13/627214) (1km up tributary of lower Maretae Stream) R13/f7 (R13/625214) (near locality R13/f6) R13/f6920 (R13/625213) (150m upstream from locality R13/f7) R13/f6811 (Rl 3/646184) (upper Huriwai Stream) R13/f7050 (R13/644183) (300m NE of woolshed) R13/f6649 (Rl 3/642184) Fossils Buchia hochstetteri Fleming Buchia hochstetteri Fleming (150m NE of woolshed) R13/f8 (R13/641185) (200 m from Huriwai Stream) R13/f9 (R13/640181) (300m west of road) R13/fl0 (R13/637185) (600m west of road) R13/fll (R13/636896) (500m up tributary of Huriwai Stream) R13/H080 (R13/632183) Buchia hochstetteri Fleming (Huriwai Stream) R13/H2 (R13/631183) (Huriwai Stream) R13/H3 (R13/633185) (Huriwai Stream) ACKNOWLEDGEMENTS The writers would like to thank Dr P.F. Ballance and Dr K.A. Rodgers of the University of Auckland for their critical appraisal and assistance in improvement of the manuscript; and Mr D.W. Pryor of the University of Auckland for his help in the tedious task of recording new fossil localities. 132

5 REFERENCES Hochstein, M.P. & Nunns, A.G. 1976: Gravity Measurements Across the Waikato Fault, North Island, New Zealand. New Zealand Journal of Geology and Geophysics 19(3). Purser, B.H. 1961: "Geology of the Port Waikato Region"'. New Zealand Geological Survey Bulletin 69(N.S.). 133

6 134