ISSN 0083-7903, 89 (Print) ISSN 2538-1016; 89 (Online)
Late Quaternary Stratigraphy and Sedimentation.. of the Canterbury Continental Shelf, New Zealand
by
R. H. HERZER
New Zealand Oceanographic Institute Memoir 89 1981 NEW ZEALAND DEPARTMENT OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Late Quaternary Stratigraphy and Sedimentation of the Canterbury Continental Shelf, New Zealand
by
R. H. HERZER
Geology Department, Victoria University of Wellington, Private Bag, Wellington
New Zealand Oceanographic Institute Memoir 89 1981
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ ISSN 0083-7903
Received for publication: March 1980
© Crown Copyright 1981
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ CONTENTS
Page LIST OF FIGURES 5 LIST OF TABLES 5 ABSTRACT 7 INTRODUCTION 8 BATHYMETRY 9 Methods 9 Geomorphic Zones of the Continental Shelf 11 Zone A 11 ZoneB 11 ZoneC 11 ZoneD 11 ZoneE 16 SEDIMENTSZoneF 16 16 Sampling 16 Analytical Methods 16 Selection of Significant Tex.turalParameters 20 Sediment Map 26 Gravel 26 Sand 28 Mud 28 Distribution of Modes ... 28 Lithic Gravel (Mode I) Distribution 28 Shell GravelDistribution 31 Concretions 31 Sand (Modes II, III, and IV) Distribution 31 Distinction of Sand Modes ... 31 Distribution of Modes III and IV 31 Some Surface Features of the Sediment 34 Origin of the Sediments 34 Provenance of the Gravel 34 Provenance of the Sands 38 The Fauna .. . 39 STRATIGRAPHY .. . 40 Instrumentation and Coverage 40 Stratigraphic Nomenclature 40 PegasusBay Formation 40 Type and Reference Profiles 40 Upper and LowerBoundaries ... 40 Lithology and Structure 51 Anomalies ... 51 Age of the PegasusBay Formation 51 CanterburyBight Formation 52 Type and Reference Profiles .. . 52 Upper and Lower Boundaries .. . 52 Lower Member: Lithology and Structure 52
3 Inset I
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Upper Member: Lithology and Structure 52 Fluvial Facies 52 Marine Facies 52 DISCUSSIONAge of the Canterbury Bight Formation 55 59 Geologic Map of the Canterbury Shelf 59 Segregation of Size Modes 59 Relative Ages of the First- and Second-order Ridges 60 Theories of Origin of Ridge-and-Swale Topography 60 Second-order Ridges on the Canterbury Shelf 60 Ridges on theConstricted Shelf offBanks Peninsula 60 Ridges on the Open Shelf of Canterbury Bight ... 61 First-order Ridges on theCanterbury Shelf 61 Relative Pleistocene Sea Levels in the Study Area 61 History of the Regressive-Transgressive Surface 63 Shelf Sedimentation during the Last 6,000Years 67 Dispersal of Modern Input Sediment 67 ACKNOWLEDGMENTS Tendency of Relict Sediments towards HydraulicEquilibrium 68 REFERENCES 68 69
4
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ LIST OF F IGURES
Page Page l. Bathymetry of the continental shelf and slope off 19. 3.5 kHz seismic profiles 1 and 2, northern Pegasus Canterbury and simplified physiography of the Bay 41 adjacent land area ... 6 20. 3.5 kHz seismic profiles 3 and 4, southern Pegasus 2. Simplified geology of the South Island 9 Bay 43 3. Direction of net water flow on the Canterbury shelf 10 21. 3.5 kHz seismic profiles 5 and 6, east of Banks 4. Echo sounding coverage of the continental shelf off Peninsula ... 45 Canterbury 12 22. 3.5 kHz seismic profiles 7, 8, and 9, northern 5. Detailed bathymetry of Pegasus Bay and the shelf off Canterbury Bight 47 13 Banks Peninsula 23. 3.5 kHz seismic profiles 10 and 11, southern 6. Bathymetry of a portion of the second-order ridge-and Canterbury Bight 49 swale topography off Banks Peninsula ... 14 24. Core H440 from the muddy facies of inner Pegasus Bay 7. Geomorphic zones of the Canterbury continental (Pegasus Bay Formation) 51 shelf 15 25. Chronostratigraphic diagram of the Late Pleistocene 8. Locations of sample stations 17 formations proposed for the Canterbury continen tal shelf 53 9. Locations of sample stations referred to by numbers in the text 18 26. Core H781 from the shelf edge terrace (lower member of the Canterbury Bight Formation) 54 10. Surficial sediment distribution on the Canterbury continental shelf 27 27. Core H777 from the ridge facies of the upper member of the Canterbury Bight Formation, showing 11. Mud distribution on the Canterbury continental traction and accumulation zones 55 shelf 29 28. Core H788 from the ridge facies of the upper member 12. Lithic gravel distribution on the Canterbury continen of the Canterbury Bight Formation, showing tal shelf 30 presumed storm deposits 56 13. Shell gravel distribution on the Canterbury continental shelf 32 29. Core H405 from the trough facies of the upper member of the Canterbury Bight Formation 57 14. Distribution of sand modes on the Canterbury continental shelf 33 30. Core H812 from the inner edge of the shelf edge terrace (upper member of the Canterbury Bight 15. Side scan sonographs of the shelf off Banks Formation) 57 Peninsula ... 35 31. Geological map of the Canterbury continental shelf ... 58 16. Distribution of the dominant rock types in the -24>to -3<1> gravel fraction ... 36 32. Radiocarbon ages and depth below present sea level of 17. Gradient profile of the crest of the Rakaia fan surface dated species from shell layers under the (Canterbury Plains) and adjacent continental Canterbury continental shelf, and proposed Late shelf 37 Pleistocene sea-level curve for the Canterbury shelf 62 18. Ratio of quartz to sedimentary rock fragments in the sand fraction as a function of modal grain size and 33. Positions of successive Late Quaternary shorelines on distance from shore ... 38 the Canterbury continental shelf 65
LIST OF TABLES
Page Page 1. Lithology of pebbles in the -3<1>to -2<1>size range 19 4. Benthic macrofaunal assemblages in selected surface samples 22 2. Lithology of pebbles larger than - 3<1> 20 5. Benthic macrofaunal assemblages in cores 24
3. Petrographic analysis of the sand modes 21 6. Radiocarbon ages of shell layers 26
5
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ ° 3 1-1·s f ° /15 o o " is � edge shelf 80//#TY Tl?Oi/GII �AOOLE The MEl?N0O area. land · 11- 1 adjacent the of \\f� \\f� \\\ · metres. in ) ) physiography I I Isobaths simplified E shown. ° ° is and /13 /13 BAY 17 . PEGAS[JS '�' Fig. in Canterbury, ,,.,., I off profile ...... r:/ : I the slope LJ : of !�/ EY and . i I . : shelf /��ff< i location I :;,/ The I 112· 112· continental line. the � I of dashed a J I ? \, by � I .JO 'f.O 50 L Bathymetry �t-�� 20 k//omelres I ' represented FtG. \J o 10 s I 0 ' ° I Ill' Ill ° · 3 44 1 . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I Late Quaternary Stratigraphy and Sedimentation of the Canterbury Continental Shelf, New Zealand
by R. H. Herzer
Geology Department, Victoria University of Wellington Private Bag, Wellington*
ABSTRACT The Late Quaternary stratigraphy and sedimentary processes are interpreted for an area of continental shelf on the eastern side of the South Island, New Zealand. The area extends from latitude 43° OO'S to latitude 44° SO'S and includes Pegasus Bay, the shelf around Banks Peninsula, and Canterbury Bight. Two formations are recognised and defined in the Late Quaternary stratigraphy: the Canterbury Bight Formation and, locally overlying it, the Pegasus Bay Formation. They are identified by extensive unconformities (visible in high resolution seismic profiles), by geomorphology, by grain-size modes, and by macrofauna. The Canterbury Bight Formation underlies the entire continental shelf in the study area, where it is subdivided into lower and upper members. The lower member, deposited during the high sea level of the Last Interglacial and the falling sea level of the Last Glacial, consists of prograded very fine marine sand and mud. It is now exposed only near the shelf edge and on the continental slope. The upper member was deposited during the Last Glacial as a littoral/fluvial blanket on the regressive marine lower member, and was then reworked during the subsequent deglacial transgression. It is composed of gravel and medium and fine sand. This member is exposed on the inner shelf of central Canterbury Bight, where vestigial fluvial geomorphology is preserved, and on the middle shelf around Banks Peninsula where both relict littoral geomorphology and relict and modern sublittoral geomorphology are recognised. The Pegasus Bay Formation forms the present shelf surface off prograding portions of the coast and on those parts of the shelf that are not swept by strong currents. It is composed of very fine marine sand and mud, and represents the net accumulation of new sediment supplied to the shelf since the Holocene return to relatively stable high sea level. Ridge-and-swale topography occurs on two scales on the shelf. Very large, coast-parallel ridges and troughs are interpreted from detailed bathymetry, stratigraphy, sediments and macrofauna to be the remains of Pleistocene spit/lagoon complexes. With the aid of radiocarbon dates, the remains of four
"Present address: N.Z. Geological Survey, DSIR, P.O. Box 30-368, Lower Hutt.
7 New Zealand Oceanographic Institute Memoir 89. 1981. ISSN 0083-7903
Inset 2 This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ well deveioped shorelines between 28,000 yr and 15,000 yr old are recognised. Smaller ridges and troughs are interpreted as submarine features formed by strong currents. Those ridges that are in a zone of constricted and accelerated currents near Banks Peninsula appear to be active, while those well removed from the peninsular constriction are partly buried by muddy sediments, and date from times of lower sea level. Studies of sediment texture and provenance show that net sediment movement on the shelf and along shore during both Late Pleistocene and modern times has been northwards. The submarine canyons of the north-eastern shelf were probably the main Pleistocene sediment sinks. The present day flow of sediments is mainly into Pegasus Bay. Sedimentation on the continental shelf has reached a state of equilibrium with the modern hydraulic regime. Relict sediments of the deglacial transgressive sand/gravel sheet are being reworked in zones of high energy (principally in the region of constricted flow around Banks Peninsula) into lag gravels, sand ridges, sand ribbons and sand waves. Modern-input sand (distinguished by its grain size mode) is restricted mainly to an active belt near shore as a result of current action, but locally this sand has replaced palimpsest sand on the middle shelf. The modern mud facies, being confined by zones of higher energy, has reached its maximum areal extent; its greatest thickness is in Pegasus Bay.
Keywords: geology, Late Quaternary, stratigraphy, sedimentation, Canterbury continental shelf, New Zealand.
INTRODUCTION
The eastern continental margin of the South Island of reviewed here. The various components are tidal ew Zealand comprises a rather narrow continental currents, the Southland Current, swell, storm waves, shelf, flanked seaward by a system of large submarine wind-drift currents, barotropic currents, and possibly plateaus and troughs which intersect the landmass at internal waves. During calm weather the north-east high angles. The continental shelf is almost linear and is going Southland Current, the semi-diurnal tidal exposed to the open ocean for its whole length. currents, and the dominant southerly oceanic swell can periodically collectively exceed the threshold velocity The portion of the eastern continental shelf dealt for fine sand entrainment at a typical inner-shelf depth with in this memoir lies off central Canterbury between of 30 m. The frequency of entrainment increases with 43 • OO'S and 44 SO'S (Fig. 1). The eastern shelf ° decreasing depth such that near shore, sand movement reaches its greatest width in this area, which includes is continual. In deeper water, the currents are too weak Pegasus Bay, the shelf around Banks Peninsula, and to move sand except on the shelf off Banks Peninsula Canterbury Bight. It descends very gently to a shelf where the flow is accelerated by the topographic break at a depth of 130-180 m, the shelf break being constriction. During stormy weather, swell, wind waves approximately 90 km off shore. The shelf lies off a and wind drift may move sand almost continuously in 40 km wide Pleistocene fluvio-glacial outwash plain, inner-shelf depths, and less frequently at a typical the Canterbury Plains, behind which rises a high middle-shelf depth of 75 m. On the outer shelf, such mountain front, the Southern Alps. An extinct volcanic shallow storm currents are largely replaced by complex, Banks Peninsula, rising to over 500 m, barotropic and baroclinic flows and possibly internal divides the shelf into Pegasus Bay to the north and waves which, when reinforced by tidal currents and the Canterbury Bight to the south. East of the continental Southland Current, may be strong enough to move shelf, the Chatham Rise, with a crestal depth of less sand on the bottom. than 500 m and shallowing locally to as little as 32 m on The net direction of mass transport of water on the Mernoo Bank, extends seaward for more than 800 km. shelf is north-eastwards (Fig. 3) because, although the North and south of the rise, the sea is over 2000 m north-east-going flood and south-west-going ebb tidal deep. Separating the rise from the continental shelf is currents are of equal strength, the other principal Mernoo Saddle with a depth of 550 m. currents move mainly in a north-eastward direction, Rivers entering the sea along the east coast of the viz., the Southland Current is a permanent north-east South Island drain a hinterland composed mainly of going oceanic current; the dominant longshore current Mesozoic greywacke and argillite in the north, and is north-eastwards in response to deep-sea swell from chlorite zone, quartzo-feldspathic schist in the south the south; and the strongest wind-induced currents flow (Fig. 2). Small intermontane areas of soft Cretaceo generally northwards in response to intense and Tertiary elastics and limestones are present and basic frequent storm winds from the southerly quarter. The and intermediate volcanics occur locally. southerly winds also generate an on-shore (westerly) The currents affecting the area have been discussed component of flow over the inner shelf and a slight off in detail by Carter & Herzer (1979) and are briefly shore (easterly) component further seawards. 8
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ 0 0 0 0 166 168 170 E 172 174
CJ Ouaternar� sediments ° 40 Cretaceous - Tertiar�
� I imesTone. mudstone. Tuff. sandstone. conglomerate
Carboniferous Jurassic
� gre�wacke, argillite 0 rnIIIIIIII Paleozoic sedimen ts 42
44 S
- Volcanic rocks
Schist
l(:·::;,':-;j:il Granite, gneiss, 46 amphibolite. gabbro
Fm. 2. Simplified geology of the South Island. Modified from NZ Geological Survey 1972: Geological map of New Zealand.
BATHYMETRY
In areas where detailed surveys have been done, such The first step in the geologic investigation of any as Pegasus Bay and west of Banks Peninsula, the METHOpiece of DSsea bottom is ( or should be) bathymetric detailed bathymetry was contoured from the collector mapping. All available bathymetric data were accord sheets of the Hydrographic Branch, Royal New ingly compiled (Fig. 4) and contoured (Figs 1, 5, 6). Zealand Navy. An obvious and fairly systematic 9
Inset 2•
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ s ° ° 44 0' 43 3 30' ° ° 1- I- I- I 75 75 1 1 \ \ Carter � "---..... from ' \._ 0 I 30' 3 / 7 J ° ° '\ 174 174 information. Modified information. ------II " J�W? / Landsat I '-\(() I ' ' I 'J� 0 0 \) I 3 3 / / drifter and drifter / ;t / I � I / I i I sea-bed 0 I / I / I i I / ,r:f:P '/5 E / / ° .§F ° / card, 3 --· 73 / !'><:) 1 (\; 17 ""' drift / .7 / ? · on / · y ·-·- . ' I / : hl I / based >- · ./ ./ ' shelf, · 0 0' I 3 3 � -- ,/ / 76). -- (19 Canterbury / al. the I FLOW �- ° ° ° et on ' I 172 172 flow SURFACE FLOW Cochrane FLOW water / net � and of BOTIOM ' ' 0 0 I 3 3 (1979) ::> SURFACE - --- BOTIOM AND Direction . 3 Herzer I ·-·- and FIG. ° ° 71 i � ...j 1 171 ---j ---j ' 0 ° ° 3 30' 43 44 . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I surveying error along some of the sounding lines on the the coastline is rocky, a rough surface of low relief collector sheets created a spurious bathymetric trend (Zone B) occupies a coastal belt about 10 km wide. that had to be filtered. The spurious nature of this trend was confirmed by echo-sounding profiles made by the author. The echo-sounding lines of the hydrographic Zone C survey, closely spaced, parallel with each other, were Zone C lies adjacent to the coast south of the Rakaia run in a north-south direction in some areas and an River, where the coastline is cliffed. The zone does not east-west direction in others. When the depth values stand high above the rest of the shelf as does Zone A. were contoured, isobaths crossing the lines at a high Its slope is slightly steeper than the more level shelf to angle formed a sharp zig-zag pattern that was always seaward, but merges imperceptibly with it at a depth of parallel with the trend of the survey lines and clearly approximately 60 m. Bathymetric contours within the superimposed on the actual gross bathymetric trend. zone (Fig. 1) describe two adjacent, broadly convex, The error was one of depth and not one of position, arcuate surfaces with a slope of 1.2-1.4 m km - ; extending 35 km off shore from the two great alluvial since a large displacementkm) of contour lines (generally in the order of 1-2 occurred where the slope is fan surfaces of the southern Canterbury Plains. It gentle - on the continental shelf - and no displacement shows on echograms as a smooth surface with local occurred where the slope is steep - in submarine wave-like features superimposed. The wave-like canyons or on the continental slope. The error was features are asymmetrical, with steep slopes facing approximately ± 0.5 m, and when allowed for, it north to north-west; they have a peak-to-trough became possible to contour with a 2 m vertical interval. amplitude of approximately 1 m and a wave length of South of Banks Peninsula no detailed survey had approximately 2 km. been made. In order to deduce the geomorphology, every available echo-sounding profile (Fig. 4) was examined, and features such as slight changes of slope, Zone D Zone D occupies a mid-shelf position and stretches areas of smooth or undulating sea bed, highs, ° depressions, terrace-like features, etc., were plotted. It from about 44 30'S northwards to Pegasus Canyon. Zone D generally has a much gentler overall slope was found that most features could be correlated from 1) profile to profile and tied in with those shown by the (0.6-1 .0 m km - than the landward zones (with the detailed bathymetry to the north. exception of Zone A in Pegasus Bay) and it terminates seaward at a line where the1 slope of the shelf again increases (2.5-5.0 m km - ). The surface of this zone is uneven and echograms display a gently rolling profile that is characteristic of GEOMORPHIC ZONES OF THE CONTINENT AL ridge-and-swale topography. Echograms and detailed SHELF bathymetry show that the ridge-and-swale topography The morphology of the shelf surface shown in Figs 5 is made up of two distinct types of ridges: small ridges and 7 is described below. Its probable mode of origin is with heights of 1-14 m crest-to-trough, widths of 1- discussed later. It consists of a series of zones which are 2 km across the base, and crest lengths of JO km or generally parallel to the strike of the shelf. more (Fig. 6); and very large ridges of generally similar height, but with widths of 9-13 km across the base and Zone A crest lengths of 30-50 km (Fig. 5). The large ridges are Zone A includes almost all of Pegasus Bay plus a termed here "first-order ridges", and the small ridges. narrow band extending south around Banks Peninsula "second-order ridges". Second-order ridges are locally and into Canterbury Bight as far south as the Rakaia superimposed upon first-order ones. River. It generally stands above the rest of the In Canterbury Bight, second-order ridges are continental shelf, its seaward limit being easily concentrated in two long, roughly parallel bands, recognised along much of its length by a relatively incompletely separated by patches of smooth bathy steeply sloping (2.0--6.0 m km·1), convex suface ter metry (Fig. 7). The bands run parallel to the general minating abruptly against the more level shelf to the 1 trend of the bathymetric contours, the landward band east (0.6-1 .0 m km - ). Within Pegasus Bay, the zone occurring along the south-eastern margin of the near comprises a broad, very shallow trough with slopes of 1 shore Zone A, and the seaward one occurring at the 0.2-0.8 m km - , open to the north, and bounded on line where the bathymetry again steepens seawards at the east by a long, low bank, projecting north from the about 80-90 m depth. north-eastern extremity of Banks Peninsula. Around Where the continental shelf is narrow south-east of Banks Peninsula it consists of a narrow, sloping apron the peninsula, Zone D occupies most of the width of 7 km wide. It widens in Canterbury Bight to a shallow the shelf. Within it, neither a two-fold areal distribution platform 20 km wide. The surface of Zone A on nor first-order ridges are evident and the zone is echograms is smooth and even. B dominated from east to west without interruption by Zone high second-order ridges with crest-to-trough In the north-western corner of Pegasus Bay, where amplitudes up to 14 m.
11
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ 43° 4 4 °S 4 4 0 ° 5 5 17 17 ) km km km km - nset so (i o 5 o · 40 1 · 1 · 0 2 10 e profile - - 1 survey o o - : edg soundings · kHz 20 30 hydrographic 4 O · 5 - 1 · o · by 10 O: B: C: Shelf A: Echo 3 5 KILOMETRES 0 spacings detail in ° ° Hydrographic line 4 174 17 covered area --'---....I..--....L.--...L---'---.,_--._----;I- - shows , / / (inset) / ' __JL._-,-' / _ ,. ,. L_ diagram '- __ /V' / .J_ / / E / ' ' ° ° ' / ' ,( 3 Reliability ' .J__....,... ' / 17 173 , " I '! __ y-/ ..J.... / Canterbury. __ off __,__ __ " shelf ...J.. " :.-- ' 1, __ ' /'>" /, 1 _,._ 1' , continental ° o / ./-...: ' ' '-/ , __ 44 the 43 /� __. ',/ �/ ,� / 0 of ° ° 2 ; v 2 ' L_ _ L_ 174 17 17 ' �/ / __ ' ' coverage / ,_ /� ' ' ' / -< _ ,_1, ' / sounding ° / '- >" � 1 ' ..J x 173 ,, ' '1" __- Echo / '> "1< ,, /_ 4. / eys. ... ,, '- < ' < >' ' < ' / v ( ' FIG. sur ' ° 1 ----L--....L.--..1...-_.J_ __ ----L--....L.--..1...-_.J_ 71 -+ 17 1 ° 43 44 . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I S 1o' � 0 · \•) , " 43' 15 35 .;5 .lQ 20 4 � 17 174 20 55 5� of 10 50 50 km locations are 5 lines 45 ·1 0 Dotted 0 I .1 40 " diagram. i, ,J ) 5 ) l 3!> .1 ,, � /f '' reliability ,1 ' Jl \ 30 .10 )) \ 1 Fig. 4 for /J IJ/ "''' J) 25 See 7 ?S J 1 / , J 1 \) ) . 1• 20 ?O Peninsula. ij} ' l \ '' ' 15 15 <::: off Banks off " metres. � shelf in 10 10 I the and !sobaths I >jf' ,, 1. Bay 5 � -2 19 /) ,,.1 E 3 Figs } /// 1 17 or Pegasus 17J in // )/ ••. PENINSULA / 1 55 55 appearing bathymetry I/ ··· ;, 1 50 profiles so' /) Detailed BANKS �� 5. kHz /I/� ' ' 5 FIG. 3.5 I · 45 4 · 40 40' ' 'I 5' 5 10 1 so't=;:;::::;:1 45' 30 35 40 20' • 43 ,.... (.;)
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I 5 ' 5' 55' 173 E 5' 10' 1 20' 2 30'
F1G. 6. Bathymetry° of a portion of the second-order ridge-and-swaie topography off Banks Peninsula. Locations of side-scan sonar lines are shown. Isobaths in metres.
North of latitude 43° 55'S the second-order ridges level. The ridge branches at about 43' 40 1S, the eastern become abruptly sparser, and within 10 km, virtually branch appearing as a very subdued high east of the disappear. Two parallel first-order ridges, each with a head of Pegasus Canyon and the western branch trough on its landward side, continue north towards forming a narrower and more pronounced ridge which Pegasus Canyon . trends into the western side of the canyon head. The eastern ridge of the first-order pair lies just The western ridge of the first-order pair has a depth inside the 90 m isobath and is broad and indistinct (Figs of 74-76 m below sea level along its crest and its 5, 7). Its crest is 84-86 m below sea level and its landward trough has a fairly consistent depth of 80 m associated landward trough is, on average, 90 m below below sea level. Ridge and trough both terminate at sea level. There are several relatively small, closed 43° 35'S where they merge with steeper bathymetry off basins in the trough, the deepest being 100 m below sea eastern Pegasus Bay.
14
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To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I At the northern end of these first-order ridge 1.3 m km - 1 south of adjacent Zone D, but is free of systems, shallow troughs leading into the canyon are ridges. It then steepens again 40 km off shore at 70 m 1 prominent in the bathymetry. East of the eastern first depth to approximately 2.5 m km - . order ridge the shelf simply slopes gently seawards for 25 km as a broad platform with an uneven surface, Zone F steepening near the shelf edge. Zone F is a terrace 4-9 km wide that skirts much of the shelf edge of the study area. North of 44° 25'S, the Zone E surface of the terrace is smooth and even; to the south In Canterbury Bight the ridge-and-swale zone passes it has a gentle second-order ridge-and-swale seaward and south-westward into a zone of smooth, topography. even bathymetry. The seaward change is marked by an increase in slope; the south-westward change simply by The depth of this terrace is variable. The depth of the a gradual disappearance of the ridges. This region of lower limit of the terrace (the shelf break) ranges from smooth bathymetry is designated Zone E. Its seaward about 130-180 m, and that of the nick point at the portion forms a relatively steeply sloping upper edge of the terrace from about 110-150 m. The 1 greatest depths are found along the distal portions of (2.5 m km - ) belt about 11 km wide running parallel to the shelf trend and tapering out north-eastwards at the shelf east of Banks Peninsula, and the shallowest on about latitude 44° 13'S. To the south-west it widens the narrow shelf at the northern edge of the study area landward to include most of the shelf, but undergoes and in the upper reaches of Pegasus Canyon. subtle changes of slope which reflect the slopes of the Intermediate depths prevail off southern Canterbury distinctive zones to the north-east. From the coast in Bight. this area down to a depth of 40 m, 20 km off shore, the The terrace is slightly anomalous in the region of 1 slope is approximately 2.0 m km - , slightly convex, Pegasus Canyon. It is 9 km wide along the south-east and reminiscent of Zones A and C in central and side of the main trunk of the canyon but is absent on northern Canterbury Bight. There are no wave-like the canyon's west and north sides. The terrace is also features on the echograms, however. The slope absent on the narrow portion of the shelf adjacent to becomes more gentle seaward, approximately Banks Peninsula.
SEDIMENTS
SAMPLING ANALYTICAL METHODS A total of 332 surface samples and 23 cores were Each of the surface sediment samples obtained on studied (Figs 8, 9). The cores and 224 of the surface the three Tangaroa cruises plus any relevant samples in samples were obtained during three cruises by the the NZOI collection was homogenised, split, and wet author on r.v. Tangaroa in 1973 and 1975. The sieved with a 4<1> sieve. The relative amounts of lithic remaining samples came from the NZOI collection. gravel, shell gravel, and sand (including carbonate) The bulk of the surface samples were obtained with a were determined by dry weight. The amount of mud modified Hayward orange-peel grab and a small was determined by the pipette method of Folk (1968). number with a Deitz-Lafond grab. Cores were obtained The colour of the dry sand fraction was noted with a 6 m Kullenberg-type piston corer. (Geological Society of America 1963). Complete The shipboard processing of samples was as follows. pipette analyses of the silt/clay fraction were performed A representative subsample (to be used later for for a small number of samples only. A complete sieve textural analysis) was taken from each grab sample. For analysis using the following mesh sizes (in units) was sandy and muddy sediments, about 400ml were taken; carried out on approximately half of the samples (Fig. for very gravelly sediments, several litres were taken. 8): -1, -0.5, 0, 0.5, 1.0, 1.25, 1.50, 1.75, 2.00, 2.25, The remainder of the grab sample was washed through 2.50, 2.75, 3.00, ::; :s. 3.50, 3.75 and 4.00 . This data a screen and the pebbles and shells retained were is presented :n the author's thesis (Herzer 1977, bagged. Living specimens were preserved in alcohol. Appendices 2, 3) orGmaybe obtained by writing to the Cores were stored vertically aboard ship in their plastic author. liners. On shore they were cut to storage size and This sieve distribution was adopted after trial runs stored horizontally prior to analysis. using a complete 0.25 interval set on a number of Surface sampling was mostly on a grid pattern with a representative samples had ..-evealed the dominant spacing of about 10 km. Coring was restricted to sites grain-size populations. Matenal smaller than - 1.0 deemed important. was brushed through the large sieve with a paint brush 16
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ S ° ° 3 44 ° 4 0 5 5 I � • 17 17 • • I 40 50 0 3 • • • • e • 20 LOMETRES 10 KI 0 • e O 4 ° 4 • • • • • 0 0 17 ; 17 I I , • ,' - - V r.\ • '\ • j'J • · • • 0 : I , / ® 0 0 0 • • 0 /, .s . • 0 , . � \ � 0 . r.\ \!I ®· • ® ® / Y Y ; /0 : ' •1 1 0 • � ' � : , .. , ,. � � @ " @' 0 � •• ,,. / -- �� 0 ®J "' .. , • ,,. 0 · .n<0 ,0 . ,,1 ; • ,,. . �I ® (!)0, . • , • • I i e (!)"/ � • • ,. • ,� • • � ® 0 f»; r.\ \V 0 0 • 0 ® (!)0 s t ations. · • r.,( • · )I!/ 0 • 0 � •• , •• 0 0 . ,.,:-' - © · • r;;( P P 00 e) • / • E • sample � O � · • • 0 0 3 ,4, • ' 0 of --@ � 17 • / " 0 - 173 0 ,,' 0 0 ., ' • • (!) 0 • tions • td< 5}!:I., • • � 0 ,- -'O Loca ' • , 0 0 0 8. • • 0 / 0 " ,, # FIG. ,, G) : • Gf , 0 • 0 0 e5 /8 JI • 0 0 0 / ' 0 0 % 0 0 0 0 _ 0 ,( I I I I I I I I I I I I I I I l 0 0 0 0 • 0 0 o 0 0 0 2 ° 2 · • 17 17 r;;( )0 0 sis 0 r.,( xv 0 analy 0 sis • e 0 g 0 ed / analy sample ofauna 0 e Grab Cor Macr Shelf Sieve • ,- • 0 I I 0 I I I I I I , _ 0 I 171 171 171 ° 3 44 4 ...... --.J 0 � ": . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I
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To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I to avoid abrasion of the commonly shelly and delicate relative abundances of visually determined rock types gravel sized material. The 16 sand sizes ( - 0.5 to in each of two size classes ( - 3 to - 2, and > -3 ) + 4.0 ) were sieved for 15 minutes on an Endecotts were recorded (Tables 1, 2). Where there was sufficient shaker which could accommodate eight sieves at a time. gravel in a sample, 100 pebbles were identified; but in Each sieve fraction was examined under a binocular many samples, gravel was rare and smaller numbers microscope and the relative proportions of elastic shell had to be used. Besides the gravel, certain sand sand, foraminifera and terrigenous sand were esti fractions obtained from the 0.25 sieve analyses were mated. The carbonate sand content was low and did not examined petrographically for the provenance study significantly affect the fraction weights. (Table 3). The pebbles that had been washed from the The shells washed from the remainder of each total remainder of each grab sample after subsampling on grab sample at 54 stations along selected sample lines board ship were examined for a provenance study. The (Fig. 8) were identified and the relative abundance of
TABLE 1: Lithology of pebbles in the -311) to -211) size range
Greywacke Other Grains Quanz Schist ( Argillite) Rocks Co unted
8552* 6 + 94 16 E28 68 32 22 E40 25 70 5 20 E99* 93 7 15 EIOO 100 11 H314 95 5 42 H316 65 33 2 11 46 H317 89 82 H318 82 18 44 H323* 70 1 25 4 100 H324• 83 1 15 1 100 H325 94 5 1 100 H335 91 l 6 2 100 H336 92 l 3 4 100 H339 23 68 9 22 H340* 83 2 13 2 100 H341 100 54 H346* 70 24 5 100 H3Sl 93 3 4 119 H360 95 1 4 100 H361 81 2 16 I 168 H364 80 20 69 H376* 80 17 3 100 H378 81 14 5 57 H380 63 37 46 H382 16 79 5 19 H383 22 I 5 • 100 H384 2 7872 7 100 H394 8913 3 8 35 H395* 93 5 2 105 H396• 94 6 100 H398• 88 11 I 100 H400 69 21 9 104 H401* 66 32 l 100 H402 92 5 3 100 H403 89 3 7 100 H408 91 8 l 100 H410 78 1 19 2 115 H412 90 2 4 4 100 H418 78 18 4 27 H421 80 15 4 100 H423 78 19 3 100 H424* 74 21 4 99 H425 80 2 14 5 63 H470 3 91 6 100 H782* 2 95 3 100 H785 75 22 3 32 H787 78 22 9 J448 93 4 3 71 J458 100 8 J459* 100 5 J460 2 83 14 79
•The sample number appears in the analysis of coarser gravel in Table 2.
19
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ TABLE 2: Lithology of pebbles larger than -30
Greywacke Other Grains Quartz Schist ( + Argil/ite) Rocks Co unted
8552 100 5 100 3 G656 2 95 3 20 E99H323 26 66 8 144 H324 50 50 16 H340 69 2 23 6 51 H346 41 9 47 3 H376 63 33 3 202 H384 2 96 7834 H395 64 2I 29.5 4.5 44 H396 67 33 I 18 H398 56 2.5 39 2.5 36 H401 65 32 3 60 H424 3 46 61 H470 3 94 3 30 H782 SI 98 2 100 J459 100 15
each species noted (Table 4). The lack of faunal remember that the sampling is by necessity crude analyses from Pegasus Bay is a reflection of the relative to the complexity of the environment being extreme paucity of shells in that area and not of bias in studied. sample selection. During the last 18,000 years the sea has transgressed Cores were split, logged and photographed (Herzer inland across the continental shelves of the world, and 1977, Appendix 4), those obtained on the first cruise the whole of the shelf environment has changed. The (H347-H474) being X-radiographed prior to splitting; normally fining-seaward sequence of elastic sediments shell horizons were sampled, the fauna identified is restricted to the inner shelf, while the sediments of (Table 5), and selected material submitted for the outer shelf comprise coarse detritus abandoned radiocarbon dating (Table 6). In most cases, only a during the deglacial transgression (see e.g., Pilkey & single species was submitted for dating, and then only if Frankenberg 1964, Curray 1965, Emery 1968). These it was a dominant species in the core. Where two so-called "relict'' sediments on the outer shelf, pecies had to be used in order to make up the required although not replenished by modern sediment input, ample weight, they were from closely related remain mobile where energy conditions are sufficiently environments. Wherever possible shells of species high, and sediment waves, sand ribbons, linear ridges, restricted to shallow water were selected. The material etc., may form from these relict sediments and migrate submitted for dating was apparently free of borings and across lag pavements or exposed semi-indurated of biological or chemical encrustations. surfaces (see e.g., Stewart & Jordan 1964, Belderson et In order to obtain an on-shore reference collection al. 1971, Stubblefield et al. 1975, and Swift et al. 1973). for provenance study, 38 river, beach, and cliff grab Conversely, where energy conditions are low, relict samples were taken along the coast from the Waipara gravels may be thinly mantled with modern mud. Low ° ° River (43 09'S) south to Dunedin (45 55'S). River or negligible rates of sedimentation on the outer shelf bed samples were collected where possible from contribute to high concentrations of shell material midstream bars and beach samples from the wet which have accumulated on the surface over thousands foreshore below the most recent high water mark. The of years. The surface of the continental shelf is lithology of the gravels was determined visually. The therefore a surface having sediments that reflect modes in the sand fraction of selected samples were different environments. For a particular sample the determined by standard sieving techniques with a sediment texture may reflect both present conditions 0.25 cJ> sieve spacing. and past conditions, the two being very different. The textural size classes used are those of Wentworth A grab sample is an indiscriminate point sample, the (1922) and Folk (1968). top 10 cm or so of sea bed being collected by the sampler without regard to microstratigraphy. Moreover, in an area where sediment composition may SELECTION OF SIGNIFICANT TEXTURAL vary considerably over a space of metres, the spacing of samples will significantly affect results. Closely spaced When interpreting the environment of continental samples or selectively located ones ( on the basis of side shelfPAR A sedimentsMETE RS from their textures, one has to scan sonar imagery or direct observation for instance),
20
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ 3 6 2 ;.- � g, � H331 H343 H4 H351 H35 H395 H436 H445 H466 H470 H782 H784 H793 H371 H458 II II � � Ill IV IV I IV I IV IV Ill IV III IV IV IV III .6 s:: s::, � 0 30.2 24.9 55.5 61.4 65.7 57.S 52.0 39.7 59.2 66.6 40.6 62.2 SI 47.4 57.7 57.7 >:: ..., 6.9 2.7 8.5 8.2 7.6 6.5 .. 5:: I:> 9.6 9.8 8.7 3.8 6.8 8.5 7.6 {l -,!., 12.8 10.4 .5 :32 s::, r 1 l 1.5 1.5 7.2 0 3.6 0 5.2 3 0.5 2.0 0 3.6 2.5 9 c :i. � 1.0 0.5 0.5 0 0 0.5 0.5 0.5 0 0.5 0 0 3 0 0.8 0 modes t:i:, 5· � :,, 0.5 0 0 0 0 0 0 0 0 0.5 0 0 0 0 0.5 0.5 sand of the � )'l c::, e �- 1.5 1 0.5 0.5 0.5 0 0 0 0 0.5 0 0 0 0.8 0 analysis Cl s s:: :s " c::, i' 1.0 1 0 0 0 0 0 0 0 0.6 0 0 4.1 0 0.8 scricite h t i w s Petrographic tives gn. in a ;:, a So .. ..., � i:l r;;- "' 3: 1.7 1.7 0.5 0.5 0 0 0.5 0 0 0 0.5 0 0.7 0.5 ins; a deriv gr cke) a TABLE · .... 1 I:> ;; � strained . n 1 1 1 1.0 1 1 0 0 0 2 0 0.5 0.5 0 0.8 0 0 (greyw � 6. -· ,: � i:l "' r;;-3 pcrthite e iti c d u an d m ed rutilc so n amm i s ocks ' s p ra e, r nd n it a st ic s::, - ;:, i;; :i:,� g �3 '.?lS �� 13.3 15.1 17.8 16.6 16.0 12.7 13.2 26.3 23.1 33.3 41.8 20.4 32.S 20.9
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Patellids Patellids
Anhropoda Anhropoda
Gastropoda Gastropoda
Xymene Xymene
Xy
T T
flabellum flabellum
G/aphyrina G/aphyrina
Nat.
Zerhalia Zerhalia Zega/ems Zega/ems
Z,:arroplwn Z,:arroplwn
Z
1 M M Neuthyris Neuthyris
Splt
Leporemax Leporemax
Denralhun Denralhun Poricria Poricria
1:./J />
Pervicac:ia Pervicac:ia
Micrelenchus Micrelenchus
Mfr:aulapex Mfr:aulapex
Maurea Maurea
Mauriro11el/a Mauriro11el/a Maoricrypra Maoricrypra
Maon
Dentalium Dentalium
� � . . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I
To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I - - + - + t + + - + ++• + + + • t t - - + + - + - - -- - + + + + + · + + + + + - -- + - + + -- -- - + - + -t + - + - + + + + + + t -- + + + - + + - + ++++- - ---+ t + - .. • + + t - t + tt+- - + + + + + + + + ++ - + • - - - - + t t - .. + + t t + + + + + cores rare - in • . + - + + • - + -- - -- + • + - + + � • + + + - - + - + + + + + • I r . r 1· I I assemblages 0 common, 0 0 0 0 0 0 0 11 7S 200 2 200 3 0 3 3 3 00 9 200 20 2 8 4 4 4 ------� - - -3 - - - = 0 0-JO 0 0 0 0 8 0 0 0 0 0-200 O+ 8+ 0 0-20 0 0-4 0 220 0-20 0 O+ 0 0 4 70 0-90 O+ 10 27S 10 10 10+ 20 20 40 , + macrofaunal abundant 1833) = Benthic rd. a • S: } Caim & } TA8LS is 183S) (Quoy } ulondiat gemmulara C BIVALVIA { C { (Gray. GASTROPODA } trgei/urm 1791) stra,rg,:; } laticosrata 1906 stra 1819 incl. delicat11/a bolto11i subrostrata anripoda ,ubrosea M australis mgora M M. { lineolara stangeri Hedley. £111,ucu/a N. Nucula (Gmclin. N. { { G. G. Lamarck, pulclrellum ooteanus · scalpel/um p. I ze/andica purpurata convexum glans sp.? s multi.striara 1 zelandica · modesra (Grandaxirrea) impacta maoriana sp. e . e 1 { aeztlandiae . sp? cancellata hu11011iona me i cybaea chorlouoe lruttoni maorio . nano m biconvexa ,wvaezelandiat rtguloris ta globu, sp. incl. sp. sp. (Zygochlamy,) (Mimachlamys} Phaco,oma) indet, ula striatula vitrea no,,aezelotrdiae vegrandis sp. me acinaces sp. subtniJ,rgu/atum sp. au!lra/e ( spisso aequiloteralis grayl ooteona sturchburyi no, sp. sinuota ae meris canaliculu, I sp. I'd ufusus ofusus sp. o . hitJ 1 rilus edulis . wero llinetla ucu n er Venericardla Thracio Bivalvia Ta Tellintlla Te Throcia Mytilidae Zenatia Ostrea N Saccella be/111/a Pullostro largillierti Limotula Pfeuro Scolpomactro Pa,ruvea Paphies Pap Paphies Ostreo Proleda /a,iceolata Spisulo My Uma colorata Myadora Glycymeris Glycymeris Notocallisto Ga Cominella Pecun 0,/amys Penra Antiso/orium egemun Myadora Modiolarca Myadara Mesopeplum Escalima Dosinia DapJmtlla Emargin Do,i11ia Argalisto Austr Austr Nemocardium Otione Caryocorbula Oirdita N, Anumia Myadora striara Dosinla Diplodo11 Buccinulwn Divaricella Epitonium Dosinla lambara Dosinla Amo/do Orlamys Bathyarca Aulacomya �
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. . To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ e I · - + - + + - + + + + + + I I I I - N -- + + I ����� + - I oo�N� +-- • + + + + I o - +- - ���b����� �� ---- ++++ + �-��� - - (cm) + ----- I I I I I OMM -NNNNN- -NMNNMN �- -- core � + ++++- -- -- + t + + in I -�������OOONNM + + - ����������oo�oo�� I I depth ONO --- + + + + t + I O - and � N N + No. -- - �N + + • NMVOM�N� ��������������������� Core - - I I I I I � - - + + OMMMM�M�� � �������������� - �M������ �� + + + I+ 0 (m) 11 70 40 - + - depth 0 20 O+ 0-60 4 0 O 0 0-20 4-130 30-200 range Approximate 1873) GROUPS (Hutton, SCAPHOPODA BRACHIOPODA OTHER ,ymme1ricus ei11a111, ze/andica Taneo hulloni a/hula roseu.s monoxyla sanguir,ea indet. Cruslacca Cirripedea tubes no11atzelandiae aotea11a sp. sp.? zelondicum 11m1um (Zeacolpu,) /e,,ticularis (Sliracolpus) sp.? tristis tenuis indet. sp.? zelandica chus sp. Species brate/la :omi11ia Echinoidea Arthropoda Bryozoa Polychaete Tere Marginellidae l'yramidcllidae Xymene Gastropoda Dentalium Dt.molium Ne01hyris aacolpus Naticidae incl. Patellids Trochidae Zeaco/pus Zegalerus Zetha/ia Phenoroma Pervicacio Glaplryrina Maoricolpus Splendrillia Fusitriton Maoricrypta Maori1onella Maurea Micrelen ·1;i.u � � ( . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I if accurately positioned, warrant detailed treatment. Widely spaced grab samples, on the other hand, are only usefully interpreted by simple statistical measures. The sediments on the Canterbury continental shelf exhibit all the textural complexities outlined above. The grab samples were, by necessity, widely spaced. 0 § V, Accordingly the following simple textural parameters tl tl were used: gravel-sand-mud ratio, % lithic gravel, % .... shell gravel, % mud, and grain-size modes within the V, � Nr- sand range. The gravel-sand-mud ratio, including a provision for shell gravel content, was determined for each grab sample and used to construct a simple sediment distribution map. The remaining parameters form part of a "quick" modal analysis which was used with some support from sediment colour p. 31) to map the principal sediment populations. Since grain-size modes tend to retain their integrity(see despite changes in transport energy or mixing with other sediment, they are an effective means of tracing discrete sediment populations with different spatial and temporal relationships through this complex zone of modern, relict and palimpsest sediments (cf. Curray 1961).
SEDIMENT MAP The method used to construct the basic surficial sediment distribution map (Fig. 10) conforms funda E E EEEEEEE mentally with that used by the N.Z. Oceanographic N r- OOri"lf'.\O r--- N N � °'OON-,°'O"
26
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ S 0 ° 43 44 ° ° 175 175 ' :,' 60 km ' , : · , , , :; ., : edge . ( ·_: / � '. <;- . : . 40 i ,: . . . · . Shelf - ·. / \:;ff{ · .. ;� ' ;c;; ·: • , I I � ·�- ; 20 , . % i . b � i ,: < l ° 0 � .�(\/ ° ,;)/ < ; 174 , , 174 _ , ,' ' ] • , . . ., , ·, . ;;){ shelf. : ···••• : ·· inental . t "' . - ' ! ,' con ', � _. ', ,:•: ,' ;_ • Canterbury E t he ° ° on 173 /73 • . ,� ibution r dist 1 t sedimen 'ficiul Slll gravel grovel 10 A . ° ° in in F IC l 172 172 ll she shell s 50% 50% (> (< 90% hic lit shelly A G 50% O 0 0 0 0 0 10% ° ° I 171 171 ° ° 3 44 4 J . N -- . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I extends from Pegasus Canyon to about 44° 20'S. The DISTRIBUTION OF MODES highest gravel concentrations occur where the shelf is Runs of complete - 4 to + 4 size analyses on a narrowest south-east of Banks Peninsula. representative number of samples defined the major Gravel is absent from most of Pegasus Bay, from a modal classes in the gravel/sand range. These modes narrow belt around and immediately adjacent to Banks were sufficiently well separated to be easily read from Peninsula, from the far northern part of Canterbury histograms. They are: Bight, and from the southern part of the Bight (except Mode I - gravel ( excluding shell gravel) - 4 to the coastal belt south of Timaru). - 1<1> Mode II - medium sand 1.1 to 1.4 Sand Mode III - fine sand 1.9 to 2.6 Sand is almost ubiquitous on the shelf. It is present in Mode IV - very fine sand 2.9 to 3.4 quantity in all but the muddiest areas. Clean sand occurs along the coast of Pegasus Bay as a near-shore Shell gravel is present in sufficient amounts in many belt about 4 km wide; and in a 13 km wide coastal zone samples to form a gravel mode. It defines areas of relict in northern Canterbury Bight. It also occurs as a sediments (low present-day sedimentation) or areas of prominent lobe that projects northwards from Banks active erosion. The parameter "weight % shell gravel'' Peninsula into Pegasus Bay. (as a function of total sediment weight) has been Clean sand also occurs, locally mixed with gravels, in accordingly treated as a mode. a zone on the middle shelf. North-east of Banks Lithic gravel also indicates areas of active erosion or Peninsula this clean sand zone extends right out to the low present-day sedimentation on the shelf, and is shelf edge and into Pegasus Canyon; south-west of significant for indicating past near-shore environments. Banks Peninsula it extends into Canterbury Bight Its concentration is expressed as weight % gravel in the where it is gradually and unevenly replaced south sediment after removal of shell gravel. westwards by muddy sand and sandy mud. A belt of In many places, mud is the dominant sediment. muddy sand occurs along the south-eastern margin of Although pipette analyses of the mud fraction were not this zone, separating it from the shelf edge. done as a matter of routine, samples that were so Isolated areas of clean sand form a chain along the analysed contained a mode ranging from 4.5-6.5 shelf break from end to end of the study area. (coarse to medium silt). For the purposes of this study the sizes within the mud range are not differentiated. Because extremely polymodal sediments are the rule Mud rather than the exception on the continental shelf, the Mud with less than 10% sand is rare on the mud content is considered here to be a more reliable Canterbury continental shelf. Sandy mud is found in a indicator of the energy of the environment than the patch on the middle shelf at the southern end of large standard deviation of grain sizes (sorting). A low mud the area. It grades laterally landwards, seawards and content in the sediment is taken here to imply a high into muddy sand. Its southern limit is northwards energy environment. unknown. Most of Pegasus Bay is covered by sandy mud which extends north of Banks Peninsula in a broad band. Mud Lithic Gravel (Mode I) Distribution with less than 10% sand is confined to the deeper The lithic gravel content of the sediment is calculated northern part of the bay and to a small patch at the thus: southern end adjacent to Banks Peninsula. It is also wt lithic gravel -;- (wt total sediment - wt shell found within the bays of Banks Peninsula. It grades gravel) x 100 laterally landwards and seawards into muddy sand. Within the sandy mud zone in Pegasus Bay is the The results are contoured at 0.1 %, 2%, 5%, 10%, tongue of sand which was described above. The 20%, 40% and 80% in Fig. 12. northern limit of the sandy mud is beyond the boundary This figure is a truer portrayal of the relict gravel of the study area at approximate latitude 42 ° 52'S distribution than Fig. 10 because shells, which include a (Cullen & Gibb 1966). The zone extends south around modern as well as a relict population, are excluded Banks Peninsula as a very narrow intermittent band from the calculation. The 0.1 % contour is used to 6 km off-shore which then trends west into Canterbury detect the slightest presence of gravel in the surface Bight. Mud less than 10 cm thick, overlying muddy sediment as this may indicate the presence of shallow sand, was sampled off the Rakaia River mouth. underlying gravel beds and help to give a more Beyond the edge of the continental shelf, the mud complete picture of the former extent of gravel deposits content of the sediment increases and most of the slope on the shelf. The gravel south of 44° 30'S off the is covered with sandy mud and mud. The sediments Waitaki River is not included here as its presence is become more calcareous with increasing distance from only known from notations on the N.Z. Hydrographic the shelf. Sandy mud and mud are common at the Chart (Hydrographic Branch, Royal N.Z. Navy) which bottom of the submarine canyons off Pegasus Bay. The are not quantitative. Lithic gravel is distributed right mud distribution is shown in Fig. 11. across the muddy shelf at the northern end of the study
28
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ °s °s 0 44 44 43 ° 5 t- I- � I- . ·• , 17 115• .. - ··· " . ... ::� .. I ��t-- 'x. I , J·• t _, ,, "'� I ------50 I\ r'I ____ r'K I'\ \ r I \--- V ', � ,_ I I RES It ' � I I KILOMET 0 10 20 30 40 I O ° I• I 174 174 I I I t I E ° ° I T �::i�=��trrrrrli"\S��ll'l���1rr1,=wntrn1111111t=====�i 173 173 __ I I .l.. 1f! • • •
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ ° 43 ° 5 17 50 40 • • • • • 20 30 shelf 10 KILOMETRES 0 ° 0 4 continental • • • • 174 17 • • '. • • • Canterbury • 1 I the I on . • • weight) �f,:;;,r- l� • • �A • • .,;:::-�� • • • • I • • sediment E I 0 • • 3 ° 3 73 total • 17 1 • • of • • • . • • • a function • (as _.,.., , / . distribution ion ,---' . . gravel ° • ° ibut • 172 172 Shell str bonate 13. edge Di car FIG. l • of shelf tion 1% tal ave 8% 2% 4% sta 16% Gr inen tours Sample Occurrence Cont , 0 , Con Shell 171 \;.) N . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ °s ° 44 43 ° ° 5 5 f 17 17 I I 40 so • 30 L 20 I 10 KILOMETRES 0 � , , � : . ., , . , :, , .. : ° , , ° : ,. , 4 :, : , : , : J 17 174 , , .,.,. . ,.,., L.,. , ,,. shelf ... J ... ,, .,.,. ,., : :, : , ! ',. continenlal 'y:=- Canterbury 7\� the E 7JJ.. ° ° 173 173 modes on sand of I I I I Distribution I 14. F1c.. ° ° 172 172 present station m Il[ m and ril edge • Somple Mode Shelf Mode II Mode Mode I I I I I ,- • ... 0 -- ESSS:sJ [22] PR®N • I I ° ° I 171 171 ° 43 44 vJ vJ . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ break, on the whole upper continental slope, and in Diameters of - 3 (8 mm) to - 5 (32 mm) Pegasus Canyon. Mode IV is usually associated with predominate. All the gravels are rounded to well greater or less amounts of mud, and, except in zones of rounded (as defined by Powers 1953) and tend to be mixing with Mode Ill, it is generally free of lithic dominantly spherical to rod-shaped. The rare pebbles gravel. High shell concentrations occur with this mode larger than - 5 (32 mm) in diameter are frequently at a few places. disc-shaped but their rounding is good. Two suites of rock contribute to over 90% of the gravel in the samples - greywacke and quartz (Tables SOME SURFACE FEATURES OF THE 1, 2; see Fig. 9 for sample locations). Pebbles SEDIMENT designated as greywacke in the table include Side-scan sonar and underwater camera observations unmetamorphosed sandstones (generally of greywacke were made of in situ sediments in the zone of strong type) and argillite. The quartz pebbles are composed of currents off Banks Peninsula. massive quartzite, foliated micaceous and chloritic It was found that the steeply sloping band of Zone A, quartzite, vein quartz, and vein or lenticle quartz with Mode IV silty sand that hugs the coast of Banks adhering schistose wall rock. The quartz pebbles are Peninsula is featureless on the sonographs, being of usually iron-stained to a bright orange. uniform texture and having no bedforms high enough Schist occurs as a rare but persistent rock type in the to cast a sonic shadow at the scale used. Fig. 15a shows gravels. It includes schist, phyllite and foliated gravel or rock outcropping locally through the sandstone or semischist, usually of greywacke origin, featureless sand off Steep Head, Banks Peninsula. In showing lenticular streaks or deformed grains. Other this zone the water is mostly too turbid for rocks present in the gravels are rare and occur photography. sporadically. They include chert, felsite, intermediate In the zone of second-order ridges off Banks and basic volcanic tuff, porphyry and scoria, Peninsula (Zone D, Mode III sand, gravel and shell) orthoquartzite, jasper, agate and indeterminate rocks. sand ribbons are conspicuous and abundant (Fig. 15b) When the relative proportions of greywacke (cf. Kenyon 1970; Belderson et al. 1972). Indistinct (including argillite) and quartz in the - 3 to - 2 sand waves with crest-to-trough amplitudes of 0.3- (4-8 mm) fraction of each sample are plotted on a map, 0.4 m and wavelengths of 20-30 m were locally a clear-cut pattern emerges (Fig. 16). The size fraction observed, especially on the peripheries of sand ribbons. chosen is the largest one that provides a sufficient In addition there are some gradual textural changes, number of pebbles to make a valid plot of the shown on the sonographs by a gradual lightening or distribution. darkening of the background, and some sharp Close to shore, the gravel is almost entirely interfingering contacts, the fingers being parallel to the greywacke, while on the middle and outer shelf the trend of the sand ribbons (Fig. 15c). The textural gravel is a mixture of greywacke and quartz. At mid distinctions of the side-scan sonographs probably shelf, off Banks Peninsula, there is an elongate zone of represent fine sand (Mode III) moving across shelly gravel that is particularly rich in quartz. sandy gravel. Only 15 samples contained enough gravel larger than The sand ribbons, which are current-parallel -3 (8 mm) to be useful (Table 2). The ratio of structures, trend roughly parallel to the strike of the greywacke to quartz in them is generally higher than in second-order ridges, i.e., north-east to north-north the smaller sizes but the distribution pattern is east. The axes of the sand waves, which are current otherwise unaffected. normal structures, trend at high angles to the long axes The rare pebbles composed of rocks other than of the ribbons and ridges. Sand ripples in bottom greywacke and quartz consist of rock types that outcrop photographs (Stn H423) have east-west long axes and on the eastern side of the South Island but do not undetermined symmetry. With the data to hand, no provide any useful data on source areas. preferred location of the ribbons or sand-waves with However, different source areas can be determined respect to the ridges ( on crests, on flanks or in troughs) for the greywacke and quartz. The gravel will have was found, but it is clear that currents on the bottom act come from the eastern side of the South Island opposite in a general northerly or southerly direction. the study area, and also because of the general north eastward drift, from some distance to the south; that is from about latitude 43° S to the southern end of the ORIGIN OF THE SEDIMENTS South Island. Provenance of the Gravel The bedrock geology of the South Island is shown in ° The gravel on the outer continental shelf is almost Fig. 2. In Canterbury, south to latitude 45 S, it is exclusively of pebble and granule size, the size almost entirely greywacke; and in Otago, from latitude ° ° commonly referred to as "pea gravel". Clasts larger 45 S to latitude 46 S, it is almost entirely schist. The than - 4 (16 mm diameter) are uncommon and most boundary is gradational and it is generally accepted that have diameters less than - 3 (8 mm). The gravel of the schist is metamorphosed greywacke and argillite. inner Canterbury Bight however, tends to be large. Quartz occurs as veins in the greywacke but is 34
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ 100 0 50 150 I 150 100 50 metres 0 ons. y i l off shel faintly sandy, locat sand arc for and IV 6 sandy, g. i trend are (light) F Mode See sand silty ribbon III the fine, nsula. i to Mode Pen very The dark areas dark The ne i f by quely i Banks een obl gravel. w off ng bet i and acts shelf t trend sand fine sand. the con rock surrounded III aves c i ng of w are i Mode nger i volcan Sand areas Peninsula. ct . i of rel ght interf i l sonographs n i ke (dark) Banks i the scan outcrop or and de bbon-l gruvcl Head, . i i ribbons r S e y bl i s 15. i Sand gravel Sand v Steep shell Gravel (c) (b) FIG. (a) a b C . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I s ° ° 3 44 ° 4 ° 5 5 7 I 1 17 I 50 S two the eywacke r 30 40 of g ion 20 tions 90% r stat opo than r KILOMETRE p e r 0 10 mo elative ° ° r 4 Sample C o n t i n e n t a t n I e n t i n f C o I e h s e g d e I \ I I • I I ' , 17 I the ontains 174 \ I • I , 1 c I ' _,,/ I ,, 1 I to I I \,, I ' , .. I field I , , ' ' • , ' / ding • r , "'\, ke \ o ' c , .... cc • t ,6\ ' a ,> I I ._ , .. , ' ;� I I I I eywa , , . I ,/ r . I ', I g 't-«, 't-«, , .. -, .. \ , , 1' , ,, C, , , '... _, '"\�� f I The �, ,,,' .. ,,, �?'" , subdivided �-- ,,, � e ,,.- r tion. • a c ,;'' ,.,_
KILOMETRES 40 20 0 20 40 60 I 80 _J I I I I L I
- + 300
- + 200 3:: f'Tl - + 100 ,,,::0-, (/) PRESENT SEA LEVEL 0
- -100
VE X 100 - -200
FIG. 17. Gradient profile of the crest of the Rakaia fan surface (Canterbury Plains) and the adjacent continental shelf stripped of modem sediment. See Fig. 1 for location of profile. 37
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ volcanics. This is probably due in part to the embayed sedimentary rock fragments is plotted against distance coastline of the peninsula whch prevents river-borne from shore (Fig. 18) there is an apparent tendency for gravel and even sand from reaching the open coast; and the near-shore sands to be more lithic than those in part to the much greater resistance to wave attack of further off shore but this trend is not altogether the peninsula's vertical lava cliffs compared to that of independent of grain size. The finer Mode IV sands are the cliffs of unconsolidated Pleistocene alluvium along quartz-rich in both locations, with a slight tendency the adjacent Canterbury coast. towards seaward quartz enrichment; the coarser Mode III sands, however, show a strong trend to lithic grain emichment near shore. The trend of the Mode Provenance of the Sands III sands is similar to that already established for the The sand grains on the continental shelf in the gravels (Mode I), that is, the sedimentary rock present area range from very angular to well-rounded (as defined by Powers 1953). They are in general better fragments tend to be most abundant on the inner shelf rounded than those in the modern river beds and in the adjacent to their source. sea cliffs of Pleistocene river alluvium. The degree of Another minor compositional difference that has rounding tends to increase with grain size. Hence, some significance is an anomalous concentration of glauconite in the sample from Stn H458. This sample is Modes III and II sands are better rounded than the finer Mode IV sands. Modes II and III contain a large typical of relatively glauconite-rich sediments found in proportion of iron-stained sand grains which Mode IV the north-western corner of Pegasus Bay. The does not. Both these facts could suggest a separate glauconite is dark green and lustrous, closely provenance for each mode. However, Emery (1965) resembling detrital glauconite found in the nearby and Judd et al.(1969) have stated that a yellow surface Wai para River which drains an area containing Tertiary rind may be acquired during Pleistocene subaerial glauconitic rocks. It is assumed here to come from that weathering, and Swift & Boehmer (1972) have source. suggested that yellow colour can be simply a function of grain size in marine shelf sands, the depositional environment of the coarser sands tending to be more agitated and therefore more oxygenated than the 6:1 environment of deposition of the finer sands. That (/) larger sand grains are often better rounded than smaller I- eH784 ones is well known. LLJ 5=1 eH445 The mineral composition of grain size fractions :E z<.!) representing the different modes from various places <{ a:: eH331 on the shelf was determined petrographically. The u... 4:1 &H351 fraction were selected from samples containing only ::.::: u eH343 unimodal sand populations in order to avoid the 0 eH458 po -ibilit� of contamination from the tail of an adjacent a:: 3,1 eH436 population. As a rule, approximately 200 grains were a:: eH466 identified in each thin section but in several slides of <{ eH793 poor quality, only about 100 grains could be identified. I->- .l.H423 LLJ eH356 Potassium feldspar was distinguished by staining with :E sodium cobaltinitrite (Bailey & Stevens 1960). zC LLJ Z:I The results are presented in Table 3 (see Fig. 9 for (/) .l.H470 .l.H395 sample locations). There are some consistent differ ...... 1:1 ences between the mineral composition of one mode I- .l.H371 and another, and in the mineral composition from one N<{ ::> 1:2 .l.H782 place to another. The great majority of the lithic sand 0 grains are from sedimentary rocks (immature sand ....0 stones and mudstones) and most of the remainder are MODE ill .l. from low grade pelitic and psammitic metamorphic 1:3 MODE Tiz: e rocks. This is consistent with the two sources mentioned above - greywacke and schist, the grey ;1 wacke being the closer source. 1:4 Sedimentary rock fragments are relatively abundant 0 10 20 30 40 50 60 in Mode III and impoverished in Mode IV, while the reverse is true for quartz. This could reflect differences DISTANCE from SHORE (k m) in source but it is hazardous to attempt such a correlation because in sand-size siliciclastic sediments, FIG. 18. Ratio of quartz to sedimentary rock fragments in the the ratio of quartz to lithic fragments usually increases sand fraction as a function of modal grain-size and distance from with decreasing grain size. When the ratio of quartz to shore. 38
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ The modes of the off-shore sands are about the same younger being more abundant that the older), and by as the input grain size modes from the south and from the accepted depth range of the shells. The total fauna the west. The spectrum of sand grain-size modes and the accepted depth ranges of the important species reported by Andrews (1973) on the Otago continental are set out in Table 4 Fig. 9 for sample locations). shelf is very similar to that on the Canterbury shelf. His The depth ranges are those of Powell (1961), Morton & modal class II (1.3-2.4
39
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ STRATIGRAPHY
INSTRUMENTATION AND COVERAGE 2, 3, 4) and skirting Banks Peninsula (Profiles 5, 6, 8). Using an EDO 3.5 kHz profiler, a basic pattern of It thinly covers the sea bed of southern Canterbury lines was run more or less at right angles to the coast Bight (Profiles 10, 11). u L (Fig. 7). The transducer is hull-mounted and it was The middle unit (CB), which is further subdivided possible to run the profiler during other operations and into upper and lower parts (CB and CB respectively), while steaming between stations. This added greatly, if extends over most of the shelf. It is best shown in the not systematically, to the coverage of the area and southern part of Canterbury Bight (Profiles 10, 11) provided abundant intersecting tie lines for the basic where the top unit (P) is thin and muddy. It is relatively pattern (Fig. 4) . In addition to the main survey, a series thin near shore and thickens seaward with well defined of short lines were run in the vicinity of the submarine foreset beds that continue down the continental slope. canyons. All the original records are held at the N .Z. It is bounded above and below by zones of apparent Oceanographic Institute, Wellington. erosion, infilling and reworking which show fairly Because of its relatively high frequency of 3.5 kHz, clearly in the profiles. the profiler has a high resolution but a fairly limited The lower unit (X) shows up clearly at a few places penetration. A strong echo is returned from sand and only (Profiles 1, 9, 10, 11) and is not discussed in detail. gravel, and penetration is virtually nil where these are The middle and upper units are considered to be at the surface. Mud is more acoustically transparent: formations and are named and described accordingly: reflectors are thin and clear. Penetration of 60 m or the Canterbury Bight Formation (CB) and the Pegasus more is possible through mud. However, gas generated Bay Formation (P). The chosen upper and lower limits from decay of organic material in the mud can produce of the formations do not conform precisely to the a strong reflection and limit deeper penetration conventions generally used in terrestrial stratigraphy, (Schubel & Schiemer 1973, Keen & Piper 1976). since the basal transgressive sequence is included in the Reflections from gas occur at places in the muddy underlying formation rather than in the overlying one. sediments of Pegasus Bay (Profile 2, Fig. 19). Thus, It is felt that the mapability of the formations would be certain characteristics of the sediment may reduce the severely restricted if normal convention had to be penetration of the 3.5 kHz system, but this very fact followed, since the basal sequence, including transgres makes it possible to infer sediment types from the sive and palimpsest sediment, is derived from the profile. underlying formation and is texturally and composi tionally distinct from the overlying progradational sequence.
STRATII GRAPHIC NOMENCLATURE The Late Quaternary stratigraphy of the continental shelf related in this paper to diachronous shelf-wide PEGASUS BAY FORMATION unconformities created by shelf-wide glacio-eustatic Type and Reference Profiles regre sions and transgressions. These are related via Profile 3 (Fig. 20) (NZOI No. 1002-18) (in Pegasus the following terminology to the oxygen-isotope Bay) is designated as the type profile and Profile 11 paleoclimatic-paleomagnetic record of Shackleton & (Fig. 23) (NZOI No. 1034-22) (in southern Canterbury Opdyke (1973) to provide a basic chronostratigraphic Bight) is designated as a reference profile. Together framework. B.P. they show most of the diagnostic features of the B.P. formation as required by the American Commission on Deglacial: 6-18 ka Stratigraphic Nomenclature (1961). Last Glacial: 18-75 ka B.P. (Shackleton & Opdyke stages 2, 3 and 4) Last Interglacial: 75-128 ka B.P. (Shackleton & Upper and Lower Boundaries Opdyke stage 5) The upper surface is that of the sea floor (smooth Penultimate Glacial: 128-195 ka B.P. (Shackleton & zones A and E already described; Fig. 7). The base of Opdyke stage 6) the Pegasus Bay Formation is uneven, being the eroded Penultimate Inter 195-251 ka (Shackleton & and reworked top of the underlying Canterbury Bight glacial: Opdyke stage 7) Formation. The formation is thickest on the inner shelf and thins landward and seaward. The maximum Figures 19 to 23 are line drawings of the profiles thickness on a given profile in southern Canterbury located in Fig. 7 set out in order from north to south. Bight is only 2-7 m but the formation is much thicker in Three major units are apparent. northern Canterbury Bight and Pegasus Bay, reaching The top unit (P) is a discontinuous blanket of a maximum of > 28 m in south central Pegasus Bay. variable thickness and is best developed on the inner Where the formation is thickest, its base is ill-defined continental shelf, particularly in Pegasus Bay (Profiles on the profiles. 40
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Older Circled Pegasus Vertical - - within the Canterbury 0 33. P. - Fig. sediment. in reflector CBL the in Glacial unit: Pegasus Bay. Pegasus appearing gas The strong by Formation; northern rock. Penultimate caused Bight across 2 is X - paleoshorelines 2 and Tertiary the 1 of Profile Canterbury member); in profiles - Probable locations CB T - (lower the seismic Formation kHz units; indicate Bay metres. 3.5 Formation in Formation; 19. F10. Bay Pleistocene Bight numbers Pegasus scale . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I -- CB
C
- � CB
� I "-.....:- -� Kilometres 10 15 0 5 20 25 l�I 2 I 2 4 1', VE X 92. S
F10. 20. 3.5 kHz seismic profiles 3 and 4 across southern Pegasus Bay. P - Pegasus Bay Formation; CB - Canterbury Bight Formation; CBu- Canterbury Bight Formation (upper member); CB - Canterbury Bight Formation (lower member); X - Penultimate Glacial unit. �ircled numbers indicate the locations of the paleoshorelines appearing in Fig. 33. Vertical scale in metres.
43
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ 0 3 40
CB
80· C B L
- - 120 &,·�,',� \ 9� 160
40
80
C B L
120 X
160
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ 0 160 80 120 40 Q 80 4 lGO 120 \- I I Q � ? 5 - L s B C Bu Bu C C 5 92. X = - u - B C VE L - in u scale indicate B member}; Peninsula. C 25 L -=-- B Venical (upper numbers C Banks 33. of Fig. 20 Circled � in Formation east u B 6 Bight --, C outlined member). 15 ._,.. ____ � ._,.. and 2 1 5 2 (lower Canterbury __ ometres u- u profiles 10 B paleoshorelines CB Kil C n; Formation the --= seismic Bight with Formatio 5 kHz associated 3.5 Canterbury p - 21. p Pegasus Bay Pegasus 1- 0 P - P Fro. CBL metres. features . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I
To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ I 160 0 40 40 160 \ 9 �\\ / \�\ X \\ L B C �\ - / L B - C ��,� � u � B X ' C: .....___ ...... ___ � � - ...... -- -. p � /� L -� / B u --: C 40 80 .... B .5 C 92 �� �� X 8 "--\r- - - E ::__ u , L B \/ , Bu B C \ C � C �lS. � � ---- � - �- - - - ; "-� in 1 - � 25 unit. l Bight. � ------:::: r B member) outlined C Glacial (upper 20 Canterbury '8 Penultimate paleoshorelines northern 4 Formation X- ; the ------Bight across 21 with 9 u member) B L and lometres B C 8, 10 15 C --=r- Ki (lower Canterbury associated 7, p u- L B CB C metres. features profiles Formation � 5 in - - Bight - seismic scale Formation; Bay 1- 0 3.5 kHz Venical numbers indicate Canterbury . 22. - I This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. 33. Pegasus . . . -
To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ g I --=:::: --- -- FIG fi P CBi Orded p ? CBu?
X -
o 20 I��Kilometres ' I 1" - 25 VE X 9 2. 5
e 1 11 Canterbury Bight. F1G. 23. 3.5 kHz s ismi_c profiles d across souther!1 _ _ � �� ( er member p Pegasus Bay Formation; CBv a erbury Bight Form���� ); e l e e �fr l unit CB _Cant rbury .B"ght Formation ( ower. memb r); X-P m Glacia _ Cir�led numi:,ers �-cat e features associa ted with pa 1 eoshorelines outlined m Fig. 33. Vert1ca 1 s��l; in metres.
49
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ 0 10 40 p
80
/ C B L 120
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II
120
160
X
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Lithology and Structure Mud and/or very fine Mode IV sand, free of gravel, cm o- form the top of the formation Figs 11, 12, 14). Three cores (H435, H440, H459) in Pegasus Bay (Zone A) and one (H350) in outer(cf. Canterbury Bight (Zone E), extend into the top 3 m of the formation Fig. 9). Inner Pegasus Bay contains a muddy facies 10 - comprising heavily bioturbated sandy (Mode IV) mud(see and silty clay beds, 10-150 cm thick, alternating with thin beds 4-10 cm thick, of finely laminated coarse silt with few burrows (H440 and H459, Fig. 24). O1,1ter Canterbury Bight contains a facies of muddy fine Mode IV sand and sandy mud that are so strongly bioturbated 20 - that bedding is entirely destroyed (H350). The sandy facies of the Pegasus Bay Formation, which is generally limited to those areas where the water depth is less than 25 m, is mainly composed of clean or coarsely silty, very fine, hard-packed Mode IV sand that cannot be penetrated with a piston corer. The only core from this 30 - dominantly sandy facies (H435) was obtained on the edge of the formation in outer Pegasus Bay where the sand contains enough mud to render it penetrable by piston coring. The sediment is bioturbated, muddy, \·ery fine Mode IV sand. Shells are rare in all the cores 40 - from this formation. The muddy parts of the formation indicate low energy conditions in which the rate of deposition is usually low enough to allow an active infauna to effectively rework the accumulating sediment. Occa sional pulses of rapid sediment input are indicated by 50 - the laminated silt beds in cores H440 and H459 of inner Pegasus Bay. The sandy parts of the formation (core H435) indicate higher energy conditions. Except along the seaward margin of the thickest deposits, where it dips gently seawards, and overlying the top of the ridge-and-swale topography of the 60 - Canterbury Bight Formation, the bedding on the scismic profiles is effectively horizontal.
70- Anomalies In southern Canterbury Bight, as the Pegasus Bay Formation thins northwards, the ridge-and-swale �pography of the top of the underlying Canterbury Bight Formation gradually emerges, the Pegasus Bay F-0rmation being too thin (Profile 10, Fig. 23) to Flo. 24. Core H440 from the muddy facies of inner Pegasus Bay smooththe profile. The southernmost part of the ridge (Pegasus Bay Formation). and-swale Zone D (Fig. 7) therefore actually appears as Pegasus Bay Formation on the geological map ?- 58). In this area the northern limit of the formation is defined by the change in surface sediment type,(see attributed to low angle submarine sliding (Herzer & ;:wncularly by the sand modes (Fig. 14). Lewis 1979). On the inner shelf in northernmost Pegasus Bay Zone B), in the area where seismic profiles show Age of the Pegasus Bay Formation colded rocks directly below an eroded sea bed (Profile The distal part of the formation, now in relatively !. Fig. 19), the Pegasus Bay Formation thins to a deep water, probably began to accumulate shortly after eaeer and is not shown separately. the beginning of the deglacial transgression some The curious pinnacled terrain in the Pegasus Bay 18,000 years ago. However, because its greatest Formation shown at the outer end of Profile 1 is thickness is on the inner shelf, most of the formation 51
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ was probably deposited within the last 6,000 years, H813; Fig. 9). The cores consist of compacted, very after the sea had risen approximately to its present level fine Mode IV muddy sand and sandy mud with a high (Fig. 25). The Pegasus Bay Formation is considered shell content (Fig. 26; Table 5). here to be entirely marine. The upper continental slope is considered here to be the progradational front of the lower member without a significant cover of younger sediment. Two 4 m cores (H347 and H349) were taken; they consist of CANTERBURY BIGHT FORMATION structureless mud with occasional silt bands and Type and Reference Profiles colonies of calcareous worm tubes. Rapid deposition in Profile 10 (Fig. 23) (NZOI No. 1012-13) is quiet water is inferred from the dearth of micro- and designated as the type profile of the Canterbury Bight macro-fauna and from the very fine grain size. The top Formation and Profiles 6 (Fig. 21) (NZOI No. 1012-19) 20-40 cm is rich in foraminifera and sandier (Mode and 7 (Fig. 22) (NZOI No. 1012-05, 06) are designated indicating a reduction in the rate of sedimentation as reference profiles. and a probable concomittant increase in reworking. ThisIV), top layer may reflect the change to the Holocene sedimentary regime where the primary sediment source Upper and Lower Boundaries is far removed from the shelf edge. If so, it represents The Canterbury Bight Formation underlies most of the Pegasus Bay Formation, but this would need to be the continental shelf in the study area. Its base is the confirmed by further work. top of the eroded and reworked upper part of the Very fine Mode IV sand and mud are the typical unnamed lower formation (X) which includes buried sediments of the shelf edge terrace and upper river channels, second-order ridges and truncated continental slope and are regarded as the typical foresets (Fig. 23). On the continental slope, the sediments of the lower progradational member. boundary between the Canterbury Bight Formation and the underlying formation is evidently a paracon formity or a low amplitude disconformity that can only be identified with certainty by tracing the reflector to its Upper Member: Lithology and Structure distinctive equivalent on the continental shelf. The upper member is thin but well defined by The top of the Canterbury Bight Formation shows in bathymetry and sediment type. Zones C and D (Fig. 7) profiles as a diachronous sequence of truncated are its surface expression, and Mode II and III sand foresets, buried river channels and ridge-and-swale (Fig. 14) and Mode I gravel (Fig. 12) are exclusively its topography. About half of it is exposed on the present sediments. The upper member records the formation of sea bed where it is represented by Zones C, D, and F, a coastal plain and the subsequent reworking of the and about half is unconformably overlain by the surface of the plain by the sea, and consists of two Pegasus Bay Formation. facies - a fluvial facies and a marine facies. The Canterbury Bight Formation is about 35 m thick on the outer continental shelf and thins to less than 10 m near the shore. On the continental slope it thins The fluvial facies is defined by small, steep-sided, seaward to 7 m, 50 km from the shelf edge. The cut-and-fillFluvial Facies featur es on the seismic profiles, interpreted formation may be divided into a lower and an upper here as buried river channels on the inner shelf (Fig. member. 23). At most places it is covered by the Pegasus Bay Formation and is too deeply buried to be sampled. Lower Member: Lithology and Structure However, it virtually reaches the sea floor in the The lower member, which makes up the bulk of the shallow water of inner Canterbury Bight, where the formation, is progradational and youngs seaward. On arcuate shapes of the 25 m and 50 m bathymetric the inner shelf it is horizontally bedded but, as it contours most likely represent the eroded seaward thickens seaward, dipping foreset beds appear (e.g., extensions of the Canterbury fan surfaces (Fig. 1). The Fig. 23). A low rate of sedimentation during a high sea sea bed is covered by coarse, rounded, greywacke level was thus followed by rapid sedimentation during a argillite gravels that would have been eroded from the falling sea level. alluvial fans. On the continental shelf most of the lower member is buried beneath the upper member. The lower member does, however, appear to be exposed on the edge of the The marine facies of the upper member appears in continental shelf off northern Cantebury Bight and on thMarinee se ismic Facies profiles and in the bathymetry as the margin of Pegasus Canyon where locally the shelf widespread second-order ridges and as the first-order edge terrace (Zone F, Fig. 7) has cut deeply into it ridges and troughs that are prominent east of Banks (Profiles 1, 3, 6, 9, Figs. 19-22). Two places where the Peninsula. Three different types of morphology 3 .5 kHz profiler showed truncated foresets with no corresponding to three different sediment subfacies apparent overlying sediment were cored (H781 and within the marine facies were cored to a depth of 3 m: 52
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ CONTINENTAL CONTINENTAL ...SHELF SLOPE PR€SEN T HIGH SEA LEVE:L --::::-=-=-=-=-=-:::: PEGASUS BAY FORMATION -=-=:::{ _-_-_-_-_-_ -:_-_-_-_-_-_-_-_ ----:._:-_-_-_-_-_-_-_--- LAST OEGLACIAL TRANSGRESSION
LAST GLACIAL LE:vELS ��� � b � ���� �����(UPPER MEMBER) ;������������� ooouoooo1 e�ooo000000GOOOOOCOOOOOOOOOOOOOOOOOOOO� 000000000000000000000000000000000?0000000000000000¥ �ooo�ococo�ooooooooooooooocoooooocoooooooooonooooooooo �COOOOO�OC'CC'OCOOOOOOOOOOOO�OOOOOOOOOOCOOOOOOOO � - ������������������������:����eoc----ococ�_�------_:_-_-_-_-=-=-=-=-=--=-----=-=-�=-=-=-;;-;--_ - oeoeceQoooooo�oooon � 0 (1 000000
LAST INTERGLACIAi.' SEA LEVEL �=����������·,���. �c����::�� PENULTIMATE DEGLACIAL o o" o .., o o o o o o o o o o o o o o o o o o o o o <- o o o c on o ��oif ] �·�'-"""-""•-"'-' TRANSGRESSION �-ll·1·1·1·l·r·r·IT·I·�·1·�·�·�·r�··1·1·1·�·rrr�n�����������������_ ����s��._������..L.J. - ..1
PENUL TIMATE SEA LEVELS
PENULTIMATE GLACIAL REGRESSION
<' 0 0 c-c · �; � � � LITTORAL AND FLUVIAL SAND AND GRAVEL
� MARINE MUD AND SAND
[I]] LOW OR NEGLIGIBLE SEDIMENTATION
� UNCONFORMITY
FIG. 25. Chronostratigraphic diagram of the late Pleistocene formations proposed for the Canterbury continental shelf.
(i) First- and second-order ridges. H777 (Fig. 27), H789 and H810 the structureless sand (ii) The long troughs adjacent to first-order ridges. horizon is underlain by dense, cross-bedded, shell beds. (iii) The inner edge of the shelf edge terrace. The top layers of sandy cores tend to be disturbed during the horizontal extraction of the liner from the Sand modes in the cores were estimated visually as barrel aboard ship, and the grading of the top unit may described earlier. It is assumed here that the minor be an artefact. The top sandy unit otherwise resembles mode (II) sometimes occurs with Mode III as it does in the "traction zone" of Powers & Kinsman (1953) and it the surface samples although it cannot be distinguished probably represents sections through modern sand visually. bodies that are moving across the shell beds of an "accumulation zone". (i) First- and second-order ridges In core H788 (Fig. 28) there are four cyclic graded The cores taken in this fades are H353, H403, H777, beds, each 45-75 cm thick. Each bed comprises a H788, H789, H790, H810, and H811 (see Figs. 27, 28; subhorizontally bedded sand with fine shell fragments, Fig. 9 for location). grading downwards to a coarsely crossbedded sedimen In most cases, the top 150-200cm is composed of tary breccia consisting of oblong clasts of soft sticky apparently normally graded, otherwise structureless, clay, bivalve shells (with their concave sides upwards) fine sand (Mode III) with small amounts of finely and sand. The structures and the inclusion of clay clasts fragmented shell material. In some (H353 and H403) indicate that these beds were formed during periods of shells and/or pebbles are found at the top. In cores very high energy that alternated with periods of
53
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ o- 80 - between each successive bed. The commonest shells in cm the core are and both of which have a wide depth range (subtidal to middle she lf).Scalpomactra The radio carbscalpeon llumages of theTa shelweral lspissa,ayers are shown in Table 6. The ages of the shells in the upper three beds increase downwards and are 10 - 90 - separated by 700--1300 years.H owever, the date for the lowest layer is younger than that for the next overlying one, implying that older and younger shell l ayers have sometimes been mixed during reworking, and that the cyclicity of the events may have beenas frequent as 400 years (since the interval between the highest and lowest 20 - 100 - beds is about 1200 years). By c omparing the present depth of the shells be low sea level (59-61 m) to the sea level in the region a t the time they were living ( - 13 to - 26 m fr om 7340 to 9370 years B.P.; Gibb 1979) and assuming no t ectonic or isostatic movement at t he site, 30 - 110 - it may be inferred that they were living on the inner shelf in a water depth of approximately 35-46 m when the beds were deposited. The style of bedding and the age span of the cored interval imp lies that preserved here are infrequent catastrophic even ts, rather than a gradual accumulation. The extreme flatness of the shelf 40 - 120 - and the coarseness of the sediments preclude a rational explanation for emplacement of the graded beds by gravity f low mechanisms. Instead it would seem more reasonable t o attribute the formation of these beds to phenomenal storms that occur only once in 400 years or more. Sediments that were laid down during more 50 - 130 - normal conditions here have evidently been totally destroyed and transformed into these storm deposits.
(ii) Troughs associated with first-order ridges The troughs eas t o f Banks Peninsula (Fig. 5 and 60 - 140 - Profiles 5, 6, Fig. 21) contain a generally muddy facies (cores H405, H778, H780, Fig. 29; Fig. 9 for location). The top 20-80 cm of these cores consist of a normally graded mud and M ode III sand unit similar to that at the t op of the co res in the first- and second-order ridge subfacies. The top unit is thought here either to have 70- 150 - been produced during handling or to represent the modern "traction z one". Below the top unit, two o f the three cores contain alternating muddy and sandy layers, but one is entirely muddy to the bottom. The mud in these cores is 80 - 160 - moderately bioturbated, burrows being either sand or mud filled. Horizontal lamination is frequen tly preserved. The sand, with few exceptions, is Mode III throughout. The muddiness of the cores indicates a l ow energy H781 environment which might, in theory, be due simply to F10. 26. Core Crom the shelf edge terrace (lower member of the Canterbury Bight Formation). the present-day protective action of the bathymetry. If so, these muddy sediments would represent a m odern infilling. However, the alternating sand layers obvi relatively lo w energy leading to the deposition of thin ously reflect o ccasional input from an adjacent higher mud layers that were later broken up. energy facies. The shell bed in H405 is dominated by The abundance of shells a t t he base of each graded black, sulphide-stained bed made it possible to use radiocarbon dating to and (Table 5), in a estimate the magnitude of the time interval represented matrix of co arse sand andPaphies grave l australe,suggesting P. thasubtrit t he angulatum Chione stutchburyi 54
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ cm 80 - mud-sand alternations could well be part of a lagoonal facies. The origin of the troughs is discussed later. (iii) Inner edge of shelf edge terrace Except where it is mantled by Pegasus Bay Formation, the inner edge of the shelf edge terrace is the zone of change between the upper and lower 10 - 90 members of the Canterbury Bight Formation(cf. on the surface of the outer shelf (Fig. 7). The upper member has a relatively high concentration of gravel Figs. 7 and 12) in Mode III sand. The lower, as mentioned above, is quite deeply eroded by the terrace and is 20 - 100 - mostly muddy Mode IV sand. The single core taken in the terrace edge facies (H812, Fig. 30; Fig. 9 for location) penetrated 2 m into the upper member. Its top 26 cm is composed of the graded mud to sand sequence that is found in other cores. The remainder is dominated by pea-gravel in a 30 - 110 - sandy (Mode III) or muddy matrix, the muddy matrix being confined to the upppr 86 cm. The gravel contains numerous intertidal and shallow-water species (Table 5). It is thus thought here to include reworked shore deposits. The muddy matrix of the gravel in the top half was most likely emplaced by mixing of Holocene mud 40 - 120- with the relict Pleistocene gravel through the activities of burrowing infauna.
Age of the Canterbury Bight Formation The Canterbury Bight Formation is bounded above 50 - 130 - and below by unconformities apparently representing shelf-wide transgression surfaces, and is strongly diachronous. The lower member of the formation has been interpreted here as a marine neritic regressive sequence; the upper as a transgressed and reworked coastal plain sequence. Sedimentation of the neritic 60 - 140 - lower member upon the lower unconformity would have begun with the deglacial transgression at the close of the Penultimate Glacial, with most of the deposition occurring from the Last Interglacial high stand of sea level through the Last Glacial regression until the shelf was again exposed by the low sea levels of the Last 70 - 150 - Glacial (Fig. 25). Deposition would then have been confined largely to the continental slope. The upper member has been well dated by radiocarbon (Table 6). Exclusively shallow-water shells, selected from cores H777, H790 and H810, range in age from 11,750 to 27,900 years B.P., while 80 - 160 - shells which can live in deeper water, selected from cores H788 and H789 , range in age from 6,400 to 9,400 years B.P. The upper member is therefore considered here· to have been deposited during the Last Glacial F10. 27. Core Hm from the ridge facies of the upper member sea-level regression and sea-level low, and to have been of the Canterbury Bight Formation, showing the sandy reworked by the sea during the last deglacial "traction zone", interpretedas part of a palimpsest sand body,· transgression (Fig. 25). At places, the reworking may overlying the shelly "accumulation zone··. still be continuing.
55
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ cm o- 110 - 220 -
10 - 120 - 230 -
20 - 130 - 240 -
30 - 140 - 250 -
40 - 150 - 260 -
50 - 160 - 270 -
60 - 170 - 280 -
70 - 180 - 290-
80 - 190 - 300 -
90 - 200 - 310 -
100 - 210 -
FIG. 28. Core H788 from the ridge facies of the upper member of the Canterbury Bight Formation, showing cyclic, graded bedsthat are interpreted as storm deposits. 56
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ cm o- cm 100 - 110 -
10 - 10 - 110 -. 120 -
20 - 20 - 120 - 130 -
30 - 30 - 140 - 130 -
40 - 150 - 40 - 140 -
50 -
160 - 50 - 150 -
60 - 170 - 60 - 160 -
70 - 180 - 70 - 170 - 80 - 190 - 80 - 180 - 90 - 200 - 90 - 190 - 100 - 210 -
100 - 200 -
220 -
210 -
FIG. 29. Core H405 from the trough facies of the upper member FIG. 30. Core H812 from the inner edge of the shelf edge terrace of the Canterbury Bight Formation, showing alternating layers of (uppermember of the Canterbury Bight Formation), containing a mud and coarse to medium sand, gravel and intertidal shells. dense deposit of pebble gravel.
57
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ S ° ° 4 3 4 ° ° I- I 4 175 175 I 50 S I 40 RE edge I 20 30 Shelf KILOMET ,. 0 10 I I / 6 __ 0 5 3 4 4 ° 4 I 17 17 metres. 7 in I I contours , �� r I. I I I I shelf. Bathymetric !?!;� - Frll - ? ° 0 3 17 1 73 E continental � , I� >2,;+rfl:MT} ' Canterbury 'Vi>0 the ' of N\. - N\. map I � J.,1 ) ) I ' Formation ° units I I I Geological Formation 2 I t 31. 17 member member Formation A FIG. Bigh ) I Bight ) �\ rock Bay ided Lower Pleistocene I upper ( ( undiv ( I undivided ( Pegasus Tertiary Canterbury Canterbury Older '-' ' D � III] � � I ° 0 1 I 17 171 ° 3 4 ...] VI 00 . This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. I To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ DISCUSSION
GE OLOGIC MAP OF THE CANTERBURY SHELF It is necessary to consider the possibility that Figure 31 is a geological map of the outcrop pattern winnowing of the very fine Mode IV sand from the fine of the formations discussed above. Although the Mode III sand, which dominates the transgressive 3.5 kHz seismic profiles provide the basis for sand/gravel sheets on the shelf, has been taking place establishing the formations, the lateral formation on the continental shelf for the last 6,000years since the boundaries can be more precisely defined in some areas return to present high sea level. However, a number of by the boundaries between the different grain size obstacles present themselves: modes, and in others by the echogram morphology and (a) This would require stronger currents on the middle detailed bathymetry. Thus a combination of methods shelf sea bed than on the inner shelf sea bed. This has been used to establish the boundaries as accurately may well be the case but remains to be proved. as possible. (b) In several places (in northern Pegasus Bay and off Banks Peninsula) relict Mode III sand is mixed with modern mud deposits but Mode IV sand is absent. SE GREGATION OF SIZE MODES Such a gap in the energy gradient indicates that The most obvious feature of the sediment distribu Mode IV sand was removed before the present tion pattern on the Canterbury continental shelf is the current system was established. virtually complete areal segregation of unimodal Mode (c) Mode IV sand is absent throughout the cores in the IV sand from the polymodal assemblage of coarser transgressive sand/gravel sheet, which contains sands and lithic gravel (Modes I-III, Figs 12, 14). It is buried shell beds 12,000-15,000 years old, domi generally agreed that the coarse relict sediment often nated by littoral species that are presumed to have found on a continental shelf was transported to its been deposited and reworked in very shallow water apparently anomalous location during glacially lowered and to be undergoing no reworking now. This sea levels and left there when the deglacial shoreline clearly indicates that Mode IV sand was removed moved landward. The coastal retreat model has been (probably in shallow water) before the present proposed by Bruun (1962) and Schwartz (1965) and current system was established. confirmed in field studies by Rosen (1978). During a It is therefore possible that the very fine Mode IV rise of sea level on a low unconsolidated coast, erosion sand was removed from the transgressive sand/gravel of the shore-face will tend to be balanced by an equal sheet as it was being laid down. A useful analogy to this volume accumulation on the adjacent sea floor. The argument is available in the present coastal and near shore-normal transfer that this process requires creates shore sedimentary environment. the thin transgressive sheet of relict materials now The present coastline of Canterbury is, at different mantling the shelf. Input and output by longshore drift places, eroding, prograding, or stable. All three sand is assumed to be steady state, and river input is not modes (and gravel) are at present being supplied to the taken into account. A modification of this model put coast by rivers. Off prograding or stable portions of the forward by Dillon (1970) and Swift et al. (1971) coast (the coast of Pegasus Bay and Kaitorete Spit) the explains coastal retreat and sediment redistribution due sea bed is made up of very fine Mode IV sand (Fig. 14 ; to sea-level rise on a barrier island coastline. The see also Campbell 1974) and mud that is accumulating barrier island moves landward by a cyclic process as a thick modern deposit (the Pegasus Bay Formation) involving storm washover of the coarse barrier on the inner shelf. The bathymetric profile of the sediments into the lagoon, their burial beneath the deposit is in equilibrium with the present coastal migrating barrier, and their re-emergence at the hydraulic regime. The beaches consist of medium and retreating shore-face. fine (Modes II and III) sand and gravel (Mode I) ( cf. In both the models, fine as well as coarse sediment is Blake 1964, Campbell 1974). Such seaward fining of carried off shore during deglacial coastal retreat. sediment from beach to off shore has been well Neither model embodies a satisfactory explanation for documented by others (see, e.g., Keulegan 1948, Scott the total absence of the finer sand (Mode IV) from the 1954, Ippen & Eagleson 1955, Ingle 1966, Johnson & relict sand/gravel sheet on the Canterbury shelf. Since Eagleson 1966, Cook 1969, Swift 1969). The sorting all the sand modes (II through IV) that occur in the takes place within and just seaward of the surf zone, rivers of the present coastal plain must have occurred in gravel and coarser sand being thrown landward up onto the rivers of the Pleistocene coastal plain as well, the the beach while mud and finer sands are carried complete separation of Mode IV from the others is seaward. (Kirk unlikely to have been achieved by river processes. Off the eroding coast - the coast of central Instead, it is much more likely to have been Canterbury Bight 1967, 1969) - the sea bed is accomplished by marine processes. made up of gravel and Modes II and III sand (Figs. 10,
59
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ 14). Very fine Mode IV sand is apparently absent, Sanders 1962, Deitz 1963, Emery 1966, Garrison & although it may be present very close to shore in water McMaster 1966, Hyne & Goodell 1967, Uchupi too shallow to sample. Earlier in this memoir (p. 11) it 1970, Kraft 1971). was mentioned that the surface of the inner continental (ii) a drowned fluvial surface (Garrison & McMaster shelf off central and southern Canterbury Bight is 1966, McKinney & Friedman 1970, McMaster & steeper than to seaward and has the form of two very Ashraf 1973) broad fans. The fans are thought here to be the eroded (iii) near-shore shoals, abandoned during deglacial and submerged extensions of the Rakaia and Rangitata coastal retreat, and subsequently modified by the gravel outwash fans shown in Fig. 1. The drowned fans inner shelf hydraulic regime (Moody 1964; Swift are well defined between the 50 m and 25 m contours, 1972a, 1972b, 1973), and indicating that the drowning was initially rapid and that (iv) modern submarine ridges formed after theet relatively little erosion took place. The later stage of al. coastal retreat has involved a large amount of cliffing. deglacial transgression (Duane 1972, Schlee It is assumed here from the vestigial fluvial & Pratt 1970). geomorphology and the absence of a modern fine Recent studies indicate that the etsediments al. on the sediment cover that the inner shelf in this area has not ridges and swales of the inner shelf are active at least in reached a profile of equilibrium with the hydraulic some areas (Duane 1972, McKinney 1974, regime, that high energy conditions prevail, and that Stubblefield 1975). Wind and wave-generated the sea bed is thus continually swept clean of fine currents caused by intenseet al. winter storms oftenet al. disturb sediment. The transport path of bedload sediment on the entire inneret al. shelf water column and are a very the inner shelf in this area is northwards and important cause of sediment movement. The currents shorewards (Carter & Herzer 1979). Very fine Mode progress parallel to the ridge axes with a helical flow IV sand introduced by the local rivers is therefore pattern that scours out the troughs and carries sand transported away towards the north-east in a near obliquely up the flanks of the ridges, thus building them shore belt. up. During weaker storms, sand is swept back from the During the rapid sea level transgression of the last crest towards the flanks. Rotary tidal currents are an deglaciation the profile of the sea bed off the whole equally effective cause for the present activity of some coast would have been continually out of equilibrium ridges (Stewart & Jordan 1965, Smith 1969). and conditions would have been unfavourable for The North Sea-English Channel-Celtic Sea region is deposition of Mode IV sand. Any Mode IV sand in the tide dominated, although meteorologic effects are still area would thus have been removed from the important e.g., Carruthers 1963; Stride 1963, transgressive sand/gravel sheet as the latter was laid 1973; Robinson 1966; Allen 1968b; Houbolt 1968; down, and would have been transported northwards Kenyon & Stride(see, 1970; Belderson 1971; Bouysse out of the area. 1976). The proffered mechanism for ridge construction in this case is also oneet al. of helical flow parallelet al. to the long axes of the ridges with scouring of RELATIVE AGES OF THE FIRST- AND the troughs and sediment transport towards the ridge SECOND-ORDER RIDGES crests e.g., Houbolt 1968, Caston & Stride 1970, Because of the complexity of the geomorphology and Caston 1972). Surface current velocities reaching 3 sediment distribution on the exposed top surface of the knots (154(see, cm/sec) seem to be required (Stride 1973). Canterbury Bight Formation, it is thought that some The constructional current is mainly tidal; degrada features are modern and some are old. tional currents are supplied by waves which winnow the sediments from the ridge crests. Ridges in shallow Theories of Origin of Ridge-and-Swale Topography water (30-40 m) are apparently active today. Ridges in The origin of the ridge-and-swale topography on the deeper water (140-170 m) of the Celtic Sea are Canterbury continental shelf is not fully understood, considered to have been most active during lower sea but much can be inferred from better known examples levels (Bouysse 1976). in other parts of the world. The best studied examples of ridge-and-swale topography are on the Atlantic Second-order Ridgeset al. on the Canterbury Shelf continental shelf of the United States of America and in the North Sea - English Channel - Celtic Sea area of The second-order ridges on the constricted shelf off Europe. The former are on a long, linear continental RidgesBanks onPeninsula the Constricted are numerous Shelf off andBanks high. Peninsula Ridge shelf that directly faces the open ocean, whereas the parallel sand ribbons and oblique-trending sand waves latter are in two semi-enclosed bodies of water and the occur with the ridges (Fig. 15b and c; Fig. 6 for channel joining them. location) and, like the ridges, they are thought here to Different parts of the ridge-and-swale topography on be related to a three-dimensional, longitudinalsee flow the Atlantic shelf of the U.S.A. have been explained pattern, where pairs of counter rotating, helical as: vortices maintain parallel strips of sand accumulation (i) Pleistocene beach ridges and barriers overstepped alternating with strips swept clear of mobile sands by the transgressing sea (Veatch & Smith 1939, (Allen 1968a, 1968b; Houbolt 1968). Ridges are
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This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ thought to form where the currents flow at > 3 knots present, active ridges apparently being confined only to (> 154 cm/sec) (Stride 1973), and sand ribbons where the middle shelf in the region of strong currents around currents have maximum near-surface velocities of > 2 Banks Peninsula. Instead, modern sediments have knots (> 103 cm/sec) (Kenyon 1970). Unidirectional accumulatd to a substantial depth along much of the surface currents higher than 2 knots and possibly as shore and have thinly blanketed many of the ridges high as 3 knots have been detected in the region of further off shore. It would thus appear that ridges are a constricted flow off Banks Peninsula (Herzer 1977, widespread feature of transgressing seas, and that p. 17). The contoured bathymetry (Fig. 6) shows that except for local occurrences where currents remain the long axes of the ridges trend north to north-east, strong, they are superseded during periods of stable sea parallel to the direction of present current flow (Carter level by progradation of a new prism of neritic elastic & Herzer 1979). Many ridges are visibly imbricated, sediments. the direction of imbrication indicating that they have migrated seawards (Profile 8, Fig. 22), this in turn First-order Ridges on the Canterbury Shelf suggesting that they are moved by the present First-order ridges occur in the area north-east of northward-going bottom current which has a frequent Banks Peninsula where the second-order ridges are wind-driven off-shore component (Carter & Herzer largely absent. The two very long, low, broad, coast 1979). It is thus probable that the high second-order parallel ridges, each backed by a trough on the ridges south-east of Banks Peninsula are maintained landward side (Figs 5, 7) do not resemble any common today by the accelerated currents in the constriction submarine bedforms. The crests of both of these ridges caused by the peninsula (Carter & Herzer 1979). were cored (H777 and H810) and in both cases dense, Belderson (1964) and Stride (1973) have shown that cross-bedded shell beds were penetrated (p. 53), the where tidal currents flow at less than 1 knot (51 cm/sec) fauna consisting exclusively of species that can be found the depositional area for sand is flat, or in very low in shallow water and dominantly of species restricted to mounds at bedload convergences. The gradual beaches and lagoons (Table 5). northward disappearance of second-order ridges east of There is no possibility that these first-order ridges Banks Peninsula probably reflects the slackening of represent tectonic features. Marine seismic reflection currents beyond the constriction. Accompanying the profiles show that there are no growing folds or faults northward change in topography is a northward change under the ridges, in fact, rocks younger than lower from gravel-rich to gravel-poor sands on the sea bed Miocene are practically undeformed (D ean & Hill (Figs 7, 12), which supports the inferred decline in 1976), and the region has subsided only about 1000- current speed. 1600 m since that time (a mean rate of 0.05- Although the ridges on the constricted shelf south 0.08 m/ka). The Banks Peninsula - Mernoo Saddle east of Banks Peninsula are thought to be active today, region is therefore, relatively speaking, very stable. they may well have originated in shallower water These first-order ridges, with their associated during the deglacial sea level rise. troughs, must therefore be of sedimentary origin. On . the basis of their sediments, geomorphology and associated fauna, they are interpreted here as being a Ridges on the Op en Shelf of Ca nterbury Bight pair of spit/lagoon complexes that have been In Canterbury Bight the outer belt of second-order transgressed and reworked by the sea. Relatively minor ridges in Zone D and the low, second-order ridges on subaerial features, such as beach ridges,· dunes, etc .. the shelf edge terrace (Zone F) occur far to the south would have been destroyed, and submarine features west of Banks Peninsula and the zone of constricted such as shoals, etc., modified during the transgression. circumpeninsular flow. Southwards the ridges become only the broad high of the barrier and the broad low of buried under a modern sandy mud deposit, indicating the lagoon being features large enough to withstand the that these ridge-swale systems, which probably once reworking. extended south of the study area, are now inactive, dating from a time of lower sea level when the local energy conditions were higher. It is thus possible that RE LATIVE PLEISTOCENE SE A LE VELS IN THE these ridges were formed as near-shore shoals by the STUDY AREA storm-current process described by Moody (1964) and The Banks Peninsula- Mernoo Saddle region, being Swift et al. (1972a, 1972b, 1973). effectively free of tectonic influence and surfaced by The inner band of second-order ridges in Canterbury plausible drowned littoral features with shallow-water Bight dies out about 74 km south-west of the peninsula faunal assemblages, is a good location in which to (Fig. 7) with no evidence from seismic profiles of any attempt to set up a Pleistocene sea level curve for the buried south-westerly extension. The available data is Canterbury shelf. The curve, derived and discussed insufficient to determine if these ridges are modern or below and presented in Fig. 32, spans the period relict. 12,000-28,000 years B.P.; for general comparison, the B.P. Ridges are restricted to depths between 50 m and curves of Curray (1965) from 0-20,000years and of 150 m in Zones D and F and do not occur on the inner Shackleton and Opdyke (1973) from 0-38,000years B.P. shelf. Near shore ridge generation is not taking place at are also included.
61
This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Two cores from the Banks Peninsula-Mernoo Saddle years old between 1.7 and 2.07 m below the surface. region, and one from about 35 km south of the The dated species in this third core has a rather wider peninsula, contained enough shallow-water shells to depth range of 0-20 m; the sea-level curve could pass allow dating of one or two selected very shallow-water through the depth at which these shells plot or up to species. 20 m above it. Here, it is arbitrarily drawn 10 m above Shells of Paphies australe and P. subtriangulatum, their plotted depth, midway through their depth range. obtained in one core from the crest of each first-order The lowest sea level on the new curve presented here ridge, have the following radiocarbon ages (Table 6): was measured from the nick point of the shelf-edge those on the outer ridge (H777) are 27,900 ± 1,550 terrace (the deepest detected shoreline onB.P. the shelf) years old between 1.22 and 1.67 m below the surface, and, in the absence of radiometric age control, is and 24,500 ± 1,000 years old between 0.8 and 1.22 m; arbitrarilyB.P., drawn at 18,000-20,000 years those on the inner ridge (H810) are 15,100± 200years On the portion of the curve younger than 20,000 old 0.95-1.42 m below the surface. The depth ranges of years the data points (H790and H810) agree well these two dated species are strictly intertidal (the with the published Late Pleistocene sea-level trend of maximum tidal range in New Zealand being 3 m), and Curray (1965) and less well with that of Shackleton and where these shells have been used the sea-level curve Opdyke (1973). However, some consistent disagree (dotted line, Fig. 32) is very precise. ment is expected since the curves were derived by Shells of Zethalia zelandica obtained in the core different methods, theC urray curve being derived from south of Banks Peninsula (H790) are 11,750 ± 250 radiocarbon-dated material on the North American THOUSAND YEARS BEFORE PRESENT 36 0 4 8 12 16 20 24 28 32 * 0 8 12 16 20 24 28 32 36 ..J 4 LI.I t > LI.I ..J 20
120 T.:f. NZ6S FIG. 32. Radiocarbon ages and depth below present sea level of dated species from shell layers under the Canterbury continental shelf. The magnitude of 14C dating error is shown for each sample by the length of the horizontal bar. The solid line is the mean sea level curve of Curray (1965) from O - 20 OOO yr B.P.; the vertically hatched zone shows part of the sea-level trend proposed by Shackleton and Opdyke (1973);5 the dotted line is the sea-level curve proposed here for the Canterbury shelf in the vicinity of Banks Peninsula. Circled numbers 1 - indicate sea-levels corresponding to shoreline positions shown in Figs 19-23 and 33. • age using 5730 U 14C half life (Godwin 1962) age us ing 5568 4C half life (Libby 1955) t 62 This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ continental shelf and the Shackleton and Opdyke curve ridge has been interpreted above as the remains of an from paleotemperatures and ice volumes inferred from old shore zone. It is inferred therefore that during the 1SO isotope ratios of foraminifera in deep-sea cores. The interstadial that gave rise to the shore dated here at trends are nevertheless all reasonably parallel. 24,500-27,900years B.P., the shoreline stood just east of On the portion of the curve older than 20,000 years the head of Pegasus Canyon (shoreline position 1, BP. the data points (H777a and H777b) support the Fig. 33-1). A large spit/lagoon complex formed, the evidence of Morner (1971) and Shackleton and Opdyke lagoon opening northwards into the canyon. Although ( 1973) that a cool climate and intermediate low sea the interstadial lagoon has subsequently been largely level prevailed prior to the Last Glacial maximum. infilled and locally buried, the original position of the However, the curve diverges significantly from that of lagoon and spit are recognisable on Profiles 5 and 6 Shackleton and Opdyke (1973); where their curve (Fig. 21) by deep cut-and-fill structures midway across records a continuously falling sea level, the new curve the shelf, flanked on the east by a broad low ridge. The presented here records a long stillstand from about positions are denoted by a circled 1 on the profiles. Off 19,000 to 25,000years B.P. Although it could be argued Canterbury Bight there is no evidence in the profiles that the positions of H777a and b to the left of the (Profiles 9, 10 and 11, Figs. 22, 23) for a barrier/lagoon Shackleton and Opdyke curve might be due to system. Instead, with a relatively stable sea level, a contamination of sample site H777 with young thick, prograding wedge of sediment, analagous to the hells when littoral conditions returned during the present-day one in northern Canterbury Bight, deglacial transgression, this would need to be Paphiesproved developed and the shore migrated very slowly by individually dating a number of single shells from seawards. the location to see if two populations of different age Straightforward progradation took place in Canter were in fact present; until this is done the ages of H777a bury Bight because this region was at the advancing and b must be accepted as valid and with them the foot of the Canterbury Plains with its copious direct occurrence of at least 4,000 years of stillstand during input of glacio-fluvial sediment. A spit/lagoon system that period. Good data on the positions of sea level formed east of present day Banks Peninsula because prior to 20,000yea rs B.P. are still rare and the validity of this latter region was then, from a fluvial aspect. in the this stillstand is strongly corroborated in this case by the depositional shadow of the mountains which now form interpreted paleogeomorphology of the outer first the peninsula. Furthermore, sediment from the order ridge as a major spit; well developed shoreline Waimakariri and Ashley Rivers would have been features of this magnitude would be expected to have longshore-drifted northwards, and therefore would formed during such a significant period of relative sea have contributed little to the area east of these level stability. mountains. Sediment input would have been almost The outer first-order ridge is proposed therefore as exclusively by longshore and shelf transport from the the shore of the last interstadial stillstand which south, and any long continued period of sea level preceded the main Last Glacial ice advance. The lower stability would have led to the development of a spit. formation (X) is then proposed to be of Penultimate The spit extended well north of the depositional Glacial age, and disconformities related to minor shadow of the mountains because Pegasus Canyon regressions and transgressions during the Last Glacial would have prevented the eastward progradation of the would thus be hidden in the Canterbury Bight northern part of the plains. Formation. As shown above (p. 59), coastal stability or The inner first-order ridge, interpreted on similar progradation leads to the accumulation off shore of geomorphological and fauna) grounds as also being a very fine Mode IV sand and mud and the accumulation spit, would have similarly required a stillstand of sea on shore of the coarser Mode II and III sands and level for its formation. Sample H810 with a dominantly gravel. The progradational portion of the Canterbury intertidal and shallow-water fauna dates this stillstand Bight Formation at this time would thus have been at around 15,000 years B.P. during the deglacial forming from Mode IV sand and mud while the surface transgression. An inflection has been put into the curve of the shoreline and spit would have been forming from at this point to show this. gravel and Mode II and III sands. Gravel and sand, carried northwards by littoral drift and currents on the inner shelf, would have poured into Pegasus Canyon HISTORY OF THE REGRESSIVE-TRANSGRESS through the small tributaries that enter it partway along IVE SURFACE its length. With the ancient and modern ridges distinguished, After the inferred interstadial, the sea level fell and a local relative sea-level curve established, a quickly Fig. 32). A continued high rate of chronological sequence of Late Pleistocene-Holocene terrigenous sediment input during this fall is implied by events on the shelf can be worked out. In the region of the persistence(see of a thick foreset sequence right to the Pegasus Canyon, the terrace truncates the end of the shelf edge (Profiles 9, 10, 11, Figs 22, 23). broad low eastern branch of the outer first-order ridge When the sea stabilised at its lowest level during the that p�sses' east of the canyon head ( cf. Figs 5, 7). The last glacial maximum 18,000-20,000 years ago, it cut a 63 This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ bench in the newly deposited unconsolidated sediments ridge would have been drastically reworked (Fig. 33- of the shelf edge while progradation continued on the 3). This event, marked by shoreline position 3, would upper continental slope. The landward margin of the have taken place, according to the curve, 17 ,OOOyears shelf edge terrace thus formed (Figs 5, 7) represents ago. shoreline position 2 (Fig. 33-2and cross-sections Figs. In Canterbury Bight, the sea bed appears to have 19-23). Along the south-east-facing portion of the been worked into low shoals probably by large waves terrace in the southern half of the study area, southerly and storm currents, generated mainly from the south storm conditions were favourable for the generation of (see p. 61). These shoals are now represented by the second-order ridges (Figs 7, 33 - 2, and Profiles 10 and present second-order ridges of outer Zone D in 11, Fig. 23). In the north, the terrace surface remained Canterbury Bight (Fig. 7; Profiles 9, 10, 11, Figs 22, smooth (Profile 3, Fig. 20). 23). Coarse sediments accumulating on beaches at this South-east of Banks Peninsula, reworking was time are now distributed in the large belt of gravel that probably so intense, because of the constricted and lies adjacent to the terrace (Fig. 12). Very fine Mode accelerated tidal and storm currents, that any pre IV sand and mud were carried both seawards on to the existing interstadial ridge was completely destroyed continental slope and northwards along the narrow and remoulded into high second-order ridges (Profiles shelf towards Pukaki and Okains Submarine Canyons. 7, 8, Fig. 22). Obliteration of the shelf edge terrace The coarse beach sediments would also have been probably continued, at least up to the time of shoreline transported northwards into the submarine canyons by 3. the dominant southerly swell. The loss is assumed to North-east of Banks Peninsula, where the off-shore have been balanced by the input of the longshore tidal and storm currents would have been reduced drifted gravel and sand from the south. relative to the area further south, reworking evidently With the onset of deglaciation, much of the terrace resulted merely in the formation of a transgressive was evidently drowned without much modification, gravel and sand sheet without shoal formation. As a suggesting that the sea level rise was initially fast. It is consequence, the bathymetry in this area has not been assumed here that the terrace once extended through diversified by second-order ridges (Figs 5, 7), and Mernoo Saddle but was destroyed during the earlier stages in the overstepping of the interstadial spit can part of the deglacial rise of sea level by constricted, thus be deciphered on the basis of the existing high-velocity currents and swell. The latter may have morphology. been focussed on to this part of the shelf by the convex The argument runs as follows. The outer first-order isobaths. ridge is forked near the head of Pegasus Canyon (see During the deglacial sea level rise, the coastal regime p. 14). The western fork is interpreted here as a post was one of erosion and retreat. As shown earlier glacial modification of the earlier-formed interstadial (p. 59), coastal retreat leads to the off-shore transfer spit which extended up the eastern side of the canyon. and accumulation of the coarse on-shore sand and When the shore of the rising sea reached the vicinity of gravel, and the removal of mud and very fine Mode IV the old spit (the present eastern limb of the outer first sand from the shelf system. Sediments eroded from the order ridge) there would have been a temporary return coastal plain, which probably consisted mainly of a to the last interstadial situation of a spit backed by an beach-ridge and dune terrain, were redistributed on the extensive lagoon (Fig. 33- 1). The lagoon opened continental shelf. Gravel and Modes II and III sand northward into Pegasus Canyon. As sea level neared would have formed a shelf transgressive sheet. Mud the crest of the spit, the spit would have occasionally and Mode IV sand would have briefly entered the been overtopped by storm wash over. A permanent transgressing littoral system and have been transported breach would have developed near the head of the northwards away from the nonequilibrium sea bed and submarine canyon, where washover sediment, carried into the submarine canyons. As the retreating beach across the spit, would have spilled into the canyon and migrated westwards, gravel and sand carried north in been lost to the spit system. With the re-entry of the the littoral drift would have poured into Pegasus canyon head into the littoral system, the crest of the Canyon at successive points along its length. The spit would have quickly shifted westwards, to a position shoreline and sea bed configurations during the sea west of the new sediment sink formed by the canyon level retreat phase are discussed below. head (Fig. 33-3); the spit segment north of the breach When the rising sea had transgressed a short distance would thus have been abandoned, although remaining across the shelf, it encountered the great high of the old as an off-shore shoal. The detailed bathymetry (Fig. 5) interstadial shore complex (outer first-order ridge) is consistent with the postulated breach and new spit (Fig. 7). As the sea overstepped, the surface of the alignment and there is some evidence in the bathymetry Flo. 33 (opposite). Positions of successive Late Quaternary shorelines (heavy lines) on the Canterbury shelf. Shoreline ages that are based on radiocarbon-dated shells are indicated by 14C. Other shoreline ages are inferred from the sea-level curve for the area (Fig. 32). Land areas are shaded. Areas of inferred active off-shore shoals or submarine ridges are indicated by a hash pattern . See Figs 19-23 for positions of shorelines on profiles. 64 This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ ° ° "- ° 171 172 173 E 174 ° :: :::: :: :: :::::: ::::::: ::::: ::: ::::::: : :::::::: ::::::: :::::::: :::.:::: .:::::::: .::::::: : ...ii.:.: :::::::: .• ...... 43 ::::::::: :::::::: ::::::::.:::::::::::::::::: :::::::::::::::: ::::::::.::::::::.:::::::::.:::::::::1.:::::::: :;�::: :::::::: ::: iiilllllili11lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllilllllJ1llllllllllllllllllllllllll�ir\ 44 °S ° ° ° ° 171 172 l73 E 174 ° 43 44°S fr o 2 o OOO , 1 ppr ox � - P. ''i\\lilliiiil!iililll\\!!llli\![!illlllliilll'lll!!li1!iiii_�F/: ���r� 65 This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ ° 171 ° 172 ° 173 E 174 ° ° 43 ° 44 S approx 17,OOO 1 1 B.P. i!!lillllllllllillllilllllllllllllllillllill!llil :f years ° ° ° 171:::::::: :::::::· ::::::::: ::::::: :::::::: 172 173 E ::::: : \ ... r I \ \ ' I I " f"' ( / I\ ,,. J" \ ' \'I�,, ' ,,) � \I' \ l II I r ° 44 S 15, 100 years B. P. (14 C) This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ ° ° 171 172 173 E 174 .....°.. . :::· ·::::::: :::::::· :::::·:· ::::::: :::::::: :::::::: ...... ° .... iii :::::::::::::1::::::::::::::::::::::::::11 ::::;·. l!::::::::.::::::::!iii�l�m111111111111111i:::::::::::: :· ! ? :::::::x::::::::::::::::: ···::::1'::::::::::::::::: 11,750 years B.P. (14 C) :11lllllllllllllll:llliill:::::::::1:::::::::::::::::. (l I \ '- "' 1 1 ,-) ', r- f-1I I I \, '\ ' ( .... \\ ....' \ v,J I I I I I I .,/ 4-4 °S / ., .,,/ / / / ., .,, ;' --- ,,,, / .,,., .,,. � Present / .,,.., " I I_., This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ that the southern end of the spit was breached as well. been part of a broad off-shore system of active shoals or The extensive infilling of the northern part of the submarine ridges at the time. lagoon that took place during the shift of the spit is Second-order ridges are assumed here to have been evident in Profile 5 (Fig. 21) where the muddy lagoonal active south-east of Banks Peninsula during this phase, facies reflectors are seen to be rather deeply buried. since they are inferred to be so today (see pp. 60-61). The old interstadial lagoon and spit positions are Banks Peninsula was already a prominent headland by denoted by a circled l. On the profile, the new spit this time, preventing the northward movement of crest appears just to the east of the lagoon and west of gravel into Pegasus Bay. The inner belt of second-order the old interstadial spit. The new lagoon and spit ridges south-west of the peninsula may have been positions are denoted by a circled 3 on the profile. A forming at this time. short distance to the south (Profile 6, Fig. 21), where In Canterbury Bight, the arcuate shapes of the the spit crest did not migrate appreciably, as shown by drowned Rakaia and Rangitata fans are visible. Their the relative positions of 1 and 3 on the figure, the good preservation implies that there was no stillstand muddy lagoonal facies is still at the surface (see p. 54), within their bathymetric range. Evidence from Foveaux only slight infilling (from the east) being apparent. Strait (Cullen 1967) supports a very rapid sea level rise during this phase, beginning about 11,000 years ago A further 10 m rise of sea level to about the present and ending about 9,300 years ago. 76 m isobath (Fig. 5) was followed by a stillstand and Subsequent shore retreat in central Canterbury Bight the formation of the inner first-order ridge - shoreline was by cliffing (Fig. 17). It is assumed that the position 4 (Figs 5, 33-4; Profiles 5, 6, Fig. 21). It is greywacke gravel now exposed on the sea bed of inner assumed here to have been a spit/lagoon complex central Canterbury Bight is that part of the eroded fans similar to the outer one and according to radiocarbon which was not moved northward along the submerging dating it formed 15,100 years ago. coast. During the formation of this spit, conditions would The final phase of sea level rise (10,000years B.P. to have been considerably different from those of the present) along the Canterbury coast has been described present day (Fig. 33-4). To the east, the drowned by others (Jobberns 1926, 1927; Speight 1930, 1950; interstadial spit (outer first-order ridge), only 10 m Blake 1964; Suggate 1968; Kirk 1969; Armon 1970, deep, would have damped the ocean swell. To the 1974; Gibb, 1979). south-west there was probab!y a shallow system of active shoals (second-order ridges) which would have reduced the swell from that quarter. The northward longshore drift may therefore have been much reduced SHELF SEDIMENTATION DURING THE LAST and even at times replaced by southward drift because 6,000 YEARS the only deep open water lay to the north-east through During the last 6,000 years sea level has stabilised Pegasus Canyon. near its present level. In some places on the continental South and south-east of Banks Peninsula no shelf, modern sediment has accumulated; in others, barrier/lagoon system formed during the stillstand. exposed relict sediments remain exposed, but have Instead, reworking of the shallow sea bed into second adjusted to the modern shelf hydraulic regime. order ridges probably continued off the cape that would have existed south-east of present-day Banks Dispersal of Modern Input Sediment Peninsula. Further south, off Canterbury Bight, where Sediment that has been added to the continental the shelf beyond the 75 m isobath slopes more steeply shelf system during the last 6,000 years is considered than it does to the north, the shore would have still here to be forming deposits in equilibrium with the been in the outer part of Zone D near the old present hydraulic regime. They consist of very fine interstadial shore line and near-shore, storm-domin Mode IV sand and mud and constitute the Pegasus Bay ated shoals probably remained active. Formation. A shelf-wide blanket of sediment derived from the Shortly after the renewed rise of sea level from the rivers south of the study area and identified as modern 15,100-year shore, the supply of littoral gravel to by its fauna and grain size modes (Mode IV sand and Pegasus Canyon would have ended. mud) has accumulated on the older transgressive sand Subsequent events of the deglacial transgression are and gravel sheet in southern Canterbury Bight. less clear. A radiocarbon age of 11,750 years B.P. was A thick prism of modern muddy sand has obtained from 0-20 m depth-range shells at a depth of accumulated in the northern part of Canterbury Bight. 63 m in Canterbury Bight (H790, Table 6). Assuming There, a secondary landward component of transport that the animals were living within created by wind drift (Carter & Herzer 1979) and 20 m of sea level, a very approximate shoreline position probably on-shore waves, confines the modern deposit for that time is drawn(Zethalia on the zelandica) surface of the Canterbury to the inner shelf. The shallow upper portion of the Bight Formation along the 50 m isobath (shoreline deposit has been kept mud free (Fig. 11) by swell and position 5, Fig. 33-5). Core H790 was taken 30 km east storm waves. From it, sand is transported eastwards of this line among ridges and swales that would have then northwards around Banks Peninsula. 67 This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ Pegasus Bay is in the lee of Banks Peninsula, and sand apparently derived from the Pegasus Bay sediment moving around the peninsula has accumu Formation to the south. In one small area, about 45 km lated in the bay along with sediment entering from the south of the Rakaia River, on the southern part of the rivers to the west. Sand and mud have built up the floor inner zone of second-order ridges, Mode III disappears of the southern part of the bay by at least 28 m and from the clean sand, which contains only Mode IV. sand has formed a banner bank at the mouth of the bay This clean Mode IV sand is interpreted to be the (Figs 5, 10, 20). The sand on the bank does not cross to northern edge of the Pegasus Bay Formation of the western shore. southern Canterbury Bight. The edge of the modern blanket of muddy sediment thus changes north of latitude 44 ° 20'S to a mobile, clean Mode IV sand Tendency of Relict Sediments towards Hydraulic facies which is mixing with and ultimately replacing the Equilibrium coarser palimpsest Mode III sands. The relict sediments on the shelf are adjusted to the Off Banks Peninsula, where the flows of currents are present hydraulic environment, the degree of rework constricted and therefore stronger than elsewhere, the ing being different in different places. The adjustment sand of the relict sediment on the middle shelf is being is shown most clearly by belts of palimpsest sand, and swept in the form of sand ribbons across lag pavements by deposits of gravel that are stripped of sand. of gravel and shells (see p. 34). These mobile sand The coarse lithic gravel of the inner part of central bodies are estimated from the variable thickness of the Canterbury Bight is considered here to have been traction zone in cores (Herzer 1977, Appendix 4) to be derived from underlying gravel outwash fans, and lies up to 0.75 m thick. Second-order ridges are high and within the depth range that is frequently swept by may still be active (see pp. 60-{il). North of the wave-induced currents (Carter & Herzer 1979). peninsular constriction, currents are somewhat slower, Dispersal paths inferred from sea-bed drifters (Carter ridges are absent, but the sand is locally mud-free in a & Herzer 1979) are all away from this area. Sand and traction zone of variable thickness up to 0.6 m thick mud is thus continually swept out of the area and the (Herzer 1977, Appendix 4) and is still apparently gravel that remains is a lag deposit. mobile. On the middle shelf in Canterbury Bight, gravel is To the north-east, in the region of submarine extremely rare (Figs 10, 12), implying that currents are canyons, the Mode III sand on the shelf edge terrace slower than on the inner shelf. Second-order ridges are and upper continental slope is ascribed here to past or low, and the more southerly ones appear to be inactive present sand spillover. Clean Mode III sand on the at the present time (see p. 61). However, the mud-free walls of the westernmost tributary of Pegasus Canyon sand belt which occurs about 40 km off shore in this suggests that spillover is taking place now. area (Fig. 10) forms a traction zone up to 1.7 m thick in The occurrence of relatively clean sand in isolation at cores (Herzer 1977, Appendix 4) and is evidently the edge of the continental shelf (see p. 28) is mobile. The sand here is made up of relict Mode III unexplained. The energy to shift sediment there may from the Canterbury Bight Formation, except near the come from internal waves or from accelerated tidal southern margin of the mud-free belt, where the sand is currents that are expected at the shelf edge (Carter & frequently bimodal (Fig. 14) and contains Mode IV Herzer 1979). ACKNOWLEDGMENTS This paper is extracted from a Ph.D thesis submitted and help with analytical work; to staff at GULF Canada to Victoria University of Wellington. I am indebted to Resources Inc., Calgary, and A. Jaegers, NZGS, for the following individuals and organizations for their drafting; to Prof. H. W. Wellman, VUW, and Dr K. B. help: to Mr J. W. Brodie, former director of the New Lewis, NZOI, for reviewing the manuscript; and to Zealand Oceanographic Institute, for ship time, use of Patricia Hodgson for typing the final manuscript. facilities, and initial encouragement; to the technical Radiometric dating was done by the Institute of staff of NZOI who built equipment and kept it running; Nuclear Sciences, DSIR. to the personnel of r.v. Ta ngaroa, NZOI, Victoria University of Wellington and the New Zealand Financial support during the research was provided Geological Survey for their willing cooperation in the by my wife, the University Grants Committee, and data collection; to my wife Margot, for drafting, typing NZOI. 68 This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ GARRISON, L. E. ; McMASTER, R. L. 1966: Sediments and KIRK, R. M. 1969: Beach erosion and coastal development in the geomorphology of the continental shelf off southern New Canterbury Bight. N.Z. Geographer 25(1): 23-35. England. Ma rine Geology 4: 27>-89. KRAFT. J. C. 1971: Sedimentary facies patterns and geologic history GEOLOGICAL SOCIETY OF AMERICA 1963: ·'Rock-color chart". New of a Holocene marine transgression. Bulletin of the Geological York, N.Y. Society of America 82(8) : 2131-58. GIBB. J. G. 1979: Late Quaternary shoreline movement in New LIBBY, W. F. 1955: "Radiocarbon dating''. 2nd edn. Universtiy of Zealand. PhD thesis, lodged in the Library, Victoria University Chicago Press. !75p. of Wellington, 217p. MARWICK, J. 1957: New Zealand genera of Turritellidae, and the GODWIN, H. 1962: Half-life of radiocarbon. Narure, London species of Stiracolpus. N.Z. Geological Survey Pa leonrological 195 (4845): 984. Bulletin 27: 55p . . 5 pls. HERZER, R. H. 1977: Late Quaternary geology of the Canterbury McKINNEY, T. F.; FRIEDMAN, G. 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Transactions of rhe American Geophysical Union 36 (4): 655--{)3. P. D. HASSELBERO, GOVERNMENT PRIITTER, WELLINGTON, NEW ZEALAND-1981 64262J-l200/5/8IPT This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ ./