Tuatara: Volume 10, Issue 1, April 1962
Determining Depths of New Zealand Tertiary Seas — An Introduction to Depth Paleoecology
Determining Depths of New Zealand Tertiary Seas
An Introduction to Depth Paleoecology
Paleoecology is the study of past environmental conditions. It is based chiefly on fossil; but also on lithologies and inferred geological history. Marine paleoecology has two primary aims: firstly to determine former surface water temperatures and secondly to determine former sea depths. Until now little effort has been made to determine depths of former New Zealand seas, even though the information is vital for geological history, and particularly for tectonic history.
Depth paleoecology is best carried out in three steps. The first is to determine fossil biofacies by noting consistent taxonomic associations. The second step is to determine the order of relative depths of the fossil biofacies from their space-time distribution. The third step is to determine absolute depths by depth-correlating each fossil biofacies with its equivalent present-day biofacies. Depth-correlation of biofacies that represent a particular depth at different times is carried out by application of methods and principles analogous to those applied to time correlation of stratigraphic zones that represent the same time at different places. A series of biofacies for any particular age is easier to depth-correlate than an isolated biofacies.
The determination of absolute depths of former New Zealand seas is hampered by lack of data on the depth ranges of present-day organisms (especially Foraminifera) in seas around New Zealand. But a basic framework of world-wide depth biofacies is defined by progressive change with depth of relative proportions of the main groups of marine organisms represented by fossils. and can be applied to paleoecology in New Zealand. This framework is supported by some well established changes with depth in molluscan and foraminiferal faunas.
The Status of Paleoecology in New Zealand
The two most important aims of paleoecology are to estimate sea-level temperatures at successive times during the past, and depth of deposition of ancient sediments. Temperatures indicate climatic history: depths indicate palcogeography and much of tectonic history.page 20
Various animals grousp have been used to estimate temperature changes at sea-level during the Cenozoic. Temperatures are generally considered to have risen during the early Tertiary, reaching a maximum at the beginning of the Miocene, and to have fallen during the late Tertiary, reaching a minimum in the Pleistocene. No attempts have been made to define temperature-controlled zoogeographic provinces for any time within the Tertiary. From the distribution of reef-building corals (Squires, 1957), however, it is clear that temperature zonation existed during the Miocene, and temperature zonation may be assumed to have existed throughout the Cenozoic. Determination of past temperature zones is vitally necessary for both depth determination and inter-regional stratigraphic correlation.
Depth paleoecology is virtually an unexploited field of research in New Zealand, and is still seriously hampered by lack of data on the depth ranges of present-day organisms in New Zealand seas. Previously only two serious attempts have been made to determine depths of deposition of New Zealand Tertiary sediments by paleoecological methods. The first (Fleming, 1953) dealt mainly with Pleistocene marine sediments deposited within a small range of shallow depths. The second (Vella, 1962) dealt with Upper Miocene and Pliocene sediments near Mauriceville. Wairarapa. deposited between 0 ft. and c5.000 ft. In a large range of depths such as that determined in Wairarapa. certain general features of the faunas can be used as criteria for broad depth divisions which are useful for a reconnaissance of New Zealand Cenozoic sea depths.
The physical factors which probably account for these depth criteria are discussed below and a practical method for applying the criteira to paleoecology is outlined. The method is based on total fossil content of rocks, and can be used by a geologist with an elementary knowledge of paleontology. It is limited in that it will give only broad and rather indefinite depth ranges, but these depth ranges can serve as a firm basis for more detailed studies by specialist paleontologists. Some additional notes are given on particular Mollusca. Echinoidea. and Foraminifera which appear to be useful depth indicators.
First Principles of Paleoecology
The primary assumption of paleoecology is that ‘the present is the key to the past’. Paleoecology is essentially the reverse of ecology. The ecologist seeks to determine the inter-relationships of living plants or animals to their environment, and is able to measure physical factors of the environment directly. The paleoecologist uses ecological data to attempt to infer physical factors of the environments of fossils. His inferences are liable to be considerably in error because the fossil record is fragmentary, and because the ecology of present-day organisms is as yet imperfectly understood.page 21
The plant and animal community is itself one of the most characteristic features of any environment because it is extremely sensitive to any change in the environment. Fossils are a partial record of past communities, and are the best index we have of past environments. They are not so reliable, however, that the paleoecologist can afford to neglect any other lines of evidence. All fossils occur in some kind of rock, and the physical and chemical characters of the rock may be diagnostic of a particular environment or range of environments. The sequence of events inferred from the stratigraphic succession is also invaluable for the paleoecologist.
The term ‘facies’ which commonly appears in geological literature strictly means general appearance. It is used in several special senses, one of which is the rock or group of rocks characteristic of a particular environment. For example we may speak of the turbidite facies meaning the kind of rock deposited by submarine turbidity currents: or the estuarine facies meaning a particular suite of rocks varying in some characters, but all exhibiting the peculiar characters of estuarine deposits. The term lithofacies specifies the physical and chemical characters of the rock of a particular range of evinronments. The term ‘biofacies’ specifies the biological assemblage of a particular range of environments. A particular marine lithofacies of one age within a limited area is generally accompanied by its own particular biolfacies. This is partly because many benthonic animals prefer to live on a particular type of sea-bottom — a hard bottom, sandy bottom, or mud bottom. But more generally it is because biofacies and lithofacies are both controlled by physical and chemical factors related to depth in the sea.
Dominant Physical Factors in Oceanic Environments
Conditions on the shallowest parts of present-day seas are strongly affected by local fatcors including the air temperature, the strength and direction of the prevailing wind, the physical and chemical nature of sediment supplied from the land, the distance from river mouths, the velocity of tidal and wave generated currents, and the morphology of the sea-bed. These produce local variations in the type of sea-bed and in the physical and chemical nature of the sea-water. As a result a number of widely different biofacies occur each with a localised though recurrent distribution. The associated lithofacies are equally variable. At the present time these variable shallow-water biofacies extend down to about 400-500 feet, although there is reason to believe that they did not extend so deeply during much of the Tertiary.
At greater depths the factors controlling the environment show much less variation from place to place. At a given depth physical and chemical conditions are relatively uniform and this results in relatively uniform biofacies and lithofacies. The main variations in page 22 conditions take place vertically as the sea tends to be stratified with regard to temperature, salinity, oxygen content, and other chemical variables. In addition depth controls hydrostatic pressure and exerts a dominant influence on light intensity. Depth is therefore a master factor which gives rise to a series of biofacies, each being characteristic of a particular depth range.
Sunlight is the ultimate source of energy for all life in the sea as on the land. The limit of effective light penetration and hence of photosynthesis is thought to be about 500 feet in temperate seas (Holmes, 1957). The quantity of growing plants available to herbivorous animals must decline rapidly with increasing depth. A decrease in variety of animals and a radical change in the composition of faunas takes place between 500 and 1,000 feet, and is inferred to be related to the availability of living plant food. Biological communities from 0 to 500 feet includes photosynthesising plants, herbivores, carnivores, and scavengers; communities from 500 to 1,000 feet are probably transitional with decreasing numbers of photosynthesising plants and herbivores; communities below 1,000 feet probably include only scavengers and carnivores.
Temperature is an important physical factor affecting plant and animal distributions in every part of the ocean, and is possibly the most important factor in depths greater than 1,000 feet. Except locally in polar regions temperature decreases with increase in depth. At the same time temperature of the surface water generally decreases with increase in distance from the equator, but this horizontal temperature gradient may be locally reversed as a result of ocean currents. The deepest parts of the ocean have a uniform temperature from equator to polar regions of about 1-2° C. The vertical temperature gradient is steepest at the equator. It is much lower and may actually be reversed in polar seas. Isothermal surfaces in the oceans are thus generally inclined, dipping towards the equator: shallower isotherms are more steeply inclined than deeper isotherms. Furthermore, at any particular place temperatures gradually changed during the past, and the space-time pattern of temperature change is highly complex. Consequently paleoecological interpretation of a temperature controlled organism is difficult and requires a knowledge of either the depth at which the organism was living or the sea-level temperature where and when the organism was living. It is believed (cf. Natland, 1957) that distributions of many deep-water Foraminifera are controlled mainly by temperature and present-day biofacies occur in zones which tend to be bounded by isothermal surfaces and thus deepen towards the equator (Fig. 4).
Little is known of the direct effects of the hydrostatic pressure gradient on animal distributions, but at least a few Foraminifera appear to occur in restricted depth ranges while tolerating large temperature ranges. Such forms are of very great value for determining past depths, but not many are known because they are page 23 detected only by ecological study over a large range of latitudes and ecological studies tend to be localised.
Fig. 1: Conventional arbitrary depth divisions of the ocean floor, showing main changes with depth of sediments, physical and chemical factors, and animals represented as fossils.
Fig. 2: Depth distribution of main present-day groups of marine organisms represented as fossils, based mainly on data in the Annotated
Conventional Names for Oceanic Depth-environments
The ocean floor has been divided according to arbitrarily chosen depths into five main depth environments — littoral (intertidal), neritic (continental shelf), bathyal (continental slope), abyssal (deep-sea-floor), and hadal (deep oceanic trenches). Figure 1 shows the conventional depth-environments, the physical factors affecting distribution of plants and animals, and the probable depth range of the main lithofacies occurring in New Zealand. The physical factors form a basic framework for the pattern of depth faunas which can be used for estimating depths of former seas.
Depth Distribution of Present-day Marine Organisms
Generalised Distribution of Main Taxa:
The most useful depth indicators are benthonic organisms. The planktonic Foraminifera and Radiolaria are also useful because they are essentially pelagic (inhabitants of the high seas) and their remains generally accumulate most abundantly in deep water.
The overall depth distributions of main benthonic marine taxa represented as fossils are shown in Figure 2. Most taxa have a large depth range, but all except the benthonic smaller Foraminifera are poorly represented in. and for paleoecological purposes are virtually absent from deeper waters. The abundance of species and specimens of most groups or organisms falls off rapidly below 500 feet, becoming very small by about 1.000 feet, suggesting that light intensity is the essential controlling factor for most organisms.
Benthonic smaller Foraminifera are most abundant between 500 and 1.000 feet (Bandy and Arnal. 1960) and presumably are mostly scavengers, their primary energy supply being organic material drifted down from above. They are well represented down to the CaCO3 solution boundary.
|0-400 feet||greater than 3/1-2/1|
The decrease in the ratios with increase in depth is well established but statistical work on recent faunas is needed to determine ratios for each depth more accurately.
Mollusca in New Zealand:
Powell (1957) listed slightly over 2,000 species of Mollusca living in New Zealand with the depth range and geographic distribution known for each at the time of writing. Nearly all are littoral or neritic. What we know of deeper water Mollusca is due mainly to Dell (1956) who listed 595 species and subspecies from 48 stations (sampling positions) between 600 and 1,800 feet. Dell called this the achibenthic fauna of New Zealand. Only 165 species were recorded at 1.800 feet. Of these 27 are typical shallow water species found in deep water close to the continental shelf only, and are inferred to have been swept down from the shelf by vigorous currents. The typical 1,800-feet fauna thus includes 138 known species. Within it Dell distinguished three important elements:
(1) Species with a large bathymetric range, extending from shallow water down to 1.800 feet — 36 spp. (includes Ncilo australis, Nernocardium pulchellum and Scaphopoda which occur in the massive calcareous mudstone facies of the Pliocene in Wairarapa)
(2) Species occurring in shallow water (0-100 feet) at the Subantarctic Islands — 9 spp. These species are depth-tolerant but are apparently temperature-controlled.
(3) Species restricted to deep water — 93 spp., representing 71 genera. This group is distinctive and of prime importance for paleoecology. It includes 13 genera which occur in the Cenozoic and are useful bathymetric indicators. Ten of these — Pectunculina. Parvamussium. Manawatawhia, Pleia, Waipaoa. Teremelon, Mican-tapex. or the murdochi group. Comitas of the fusiformis group. Scaphander, and Planipyrgiscus — have no known shallow-water species. The other three — Galeodea, Ellecea. and Iredalina — are known only rarely above 600 feet. Dell included in his list of archibenthic Mollusca 14 species recorded from 4.000 feet most occurring at one station collected by the ‘ Challenger ’ Expedition, and 3 species from 6.600 feet at another station collected by the ‘ Challenger ’ Expedition. Seven of these species are also recorded from 1.800 feet or less.
Apart from the rapid decrease in number of species with increasing depth between 500 and 1.000 feet (Fig. 3). shells tend to become smaller and thinner. The absolute depth range of individual species, and the geographic range of genera tend to become larger. A large proportion of the genera represented at 1,800 feet are not endemic to New Zealand.
Figure 3 is based on New Zealand occurrences down ot the depth of 6,600 feet. The number of species in hadal depths is filled in from data given for the Kermadec Trench by Bruun (1957).page 27
Data on the number of species on a typical fauna of any particular depth has not been analysed. It is assumed that the number of species at any place will show approximately the same relationship to depth as the total New Zealand fauna, and experience with fossil depth-biofacies supports this assumption.
Deep-water Mollusca of the New Zealand Tertiary are comparatively little known. They are generally rare and poorly preserved and have been neglected by paleontologists in favour of neritic fossils which are at many places abundant and well preserved. They offer a fertile and undeveloped field of research as they promise to be the most reliable indicators for depths between 1.000 and 2.000 feet. Sediments deposited within this depth range form a large volume of New Zealand Tertiary rocks.
Echinoidea in New Zealand:
Of the genera of Echinoidea which occur in the New Zealand Tertiary, a number are still represented in Recent seas, either in New Zealand itself or; in the case of the warm-water genera which no longer occur here, in north-eastern Australian waters.
Goniocidaris ranges the outer part of the shelf, apparently on both hard and soft bottom, but the slender-spined species also enter the archibenthal fauna. Ogmocidaris is at present unknown from the Tertiary, but it may be expected to occur in the deeper-water facies of the Castlecliffian; it appears to be mainly archibenthal. Of the genera now extinct in New Zealand, notable cidarids are Phyllacanthus and Eucidaris: both are indicative of shallow-water hard-bottom, especially the outer parts of reefs including coral reefs), in waters not cooler than those of Norfolk Island and the Kermadecs.
Of the other regular genera. Pseudechinus is the only important one in the Tertiary: most species prefer hard bottom, on the outer two-thirds of the shelf (30 to 100 fathoms), though dead immature tests are constantly encountered in muds. Similar immature tests are common in the Castlecliffian. and are thought to indicate specimens which have been overcome by muds, whilst originally living on some temporary hard-bottom, such as shell-beds would provide. One species which occurs in the Castlecliffian. P. flemingi. is otherwise known only as living specimens in about 30 to 300 fathoms, east of the South Island: this is evidently a deeper-ranging form than the other members of the genus. Evechinus chloroticus is a eurythermal. strictly littoral speices. ranging at present from the Kermadecs to Stewart Island: typical of reefs and rock-platforms in the extant fauna, it occurs fossil in the same inferred facies in the Nukumaruan.
Of the irregular genera, all indicate a moderately soft bottom such as shell-grit), and most require a mud. or sandy mud. bottom. The commonest is Echinocardium, which tolerates all depths. The sand-dollars inhabit shallow water on the upper part of the shelf, below low-tide, resting on sand or sandy mud. Other genera, such as Brissopsis and Spatangus. among the heart-urchins, are soft-bottom indicators, occupying most of the shelf: they avoid rough water, and do not come into the uppermost 10 fathoms or so. but may extend well into the archibenthal zone.
Summaries of data referring to the bottom ecology of extant and fossil echinoderms in New Zealand will be found in Fell 1952, pp. 3-4: 1954 (passim); 1958 (complete checklists for species occurring in the ranges 1.000 fathoms or deeper. 300-1.000 fathoms, and 100-300 fathoms).
The only account of distributions of Recent Foraminifera in New Zealand (Vella. 1957) describes faunas from 0 to 1.500 feet in Cook Strait. There strong bottom currents have transported shallow-water shells into deeper waters, and only the upper limit of the depth range of each species is useful for paleoecology. Data from ecological studies carried out in other parts of the world must be used with caution for two reasons: In the first place there is good evidence that many Foraminifera are depth-tolerant but temperature-restricted, and have different depth ranges in different parts of the world. In the second place many Foraminiferal species have few distinguishing characters and there may be more homeomorphs than we now realise. Homeomorphs are different species which are morphologically similar and therefore difficult to distinguish. For example, a form called Pullenia bulloides is characteristic of abyssal depths off the coast of California, while a form with a similar shell known by the same name occurs in quite shallow water in New Zealand. Hitherto pafeontologists have recognised homeomorphs of different ages (heterochronous): it may also be necessary for ecologists to recognise homeomorphs of different depths (hetrobathyal).
In the United States use of fossil Foraminifera by oil companies has stimulated all phases of research on Foraminifera. including their ecology. The Gulf of Mexico and the coast of California are probably the most throughly examined areas in the world. The biofacies distinguished off the west coast of the North American continent from Alaska to Panama, summarised by Natland (1957. pp. 554-8). give a good idea of faunal variation with depth and with latitude (Fig. 4).
Biofacies I. that of lagoons and estuaries, includes only species capable of tolerating a large range of temperatures and shows no significant change with latitude. Biofacies 2 — intertidal and near-shore (upper neritic) — includes a greater number of species, all of which are capable of tolerating a moderate range of temperatures: this biofacies also shows little change with latitude. Biofacies 3 — 125-900 feet — is divided into sub-facies 3A and 3B: light penetration seems to be the depth-controlling factor: the boundary between the sub-biofacies 3A and 3B is a latitude, and the difference between them is due to temperature difference. The five depth biofacies from 900 to 15.000 feet (the CaCO3 solution boundary)
are controlled by temperature, their boundaries corresponding to isothermal surfaces. The depth ranges of biofacies increase approximately logarithmically with increasing depth.
The value of pelagic (planktonic) Foraminifera for depth determintation has been a controversial subject, and is still contested by some micropaleontologists. The number of pelagic Foraminifera living in the surface water generally increases with distance from the page 30 shore, and as depth generally increases with the distance from the shore there is usually a relationship between depth and the number of shells of pelagic Foraminifera falling to the bottom. Pelagic oozes (including Globerina ooze) as their names indicates, are essentially restricted to the deep sea floor. The number of shells of pelagic Foraminifera falling to the bottom generally decreases towards land becoming very small near the shore. Pelagic shells may accumulate abundantly near the shore in two kinds of exceptional circumstances. Phleger (1960) notes that they may be abundant along coasts where the run-off of fresh water is extremely low. thus suggesting that the pelagic Foraminifera are sensitive to a slight decrease in salinity: it is unlikely that run-off from New Zealand was low at any time during the Cenozoic. On high run-off coasts on-shore winds may periodically blow pelagic water masses towards the shore causing local abundant accumulations of pelagic shells in shallow water and this may well have happened on former westward continental shelves of New Zealand.
Abundance of pelagic Foraminifera is expressed as the percentage of pelagic foraminiferal shells (%P) in the total number of foraminiferal shells, benthonic and pelagic. The number of pelagics increases fairly regularly from 0% in the littoral biofacies to 90% or more in Globigerina ooze on the deep sea floor.
Phleger (1960) described seven generally applicable foraminiferal depth biofacies (Fig. 5). using the following ‘Population characteristics : (1) Number of benthonic species. (2) Number of benthonic genera. (3) Percentage of arenaceous specimens. (4) Characteristic benthonic genera. (5) Percentage of pelagic shells. (6) Other features.
Phleger's depth biofacies are defined in much more general terms than those of Natland. They are more useful to us than Natland's because they are controlled solely by depth: they can be recognised in the New Zealand Cenozoic. but are not as reliable for depth determination as biofacies based on total fossil faunas.
Depth Restricted Groups of Benthonic Smaller Foraminifera:
The following groups of benthonic smaller Foraminifera are restricted in depth range except for a few species, and appear to have been restricted to the same depth range throughout the Cenozoic.
Miliolidae: Most genera are restricted to above 400 feet. Some genera, for example Biloculina. are locally abundant between 400 and 1.000 feet. Only a few species live or lived in deeper water, probable Tertiary examples being Sigmoilopsis schlumbergeri and Praemassilina tenuis.
Fig. 4: Diagram to show foraminiferal biofacies from Alaska to Panama in relation to depth and temperature; constructed from data summarised by Natland (1957). Slope of isotherms diagrammatic.
Elphididae: Large ornate species of Elphidium are restricted to intertidal and nearshore environments. Elphidium charlottensis and related Cenozoic forms appear to have occurred down to about 1.000 feet. Elphidium is not as common in New Zealand as elsewhere, being largely replaced in both Recent and Cenozoic faunas by Notorotalia. Notorotalia depressa and inornata are essentially intertidal. Other large species are restricted to depths less than 400 feet. Notorotalia finlayi occurs abundantly in estuaries and in the open sea from O to about 1.000 feet. Notorotalia taranakia occurs from about 1.000 feet down to an undetermined depth. Corresponding Cenozoic species occur in the same order relative to the shore-line, and probably lived in similar depths.
Rotaliidae: The genus Streblus is abundant in estuaries in New Zealand and other parts of the world.
Discorbidae: This family includes a host of genera and species From Tertiary occurrences most appear to have lived in less than 400 feet.page 33
Planispiral Lagenidae: Many species occur between 1.000 and 2,000 feet (Brady. 1833). Robulus calcar seems to be the only species which occurs abundantly in late Cenozoic sediments containing abundant macrofossils, and is probably limited to depths less than 1.000 feet. Most species of Robulus, Lenticulina, and Sarecenaria are abundant in massive calcareous mudstone with rare Mollusca such as Neilo, Parvamussium, etc. They define the Robulus biofacies which is considered to indicate depths between 1.000 and 2.000 feet.
Lituolidae: Cyclammina cancellata and a large Haplophragmoides are common in New Zealand Tertiary calcareous mudstones without macrofossils. Phleger (1960) noted that Cyclammina cancellata is common in Tertiary mudstones (presumably in the United States). C. Cancellata now lives deeper than 6.000 feet (Akers. 1954) and is probably a good world-wide deep water indicator.
A simple method of depth determination which may be called depth range analysis ‘makes direct use of depth ranges of Recent species that occur as fossils or are closely related to fossil species. This method works fairly well for Pleistocene and perhaps Pliocene faunas, but is ineffective for older faunas because of their small number of Recent species. Other limitations are that some Recent species have different depth ranges at different places, that closely related Recent species commonly have different depth ranges, and that until depths of past faunas are determined we have no means of knowing whether any particular species has maintained the same depth range with time.
Bandy (1956. pp. 189-191). working in Miocene and Pliocene sediments of Florida, estimated water temperatures from fossil benthonic Foraminifera, and scaled off depths from the present day bathythermal gradient in the adjacent north-eastern part of the Gulf of Mexico. This method is subject to the same kinds of limitations as the depth range analysis method, and is further complicated by the decrease in sea-level temperatures which took place all over the world in the late Tertiary, and which Bandy did not take into account.
The method suggested here is carried out in three stages: First, determine biofacies: second, determine the order of relative depth of the biofacies: third, attempt to determine absolute depth range of each biofacies by comparison with present day biofacies.
Determination of Fossil Biofacies and Their Depth Order:
Biofacies are simply natural biological assemblages distinguishable by taxonomic content from other natural biological assemblages. The biofacies may be defined by either the total or part of the page 34 total biological assemblage. All fossil biofacies are only partial assemblages, and even these may be defined by reference to only one taxonomic group — say Foraminifera, or Mollusca. They are recognised by noting consistent associations of species or other taxa. Generally innumerable biofacies can be recognised in extensive fossiliferous rocks and the paleoecologist must sort out the significant associations by experience.
Two methods are available for determining the depth order of fossil biofacies — that is the relative depth represented by each — without reference to present-day biofacies: (1) by considering the change of biofacies with distance at one time; (2) by considering the change of biofacies with time at one place.
The first method is ideally the same as that used for present-day ecology. but in practice is limited by the difficulty of precise dating. Unless redeposited the fossils in lateral geological sections (sections exposing rocks representing one time) have approximately their original aerial distributions, except where they are displaced by transcurrent faults. The faunal changes in the section can be mapped, and the position of the shore-line and direction of deepening can usually be determined fairly easily.
The second method of determining the depth order of fossil biofacies may be limited by difficulty in separating the faunal changes which are due to depth change form those irreversible faunal changes which are due to evolution and ante-Pleistocene cooling (cf. Vella. 1962. Fig. I). At various times during the Tertiary large areas of land in the New Zealand region dropped down very rapidly to become deep-water basins. First, shallow-water, then progressively deeper-water facies were deposited. Many Tertiary basins deepened so rapidly that shallow to deep transitions are represented within one stage. As the stage is generally the smallest time division that can be recognised time-controlled faunal changes are non-existent or negligible in these sections.
|Massive blue-grey calcareous mudstone with rare macrofossils grading down to||2.000 ft.|
|Massive blue-grey muddy sandstone with abundant scattered macrolossils grading down to||c500 ft.|
|Well-bedded, well-sorted sandstones with shell-beds — many kinds of macrofossils||c300 ft.|
|Basal Conglomerate||c2 ft.|
|Angular unconformity: erosion surface on Mesozoic greywacke.|
The basal conglomerate, at some places barren of fossils, at others containing broken shells, is the littoral facies of the advancing sea. The well-bedded sandstone with shell-beds is the inner neritic facies deposited in shallow water just off-shore. The massive muddy sandstone and massive calcareous mudstone represent successively greater depths. There is considerable difference of opinion as to the absolute depths represented by all except the littoral facies.
Generalised descriptions of the faunas of each facies are as follows:
Basal Conglomerate: Fauna not known.
Shell-beds in well-bedded sandstone; Abundant Mollusca including Pelecypoda. Gastropoda, Scaphopoda. and a Nautiloid. rare Brachiopoda, locally abundant Scleractinian corals, fragments of Echinoidea. abundant Cirrepedia, locally abundant Bryozoa. abundant Ostracoda. relatively infrequent benthonic Foraminifera page 36 But locally abundant Notorolalia and Elphidium, rare pelagic Foraminifera. The dominant fossil is generally the thick-shelled gastropod Callusaria callosa. No other facies contains so many phyla or so many species.
Massive muddy sandstone: Abundant Pelecypoda, gastropoda, and Scaphopoda, moderately abundant benthonic Foraminifera. infrequent pelagic Foraminifera. and rare Ostracoda. Dominant molluscs in the shallower phase are Cucullaea n. sp., Dosinia cottoni, Kuia macdowelli. and Marama hurupiensis, and in the deeper phase are Limposis lawsi and many species of Turridae.
Massive calcareous mudstone: Rare Mollusca. mainly small and delicate gastropoda, and thin-shelled Pelecypoda such as Neilo, Myrtea and Parvamussium: abundant benthonic and pelagic Foraminifera. the pelagic percentage increasing upwards from about 30 to about 90%.
A similar but more complete series of biofacies determined for the Pliocene in northern Wairarapa (Vella. 1962) is shown in Fig. 6. This series was determined during the examination of some hundreds of fossil faunas, mainly microfaunas, from a large area, and is based on many vertical and many lateral sections.
Estimation of Depths Represented by Fossil Biofacies:
Estimation of the absolute depth range of each fossil biofacies is a process of faunal matching akin to age-correlation, and may be called depth-correlation. Faunas of several different consecutive ages are always easier to age-correlate than an isolated fauna. Similarly faunas of several different consecutive depths are easier to depth-correlate than an isolated fauna. Depth correlation is easiest when a complete series of depth biofacies for a particular age is available. Depth correlation of the Wairarapa Upper Miocene (Hurupi Formation and overlying sediments) and Pliocene facies is shown in Fig. 7 together with some important isolated facies in the early Tertiary.
In each of the upper four biofacies of the Pliocene about 90% of the genera and 70% of the species of Foraminifera are the same as in equivalent present-day biofacies. When the possibility of transportation of shells from shallow to deep water is allowed for Foraminiferal index genera are useful. Not enough is known about Foraminifera deeper than 1.000 feet in present-day New Zealand seas to allow a comparison to be drawn with equivalent Pliocene faunas.
The shallower Upper Miocene biofacies contain many different genera (such as the Mollusca Cuculaea and Conospirus and the Foraminifer Amphistegina) due to the sea-level temperatures being appreciably warmer than at present. Deeper Upper Miocene biofacies are generally similar to deeper Pliocene and present-day biofacies.page 37
Lower Miocene shallow water biofacies are markedly different, including reef building calcareous algae, locally reef building corals, and larger benthonic Foraminifera, because sea-level temperatures at the time ranged from subtropical in the south to tropical in the north.
Pelagic biofacies are best represented in the early Tertiary. The Amuri Limestone is white, fine-grained, locally muddy and locally highly siliceous, ranges in age from Paleocene to lower Oligocene, and extends from northern Canterbury through Marlborough to Southern Wairarapa. Thin sections generally show abundant globigerinid shells, and siliceous phases contain many Radiolaria. As pointed out long ago by Marshal (1916) the Amuri Limestone is a fossil Globigerina ooze, and the siliceous phases probably were deposited near the CaCO solution boundary and are transitional to Radiolarian ooze. The siliceous phases are mostly Paleocene in age; surface temperatures at the time may have been subglacial (Squires, 1957, Fig. 5). and the solution boundary may not have been as deep as 15,000 feet.
Bands of red clay up to 20 feet thick occur in red and green mottled fine-grained mudstones (‘ bentonite ’) of Paleocene age in Raukumara Peninsula. The red clay contains entirely non-calcareous microfaunas including Radiolaria and siliceous and arenaceous Foraminifera. The association of red clay with siliceous faunas is known at present on only the deepest parts of the ocean floors.
In New Zealand during each stage of the Cenozoic, sediments were deposited in a large range of depths. Broad depth divisions can be differentiated in them without a detailed knowledge of the depth distributions of present-day organisms. Depth divisions are defined essentially by biofacies, though lithofacies are also useful depth indicators. Fossil biofacies should be determined from the spacetime distribution of fossil species without reference to present-day biofacies. Methods of recognising fossil biofacies are described by Imbrie (1955).
At this stage fossil biofacies for each age can be placed in order of increasing depth, and can be used as indicators of relative depth in the same way that fossil zones are used as indicators of relative ages. Relative depth determinations can be extremely useful to the geologist for paleogeographic reconstruction, and for determining some kinds of tectonic events. In the light they may shed on faunal succession they may prove to be of considerable value to the ecologist.
Once fossil biofacies are defined absolute depth determination may be attempted by depth-correlation with prsent-day biofacies. page 39 Absolute depths are better determined from total fossil biofacies than from single taxonomic units, such as Mollus or Foraminifera within the fossil biofacies. They can not yet be determined with great precision, and consequently it is essential that fossil biofacies be defined and name as distinct units from present-day biofacies. To this may be applied the principles and rules already laid down for defining and naming fossil (biostratigraphic) zones.
Two neglected lines of research needed for depth determination in the New Zealand Tertiary are studies of distribution of Recent Foraminifera in New Zealand, and taxonomic studies of Tertiary deep-water Mollusca.
Dr. H. M. Pantin. New Zealand Oceanographic Institute, criticised and suggested alterations to the section on oceanography. Professor H. B. Fell supplied the comments on Echinoidea.
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Marshall, P., 1916. The Younger Limestones of New Zealand. Trans. N. Z. Inst. 48, pp. 87-99, pls. 8-10.
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—— 1962. Biostratigraphy and Paleoecology of Mauriceville District New Zealand. Trans. Roy. Soc. N.Z. Geology I.