Sedimentary Geology, 60 (1988) 301-322 301 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Application of bryozoan zoarial growth-form studies in facies analysis of non-tropical carbonate deposits in New Zealand

C A M P B E L L S. N E L S O N 1, F I O N A M . H Y D E N 2 . , , S A N D R A L. K E A N E t,

W I L L I A M L. L E A S K 3 , , , a n d D E N N I S P. G O R D O N 4

i Department of Earth Sciences, University of Waikato, Private Bag, Hamilton 2001 (New Zealand) 2 Department of Geology, University of Otago, P.O. Box 56, Dunedin (New Zealand)

3 Department of Geology, Victoria University, Private Bag, Wellington (New Zealand)

New Zealand Oceanographic Institute, Division of Marine and Freshwater Science, DS1R, Private Bag, Kilbirnie, Wellington (New Zealand)

Received September 16, 1987; revised version received March 20, 1988

A b s t r a c t

Nelson, C.S., Hyden, F.M., Keane, S.L., Leask, W.L. and Gordon, D.P., 1988. Application of bryozoan zoarial growth-form studies in facies analysis of non-tropical carbonate deposits in New Zealand. In: C.S. Nelson (Editor), Non-Tropical Shelf Carbonates--Modern and Ancient. Sediment. Geol., 60:301-322

The fragmental remains of bryozoan colonies are the dominant skeletal contributor in many modem and ancient occurrences of non-tropical shelf carbonate deposits. The taxonomic complexity of the Bryozoa, the difficulties of systematic classification except by specialists, the wide range of particle sizes and of preservation state in the carbonate deposits, and the often well-cemented nature of host limestones, all limit the sedimentological interpretation of the commonly diverse bryozoan component. However, following the pioneer work of Stach in the 1930's, the potential exists for sedimentologists to obtain useful (paleo)environmental information about the deposits by recording simply the nature and relative abundances of the various growth shapes of the bryozoan material, known as their zoarial (colonial) growth forms. Here we propose a simple descriptive terminology, covering both cheilostome and cyclostome bryozoans, that involves four main growth forms (encrusting, erect rigid, erect flexible and free-living) and a selection of subcategories based on shape. Identification of these habitat-related growth-form types is aided by reference to line drawings, specimen photographs and photomicrographs of thin-section slices. The scheme is illustrated by examining briefly some possible environmental controls on the spatial and /or time variations in the nature and distribution of colonial growth forms in several examples of modem and Tertiary bryozoan-dominated carbonate deposits in New Zealand. We anticipate that the routine description and quantification of bryozoan colonial growth types in non-tropical carbonates generally will facilitate the recognition and interpretation of contrasting (sub)facies within the global foramol/bryomol group of carbonate deposits.

Present address: * 18 Rosemary Drive, Bromham, Bedford, England MK43

8PL, U.K.

* * Ian R. Brown Associates Ltd., P.O. Box 9043, Wellington, New Zealand.

0037-0738/88/$03.50 © 1988 Elsevier Science Publishers B.V.

I n t r o d u c t i o n

B r y o z o a n s a re m a i n l y m a r i n e , sessi le, b e n t h i c ,

s u s p e n s i o n - f e e d i n g c o l o n i a l a n i m a l s t h a t l ive o n

d i n o f l a g e l l a t e s , s m a l l d i a t o m s , n a n n o p l a n k t o n ,

302

ciliates, non-living organic particles and possibly bacteria (Duncan, 1957; Bullivant, 1969). They are typically stenohaline (32-37%o), eurythermal (10-30°C) and most abundant at shelf depths (0-200 m), but range as deep as 6000 m or more (Duncan, 1957; Lagaaij and Gautier, 1965; Cuf- fey, 1970). Some species are cosmopolitan (Lagaaij and Cook, 1973) and the phylum has a prolific world-wide geological record from Paleozoic to Recent (Bassler, 1953; Ryland, 1970).

The individual units, or zooids, of a bryozoan colony are about 0.5-1mm in size. Each comprises soft parts (polypide) and a calcified (typically high- or low-Mg calcite, more rarely aragonite-- Rucker and Carver, 1969; Sandberg, 1977; Keane, 1986) or sometimes non-calcified body wall (cystid). The calcareous zooid skeleton is called a zooecium. Zooecial architecture distinguishes the two orders dominant since the Cretaceous. These are the Cheilostomata, with boxlike zooids bearing a subterminal opening and operculum, and the Cyclostomata with tubular zooids having a termi- nal, typically circular orifice without an oper- culum (Bassler, 1953). A collection of zooids (zoo- ecia in fossil colonies) constitutes the bryozoan colony or zoarium. The colonies have extremely diverse forms, ranging from flat encrusting habits, where all zooids adhere to a hard substrate, to erect, rigid colonies with foliaceous, arborescent or fenestrate shapes, in which only a few individual zooecia attach the colony to the substrate by direct cementation or by rootlets. Others build erect, flexible colonies having chitinous joints be- tween stem segments and chitinous rootlets for attachment. Still others are free-living and are disc- or bun-shaped. These various colonial shapes are known as zoarial growth forms in the litera- ture.

Here we note the application of the study of bryozoan colonial growth forms to a selection of non-tropical carbonate deposits from New Zea- land, both modern and ancient (Fig. 1). Because bryozoans are a major skeletal contributor to carbonate sediments in temperate regions gener- ally, the relatively straightforward determination of growth forms has the potential for defining environmentally-sensitive subfacies within the otherwise bryozoan-rich deposits. Our principal

!

170"E 17~i'E

S

S-

Fig. 1. Map of New Zealand showing the general location of the modern and Tertiary bryozoan carbonate shelf deposits discussed in the text.

intention at this time is to provide a simplified classification, identification and interpretive scheme for the colonial forms that may have some practical value for sedimentologists working on non-tropical carbonates.

Bryomol carbonates

In warm shallow seas, competition with corals and calcareous algae invariably restricts bryozoans to a minor role (Duncan, 1957), although there are local exceptions (e.g., Hoffmeister et al., 1967; Cuffey, 1973). The major skeletal contributors to modern subtropical and tropical carbonates are typically hermatypic corals, calcareous green and red algae, benthic foraminifers, bivalves and gas- tropods. These sediments, which often also in- clude a variety of non-skeletal grains such as ooids, aggregates and pellets, have been named "chlorozoan" carbonates by Lees and Buller (1972) because of the consistent presence of Chlorophyta (algae) and Zoantharia (corals). Ancient analogues are common, usually involving reefal and associ- ated lagoonal and forereef facies (Wilson, 1975).

Active growth of hermatypic corals and calcareous green algae is strongly light- and tem- perature-dependent (Stoddart, 1969), so that chlo- rozoan carbonates do not develop in waters in which the mean annual surface temperature falls below about 20 ° C, typically beyond about 30 °N and 30°S latitude. However, carbonate sedi- mentation persists in the more temperate regions wherever the input of terrigenous material to shelves is low (Chave, 1967). In these cases the principal skeletal contributors are foraminifers, molluscs, calcareous red algae, bryozoans and barnacles, named "foramol" carbonates by Lees and Buller (1972) because of the consistent occur- rence of foraminifers and molluscs in the deposits. However, the rapidly expanding database for modern non-tropical shelf carbonates over the past 15 years or so has high-lighted the very major contribution made by bryozoans to the deposits, particularly in comparison to foraminifers (e.g., Boillot, 1965; Chave, 1967; Wass et al., 1970; Lees, 1975; Marshall and Davies, 1978; Probert et al., 1979; Wilson, 1979, 1982; Scoffin et al., 1980; Leonard et al., 1981; Rao, 1981a; Nelson et al., 1982, 1988b; Nelson and Bornhold, 1983; Wass and Yoo, 1983). This is true also for many ancient non-tropical limestones (e.g., Siesser, 1972; Schlanger and Konishi, 1975; Nelson, 1978a; Hy- den, 1979; Barbera et al., 1980; Rao, 1981b; Bal- san and Taylor 1982; Kamp and Nelson 1987, 1988). Consequently, Nelson et al. (1988b) have suggested the new name "bryomol" carbonates for this distinctive non-tropical carbonate facies.

The taxonomy of bryozoans is both diverse and complex, commonly requiring binocular or even SEM examination for positive classification, and is generally the realm of the specialist biologist or paleontologist. In New Zealand, detailed taxo- nomic work on bryozoans has, until recently (see Gordon, 1984, 1986), been hindered by a lack of comparative material and by the absence of com- prehensive published descriptions of Australasian species. Poor preservation of some of the bryozoan remains, or their incorporation in well-cemented, tenacious host rocks can limit the application of detailed systematic studies. As an alternative, the recognition of bryozoan colonial growth forms offers a relatively straightforward way for sedi-

303

mentologists to classify the bryozoan component of non-tropical carbonate deposits, including core material. In turn the growth-form types repre- sented can provide useful (paleo)environmental information concerning the deposits.

Zoarial growth forms--classification and environ- ment

Stach (1935, 1936, 1937) pioneered the study of bryozoan zoarial forms in Recent and fossil as- semblages. He proposed that there is a close rela- tion between the colony form of modern cheilo- stomes and the depth and degree of water agita- tion of their environment, from the discovery that certain species developed different growth forms in different habitats (see also Flor, 1973; Harme- lin, 1973; Thomsen, 1977; Kopajevich, 1978).

Lagaaij and Gautier (1965), working on surfi- cial sediments from the Rh6ne delta, verified the interrelations between colony form and habitat, but emphasized also the effects of substrate type and sedimentation rate on the distribution of growth forms. They defined additional growth- form types and extended the approach to include cyclostome zoaria. Many subsequent studies (e.g., Rucker, 1967; Schopf, 1969; Ryland, 1970; Wass et al., 1970; Caulet, 1972; Milliman et al., 1972; Annoscia and Fierro, 1973; Labracherie, 1973a, b; Cuffey and Turner, 1987; Reguant and Zamarreho, 1987) have provided more detailed zoarial classifi- cations and/or more specific habitat controls, but the general conclusion remains that a broadly consistent relation exists between colony form and environment, thus confirming the usefulness of bryozoans in paleoenvironmental analysis (e.g., Brown, 1952; Cheetham, 1963; Labracherie and Prud'homme, 1967a, b; Askren, 1968; Leitch et al., 1969; Labracherie, 1970, 1972, 1973b; Crabb, 1971; Brood, 1972; Siesser, 1972; Pedley, 1976; Cuffey et al., 1981). No one growth form is con- fined to a single habitat (e.g., Gordon, 1987), and specific growth forms will tend to achieve domi- nance only under optimum conditions. In prac- tice, it is the association and relative abundances

of the various growth forms in samples that are probably most useful as a monitor of environment and environmental trends with time.

304

Bryozoa Mollusca Foraminifera Rhodophyta Echinodermata Annelida Arthropoda Coelenterata Porifera Brachiopoda

Bryozoa Mollusca Foraminifera Rhodophyta Echinodermata Annelida Arthropoda Coelenterata Porifera Brachiopoda

Bryozoa Mollusca Foraminifera Rhodophyta Echinodermata Annelida Arthropoda Coelenterata Porifera Brachiopoda

A) MODERN

I

Three Kings (34°S)

I o lO 2'0 3'0 '

l

I

Otago [46°S)

0 110 2KO 30

I I

Snares (48°S)

, • , I 0 10 20 30 40

Average percent

[B] A N C I E N T l

1 Te Kuiti Group

limestones (Oligocene)

0 lX0 2'0 ~ 6'0 7'0

__ ] __ _ l __1

2 - - q

Forest Hill Formation

(Early Miocene)

I • I t / V ~ i 60 10 20 60 70

J - - - - ]

i ] Takaka Limestone (Oligocene -

Early Miocene)

i ~ I ,J 5J0 O lf0 20 60 70

Average percent

Fig. 2. Average skeletal composition of the dominant shelf carbonate facies for each of the modern and Tertiary New Zealand occurrences (see Fig. 1) discussed in the text. The Arthropoda include mainly barnacle remains. Data from Nelson (1973), Hyden (1979), Leask (1980) and Keane (1986).

The majority of skeletal carbonate shelf de- posits in New Zealand, both modern (e.g., Carter, 1975; Probert et al., 1979; Nelson et al., 1982, 1988b; Nelson and Hancock, 1984; Carter et al., 1985; Head, 1985; Keane, 1986) and Cenozoic (e.g., Leitch et al., 1969; Nelson, 1978a; Hyden, 1979; Leask, 1980; Anderson, 1985; Smaill, 1985; Nathan et al., 1986; Kamp and Nelson, 1987, 1988), contain appreciable bryozoan material. Not uncommonly, bryozoans are the principal sedi- ment contributors (Fig. 2).

In dealing with the bryozoan component of the sediment, we have attempted to reduce the in- creasingly complex classification and nomenclatu- ral schemes suggested for bryozoan zoarial forms (e.g., Stach, 1936; Lagaaij and Gautier, 1965; Labracherie and Prud'homme, 1967b; Schopf,

1969; Brood, 1972) to a simple descriptive terminology involving four main growth forms and covering both cheilostomes and cyclostomes: encrusting (EN); erect rigid (ER); erect flexible (EF); and free-living (FL). Further subdivision of categories according to shape is possible and for graphical purposes these can be referenced by abbreviated letter notations, such as ENml and ERfe (Figs. 3 and 4). Correlation between this nomenclature and the rather cumbersome growth form names in the literature is indicated in Figs. 3 and 4. Illustrations of several of the more im- portant bryozoan growth forms in New Zealand deposits are given in Fig. 5, which includes also thin-section photomicrographs of samples.

The four-part scheme parallels that used by Gordon (1987) to encompass all the Recent species

305

z :LLI

© Z

z UJ

ZOARIAL SHAPE CLASSIFICATION TRANSVERSE LONGITUD. AND APPEARANCE AND DESCRIPTION SECTION SECTION

Unilaminar (ul]

(A]

Multilaminar [ml) [B]

CO]

Foliaceous [fo)

Robust branching

[ro] r'r-

Delicate branching

[de) UJ

Fenestrate (fe)

ERECT FLEXIBLE

(EF)

FREE -LIVING (FL)

M E M B R A N I P O R I F O R M Unilaminar sheets encrusting solid, sometimes flexible substrate; in latter case dorsal wall poorly calcified

C E L L E P O R I F O R M A Colonies heaped irregularly in multil aminar masses of variable shape on or around flexible, sometimes rigid sub strate; zooecia open on all sides

C E L L E P O R I F O R M B Encrusts pagurid-inhabited gastropod shells forming cylindrical tube from aperture; (sub)spberica~ nodular zoaria encapsulate substrate

CELLEPORIFORM C Hemispherical, multilaminar, heavily calcified basal lamina; zooecia open on outer face

E S C H A R I F O R M Erect, rigid, foliaceous, convoluted, bi- lamellar colony; zooecia face opposite directions; calcareous base or chitinous rootlets

A D E O N I F O R M Erect. robust, lobate to flattened, bi- furcating branches; zooecia open on all sides; calcareous base

V I N C U L A R I I F O R M Erect. delicate, (sub)cylindrical, bifur- cating branches; zooecia open on all sides; calcareous base

R E T E P O R I F O R M Erect, rigid, strongly calcified, fenes- trate or reticulate; zooecia open on frontal side; calcareous base

C E L L A R I I F O R M Erect, (sub)cylindrical branches, flex- ible (jointed) with chitinous nodes; zooecia o p e n on all sides; chitinous rootlets

LUNULIT IFORM Disc shaped, unilaminar, heavily cal- cified basal lamina; zooecia open on outer face; setae allow motility

~ C 2 2 3 t

@

@

t

/

Fig. 3. Simplified zoarial growth-form terminology, coding and sketches (not to scale) for cheilostome bryozoans compared with established+ but generally cumbersome, zoarial classification names (in central column). See text for explanation and references.

of bryozoans (> 800) known in the New Zealand fauna. It has proved useful for aiding the classifi- cation of the diverse growth forms in the fragmen- tal skeletal hashes forming the modern New Zea- land shelf carbonates (e.g., Nelson et al., 1982). It has also been applied to growth-form identifi- cations in thin-sections of impregnated sediments and ancient limestones, despite the fact that the same form can vary in appearance with the plane of section, whether transverse, longitudinal or ob- lique. This problem is compensated partly by ref- erence to thin-section "standards" (e.g., Fig. 5)

that reveal the kinds of variation that can be expected under the microscope for any particular growth-form type. With experience and care, it is often possible to distinguish many of the growth forms, although sometimes only with supporting evidence from associated macroscopic bryozoan material. We have been interested more in de- termining the major classes of growth form pres- ent, and their relative abundances, rather than positively classifying every bryozoan fragment in a sample.

Some suggested relations between bryozoan

306

ZOARIAL SHAPE CLASSIFICATION TRANSVERSE LONGITUD. AND APPEARANCE AND DESCRIPTION SECTION SECTION

STO.ATO.O. , .ORM ~-. Encrusting, rarely partly erect; colony

i z Unilaminar linear or flabelliform; tubuliporidean (u0 wa,

C.9 Z ~ . ~ LINILAMINAR DIA$TOPORIFORM ~__ ~ Encrusting, unilam{nar p[ates corn- CO monly building multilaminar zoaria; ::) tubuliporidean wall rr o Multilaminar ATRACTOSOECIFORM

Z (ml) (B) pod shell; (sub)spherical nodal . . . . . . ria encapsulating shell fragment; ceri- 3porid wall

BILAMINAR DIA$TOPORIFORM FolJaceous Erect, foliaceoas, bilamellar colony;

(fo] zooecia irregularly developed on oppo- site sides: tubuliporidean wall

~ HORNERIFORM ~ '~ : I Robust Erect, robust, triangular or rounded branching ~ '~ bifurcating branches; zooecia front or

(ro) lateral: cancellate wall

Erect, delicate, triangular, oval or rounded bifurcating branches; zooecia

ILl front or lateral; attached to substrate by kenozooids; tubuliporidean wall

Delicate ~ PUSTULIPOFIIFORM ~ ~ I I ~ q branching Erect, delicate, subcylindrical branch- l © (de) ,ng stems; zooecia open on all sides

tubaliporidean wall

er ~ . ~ . ~ CAVARID DIASTOPORIFORM @ ~ Erect, delicate, hollow, subcylindrical

~-- branching stems; zooecia open on all Q) sides; tubuliporidean wall

ILl ~ j~s-4-,,i~?j~-~ RETICULATE DIASTOPORIFORM t~ I] ~ ~ 9~ ~(~k Erect, fenestrate, bilaminar fronds; "v~ O D a ~f o 4 LU (re) "°%~t] ~ ~n~" ~ zooecia open in lateral rows; tubuli- ~ >~,~@ poridean wall

Radiate . , ~ Erect, branches radiate from crown of (ra) .~" ' " stem to form a 'cup'; zooecia open on

inner surface; tubuliporidean wall

Massive " . : ' " : - - :~: Erect, massive, branched; zooecia (ma) " ? open on all sides; cerioporid wall

CRISIFORM 1 ERECT ~ 'Erect dedtriangul hortseg- ~ t . . . . . . . . . FLEXIBLE ments separated by chitinous joints:

(EF) zooecia open on one side; tubulipori- dean wall

Fig. 4. Simplified zoarial growth-form terminology, coding and sketches (not to scale) for cyclostome bryozoans compared with established, but generally cumbersome, zoarial classification names (in central column). See text for explanation and references.

zoarial growth forms and preferred environmental habitat conditions are summarized in Fig. 6 fol- lowing Stach (1936, 1937), Lagaaij and Gautier (1965), Schopf (1969), Brood (1972) and Probert et al. (1979), among others. We stress that, where possible, taxonomy is important in refining

(paleo)ecological relationships (cf. Kelly and Horowitz, 1987).

Modern New Zealand examples

An overview of the distribution and nature of carbonate deposits on the modem New Zealand

Rad

iate

4 ,~

.~.

~

ER

EC

T

Rob

ust

I

RIG

ID

Rob

ust

Folia

ceou

s I

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CR

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Mul

tilam

inar

U

nila

min

ar

I

r

'-rl

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ING

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enes

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Unilarninar [ul]

Multi- [A] laminar

[B] © [ml] z w (c}

ZOARIAL GROWTH FORM

S H A P E CHEILOSTOMATA CYCLOSTOMATA

Mernbraniporilorm Stomatoporiform

Bilaminar [fo] foliaceous

~- Robust (ro) branching

D e l i c a t e C branching C [de] E Fenestrate

(re) Radiate

[ra) L, Massive

(ma)

Celleporiform A

FREE-LIVING (FL)

Celleporiform B

Celleporiform C

Eschariform

Adeoniform

Vinculariiform

Reteporiform

Unilaminar diastoporiform

Atractosoeciform

Bilaminar diastoporiform

Horneriform

Idmidroniform Pustuliporiform

C~varid dias[oponform

Reticulate diastoporiform

Lichenoporiform

PREFERRED ENVIRONMENTAL CONDITIONS

SUBSTRATE Hard Particul Flexible

i i i , • ©

0 •

WATER ENERGY LOW Moderate High

0 (!I •

O • O 0 3t (11

SEDN. RATE Low Moderate High

0 ' '

SHELF DEPTH Inner Middle Outer

i i i • ©

• 3t • ~ ~t (D •

(11 • 0 • O

(11 • (1t •

(11 31

0 • 0

• 0

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• 0

• 0

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• 0

• 0

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0 • 0

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O • 0 0 • O 31 (!) Cerioporiform

ERECT FLEXIBLE Cellariiform Crisiform O • (~ • (~ (~ (~ • • ~ (~) (EF]

Lunulitiform [ • O

309

• Very common (~Comrnon (~Moderately common

Fig. 6. Some generalized relations between the zoarial growth form of bryozoans and their preferred environmental habitat conditions. See text for explanation and references.

shelf is provided by Nelson et al. (1988b). Bryozoans are important and commonly the domi- nant sediment contributors (Fig. 2A), their abun- dance relating mainly to suitable substrate availa- bility and, to a lesser extent, hydrologic regime. Over 70 species have been identified as dominant or subdominant types (Keane, 1986), the more important of which are listed with their growth- form type(s) in Table 1. Overall, cheilostomes dominate over cyclostomes, typically forming 70-80% of the main bryozoan species in the carbonate-dominated shelf sectors (Keane, 1986), although locally, as on the Otago shelf (Fig. 1; Probert et al., 1979; Carter et al., 1985), cyclo- stome colonies are numerically dominant.

Three Kings platform, northern New Zealand (34 °S)

Nelson et al. (1982) describe the submarine morphology and bottom sediments of the exten- sive (> 10,000 km 2) carbonate platform centred

on Three Kings Islands off northernmost New Zealand (Fig. 1). The content of bryozoans in the skeletal fraction of the carbonate sediments is shown in Fig. 7A. Maximum abundances occur on and about the rugged sea floor associated with the offshore banks and islands in depths of 50-300 m, as well as immediately off the far north of North Island at depths of 60-80 m. A map of the domi- nant bryozoan growth forms in samples (Fig. 7B) demonstrates that particular forms dominate dif- ferent areas of the platform, presumably reflecting particular combinations of ecological and environ- mental conditions (e.g., see Fig. 6). We presently have little information on the hydraulic and sedi-

mentary processes operating at the sea floor over Three Kings platform, other than the existence of strong tidal flows, frequent storms and prominent areas of up-welling currents in the region (Nelson et al., 1982), and the topographic and substrate diversity of the bottom (Summerhayes, 1969: Nel- son et al., 1982). Consequently, specific environ-

Fig. 5. Photographs (left; underlying bar scales represent 5 mm) and photomicrographs (dark and light backgrounds in reflected and transmitted light, respectively; bar scales represent 0.5 mm) of a selection of zoarial growth-form types in the fragmental bryozoan component of skeletal carbonate samples from Three Kings platform, New Zealand (see Fig. 1). Only the reflected-light thin-sections were prepared directly from the materials photographed at left. Cyclostome bryozoan material (Fig. 4) represented in E, F, H and I: the remainder are mainly cheilostomes (Fig. 3).

310

TABLE 1

Some of the more important bryozoan species contributing to modem carbonate deposits on the New Zealand shelf

Bryozoa a Type b Distribution ¢ Zoarial growth form(s)

Galeopsis polyporus (Brown) Ch V

Hippellozoon novaezelandiae (Waters) Ch V

Mecynoecia purpurascens (Hutton) Cy V

Telopora digitata (Busk) Cy V

Celleporina spp. Ch V

Celleporaria spp. d Ch M

Foveolaria (Odontionella) cyclops (Busk) Ch M

Galeopsis grandiporus (Waters) Ch M

Cellaria imrnersa (Tenison-Woods) Ch M

Cellaria tenuirostris (Busk) Ch M

Hippomenella vellicata (Hutton) Ch M

Hornera spp. Cy M

Cinctipora elegans Hutton Cy M

Idmidronea sp. Cy M

Otionella squamosa (Tenison-Woods) Ch M

Otionella affinis Cook & Chimonides Ch R

Adeonellopsis yarraensis (Waters) Ch R

"'Idmonea" sp. Cy R

Steginoporella neozelanica (Busk) Ch R

Steginoporella perplexa Livingstone Ch R

lodictyum yaldwyni Powell Ch R

Phidolopora avicularis (MacGillivray) Ch R

erect delicate

erect fenestrate

erect delicate

erect radiate

encrusting multilaminar

encrusting multilaminar

erect delicate, erect robust or encrusting unilaminar

erect delicate

erect flexible

erect flexible

erect bilaminar foliaceous

erect robust or erect fenestrate

erect delicate

erect delicate

free-living

free-living

erect robust

erect delicate

erect flexible

erect bilaminar foliaceous or encrusting unilaminar

erect fenestrate

erect fenestrate

a Most of these species are illustrated in Nelson et al. (1988b, fig. 6).

b Ch: cheilostomatous bryozoans; Cy: cyclostomatous bryozoans.

" V: very widespread; M: moderately widespread; R: reasonably widespread.

d Particularly C. cf. tridenticulata (Busk) and C. agglutinans (Hutton).

mental conditions controlling the growth-form distribution patterns remain uncertain and can only be inferred from the habitat relationships suggested in Fig. 6. For example, the broad west- to-east transition at Three Kings from the pre- dominance of delicate branching (e.g., Galeopsis polyporus) to fenestrate (e.g., Hippellozoon novaezelandiae) and (or to) encrusting (e.g., Cel- leporina spp.) growth forms relates to the increas- ing amount of water agitation as depth decreases across the platform, while free-living varieties (e.g, Otionella affinis and O. squamosa) are con- centrated only in an area of more mobile, finely particulate, mixed terrigenous-skeletal sands in water depths less than about 100 m on Reinga Shelf.

A complicating factor on Three Kings platform is the occurrence of both modern and relict skeletal deposits, the latter associated with the lower stands of sea level accompanying and following the

culmination of the Last Glaciation (Nelson et al., 1982; Nelson and Hancock, 1984). While relict sediments generally dominate at the deeper-water sites (over 100 m depth on Reinga Shelf and over 200 to 250 m elsewhere on the platform; Nelson et al., 1982), large tracts of the sea floor contain admixtures of both modem and relict bryozoan material. By considering only the modern bryozoan components in samples from a range of depths on South Maria Ridge, Hancock (1980) found that the peak abundance of different growth forms varies with depth: 0-30 m, unilaminar encrusting (e.g., Steginoporella magnifica); 20-50 m, multi- laminar encrusting (e.g., Celleporina and Cel- leporaria spp.); 50-80 m, erect fenestrate (e.g., Hippellozoon novaezelandiae, Iodictyum yaldwyni and Phidolopora avicularis); 70-160 m, erect rigid delicate (e.g., Galeopsis polyporus and G. gran- diporus); and 160-200 m, erect rigid delicate to robust (e.g., "Idmonea" sp., Hornera sp. and

i i i . ~ ! 73"E

:..'%'.. .10: %%

En:'ustln~ [!TC Em ;t rigK

I - Z F E~le×

Fr el,-hvlnc

Fig. 7. Percent abundance of bryozoans (A) and dominant zoarial growth-form types (B) in skeletal carbonate deposits on Three Kings platform, northern New Zealand. Adapted from Keane (1986). NMR = North Maria Ridge; SMR =South Maria Ridge; TK = Three Kings Islands; RS = Reinga Shelf (see Nelson et al., 1982).

Metroperiella mucronifera). Levels of m a x i m u m

abundance of relict grains with these growth forms

typical ly occur 100-150 m below their mode rn

coun te rpar t s (Nelson and Hancock , 1984), sup-

por t ing their fo rmat ion dur ing the low sea-level

s tand accompany ing the Last Glac ia l max imum.

In conjunc t ion with da t a f rom other sources,

Hancock (1980) used this in fo rmat ion to suggest a

general scheme relat ing the relat ive abundance of

g rowth- form types in the b ryozoan c o m p o n e n t of

Three Kings sediments to depth, essent ial ly as it

relates to var ia t ions in the degree of open-wate r

agi ta t ion (Table 2). The scheme highl ights the

wide "env i ronmen ta l " range and over lap of

specific g rowth- form types and emphasizes that it

is p r inc ipa l ly the associa t ion and relative abun- dance of forms that are l ikely to be of most value

for provid ing paleoecological in format ion .

Otago shelf southeastern New Zealand (46 °S)

The mixed te r r igenous-carbona te sed iments on

the open, na r row (10-30 km) shelf off Otago

Peninsula, southeas tern South I s land (Fig. 1), are

d i s t r ibu ted in four major shore-para l le l facies

311

(Andrews, 1973; Car te r et al., 1985): (1) an inner-

shelf bel t ( 0 - 7 0 m dep th ) of mode rn terr igenous

sand (and mud) ; (2) a middle -she l f belt (30 -80 m

depth) of rel ict te r r igenous gravel; (3) a middle- to

ou te r - she l f s t r ip ( 3 0 - 1 2 0 dep th ) of r e l i c t /

pa l impses t te r r igenous sand; and (4) a (middle- to)

outer -shel f zone (50 -120 m depth) of mixed mod-

ern and relict skeletal sand and gravel (Fig. 8).

Car te r et al. (1985) e loquent ly expla ined the devel-

o p m e n t of these j u x t a p o s e d cont ras t ing sediment

facies as a response to a succession of sea-level

s t i l l s t ands dur ing the pos t -g lac ia l t ransgression,

augmen ted and modi f i ed by the modern hydraul ic

regime since abou t 6500 years B.P. The hydraul ic

regime and sed imen t - t r anspor t mechanisms are

much be t te r known than at Three Kings, with

p r e d o m i n a n t l y nor theas te r ly dispersal of sediment

under the c o m b i n e d inf luence of s t rong souther ly

swells, tides, the Sou th land Curren t (mixing sub-

antarc t ic and sub t rop ica l oceanic waters) and

s to rm- induced mot ions (Fig. 8). The inner shelf is

ba thed by neri t ic water of more var iable tempera-

ture and lower sa l in i ty than the mid-ou te r shelf,

where the Sou th land Curren t is character ized by

salinit ies in excess of 34.5%0 and a water t empera-

ture over 9 . 5 ° C (Prober t et al., 1979).

TABLE 2

Variations of major bryozoan growth-form associations with water depth (presumably reflecting mainly environmental en- ergy level) in the vicinity of South Maria Ridge (see Fig. 7A), Three Kings platform (after Hancock, 1980)

Depth (m) Bryozoan growth-form associations a

Intertidal 0- 10

10- 25 25- 50 50- 75 75-100

100-125 125-150 150-175 175-200

ENul ENul > ENml > EF ENml > ENul > ERde > ERro > ERfe ENml > ERde > ENul > ERfe > ERro ERde > ENml > ERfe > ENul > ERro ERde > ENml > ERfe > ENul > ERro ERde > ENml > ERro > ENul > ERfe ERde > ERro > ENul > ENml > ERfe ERro = ERde > ENul > ENml > ERfe ERro > ERde > ENul > ENml > ERfe

ENuI: encrusting unilaminar (membraniporiform); ENmI: encrusting multilaminar (celleporiform); EF: erect flexible (cellariiform); ERde: erect rigid delicate (vinculariiform); ERro: erect rigid robust (horneriform); ERfe: erect rigid fenestrate (reteporiform).

312

~ ~ ~ I Canyon headwalls (200-500m)

Mu 77/43 (109 m) ~-~ r -~

r =

P153 (92 m)

-~ Pt52 (69 m) ~

_ ~ - ~ ~-, ~

{ ~ " I Tide-dominated shelf 0 <: m O

~_...~ Sand wave field ~ ~ -~ ~ o o ~ Sand ribbons z z z z ~ ~ ~ ~ ~j

[ _ ~ Gravel ridges Bryozoan Zoarial Forms

Fig. 8. Schematic diagram (after Carter et al., 1985, p. 40, fig. 20) of sediment facies and hydrodynamic conditions on the Otago shelf (see Fig. 1) showing also the nature of bryozoan zoarial growth forms in samples from a selection of mid-outer shelf and canyon head localities. Data from Hyden (1979). Letter abbreviations for growth forms defined in Figs. 3-6.

The CaCO 3 content of the sediment gradually increases offshore across the facies belts, reaching as much as 75% in the mid-outer shelf carbonate deposits. Bryozoans are widespread skeletal con- tributors in all sediments of the middle and outer shelf (Fig. 2A), and often dominate the fauna to such a degree that their prolific growth has mod- ified the gross structure and morphology of the substrate and provided support and shelter for marly other organisms (Probert et al., 1979).

Two distinct bryozoan assemblages can be rec- ognized, though the distribution of colony forms across the shelf has not been studied in detail (Fig. 8). The two groups correspond to the middle-shelf tongue of coarse relict gravels and the slightly deeper mid-outer-shelf belt of gravelly muddy sand. The gravel-dominated mid-shelf belt is dominated by Cinctipora elegans, a delicate branching cyclostome (ERde) that attaches to the larger pebbles by means of a calcareous base. Two other rigid branching cyclostomes are also com- mon (i.e., Telopora digitata (ERra) and Hornera

robusta (ERro)), together with the bilaminar folia- ceous cheilostome Hippomenella •ellicata (ERfo). Many of the pebbles are encrusted by unilaminar cheilostomes. Box cores and bottom photographs show that the colonies can rapidly become com- pletely interwoven and stabilized, reaching diame- ters of 20 cm or more. Large clumps are sporadi- cally distributed over the pebbly sandy substrate with abundant fragmented colonies accumulating in the interstices.

The mid-outer shelf belt of carbonate-rich sedi- ment is dominated by Hippomenella oellicata (ERfo), the erect flexible Cellaria immersa and the multilaminar branching Celleporina sp., but Cinc- tipora elegans (ERde) is also very common (Fig. 8; station 77/43). Cellaria attaches to particulate substrates by means of chitinous rootlets; Cel- leporaria commonly encrusts flexible, non-calci- fied substrates, often becoming secondarily free-living; Hippomenella can start life by encrust- ing hard, gravel-sized particles surrounded by mud. Clearly, the presence of extensive hard substrate is

not critical in the slightly deeper, finer-grained

substrates. Support is not the only limiting factor, how-

ever, because on the tongue of mid-shelf gravels bryozoan dominance is not achieved south of Otago Peninsula. Probert et al. (1979) ascribe this distribution to the strength of the Southland Cur- rent. As it passes the Otago Peninsula, the Current is constricted by inner neritic water and the off- shore subantarctic water, thus increasing its veloc- it,, and providing a greater nutrient supply for the bryozoan colonies.

Hermit bryozoans (ENmlB in Figs. 3 and 4) are common throughout the middle to outer shelf.

Both cheilostome and cyclostome species are rep- resented, encrusting gastropod shells and calcare- ous worm tubes that are inhabited by hermit crabs. The widespread distribution of these bryozoans can be attributed to a suitable (if mo- bile) substrate and a constantly replenished nutri- ent supply.

Beyond the shelf, on pebbly muddy substrates at depths of 200-300 m or more in canyon heads, muhilaminar, free-living hemispherical colonies up to 3 cm in diameter are conspicuous (Celleporaria cf. tridenticulata), together with multilaminar branched forms, chitinous-rooted bilaminar folia- ceous forms and erect delicate cheilostome forms.

Snares platform, southern New Zealand (48 °S)

Snares platform, over 40,000 km 2 in extent, is

the largest area of skeletal carbonate deposition on the New Zealand shelf (Fig. 1; Nelson et al., 1988b). Bryozoan remains are the major sediment contributor over much of the platform (Fig. 2A), increasing in abundance offshore to the south (Fig. 9A; Head, 1985). Spatial variation is evident in the distribution of the dominant bryozoan growth forms (Fig. 9B). Overall, erect rigid del- icate bryozoans are the most widespread type, concentrated in the coarse skeletal hashes blanket- ing the central and southern portions of the plat- form in 100-150 m water depth. The important species (all ERde) include the cheilostomes Galeopsis grandiporus, G. polyporus and, less com- monly, Foveolaria (Odontionella) cyclops, and the cyclostomes Cinctipora elegans and Mecynoecia

313

1 ~ t e w a r t I. 7 -47°S

• . . 25

0 10 20 30 "

- / . . / " \ . ~ 5 o

l . / . . . . Snar s,sJ_..___ - - - - - - 75 '

16, E / l

Encrusting Multilaminar

Erect rigid Dehcate branching RObUSt branching

Erect~flex~ble I

Free4ivlng Comolex

Fig. 9. Percent abundance of bryozoans (A) and dominant zoarial growth-form types (B) in skeletal carbonate and mixed terrigenous-carbonate deposits on Snares platform, southern New Zealand. Adapted from Head (1985) and Keane (1986).

purpurascens. Along the western margin of the platform, where the prevailing strong westerly drift induces high environmental energy conditions at the shelf edge (Head, 1985), erect robust (Hornera spp.) and encrusting multilaminar (Celleporina spp.) bryozoans are notably dominant. Not un- commonly, the latter are manifest as hermit bryozoans, encrusting small gastropod shells (type ENmlB). Due east and west of Stewart Island the bottom sediments are current-tippled, fine to medium terrigenous (and mixed carbonate-terrig- enous) sands. Here the bryozoans are predomi- nantly erect flexible (Cellaria immersa and C. tenuirostris) and free-living (Otionella spp.) growth forms, reflecting both the more persistent degree of current agitation and sediment shifting in shoal waters.

Ancient N e w Zealand examples

New Zealand Cenozoic limestones are all non- tropical deposits (Nelson, 1978a). The majority

314

contain common to abundant bryozoan remains in their skeletal fraction, just like their modern analogues (Fig. 2B). The bryozoans are seldom in situ, and only rarely are large pieces of broken or tumbled colonies conspicuous in outcrop. Most material is considerably fragmented and small in size, and while evident with the naked eye or with a hand lens in the less indurated, more porous carbonates, it is more difficult to identify in hand-specimens of well-cemented "crystalline" limestones. However, in thin-section, the bryozoan remains are usually conspicuous because of their zooecial chambers, typically filled with calcite spar and/or carbonate mud. By noting the general relations between the geometry of bryozoan frag- ments in random sections and their growth form (Figs. 3-5) it is often possible to roughly gauge the contribution made by the various zoarial types in even the most indurated of the limestones.

Previous New Zealand studies using bryozoans and their growth forms to assist with the paleoeco- logical and paleoenvironmental analysis of sedi- mentary formations include those of Brown (1952), Leitch et al. (1969) and Crabb (1971). Here we give three examples of applications in limestones of Late Oligocene-Early Miocene age in southern South Island, northern South Island and western North Island, respectively (Fig. 1).

Forest Hill Formation, southern South Island (Fig. /)

Mid-Tertiary temperate-water shelf carbonates are extensively developed in southern South Island of New Zealand and provide excellent examples of bryozoan-dominated sediments (Fig. 2B) that can be matched with similar bryozoan-dominated as- semblages accumulating on the Otago shelf today (Fig. 8).

The stratigraphy of the Forest Hill Formation and associated rocks has been described by Hyden (1979, 1980). Essentially they comprise a sequence of bryozoan-dominated, carbonate and mixed carbonate-terrigenous sediments (Fig. 10) de- posited on a narrow shelf that prograded west- wards across an infilled fault-bounded basin to- wards the larger Waiau Basin. The Forest Hill Formation overlies shallow-water glauconitic

8 E

H co ~ ~ 3 ~ ~ • • w * a) Inferred ~ q ~ ' - ~ . ~ uJ Y. . . . . . . . . . . environment ~ ~ : ; 60LMid-Outer Otago Shelf (67%)~

0 20

F o ,,oF ~

-~ . ~ C 60#F[70%) ~ 4

20 ~ cl '~ 0

60

l - -" ) (Late OlEjoJNene)

Fig, 10. Distribution of bryozoan zoarial forms within the

Forest Hill Formation, southern South Island (see Fig. 1) and

a comparison with Recent bryozoan faunas on the Otago shelf

(uppermost histogram; see also Fig. 8). Column about 40 m

thick. Bracketed percentages indicate the abundance of

bryozoans in the skeletal fraction of samples A to H. Data from Hyden (1979).

sandstones and sandy limestones of the Chatton Formation and its correlatives (Fig. 10). The basal sediments of the Forest Hill Formation (Sharks Tooth Hill Member) are often conglomeratic, rep- resenting channelized and sheet-like conglomer- ates that incised into or spread out across the shallow, glauconitic shelf. Once a bryozoan- brachiopod and thence bryozoan-dominated as- semblage became established through an ecologi- cal succession, bryozoans and associated faunas covered considerable areas of the sea floor (Woody Knoll Member). Subsequent shoaling was re- flected in a gradual increase in terrigenous mud and sand towards the top of the formation.

Quantitative analysis of bryozoan growth forms was undertaken by Hyden (1979) on wind- and

water-etched bedding planes and vertical surfaces in coarse bryozoan limestones, using a 10 cm grid square, and on disaggregated samples of bryozoan-rich terrigenous mudstones. Three en- crusting and six erect growth forms have been recognized. The encrusting forms are unilaminar (ENul) or multilaminar, the latter encrusting flexi- ble substrates (ENmlA) or encrusting gastropod shells inhabited by hermit crabs (ENmlB). Five of the erect forms are rigid (i.e., bilaminar foliaceous (ERfo), robust (ERro) and delicate (ERde) branching, fenestrate (ERfe) and massive (ERma)); the sixth erect growth form has flexible, jointed branches (EF).

Erect delicate branching bryozoans are over- whelmingly the dominant growth form in the stratigraphic succession, with a significant contri- bution of bilaminar foliaceous colonies, particu- larly in the terrigenous mudstones (e.g., levels D and G, Fig. 10). This distribution agrees well with that for the bryozoan-dominated faunas accu- mulating on the modern Otago shelf (Fig. 8; cf. also typical Otago shelf distribution in Fig. 10), with the analogy between delicate branching col- onies in the (pebbly) bryozoan limestones and the mid-shelf gravels, and bilaminar foliaceous col- onies in the terrigenous mudstones and outer shelf gravelly muddy sands. Support in the bryozoan limestones was probably provided by fragmented bryozoan skeletons.

The distribution of zoarial forms within the bulk of the Forest Hill Formation is thus con- sistent with mid-outer-shelf depths and mod- erately strong, mostly unidirectional currents that promoted a good nutrient supply and which grad- ually fragmented and locally redistributed the skeletons to provide support for successive col- onies. It is noteworthy that the distinctive bryozoan assemblage at nearby Castle Rock (Fig. 1), where correlative limestones occur, matches the equally distinctive and characteristically shallow-water as- semblage of decapod crustaceans, large for- aminiferids (mostly amphisteginids), large cidarids and coralline algae. The bryozoans, whilst still dominated by erect, cylindrical forms, contain a significant component of heavily calcified fenestrate zoaria and large (up to 48 cm diameter) multilaminar foliaceous zoaria. Both growth forms

315

could withstand high-energy conditions, consistent with shallower water than for the more southerly Forest Hill Formation limestones.

The distinctive hermit-bryozoan growth form (ENmlB) occurs throughout the formation, as it does on the modern Otago and Snares shelves. Hermit bryozoans are particularly abundant and even dominant in the basal (non-conglomeratic) shelly, glauconitic, sandy limestones of the Forest Hill Formation (except at Castle Rock). Their success probably reflects opportunistic qualities of the symbiotic relation between hermit crab and bryozoan colony and the fact that the bryozoans are not reliant on a suitable bottom topography and substrate or on a constant nutrient supply.

Takaka Limestone, northern South Island (Fig. 1)

The Takaka Limestone is a skeletal-rich shelf deposit which formed in the final stages of a New Zealand-wide marine transgression in late Oligo- cene time. It was deposited on a structurally stable plateau surrounded by relatively deep-water basins (Leask, 1980; Nathan et al., 1986). The formation rests unconformably on Paleozoic basement or paraconformably on small basins of Eocene Brunner Coal Measures. Typical facies sequences range from calcareous sandstone up to mollusc- dominated limestone (both packstone and grain- stone) grading up to bryozoan-dominated lime- stone (grainstone) (Fig. 2B). The top of the forma- tion is an abrupt, although probably conformable change to muddy sandstone and mudstone of the Miocene Tarakohe Mudstone.

The limestones are strong, well-cemented rocks which have undergone intense compaction and pressure-solution. While occasional very large, robust bryozoan colonies can be observed in out- crop, most bioclasts are in the 0.5-5 mm size range, and therefore thin-section examination is essential for their identification. The difficulty of extracting microfossils has resulted in inadequate age control and paleoecological data, and in most facies bryozoan growth forms offer the best source of potential paleoecological information.

Leask (1980) recognized eight lithofacies of the Takaka Limestone, based on field, polished slab and thin-section observations (Table 3). Facies

316

TABLE 3

Facies of the Takaka Limestone (after Leask, 1980) in northern South Island (Fig. 1)

Facies Description Average Average % bioclast bryozoans grain size in skeletal (mm) fraction

1 glauconitic limestone 0.l 0.5 5 2 calcareous coarse

sandstone 0.5-1.0 < 1 3 mollusc packstone 0.5-4.0 60 4 bryozoan-bivalve

grainstone 0.5-3.0 40 5 bryozoan grainstone 0.1-10 80 6 algal packstone 0.1-50 40 7 fine sandy calcarenite 0.2-0.3 5

8 foraminifer-echinoderm grainstone 0.5 0

1-6 are listed in generalized order of younging, although at no section do all six occur together. Facies 7 and 8 form a distinct facies association in the northwest of the Takaka Limestone region. Here we examine bryozoan assemblages from rep- resentative measured sections of the two contrast- ing facies associations (Fig. 11),

Bryozoan zoarial forms in the lower Takaka valley (Fig. 1) are dominated by erect rigid, robust and delicate branching forms, with multilamellar encrusting and erect fenestrate forms making only minor contributions (Fig. l lA). Cyclostome col- onies appear to exceed cheilostomes numerically by a ratio of 2 :1 , although the possibility of abrasion and pressure-solution favouring pres- ervation of the more heavily calcified cyclostorne colonies cannot be overlooked. The dominance of delicate forms in the lower half of the formation is gradually replaced by a dominance of robust forms in the upper part, especially in facies 5. Robust forms are dominated by Horneridae and, towards the top of the formation, cylindrical branching Heteroporidae. The latter range up to 20 mm diameter and up to 150 mm in length.

The older facies (2, 3 and 4) contain moderately well sorted and abraded bioclasts which are inter- preted as having been intensively winnowed and transported across a current-swept shelf. It could be argued that the paleoecological value of

bryozoan growth forms in such facies is limited as they could have been transported from different shelf environments. However, the dominance of

delicate growth forms and the presence of delicate erect flexible forms are noteworthy in an environ- ment which could have favoured bryozoan forms with greater preservation potential and therefore

A

g

Tara :ohe Mud ;tone ~

: I T 5C ~ ' - ~

4 0 - ~ o" N ~ " co

E - - T

3C J

T 215_ ? , i ,

2 C - ~: - -

F- z - : ~

1C - - = . k ,an

Bru Iner C~ a ~

Mea

N

s ~ - "6 &

o

50

~ ~ o - - - - - o

i o ~

.

_ I ~ U _ I ~ 7 ~ --':

~ . ~ .

loo

50

~1oo

4150

Fig. 11. Distribution of bryozoans and their zoarial forms within the Late Oligocene-Early Miocene Takaka Limestone of the lower Takaka (A) and middle Aorere (B) valleys, northwestern South Island (see Fig. 1). Facies defined in Table 3 and abbreviations for growth forms in Figs. 3-6. The upper- most histogram in A is based on cheilostome specimen counts by Brown (1952): the remaining data are from Leask (1980).

we accept that these forms are reasonably reliable paleoenvironmental indicators.

Facies 5 bioclasts are characterized by poor sorting, a relative lack of abrasion and abundant large intact colonies in a mixed packstone-grain- stone fabric, and are interpreted as an in situ "bryozoan meadow" community. The gradation from delicate to robust growth forms in the same interval as waning current intensities from facies 4

to facies 5, suggests a gradual deepening of the sea floor. Using the modern Three Kings platform as

an analogy (Table 2), mid-shelf depths are sug- gested for facies 4, and mid-outer-shelf depths for facies 5.

Immediately overlying the Takaka Limestone is a 2 m thick bed of friable muddy-sandy limestone, enshrined in the literature as the "Bryozoan Bed" (Brown, 1952). This contains a similar bryozoan- dominated fauna, and is interpreted as an in situ bryozoan meadow community gradually choked by terrigenous sediment. Brown (1952) records 35 cheilostome species in this bed but makes only passing reference to the cyclostome species. In- creasing proportions of encrusting hemispherical and erect flexible growth forms may indicate rapidly shallowing water depths, possibly during an episode of global sea-level fall. Bryozoans are absent in the overlying Tarakohe Mudstone.

The Takaka Limestone in the Aorere valley (Fig. 1) is dominated by the fine-grained, sandy facies 7 and 8, although coarse- grained bryozoan- and mollusc-dominated facies 3, 4 and 5 occur locally (Fig. 11 B). The formation rests directly on Paleozoic basement, with multilamellar encrusting bryozoans colonizing the wave-cut rock platform. The overlying facies 3 packstones are dominated by robust forms, both Horneridae and cheilo- stomes, but this facies also contains a higher pro- portion of erect flexible, fenestrate and bilamellar foliaceous forms than encountered elsewhere.

Facies 3 is overlain by facies 7 which is essen- tially devoid of bryozoans until the upper few meters of the formation. From there into the lower part of the Tarakohe Mudstone, common thin shell beds are crowded with brachiopods and large inverted-bowl shaped, multilaminar bryozoan col- onies. These have been identified as Celleporaria papiUosa Tenison-Woods or "Cellepora" sp. 2

317

(Brown, 1952). With maximum dimensions of 70 mm diameter and 30 mm height, these colonies are considerably larger than typical lunulites, but nevertheless appear to be free-living forms. They occur in jumbled, disorientated accumulations and have obviously been flipped over, rolled or sus- pended during periodic high-energy events and concentrated in lag deposits. A possible modern analogue is the occurrence of similar colonies of Celleporaria of. tridenticulata about the canyon heads incising the outer Otago shelf (Fig. 8).

Te Kuiti Group, western North Island (Fig;. 1)

The stratigraphy of the Oligocene Te Kuiti Group is discussed by Nelson (1978b). Rocks are mainly mixed terrigenous-carbonate and purer carbonate deposits (40-100% CaCO)) that accu- mulated at shelf depths (< 200 m) in a partly land-locked, tide-dominated embayment or sea- way (see Nelson, 1978a, figs. 6 and 7). The upper- most four formations of the Group, well exposed in the vicinity of the well known glow-worm caves at Waitomo (Fig. 1) where they have an aggregate thickness of about 100 m, comprise 40-80% skeletal carbonate grains (mainly bryozoans, echinoids, benthic foraminifers, barnacles and oysters; Fig. 2B), between 25 and 80% of which is bryozoan material (Fig. 12).

The rocks are tightly cemented (see Nelson et al., 1988a) and estimates of the major growth

forms contributing to the bryozoan count can usually only be gauged from careful, often tedious, thin-section analysis (cf. Fig. 5). As many as eight growth forms have been identified: unilaminar encrusting; multilaminar encrusting; erect bi-

laminar; erect robust; erect delicate, erect fenestrate; erect radiate; and erect flexible (Nel- son, 1973; Hancock, 1980). Free-living bryozoans (lunulites) appear to be absent, possibly because, like their modern counterparts (Nelson et al., 1988b), they secreted an aragonite skeleton, and most shell aragonite was dissolved from the Te Kuiti Group sediments before they were lithified (Nelson, 1978a). Following point-counting of thin-sections, Hancock (1980) ranked the growth forms in order of numerical importance (Table 4) to enable paleodepth trends as related to degree of

318

Otorohanga Limestone

c

O O o~ Waitomo © Sandstone

d 2 Orahiri

Limestone "O

Aotea Sandstone

OtClr

OtB2}

'OtA21

OtA31

Wt I

IOrB1 i

!ORB2)

0 50 100 % Bryozoans in skeletal fraction

I L I i

i

C~ ~ and paleontology

I 1 [ ,

Inner - Mid - Outer shelf depths (0] (50] (100) [150] [200]

Possible water depth (m)

Fig. 12. Stratigraphic variation in the abundance of bryozoans in lithofacies (see Nelson (1978b) for descriptions) within the upper

four formations of the Oligocene Te Kuiti Group near Waitomo, western North Island (see Fig. 1). At right a comparison is made

between the depths of sediment deposition inferred from certain bulk-rock properties (Nelson, 1973) with those based on the

variations in bryozoan growth forms given in Table 4 and interpreted in terms of the energy-related water-depth zonations recorded

in Table 2 for modern bryozoans on South Maria Ridge, Three Kings platform (see Fig. 7A). See text for discussion.

TABLE 4

Average ranking of major types of bryozoan growth forms in

thin-sections of samples from the Oligocene Te Kuiti Group

(see Fig. 12). Data from Hancock (1980)

Formation Bryozoan growth-form associations a

Otorohanga Limestone

OtCl ERro > ERde > ERfe > ENml > ENul

OtB2 ERro = ERde > ENml > ERfe > ENul

OtA2 ERro = ERde > ENml > ERfe > ENul

OtA3 ERde > ERro > ENml > ERfe > ENul

Waitomo Sandstone

Wt ERde > ERro = ENml > ERfe > ENul

Wt ENml > ENul > ERfe > ERde > ERro

Orahiri Limestone

OrB1 ENml > ERde > ERro > ENul > ERfe

OrB2 ENml > ERde > ERro > ERfe > ENul

OrA5 ERfe > ERde > ENml > ERro > ENul

OrA3 ERde > ERfe > ENml > ERro > ENul

A otea Sandstone

Ao-5

Ao-2

ERfe > ERde > ENml > ERro > ENul

ERde > ERfe > ENml > ERro > ENul

~' ENuI: encrusting unilaminar (membraniporiform); ENmI:

encrusting multi laminar (celleporiform); EF: erect flexible

(cellariiform); ERde: erect rigid delicate (vinculariiform);

ERro: erect rigid robust (horneriform); ERfe: erect rigid

fenestrate (reteporiform).

water agitation to be estimated on the basis of the modern data recorded from Three Kings platform (Table 2).

Results are summarized in Fig. 12. At least two major shallowing-upward cycles are suggested, within the Aotea Sandstone and the Orahiri Limestone-Waitomo Sandstone, followed by rapid deepening during deposition of the Otorohanga Limestone. Previous estimates of the depositional depth of these sediments, made on the basis of textural and limited paleontological information (Nelson, 1973, 1977, 1978b), are shown by the open circles and fine line. While broadly similar in trend to the bryozoan growth-form curve, the latter suggests overall deeper-water shelf environments and generally greater variations in the depth of deposition between several of the stratigraphic units. The analogy with the modern depth (en- ergy)-related habitats at Three Kings platform (Table 2) remains to be critically assessed because we know that other factors, such as substrate and sedimentation conditions, also strongly influence growth-form development (Fig. 6). Nevertheless, the systematic changes in the proportion of differ- ent bryozoan growth forms must reflect important

paleoenvironmental changes in time and space in these widespread Oligocene carbonate deposits, changes that would mainly be masked if the major bryozoan component was simply collectively grouped as "undifferentiated Bryozoa', as has previously been the case (e.g., Nelson, 1978b).

Conclusions

Interpretation of paleoenvironments and paleo- communities using bryozoan colonial morpholo- gies can utilize two approaches--systematic diver- sity (numbers of species) and numerical abun- dance (numbers of colony fragments or of individ- uals). Gordon (1987) has underscored the value of taxonomy in this regard in his analysis of species numbers in a variety of Recent habitats from the New Zealand region. For example, for hard-bot- tom areas of sea floor (rocky and terrigenous or shell gravel) he consistently found about 70% of the living bryozoan species to be "two-dimen- sional" encrusters, whether at one station or over a large area, and about 10% to be erect-rigid species (the balance being made up of shell-bor- ing, free-living and a variety of rooted and /o r flexible species). For the New Zealand region at least, these results appear to have predictive value in terms of what one might expect to find for any area of hard bottom which has reasonable diver- sity or may be well sampled. Since many extant New Zealand bryozoan species have long ranges throughout the Cenozoic, this has important pre- dictive value to paleodiversity.

However, this approach does not include infor- mation about numerical abundance of the various colony morphologies, irrespective of species num- ber, and therefore cannot be a predictor of the environmental setting. Here, the second approach, not dependent on a precise taxonomy, and there- fore useful to the non-specialist, is of importance. This is the approach dealt with in this paper. particularly in relation to temperate-shelf, bryomol carbonate deposits. While much more data are needed, it appears that the relative abundances of different bryozoan growth forms in the skeletal fraction of carbonate-dominated sediments on the modern New Zealand shelf can be related broadly to environmental habitat conditions. The occur-

319

rence of comparable assemblages of bryozoan col- onial forms in New Zealand's bryozoan-dominated Tertiary limestones highlights the potential of growth-form studies for interpreting the deposi- tional paleoenvironments of the limestones. We believe the four-part scheme of classifying bryozoan colony form presented here. with its simplified terminology, provides a utilitarian ap- proach to evaluating morphology in relation to environmental setting. The same four-part scheme is useful also in correlating colony morphology with taxonomic diversity (Gordon, 1987).

Acknowledgements

We thank several people, including particularly Keith Probert (Portobello Marine Laboratory, Dunedin), Roger Grace (Biological Consultant, Auckland), Jack Grant-Mackie and Ron Whitten (University of Auckland, Auckland), Bob Carter (James Cook University, Townsville) and Peter Barrett (Victoria University, Wellington), for dis- cussion and advice about bryozoans and for en- couraging us to persist with the application of zoarial growth-form analysis as a potential (paleo)environmental tool in temperate-region carbonate studies. Access to the unpublished the- sis information on bryozoans by Graeme Hancock and Philip Head is gratefully acknowledged.

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