Mecabrecial

Nubrigyn algal reefs (Devonian), eastern Australia: Allochthonous blocks and megabreccias P. J. C O N A G H A N School of Earth Sciences, Macquarie E. W . M O U N T J O Y Department of Geological Sciences, J^ A^ TALENT^^ D. E. O W E N

J Department

School

of Earth Sciences,

of Geology,

Bowling

University, North McGill University,

Macquarie

University,

Ryde, New South Wales, 2113, Montreal, Quebec H3C 3GI,

North

Green State University,

Bowling

ABSTRACT The widely known Lower Devonian "algal reef' limestones of the Nubrigyn Formation, New South Wales, are enormous allochthonous blocks contained within a 400-m interval of interbedded mudstones, allodapic carbonates, and megabreccias that form part of a 5,000-m succession of Lower Devonian volcanics and flysch. Previous workers have interpreted these massive limestone bodies to be algal bioherms that developed in sublittoral to littoral environments around volcanic pedestals on a "Nubrigyn shelf."

The allochthonous nature of the limestone bodies is clearly indicated by (1) occurrence of a wide range of clast sizes, as much as 1 km across; (2) presence of a wide range of clast types and sizes in close juxtaposition; (3) discordance between stratigraphic facing of the large limestone bodies and stratification in surrounding beds; (4) lack of distinctive and regular facies changes within the limestone bodies, particularly near their margins; (5) abrupt and random truncation of internal fabrics at block margins; (6) lack of an autochthonous volcanic foundation for the "reefs"; and (7) anomalous lithofacies association of the massive bodies of shoalwater limestone with enclosing flysch.

The limestones initially formed in a shoal-water carbonate complex to the west upon a geologically persistent volcanic archipelago, the Molong Arch, where source rocks for the Nubrigyn megaclasts and megabreccias crop out in the Lower Devonian Garra Formation and Cuga Burga Volcanics. The Nubrigyn megaclasts were transported eastward as debris flows into the adjacent and relatively deep water Hill End Trough after dislodgement from the eastern margin of the Garra shelf. Megaclasts isolated within hemipelagic mudstones and flysch were presumably transported by sliding or rolling. The loci of accumulation of the debris flows and exotic blocks occupy a meridional basin-margin position between the Molong Arch to the west and the predominantly turbidite-filled Hill End Trough to the east.

Other debris-flow megabreccias, many previously unrecognized as having been transported and deposited in this manner, occur in the Paleozoic rocks of the Tasman mobile belt of eastern Australia.

Key words: sedimentology, stratigraphy, algal limestone, submarine debris flow, limestone megabreccia, megaclast, turbidite, allodapic limestone, Tasman mobile belt, Australia, Devonian, Nubrigyn.

INTRODUCTION K. H. Wolf (1965a, 1965b, 1965c, 1965d; Chilingar and others, 1967) has brought to prominence some Australian Lower Devonian limestones that he interpreted as algal reefs because of their distinctive textures. Wolf recognized algal components in three contemporary Lower Devonian units in the area between the towns of Orange and Wellington in east-central New South Wales (Fig.

Ryde,

New South

Green,

Ohio

Australia Canada

Wales, 2113,

Australia

43403

1): the Red Hill Formation, the Nubrigyn Formation, and the Tolga Calcarenite. He envisaged the first two of these units as consisting of shoal-water reef complexes and the third as an associated flanking accumulation of basin-margin carbonate turbidite. The Nubrigyn Formation is the most extensive and best exposed of the three units (Figs. 1, 2).

Because of the international prominence of the Nubrigyn Formation in the carbonate literature and because of our interest in reef problems, the "algal reefs" and associated sediments of this formation were re-examined. Our studies indicate that the "algal reefs" are allochthonous (Mountjoy and others, 1972a). Thus, although important in themselves, the earlier petrographic and paleoenvironmental studies of the "algal-reef' limestones (Wolf, 1965a, 1965b, 1965c; Johnson, 1964) apply to former bank-margin situations, not to an autochthonous complex of algal patch reefs, as was widely reported or inferred (Packham, 1958, 1968a, 1969; Wolf, 1963, 1965a; Johnson, 1964; Strusz, 1967; Brown and others, 1968; Hatch and Rastall, 1971; Pogson, 1972).

The carbonate terminology used throughout this paper is largely that of Dunham (1962) and Folk (1959). The terminology used in reference to breccias, megabreccias, allodapic carbonates, carbonate buildups, and the like follows that employed by Jones (1970), Cook and others (1972), and Mountjoy and others (1972b). The definitions given in Appendix 1 clarify the meanings used here.

This paper is based on field work on the Nubrigyn Formation in July 1970 by Talent and in the first half of 1971 by Mountjoy, Talent, Conaghan, Owen, and Edgecombe and on work during and since 1972 by Edgecombe and Conaghan. Some additional data were kindly supplied by J. G. Byrnes (1972, unpub. maps; see also Morton, 1974). The best outcrops of the formation occur in the vicinity of Canobla (the type region; Fig. 3), and much critical data come from this area.

REGIONAL SETTING The Lower Devonian formations of the Orange-Wellington area of New South Wales occur within the Paleozoic Tasman mobile belt — a region of eastern Australia characterized by a history of precratonic development prior to Late Devonian time. These units crop out along the flanks of the meridional Molong Arch, a remarkably persistent north-south structure with a well-documented history of recurrent volcanism and carbonate buildup throughout Ordovician to Early Devonian time (Figs. 1, 2). The paleogeography of the Tasman mobile belt in Silurian and Early Devonian times comprised several such arches or volcanic archipelagoes separated by intervening sedimentary troughs. Contrasts in lithofacies, aggregate thickness, and degree of tectonic disturbance characterize sedimentary deposits regarded as having formed respectively within the troughs and on the highs and serve to define the areal extent of the major paleotopographic elements (Fig. 1, A).

Geological Society of America Bulletin, v. 87, p. 5 1 5 - 5 3 0 , 14 figs., April 1976, Doc. no. 60404.

515

km 0

A

75 150 225 300

20 km 10 mis

-I

super-MIDDLE

DEVONIAN

Ca ; CENOZOIC (mainly Quaternary) ALLUVIUM Tv

:

TERTIARY

VOLCANICS(mainly basalts)

M

• MESOZOIC

UNDIFFERENTIATED

P

• PERMIAN

UNDIFFERENTIATED

Due- UPPER

DEVONIAN(Catombal Group)

LOWER

DEVONIAN

Dlr • RED HILL Dlt : TOLGA

FORMATION

CALCARENITE

Din = NUBRIGYN Dig : GARRA

FORMATION

FORMATION

Dly ° CUNNINGHAM FORMATION Dim • MERRIONS Dlc

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Dlb ° BAY FORMATION Dlu ° LOWER DEVONIAN UNDIFFERENTIATED

sub - DEVONIAN

I f i S p ^ '

S

: SILURIAN

0

'ORDOVICIAN

G

:MIDDLE DEVONIAN AND INTRUSIVES FAULTS

UNDIFFERENTIATED UNDIFFERENTIATED

—t—

CARBONIFEROUS FOLD AXES

Figure 1. A, Paleogeographic elements of Paleozoic Tasman mobile belt in N e w South Wales (after Branagan and Packham, 1970, p. 5); framed area is location of B. B, Regional geology of Nubrigyn field area, compiled from maps by Packham (1968b), Brunker and others (1970), Offenberg and others (1971), Byrnes (1972, unpub.), and Morton (1974, unpub.), and from data in Strusz (1967, Fig. 1; 1972, Chart 1). In B, A - B (through Stuart T o w n ) is location of sections shown in Figures 2 and 14; framed area southwest of Stuart T o w n is location of area of Figure 3.

NUBRIGYN ALGAL REEFS, EASTERN AUSTRALIA

The Molong Arch is thus flanked by thick linear accumulations of Silurian and Lower Devonian sedimentary rocks and volcanics — the Cowra Trough on the west and the Hill End Trough on the east (Figs. 1, A and 2). The clastic fill of both troughs is largely of volcanic origin and provenance and was shed from the neighboring topographic highs.

The carbonate units studied by Wolf occupy a basin-margin position along the western flank of the Hill End Trough and interfinger eastward with some 5,000 m of Lower Devonian

- COWRA

517

flyschoid graywacke, mudstone, and volcanic sedimentary rocks of deep-water aspect (Fig. 2). A tract of contemporary Lower Devonian shoal-water carbonates lies to the west on the Molong Arch, constituting erosional remnants of a platform succession of originally greater extent (Figs. 1, B and 2). Within the Lower Devonian succession of the Hill End Trough, paleoslope and paleocurrent evidence (Packham, 1958, 1968a) suggests that (1) the trough's bathymetric axis fluctuated between an essentially median position and locations farther east, and (2) sediment influx was predomin-

TROUGH -

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A (WEST)

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STUART TOWN

w

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NO SUBSURFACE 10 mis

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^s-XS

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CONTROL

DIAGRAMMATIC

NO VERTICAL

, ,

^ -

• e

•

EXAGGERATION

Figure 2. Geological cross-section of Nubrigyn field region, modified from Offenberg and others (1971); location of section ( A - B ) is shown in Figure 1, B. Stratigraphie units keyed as for Figure 1, B. Note relationship of Molong arch to flanking Cowra and Hill End troughs.

Figure 3. Geology of northern part of Nubrigyn Formation, modified from Wolf (1963, map 2); location shown in Figure 1, B. White = Nubrigyn Formation; black within Nubrigyn Formation = massive bodies of limestone and volcanic rock and prominent exposures of megabreccia (see map legend); dots within Nubrigyn Formation = newly recognized lateral extent of megabreccias (see text). Other stratigraphic units keyed as for Figure 1.

Dln a . d indicate WolPs four subdivisions (members) of Nubrigyn Formation. Circled numbers are W o l f s reference numbers to prominent exposures.

Localities A - D referred to in text herein. Type area of Nubrigyn Formation is at Canobla (top left corner of map). M - N is line of measured section shown in Figure 4.

518

CONAGHAN AND OTHERS

antly from the flanking topographic high on the east, the Capertee Arch (Fig. 1, A). Additionally, syntaphral events are known to have occurred on the western side of the trough but in paleoslope locations downslope from those studied by Wolf. Thus, the mudstonegraywacke succession here contains beds of pebbly mudstone with limestone clasts of shallow-water origin and large slump sheets involving sediment thicknesses exceeding 300 m (Packham, 1958, 1968a).

As a consequence of the onset of regional cratonization, marine sedimentation effectively ceased throughout the area during latest Early or Middle Devonian time and gave way during the Late Devonian to widespread deposition of terrestrial quartzose clastics.

PREVIOUS WORK A N D BACKGROUND

SEDIMENT TYPES The Nubrigyn Formation has been subdivided by Wolf (1965a, p. 154, 1 5 8 - 1 6 3 ) into four members (members A to D — Figs. 3, 4). The members contain different proportions of several contrasting lithologies: allodapic carbonate, fine rudite, megabreccia, isolated blocks, and enclosing sediment. The megabreccia and isolated blocks were designated "algal reefs" and the enclosing strata "terrigenous sediments" by Wolf.

The enclosing strata mainly consist of "very impure algal-detrital limestone beds" — essentially impure calcarenites, commonly graded. Noncarbonate constituents in these rocks commonly exceed 50 percent, range in size from silt to blocks, and consist of andesite fragments, detrital quartz, and plagioclase accompanied by authigenic chlorite (Wolf, 1965a, p. 158; 1965c, p. 187, 190).

The pebble-sized noncarbonate material is predominantly andesite, with smaller amounts of basalt, dolerite, rhyolite, quartzite, and granite (Packham, 1969, p. 140). The strata surrounding the blocks also include mudstone beds, fine rudite, and megabreccia (the two latter "limestone breccia" and "limestone boulder deposits," respectively, of Wolf, 1965a, Figs. 34 and 35; 1965c, p. 192). In the northern outcrop area of member B, two thick deposits "composed of a mixture of mud, limestone rock fragments, and volcanic rock pebbles" were termed "dump" deposits by Wolf (1965a, p. 161).

Many of the sedimentary rocks that make up the Nubrigyn Formation are allochthonous, as shown in the following discussion, and are intimately associated with interbeds and thicker bedded intervals of dark argillaceous mudstone. Inherent data concerning the origin of the mudstones, particularly the water depths in which they accumulated, are not available. However, the mudstones form integral strata of a thick flysch succession of wide areal extent, suggesting emplacement phenomena in uniform and areally extensive conditions of relatively deep water (see discussion below). The bedded mudstones are therefore presumed to represent open-water "basin" deposits of hemipelagic, nepheloid, or related origin, and the term "basin mudstone" is used hereafter in this sense. (Therefore, we use the term "basin" only in a general sense for the depositional feature in which these relatively deep water sediments were formed.)

In order to clarify the nature and origin of the sedimentary rocks, it is important that they be classified systematically in terms of clast size and gross form. Four main types are recognized in addition to the bedded mudstones mentioned above; in approximate order of abundance they are (1) impure allodapic carbonate (graded calcarenite), (2) fine rudite (clasts smaller than 1 m), (3) megabreccia (clasts from 1 to 100+ m), and (4) isolated blocks (clasts as large as 1 km, enclosed in mudstone and calcarenite).

We emphasize, for the sake of clarity in subsequent discussion, that the blocks or megaclasts (Wolfs "reefs") occur within two of the above categories of sediment types: (1) as integral large clasts of the megabreccia deposits (category 3 above, see Figs. 7, 9), and (2) as essentially isolated entities within basin mudstones and interbedded graded grainstones and packstones (category 4 above; see Fig. 12). Similar if not identical clast lithologies characterize the megabreccias and isolated blocks. Megabreccias and isolated blocks are equally common overall, but megabreccias are more abundant in member A (see Figs. 3, 4).

The Nubrigyn Formation (Packham, 1968a; 1969, p. 1 4 0 - 1 4 1 ; Wolf, 1965a), consists of approximately 400 m of clastic carbonate and noncarbonate (epiclastic and volcaniclastic) strata that range in grade from conglomerate to shale and include lenticular bodies of massive limestone composed largely of algal material. The whole formation is developed over a strike length of about 20 km and is exposed in the limbs of a series of north-trending folds (Fig. 3). The Nubrigyn Formation intertongues eastward with flyschoid graywackes of the Cunningham Formation; it conformably overlies the southward-thinning Lower Devonian Cuga Burga Volcanics (= Eadvale Volcanics of Wolf), and toward the north, its basal beds overlie the flaggy calcarenites and interbedded shales of the Tolga Calcarenite (Figs. 1, B, 2, and 3). The latter formation wedges out sharply south of its contact with the Nubrigyn Formation (Wolf, 1965a), extends northward for 30 km or more, and has turbidite characteristics (Wolf, 1965a, 1965b). Packham (1968a; 1969, p.

141) emphasized the remarkable lithology of the Nubrigyn Formation, contrasted it with platform carbonates of the approximately coeval Garra Formation to the west, and suggested it could be the younger of the two, although age relationships were not clear. Wolf (1965b, p. 2; 1965c, p. 184) interpreted the Nubrigyn Formation as a reef complex, using the term to embrace the entire formation of more than 300 individual (inferred) bioherms and biostromes and its laterally equivalent or associated carbonate units, the Red Hill Formation and the Tolga Calcarenite. Wolf interpreted these "reefs" as "massively constructed" presumably algal1 limestones.

In the field these massive limestones are comparatively resistant relative to their surrounds and in many places give rise to a torlike topography. Exceptionally (for example, limestone body 62, the largest body known in the Nubrigyn Formation), there is excellent exposure of the internal structures and fabrics of the massive limestone bodies in cleanly washed pavements that are continuous throughout the entire area of outcrop of the rock mass. More commonly, however, the surfaces of the limestone masses are extensively veneered by a weathering patina, which obscures internal textures and fabrics and makes observation of critical areas difficult. Similarly, contact relationships between the "reefs" and surrounding sediments are only locally well exposed. Scoured creek-bed pavements are generally the places where best exposures of these fabrics and relationships are to be found. The "reef complex" was interpreted as having accumulated in a littoral environment along an inferred north-south coastline of a predominantly volcanic land mass to the west (Wolf, 1965a, p. 1 6 9 - 1 7 5 ; 1965c, p. 1 8 5 - 1 8 6 ) .

Impure Allodapic Carbonates

' W o l f (1965a, 1965b, 1965c, 1965d) interpreted many of the features in these limestones, especially irregular areas of dark cryptocrystalline calcite, to be of algal origin, even though definite algal remains are often rare or absent. Our thin sections indicate that some samples definitely are most appropriately described as algal boundstone, but others are mudstone or wackestone of uncertain origin. Henceforth in this paper we use the term "algal limestone" in reference to the Nubrigyn Formation only when quoting or referring to Wolf s descriptions.

Allodapic carbonates form successions of well-bedded, laterally persistent grainstone- and packstone-calcarenite interbedded with thin mudstones. Grain size varies widely from rubbly granule-sized detritus in many of the thicker beds to coarse, medium, and finer sand grades in most others. Volcaniclastic constituents are the dominant noncarbonate impurity. Beds range in thickness from 0.3

PRESENCE SCALE (M)

SEDIMENTARY LITHOLOGY

MEGA-

STRUCTURES BRECCIAS

OF

PERCENT

UNIT

MEGA-

CARBONATE

GRAIN

CLASTS

(APPROX.)

SIZE

eo+ _Jt_

S

5

(WOLF, ADDITIONAL

SOME

DESCRIPTION

1963)

INTERBEDS OF CALCARENITE AND MUDSTONE

90

l/l

—L 5 0

S

ARENITES

E

VOLCANIC ARENITE

I

THICKNESS FROM 5 TO 5 0 CM, AVERAGING 2 0 TO 3 0 CM,

•g

ARE ABUNDANT

7 0

RANGE

IN COMPOSITION FROM ALMOST PURE CALCARENITE AT TOP OF SECTION TO NEAR BASE.

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SHOW STRONG LATERAL CONTINUITY

IN THE VOLCANIC ARENITES AT BASE OF

PROGRESSIVELY

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AND RANGE

IN

OF DARK CLAYSTONE

INTERVAL BUT DECREASE

UPWARD AS ARENITES BECOME MORE CARBONATE

RICH

n 15 _ BEDDING 30

5

S C A L E S 1 M,

INTERBEDS ROCKS,

BLOCK

BRECCIAS

NEAR TOP.

CALCARENITE,

BECOME FINER AND LESS MASSIVE

CLAST L I T H O L O G I E S : VOLCANIC

5 1 ONLY OBVIOUS

MASSIVE LIMESTONE,

UPWARD.

CALCARENITE

DARK SHALE,

VOLCANIC

SANDSTONE

EXPOSURE

INTERBEDDED CALCARENITE AND SHALE

THE

FEW DISCONTINUOUS

EXTENSIVE)

EXPOSURES ARE MAINLY BRECCIA BUT SOME SANDSTONE AND

MUDSTONE ARE PRESENT,

MUDSTONE WITH MINOR BRECCIA AND

DOMINANT MATRIX

CLAST

SANDY.

SANDSTONES

LITHOLOGIES: CRUDELY

ARE

(WEST

OF BLOCK 6 1 T H I S

UNIT

IS

(POSSIBLY

DOMINANTLY

CALCARENITE)

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DARK MUDSTONE, QUARTZITE AND RARER

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IMPURE VOLCANIC

ARENITES

70

60 PROMINENTLY

OUTCROPPING CALCARENITES,

DETRITUS,

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INTERBEDDED

MOSTLY GRADED AND R E L A T I V E L Y

WITH DARK SHALES.

THIS

UNIT

AREA BUT

INTERTONGUES WITH BRECCIAS TO THE WEST.

PEBBLY,

BEDS

CALCARENITE

VERY PERSISTENT

BEDS RANGE

SILTSTONES

LATERALLY;

IS

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BLOCKS ABSENT EXCEPT

AND STRONGLY

IN

THIS

BoTH TOP AND BASE OF U N I T MORE IN

IN THICKNESS FROM 1 5 CM TO 1 M , AVERAGING

GENERALLY THINNER

FREE OF VOLCANIC UNIFORM

BRECCIAS. 5 0 CM;

SHALES AND

RECESSIVE

82_

_ A A A A A A A A

35 CLASTS

DOMINANTLY

LIMESTONE AND SHALE;

ACCESSORY CLASTS OF VOLCANIC

ROCK AND

SANDSTONE

INTERBEDDED

BRECCIAS,

CALCARENITES.

PEBBLY MUDSTONES, VOLCANIC

BRECCIAS BEDDED;

WITH FEWER SHALE CLASTS;

LOWERMOST

BRECCIA

MATRIX MOSTLY

I N SECTION,

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MUDSTONES.AND

CLASTS DOMINANTLY LIMESTONE AND VOLCANIC

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pebble

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Figure 4. Measured section o f N u b r i g y n Formation at Canobla, modified from Edgecombe (1972, Fig. 4) and shown in Figure 3 ( l i n e M - N ) . In detail, section line is segmented and offset small distances at contacts of some units. Section line approximates that of type section of Nubrigyn Formation as defined by Wolf (1963, v. 1, map 2).

520

C O N A G H A N A N D OTHERS

to 1 m and are almost invariably graded (Figs. 5, 12). Sequences of sedimentary structures of the type AE, AB(D)E, and B(D)E (Bouma, 1962; see also Walker, 1967; Walker and Mutti, 1973) characterize most beds, except that some of the thicker beds of more rubbly calcarenite are massive or show crude horizontal layering and, in places, occur in amalgamated bundles. Rare single thin sets of microâ&#x20AC;&#x201D;cross-lamination are the only type of crossbedding encountered, but these sets overlie small-scale erosion surfaces and have not been seen to occur as the Bouma division C. Basal scour-and-fill structures and load casts (Fig. 4) are the only bedding-plane features.

Although compositionally different from the calcarenites, beds of relatively pure volcanic arenite occur in intimate association with the allodapic limestones (Fig. 4) and are also allocated to this category herein. The volcanic arenites typically form graded beds characterized by grainstone or packstone fabrics and are thus texturally similar to the impure calcarenites.

Studied in thin section, the packstone and grainstone calcarenites contain a range of skeletal and nonskeletal carbonate grain types,

including echinoderms, mollusks, brachiopods, bryozoans, stromatoporoids, corals, trilobites, ostracodes, segments of green algae (including ?Litanaia and ?Lancicula), a variety of intraclasts (some containing the algae Renalcis and Girvanella), and other Iimeclasts (Pextraclasts) that show diagenetic effects (mainly cementation) predating emplacement in their present host sediment (see also Wolf, 1965b, regarding the last of these grain types).

Impure allodapic limestones occur in each member of the formation but predominate in member B, where, together with thin interbeds of mudstone, they make up a 100-m flyschoid interval essentially devoid of other sediment types (Figs. 3, 4). Elsewhere they make up thinner intervals of flyschoid sediment (member D), occur as single beds isolated in thick intervals of mudstone (member C), and form the top of continuously graded units of megabreccia and fine rudite (members A and D; Fig. 10).

The Nubrigyn allodapic carbonates exhibit many features characteristic of flysch: sharp bedding-plane contacts, lateral persistence and uniformity of bed thickness, rhythmic interbedding with thin mudstones, ubiquitous graded bedding, and presence of allochthonous carbonate clasts of shallow-water origin.

Figure 5. Graded bed of impure rubbly calcarenite in member C (loc. C, Fig. 3). Carbonate components mostly shelly allochems (see text); noncarbonate constituents are fragments of volcanic rock. Stratigraphie facing of bed is toward left.

Figure 6. Fine rudite (pebble matrix) within recessive bedded lower diagram of Figure 12, top subrounded pebbles of quartzite clasts occur but are recessive.

paraconglomerate with argillaceous sandy mudstones of member C; for location, see right corner. Clasts are mostly rounded to and volcanic rock; some angular limestone

Figure 7. Exposure of megabreccia at locality B (see Fig. 3 for location); view west across Nubrigyn Creek. Bedding vertical to slightly overturned facing east. Prominent horizontal surfaces are joints; vertical walls running north-south across and behind pavement are crudely defined bedding planes within megabreccia. Megabreccia is approximately 5 0 m thick.

Megabreccia clasts reach 10 m in size (block I) and comprise various types of limestone (L) and basin sedimentary rocks (blocks IV and V) in a dark argillaceous matrix (see text for more detailed descriptions of block lithologies). Geopetal fabrics in larger limestone blocks (see Fig. 11) demonstrate attitudes of stratigraphic facing of blocks to be completely random relative to facing of host sediment; for example, bedding in block I is vertical and faces north-northeast. Seated figure at block I gives scale. Probable lateral extent of this megabreccia indicated by dotted area in Figure 3.

N U B R I G Y N ALGAL REEFS, E A S T E R N

Fine Rudites The fine rudites in which the predominant clast size is smaller than 1 m are highly variable texturally and compositionally. In section, fine rudites have laterally continuous sheetlike or laterally discontinuous and intermittently developed lenslike geometries, some of which might represent channels. Individual beds can be traced over strike distances from a few metres to several hundreds of metres â&#x20AC;&#x201D; possibly more. Thicknesses range from 1 to 10 m but probably average less than 5 m. These deposits can be massive or graded or exhibit crude layering (Figs. 5, 6, 10; Wolf, 1965a, Fig. 35).

Clast shape, size, and lithology vary considerably. Pebbles and cobbles predominate, but boulders and even small isolated blocks 2 occur in some examples. Clast roundness or angularity is related primarily to lithology: limestone and intraformational mudstone clasts are angular to subangular, whereas clasts of all other lithologies are characteristically well rounded (Fig. 6; Wolf, 1965a, Fig. 35). A dark argillaceous matrix is characteristically present.

2 "Block" is used here as a descriptive term for exotic entities (of any lithology and of any shape) larger than 4 m and as large as 1 km (see Appendix 1).

Figure 8. Details of megabreccia exposure at locality B (see Figs. 3 , 7 for location). A , Angular limestone clasts of diverse size in dark argillaceous matrix. Predominant clast lithology is algal-stromatoporoid boundstone. B, Large clast o f fine-grained basin sediment (block V in Fig. 7; long dimension is approximately 3 m; see text for details o f lithology). Smaller limestone clasts are mostly entire and broken heads o f bulbous tabulate corals and stromatoporoids.

AUSTRALIA

521

Textural varieties range from paraconglomerate and parabreccia to orthoconglomerate and orthobreccia (see Appendix 1), but the matrix-supported fabrics probably predominate (Figs. 6, 9; Wolf, 1965a, Fig. 35). Well-rounded clasts of noncarbonate lithologies (mainly volcanics and quartzite) predominate in the conglomerates, whereas angular clasts of limestone and intraformational mudstone predominate in the breccias.

Fine rudites occur in all members of the formation but are more common in members A and D. In these two members they form laterally extensive sheetlike beds 1 to 10 m thick and are associated with megabreccias (Fig. 4).

Megabreccias The megabreccias are deposits in which angular clasts larger than 1 m are conspicuous elements but which may contain clasts well in excess of a few metres; clasts smaller than 1 m may also occur and may be rounded rather than angular. The Nubrigyn megabreccias comprise a variety of admixed large and small clasts of diverse shapes and lithologies set in a dark argillaceous matrix or, less commonly, a matrix of admixed volcaniclastic and carbonate sand (Figs. 7, 8, 9, 10; Table 1).

Megabreccias are developed throughout the formation as rela-

Figure 9. Megabreccia exposure at locality D (see Fig. 3 for location).

V i e w is south over Boduldura Creek toward central part of one of W o l f s unnumbered "reefal" outcrops (black area at locality D in Fig. 3). Three limestone blocks (I, II, and III) occur on far side of creek. Block I is many tens of metres across. Block II, 8 m high, is split and solution-weathered into several subelements along joints. Block III is c o m p o s e d of algalstromatoporoid boundstone; blocks II and III are c o m p o s e d of massive fine-grained sediment (lime mudstone and [or] algal boundstone). Area between these and other neighboring limestone blocks (outside picture) and extending across creek b e y o n d lower right corner of diagram comprises megabreccia host (hachured) containing angular clasts of varied lithology in dark argillaceous matrix. Probable (but incomplete) lateral extent of megabreccia is indicated by dotted z o n e in Figure 3.

522

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Figure 10. Continuously graded unit in creek exposure immediately south of outcrop 8 (see Fig. 3 for location). Megabreccia at left grades stratigraphically upward into fine rudite and calcarenite (splayed opening between calcarenite and fine rudite due to recent parting and tilting effects).

Calcarenite is overlain by mudstone (recessive). Approximately 5 m of stratigraphic thickness is shown in photograph. N o t e 2-m-long mudstone clast at base of fine rudite (behind sledge hammer). Hammer is approximately 1 m long.

Figure 11. Geopetal fabric (probably formed in body whorl of gastropod), one of many diverse kinds of geopetals that occur in the massive limestone bodies of the Nubrigyn Formation. Floor of internal sediment (S) is at lower right; rest of cavity is filled with calcite cement (C) and additional small deposits of internal sediment. Location: horizontal joint surface in block I, locality B (see Figs. 3, 7).

tively thick sheetlike bodies (dotted zones in Fig. 3; Fig. 4) whose wide lateral extent has not been recognized previously, except by Byrnes (1972, unpub. maps). Packham (1958) and Wolf (1963, 1965a) described isolated exposures of megabreccia surrounding some of the large massive limestone bodies but did not infer continuity between these and similar exposures aligned on strike.

Hence, many of the numbered and unnumbered "reefs" depicted on existing maps of the area (Wolf, 1963, maps 2 and 3; 1965a, Fig. 27), particularly the "biostromes" (Wolf, 1965a, p. 1 6 1 - 1 6 3 ) , are actually large exposures of megabreccia dominated by one or more megaclasts of massive limestone (Fig. 9). Within the northern part of the formation (Fig. 3), such exposures include the following localities: 8, 9, 10 (member A), 15 (member B), and D (member C).

Megabreccias occur in each of the four members of the Nubrigyn Formation, but in the vicinity of the formation's type area near Canobla, they are most common in members A and D (Figs. 3, 4).

Particularly good exposures occur here at locality B (Figs. 4, 7, 8) and east of Canobla near Merrimount at locality D (Figs. 3, 9).

The finer host breccia that surrounds the limestone megaclasts in the megabreccias is recessive and poorly exposed, except in places of active mechanical weathering in creek beds and on the steeper hillsides. Accordingly, assessment of the shape and degree of lateral continuity of the megabreccias is difficult and is based on aligned but discontinuous exposures of the constituent limestone clasts, with or without exposures of the surrounding and contiguous finer breccia host. These data indicate that the megabreccias have sheetlike or lenticular shapes, their thicknesses range from a few metres to several tens of metres, and they have lateral dimensions of at least several kilometres (Fig. 3). N o pronounced or obvious channel features have been observed. Contacts with underlying and overlying strata are only locally exposed but are evidently sharp and essentially planar; underlying beds show little or no disruption of primary structure. Upper contacts are either gradational with overlying fine rudites and calcarenites (Fig. 10) or pass more abruptly into these or other lithologies.

Texturally, these rocks are essentially parabreccias (or megaparabreccias) in that the predominant internal fabrics are those of matrix support rather than clast support (Figs. 7, 8). They are either massive or display crude layering on a scale upward of approximately 1 m. Imbrication of elongate clasts parallel with bedding is common (Fig. 7). A complete size spectrum of clasts and

blocks occurs, although the upper size limit of blocks varies from place to place along strike within any one megabreccia. The maximum size attained by blocks within the megabreccias in some places is on the order of 100 m or more (as at locality D, Figs. 3 and 9), but elsewhere it is considerably less (10 m at locality B, Figs. 3 and 7). The size range of clasts in the finer host breccia surrounding the blocks is about 0.1 to 1.3 m.

The shape of megaclasts in these breccias appears to be mainly tabular, although this feature is in most cases difficult to assess. The megaclasts are characterized by sharp, angular boundaries that randomly truncate internal block fabrics, or alternatively, the block margins exhibit a disrupted zone of autoclastic breccia of variable thickness (0.5 to 5 m). Formation of the autoclastic breccias was evidently accompanied by injection of argillaceous matrix into the fractured surfaces or walls of the blocks, with consequent dilation and eventual dispersion of angular limestone fragments into the surrounding host sediment. As with fine rudites, clast shapes are related to lithology: limestone and i n t r a f o r m a t i o n a l basinmudstone clasts have diverse shapes and tabular shapes, respectively, and are angular; clasts of volcanic rock are mostly equant and are rounded.

Limited observations of geopetal fabrics (Fig. 11) at localities B and D (Figs. 3, 7, 9) indicate that megaclasts evidently have random-facing orientations relative to that defined by graded calcarenites in the immediately underlying and overlying bedded intervals.

The megaclasts are invariably limestone; massive wackestone, mudstone, and algal boundstone are common, but petrological data do not allow inferences about which of these predominates.

The smaller clasts have more diverse lithology, including massive to irregularly laminated algal-stromatoporoid boundstone (block I, Fig. 7), nodular bedded coral-stromatoporoid boundstone (block II, Fig. 7), stromatactis-bearing mudstone (block III, Fig. 7), and pebble- to boulder-sized bulbous heads of colonial corals (mostly favositids) and stromatoporoids (Fig. 8). Other clast lithologies include andesitic volcanics and two different types of basin sediment: dark shale slabs as much as 6 m long (block IV, Fig. 7) and regularly interbedded light carbonate mudstone and dark claystone in beds 2 to 5 cm and 8 to 10 cm thick, respectively (block V, Figs.

7 and 8, B), a lithology similar in many respects to basin-margin argillaceous wackestone and mudstone flanking the Upper Devo-

523

NUBRIGYN ALGAL REEFS, EASTERN AUSTRALIA TABLE 1.

CHARACTERISTICS OF NUBRIGYN MEGABRECCIAS A N D INFERRED PRIMARY LITHOFACIES AFFINITIES OF COMPONENTS OF CONTEMPORARY SEDIMENTARY ORIGIN Descriptive features

Inferred lithofacies affinities

Shape

Sheetlike and lenslike (available exposures two-dimensional only)

Thickness

3 0 to 1 0 0 + m

Lateral extent

As much as 7 km, possibly more

Bounding surfaces

Commonly poorly exposed; base generally sharp and planar, top either abrupt and planar to irregular or internally graded and gradational with overlying fine rudites and calcarenites

External form â&#x20AC;˘

Internal organization

Parabreccia fabrics predominate (that is, clasts are supported by matrix); commonly massive overall, but other fabrics present include (1) bedding-parallel imbrication of elongate clasts, (2) crude bedding (clustering of clasts relative to matrix into clast-rich and clast-poor bedding-parallel layers), (3) continuous grading at top into overlying fine rudites and calcarenites Size

All sizes from millimetre- to centimetre-sized fragments up to 1 0 0 + m

Shape

Predominantly angular, smaller clasts of noncarbonate and extraformational origin (mainly igneous lithologies) commonly rounded

Geometry

Limestone (dominant, and mostly angular)

Lithology

"Extraformational" (accessory, and mostly rounded)

(1) Algal, stromatoporoid, and coral boundstones, commonly (1) Shelf-edge environment (reef and associated facies) massive, in other cases bedded (2) Massive lime mudstone, wackestone, and packstone, some with stromatactis

(2) Back-reef environment (lagoon)

Volcanics (andesite and others); volcanic arenite; quartzite, plutonics (1) Dark argillaceous mudstone and shale

(1) Basin-margin slope (?deeper water)

(2) Thinly interbedded calcilutite-calcisiltite and dark argillaceous mudstone

(2) Basin-margin slope (Pshallower water)

Other

Orientation (megaclasts)

Primary stratigraphie facing of megaclasts is random relative to that of surrounds

(1) Dark argillaceous mudstone (dominant)

(1) Basin-margin slope

Matrix

(2) Volcaniclastic and mixed carbonate-volcaniclastic sand (subordinate)

(2) Shelf and shelf-edge environment (littoral and neritic sands)

Stratigraphie distribution

Present in each member of the formation, but particularly in members A and D

nian reef complexes at Alberta (Mountjoy, 1965, 1967). Many of these slabs of fine-grained basin sediment are penecontemporaneously bent and folded.

Isolated Blocks The essential difference between these megaclasts and those in the megabreccias is that the isolated blocks are wholly enclosed within the normally bedded basin mudstone and flyschoid arenite instead of occurring within units of megabreccia (Fig. 12; Table 2).

The host strata consist of argillaceous mudstones with interbeds of allodapic limestone and fine rudite.

Isolated blocks occur within each member of the formation but

are most conspicuous in members B and C (Figs. 3, 12). The largest blocks in these members are on the order of 1 km (blocks 57 and 62; Fig. 3). In the case of block 62, its excellent continuous exposure in pavements, uniform attitude of internal bedding, consistent direction of stratigraphic facing throughout the entire outcrop area, and sharply defined marginal contact with enclosing basin mudstone and associated flyschoid sediment clearly show it to be one single isolated limestone mass. The same can be said of some of the other large blocks (for example, blocks 57 and 74), except that conspicuous and laterally extensive bedding structures are absent, and smaller scale fabrics are less extensively exposed in the rock pavements because of the presence of a weathering patina. The isolated megaclasts vary in size from boulders (4 m) to several tens

524

C O N A G H A N A N D OTHERS

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Figure 12. Oblique aerial view of relationships between some massive limestone bodies and their surrounds (loc. A, Fig. 3). Limestone masses (referenced with W o l f s outcrop numbers) occur at top of succession of graded calcarenites (top part of member B) and are partly mantled by overlying mudstones and interbeds of graded calcarenite (basal part of member C). W o l f s outcrop 61 (see Fig. 3) comprises three apparently separate limestone masses, shown here as 61A, 61B, and 61C. Limestone body 61A is 8 0 m long and has a maximum exposed thickness (small axis) of 10 m on its eastern side (in lower diagram, distance between two small circled crosses on body 61A is 3 0 m). N o t e strike and dip discordance between stratigraphic facing in body 61A and that of surrounding strata (broken circles, lower diagram; see also text and Fig. 13). Facing attitudes of other limestone bodies unknown.

of metres and beyond (Table 2). The largest isolated blocks (for example, block 62) occur in member C, where they comprise both limestone and volcanic lithologies (Figs. 3, 12). Isolated blocks in member B are no more than a few tens of metres in size and consist exclusively of limestone. Most of the "reefal" and volcanic outcrops shown within members B and C in the western one-third of the area shown in Figure 3 (type area of the Nubrigyn Formation) are single isolated blocks, in contrast to the aggregate (megabreccia) nature of exposures depicted within the same area in members A and D. Limestone Megaclasts. The isolated limestone blocks are lithologically similar to component blocks of the megabreccias and consist mainly of massive carbonate mudstone and fine-grained algal boundstone ("algal calcilutite biolithite" of Wolf, 1965a, p.

113). Additionally, bedded lime mudstone and massive limestone

breccia occur in some isolated blocks, both separately and in association with the massive carbonate mudstone. Bedded limestone is known to occur only in blocks 60, 62, and 67. Bedding is 0.05 to 2 m thick and varies markedly from indistinct to well defined. In block 62, massive coral-algal and stromatoporoid boundstones, beds of limestone pebble breccia (containing a few clasts of volcanic material), and beds of dark-gray calcilutite occur in distinct lithofacies zones. The bedding and the lithofacies zones are truncated at the edge of the block, in some places with attendant brecciation of the block margin. A few isolated blocks (52 and 60, Fig. 3) consist wholly or partly of limestone pebble breccia similar to that contained internally within block 62. These breccias are easily distinguished from all other types of breccia discussed herein because they are orthobreccias that lack interstitial matrix, the clast boundaries being mainly stylolitic.

One of the most important diagnostic criteria for establishing the allochthonous nature of suspected megaclasts is the detection of stratigraphic discordance between them and the surrounding strata. In the Nubrigyn Formation, many kinds of geopetal fabrics occur within the massive limestone bodies (Fig. 11; see also Wolf, 1965c) and can be utilized to this end. Work by Edgecombe and Conaghan (unpub. data) has demonstrated significant stratigraphic discordance between several isolated limestone blocks and their enclosing strata. These include blocks 25, 57, 60, 61, 62, and 74, in addition to others outside the area illustrated in Figure 5. A typical example of the discordance revealed by these studies is illustrated in Figures 12 and 13 for block 61A, an isolated megaclast that is 80 m long and has an angular discordance of 58° with the enclosing flyschoid strata. Nearby block 62 (Fig. 3), the largest and best exposed megaclast in the formation, has a similar discordance with the enclosing beds of mudstone and calcarenite.

Available evidence shows that block margins are either (1) sharp with random truncation of internal block fabrics (eastern margin of block 61A [Fig. 3] and part of the margin of block 62) or (2) disrupted (over variable thicknesses) through formation of autoclastic breccia (parts of block 62 and eastern margin of block 74).

Disruption of surrounding or underlying bedded sedimentary rocks also varies from little or none to pronounced. For example, along the eastern margin of block 61A, the underlying flyschoid strata terminate sharply against the base of the block without disruption (Fig. 12). By contrast, soft-sediment deformation of various kinds (small-scale folding, clastic injection, and a general disruption of primary sedimentary fabrics) occurs throughout a 5-m-thick zone in graded beds underlying blocks 74 and 75 near Merrimount (Fig. 3). It appears that the surrounding beds are disturbed beneath but not adjacent to the isolated blocks.

Volcanic Megaclasts. N o laterally extensive outcrops of volcanic rock are known within the Nubrigyn Formation, but volcanic bodies of limited size occur within member C and have dimensions similar to those of nearby blocks of limestone. All are isolated and surrounded by the same kinds of basin rocks that surround the limestone blocks. No definitely in situ intrusive and extrusive volcanic rocks are present.

The volcanic rocks tend to be strongly weathered and hence poorly exposed, particularly their contacts with surrounds. Where contacts can be observed, the volcanic rocks are isolated and are surrounded by bedded mudstones and calcarenites. Volcanic lithologies also occur as generally smaller clasts (pebbles, cobbles, and boulders) in the megabreccias. Thus, all volcanic materials observed consist of allochthonous blocks or clasts. However, the larger volcanic bodies have been previously interpreted (Wolf, 1963, p. 1 4 4 - 1 4 6 ; 1965a, p. 1 5 8 - 1 6 1 ) as "penecontemporaneous erosional remnants of andesitic lava flows," which, in certain places (for example at outcrops 59 and 60, south of Canobla, Fig. 3) acted as pedestals for the development of fringing reefs. Apparently this interpretation was influenced by the proximity of a body of volcanic rock to partially encircling exposures of massive lime-

525

NUBRIGYN ALGAL REEFS, EASTERN AUSTRALIA TABLE 2.

CHARACTERISTICS OF NUBRIGYN ISOLATED BLOCKS A N D INFERRED PRIMARY LITHOFACIES AFFINITIES OF THE LIMESTONE BODIES Descriptive features

External

Inferred lithofacies affinities

Shape

Variable, commonly tabular

Size

4 to 1,000 m; commonly between 10 and 4 0 0 m

Margins

Mostly sharp and discordant with internal block structures; in places brecciated

Large scale

Mostly structureless (massive); bedding rare; syndepositional sheet and prism cracks present in some blocks

Small scale

Various kinds of geopetals, including pockets of discontinuous bedding, stromatactis, cement- and clasticfilled prism and sheet cracks, organic layering, and other cavity structures of diverse origin

Geometry

Internal

(1) Massive algal, stromatoporoid, and coral boundstones (2) Pebble and cobble orthobreccias Limestone (dominant) Lithology

(3) Massive lime mudstone, wackestone, and packstone

Shelf-edge environment (reef and associated subfacies) Back-reef environment (lagoon)

(4) Bedded lime mudstone Volcanic rock (subordinate)

Massive lava (andesite)

Surrounds

Bedded argillaceous mudstones and flyschoid allodapic calcarenite and volcanic arenite; minor fine rudite

Stratigraphie relationship between block and surrounds

Primary stratigraphie facing of limestone blocks is typically discordant with well-bedded flyschoid surrounds; flyschoid surrounds disturbed only in locations that are stratigraphically beneath the blocks but not in every such place

Stratigraphie distribution of blocks

Present throughout formation but particularly in members B and C

stone (including limestone outcrops 59 and 60; see Fig. 3); thus, Wolf (1963, p. 145) stated that "In at least one case, a volcanic rock outcrop [immediately west of outcrop 59] has been encroached or overlapped upon from various sides by algal bioherms." Wolf did not document the contact relationships between the "algal bioherms" and the volcanic rock outcrop west of block 59. These contacts are not well exposed, and although limestone bodies do occur close to the volcanic outcrop, the contact is not exposed. Hence one cannot demonstrate encroachment or overlapping by the limestones. We have not examined all of the Nubrigyn volcanic outcrops depicted on Wolf's maps, but their discontinuous or isolated stratigraphic distribution within basin strata, as mapped, suggests that all are probably allochthonous blocks similar to those cropping out in the vicinity of outcrops 60 and 61.

LATERAL AND VERTICAL RELATIONSHIPS Geometrically these sediment types, except for the isolated blocks, are sheetlike or discontinuous and have maximum lateral dimensions of several kilometres. Although minor local truncation of flyschoid beds occurs stratigraphically beneath one isolated

block (Fig. 12), other geometrical relationships suggestive of channeling have not been observed. The shape of isolated blocks is commonly more difficult to assess, because many blocks are only partially exhumed and exposed parts have been modified by recent erosion. Many appear to be essentially tabular and oriented parallel or subparallel to bedding. As previously mentioned, maximum dimensions range from 3 or 4 m to approximately 1 km.

The four different types of deposits are intimately associated in some places where megabreccias grade upward continuously through fine rudites to allodapic limestone containing sporadic isolated limestone blocks. This kind of gross textural change may involve stratigraphic intervals 1 to 5 m or more in thickness. Good exposures of the complete transition are uncommon, and detailed vertical relationships are difficult to obtain. In one exposure, continuous vertical grading from megabreccia through fine rudite to allodapic carbonate and basin mudstone occurs over an interval of approximately 5 m, presumably constituting a single complete sedimentation unit (Fig. 10). Elsewhere, similar gross vertical changes may represent multiple depositional episodes rather than a single depositional event. The most common associations and their stratigraphic distribution in the vicinity of Canobla (Fig. 3) are (1) megabreccias associated with fine rudites and impure allodapic

526

C O N A G H A N A N D OTHERS

carbonates, minor basin mudstone, rare isolated blocks (characteristic of members A and D; Figs. 7, 8, 9); (2) monotonous flyschoid intervals of interbedded impure allodapic carbonates and thin-bedded basin mudstones, minor fine rudites, absent or sparse isolated blocks (characteristic of member B; Figs. 5, 12); and (3) large isolated blocks of limestone and andesitic volcanics (4 to 1,000 m across) in basin mudstones and minor fine rudites and allodapic carbonates (characteristic of member C, Fig. 12).

REEF INTERPRETATION OF MASSIVE LIMESTONE BODIES AND MEGABRECCIAS Wolf (1965c, p. 1 9 1 - 1 9 2 ) inferred that the Nubrigyn limestone bodies consist of massively constructed algal bioherms deposited "in turbulent shallow water (if not intertidal)." He (1965a, p. 163) interpreted the massive limestones as being predominantly algal "with inter- and intra-biolithite voids and channels (possibly tidal surge channels) filled with contemporaneous and internal sediments."

When compared with other Paleozoic algal reefs, many features of the Nubrigyn "algal reefs" as described by Wolf (1963; 1965a, especially p. 158—163) are unusual and puzzling: (1) Regular distribution of organic zones and lithofacies, especially annular reefedge and central lagoonal facies, is lacking. (2) Reef distribution throughout the formation is evidently random or erratic. (3) Surrounding beds contain abundant graded calcarenites and volcanic sandstones, many apparently deposited by turbidity currents. (4) There are no clear-cut relationships to presumed volcanic islands.

(5) Deeper water basin sedimentary deposits, including laterally continuous turbiditelike sandstone and breccia, sharply abut the "algal bioherms." (6) Limestone megabreccias with angular clasts enclose many of the "algal bioherms." (7) Limestone breccias cannot be traced back to their sources or to bioherms. (8) The reefs have no known modern analogues in terms of facies variation and contiguity of sediment types. These unusual features are readily

explained if the "algal reefs" and associated breccias and conglomerates are considered to be allochthonous.

CRITERIA FOR RECOGNITION OF ALLOCHTHONOUS CARBONATE DEBRIS Many geologists are inclined to interpret limestone bodies with frame-building organisms as reefs or bioherms without carefully determining whether they are in situ or not. In the case of the Nubrigyn Formation, all four categories of constituent sediment contain features indicating that the carbonate components are allochthonous. It is therefore important that the criteria for identification of such sedimentary rocks be emphasized.

Carbonate breccia and conglomerate occur in some mudstone successions near carbonate shelves or reef complexes, and some contain large exotic blocks composed predominantly of framebuilding reef organisms. Examples include the coarse megabreccias on the south margin of the Devonian Ancient Wall complex of Alberta (Mountjoy, 1962; Cook and others, 1972) and similar deposits associated with the Devonian reef complexes of the Canning Basin, Western Australia (Playford and Lowry, 1966, p.

65—66; Mountjoy and Playford, 1972), and the Permian reef complex of the Delaware Basin, U.S.A. (Newell and others, 1953; Rigby, 1958). In all of these areas, earlier workers misinterpreted megabreccias of back-reef and fore-reef limestones as bioherms (see Mountjoy and others, 1972b, Table 1). In such deposits limestone megaclasts are readily confused with small reefs, but the use of geopetals to detect stratigraphic discordance between the massive limestone bodies and their surrounding strata and scrutiny of the contact and facies relationships with adjacent beds clearly establish that they have been displaced basinward, downslope. Criteria for the recognition of allochthonous carbonate deposits have been formulated by Pray and Stehli (1962), Pray and others (1968), Cook and others (1972), and Mountjoy and others (1972b): (1) Beds containing carbonate breccias are enclosed within basin

Figure 13. Equal-area stereograms (plotted on lower hemisphere) of (a) geopetal fabrics in limestone body 61A and (b) bedding in surrounding strata (see Figs. 3 , 1 2 for location), (a), Apparent lineations of geopetal floors on randomly oriented exposures; 65 points, contoured 1, 2, 4, 6, 8, and 10 points per 1 percent area; mean S 0 strikes 141°, dips 87° southwest facing southwestward. (b), Poles of bedding; 28 points, contoured 1, 2, 4, 6, 8, 10, and 14 points per 1 percent area; mean S Q strikes 178°, dips 38° west, facing westward.

NUBRIGYN ALGAL REEFS, EASTERN AUSTRALIA

fades. (2) Dark carbonate mudstone and basin shale or mudstone surround individual clasts and blocks. (3) Large blocks and masses of dominantly skeletal materials that could be mistaken for bioherms and reefs have tilted, aligned geopetal fabrics and stratification that indicate they are not in growth position. (4) Primary depositional facies within the blocks are commonly randomly and sharply truncated at the block margins, regardless of whether rotation of the blocks relative to enclosing sediment is demonstrable through use of geopetals. (5) A wide variety of mostly angular clast types of diverse size are juxtaposed. (6) Breccias are commonly covered by a thin discontinuous layer of graded (allodapic) limestone. (7) Plausible intraformational sources that might account for generation of the debris of shallow-water origin are absent.

Readers are cautioned that these allochthonous carbonate rudites that form in slope and base-of-slope settings are superficially similar in some respects to carbonate breccias and megabreccias of other, but mostly autochthonous, intraformational origin â&#x20AC;&#x201D; solution (evaporitic), collapse (karst), tectonic (fault), caliche, impact, reef (rock-fall talus), and others. Allochthonous carbonate deposits of the basin-margin type discussed in this paper are readily distinguished from the latter kinds of carbonate rudite on the basis of a number of criteria outlined in detail by Cook and others (1972).

Particularly diagnostic of the slope and base-of-slope carbonate rudites is their occurrence as discrete units within marine sequences, the presence of chaotically oriented clasts of both shallow-water and deeper water origin, and the prevalence of parabreccia rather than orthobreccia fabrics (see Cook and others, 1972, for additional discussion).

INTERPRETATION OF MASSIVE LIMESTONE BODIES AND MEGABRECCIAS AS ALLOCHTHONOUS DEBRIS If the criteria for the recognition of allochthonous carbonate debris and the unusual features of the Nubrigyn Formation are compared, the Nubrigyn Formation clearly contains many features compatible with an allochthonous origin.

The graded impure calcarenites and internally less organized, more rubbly variants exhibit many of the features of proximal turbidites and "fluxoturbidites," respectively (facies C and facies A 3 - A 4 , respectively, of Walker and Mutti, 1973) and are tentatively interpreted as such.

Available data regarding the fine rudites do not allow very detailed comment about their origin. Their compositional and textural diversity obviously reflect a range of different provenance situations and also, presumably, different phenomena of emplacement (see discussion of these topics by Jones, 1967). Together with their sheetlike geometry, the intimate admixture of angular limestone clasts with argillaceous matrix and the prevalence of parabreccia fabrics in the majority of fine carbonate rudites (see Wolf, 1965a, Fig. 35) suggest that they are probably deposits of s u b m a r i n e debris flows (see below). Similar e m p l a c e m e n t mechanisms (see Middleton and Hampton, 1973) presumably also apply to the predominantly noncarbonate resedimented conglomerates.

The most diagnostic and distinctive allochthonous rocks are the sheetlike megabreccias. The wide variety of juxtaposed lithologies (shallow-water carbonates, volcanic fragments, basin-derived clasts), chaotic orientations, and matrix-supported fabrics indicate mixing with argillaceous basin muds and transport en masse to their present positions (mechanisms discussed below).

The isolated limestone blocks are not in situ bioherms, because they lack the annular facies zonation that one finds in reefs, because their geopetal fabrics are disoriented with respect to the enclosing strata, and because these strata largely comprise resedimented deposits with characteristics suggestive of relatively deep water emplacement. The large bodies of volcanic rock are not demonstrably

527

intrusions or extrusions and occur isolated within terrains of flyschoid sediment like the limestone blocks. Clearly, both limestone and volcanic rock bodies are allochthonous blocks that have rolled or slid considerable distances to their present outcrop positions. Thus, all four sediment types â&#x20AC;&#x201D; allodapic limestone, fine rudite, megabreccia, and isolated carbonate and volcanic blocks â&#x20AC;&#x201D; are unequivocally allochthonous and were transported from an environment dominated by shoal-water carbonates into deeper water.

GENESIS OF MEGABRECCIAS AND ISOLATED BLOCKS The genesis of the megabreccias and similar breccias has intrigued geologists for many years. Fine rudites and megabreccias of the Nubrigyn Formation exhibit many of the features described from carbonate breccia deposits elsewhere (Newell and others, 1953; Jones, 1967; Mountjoy and others, 1972b; Cook and others, 1972) and are attributed to emplacement by submarine debris-flow phenomena (Mountjoy and others, 1972b). Of the various criteria regarded as diagnostic of submarine carbonate debris-flow deposits by the above authors, the following appear to be characteristic of the Nubrigyn Formation: (1) The breccias form tabular or lens-shaped bodies that have a planar base, except for local channels and small areas of minor erosion and deformation.

The upper surface of some deposits is irregular because of projecting clasts, but it is commonly planar. (2) The coarser debris (megabreccia) is commonly associated with thinner beds of fine breccia and allodapic calcarenite. (3) Many deposits lack internal bedding planes, textural gradation, or other features suggestive of particle-by-particle deposition. (4) Several different carbonate rock types occur, varying f r o m lagoonal limestone, coral, and stromatoporoid fragments (probably skeletal framework) to minor amounts of basin-derived mudstone (that is, clasts of buildup and basin sediment are mixed). (5) Texture is obviously clastic and chaotic: disoriented clasts of carbonate-buildup material and basin mudstone are mixed and are supported by a finer matrix of basin mudstone. (6) The enormous size of many of the megaclasts probably precludes grain flows and turbidity currents as plausible transport mechanisms. Hence the Nubrigyn deposits have the principal features indicative of submarine debris flows.

The origin of isolated blocks is much more difficult to deduce than that of the parabreccias, as few textures diagnostic of emplacement phenomena are either preserved or exposed near these blocks; but the blocks somehow moved from shallow-water carbonate shelves into deeper water. Because they are each discrete entities and are associated with turbidites and deposits of submarine debris flows, it is clear that they must have rolled and (or) slid(?) to their present positions.

Isolated limestone blocks analogous in most respects to the Nubrigyn examples are known from other basin-margin environments adjacent to carbonate buildups (for example, see Playford and Lowry, 1966; Mountjoy and Playford, 1972). The general nature of such isolated blocks, their relationship with surrounding basin sediments, and their genetic significance have been reviewed by Mountjoy and others (1972b). They are commonly large angular bodies of variable size and comprise several different carbonate rock types, including reef, fore-reef, and back-reef lithologies. In some areas (such as the Upper Devonian reef complexes of the Canning Basin, Western Australia), the isolated blocks were deposited either on fore-reef slopes, near the base of such slopes, or short distances basinward (Mountjoy and others, 1972b). These allochthonous isolated limestone bodies have been referred to either as "isolated rolled blocks" or "isolated slumped blocks" and have been interpreted as material displaced into adjacent basins from carbonate buildups following slumping of the buildup margins. In the Canning Basin examples, maximum transport distances were on the order of 4 km, and it would seem that the blocks were moved as separate entities by rolling or sliding.

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SOURCE A N D DISLODGEMENT The Nubrigyn Formation is in part contemporaneous with the Cunningham and Garra Formations to the east and west, respectively (Figs. 1, B and 2), although precise time relationships are difficult to establish, as only allochthonous fossils are known from the Devonian successions in the Hill End Trough (Strusz, 1972, p.

444). However, recent conodont biostratigraphic work by Johnson (1975) and Morton (1974) has demonstrated the contemporaneity of (1) initiation of carbonate accumulation in the basal Garra and underlying transition beds with the Cuga Burga Volcanics and (2) the onset of emplacement of allodapic carbonates in the flyschoid Tolga Calcarenite, a unit contiguous and probably in part correlative with the Nubrigyn Formation in the north. Morton showed that both of these events occurred during late early or late Gedinnian time. The Garra Formation (Strusz, 1967; Packham, 1969, p. 138; Druce, 1970; Johnson, 1975) consists of a 900- to 1,200-m platform succession of shallow-water carbonates, with subordinate interfingering shale and minor arenite, cropping out as a 100-km-long meridional belt roughly 13 km west of the Nubrigyn Formation (Figs. 1, B and 2).

Reappraisal of Strusz's (1965, 1967) investigations, together with work by Johnson (1975), J. G. Byrnes (1972, personal commun.), and Morton (1974), indicates that the preserved part of the Garra Formation can be interpreted as a platform association of (1) pelletoid, oolitic, and algal limestones and dark calcarenites with abundant brachiopods and mollusks; (2) noncarbonate strata of the Cowra Trough interfingering from the west with Garra carbonates at several localities; and (3) allochthonous deposits forming part of the platform margin preserved in outliers east of Boree (west of Orange) and limestone bodies suspected to be allochthonous at Apsley (south of Wellington, Fig. 1, B). The deposits near Boree (Strusz, 1965) consist of calcirudites largely dominated by clasts of frame-building organisms. Those at Apsley consist of limestone masses enclosed within volcanics and interbeds of tuff and shelly limestone forming a conformable transition zone between the Cuga Burga Volcanics and the Garra Formation. The Garra Formation, therefore, is a carbonate platform succession with proximal platform-margin debris (?reef-edge talus) evidently preserved only in the Boree and perhaps Apsley areas.

Collapse of the carbonate platform margin and underlying volcanic foundation occurred intermittently along much or all of the platform's eastern edge, generating the various types of allochthonous carbonate and noncarbonate sediments that occur in the Nubrigyn Formation. Because of subsequent tectonic and ero-COWRA Parkes Plafform (stable shelf) 50kms ^

-M0L0NG

TROUGH-

» - 0 E V . ROCKS ERODEO-

- D E V . ROCKS EXPOSEO-

sion al events, nearly all of the former Devonian platform margins have been lost (compare Figs. 2 and 14). The reconstructed cross section of the region (Fig. 14) suggests that the former Garra platform margin lay approximately 10 km west of preserved Nubrigyn terrane, so that transportation of the Nubrigyn allochthonous carbonate debris involved distances of at least this order of magnitude.

The debris deposits of the Nubrigyn Formation comprise a large volume of carbonate sedimentary and associated volcanic rocks.

Either these deposits are more common at platform margins than now known or special and recurrent triggering mechanisms such as tsunamis or earthquakes were involved during accumulation of the Nubrigyn deposits. We visualize the unstable platform margin as periodically shaken by earthquakes, resulting in repeated dislodgement of carbonates and associated sedimentary and volcanic rocks, generating sediment gravity flows into the adjacent Hill End Trough. Other triggering mechanisms for initial dislodgement probably included storm waves and mechanical failure and gravitational collapse of slopes overloaded and oversteepened by outbuilding reefs (theoretical aspects of these and other mechanisms have been discussed in detail by Cook and others [1972] and Mountjoy and others [1972b]).

PALEOGEOGRAPHY Recent paleogeographic reconstructions of the Paleozoic eastern Australian Tasman mobile belt view its history in terms of tectonic analogues in modern marginal-sea areas of the southwest Pacific (Oversby, 1971; Scheibner, 1972, 1973; Packham, 1973; Packham and Leitch, 1974). The mobile belt is seen as a compound accretionary welt of deformed island-arc, trench, and associated marginal sea and shoal-water sediments, largely generated and eventually welded onto the eastern edge of the Australian craton through prolonged and continued interaction between a paleo-AustralianAntarctic and a paleo-Pacific plate.

The reconstructed Molong Arch—western Hill End Trough paleogeography during the time of deposition of the Garra and Nubrigyn Formations (Fig. 14) incorporates features of modern marginal-sea—island-arc areas in regions of associated active carbonate buildup (such as occur, for example, in Melanesia; see Ladd, 1971, p. 1 2 8 3 - 1 2 8 5 ) . This reconstruction is based largely on information in Packham (1969) and Webby (1972) and on new data reported herein. Silurian and Devonian successions are essentially conformable throughout the Hill End Trough region, but Silurian and Ordovician rocks are shown undifferentiated in Figure 14 as "basement." The depicted normal faulting is consistent with

ARCH-

—DEVONIAN ROCKS ERODED-

-HILL

END

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- D E V O N I A N ROCKS EXPOSED-

active volcanic ridge (Capertee Arch) 5 0 kms ^

STUART TOWN

L

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PLATFORM FACIES bioherms limestone shale and mudstone dostics

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volcanic sandstones allodapic limestone megodasts breccias 8 megobreccias pillow lavas pyroclostics lavos 8 volcanic intrusions

0 1 2 3 I—H-1—• 0 1 2 3 4 3

4 '

51

1 10

NO VERTICAL EXAGGERATION

Figure 14. Palinspastic reconstruction of section shown in Figure 2 for time of deposition of Garra and Nubrigyn Formations. Topographic relief on "basement" hypothetical and schematic, but depicted normal faulting is consistent with currently popular view that Hill End Trough developed during Early Silurian time as an extensional interarc basin through rifting. Water depths analogous to those of modern marginal seas are assumed and incorporated in the reconstruction. Reconstructed submarine slopes approximate those flanking modern Pacific atolls and other oceanic carbonate buildups. Stratigraphic unit labels as in Figure 1.

NUBRIGYN ALGAL REEFS, EASTERN AUSTRALIA

current interpretations that (1) the Hill End Trough developed during Early Silurian time as an extensional interarc basin through rifting and lateral separation of Ordovician rocks of the Molong Arch (Scheibner, 1972, Figs. 5, 6), and (2) continued eastward extension of the region later in Silurian time led to transformation of the Hill End Trough into a marginal sea prior to the period in which the Garra and Nubrigyn Formations accumulated (Scheibner, 1972, Figs. 7, 8).

ALLOCHTHONOUS CARBONATES ELSEWHERE IN EASTERN AUSTRALIA Carbonate megaclasts and (or) megabreccias occur in a number of Upper Ordovician, Silurian, and Lower Devonian formations elsewhere in the region of the Hill End Trough — for example, in the Upper Ordovician Malongulli Formation, the Silurian Wallace Shale, the Devonian Turondale and Cunningham Formations, and within the Cuga Burga Volcanics. The displacement of allochthonous shoal-water carbonate debris into deeper water facies provides a possible explanation for other Lower Devonian carbonate lenses and breccias within the Tamworth Trough to the northeast, particularly in the Drik Drik, Seven Mile, Silver Gully, and Yarrimie Formations. There are occurrences of similar carbonates farther afield in Silurian and Devonian rocks of Queensland and central and eastern Victoria. In the Walhalla synclinorium of east-central Victoria, several lenticular limestones are discordant with and clearly were lithified before incorporation in the enclosing strata; examples are at Marble Creek near Toongabbie (Kitson, 1925; Talent and Philip, 1956) and at Loyola. In some cases they are associated with large-scale megabreccias and debris flows or slides (Talent, unpub. data); examples occur at Coopers Creek (Thomas, 1942), at Tyers River (Kenny, 1937; Philip, 1962), and in the Silurian Cowombat Group of the Limestone Creek area of eastern Victoria (Talent, 1965). Condensed details of these and other examples of allochthonous carbonate deposits of eastern Australia — many previously unrecognized as such — will be given elsewhere (Edgecombe and others, in prep.).

In many of these examples, the allochthonous nature of component limestone bodies has previously gone unrecognized, probably, in part, because of their large size and poor exposure — particularly exposure of contact relationships with contiguous sediments.

Accordingly, studies of facing relationships using geopetals (like that reported here for the Nubrigyn Formation) have never been attempted, to our knowledge; certainly we know of no reports of such studies on the stratigraphic units cited above. However, even in the absence of more detailed supporting evidence, the widespread anomalous association of shallow-water "reefal" limestones with rocks of relatively deeper water aspect (radiolarian cherts, spilitic pillow lavas, thick successions of flysch) in the examples given provides grounds for suspecting that many of these limestone bodies are indeed allochthonous.

ACKNOWLEDGMENTS This work has been made possible by financial and technical assistance from Macquarie University. Additionally, Mountjoy received financial support from the British Council and the National Research Council of Canada, and Edgecombe received support from an Australian Government Commonwealth Scholarship. We thank John G. Byrnes for access to his unpublished maps of the Wellington-Stuart Town area. Numerous colleagues contributed beneficial discussion and comment on various drafts of the manuscript, including J. G. Byrnes, R. Cas, N. James, J. G. Jones, C. F. Kahle, C. McA. Powell, J. J. Veevers, and B. D. Webby. Jack Harris of Canobla, Hal Harris of Merrimount, Col Milne, and George Eade, all property owners in the Nubrigyn field area, gave free access to their properties and helped in other ways during field work. Oblique aerial photographs of Nubrigyn Formation exposures have proved an invaluable aid and result from the skills of

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Ross Blackwood (pilot) and Chris Gregory (photographer) of Macquarie University.

APPENDIX 1. DEFINITIONS OF TERMS USED Allochthonous — denotes any materials in which the dominant constituents have not formed in place but rather have been transported from their original depositional site into a significantly different depositional environment.

Allodapic limestone — a term proposed by Meischner (1964, p. 173) for intercalations of detrital limestone deposited in quiet waters within clayey rocks. They are made up of material mainly derived from distant areas of different facies, mostly shallow-water, marine, reefcarbonate detritus. Meischner interpreted these allodapic carbonates to be deposits of turbidity currents.

Clast — a general term for clastic fragments of any shape and size. The term is used here, variously, both in the above general sense, and in a more restricted sense to denote clastic fragments smaller than blocks, that is, smaller than 4 m in diameter.

Block or megaclast — a clastic fragment larger than boulder size, that is, larger than 4 m in diameter. Block and megaclast are used here in a purely descriptive sense for sedimentary components that are exotic.

Accordingly, the terms are, in practice, synonymous with olistolith as recently redefined by Abbate and others (1970), although the latter term is not used herein. Block and megaclast are used interchangeablyParabreccia and orthobreccia — the distinction is essentially similar to that between orthoconglomerate and paraconglomerate (see Jones, 1970, p. 128). Breccia denotes fragmental rocks consisting substantially (more than 25 percent) of angular clasts larger than 1 cm, and two basic types are distinguished: parabreccias characteristically display the texturally heterogeneous aspect of phenoclasts in a matrix (that is, the clasts are matrix supported), whereas orthobreccias have a texturally homogeneous aspect insofar as no such division is apparent (in other words, the fabric is one of clast support).

Megabreccia — deposit in which angular clasts larger than 1 m across are conspicuous components and which may contain some clasts many metres across. Many clasts of these deposits are smaller than 1 m and may be rounded rather than angular. This definition follows that used

by Cook and others (1972, p. 443). The megabreccias described herein exhibit predominantly matrix-supported fabrics and therefore may be termed megaparabreccias by extension of the breccia terminology given above.

Flysch (noun) and flyschoid (adjective) — these are used here simply in the sense of "graded bedded facies," that is, for sedimentary terranes characterized by sharply defined graded arenite and rudite beds of strong lateral persistence and uniformity of bed thickness rhythmically interbedded with thin lutites. N o other connotations are implied.

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1967, Factors governing the development of the Frasnian, Miette and Ancient Wall reef complexes (banks and biostromes), Alberta, in Oswald, D. H., ed., International symposium on the Devonian System: Calgary, Alberta Soc. Petroleum Geologists, v. 2, p. 3 8 7 - 4 0 8 .

Mountjoy, E. W., and Playford, P. E., 1972, Submarine megabreccia debris flows and slumped blocks of the Devonian of Australia and Alberta — A comparison [abs.]: Am. Assoc. Petroleum Geologists Bull., v. 56, p. 641.

Mountjoy, E. W., Conaghan, P. J., Edgecombe, D. R., Talent, J. A., and Owen, D. E., 1972a, Devonian algal 'reefs' (Nubrigyn) eastern Australia — Allochthonous blocks in debris flow megabreccias: Geol. Soc. America Abs. with Programs, v. 4, p. 6 0 5 - 6 0 6 .

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Newell, N . D., Rigby, J. K., Fischer, A. G., Whitenian, A. J., Hickox, J. E., and Bradley, J. S., 1953, The Permian reef complex of the Guadalupe Mountains region, Texas and N e w Mexico — A study in paleoecology: San Francisco, W. H. Freeman & Co., 236 p.

Offenberg, A. C., Rose, D. M., and Packham, G. H., 1971, Dubbo, Sheet Sl/55-4: N e w South Wales Geol. Survey, 1:250,000 Geol. Ser. Oversby, B., 1971, Palaeozoic plate tectonics in the southern Tasman geosyncline: Nature Phys. Sci., v. 234, p. 4 5 - 4 7 , 60.

Packham, G. H., 1958, Stratigraphic studies in the older Palaeozoic rocks of the Tasman geosyncline in central-western N e w South Wales [Ph.D. thesis]: Sydney, Univ. Sydney, 2 0 0 p.

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