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Volume 85, 1957-58
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The Geology of the Clarence Valley from Gore Stream to Bluff Hill

[Recived by the Editor, May 30, 1957.]

Abstract

Cretaceous rocks, Motuan (Turonian) to Haumurian (Maestrichtian) in age, rest with major unconformity on indurated sandstone and mudstone, known to be at one place Jurassic (Heterian?).

The Split Rock Formation (Motuan—?Ngaterian) is mainly sandstone and mudstone lithologically similar to Clarence Series sandstone and mudstone at Coverham. The Gridiron Formation (Ngaterian), resting unconformably on the Split Rock Formation or on Jurassic Rocks, is mainly fresh-water or marine sandstone interbedded with volcanic rocks and is different from either the Clarence or Raukumara Series rocks of Coverham. The Seymour Formation (Arowhanan–Haumurian), conformable on the Gridiron Formation, is marine sandstone overlain by sulphureous siltstone and sandstone that are similar to Haumurian beds at Coverham. The Cretaceous sequence as a whole is transitional in character between the lower Clarence valley and North Canterbury sequences.

The Tertiary marine formations, unconformable on the Cretaceous, are briefly discussed.

Introduction

McKay examined the Clarence Valley in 1885 during a reconnaissance survey of a great part of Marlborough. His report (1886) remains an important source of information even though his interpretation of the Cretaceous and Tertiary sequences is now difficult to follow because his correlations were fitted into the classification then used by the Geological Survey. Thomson's visits about thirty years later were followed by determination of fossils by Woods (1917) which encouraged Thomson (1919) in his revision of the stratigraphy to group all the Cretaceous rocks throughout the Clarence Valley as “Clarentian”. The apparently unbroken sequence that he described fitted the then new stratigraphic concept of the Notocene (Thomson, 1917), and its unity was accepted despite:

  • (1) the differences between the comparatively thin “Clarentian” section in Seymour Stream, comprising mainly coal measures, basalts and muddy sandstones, and the thick “Clarentian” marine sandstones and mudstones of the Coverham area:

  • (2) the absence of recorded fossils common to both the “Clarentian” of Coverham and the “Glarentian” of Bluff River and Seymour Stream, apart from the undiagnostic Belemnites superstes; and.

  • (3) Thomson's record of Inoceramus in sandstone and mudstone unconformably below the coal measures in middle Seymour (Herring) Stream—because of the unconformity he was forced by his concept of the Notocene to place these Inoceramus-bearing beds in the pre-Notocene.

Consideration of these three difficulties led to the hope that re-mapping of the district from Gore River to Bluff Hill (the area indicated in Fig. 1) would provide important stratigraphic information, and a party led by Mr. H. E. Fyfe spent three weeks in the area in 1953, being based on the old homestead at Quail Flat. Detailed mapping on a scale of 20 chains to an inch required the addition of topographical detail obtained by pace and compass traverses to an outline of main rivers and streams taken from the N.Z.M.S. 1 (one mile to an inch) topographical maps.

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Fig. 1.—Locality Map.

The greater part of the time was devoted to mapping the Cretaceous and Tertiary rocks, but many observations of the pre-Cretaceous rocks were also made. Geological observations by Messrs. H. E. Fyfe, G. W. Grindley, D. R. McQueen, R. A. Couper, and the author have been incorporated in the geological map, Fig. 2. The important lower Seymour Stream section was examined by the whole party; the first three members examined the Gore Stream area and, more briefly, the Bluff River area; and the latter two members examined the middle Seymour Stream section and the area from Limestone Hill to Dubious Stream.

After the completion of the mapping, the sequence of the Cretaceous rocks in these areas was not easily fitted into the New Zealand sequence, and the uncertainties are not entirely resolved as a result of the recent revision of the Cretaceous sequence at Coverham by Wellman (1955) and of the New Zealand Cretaceous Inoceramus species, of which preliminary notice has been given by Wellman (1956).* Nevertheless the district contains an important part of the record of the Cretaceous geological history of the north-east part of the South Island.

Stratigraphy

Introduction

The Tertiary formations used by McKay and Thomson are satisfactory units, but new names are required for the Cretaceous rock units. Fossils are too few to allow mapping of Cretaceous rocks in time-rock units, as Wellman did for rocks of the same age in the Coverham area; consequently three formations are used, even though rapid changes in lithology are found within the formations. The pre-Cretaceous rocks are not subdivided, although this might be possible with more detailed mapping. Table I (p. 406) shows the sequence of formations with their ages in terms of New Zealand and European units.

[Footnote] * The designations Inoceramus sp. R., etc., used in the present paper are those given by Wellman to species yet to be described.

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Jurassic Rocks.

The greater part of the area is formed of indurated sandstone and siltstone (greywacke and argillite) many thousands of feet thick. Within the mapped area the rocks of the Inland Kaikoura Range appear in general to be somewhat more indurated than those of the Seaward Kaikoura Range but no differences sufficient to justify distinguishing the rocks of the two ranges were observed. The rocks of the Inland Kaikoura Range were, however, less closely examined. Locally the rocks are intensely sheared, particularly where sandstone bands are thin and comprise the minor portion of the sequence.

Sandstones with “cannon-ball” concretions are widely distributed in the south-west part of the district, where the rocks are least indurated, and some rocks, for example 1 mile north-east of Trig GZ and in the extreme south-west, are no more indurated than early Tertiary rocks in bituminous coalfields of the West Coast.

In Gore Stream, in the extreme south-west, black mudstone with minor sand-stone contains several thin beds of a dark, fine, green-weathering rock that appears to be an altered volcanic flow or tuff. Fifty feet above these volcanics, dark shattered argillite contains fossiliferous concretions and a thin shell bed. A collection (GS 5828) by Messrs. G. W. Grindley and D. R. McQueen contained Buchia aff. subpallasi (Krumbeck) (ex gr. mosquensis), Anopeia n.sp. and Inoceramus sp. from the concretions, and Buchia cf. malayomaorica (Krumbeck) from the shell bed. In the.

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Fig. 2.—Geological Map of Clarence Valley from Gore Stream to Bluff River.

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opinion of Dr. C. A. Fleming these fossils indicate an upper Jurassic age, and are probably Tithonian. Judged by induration, most of the sandstone and siltstone of the ranges is older than the fossiliferous beds, but none may be older than Jurassic. Some of the least indurated rocks may perhaps be of early Cretaceous age.

The structure of the Jurassic rocks appears to be a series of anticlines and synclines with dips of strata averaging 55° to 60°. No evidence of overturning was obtained. Detailed observations were made in only a minor part of the district on the south side of the Clarence River from Gore Stream east to Tytler Stream, and in somewhat less detail further north-east to Limestone Stream. A syncline trends north between Limestone Stream and Tytler Stream, its west flank being separated from the next main structure to the west by a major fault—the Split Rock Fault.

Split Rock Formation.

The following important sequence in the type locality of the Split Rock Formation, in the extreme south near the junctions of Split Rock and Black Rock streams with Seymour Stream south of Trig GZ, shows the unconformable relations of the Split Rock Formation to both the underlying and overlying rocks.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

(Split Rock Fault) Feet
Gridiron Formation Basalt flows 150
Coal Measures 60
Conglomerate of well-rounded pebbles of Jurassic rock up to 2in Angular Unconformity 35° 10
Split Rock Formation Alternating sandstone and mudstone 25
Weathered tuff (or basalt) 5
Sandstone and mudstone 8
Weathered tuff (or basalt?) 5
Alternating green sandstone and blue-grey siltstone; shelly grit with Inoceramus sp. I near base 125
Conglomerate (Jurassic rocks and “cannon-ball” concretions) and mudstone 5–15
Dark blue-grey siltstone with concretionary bands 80
Conglomerate of angular and sub-angular blocks of underlying rock (up to 9ft); many “cannon-ball” concretions 0–50
Angular Unconformity 60°
Jurassic Rocks Siltstone and indurated sandstone with “cannon-ball” concretions.

The two unconformities are well exposed, the lower one on the spur south of Bluff Dump Hut, and the upper a few chains south of the mouth of Black Rock Stream.

Boulders on the hillside south-west of the hut, derived from the shelly grit (see table above), yielded the following fossils (GS 5830): Arca sp., Aucellina sp., Inoceramus sp. I, Variamussium sp., Syncyclonema sp. A collection by McKay (GS 572), although marked “Quail Flat, Clarence Valley,” has an identical matrix, and contains the same genera with the exception of Inoceramus, and with the addition of Nuculana sp. The probability that GS 572 is from this locality is strengthened by McKay's record of “Conglomerate with shells” in this area (1886: 104). Thomson (1919: 306) noted presence of Inoceramus in “the rocks lying unconformably below the eastern Clarentian outcrop” in Seymour Stream, and his specimens at the Geological Survey in Wellington are similar to the Inoceramus sp. I of GS 5830. Thomson knew of the unconformity above the Split Rock Formation but not of the unconformity below; hence his inclusion of these beds as part of the “pre-Notocene” rocks. Inoceramus sp. I is of Motuan (early Turonian) age. Samples (F 7963–6), from a few feet above the base of Split Rock Formation up to a horizon 80 feet above the shelly grit, yielded poor foraminiferal faunas that Mr. N. de B. Hornibrook states are similar to those obtained from the Clarence Series in the Coverham area.

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The sediments of the Split Rock Formation resemble the Clarence Series mudstones and sandstones of the Coverham area, and contrast markedly with the sediments of the younger Gridiron Formation. Judged by lithology, three areas in the extreme north-east contain rocks of the Split Rock Formation, as described below.

On the north-west side of the syncline at the mouth of Dubious Stream the Split Rock Formation appears to be unfossiliferous, comprising:

Feet
Mudstone with concretionary bands; carbonaceous flecks 240
Sandstone with mudstone fragments, and bands of mudstone 80
Conglomerate (Jurassic rocks including quartz-veined greywacke and jaspilite) 3

The basal conglomerate rests on well-jointed indurated sandstone and siltstone, which presents a strong contrast with the softer little-jointed sediments of the Split Rock Formation. The Split Rock Formation is succeeded by the Gridiron Formation without angular unconformity.

Two and a half miles to the south, in the creek entering the south of Goose Flat, interbedded sandstone and mudstone dip steeply west between the Jurassic rocks and the moderately north-west-dipping conglomerate of the Gridiron Formation. No contacts are exposed, and on lithological grounds a narrow strip is mapped as Split Rock Formation.

Thomson (1919: 323) records the greatest available detail of the section on the north bank of the Clarence River immediately downstream from Bluff River. Beneath the massive sandstone with pebble beds and sparse fossils of the Gridiron Formation, he gives the following sequence, indicating a thin section of probable Split Rock Formation:

Feet
Thin-bedded alternating sandstone and mudstone 40
Grey sandstone with occasional pebble beds 26–30

Contact not seen

Jurassic rocks

In Bluff River, a little more than a mile to the north of the above section, several hundred feet of alternating thin-bedded sandstone and mudstone dip south-east. From the general structure of the Gridiron and overlying formations, it would be expected that if these beds were conformable beneath the Gridiron Formation they would dip north-west. An unconformity is inferred, and these beds are mapped as Split Rock Formation. Mr. H. E. Fyfe collected specimens of Inoceramus sp. F (GS 5829), of Ngaterian (late Turonian) age.

Gridiron Formation

Where well exposed on the bluffs south-east of trig Gridiron, and in the Seymour River one mile south-west of the trig (type locality), the Gridiron Formation comprises volcanics and a minor proportion of freshwater sediments. In the north-east part of the district the volcanics form the minor part, and the sediments are marine.

Over the greater part of the district the Gridiron Formation rests unconformably on Jurassic rocks, and where it rests on the Split Rock Formation it is also usually unconformable, although no unconformity is evident at Dubious Stream. The Gridiron Formation is taken to extend up to the top of the highest basalt, and to include also any immediately overlying freshwater beds similar to those interbedded with basalt; these are known only in the extreme south-west where they are a few feet thick. The following sequences show the extreme variations of the Gridiron Formation:

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Gridiron Formation, Seymour Stream From 1.7 To 1.3 Miles Upstream from Clarence River.

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Feet
Basalt flow 30
Sandstone and siltstone 20
Basalt flows, totalling 300
Basalt agglomerate 50
Basalt flow, columnar 80
Coal measures, sandstone and mudstone 5
Basalt flow, discordant contact at base 80
Coal meaures—sandstone, mudstone and thin stony coal 60

Gridiron Formation, Dubious Stream, 0.1 To 0.3 Miles Upstream From Clarence River.

Feet
Basalt flow 10
Sandstone 2
Basalt flow 40
Fine sandstone, large concretions, fragments of Inoceramus 160
Sulphureous sandstone and mudstone, fragments of Inoceramus 70
Conglomerate—pebbles of Jurassic rocks, including jaspilite, and mudstone fragments 1

Gridiron Formation: Generalised Section S.W. End of Goose Flat.

Feet
Basalt 250
Sandstones, many with pebble bands, many with concretions containing fragments of Inoceramus; some fossils towards base 500

In the lower Seymour Stream area, and particularly to the north between trig Gridiron and the Clarence River, the Gridiron Formation consists mainly of contemporaneous volcanics. Agglomerates are known to the south-west of Seymour Stream, tuffs in Red Hill, and volcanic conglomerates north-west of trig Gridiron and in the Tytler Stream outlier. From Limestone Hill north-east to Dubious Stream only flows are known. Fig. 3 shows the thickness of Gridiron Formation volcanics in the district. The axis of maximum thickness is parallel to the valley, with the general thickness diminishing north-eastwards presumably from a centre of eruption in the south-west; the number of flows also decreases north-eastwards, only two being known in the extreme north-east, in contrast to at least seven in the lower Seymour Stream. It seems certain that these volcanics represent a single period of igneous activity, freshwater or marine sediments being laid down during pauses between eruptions. Not all the volcanics are, however, completely concordant with the beds above and below. In the lower Seymour Stream section, coal measures underlying the lowest basalt and between the first two flows are truncated slightly, but as the coal measures do not appear to be “baked” they were presumably watersaturated at the time the basalt flowed over the top. More difficult to account for is a “raft” of sediment about 10 chains long apparently separated from the main mass of sediment, in the cliff in the south bank of the Clarence River at the south-west of Goose Flat. The contacts could not be examined, but one flow appears to have forced its way beneath the surface of recently deposited sediment. Despite the discordant contacts the volcanics are accepted as representing a period of activity substantially contemporaneous with those sediments.

Marine macrofossils are found in the north-east and plant fossils in the south-west. Fossils collected from Quail Flat by McKay (GS 571) and described by him as fresh-water shells are poorly preserved and not diagnostic of age.

Pteridophytes and cycads from the coal measures of Quail Flat and Seymour Stream have been recently described by McQueen (1956), who lists the following from collections B125, B166, B351, B356 and B358: Cladophlebis australis (Morr.),

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Fig. 3.—Thickness of Ngaterian Volcanics (flows, agglomerates and tuffs).

Coniopteris? lobata (Oldham), Phyllopteris expansa Walk., Lycopodium cf. volubile Forst. f., Sphenopteris mackayi n.sp., Taeniopteris stipulata Hect., Pterophyllum clarencianum McQueen, Ptilophyllum seymouricum McQueen.

The angiosperms and conifers from these collections have not yet been described, nor have floras from the middle Seymour Stream and from the creek west-north-west of Warder trig. Samples for plant microfossil study, from lower and middle Seymour Stream, from Quail Flat, from the Clarence River west of the mouth of Tytler Stream, and from Red Hill Stream, some of which contain rich floras, have yet to be fully examined.

McQueen (1956; 675) concludes that although Pterophyllum “is apparently an archaic element”, the coal measures at the base of the Gridiron Formation (described as “Seymour River coal measures” by McQueen), “are probably post-Cenomanian and may well be Turonian. …” It is clear, however, that the stratigraphic relations to the older Split Rock Formation and younger Seymour Formation provide more conclusive evidence of the Ngaterian age of the coal measures in the Seymour Stream area.

McKay's collection (GS 615) from “Bluff River” was probably obtained (McKay, 1886: 97) primarily from the Clarence River below the junction of Bluff River, on the south-east side of the anticline, since McKay's list of fossils from the Clarence River is similar to that for the collection numbered 615 now in the Geological Survey. Some may, however, have come from Bluff River itself, on the

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north-west side of the anticline, where McKay noted “fragments of a large Inoceramus and great numbers of a large species of Trigonia”. Dr. C. A. Fleming gives the following list for GS 615: Iotrigonia glyptica (Woods), Inoceramus cf. porrectus Woods, Modiolus kaikourensis Woods, Thracia sp., Dimetobelus superstes (Hector), Natica sp., Perrisoptera sp., Dentalium sp.

Of these, Iotrigonia, Modiolus, Thracia, and Dimetobelus were recorded by Woods (1917), and Natica and Perrisoptera by Wilckens (1922), but Wilckens wrongly gave the locality as Cover Creek, in the Coverham area. The Inoceramus cf. porrectus which indicates a Ngaterian (late Turonian) age for this assemblage, was apparently not submitted to Woods. Although not mentioned by Thomson, his collection (GS 855) from the north bank of the Clarence River below the junction of Bluff River, now at the Geological Survey, includes an Inoceramus, possibly I. sp. F. Both I. porrectus and I. sp. F are key fossils of the Ngaterian stage (Wellman, 1956). Since I. sp. F was also collected from the Split Rock Formation in Bluff River (supra), the change of facies, from thin-bedded alternating mudstone and sandstone of the Split Rock Formation to the massive sandstone with pebble beds of the Gridiron Formation, appears to have been particularly rapid. The sequence in the Bluff River area clearly needs detailed re-examination.

Seymour Formation

Seymour Stream crosses the Seymour Formation from 1.3 to 1.0 miles upstream from Clarence River, exposing the following type section:

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

Feet
Well-bedded limestone (Amuri Limestone)
Unconformity3°
Seymour Formation Dark glauconitic sandstone 10
Greensand and dark siltstone 200
Dark siltstone and sandstone, sulphureous 600
Not exposed 60
Bentonitic sandstone and fine conglomerate 30
Not exposed 60
Green sandstone with concretionary bands, tuffaceous and fossiliferous 40
Basalt (Gridiron Formation)

The dominant lithology is fine sandstone; dark siltstone forms the minor part of the sequence; sulphur efflorescence shows in the greater part. Three miles to the south-west, in a small tributary of Gore River, a syncline shows the following sequences of the Seymour Formation:

North-West Side of Syncline South-East Side of Syncline
Feet Feet
Sulphureous sandstone 800 Sulphureous sandstone 800
Green sandstone with pebbly sandstone bands 300 Conglomerate 10
Shell beds in glauconitic sandstone 40
Green sandstone; dark siltstone partings 500

The sulphureous appearance of the beds is considered a facies indicator and not necessarily of age significance Since the 800 feet of sulphureous sandstone is in the centre of the syncline and there is no reason to suppose an unconformity within the Seymour Formation and the underlying Gridiron Formation, the section is considered to thin south-eastwards from 1,600 to 800 feet from the north-west side to the south-east side of the syncline. In the Fell, half way between this section and the lower Seymour Stream section. Trigonia shell beds are exposed in the creek 0.6 miles from the Clarence River, and fossiliferous sandstone on a ridge 0.6 miles further south-south-east, to which it extends from Seymour Stream. Iotrigonia glyptica (Woods) and Esalaevitrigonia meridiana (Woods) are the commonest fossils in both the shell beds and in the fossiliferous sandstone, which must be several hundreds of feet apart stratigraphically even allowing for probable gross south-eastward thinning of the beds.

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In the north-east, from Limestone Hill to Bluff Hill, the basalt at the top of the Gridiron Formation is followed by poorly bedded fine sulphureous sandstone which is glauconitic close beneath the base of the overlying Amuri Limestone. The syncline in Dubious Stream shows only 100ft of the Seymour Formation, in which no fragments of Inoceramus and no large concretions were noted, in contrast to those seen in the sulphureous sandstones of the Gridiron Formation in this section.

It appears that sulphur efflorescence, which Wellman (1955; 103) has insisted is distinctive of the Whangai Shale (Haumurian) of the Coverham area, is an indicator of facies rather than of age. Wellman noted that in the Clarence Valley between Coverham and the district now being described, the Whangai Shale progressively changes south-westwards into sulphureous sandstone with large concretions, but the sulphureous beds that contain the concretions in the Dubious Stream section are about Ngaterian in age, considerably older than the Haumurian Whangai shale. The sulphureous beds without concretions and above the basalt are probably little younger than Ngaterian.

Macrofossils have been collected within the lowest 40 feet of the Seymour Formation on both the south-west and north-east sides of Sevmour Stream, and from the shell beds in the Fell. Dr. Fleming has identified the following fossils:

GS 5825 GS 5826 GS 5827
S. W. Seymour N. E. Seymour The Fell
Esalaevitrigonia meridiana (Woods) x x x
Iotrigonia glyptica (Woods) x x x
Inoceramus sp. x x x
Modiolus kaikourensis (Woods) x
Pleurotomaria sp. x
? Astarte sp. x
? Ostrea sp. x
Dimetobelus sp. x x

Only fragments of Inoceramus were collected, but Dr. H. W. Wellman considers those from GS 5825 to be an unnamed giant species (I. sp. R) that marks the Arowhanan stage (Coniacian). The remainder of the fossils are not diagnostic of age.

About 150 feet higher in the Seymour Stream sequence a sample (F 7967) from the lowest exposed sulphureous sandstone beds yielded Foraminifera considered by Mr. N. de B. Hornibrook to be Haumurian (Maestrichtian) in age: similar faunas containing Gaudrvina healyi and Dorothia elongata were obtained higher in the formation (F 8244), but samples within 200 feet of the base of the Amuri Limestone were unfossiliferous. In the Fell, the Amuri Limestone is only about 50 feet above the? Arowhanan shell band, and the younger part of the Seymour Formation appears to be cut out by unconformity.

Thus the Seymour Formation ranges from Arowhanan (Coniacian) to Haumurian (Maestrichtian) in age, but the Mangaotanean. Teratan, and Piripauan stages (Santonian to Campanian) are not recorded by fossils. There being no reason to suppose an unconformity within the Seymour Formation, it may be suggested that slow deposition may have continued through this time; unfossiliferous bentonitic sandy silt and minor conglomerate, together with obscured parts of the section, perhaps represent these stages in the Seymour River, but in the north-east the whole sequence appears to be sulphureous sands and silts.

Tertiary Rocks.

The sequence of Tertiary beds, from the Great Marlborough Conglomerate at the top to the base of the Flint Beds or, in their absence, the base of the Amuri Limestone, is as described by McKay and Thomson, but some new detail is worth recording, in particular the more precise ages now available from new fossil determinations. The formation names used by both McKay and Thomson are retained, the only one over which confusion might arise being Amuri Limestone. Since, however, the

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lithology of the Amuri Limestone is usually similar to, and the age range within that of, the Amuri Limestone at Amuri Bluff, the retention of the name seems justified. Table I shows the age range of the full Tertiary sequence.

Flint Beds. These are recorded only by Thomson (1919: 328) from Bluff River and Gentle Annie, but they are absent from Limestone Hill and further to the south-west. They lie conformably beneath the Amuri Limestone, are only about a hundred feet thick so that may represent no more than about one older stage, and are shown as Mangaorapan in Table I.

Amuri Limestone. This follows the Flint Beds in the extreme north-east, but is the basal bed of the Tertiary sequence in other areas. As recorded by Thomson (1919: 329) it rests unconformably on the Seymour Formation on the south-east side of the syncline in the lower Seymour Stream section. Further downstream, on the north-west side of the adjoining anticline, the contact between the Amuri Limestone and the Seymour Formation is difficult to define, possibly owing to re-sorting of the uppermost part of the Seymour Formation. No angular unconformity was seen there. Microfaunas from the Amuri Limestone of the lower Seymour Stream area represent the Heretaungan, Porangan, and lower Bortonian stages.

In a small area of Limestone Hill tuff within the Amuri Limestone is locally up to 300ft thick and contains lumps of basalt up to a foot across and churned-up masses of red and green siltstone. Microfaunas from close below, within, and close above the volcanics show them to be upper Bortonian in age, as is vesicular basalt in the Kekerangu area (Macpherson, 1951: 268).

Weka Pass Stone. Although no angular unconformity can be observed, the contact between the Weka Pass Stone and the Amuri Limestone is markedly irregular at the well-exposed contact on the north-west side of the anticline in the lower Seymour Stream, where the lowest bed is a worm-bored greensand with phosphatic nodules at the base. Microfaunas show the base of the Weka Pass Stone to be Whaingaroan, so that the upper part of the Bortonian and the Kaiatan and Runangan stages are missing. Thin tuff beds in the lower part of the Weka Pass Stone show in lower Seymour Stream but are apparently absent elsewhere. Microfaunas from the top of the Weka Pass Stone are Waitakian.

An important small area of Weka Pass Stone lies north-west of Quail Flat Fault near trig Quail, south-west of the mouth of Seymour Stream. In this section a.

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Table I
Formation Principal Lithology Stage Series Age
Great Morlborough Conglomerate Conqlomerate Cllfdenion Altoman Southland Miocene Tertiary
Gray Marl Calcareous silfstone Awamoan Hutchinsoman Otalan Pareora Oligocene
Weka Pass Stone Calcareous sandstone Waltakian Duntroonian Whaingaroan London
Disconformity Runangan Kaiaton Bortonian Arnold Eocene
[ unclear: ] [ unclear: ] Porangan Heretoungan Monogropan Waipowan [ unclear: ] Paleocene
Flint beds Unconformity Bedded flint (Wongoloon) Teurlan Houmurlan Piripauan Mala Danian Moest [ unclear: ]
Seymour Formation Glaucomic sandstone Sulphurous siltstone Sulphurous sandstone Sandstone and congiom. Sandstone [ unclear: ] Teratan Mongoaotanean Arowhanan Roukumara Senonian
Gridfron Formation basalt and tuff cool measures, conglom Unconformity Sandstome and mudstone Ngoterian Motuan Clarenc Turonian
Split Rock Formation Unconformity Urutawan Coverian Cenomanian Albian
Pre-Clarence Inducted sandstone and mudstone [ unclear: ] Puaroan Jurassic
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conglomerate containing blocks of Amuri Limestone rests unconformably on Jurassic rocks and is followed by conglomerate and glauconitic sandstone (Whaingaroan and Duntroonian on microfaunas, F 8702–3) 70ft thick, and then by typical Weka Pass Stone. Within the conglomerate beds is one composed solely of angular fragments of pre-Cretaceous basement rocks. This section contrasts strongly with the adjacent section south-east of the fault, so as to suggest that considerable lateral movement has taken place at the fault.

Grey Marls and Great Marlborough Conglomerate. The Grey Marls grade up from the Weka Pass Stone, and contain Otaian microfaunas. No younger faunas have been obtained from the Grey Marls or from the Great Marlborough Conglomerate. For convenience the Grey Marls are shown as representing the whole of the Pareora Series in Table I and the Great Marlborough Conglomerate is shown as lower Southland Series, although a mid-upper Southland microfauna has been found further north-east, in sheet S 35, 600ft above the base of the great Marlborough Conglomerate (F 8188).

Observations on the Cretaceous Geological History

In New Zealand the Cretaceous Period still remains unsatisfactorily interpreted. At the beginning the period of Hokonui (Triassic-Jurassic) sedimentation was ending; in the late Cretaceous a new period of sedimentation, that which continued through the Tertiary, had already begun in some areas. Contrasts between these two periods of sedimentation lie in the types of sediment, the patterns of deposition, and the patterns of deformation. Sediments recording much of the intervening Cretaceous are now known, however, and they provide the clues to an important series of events in New Zealand's geological history generally known as the post-Hokonui orogeny. Throughout the Clarence Valley both the youngest of the Hokonui sediments and the oldest of the Clarence sediments are marine; there is no evidence in this region of orogenic movements producing land areas in the intervening time, and consequently sedimentation may well have been continuous at some places. In the Coverham district “the distinction between Clarentian and pre-Clarentian is not everywhere easily made” (Wellman, 1955: 110), and Thomson also appears to have had difficulty in distinguishing Clarence and pre-Clarence rocks (1919: 307).

In the Gore Stream–Bluff Hill district, however, no such difficulty confuses the stratigrapher. The distinction between the Split Rock Formation and the Jurassic (Hokonui-type) sediments is clear. The structural trends (Fig. 2) are different, and the unconformity at the base of the Split Rock Formation on the spur south-west of Bluff Dump Hut is unmistakable. Orogenic movements certainly took place before the Motuan (early Turonian), and the unconformity between Split Rock and Gridiron Formations, despite no great difference in age, indicates continued though probably waning movement. The outpouring of great thicknesses of basalt shows that instability continued into the Ngaterian (late Turonian). The probable conformity between Hokonui-type sediments and Clarentian sediments in the Coverham district, despite the contrast between the types of sediment except for transition beds, does not eliminate the need for postulating a post-Hokonui orogenic period; it merely indicates that sedimentation was locally continuous throughout the orogeny.

The full original extent of the Split Rock Formation is not known, owing to the unconformity above it, but the formation is sufficiently widespread to suggest that it may once have covered the whole of the district; as soft sediment, it would be rapidly removed if later movement brought it into the zone of erosion, either subaerial or submarine. The earliest evidence of a land area is in Ngaterian times, and land then appears to have been fairly local. The Jurassic rocks underlying the freshwater Gridiron Formation in the lower Seymour Stream section are leached, and the coal measures, the basal Gridiron sediments, are quartzose. These, the oldest of the quartzose-type coal measures so widespread in New Zealand, cannot have been

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the product of a prolonged period of subaerial erosion as is commonly assumed for quartzose coal measures. They appear to have been formed locally in a short time, during the late stages of the post-Hokonui orogenic period.

The Gore River–Bluff Hill district is small, and does not record an unbroken sequence of rocks throughout the post-Hokonui orogenic period as some other parts of Marlborough appear to do. But it is a transition area between these Marlborough areas and North Canterbury, where quartzose coal measures which underlie Piripauan or Haumurian marine beds rest with obvious unconformity on Hokonui-type sediments. The progressive change in stratigraphic sequence should be paralleled by a progressive change in structural relations between the Hokonui-type sediments and the younger rocks, and in future intensive study of the stratigraphy should be accompanied by similar study of the structures.

References

McKay, A., 1886. On the Geology of the Eastern Part of Marlborough Provincial District. N.Z. Geol. Surv. Rep. Geol. Explor. dur. 1885, 17: 27–136.

Macpherson, E. O., 1951. The Stratigraphy and Bentonitic Shale Deposits of Kekerangu and Blue Slip, Marlborough. N.Z. Jour. Sci. Tech., 33: 258–286.

McQueen, D. R., 1956. Leaves of Middle and Upper Cretaceous Pteridophytes and Cycads from New Zealand. Trans. Roy. Soc. N.Z., 83: 673–685.

Thomson, J. A., 1917. Diastrophic and Other Considerations in Classification and Correlation, and the Existence of Minor Diastrophic Districts in the Notocene. Trans, N.Z. Inst., 49: 397–413.

—— 1919. The Geology of the Middle Clarence and Ure Valleys, East Marlborough, New Zealand. Trans. N.Z. Inst., 51: 289–349.

Wellman, H. W., 1955. A Revision of the Type Clarentian Section at Coverham, Clarence Valley. S35. Trans. Roy. Soc. N.Z., 83: 93–118.

—— 1956. Cretaceous Rocks of New Zealand. Proc. XX Int. Geol. Cong. (Mexico).

Wilckens, O., 1922. The Upper Cretaceous Gastropods of New Zealand. N.Z. Geol. Surv. Pal. Bull., 9.

Woods, H., 1917. The Cretaceous Faunas of the North-Eastern Part of the South Island of New Zealand. N.Z. Geol. Surv. Pal. Bull., 4.

Mr. R. P. Suggate, N.Z. Geological Survey, Christchurch.