Environments and palynofacies of a Dinantian (Carboniferous) littoral sequence: the basal part of the Navan group, navan, country meath, Ireland

Pulaeogeography, Palaeoclimatology, Palaeoecology, 96 (1992): 175 193

175

Elsevier Science Publishers B.V., Amsterdam

Environments and palynofacies of a Dinantian (Carboniferous) littoral sequence: the basal part of the Navan Group, Navan, County Meath, Ireland A. McNestry and J.G. Rees British Geological Survey, Keyworth, Nottingham, NG I2 5GG, UK (Received June 20, 1991; revised and accepted May 26, 1992)

ABSTRACT McNestry, A. and Rees, J.G. 1992. Environments and palynofacies of a Dinantian (Carboniferous) littoral sequence: the basal part of the Navan Group, Navan, County Meath, Ireland. Palaeogeogr., Palaeoclimatol., Palaeoecol., 96: 175-193. The basal, dominantly littoral, part of the Courceyan (Dinantian, Early Carboniferous) Navan Group is described in borehole N866 from the Tara Mines area, Navan, County Meath, Ireland. The kerogen assemblages of 17 samples from the sequence are described in terms of the palynofacies they represent. Both the depositional environments and palynodebris deposited within them reflect distance from terrestrial source, which became more distant with time. The change in littoral sedimentation from dominantly siliciclastic to dominantly carbonate, near the top of the Liscartan Formation, represents drowning of the principal siliciclastic source areas on the margins of the Irish Midlands during the Courceyan (PC Miospore Biozone time). Spore colours and UV fluorescence indicate that the Navan area was not heated as much as other areas of the Irish Midlands.

Introduction

Navan, approximately 50 km NW of Dublin (Fig. 1), lies on the edge of the Lower Palaeozoic Longford Down Inlier. South of the inlier is a sequence of Carboniferous rocks, the lower part of which, the Navan Group (Ashton et al., 1986), unconformably overlies Lower Palaeozoic rocks and is composed of alluvial and shallow marine siliciclastics and carbonates. The Navan Group is poorly exposed along the margin of the inlier, but has been extensively drilled within the area of the Tara Zn-Pb Mines, west of Navan Town. The Navan Group is overlain by limestones deposited in deeper water environments, The oldest marine rocks of the Navan Group, comprising varied, transitional marine, supra- and inter-tidal deposits (Ashton et al., 1986), were Correspondence to: A. McNestry, British Geological Survey, Keyworth, Nottingham, NGI2 5GG, UK. 0031-0182/92/$05.00

((~, 1992

shown by Philcox (1984) to be more varied in the mine area than elsewhere in the North Midlands Province. This variability encouraged a study of the distribution and preservation of palynodebris within transitional marine facies. Prior to this the only detailed palynofacies study of a Carboniferous littoral sequence was that described by Davies et al. (1991) of an alluvial to fully marine transition, in which the littoral section is less well developed than that at Navan. Borehole N866 (stored by Tara Mines), drilled on the northwestern margin of the mine area, was selected for study. The part of the borehole below 535 m (downhole), hereafter referred to as "the section" (Fig. 2), contains a dominantly littoral sequence between alluvial and principally open marine sequences. Seventeen samples from the section were prepared using standard processing techniques, including treatment with HCI and HF; hot HC1 was used to dissolve any residual mineral. The

Elsevier Science Publishers B.V. All rights reserved

176

A. M c N E S T R Y A N D J.G. REES

Longford Down Inller J

N J

0

1

2

3

BoRE.o,E

N866

Tara mine area (//

~

-Dn ia N ,nAa tiVnAN

~ ~,_ •

•

Fingal Group

II

[~

C r u i c e t o w n Group Navan Group Lower Palaeozoics

s

Fig. l. Map showing location and geological setting of Navan.

organic residues satisfy the definition for kerogen of Durand (1980) as they were immersed in methanol benzene mixtures. Here the term palynodebris (e.g. Boulter et al., 1986; Highton et al., 1991) is used to replace the term kerogen which is cornmonly inaccurately used by palynologists. The residues were oxidised using Schulze's Reagent, but the palynofacies analysis presented below was performed on unoxidised residues. This analysis was carried out in the same fashion as in Davies et al. (1991). Unoxidised residues, mounted on glass slides, were examined and percentages of palynodebris class types estimated by performing traverse counts covering 500 particle points per sample. The classification used is similar to that presented in Davies et al. (1991) shown in Fig. 3. The miospore assemblages are assigned to the PC Miospore Biozone of Courceyan (=Tournaisian) age (Appendix 1). Maturation levels are discussed in Appendix 2. Description of the section The section studied comprises rocks of five formal stratigraphical divisions within the Navan Group (Table 1).

These divisions, originally described at Tara mines by Andrew and Ashton (1982), have been formally named and defined by Strogen et al. (1990) and Rees (in press a). The descriptions below refer only to the units within the section in borehole N866, shown in Fig. 2. The following environmental interpretations take into consideration other boreholes in the Navan area.

Baronstown Formation (Rees, in press a) 678.6-676.3 m

The base of the formation, which unconformably overlies Lower Palaeozoic rocks, was not cored in borehole N866. Within the mine area the formation ranges from 0 m to over 15 m thick (Philcox, 1984). This interval is dominated by conglomerates containing greywacke, quartz, and cherty volcaniclastic granules and pebbles in a generally coarsegrained strained quartz matrix, interbedded with variably pebbly sandstones. These rocks are reddened, with erosional bases, cross-stratification, mud-wisps, and imbricated clasts. The silty red to

ENVIRONMENTSANDPALYNOFACIESOF DINANTIANLITTORALSEQUENCE:NAVANGROUP.IRELAND y ~

_606

<~ (9

~ ,

25"

<~

,9

_614

~

• 7 i ~ + I I1~I _536 ,~,

"7

_620

~o ~d

~

L

~-

"dS( I ~

t.

--

I I I //2

i

-~ ~7

~

Argillite Arenite Rudite

II~-[I ~ :::~:!?"; "~" -542 :~.~

=

i

I- %Shale

]

V

i

i

177

Micrite

:~o:: :~o::

- 548

Shale Interlaminated sandstone/shale

i~ ~ -6~226

!

, ff ,. 2~I ~ _ i

~

3

yv * ¥

_554 /.4,

~

i [ " fii::ii:!:iI]

~

)oO;

'% 0:

Sandstone Conglomerate (carbonate clasts)

,e..

(lithic clasts) Conglomerate

560

t~

Burrows

k

Rootlets

~

Shale wisps

,~ 2 5 ' ~

i a:

Y~O 111,

/ . Z6 4 ,~!/ ~3.I 8• -

~

~l

-

/A

o o

cr --

v*

l~it~

A

~.~

Q

0 uJ ~ 0

~ {~....

;:lJ

'~

;~

30"

I) •

~

_

_

~_644

Z

(~

•

~ 6 ~ 5:i5 :

O

25" ~;----- e,~;:.:~.~

~:~

)~{;:~......

~

0 ~.

~ ~

- 572

t ~'o ~ ~

• _578

•

~/

/,~ __-

584

Y

{~ ~ ~'~ ,+

~'~%= 5-~ ":;:

.....

~=

!i!.~:~:::i.3

"r

v:

:..:ii:i!;~!:i

,/

5= ..:......: "d-(-?}~=i~{: --

@

20

Sandy Oolitic

- -

i

. =

-566

I

=

;

Oolite Bioclastic/ peloidal arenite Sandy dolomite

41

5.

662

~

.

5--- :iii~}:.::..: -,

~

......

590

Erosional ~

Parallel laminated

~ ~, 5

Cross laminated Calcretes Bioturbated

/

Fault (?)

A

Anhydrite

Q

Quartz Marker

o

Fenestral

~

Foraminifera

~

Echinoids

G~

Gast ropodS

v

Bryozoa

~

Crinoids

~ ~

Rugose corals Tabulate corals

~

Brachiopods

668

h/ i _

-

30"

- 596

?:::.-:~3.: --=

10"

~1

@

~ 'f~&:---=== - 674 5" ~ . ~ " ~ ' ~ ~ [i "11 TD

~L]_~-~

"~

_602

<~1[ -JJ . . . . . ]' rl"ll . m

678.6 •

Palynofacies sample

Fig. 2. Log of part of borehole N866. B F = Baronstown Formation,

LDM=

Ostracodes /--'

Lower Dark Marker.

Bivalves

[78

A. M c N E S T R Y A N D J.G. REES

SCOLECODON~

?Botryococcus

Tornacia

(Order ChLorophgta)

sarjeantii

Stockmans & Willi~re ex

Wicander 1974

L e f o s p h a e r f d f a (<25 ~tm) (Eisenack) Downie & ~ m o re

Z O

Sarjeant 1963 Navffusa Combaz. Lange & Pansart 1967 T e t r a ~ e t e s C r a m e r 1966

Z

o x

~ x

~

o re

<

o

Cymatfosphaera O.Wetzel ex Deflandre 1954 DasypiZula Loeblich & Wicander 1976 E l e k t o r i s k o s Loeblich 1970 Unellfum Rauscher 1969 Tyligmasoma alargadum (Cramer) Playford 1977 Chomotriletes

Naumova ex Naumova 1953

<

Veryhachfum Deunff ex Downie 1958 Ste~Zfnium Jardin~ et al. 1972 Mierhy$trfdfum Deflandre 1937

~ re o

(>25 ~m) (Eisenack) Downle 1963 Gorgonlsphaerfdium Staplin, Jansonius & Pocock 1965 Lefosphaeridfa

o

m m

Thin-walled, elongate'tube-like structures, 20-i00 ~m in diameter; smooth or rarely displaying equatorial striae; generally openended but rarely with one closed, rounded end.

co m o e~

MIOSPORES (<200 ~m) MEGASPORES (>200 ~m) TN'rI~DS

cc

,..-1

~

<

o

& Sarjeant

m m

!

co r.~

z m O

~e re r.1

Z

cc "I C~ n~

z < >~ ca O o 3

Platy or sheet-like fragments of organic matter presumed to represent non-woody and/or degraded plant material

MEL~OGEN ) Structured: Black, opaque, ) conductive elements of carbonised fragments ) terrestrial plants

) HYLOGEN Brown, uncarbonised or partially carbonised fragments

Fig. 3. Palynofacies classification (after Davies et al., 1991).

) ) ) ) )

Unstructured: displaying a similar texture to the above but lacking recognlsable structure

ENVIRONMENTS AND PALYNOFACIES OF DINANTIAN LITTORAL SEQUENCE: NAVAN GROUP, IRELAND

TABLE I

Two palynofacies samples, taken at 675.55 m and 675.50 m, contain similar assemblages consisting of approximately equal amounts of melano-

Formal nomenclature

Tara Mines nomenclature

Meath Formation (undivided)in part Stackallan Member Liscartan Formation

Pale Beds (undivided)

Bishopscourt Member Portanclogh Member

Muddy Limestone

Baronstown

Red Beds

Formation

17 t)

Micrite Unit

Laminated B e d s

gen, hylogen and miospores (Plate I, 7, 8). Both assemblages contain very coarse fragments, are well-preserved and have very low levels of pyritisation. Miospore tetrads (Plate II, 4) spore clusters and megaspores are common. Rare specimens of phytoplankton are present (Plate II, 9, 10). Scolecodonts are absent.

674.3 657.2 m CG unit. This interval consists of

green sandstone at 677.6 m contains dark rootlet traces and the sandstone below this includes horizons of caliche." No palynofacies samples were taken from the Baronstown Formation. Liscartan Formation (Strogen et al., 1990) 676.3 630.7 n7

This formation, divided into the Portanclogh and Bishopscourt Members, consists of the dominantly siliciclastic sequence at the base of the Dinantian marine succession, Portanclogh Member 676.3-638 m

(Strogen et al.,

1990)

This member consists of a sequence of heterolithic siliciclastics which at Tara Mines are informally referred to as the "laminated beds", and subdivided into discrete units, CG to CB, representing different lithofacies (Fig. 2). It is dominated by grey, fine to coarse calcareous sandstones, micritic limestones, interlaminated and interbedded with shales, 676.3 674.3 m CG unit "transitional base". This

unit is dominated by sandstones, conglomerates and calcareous ~udstones. The sandstones are pebbly, laminated, and have mixed siliceous and calcareous cements. The conglomerates have erosional bases and are similar to those of the underlying Baronstown Formation (except that here only the clasts are reddened); they contain micrite clasts and mud wisps and have a dolomicrite matrix,

sandstones interbedded and interlaminated with calcareous mudstones and occasional sandy limestones. The sandstones and limestones compositionally grade into each other, and generally form partings, mostly 1-2 mm thick, but which reach 3 mm thick. The sandstones consist mostly of fine to medium-grained strained quartz and have calcite cements. The limestones contain aggregate grains, radial-fibrous ooliths and have a micrite or dolomicrite matrix. Most contain abundant fragments of ramose bryozoa, bivalves, ostracodes, gastropods and algae, but brachiopod and crinoid debris is notably rare. The sandstones are commonly mudwisped. Well-sorted limestones form beds between 10-50ram thick in which intensively micritised mollusc fragments lie convex surface upwards. These sandstones and limestones are crosslaminated, with the azimuths of dip of laminae varying considerably. Bioturbation is extensive, and discrete 1-5 mm diameter near-vertical burrows occur throughout. Palynofacies samples taken at 671.4 m and 658.9 m contain well-preserved, coarse palynofacies assemblages (Plate I, 5, 6), though they are finer and better sorted than in the samples from the base of the CG unit. Non-woody tissue (Plate I1, 5), which is well preserved and notably abundant at 671.4 m, appears to be dominant mainly at the expense of woody fragments. Megaspores and spore clusters are absent and miospore tetrads are present only at 658.9 m. Levels of pyritisation are low at 658.9 m and low to absent at 671.4 m. Scolecodonts (Plate II, 7) are uncommon in both assemblages. Rare organic-walled microplankton occur at 671.4 m.

180

657.2-654 rn C F u n i t . The CF unit consists of buff-

brown, fine- to medium-grained, generally wellsorted sandstone. It is muddy and calcareous towards the top and contains three greenish, micaceous siltstone beds. The sandstone is wave ripple laminated, the laminae consisting of coarse and fine quartz sand dipping at up to 15°. The finer laminae contain argillaceous material, the carbonate component of which has been altered to ferroan dolomite, giving the sandstone its distinctive colour. Strogen et al. (1990) note flasers in the CF unit elsewhere in the mine area. The skeletal component of the unit consists of occasional, micritised, crinoid grains, The palynofacies sample from 656.1 m contains a very coarse, mainly woody assemblage showing moderate to good preservation and moderate pyritisation. Rare scolecodonts are present. 654-647.5 m C E unit. This unit is broadly similar

to the CB unit, though most laminae in the CE unit are thicker (except those between 651.4-650.1 m) being mostly 2-3 mm thick. The laminae are made conspicuous by concentrations of crinoids and bryozoa, much of which has been extensively micritised, or encrusted by algae or bryozoa. Bioturbation is less extensive than in the CB unit. Micrite and dolomicrite with aggregate grains form discrete beds and the matrix of some sandstones, Algal colonies, notably of Ortonella sp., are common and form a conspicuous bed at 649.6 m. The palynofacies sample from the CE unit at 649.4 m contains a coarse assemblage dominated by unstructured melanogen. Preservation is moderate to good and pyritisation is moderate. The sample at 649.6 m was taken from the conspicuous

A. McNESTRY AND J,G. REES

Ortonella bed (see above) and contains a high proportion of "amorphous" (unstructured), nonwoody tissue which may be algal in origin and, in comparison with other assemblages, occurs at the expense of woody material. Rare megaspore fragments are present. Scolecodonts and organicwalled microplankton are absent. 647.5-645.6 m CD unit. The CD unit consists of well sorted, silty, sandy limestones with wisps and thin beds of mudstone. They contain an impoverished fauna of bivalves, gastropods, crinoids and brachiopods. The coarser, upper, part of the unit is wave ripple laminated, fines upwards, and is less bioturbated. No palynofacies samples were taken from the CD unit. 645.6-641.0 m C C unit. This unit consists of greenish-black, extensively burrowed, calcareous mudstones. Laminae and lenticles of fine-grained sandy calcarenites or micrites are common near the top and base of the unit. Most are 2-3 mm thick, cross-laminated, and contain radial-fibrous ooliths and rare fragments of ramose bryozoa, bivalves, brachiopods and crinoids. The brachiopod and crinoid detritus is notably micritised and concentrated in isolated laminae. The micrites are dolomitised, bioturbated and fenestral, and include lithic sand and aggregate grains. They contain calcitised and dolomitised anhydrite pseudomorphs, after gypsum. Gustafson (in Andrew and Ashton, 1982) suggests that the "Quartz marker" of Tara Mines, a 30 mm thick bed of chalcedonic silica at the base of the unit (see Fig. 2), represents an horizon of replaced evaporites.

PLATE I Specimens figured on Plates I and II are housed in the PalaeontologicalCollection of the Geology Department, Trinity College Dublin, Ireland. 1. 575 m (TCD 33668) 2. 625.9 m (TCD 33669) 3. 633.6 m (TCD 33670) 4. 645.4 m (TCD 33671 5. 658.9 m (TCD 33672) 6. 671.4 m (TCD 33673) 7. 675.5m (TCD 33674) 8. 675.55m (TCD 33675)

ENVIRONMENTS AND PALYNOFACIES OF DINANTIAN LITTORAL SEQLENCE: NAVAN GROUP, IRELAND

181

PLATE I

~," ~

"'"

u~

i[il-',,,,r

' ........

"

.......

......

.... **

......

,, '~-,

- ~,

'

d

@

q! '

.

',

~

~.,,

.~-

Dm~BNPF'TmJ

~

1

~

~

i

3

~

,5

,

....

~

7

-~

-,,,i,,~

6

~

B

182

A palynofacies sample from the CC unit at 645.4 m is dominated by abundant non-woody tissue (Plate I, 4). Particle size is medium, preservation is very poor and pyritisation is extensive. Scolecodonts and phytoplankton are present but rare.

641.0-638 m CB unit. This unit is similar to the CG unit. Rare limestones, composed almost exclusively ofbryozoa and brachiopods are bioturbated, though burrows are most common in mud-rich horizons. Most beds contain 1-3 mm thick mud wisps and laminae. Compared with the similar facies in the CG unit, and to a lesser extent the CE unit, the CB unit contains more, coarser, crinoid and brachiopod detritus, and a greater range of sediment grade and degree of sorting, The palynofacies assemblage from 639.2 m, consists predominantly of coarse to medium sized woody fragments and shows poor to moderate preservation and moderate pyritisation. No scolecodonts or phytoplankton are present. Bishopscourt Member (Strogen et al., 1990) 638.0-630.7m This member, which represents the transition from predominantly siliclastic to carbonate deposition, normally consists of dark, shaly bioclastic limestones. It is limited to the Navan area (Philcox, 1984; Ashton et al., 1986; Andrew and Poustie, 1986; Strogen et al., 1990). The member in borehole N866 is somewhat atypical of the member in the mine area where it is normally between 15-16 m thick, relatively sand-free and contains corals, including abundant Syringopora sp. The member in borehole N866 can be divided informally into

A. M c N E S T R Y A N D J.G. REES

two units, the upper part being transitional with the overlying Stackallan Member of the Meath Formation.

638.0-634.7 m Bishopscourt Member (lower part). This interval is dominated by interlaminated, bioturbated, calcareous sandstones, sandy limestones, micrites and calcareous siltstones. The sandstones are mostly fine- to medium-grained, and contain molluscs, brachiopods, ostracodes and crinoids. The limestones are micritic and contain abundant bryozoa and crinoids. The mudstones occur principally as wisps or laminae, though they also form discrete beds and are usually bioclastic. A palynofacies sample, from 637.5 m, contains a medium to coarse sized assemblage with abundant non-woody tissue present. Preservation and pyritisation are moderate. Scolecodonts are present. 634.7-630.7 m Bishopscourt member (upper part). This interval is similar to the lower part of the Bishopscourt Member, though is less sandy, being dominated by micritic, intraclastic limestones which are interlaminated with calcareous mudstones over 2-3 mm. Bioclastic debris is common and is dominated by gastropods and ostracodes. Most crinoid and brachiopod debris is fragmented, extensively micritised and occurs in distinctive, coarse, coarsening upwards beds. Oncoliths and micritic, bored, algally encrusted, hardgrounds occur at several horizons. Three conglomerates (Fig. 2), with erosional bases, contain intraclasts and pebbles of micritic and pelmicritic limestones in a sparry cement, and calcarenite fragments in

PLATE II Palynomorphs and palynodebris Depths, slide numbers and England Finder references are shown 1. Rugospora polyptycha Neves and Ioannides 1974, 675.55 m ox2 (H 55), x 500 [TCD 24946]. 2. Grandispora echinata Hacquebard 1974, 675.55 m ox2 (B 47), x 500 [TCD 33676]. 3. Spelaeotriletes pretiosus (Playford) Neves and Belt 1970, 675.55 m oxl.1 (J 70), x 500 [TCD 33677]. 4. Miospore tetrad, 675:55 m oxl (R 57), × 500 [TCD 33678]. 5. Non "woody" tissue (structured), 671.4 m unox 1 (J 57-1), × 500 [TCD 33679]. 6. Non "woody" tissue (structured "tube"), 671.4 m unox 1 (J 57), × 500 [TCD 33680]. 7. Scolecodont, 658.9 m unoxi (C 39-2), × 500 [TCD 33681]. 8. Pyritised miospore, 633.6 m (R-S 49), x 500 [TCD 33682]. 9. Tornacia sarjeantii Stockmans and Williere ex Wicander 1974, 675.5 m unox 1 (F 51), × 1000 [TCD 25007]. 10. Navifusa sp., 675.5 m oxl.2 (L 69-2), × 1000 [TCD 33683].

183

ENVIRONMENTS AND PALYNOFACIES OF DINANTIAN LITTORAL SEQUENCE: NAVAN GROUP, IRELAND

PLATE II

i i

,

2

,:~ ~V6

8

4

9

I¸,

7

I I •

~ 0

[84

a calcisiltite matrix. They are bound by algal laminae, Palynofacies samples from 633.6 m and 6 3 2 . 5 m contain assemblages similar to those from the main part of the Bishopscourt Member, but the preservation is marginally better and pyritisation is less marked (Plate I, 3). Miospore tetrads and megaspore fragments are present and miospores are common. Scolecodonts are present. Meath Formation (pars) (Strogen et al., 1990) 630.7-535 m The Meath Formation consists of a limestone sequence containing a basal micritic member, the Stackallan Member. Only the lowest 23.5 m of the overlying part of the formation is described here. Stackallan Member (Strogen et al., 1990) 630.7-558.5 m The Stackallan Member consists of pelmicrites and micrites with subsidiary dolomicrites, oncolitic limestones, oolites and thin shales. The micrites are dark grey, weather pale grey, contain fenestral fabrics and, where shaly, can be seen to be extensively bioturbated. Oncoliths, and intraclasts are common, particularly below 608 m, and ooliths are common in the transition between the Stackallan Member and the overlying part of the Meath Formation. Siliciclastic sand is most common in the upper part of the member, especially in the grainstones and dolomicrites. Shale occurs as wisps, mostly in the lower part of the member, though it also forms discrete beds. Bioclastic debris is extensively micritised, and though most brachiopod, crinoid and echinoid detritus is finely comminuted, mollusc valves remain relatively intact. The micrites contain abundant Archaesphaera and a rich fauna of ostracodes and foraminifera consisting mainly of Eoforschia spp. Conodont faunas from the Stackallan Member, collected at and near to Navan (Rees 1987, in press a) consist mostly of cavusgnathids and taphrognathids, Palynofacies samples from 630.5 m, 625.9 m, 576 m and 575 m are all dominated by non-woody tissue, though woody fragments are more abundant at 575 m where miospores are common and miospore tetrads occur (Plate I, 1, 2). Particle size

A. McNESTRY AND J.G. REES

is mainly medium to fine, preservation is very poor and pyritisation consistently moderate. Structured hylogen is rare and was not recorded at 625.9 or 576 m. No scolecodonts or organic walled microplankton were recorded. Meath Formation sequence above the Stackallan Member 558.5-535.0 m This sequence is dominated by oolitic, intraclastic and peloidal grainstones, and calcareous sandstones. The oolites are mostly well sorted, crosslaminated, burrowed, and contain zaphrentoid corals. The peloidal limestones contain few bioclasts, though they are extensively bioturbated. The sandstones contain laminae of ooliths, peloids and bioclasts at 10-20 mm intervals. A fossiliferous, intraclastic conglomerate containing shale clasts occurs at 542 m. Crinoids, brachiopods and bryozoans occur throughout the interval. Some horizons are very silty, particularly the oolites which represent the Lower Dark Marker of the Tara Mines sequence (Fig. 2), and the micritic limestones at the top of the section. A single palynofacies sample from 535 m conrains predominantly woody debris in the form of unstructured hylogen. Particle size and pyritisation are moderate and preservation is very poor. Scolecodonts are present. Environmental interpretation of the section Sedimentary facies Baronstown Formation The Baronstown Formation was deposited in an alluvial environment, indicated by the lack of micrite or marine biota, the red colouration and rootlet traces. It may have been deposited on an alluvial fan, or in braided streams in proximal alluvial environments (Rees, 1987; in press a; Strogen et al., 1990; Pickard et al. 1992). This is suggested by the high proportion of conglomerates relative to sandstones and siltstones, and by the variation in sorting and rounding of the sediments. The fine sandstones and siltstones probably represent ephemeral overbank deposits which became desiccated and vegetated.

ENVIRONMENTS AND PALYNOFACIES OF DINANTIAN LITTORAL SEQUENCE: NAVAN GROUP, IRELAND

Liscartan Formation Portanclogh member. The lamination, colour and clast composition of the CG unit basal transition suggests that it contains the earliest marine sedimerits of the Dinantian section. It probably represents mixing of alluvial and marine regimes where coarse alluvial sediments are interbedded with tidal sands and muds. The CG and CB units consist of similar lithofacies, interpreted as having been deposited on supratidal or inter-tidal mudflats (Shinn, 1983), marginal to a lagoon, onto which clastic and lime sands were periodically deposited, possibly during spring or storm tides, or by winds. This is suggested by the conspicuous laminae in borehole N866, and by flaser and linsen bedding elsewhere in the mine area (Strogen et al., 1990). These structures have been preserved because burrowing faunas were restricted by periodic exposure and fluctuating salinities. The cross-laminae within these suggest varying wave, wind and tidal influence. The thicker limestones were probably accumulated by storms, The broadly similar, though more algae-rich facies of the CE unit was probably less frequently exposed. Although the lithofacies of the CG, CE and CB units are similar, the environments they represent attained a more normal-marine salinity upwards through the member. The fauna in the CG unit suggests that the source of debris was derived either from hypersaline or brackish sources. However, the calcareous algae and radial fibrous ooliths, neither of which are obviously indigenous, suggest the source of carbonate detritus in the CG unit was hypersaline and warm. By comparison, the allochthonous faunal elements of the CE and CB units, composed largely of crinoid and brachiopod detritus, suggest that allochthonous sediments were derived from sources of near normal-marine salinity. Extensive bioturbation in the CB unit., and abundant algae in the CE unit suggest that the environments in which these units were deposited were probably less prone to long periods of desiccation. The sorting, sparse faunas and lack of bioturbation within the CF and CD units suggest they were deposited in higher energy environments than those discussed above. As the CF unit contains no

IN5

features indicative of deposition in a tidal channel (e.g. large scale cross bedding, or asymmetrical ripples), it probably represents an off-shore bar (Rees, 1987: Strogen et al., 1990). The silts and fine grained sands of the CD unit were probably deposited marginal to such a sand bar. This bar is most likely represented by flaser-bedded sandstones in the upper part of the Liscartan Formation recorded by Strogen et al. in borehole EP30, to the south of Navan. The CC unit was deposited on a tidal mud flat, or in a hypersaline muddy lagoon which was prone to periodic desiccation. This is suggested by limited bioturbation, fenestral fabrics, evaporites and lack of an indigenous fauna. The lithic and carbonate sands reflect influxes of allochthonous sediment. The small sand content of the CC unit, which distinguishes it from the otherwise similar CG, CE and CB units, suggests that the CC unit was deposited in a position distant from, or by-passed by, sediments from siliciclastic sources. The radial fibrous ooliths and faunas suggest that the carbonate debris was derived from a hypersaline source. The occurrence of extensively micritised crinoid and brachiopod debris in isolated laminae may result from influxes of sediment from more distant, more normal-marine sources.

Bishopscourt member. The lower part of the member was deposited in an inter-tidal or sub-tidal environment of near normal salinity, as is suggested by its varied fauna and extensive bioturbation (Andrew and Ashton, 1982: Strogen et al., 1990). The part of the member which is transitional with the Stackallan Member may have been deposited in an inter-tidal environment with increased salinities, as is suggested by the abundance of gastropods, less extensive bioturbation, micritic hardgrounds and intraclastic horizons. However, storms still washed in bioclastic debris from more open-marine sources. Meath formation Stackallan member. The Stackallan Member was deposited on tidal carbonate fiats, suggested by the fenestral fabrics, extensive bioturbation,

186

numerous oncoliths, large molluscan faunas, and characteristic near-shore conodont faunas (Austin and Davies, 1984). The crinoid and brachiopod detritus may have been transported from more open marine sources by storms or tidal channels,

Sequence above the Stackallan Member. This part of the sequence represents the establishment of the first, widespread, Dinantian open-marine environment in the area. This is suggested by the predominance of oolites, the degree of sediment sorting, and the open-marine fauna, Sedimentary facies." discussion A facies distribution block model is shown in Fig. 4. All of the facies recognised in the Liscartan Formation fall within a very simple facies model consisting of a sequence of on-shore to off-shore facies belts. These constitute supra- and restricted intra-tidal mudflats marginal to shallow lagoons (CG, CE, CC and CB units), near normal marine environments (Bishopscourt Member) and sand or silt barrier complexes (CD and CF units). The sequence of facies recognised over the mine area contains three "cycles", lower, middle and upper (Fig. 4) in which marginal, restricted marine lagoonal facies are succeeded by off-shore sandsilt facies which transgressed over them to the north (Clayton and Higgs, 1979). The boundaries between the cycles represent regressive events in which the facies belts were pushed off-shore, Smaller scale, probably autocyclic, cycles in the Portanclogh Member have been recorded by Strogen et al. (1990). The recognition of cyclicity in Courceyan transgressive successions is not new, and has been recently documented from South Wales (Burchette, 1987; Davies et al., 1991). The large amount of siliciclastic sediment in the Liscartan Formation was derived from erosion of continental facies. The increasingly normal-marine trend of the formation suggests that the sand-silt barrier complexes, represented by the CF and CD units, diminished in size, or broke-up because the on-going transgression caused increasing distance from sediment source. The Bishopscourt Member represents the greatest extent of the marine trans-

A. McNESTRY AND J.G. REES

gression of the Liscartan Formation in the Navan area. The same event is distinguished by near normal-marine facies in a similar position, immediately below the Stackallan Member, further west in the Irish Midlands, such as at Sion Hill and Moate (see Philcox, 1984). The switch from near normal-marine environments of the Bishopscourt Member to the carbonate littoral environments of the Stackallan Member represents a major regressive event that can be recognised over most of the Irish Midlands (Philcox, 1984). Littoral facies were finally pushed north of Navan at the top of the Stackallan Member when the continuing marine transgression pushed openmarine facies, including an oolitic barrier system, into the area (Fig. 4). Boreholes in the Navan area show that at the top of the Stackallan Member several facies, including micritic mudflats and oolitic barrier systems existed contemporaneously, proving the top of the Stackallan Member to be diachronous (Philcox, 1984). This suggests that the transgression was not a simple transgressive pulse.

Palynofacies The abundance and preservation of palynodebris is influenced by depositional, taphonomic and diagenetic factors. Several approaches at evaluating these have been made in interpreting Carboniferous palynofacies, such as Coleman and Clayton (1987), though the interpretation below follows most closely that of Davies et al. (1991). Palynodebris composition can be considered to reflect primarily the environment in which it was deposited, and proximity to sources. General assumptions are that particle size and abundance decrease with distance from source, that the condition of the palynodebris is related to the preservation potential of the environment, and that pyritisation is most extensive in marine conditions. Palynodebris class abundances are shown on Fig. 5, palynofacies on Plate I and palynodebris on Plate II. The size of palynodebris particles is presumed to mirror distance from terrestrial sources and the degree of degradation in the depositional environmerit. Although correlating particle size and dis-

~

J

[

~

j

~

[

-

'

:

~

t

~_

I .

__

8-

.

~

: [:::

u

:::

~ .:'~ .

~

BARRIER COMPLEX

.. . . . ' '

.

•

I

Alluvial

": .-~

Y

/ i(G)

°

i--- t. . . . .

Plant

-

.o~

.

.

BARONSTOWN FORMATION

,')

__

~. {j"(..2o "''

~

~

~°

'

,7..

.

.

[]

.

.

.

.

.

Op . . . . . . . bayment (?) (near normal marine)

Open sea (normal marine) Oolitic barrier Micritic mudflat

~

F

EO

c O

t

~

cC:t~:iitt j

CF Unit

CO Unit ~O OiJ~::

ccoo.

e etation

i

i

I

0,

. I v .

y:r~.:_:-

STACKALLAN MEMBER

i

~ PORTANCLOGH [MEMBER TIMHs]

,

TACKALLAN EMBER TIMES J

i

PORTANCLOGH MEMBER

~

i

t

I

~

- . . . . . .

~

P Ia nt d e b r i s

NORTH

IIIlj/lll111tIttllltllt It {II I IIIll IIIIIIIIlIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIINIIIIIIIIIIIIIIIII ,,~'>" IIIlIIIIIIIIIIIIIIIIIIIIIItll

. .

debris

NAVAN SECTION

V-_"~'-~IIIIIIIIIII

Le

OPEN MARINE LIMESTONES (Not seen)

1 ~L

~ ] | ]

~

debris

[]

Sand/ silt barrier Marginal lagooflal Marginal lagoonal/ evaporitic

Fig. 4. South to North diagrammatic representations of the Navan area in PC Biozone Times, based on several boreholcs. Horizontal scale 5 50 kin.

~

[

[~l

Brachiopods

SOUTH

~'~~J&.

,I:j

Plant

BARRIER COMPLEX

KEY TO LITHOFACIE~

--4

2~ > > 2T

>r-:~

--

.~ >

~>

>

~' m

188

tance from source area is simplistic, a meaningful evaluation of complex transport and sorting, mechanisms is virtually impossible in the absence of extremely detailed palaeoenvironmental work.

Liscartan Formation Portanclogh Member (Plate I, 4-8). The palynofacies profiles of samples from the Portanclogh Mereber are broadly similar (Fig. 5). Miospores are most common and best preserved in the lowermost assemblages in which miospore tetrads, megaspores and megaspore framents are also common, and scolecodonts and phytoplankton are rare. The occurrence of coarse, well preserved palynodebris showing low to absent pyritisation and including numerous miospore tetrads, spore clusters and megaspores, in the basal part of the CG unit, indicates proximity of the depositional evironment to terrestrial plant sources. That the environment was probably marine is indicated by the presence of organic-walled microplankton mainly from Group 111 of Davies et al. (1991). Similar assemblages are shown by Davies et al. (1991) to occur in proximal to very near shore marine environments, The part of the CG unit above the basal transition sequence, contains similar, though finer, palynofacies assemblages, with scolecodonts (Plate I, 5, 6) indicating deposition in environments further from terrestrial sources and becoming increasingly marine. This trend is also observed in the assemblages of the CB and CE units which, compared with the CG unit, are poorly preserved and more pyritised. However, the variable nature of marginal marine environments is evidenced by the absence of scolecodonts and organic-walled microplankton in the CE and CB units. The sample at 649.6 m in the CE unit was taken to see if amorphous palynodebris of the type associated with algae (amorphogen) would be present. It is difficult to positively identify amorphous palynodebris as algal in origin (particularly where fluorescence cannot be used) but the amorphous non-woody tissue from this sample may be algal. The abundant wood (predominantly melanogen) in the sample at 649.4 m may reflect reworking within transgressive units (cf. Staplin, 1969).

A. McNESTRY AND J.G. REES

The palynofacies assemblage from 656.1 m in the CF unit contains a high proportion of very coarse woody debris. The particle size, and abundance of woody material at the expense of nonwoody tissue and miospores may be the result of winnowing in a high energy environment. The composition of the palynofacies sample from near the base of the CC unit is very similar to the sample from the CG unit at 671.4m. This is notable as both units were deposited in probably similar environments. The finer, more poorly preserved palynodebris in the CC unit suggests that the terrestrial source was, during deposition of the lower part of the unit, more distant from the site of deposition than during deposition of the CG unit. This may reflect a delay in the shift in the vegetational belt, caused by the regression at the base of the CC unit. The extensive pyritisation of debris, and presence of scolecodonts and organic walled microplankton in the CC unit indicate deposition in a marine environment.

Bishopscourt Member (Plate I, 3). Non-woody tissue, most abundant in the lower part of the member (637.5 m), and the consistent presence of scolecodonts suggest a marine depositional environment. The two samples from the transition with the Meath Formation contain greater proportions of miospores and woody fragments and also miospore tetrads which possibly suggest increased proximity to a source associated with the regression at the base of the Stackallan Member. Meath Formation Stackallan Member (Plate I, 1 and 2) The palynofacies profiles of the samples from the Stackallan Member are consistent but change slightly near the top of the member. The assemblages differ markedly from those lower in the borehole and are characterised by moderate to fine palynodebris, consistently moderate pyritisation and poor to very poor preservation. The palynodebris contains an abundance of non-woody tissue and very low numbers of miospores. This suggests that during deposition of the Stackallan Member the preservation potential of the micritic mudflats was low, and that, unlike the littoral facies of the Portan-

--

, , -~ . . . .

--

---

,. . . . .

I

j

~

.

~

i

~

.

~

~

.

Z

~

J

--

.-4.

c~

'

'

.

l

=

M

/!

~

::

:::::::::--

~

<

.

°

:

i

~

o

o

-

.::

)

o

~

~

,

c

tOo

. . . . . .

":'

~

o

o o o

o

o

,

'

,

-

"

o

ooo

o°o

....

o o

o

oo°

1000

ol

VC

VG

o ~o 0

co ° oo

o~0

~ oo ~ ooo

VG

VG

VG

o

C

C

G-m

M-p

G-m

VP

M-p

M-p

o o o OoO o V C

:'#5~ o<° o. . .o °.o . .o°o ..

,-,

-'.'.' i [.,....-

~,..,-,- ......

C

C-m

o°o: o 0ol V C

OoO

o

C-m

VP -M

VP

VP

VP

13_

~.

~ ~

M-f

l

-C-m

I C

Oo0 o

-~ o o ooo Oo% °cO

~o

M-f

oo,C

< , _ f ~ o o OOo OOo co

Fig. 5. P a l y n o f a c i e s d i a g r a m s h o w i n g p a l y n o d e b r i s class a b u n d a n c e s .

09'o

o~J::::::::::::::: ~ ~ :

o

oo

~ ° ° ;°_°~-!

OoO

,- #

~

]

~

o o CoO

o

o

o°

. . . . . ~-)o o ° ° ~ co L';--" : ' ' 7 " i " ' ~ oo o oo o .,.',..,\~:r'o o o o o

-,:'.-,',

: ,":',:

~ .......

-,~_~

~ .....

~:/.~

°

o

~ "-~/~S'~"

,

<,>,,_,<,,_;,I~OoO

I e l l : ~ ' - - ' , ' - ; , ' : : / t : o

. . . . . . . ::: . . . . . . ::

. . . . .

O~14.:::::::~7,',,-,,,:~-,,,!c,9<:q~ fl: ::::: ~ . " 7-,"'. H. . . . . . . . ~ : ~ : : : ~ : : 1 . , : . . ~ - 7 - ' - , i , ' : , ,

.... : . . . . . . . . . . .

::: .... : : : : ' : : : : : : : : : : i

,.H.::::

c

: : -

,-~,z,'--',-'-\'r,7,-h ~ i~:, - ; L ' : , , - , ~ , , , - , ~ _,:,/,,~ -:~ ~- - , ~. . . . , - , , , , -~,

oo ~ ~ ~ o ~ o ° ° ° _ % o ~ % 1

"" "-q;"-',~/-x ~-,,',,~,_,/ - ,'- -~,,,-,-

H ....................

r::t

~

~

{

.

................. ................. .......

................. :::::::::::::::::

........ : :: ..

, _: . : ,. . . . . l. . . . . .t. . . . . .~. . . . . . .,. , > < ~, t e O o o ~ ~O- _j_ . - ~ . ' /

(~

~

M

~

I~

t,~.-" I \ \ ~ . , ' ..... ," . . . . I M - f

,'.,Z~,~j_,_?/_-:j . C~,.~

-~ - ' - " ' - , , -- ,'--~/ - - - , - : , , - ,, - , ' , - : - , ' - , / , ; '

,~

"1-

,m #"

5

......

"-''q"-'ff'"'~-\

~'~-'/" _,,

,,7,,[,~:.;~1%o

o

~-

37/55

i-

, : :\

,;,C.,.7,,/,_

,'-. ',-. , '-, , , , "--.,'-",-...',-., .......... ; ... '. -. . . . -. . . . . . "-it

. . . . . . . . . . ~ . , ~ , / _

1 : ' . ~ " : ~ ' ~ 2 ,,~-':,:,:'---/-'~,q/i-,'t:['--'.t'~c~'2;:,.L'~..':,~i,L~

~'.'1~<~'I:'.~',;,-,"

.. "'"""""

.......

,I]..'.'.'.'1==:=1...'-,--' ..........~, , , -<' . . . ....... .. .

~ .....

<

rr

-

--

HYLOGEN

o

~

~

............ ~, -, .... . ,:j. ~i............ ~'.~'~,-!,<'<-"~';'~:""'//,'-'.~:~t ° u..: . . . . . . . . . . . . . . . . . . . . . . . . . -'.;,'~-7 . . . . . , , ; ,-~,o, ~................-....-.....~Ez~ ,,_/z-,,-~,_,-<,,,. ,>...................imc=~=:~':%,% -,,,,,L,./-_-:L,:_-~:,'-I ~

,.HYLOGEN

S~RUCTUREDMELANOGEN ~~IJNSTRUCTURE[;::::::::::::::::::::: ME/ANOGEN

(E,-TL',[-',;'~'-,[~[,~; 3 NON 'WOODY' Io~^oo %~o o o^-Oo 1 MIOSPORES

~

~ :::::: .... : . . . . . . . . . . . . . . . . .

c~

~

~

OF

I-- ~

z

u_ cr

~71.4

~oo.~

....

356.1

]

~

349.4 -- - -

~49.6

O

345.4

0

Z

~39.2

. . . . . .

~

333.6

~m

~:

~ ,~

-

-

;32.5

305

-J <

O

~

Z

or"

~

~.

<

~

__j ~ m

l

~-- i

CLASSES

ABUNDANCES

I................. ' ' ' ~ T R U O T U R E D ] ' " : : : : : : ' : ' : : : : ' : ' , I N S T R U C T U""R E E

PALYNODEBRIS

PERCENTAGE

I1~-~ ~ ~ ~ : : : 1

~- ! ~

~-

o_ ~,r"

~i

z o

_~

__

,25.9

•76

.~

;35

< ~

~

"-;

E

~_-J

%~

[

O ~-'r

~.lJ' ~ ~

'z

I

"

•

*

~

*

~"

•

-II1"

•

.

•

"=

m

~

m

Q.

~ ~.@

"4"

+

4-

~

+ +

-

+

~

~

4-

"4"

+

'4=

4-

4-

+

4-

4"

"1"

~ ~ 8~ ~_ ~

(~ ~ ~C~ ~ c~ ~)O~ 2 ~ ~

i

+

+

4-

+

mO Oo~

~Z ~3

Lag . . . .

Marginal

Barrier

Sand

Lagoonal

Marginal

ELva gpo° roi nt iac l

l

Marginal Lagoonal

Embayment?

Marine

Near-normal

Mudflat

Micritic

8arrierO°litic

LLI

>Z

O n"

Z

ca

z~ SIZE u p to 4 0 0 u r n

: Very

: Low low

Extensive

Rare

Common

: Abundant

LBM'

part) Bishopscourt

(upper

UBM : Bishopscourt

LITHOSTRATIGRAPHY

+

"~':

"1" : P r e s e n t

4-:

PALYNOMORPHS

,)

•

.

"~:

Member

Member

: Very poor : Poor :Poormoderate :Moderate-poor : Moderate :Goodmoderate : Very good PYRITISATION

VP P P-m M-p M G-m VG

PRESERVATION

M : Medium M-c : Medium - coarse C-m : Coarse - medium c :coarse VC : Very Coarse

M-fC°arse:: M eudpi u m to 1400urn_ fine

Medium:

Eine:upto200um

KEY

z

~C) ~-

z~;

"

r-

0 >

z -

7_ > z

t~

"~ o m>

zo L<

<

190

clogh Member, local terrestrial sources were few. The occurrence of rare miospore tetrads and increased amount of woody palynodebris in the uppermost sample (575 m) may indicate the initiation of new terrestrial palynodebris sources in the approaching oolitic belt, represented by the oolites in the Meath Formation, above the Stackallan Member (although no direct evidence of this exists). The absence of scolecodonts in the member probably reflects the restricted, hypersaline, nutrient-poor nature of the carbonate mudflat environment.

Sequence abovetheStackallanMember. Thesample from above the oolitic interval which overlies the Stackallan Member contains a large proportion of woody material. Like the sample from the top of the Stackallan Member, on the landward side of the oolitic belt, this sample, from the seaward side, may suggest that the oolitic belt was vegetated in part.

Palynofacies: discussion The Navan section draws attention to a problem in palynofacies: the significance of preservation. The decomposition of palynodebris through pyritisation is considered to be of little importance, as the distribution of pyrite in the rocks and samples is limited. As can be seen from the Navan palynofacies profiles it appears that the more poorly preserved palynodebris occurs in parts of the sequence which were frequently subaerially exposed, and oxidised, such as the CC unit, which contains evaporites, and the peritidal carbonate mudflats of the Stackallan Member. However, similar exposure may be expected to have occurred during deposition of the CG CE and CB units, yet the palynodebris they contain is relatively well preserved, This suggests that the palynodebris in these environments was deposited near to its source, and was rapidly deposited and buried before much physical degradation by inorganic or organic processes during seaward transport occurred, or that it was preserved by a high water table. As a general rule, it appears that preservation deteriorates up through the section with increased distance from the major palynodebris source area (to the north

A. McNESTRY AND J.G. REES

of the coastal belt). Probably distance from source had a greater infuence on palynodebris preservation than in situ degradation. The Navan section provides a comparison to that described by Davies et al. (1991) of a marine transgressive sequence of similar age from South Wales where the marine environment was proximal to a rich source of palynodebris material and where fully marine conditions were more rapidly established over the Old Red Sandstone facies. Compared to the palynofacies assemblages described by Davies et al. the Navan samples contain less abundant and more poorly preserved palynodebris. However, the overall distribution pattern is similar, with wellpreserved palynodebris recovered from the basal, most marginal marine rocks and the most poorly preserved palynodebris from rocks deposited at some distance from terrestrial sources.

Conclusions The sequence between the dominantly alluvial and open-marine parts of the Navan Group consists of the Liscartan Formation and the Stackallan Member of the Meath Formation. These were deposited in and around lagoons and tidal flats separating the coastal plain from offshore barriers. The change from dominantly siliciclastic to carbonate sedimentation at the base of the Bishopscourt Member of the Liscartan Formation reflects drowning of the principal siliciclastic source areas on the margins of the Irish Midlands during PC Biozone time. Several regressive events can be recognised in the course of the overall transgression, especially that at the base of the Stackallan Member, first recognised by Philcox (1984). Given that abundances of different palynodebris classes, palynodebris particle size and preservation can be taken as indicators of the distality of the depositional environment to major palynodebris sources; the palynofacies in the Navan section add to the interpretation of sedimentary facies. The following, identified in the sedimentary facies, can be illustrated by the palynofacies: (1) The CG, CE, and CB environments of the Portanclogh Member becoming more distal to terrestrial sediment and palynodebris sources with

ENVIR()NMENTS AND PALYNOFACIESOF DINANTIAN LITTORALSEQUENCE: NAVANGROUP, IRELAND

time. This is best demonstrated by particle size and preservation. (2) The higher energy depositional facies of the CF unit of the Portanclogh Member, compared with adjacent units, identifiable by the palynodebris particle size. (3) The increase in the distance of the principal terrestrial sediment and palynodebris sources from Navan, and poor preservation potential of the micritic mudflats accounts for the lack of woody tissue in the Stackallan Member at the base of the Meath Formation. This is most clearly identified by the significant increase in the content of n o n woody tissue passing upwards from the Bish-

opscourt Member. Proximity to vegetated barrier systems are suggested by increased amounts of woody palynodebris during the regression at the base of the Stackallan Member, and during continued transgression at the top of the member, Interpretations of other, more subtle, events recognised in the sedimentary facies, such as the cycles in the Portanclogh Member, may also be identified in the pal.ynofacies, but would require further sampling. It has yet to be seen whether the palynofacies profiles derived from the depositional environ-

191

and a contract with the Geological Survey of Ireland (JGR). This paper is published with the permission of the Director of the British Geological Survey (NERC).

Appendix 1 The following stratigraphically significant miospore taxa have been recorded from the Tara N866 borehole 535-675.55 m. The taxa listed below are characteristic of the PC Miospore Biozone (see Higgs et al., 1988) of Courceyan age and include the index taxa of the biozone Spelaeotrilete~s pretiosus and Raistrickia clavata. S. pretiosus is not recorded above 649.4 m, the taxa S. baheatus, Crassispora trychera and Schol?fites spp. are recorded up to 575 m and the diagnostic taxon of the overlying CM Miospore Biozone (Schol~fites clav(~er) is not present in the sequence discussed in this paper. Therefore, although it is likely that the sequence is of PC Miospore Biozone age between 649.4-675.55 m it m a y be younger above 649.4 m and is most probably of CM Miospore Biozone age at 440.4 m ( A M c N unpublished information). The miospores of borehole N866 and two adjacent boreholes are discussed in a publication currently in prep. (AMcN). Selected specimens of miospores and organic-walled microplankton are figured on Plate 2.

Miospore taxa recorded

holes

Cvrtospora cristffer (Luber) emend Van Der Zwan 1979 Knoxi~porites literatus (Waltz)Playford 1963 Spelaeotriletesobtusus Higgs 1975 Hymenozonotriletes explanatus (Luber) Kedo 1963 Spinozonotriletes uncatus Hacquebard 1957 Verrucosisporitesnitidus (Naumova) Playford 1964 Spinozonotriletes impensus Higgs et al. 1988 Grandisporaechinata Hacquebard 1957 Crassisporatrvchera Neves and Ioannides 1974 Vallatisporitesverrucosus Hacquebard 1957 Kraeuselisporiteshibernicus Higgs 1975 Umbonatisporites abstrusus (Playford) Clayton 1971 Crassispora maculosa (Knox)Sullivan 1964 Schopfites delicatus Higgs et al. 1988 Convoluti.q~ora major (Kedo) T u r n a u 1978 Rugospora polyptycha Neves and Ioannides 1974 Knoxisporites triradiatus Hoffmeister, Staplin and Malloy 1955 Granulatisporites microgran([Er Ibrahim 1933 Aratri,vmrites cf. saharaensis Loboziak. Clayton and Owens

lege, Dublin, under Department of Education and National University of Ireland funding (AMEN),

Spelaeotriletes balteatus (Playford)Higgs 1975 Raistrickiaconclvlosa Higgs 1975 Spelaeotriletes pretiosus (Playford) Neves and Belt 1970 Raistrickia clavata Hacquebard emend Playford 1964 Vallati,~poritesvallatus Hacquebard 1957 Schol~fitescf. claviger Sullivan 1968

ments at Navan will typify similar environments in other Upper Palaeozoic or younger settings. However, it is clear that the variations in palynofacies illustrated above demonstrate that changes in depositional environments can be identified within a small range of facies extending between a coastal plain, and off-shore barrier system. Clearly palynofacies has the resolution to detect relatively minor environmental changes in a restricted range of closely related facies. Acknowledgements

We thank Tara Mines Ltd. for access to borein the mine area and permission to publish our findings, and to G. Sevastopulo, G. Clayton, M. Philcox, R. Knox, N. Riley, I. Somerville, P. Strogen and N. Pickard for reading the manuscript. This work was carried out at Trinity Col-

1986

192

Organic-walled microplankton taxa recorded 675.55 m Cymatiosphaera sp., Leiosphaeridia sp., Pterospermella sp., Stellinium sp. 675.5 m Leiosphaeridia sp., Navifusa sp., Tornaeia sarjeantii Stockmans and Williere ex Wicander 1974 671.4 m Stellinium sp., Chomotriletes sp. 645.4 m Leiosphaeridia sp.

Appendix 2 Spore colour and maturation TAI (Thermal alteration index) values are approximated from Staplin 1982. Spore colours at and below 658.9 and at 630.5 and 576 m are brown (TAI= 3) while spore colours in assemblages between 656.1-632.5 and at 575 and 535 m range from light brown through brown to orange/yellow (TAI=2-3). Fluorescence studies show the orange/yellow spores to be weakly fluorescent. This fluorescence is enhanced by oxidation using Schulze's Reagent, but is rapidly extinguished when specimens are left under the UV light, and is not regained. The spore colour variations and the fluorescence properties of palynodebris in these samples indicate minor variations in the thermal maturity as expressed using spore colour [see Batten (1980) as quoted in Smith (1983]) and a fluorescence level close to that at which fluorescence is extinguished (Stach et al., 1982, p. 81). This is a level proximal to the oil 'window' and unusually low for the Irish Midlands (see Clayton et al., 1989; Rees, in press b).

References Andrew, C.J. and Ashton, J.H., 1982. Mineral textures, Metal zoning and ore environment of the Navan Ore Body, County Meath Ireland. In: A.G. Brown and J. Pyne (Editors), Mineral Exploration in Ireland: Progress and Developments, 1971-1981 (Wexford Conference 1981). Irish Assoc. Econ. Geol., Dublin, pp. 35-46. Andrew, C.J. and Poustie, A., 1986. Syndiagenetic or epigenetic mineralization--the evidence from the Tatestown zinc lead prospect, Co. Meath. In: C.J. Andrew, R.W.A. Crowe, S. Finlay, W.M. Pennel and J.F. Pyne (Editors), Geology and Genesis of Mineral Deposits in Ireland. Irish Assoc. Econ. Geol., Dublin, pp. 281-289. Ashton, J.H., Downing, D.T. and Finlay, S., 1986. The Geology of the Navan Z n - P b orebody. In: C.J. Andrew, R.W.A. Crowe, S., Finlay, W.M. Pennel and J.F. Pyne (Editors), Geology and Genesis of Mineral Deposits in Ireland. Irish Assoc. Econ. Geol., Dublin, pp. 243 280. Austin, R.L. and Davies, R.B., 1984. Problems of recognition and implications of Dinantian conodont biofacies in the British Isles. In: D.L. Clark (Editor), Conodont Biofacies and Provincialism. Geol. Soc. Am. Spec. Pap., 196: 195-228. Batten, D., 1980. Use of transmitted light microscopy of sedimentary organic matter for evaluation of hydrocarbon

A. M c N E S T R Y A N D J.G. REES

source potential. Proc. 1Vth Int. Palynol. Conf., Lucknow, 1976-1977, 2: 589-594. Boulter, M.C. and Riddick, A., 1986. Classification and analysis ofpalynodebris from the Palaeocene sediments of the Forties Field. Sedimentology, 33: 871-886. Burchette, T.P., 1987. Carbonate barrier shorelines during the basal Carboniferous transgression. The Lower Limestone. Group of South Wales and Western England. In: J. Miller, A.E. Adams and V.P. Wright (Editors), European Dinantian Environments. Geol. J. Spec. Issue, 12: 239-263. Clayton, G., Haughey, N., Sevastopulo, G.D. and Burnett, R., 1989. Thermal maturation levels in the Devonian and Carboniferous rocks in Ireland. Geol. Surv. Ireland, Dublin, 36 pp. Clayton, G. and Higgs, K.T., 1979. The Tournaisian marine transgression in Ireland. J. Earth Sci. R. Dublin Soc., 2: 1-10 Coleman, U. and Clayton, G., 1987. Palynostratigraphy and Palynofacies of the Uppermost Devonian and Lowermost Mississippian of Eastern Kentucky (USA) and correlation with Western Europe. Cour. Forschungsinst. Senckenberg, Frankfurt am Main, pp. 75-93. Davies, J.R., McNestry, A. and Waters, R.A., 199l. Palaeoenvironments and palynofacies of a pulsed transgression: The Late Devonian and Early Dinantian (Lower Carboniferous) rocks of South East Wales. Geol. Mag., 128 (4): 355-380. Durand, B., 1980. Kerogen: Insoluble organic matter from sedimentary rocks. Technip, Paris, 519 pp. Higgs, K.T., Clayton, G. and Keegan, J.B., 1988. Stratigraphic and systematic palynology of the Tournaisian rocks of Ireland. Geol. Surv. Ireland Spec. Pap., 7, 92 pp. Highton, P.J.C., Pearson, A. and Scott, A.C., 1991. Palynofacies and palynodebris and their use in Coal Measure palaeoecology and palaeoenvironmental analysis. Neues Jahrb. Geol. Palaontol., 183: 135-169. Philcox, M.E., 1984. Lower Carboniferous lithostratigraphy of the Irish Midlands. Irish Assoc. Econ. Geol., Dublin, 89 pp. Pickard, N.A.H., Jones, G.L.L., Rees, J.G., Somerville, I.D. and Strogen, P., 1992. The Lower Carboniferous (Dinantian) stratigraphy and structure of the Walterstown-Kentstown area, Co. Meath, Ireland. Geol. J., 27: 35-58. Rees, J.G., 1987. The Carboniferous geology of the Boyne Valley area, Ireland. Thesis. Univ. Dublin (unpublished). Rees, J.G., in press a. The Courceyan (Lower Carboniferous) succession at Slane, Co. Meath. Irish J. Earth Sci. Rees, J.G., in press b. Carboniferous to Recent heat flow in the Boyne Valley area Ireland, based on the mapping of conodont colour. Irish J. Earth Sci. Shinn, E.A., 1983. Tidal fiat environment. In: P.H. Scholle, D.G. Bebout and C.H. Moore (Editors), Carbonate Depositional Environments. Am. Assoc. Pet. Geol. Mem., 33: 171-210. Smith, P.M.R., 1983. Spectral correlation of spore colour standards in petroleum geochemistry and exploration of Europe. Geol. Soc. Spec. Publ., 12: 289-294. Stach, E., Mackowsky, M.-T., Teichmuller, M., Taylor, G.H., Chandra, D. and Teichmuller, R., 1982. Stach's Textbook of Coal Petrology. Borntraeger, Berlin, 535 pp. Staplin, F.L., 1969. Sedimentary organic matter, organic meta-

ENVIRONMENTS AND PALYNOFACIESOF DINANTIAN LITTORAL SEQUENCE:NAVANGROUP, IRELAND morphism and oil and gas occurrence. Bull. Can. Pet. Geol., 17: 47-66. Staplin, F.L., 1982. Determination of thermal alteration index from colours of exinite (pollen, spores). In: F.L. Staplin, W.G. Dow, C.W.D. Milner, D.I. O'Connor, S.A.J. Pocock, P. Van Gijzel, D.H. Welte and M.A. Yukler (Editors). How

193

to Assess Maturation and Palaeotemperatures. SEPM Short Course, 7:7 9. Strogen, P., Sommerville, I.D. and Jones, G.L., 1990. Stratigraphy and sedimentology of lower Carboniferous (Dinantian) boreholes from West Co. Meath, Ireland. Geol. J., 25: 103 137.