Coral-rich bioconstructions in the Viséan (late Mississippian) of Southern Wales (Gower Peninsula,...

Coral-Rich Bioconstructions in the Visean (Late Mississippian) of Southern Wales (Gower Peninsula, UK)

Markus Aretz and Hans-Georg Herbig, Koln

KEYWORDS: CORAL REEF- REEF MOUND RUGOSA- MK'R(.)BIALI.IMI{STON'~{ MICR()FACIES (GREAT BRI'fAIN) CAI~,BONIFERC)L1S {VISI:bkNI

S u m m a r y

The Holkerian(?)/early Asbian succession of Blue Pool (northwestern end of Gower Peninsula, southern Wales) includes two individual, decameter-sized, low reliefbioconstructions: Ca) a microbe-bryozoan-coral reefmound and (b) an overlying coral reef. The bioconstructions are embedded within a grainy, hioelastic and oolitic inner shelf facies. They occur in two successive shallowing_ upward cycles. Especially the coral reel" is unique within the metazoan-dominatcd shallow-water bioconstructi(ms of the European Visdan-Seqmkh(wian, which diversified remarkably during this time sl ice.

Various descriptive and semi-quarHitative techniques were applied during field work and microfacics analysis was used to describe the complex interaction of macro- and microenvironments in the bioconstructed facics~ to evaluate potential framework building, and to dcm(mstrate the functional position ofrugose corals in Carboniferous shallow-water bioconstrucliuns. In particular, quantitative spatial plots of the distribution of framchuiMcrs in the field and a stratigraphic log of the quantitative distribution of allochems/earbonate textures according to abundance classes were used and critically evaluated.

The microbc-bryozoan-coral reef mound is characterized by dominance of rigid non-skeletal microbial irame- work, enforced by diverse bryozoan morphotypes and few syringoporid corals. Colonial rugose corals occur isolated. becoming more abundant and clustered towa, rds Ihe top. Thus, their limited contribution to framework building is passive, providing hard substrate for microbial framework.

Stabilisation and binding of loose sediment by microbes is also essential R>r the initiation of the coral reef developing during the next shallow-upward cycle. Pri- mary rigidity of the reef core resulted froth closely spaced, intergrowing cetioid rugose corals (Litho+troticm). Red uc- tion of open space in the coral meshwork resulted from microbial encrusting, formation of non-skeletal framework, and bioclastic infill. Reduced coral abundance. increased number of fasciculate rugose corals (Siphot~o- dendron), increased colony sizes of syringoporids, and a high amount of coarse bioclastic debris characterisc the reef flanks. Reef growth stopped due to shallowing-upward and erosion. Locally. microbial precipitation per- sisted in post-constructional phases.

GOWER PKNINStJI+A. WAI..I-tS

Ma.ior Factors controlling the development of both bioconstructions are changing water depth, expressed by a potential interplay of changing hydrodynamic conditions. reduced aecominodation space and lateral facies shifts. The general fragility of delicate Carb(miferous corals and the low fixing potential of tugosa arc additional palacobiological restraints.

In a final chapter, both l?,tuc Poet hionconsLruction:, are compared ~ith different Visdan bh>constructions from the British Isles and with eoralIrich bioconslructions elsewhere. The Welsh coral reef is most similar to L,ate Visdan micrr coral-patch reefs from Queensland, Australia.

1 INTRO1)I.;(711()N

After the decline of 1he extended middle Pahteozoic stromatof~orcficl-cc~ral recfcomnlunities during Prasnian lime, Ihc t:amennJan and Mississippian bioconstructions grew during a period generally accepted as a recovery stage. Their interpretation is hampered hy the c(+mmon belief that they would be dominated by txm-skctctal types (Heekel, 1974; West, I 9gg) conqn~on [y lumped under the label "Waulsortian" (mud-)mound. However, the strict application of the term Waulsortian bank (l~ees and Miller 1995) exclude:,; numerous Early Carbunitcr(ms bioc(mstructiuns.

A more distinctive analysis reveals that a wide range of hioconstructions existed during Mississippian time. Their spatial and temporal distribution did not follow a single evolutionary lineage (Webb. 1994, 2(/02 ). ]:ks a rule, they w,ere sn'H:tllCl'-Si/.C(t than the i r Devonian ancestors and occupied short-term, local niches. This rcsuhed in the common tinder estimation of their abundance (Aretz 2002a1. It has to be stressed that the absence of skeletal components ,,lr not imply a missing rigid framev~ork. Pickard (1996) and Wchb (1996) demonstrated thai microbial facies, which is found in various Visdan hioconslructions (Attains, 1984; Wehb, 1994, 1996, 2002). can form a true framew(+rk.

Compared to other Mississippian carbonate plat f(+rms and shelves, the number of bioconstruclions known ftOlll the British Isles is very high. Wauts(~rtian m()tlrlds at',,J banks (Lees and Millet, 1995). t+dcrohialite-spongc-hryozcmn-cural buildup>, (apron- and Cracoen reeEs: Woltcndcn 195S, M undy 1994/, as welt as late Ashian-Brigantian carbonate mud- mounds (Warnkc. I994: Guttcridge. 1995: Mcischner and Warnke. 1996) were characleristic slructurcs in distinctive

Address: Dr. M. Aretz. Prof. I)r. H.-(]. Hcrbig, Universil.~it/u K(iln, (;eologi~,chcs lnstitul, Ztilpichcr Str. 49a. I)-5C)674 K61n, e-mail: [email protected], herbig, [email protected]

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Fig. 1. Palaeogeography of the Late Visdan (Asbian) of the British lsles and distribution of known bioconstructions: 1: South Wales. this paper, Wright and Wright (198l). 2: Derbyshire, Wolfenden (1958), Gutteridge (1995). 3: North Wales, Bancroft et al. (1988). 4: Askrigg Block. Mundy (1994). 5: Lancaster and Cumbria, Adams (1984), Horbury (I992). 6: Scotland (Jameson. 1987). 7: Northwest Ireland, Wamke (1994), Meischner and Warnke (1996), Kelley and Somerville (1992). 8: Dublin Basin, Somerville et al. (I 992), Somerville et al. (1996). Map according to Ramsbottom (1981 ). Bridges et al. (1995) and Cope et al. (1999).

environmental settings and time slices. Besides, unique or rare bioconstructions, often known only from single locali- ties. were described (Taylor, 1972: a coral patch-reef from Derbyshire; Wright and Wright; 1981: algal-gastropod bioherms from S Wales; Adams, 1984: an algal-foraminiferal- coral reel' from NW England; Jameson;1987: an ecologi- cally stacked buildup developing from (I) a low diversity bryozoan-brachiopod-assemblage to (2) a high diversity siliceous sponge (Hyalostelia)-brachiopod-rostrochonid- assemblage and finally to a low diversity crinoid-dominated assemblage; Bancroft et al., 1988: a bryozoan reef from N Wales; Horbury, 1992: a peloid cementstone-palaeoberesellid buildup from North Lancashire; Kelly and Somerville, 1992: Arundian carbonate mudbanks from NW Ireland; Somerville et al., 1996: microbial buildups grading into algal boundstones with abundant bryozoans and rugose corals from Kingscourt, a NW outlier of the Dublin basin). (Fig. I).

Rugosc corals did not contribute to framework building in those bioconstructions, except for the insufficiently described patch-reef at Combsdale, Derbyshire (Taylor, 1972). However, in May 2002, the locality was found to be mostly overgrown. Only a limited number of corals were recovered. Evidence for a coral-dominated patch-reef could not be substantiated.

The contribution of rugose corals to Lower Carbonifer- ous bioconstructions has commonly been misinterpreted. The mere occurrence of rugosa within bioconstructions or massive limestone is not a justification for a "coral reef", unless they occur in sufficient number and their contribution to framework building has been demonstrated. Up to now, this has been proved only in few Mississippian bioconstructions (Ota, 1968; Webb, 1989; Aretz, 2002a; Aretz and Herbig, 2003).

The aim of this paper is, (I) to demonstrate the contribution of rugose corals to

two stratigraphical successive Mississippian bioconstructions in a section in southern Wales,

(2) to discuss factors controlling growth and demise of the corals and of the bioconstructions as well,

(3) to integrate the bioconstructions and the palaeoecological function of the inherent rugosa in a general scheme.

Additionally, the paper evaluates some techniques used for the description and interpretation of such bioconstructions. Herein, we use "bioconstruction" as a general term for varied rigid, biological-induced and biological-controlled structures with a topo~aphic relief. Concerning morphol- ogy, this includes biohenns and biostromes in the sense of Cumings (1932). He defined "bioherms" as "reel] ike, mound- like, lenselike or otherwise circumscribed structures of strictly organic origin, embedded in rocks ofdi fferent lithol- ogy" (Cumings 1932:333). Following Aretz (2002: fig. 55), Mississippian shallow-water bioherms am subdivided into a continuous succession from reefs via reefmounds to microbial-induced mud-mounds, aside from the typical deep- water Waulsortian mounds. The framework of reefs is dominated by skeletal organisms. Reefmounds are bioherms with significant proportions of skeletal and non-skeletal framework. Mud-mounds, in general, describe micrite dominated, mound-shape bioconstructions with non-skeletal frame works in a wide range of environmental settings (James and Bourque, 1992; Pratt, 1995). Their importance in shallow-water environments was recently stressed by Wood (2001). Framework is the meshwork of a bioconstruction that leads to rigidity. It is due to skeletal organisms as well as to a broad range of non-skeletal organisms and processes (Pratt, 1995; Webb, 1996).

2 GEOLOGICAL AND STRAT1GRAPHICAL SETT1NG

During Mississippian time, the British Isles belonged to the southern shell' of Laurussia. The shelf was a complex array ofintraplatform basins and surrounding shallow-water platforms. Some were emergent, like St. George's Land (Fig. 1). Most shallow-water facies was carbonate; siliciclastics were restricted to the vicinity of emergent areas and to deeper intraplatform basins. In general, the shelf system developed from a ramp to a rimmed shelf during that period (Gawthorpe et al. 1989).

The Gower peninsula was part of the extensive carbonate shelf south of St. George's Land, which continued into the London-Brabant massif, a structure extending from Eastern Ireland to Western Germany. Based on the faunal zonation

223

Fig. 2. Geological map of the western and cemral part of Gowcr Peninsula. (South Wales) ,xith m~crvicw of Ihc stratigraphical column and localisation of Blue Pool. Redrawn fnml l),~,msav I1985 ).

of Vaughan (1905), the Dinantian succcssion of Gower was established by Dixon and Vaughan (191 I) and translcrred into the British chronostratigraphical scheme by George el al., (1976; see legend of our l:ig. 2). P, amsay (I 987. 1989) provided a detailed carbonate petrographic description and interpretation of the entire Di nantian succession of Gower. The Gower peninsula is situated west of Swansea. It extends for about 30 km into thc Bristol Channel. Blue Pool is the name for the studied outcrop (National Grid coordinate 2409 2931, Royal Ordnance Survey, Explorer 164: Gowcr) at the eastern end of Blue Pool Corner, a small bay at the northwestern tip of the peninsula, about 2 km northwest of Llangennith. The bay is situated west of Broughton Bay and north of Rhossili Bay (Fig. 2). The formerly used, unprecisc name "Blue Pool Bay" (Aretz and Herbig, 20001 should bc abandoned.

Blue Pool section is named after a natural pnol at the bottom of the cliff. The section ranges from the pool to thc second bench of the clil:l: it incorporates short disconticmous successions beside it, where lateral equivalents are mostly grass-covered. The best exposures are 10 rn above lhe pool with a several in 2 large surface exposed. The outcrop is the type locality for Siphop~odendron multiradiale Nudds and Somerville, 1987. Previous brief descriptions and interpretation of the reef el interval of the succession can be found in Ramsay (1987), Aretz and Hcrbig (2000) and Aretz (2002a). Based on lithostratigraphic criteria, the outcrop was placed into the Holkerian Hunts Bay Oolite (Fig. 2), eilher in its upper or its middle part (Ramsay, 1985, respectively Ramsay, 1987). Since Blue Pool section cannot be incorporalcd into a continuous succession (Ramsay, 1987: figs. 14, 15)and ooids dominate also in overlying strata (Ranlsay, 1989). this remains somewhat doubtful.

Also the biostratigraphic age of the Blue Pool exposurc is still poorly constrained. Due to unfavourablc tacics.

foramini lers ~,l.lC -;caret and diagnostic laxa havc nol ,,,, cl been recorded. The biostratigraphic value of the rugose coral /,it/Iostrotiotl decipiens (McCoy, 1849) ( Pl. 37/I -2 ). which is ,,cry abundant in lhc middle parl of the section, is equivocal. Mitchell (1989)reported it from the British Holkerian, but in Bclgium the species appeared not earlier than Warnantian (Poty 1985), which is equivalcnl to thc British Asbian-Brigantian stages. In Ireland it also appeared in the Asbian (pcrb. com. I. Somerville, Dublin).

()thor Blue Pool cural [axa, Siphof~odendrotz martbF (Milnc-Edwards and Haime, IN51) (Pl. 37/3), Sipho,zo deeming2 sociale (Phillips, 1836), ,~iphon,,~demtr<m of. l,'le/.fi,ll ~e ( Schi ndcwol f. 1928), Si/#7onodendro~l multiradiale Nudds and Somerville, 1987. Axol#Tyllum sp. A. AxoptG'llum sp. B, Rc~tiphylhml re.rhenium (Vaughan. 1906). and SyriH'r pore sp. (PI 37/.4) werc common in both Holkerian and Asbian limes. None of the biostraligraphicall.y importanl /\sbian coral laxa, SipITcmr~de~Mrot@mceum (Flcming, 1828). Sipholzodetldron pauciradiale ( M c C o y , 1844) and Dil~umV#tyllHm bil~artitum (McCoy, 1849)lnave been reported from Blue Pool so far. It has to be stressed, that those corals do not appear beforc the late Asbian.

Favouring an Asl-,ian appearance of L. decipie~r; in the UK. as delmmstrated for Ihc neighbouring counlries, the succession is likely early Asbian in age. However, the separation of the lale Holkerian and early Asbian is commonly di ITicutt due to lhc absence of useful markers {Jones and Somervillc. 1996).

3 M E T I I ( ) i ) S

The bibliography of F!iigcl and Fli.igeI-Kahlcr I 1992) on Phanero/oic reel~ showed that the temporal and spatial Colnparison of reef environments is based mostly on llOn- quantitati \e datasets. 'Yhus, the individual and non-stand-

224

ardised description of reef-environments is the rule and, hence the main source of our current understanding of this highly complex system and the processes linked to it.

Efforts to generate reproducible, more comparable datasets in reef studies have been made for many years (see Bernecker et al., 1999, for a longer review). Point-counting methods are the most common approach (e.g. Zankl, 1969), but these methods are very time-consumptive, commonly cover only small areas of the reef and provide only poor information on the complex interaction of greatly different scaled structures, which characterise the reef facies and the distribution of the organisms.

Webb (1999) introduced an improved method based on point-counted samples taken from larger grids. This approach revealed very reliable data for detailed quantifica- tions and showed the importance of microbial communities forming a non-skeletal reef framework. However, despite numerous advantages, also this method is very time-consumptive and only suitable for the description of small Iocalised surfaces of up to a few square meters.

In various studies of Mississippian reefs and mounds, techniques have been introduced to standardise descriptions (e.g. Hennebert and Lees, 1985). Most common is the hierarchic division of bioclasts and textural features into semi-quantitative classes according to abundance. Com- monly, 4-5 classes, ranging from absence to dominance, are plotted against a log (e.g. Lauwers, 1992; Somerville et al., 1996).

A combination of various techniques has been introduced and tested in case studies (e.g. Weidlich et al., 1993; Bernecker et al., 1999; Schneider and Ausich, 2002). ] 'he numerous results, both quantitative and descriptive, ob- tained from the impressive study of the latest Triassic Adnet reef (Bernecker et al., 1999) showed the high quality of this approach. The methods of that study would be ideal as a normalised approach, but it has to be stressed that Adnet is a very special locality with sawed quarry faces. Moreover, similar exposures are rare, and a continuous observation and documentation of an active quarry over a long time is unusual. Thus, the complexity of the Adnet study cannot be repeated in most cases. However, the application and/or modification of some of the techniques used there would help to base the current reef research on a more standardised approach.

The present study is an attempt to combine some of the previously described techniques and to evaluate their potential for further studies.

The Blue Pool section was logged and about 90 random samples were taken to get representative microfacies data for the single units and the carbonate facies recognised in the field. Additionally, samples were randomly taken from the succession (a) to gain a representative dataset for the corals, and (b) to characterise vertical and horizontal variations in fine-grained limestone. The aim of this sampling style was to reduce primary biases resulting from sampling of the "nicest" reefbuilders. However, a bias cannot be excluded, since the sample intervals changes and therefore smaller facies shifts might not be represented.

Similar to Lauwers (1992) and Somerville et al. (1996), the quantities of maj or litho- and bioclasts and abundance of textural features were evaluated and logged against the succession. Five classes (Tab. 1) were established to gain semi-quantative data. They are based on frequencies that could easily be delermined in a single thin section (7x 10 cm). hnportant facies variations were observed during fieldwork on the large rock surfaces in the middle part of the section. Since these coral-rich surfaces were interpreted to represent the proper reef facies, an attempt was made to document the distribution and size of the corals and all additional identifiable organisms and facies types on the surface. Only objects sized at least I cm-" were included. A grid of 15 x 15 cm or 40 x 40 cm (using a ruler) was used to redraw the surface on a smaller scale (preferred 1:7.5 or 1:10). The areas of the redrawn surfaces (areas A-D, Fig. 6) vary from 1.3 - 7.3 m ~- according to exposure quality.

4 DESCRIlrI'ION OF OUTCROP AND SUCCESSION

The carbonate rocks at Blue Pool Corner are mostly dolomitised; the exposures are not continuous throughout the cliff sections of the bay. The best exposures occur at the western and eastern ends. The eastern end, Blue Pool, displays a partly dolomitised section, which is rich in macrofauna, especially corals (Fig. 3).

The dolomitisation of the Blue Pool section varies. The least dolomitisation occurs on the previously mentioned surfaces in tile middle part of the section. Dolomitisation increases gradually away from them.

The lower part of the succession is complete ly dolomitised, consisting at least of 3.25 m of dolostones (unit A). Apparent channel-like structures and cross-stratification were observed, but their primary origin is uncertain.

The overlying unit B consists of 5.25 m of commonly poorly bedded carbonate rocks. At the base of the unit, a 35 cm thick coral bed with Siphonodendmn sp. andAxophyllum sp. is completely dolomitised. The corals appear to be arranged in small clusters. Bioclasts are abundant, including coral fragments, pelmatozoans, brachiopods and large gastropods. Above the coral bed, dolomitisation gradually decreases and fossil content increases. Silt- to sand-sized dolomite crystals occur throughout the unit within the matrix or as clusters in small pockets. Cross-bedding and channels decreased in size towards the top of the unit. Coral fragments and overturned coral colonies are widespread; colonies in

C l a s s m i c l o - s c a l e d m a c r o - s c a l e d

0 = a b s e n t 0 0

= t a l e 1 3 1

2 = p r e s e n t 5 - 20 2 3

3 = a b u n d a n t [ 20 50 5

4 = d o m i n a n t ! > 100 > 10

Table I. Abundance categories and corresponding absolute hits per thin-section, used in the semi-quantitative log~ng of the Blue Pool succession (Fig. 4). Macro-scaled components were corals, brachiopods, echinoderms, fenestellid bryozoans and sponges. Further allochems are ranked as micro-scaled.

225

Fig. 3. The Blue Peal succession sinew- ing litholog 5 4rid lll~ljOr mncrolaunal cle- menl>. Facies 13pes are indicalcd, hut the ox eral l imago is more complex. The dom i I1LIIll Facies type for the particular tlnJl iS in bold letter. Modified from Aretz (20()2a).

growth position are rare. The coral fauna is dominated by fasciculate colonies of Siphonodendron and solitary corals. Massive (cerioid) mgosa and .~3ringcq~ora appear al the lop o f the unit.

Unit C, 1.9 m in thickness, is differentiated b3 reduced, but coarser input of bioclasts, and more massivc character, compared to the underlying units. The macro fauna is char- acteriscd by brachi opods, lo n g - stem m ed c rin old flag ments, large coral fragments, and colonies of Lithost~wtion. Low- growing colonies of ,Siphonodendron in growth position, with diameters of up to I m, are restricted to the top el" the unit.

The overlying unit D consists of 5 m of massive, fine- grained limestone. I,argc colonics ot: Sil)honodendron and Lithostrotion are common, l, ithostrotion colonies reach maximum diameters of 0.9 m and heights of 0.5 m. Tile percentage of colonies in growth position versus coral debris varies horizontally and vertically. Double-valved brachiopods, pelmatozan debris and some chaetelid sponges occur; bioclasts become coarser towards the top of the unit. Densely packed, large Siphot~odendt-ott colonies in growth position are concentrated in the upper part of the unit. Thcy form a mostly dolomitised thicket, about 2.5 m high and 7.0 m wide, extending 1.5 m into the overlying unit E. SinGle colonies are up to 1 m in diameter and 0.7 m in height. The lateral equivalents of file 5iphonodendron thicket arc dolomitised.

The boundary to unit E is wax.y and coincides with thc change from limestone to fine-grained dolostone devoid of macrofossils. Significant thickness changes occur in unit E. varying from 2.5-4.0 m within a distance of 10 m. Cross- stratification and channels, both of various sizes, arc common.

Unit F, 2 m in thickness, consists (fl cross-bedded and commonly laterally discontinuous, fine- to mcdium-graincd grainstone layers. Grain-sizes alternate commonly. Single lumachelle layers, mostly deriving from brachiopod and gastropod shells, are intercalated. Partial dolomitisation is more common in the lower part e l the unit.

The following units, G-I, are bcst exposed on the pro\ i- ously mentioned central surlaccs. They arc rich in macrofauna

and dilTer significantl~ lrom the lower grainslone layers. Unit G mcasures 5 m in thickness. The finc-grained.

massive limestone contains nun~erous corals. At Ihe base. fragmcnls of lJtho.slroriot~ and .S'il#'tcmr*fletMrrm are still rarc, but the abundance nf corals and Iheir dimensions increase Iowards the lop. [,ai'ge colonies in growth position occur in the upper part of the unit, ha~ ing diameters up Io 1.2 ill. S YI'JIlgO].N~I'{I Occurs lhIouohol.lI , c o n l m o n l y in small patches, but it is uncertqin whether these arc small cokmics OI" I ra~lncnls . Patch}, c o n c e n t r a t i o n s o f brachiopods and pelmatozoan debris, parlly associaled with collccnlraled coral bioclasts, fire also COlllIllon Ihroughoul lhe unit.

Massive limestone in trait H (5.6 m thick') can be scpa- rated inlo lwo di.,,tinctivc Facies on Ihc basis of Faunal differences, l,argc in sig,-colonics (up to 1.5 m in diamcter) of Litho.strotion. ~ro\ving {m e a c h el.her, dominale the central part of the outcropping rock surface {compare t:ig. 6B). in between, colonies of .Sil#~onodend*v,1 and .S'3,ringr~pora occur. Solitary corals are rare. Spaces between colonies and between corallites of fasciculatc laxa arc partly filled by brachiopod, pelnqatozoan, and coral rubble. The nuinbers and dimensions (>1 l, iHm~rr<,Hon colonies decrease drastically to both sides, hut SiphrmodetMron. ,S'ydJzgOl)Ota, anti solitary corals becnme i]lOl'C common. Imterall3.. also lhe amount of debris increases find becomes coarser. In the upper parl of the unit. the LitheJ.vtr:~Hcm-rich central area is surrounded by large Sil)h(modendr(m colonies in grovvlh. position, measuring up to I.() m in height and 1.3 m in diameter. Howcvcr. lhc thickness of lhc coral dominated facies in thc central part i,, abou t 1.5 - 2 Ill t h i cke r than ill

Imeral equivalents. L!nit 1, the uppermost unit exposed on the rock bench, is

4.5 m thick. It sialIs with a single dolostonc layer. Compared to units G -- H, unit I is loss lnassjvc, with some faint buddin G. lls lower, ~1.5 2.0 m-thick part consists o1" fine-grained wackeslone/grainstonc, inlcrfingcring wilh chaoticalh, distributed, coarse-grained bioclastic ~rainstonc ttnd rudstone layers. Rugosc corals arc less abt.ndant, bul single fragments and mostly oxerlurncd colonies occur. This Iowcr part is o~crlain by a 0.4-0.6 m-thick horizon with a v c r y sparse fauna in linc-graincd wackcstones/grainstones. File upper

226

part of the unit starts with an entirely dolomitised, ~0.4-0.5 m-thick brachiopod lmnachelle. It is overlain by fine-grained limestone yielding a rich fauna of brachiopods, fistuliporid bryozoans, large crinoid ossicles and articulated sterns, few chaetetids, solitary corals, and Siphonodendron colonies. Beginning in the middle part of the unit and laterally discon- tinuously, dolomitised and non-dolomitised horizons alternate frequently. Partially dolomitised, fine-grained grainstones with an exposed thickness of 5 m follow on top (unit J). Cross-bedding is common; graded bedding occurs.

5 FACIES ANALYSIS

Twelve microfacies types (MFF) were differentiated at Blue Pool (Tab. 2). As seen from the macroscopic description above, the succession is highly variable; microfacies types grade into one another even within single thin sections (Pl. 38/3) or display erosional contacts (P1. 39/7). Hence, five more generalised facies types (FT) were defined, which sum up microlacies analysis and field observations.

In general, two sources are responsible for production and accumulation of carbonate sediment in the Blue Pool succession. These are (1) the autochthonous source of the microbial and coral boundstone facies (FT 1, 3), and their erosional products and (2) the allochthonous shallow-water source of adjacent ooids shoals, bioclastics bars, etc., sup- plying diverse bioclasts, ooids and other transported carbonate grains.

FT 1: Microbial boundstone (P137/8, PI. 38/1-3, P1.39/1-2, 6-8)

Microbial boundstones are widespread in the section. They are very common in units C and 1 and even more abundant in units B, D, G, H.

The microbial boundstone facies includes four micrite- dominated microfacies types (MFF A-D), which could be classified either as framestone, bindstone or bafflestone. More detailed subdivision was not supported, because of the wide variation within single samples and the unknown

original palaeoecological significance of the organisms within the facies type. Fasciculate corals are commonly regarded as sediment bafflers. If individual specimens were incorporated in a microbial boundstone matrix, the coral may not have been the dominant constructor of rigidity (Webb, 1996) and a classification would be more appropriate as bindstone. Composition and structure of the facies type is highly variable. In general, it is characterised by a micro-peloidal matrix with abundant small cavities. The peloids are small and have difuse/irregular margins. They occur (a) multilayered, laminated (=stromatolitic, PI. 38/2, 39/1, rarely oncolitic), (b) clotted and irregular, without internal layering, but with a preserved outer dark rim in some cases (=thrombolitic, P1.38/1 ), and (c) as peloidal (Pl. 38/3, 39/2), partly graded cavity filling. Automicritic matrix is in some cases associated with collapsed spiculm" sponges (Pl 37/8, 39/8).

Dark micritic crusts are common. The micrite is interpreted as a product of microbial communities. Additional organisms are incorporated as well. Thin laminae in some multi-layered crusts include Aphralysia and encrusting bryozoans (PI. 37/6, 39/6). Aphralysia also tk~rms individual crusts up to 3 cm thick (PI. 37/5). Other encrustors include porostomatc "cyanobacteria", Ortonella, Fasciella, Asphal- tina, bryozoans, serpulids and vermetid gastropods. Lo- cally, the issinellid alga Luteotubulus forms tiny patches of bafflestones (P1.37/8).

Cavities are widespread in the different microbial boundstone microfacies types, but commonly they are volumetrically restricted (P1 39/1-2). Primary cavities of various sizes, which are almost totally filled with peloidal matrix, are very common. Incompletely filled cavities are common in some place. Smaller, irregular shaped, spar- filled cavities arc less abundant. Ooids are very rare, and seem to be incorporated within cavity fillings. Exceptional are coelobitic fistuliporid bryozans, attached to the roof of microbially constructed cavities (PI. 39/2).

Flora and fauna are diverse. An analysis of the distribution (Fig. 4) shows that echinoderms and brachiopods are

P l a t e

Fig. 1. Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

37 Coral-rich bioconstructions from the Visdan of Blue Pool, South Wales: typical skeletal organisms

Lithostrotion; unit H (coral reef, core facies). Outcrop photograph, natural size. Lithostrotion decipiens (McCoY, 1849), transverse section of the cerioid colony; unit H (coral reef, core facies). Sample BP 23, x 6. Siphonodendron martini (Mn.NE-EDwAm)S AND HMME, 1851), transverse section of the fasciculate colony in dense micritic matrix including some fine-~ained bioclastic lenses; unit H (coral reef, flank facies). Sample BP 28, x 4. Syringopora cf. geniculata (PHmLIPS, 1836), fragment of a colony in a mostly dolomitised, bioclast-bearing matrix; unit I (post-constructional phase). Sample BP 61, x 5. Internal structure ofthickAphralysia-crust showing small spar-filled cavities (S) and thin micritic layers (M); unit I (post-constructional phase). Sample BP 17, x 12. Fenestellid bryozoan encrusted by Aphralysia in a crust,up to 1.5 cm thick; unit I (post-constructional phase). Sample BP 17, x 8. Large bryozoan from the bottom level community on top of the succession; unit I (post-constructional phase) Sample BP 67, x 6. Facies type FT 1: Microbial boundstone. The issincllid alga Luteotubulus sp. (i), forming local bafflestone texture, calcareous sponge (s), bryozoan (b), and cavities (c) within matrix formed by clotted (microbial) peloids (m); unit d (reefmound). Sample BP 7d, x 6.

P l a t e 37 227

228

ubiquitous, but volumetrically inconspicuous. Bryozoans are abundant and might exceptionally become dominant. Sponges are locally abundant, but only chaetetids have some volumetrical importance, whereas other groups, like lithistids and spicular sponges remain scarce. The high abundance of corals (cf. Fig. 4) normally reflects the occurrence of a single colony within the sample, but there is also a fairly continuous, small content of coral bioclasts. Foraminifers, gastropods, and calcareous algae occur locally in varying amounts: they are commonly absent or inconspicuous. The amount of transported bioclasts versus organisms in place is highly variable. Corals, chaetetid sponges, bryozoans and brachiopods are mostly in place, but also occur as bioclasts.

In some cases, fine- to medium-grained bioclasts a.'e concentrated in grainstone or packstone pockets within the microbial boundstone. The composition of the bioclasts does not differ significantly from the spectrum described.

According to the diverse fauna and the algal flora, the facies type formed within the photic zone. It represents varied subtidal, low-energy conditions within an open shelf lagoon. The microbial boundstones mostly represent a non- skeletal framework, hnportant framework builders, besides microbial fabrics, were Aphralysia, bryozoans, and to a lesser extent, spicular and chaetetid sponges. The origin of the widespread cavities remains somewhat unclear, but primary porosity within the microbial framework and the dissolution of organic material (e.g. of sponge tissue) were certainly included. The rare input of ooids indicates adjacent ooid shoals and underlines the shallow-water setting of the facies type.

In some parts of the succession, dolomitisation and recrystallisation of the matrix hampered the volumetric quantification of components. This holds true especially for the fine-grained microfacies type D (peloid-bioclast boundstone). Compared to the other microfacies types of the microbial boundstone facies, it is characterized by the high

percentage of fine-grained bioclasts, the low content of micritic crusts, and the obvious thickerAphralysia crusts. As in other microbial boundstones, packstone-pockets wilh accumulations of larger, transpolnted bioclasts occur. Since MFT D commonly occurs next to FT 5, dolostone facies within the succession (cf. Fig. 4), dolomitisation and recrystallisation may have obscured most of the small microbial components and support is given to its strong, apparently early diagenetic overprint.

FT 2: Coral boundstone (PI. 37/1-4, P1.38/4) The coral boundstone facies dominates units G and H,

and partially the top of unit D. Isolated occunences of colony clusters are found in units C, D, I. Coral boundstones were also identified lateral to the measured succession; even after dolomitisation and intense dissolution of skeletal components, ghost structures of stacked colonies are preserved in dolostones.

Large colonial rugose corals in growth position dominate. Most abundant are Siphonodendron martini and Litho- strotion decipiens. Svringopora and some other species of Siphonodendron occur in smaller amounts. The facies type is restricted to areas with a clear dominance of colonial corals: distances between single colonies are small (< 15 cm), and, ideally, they overgrow each other. A single, isolated colony does not represent the coral botmdstone facies type.

Spaces between colonies and between corallites of fasciculate taxa contain various sediments ranging from fine-grained bioclastic grainstone to microbial boundstone. The components incorporate coarse crinoid and coral fragments and less common ooids. Variability is extremely high. Corallites are partly surrounded by dark micritic layers of unknown origin. The layers range in thickness from a few micrometer thick single-layered to some millimeter thick, multilayered crusts. Unlike in FT 1, microbial boundstone, At)hralysia is not involved in entrusting.

P l a t e 38

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Coral-rich bioconstructions from the Visdan of Blue Pool, South Wales: facies types

Facies type FT 1: Microbial boundstone: thrombolite. Small, irregtflarily occurring, clotted peloids without layering. Some irregular, smaller spar-filled cavities: unit G (diversification stage of coral reef). Sample BP 68, x 10. Facies type FT 1: Microbial boundstone: the diffuse character of the thrombolitic matrix figured before turns locally into more laminated (stromatolitic) texture: unit G (diversification stage of coral reef). Sample BP 68. x16. Facies type FT 1 : Microbial boundstone: partly dolomitised, complexly distributed non-skeletal and skeletal

framebuilders, additionally a multitude of cavities partly filled with peloids and bioclastic fragments: Siphonodendron (top right), bryozoans; unit G (diversification stage of coral reef). Sample BP 50, x 7. Facies type FT. 2: Coral boundstone: phaceloid Syringopora colony in pelmicritic matrix; secondary

dissolution and infill of pelmicritic internal sediment; unit D (reefmound). Sample BP 8, x 4. Facies type VI" 4: Fine-grained bioclastic grainstone consisting of spheres/calcispheres and tubular algae.

Dark burrow in center: unit J (very top of section). Sample BP 20, x 16. Facies type FT 3: Coarse-grained crinoidal rudstone with Rotiphyllum rushanicum VAt;GHAN (I 906); unit H (coral reef, flank facies) Sample. BP 34, x 6. Facies type FT. 4: Fine-grained bioclastic grainsione containing strongly micritised ooids, ooid aggregates

and cortoids; bioclastic nuclei are mostly echinoderms; unit J (very top of section), Sample BP 22, x 24. Facies type FT 4: Fine-grained bioclastic grainstone. Coarse layer with micritized ooids, aggregate grains

and cortoids; unit F (bioclastic unit separating the bioconstructions). Sample BP 13, x 16.

P l a t e 38 229

230

The coral boundstone facies forms the core of a bioconstruction. The dominance of cerioid colonies, the partly grainy bioclastic in fill, and the scattered input of ooids indicates a higher energy shallow-water environment.

FT 3: Coral-pehnatozoan packstone/rudstone (PI. 38/6, 39/7)

The facies type occurs in units G, H, I. and less abundantly in units C and D.

Carbonate sediment consisting of numerous, poorly sorted and angular to subangular coral and pelmatozoan flagments is characteristic. Other bioclasts include brachiopods, bryozoans, gastropods, calcareous algae, and foraminifers. Dark. thin micritic crusts occur mostly on corals and. thus, point to their origin prior to reworking. Tile size of the components is variable, but generally they tend to be coarse. Later infilling by small clasts occurred. Micritisation of small clasts is very conlmon, and may be a partial source for the micrite, which occurs patchily in most samples. Early sparitic cements were partly transformed into rholnbohedral dolomite crystals.

The facies type is characterised by three factors: (a) occurrence of corals, (b) proximity to in situ colonies, (c) input of various bioclasts. Therefore, it is not restricted to the upper part of the succession, as indicated by Aretz (2002a). The facies type represents the reef debris facies - partly flank facies - in the units G, H, and I. In the lower units C, D it is associated with coral thickets.

FT4: Fine-grained, mostly bioclastic grainstone (P1.38/5, 7- 8, Pl. 39/4-5)

This facies type dominates units F and J. and is fairly common in unit I.

Silt- to sand-sized bioclasts, cortoids, aggregate grains, and ooids are characteristic within common cross-stratified and graded, often discontinuous beds. Bioclasts dominate and occur throughout, but local accumulations of other allochems are common. Ooids are partly poorly preserved: ooid aggregates occur in places (P1.38/7).

Grain sizes and packing vary widely. Very fine-grained, well-sorted and partly bioturbated grainstones, consisting mostly of spheres/calcispheres and tubular algae represent one end-member (MFT K, PI. 38/5), heterogeneous, poorly sorted grainstones with subangular clasts the opposite end (MFT J). Locally, secondary infill of smaller grains (foraminifers, spheres/calcispheres, tubular algae, unidentified microbioclasts) reduced the primary pore space and resulted in very dense packed areas, whereas nearby less densely packed grainstones with spar-filled pores persist. Rare parts may represent cementstones. Partial dolomitisation is common: mostly showing rhombohedral doloinite crystals.

Grain sizes are clearly related to the predominating biota, respectively bioclasts. Most grains are small, with a common spectrum of foraminifers, spheres/calcispheres, calcareous algae (palaeoberesellids, ungdarellaceans, Konincko- pora), and extensively micritised bioclasts. Some horizons have a remarkable abundance of thin, wavy, micritic lanai- nae. Tile existence of the laminae invariably conelates with abundant aggregate grains and coated grains (MFT L, M, Pl. 38/8). Large grains include fragments of brachiopods, pehnatozoans, and gastropods.

The facies type is interpreted to represent various shallow subtidal conditions above wave-base. Cross-bedding and ooids indicate high-energy, turbulent levels. Clotted ooids, bahamite grams and micrite laminae indicate very shallow, non-turbulent, but intermittently agitated water.

P l a t e 39

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig, 8.

Coral-rich bioconstructions l'ronl the Vis6au of Blue Pool, South Wales: sedimentary textures

Facies type FT 1 : Microbial boundstone. Two laterally spaced columnar micro-stromatolites and overgrowing microbial crusts; sparse bioclasts (crinoid - c: serpulid - s) and few irregular fenestrae. Stromatolitic growth unequivocally demonstrates rigidity: unit C (base of reefmound). Sample BP 5, x 8. Facies type FF I: Microbial boundstone. Coelobitic living fistuliporid bryozoans in microbial pehnicritic matrix. Note growth direction of bryozoans opposite to geopetal infillings of irregular fenestrae and shelter porosity below specimen in top left. This proves attachment within cavities constructed by microbial framebuilding. Unit H (coral reef, flank facies). Sample BP 58, x 5,6. Facies type FT 5: Dolostone consisting of late diagenetic idiomorphic dolomite crystals. Only few coarse bioclasts (crinoid ossicle - center) preserved; unit B. Sample BP 2, x 12. Facies type FT 4: Fine-grained bioclastic grainstone. Upward fining and current orientation of elongated fragments (Koninckopora sp., pehnatozoan): unit F (bioclastic unit separating tile bioconstructions). Sample BP 14, x 16. Top to the right! Facies type FT 4: Well sorted, current aligned, fine-grained bioclastic grainstone; unit F (bioclastic unit separating the bioconstructions). Sample BP 14, x 3,5. Facies type FT 1: Microbial boundstone. Multicrust on crinoid ossicle consisting of alternation of Aphralysia and bryozoan layers; note increasing dominance of Aphralysia; unit I (post-constructional phase). Sample BP 63, x 8. Erosional contact between FT 1, Microbial boundstone facies and FT 3, Coral-pehnatozoan-pack-/rudstone facies, consisting mainly of Sil)honodendr(m fragments. Faint overprint by pressure solution: unit D (reefinound). Sample BP 7d, x 24. Facies type FT 1: Microbial boundstone. Multiple encrustation of a brachiopod shell: below, growth of a decayed spicular siliceous sponge (s), above, settlement of bryozoan (b), rapidly overgrown by a chaetetid sponge; unit D (reefmound). Sample BP 10, x 7.

P l a t e 39 231

232

MFT

A

B

C

D

G

K

L

IVl

Name main characteristics depositional (texture, clasts) environment

reefal facies peloid-sponge boundstone

peloid-bryozoan boundstone

peloid-bryozoan- issinellid boundstone

peloidal matrix, sometimes automicritic, sparfilled cavities; lithistid and coralline sponges. calcimicrobes, bryozoans, some bioclastic debris, including corals

peloidal matrix, irregular, sparfilled small cavities; encrusting and fenestellid bryozoans, micritic crusts around bioclasts, diverse bioclasts often lense-like accumulated, few corals

like B, but cavities slightly larger, calcareous algae (Luteotubulus) abundant, crinoid stems frequently encrusted

reefal facies

reefal facies,

peloid-bioclast peloidal-bioclastic matrix, often dolomitised and recrystallised; transition bioclastic to

boundstone bioclasts in layers and lenses, [ peloidal facies. common thick Aphralysia crusts

numerous coral colonies in growth reefal facies coral boundstone 0osition. between colonies and

corallites unsorted bioclasts and aeloids

coral fragments of various sizes reef debris facies coral grain/rudstone within bioctastic grainstone,

coral-pelmatozan like F, but pelmatozoan/coral ratio reef debris facies grain/rudstone 1:1

coral-pelmatozan like G, but partly micritic matrix, reef debris facies packstone

pelmatozan diverse, pelmatozoan-dominated reef debris facies, grainstone bioclasts, often fine- to medium- bioclastic shoals

grained, rare colonial rugosa

fine-grained bioclastic very small diverse bioclasts, incl. shallow marine open calcareous algae (Koninckopora, water facies, within grainstone Palaeoberesellidae), foraminifers turbulent water of photic

zone ooid grainstone ooids, often poorly rounded, ooid ooid shoal, sometimes

agg regates restricted

fine-grained diverse bioclasts, numerous crusts, transition beeded to bioclastic-microbial microbial boundstone clasts, rare reefal facies grainstone ooids and oncoids

FT

2

4

Table 2. Microfacies types (MFT A-M) differentiated in the Blue Pool succession and assignment to facies types FT 1-4, as discussed in the text.

Episodic input of coarser material is evident from the pres- ence of larger bioclasts and the heterogeneous structure of some horizons. The very fine-grained spheres/calcispheres- tubula," algae-grainstones indicate more restricted conditions in sheltered areas. FT 5: dolostone (Pl. 39/3)

Dolostones dominate in units A and E, and are common in units H and J. They are abundant to dominating in the lateral equivalents of the measured succession. Thin layers or isolated patches occur in all units.

Two types of dolostones were observed: first, with a fine-grained dolomite matrix, and second, with large, more or less idiomorphic dolomite crystals. Some ghost structures

of the macrofauna are still recognisable. Cross-stratification and bedding plains, observable in most dolostones, are presumably of primary origin. Since partial dolomitisation occurs in all facies types, it is likely that the dolostone facies can substitute all of them.

6 CONSTRUCTED FACIES

The most important criterion to identify a bioconstruction is the abundance of framework builders and their spatial distribution. Massive or poorly bedded limestone that in- dudes some potential tee fbuilders is not sufficient. Informa- tion on rigidity, geometry, relief, and the influence/interac-

233

Fig. 4. Semi-quantitative log (fl the Blue Pool :.,uccession. For definition of aburldance categories see Table I.

tion with the surrounding environment are also required. However, poor exposures may linlit that evaluation.

[t has to be stressed that framework builders arc not

restricted to large, mostly clonal metazoans. Webb (1996) highlighted the importance of non-skele!al framework builders and differentiated frameworks deriving l:rom calcimicrobes, non-skeletal microbialites, and biocementstones. Aretz (2002a) and Aretz and Herbig (2003) showed the existence of a delicate balance between non-skeletal and skeletal framework in diflierent Late Mississippian bioconstructions.

AI Blue Pool, macro- and microlYcies analyses indicate three candidates that may fulfil those criteria: ( I ) units t3 I) with numerous microbial boundstone: (2) the coral dominated top of unit D: and (3) the eoraJ rich units O-t.

6.1 Elucidating rigidity

In the conlexl of this sIudy, the evaluation of the rigidity of the microbial boundstoncs (PT 1) is essential. Potential non-skeletal framework builders are very common in the microbial boundslone tholes, :rod in some cases resemble structures figured t,\: Webb ( 1996. fig. 2.3). Several criteria can be used to rccognise rigidib..

(I) Important are abundant, but not exclusive, well developed micritic encrustations. In otlr material, they commonly form a somcwhal open, very irregular meshwork. Main paris of the crusts are dark dense layers, not preserving the original (microbial?) encrustor. Additionally, diiTcrcnt organisms like bryozoans or Aphral.v.via are associated. P, ioctasls v, ere successively incorporaled into the meshwork.

234

The nuclei of the crusts are either a pre-existing crust, bioclast, or corallite of a coral colony (PI. 39/6). The rigidity of the meshwork resulted from the interaction and rapid synsedimentary [ithification of the single layers and components. This is well expressed by "free-standing", vertically orientated crusts (P1.38/2, 39/1 ). The density of the meshwork varies. Open spaces are generally filled with fine, commonly peloidal matrix, thus reducing porosity/fragility and enforc- ing the rigidity of the meshwork.

It is obvious that certain dimensions of the meshwork are required for the formation of a rigid framework. Some small- scaled microbial crusts incorporated into "grainy" matrix do not imply rigidity.

(2) Another tool to determinate rigidity is provided by cavities with sharp boundaries within fine-grained peloidal, thrombolitic matrix, which might be partly or completely filled by internal sediment (P1.39/2). Without synsedimentary rigidity, any cavity would collapse.

(3) The same holds true for coelobitic organisms (bryozoans) attached to the roof of cavities (Pl 39/2). (4) Sharply outlined burrows of soft-bodied organisms also indicate a firm to semi-lithified substrate.

In conclusion, non-skeletal framework, as elucidated in (1)-(4), is recognized in parts of the microbial boundstone facies.

6.2 Evidences of constructed facies in units B-D

Rugose and tabulate corals, fenestellid bryozoans, lithistid and chaetctid sponges occur abundantly as potential framework constructors in units B-D (Fig. 4). However, field observations indicate a large amount of skeletal fragments and only widely spaced, disperse distributed skeletal organisms in growth position. Therefore, a rigid skeletal framework was not developed.

The existence of non-skeletal framework in the microbial boundstones of the units has to be tested. It would have been locally supported by the invertebrate skeletons, which would provide additional stability and primary hard substrates.

In unit B, common cross-stratification, numerous channels, clear bedding, and relatively small, centimeter-sized microbial meshworks do not support the existence of a bioconstruction.

In units C-D, the existence of a bioconstruction is supported by different facts. Beginning with unit C, the limestone appears more massive, the input of bioclastic debris decreases, the number of skeletal organisms and their ar- rangement in clusters and thickets increases, and a complex, rigid non-skeletal meshwork is developed. In unit C, framework building and stabilisation is contributed by the volumetrically dominating microbial boundstone community, which includes encrusting bryozoans. Relatively few corals and sponges lived on/in the bioconstruction, but they are not primary constructors. They were encrusted later, and thus were passively incorporated into the framework. The amount of coral clusters increases in unit D. They are of limited extension and comprise only two to lbur colonies.

Non-skeletal framework dominates in between. Thus, corals contributed on a very restricted, local scale to the formation of framework and stabilisation still depended on the microbial boundstone community.

The spectacular Siphonodendron thicket on top of unit D is of biohermal geometry (chapter 4). Due to strong dolomitisation, the lateral relations within unit D, respectively with the lower part of unit E remain obscure and hamper its interpretation. The thicket could be part of the bioconstruction described and, thus, would highlight the increase of the dimensions of the coral clusters towards the top of the latter. It could also be regarded as an independently developed Siphonodendron-patch tee f postdating formation of the bioconstruction or developing laterally.

In summary, the bioconstruction of unit C-D is termed a microbe-bryozoan-coral reefmound (Fig. 5). Its geometry, relief and influence on the surrounding depositional environment are difficult to evaluate due to very limited exposures. The wavy boundary to unit E may indicate a certain relief, especially, if the Siphonodendron-thicket at its very top would be included. A tabular form with very low relief is likely.

The continuous input of bioclasts and few ooids indicates a position adjacent to bioclastic dominated areas and ooid shoals. The increasing amount of coarse debris towards the top of the reefinound illustrates increasing turbulence during upward shallowing. Shallowing upward might be evidenced also by the increasing amount of skeletal framebuilders. However, rapid upward-growth of the bioconstruction or falling sealevel could not be discrimi- nated.

6.3 Evidences of constructed facies in units G - I 6.3.1 Distribution of frame builders and quantitative

facies mapping

Fine-grained grainstones of unit F are replaced by microbial boundstones rich in bryozoans (FT 1 ) at the base of unit G. There, only single corallites of Siphonodendron and a few small fragments of Lithostrotion occur. Up-section, bioclastic lenses and layers increase as well as the abundance of corals. The lowermost colonies in growth position are small Syringopora. First large fragments of Siphonodendron and Lithostrotion, yielding more then 20 corallites, occur shortly afterward; some solitary corals (Axophyllum sp., and Rotiphyllum rushanium) are found in bioclastic lenses.

The first colonial rugosa in growth position appear 2.5- 3 m above the base of unit G. Some decimeters above, colonies already measure up to 1.2 m in diameter. There, in ~he uppermost part of unit G (Fig. 6: mapping plane A) about 25% of the surface is covered by cerioid and fasciculate corals, compared to an average of less than 10% for the total unit. Most of the mapping plane consists of microbial boundstone (FT 1), which includes thin bioclastic layers. Patches of coarse bioclastic debris, commonly rich in echinoderms and solitary corals, are significant as well.

The abundance of colonial rugosa continuously increases into and within unit H. The central part of mapping plane B

235

(Fig. 6) is dominated by large, intergrown, cerioid colonies of Lithostrotion decipien.v. S/phonodendron mormfi. S. ct'. kleffense and Syringoporo sp. are less abundant: scaltcrcd solitary corals occur. Coral coverage varies from about 25r/~ , to more than 50%; mapping plane B has coverage or 3 g ~ .

Surface-ratio ofmassive to lasciculate forms is 1:0.78. This, however, is not infonnalivc due to very, different sizes of cerioid and fasciculalc taxa, especially due to the very small- sized fasciculate syringoporoids. The ratio of similar si/cd Litho,strotion to Siphot~odendr,n is 1 : 0.44 and clcarly indicates the dominance o[cerioid colonies. Coral boundstonc (FT 2) dominates within the mapped area. Microbial boundstone (FT 1) and coral-pelmatozoan rudstonc (FT4) occur between colonies. Isolated accumulalions of coarse bioclastic material, rich in pelmatozoans. OCCLII- irl the upper part of the mapped area.

Lateral variations in unit H have been described already in chapter 4. Mapping plane C (Fig. 6) is situated north (seaward) orB. In comparison, the amount elcoarse bioclastic debris increases. Vice versa, the overall abundance elcorals decreases and coral coverage drops to 25-30 <~. Deviation from the average is greater due to very irregular cohmy shapes. The ratio of cerioid to fasciculatc colonies is in- verted. Although massive forms are still abundanl, fasciculate taxa(Siphonodendr<,~ and SyHn,~oponOdominate in numbc) and surface coverage. The colony sizes of Syrin<v~,pora increased remarkably, but specimen abundance rcmaincd almost unchanged. The ratio of massive to fasciculate taxa is 1 : 2.67; thal of Litho,vtrotion to Siphcmodendron I : 1.83. Facies analysis indicates a small-scaled mosaic of microbial boundstone (FT 1). coral boundstonc (FT 2) and coral- pelmatozoan rudstonc (FT 4). Large coral fragments arc fairly common. Somc coral colonies arc not in gr(}w[I] position.

The coral abundance decreases drastically in unil I. Arctz (2002a) calculated a coral cover of t 0~?b. Mapping plane I) (Fig. 6) shows coverage of ahnosl 20 <){. The diflercnce depends on choice and outline ot" the mapping plane. None of the colonial rugose corals in unit I is in growth position. Some colonies are overturned, the maioriiy are fragments. ranging from a few corallites to almost colon.v-sized Irag- ments. Breakage during reworking is well observed at the large colony in the cenlrc of mapping plane D (Fig. 6). whele

filling ~\Ii[Ii c(.~aisc crinoidal debris substitutes a broken-off part.

Most ofunh I is dominaled by microbial boundstone (FT 1), with Iligh contents o1" bioclastic debris (FT 3). A chaotic mi• elboth facies types was mapped, even complicated by horizons with different degrees of dolomitisation. A inajor break is the dolomitic brachiopod lumachellc in the middlc of the unit, about 0.8 m abovc tile lop of mapping plane D.

An intensive facies survey in the doh)mitiscd and widely oxergrown lateral equixalents of units G-I was onl', partly successful. [~raglnents of massive and fasciculatc colonies were found northward (seaward) adiaccnt to mapping plane C. Also southward (eli ffw~ud ) ad}aceal lo B, the l, idlo.~troHcm dominated facies persists for 2-4 mclers: farther on and in art joining parts of the cliff, dohmlitised Siph<modendre>n colonies are abundant, re<~cmblin~ the spatial distribution in mapping plane C. Transition into bcddcd limestone lacies could nol bc observed, lsohtted paIchcs of bedded limestones occur within the cliff, but their rclatitm to unit G-[ remains equivocal. Ramsay (1987:300) reported that "'tile bioherm limcstoncs ... pass latcralh, into trough cross- bedded dolomitic, oolitic and bioclaslic g/ainstoncs and packstoncs wifl~ large ovcrIurned c<~rat ctasts.'" However. this seems to be a generali/ed sumnlary f[IOl|l both sides of the structure.

6.3.2 Intcrprctado)l

According to the spatial dislnbution ( ~ I l I'acies and fauna, units G-H of the Blue Pool succession arc interpreted as a bioherm. Beginnin~ gr()wth is recorded m the basal part of unit G. There. microbial communities chert,steal loose sediment+ Rigid, ram-skeletal franlework occurs, but its spatial distribution is limited. The rcpcatcd influx of bioclastic debris interrupled its devclopnmnt and reduced its volumetrical importance. H(+we'ver, microbial boundstone fabrics and coral settlement intensified. The area with maximally dex eloped coral boundstorlc faces ( .c., mappirlg plane B with dominance of I.#h<;.wrotio,z)represents the climax, or core facies, of the bioherm. [:lank Iacics adioin laterally (mapping plane C : the general decrease in coral

I : ] ~ " ~ I M~dcl Of lhc mwrohc-brv(~zoan-coral

reefmound oI t!nJts C- [). Not ~),~calc.

236

Fig. 6. Distributional pattern of the macrofauna on selected surfaces in the coral reef of units G-It. Modified from Aretz (2002a). A: Diversification stage. B: Dominance stage: coral core facies, C: Late diversification stage: flank facies, I): Post-constructional, reworked facies

abundance, the increase of fasciculate colonies, and the increase of bioclastic debris including coral fragments and dislocated colonies as well are typical of the flank. Appar- ently, the core facies constructed by massive, cerioid colonies is surrounded by large, fasciculate Siphonodendron, as observed in the upper part of unit H.

Outcrop conditions limit interpretation of geometry. Maximum thickness of the bioherm is about 12 m, that of the core about 7 m. A relief of about 1.5-2 metres can be postulated for the bioherm due to the exaggerated thickness of the core compared to the flanks (cf. chapter 4).

The influence of the bioherm on the sedimentation is levelled out in unit I; deposition of unit J was no longer influenced by the earlier relief.

The rigid framework of the core facies was achieved primarily by the densely intergrowing cerioid corals. It was secondarily stabilised by microbial communities, including encrusting bryozoans, which live either within cavities of the primary framework or encrust the corals. Locally, rigid non-skeletal framework was developed. The accumulation ol'bioclastic debris further reduced the primary porosity and increased rigidity. Only small spar-filled cavities remained.

237

Coarse bioclastic debris and dislocated coral colonies indicate higher energy levels at the top of unit H (mapping area D). This caused an end to coral growth, but. patchy constructional activity continued by microbial boundstones communities at the base of unit I. However, rapid changing lithologies and increased bedding indicate growing hydrodynamic instability and the collapse of bioconstruct ional activity. Deposition of the brachiopod lumachelle marks ama, jor break.

At the top of unit I, rccoveFing microbial communities and encrusting bryozoans formed local non-skeletal framework clusters. These lirmgrounds were coloniscd by a diverse fauna and algal fh)ra. Afterwards, the dcpositional reahn was buried by a mobile ooid shoal, causing the breakdown of the benthic communities.

Ooids in the bioclastic matrix of the core facies indicate somewhat distant shoals during biohenn growth. The continuous import of bioclastic material during biohcrm growth is due to organisms living on the bioherm, e.g. brachiopods and crinoids, ,'.is seen in holdfasts of the latter, but also to the import of bioclasts from nearby bioclastic sands (Fig. 8).

The bioherm is classified as coral reef due to the coral framework in the core facies of unit H (Fig. 7). It shows an ecological zonation according Io Walker and Alberstadl (1975). The stabilisation anti colonisation stages arc recorded by the onset, increase and final dominance oF microbial framework in unit G (Fig. 7), and the simullane()usly decreasing bioclastic input. Microbial framev,:ork provided a rigid non-skeletal foundation and, probably, aheady a faint elevation on the sea-floor, swept by' bioclastic sands. The colonisation stage merged gradually into the diversification stage, as seen in the appearance of colonial corals and a diverse fauna within rigid non-skeletal framework in unit G. Diversification continucd t o w a r d s the top o1 unit G, documented by increasing size and abundance of colonial corals. The dominance stage is represented by the Lithostrotion-core tilcics, whereas tile flank facies typically lapses behind. It is dominated by large lasciculate taxa and coarse bioclastic debris, reminding a late diversification stage.

The small-scaled continuation ( f l hie-constructional activity on top of the reef is the aftermath of reel" growdL

7 FACTORS CONTROIJ.IN(; GROWTH AND I)I,:CIJNE ()F 'FILE ItlO(.()N,N I RLC I IONS

Blue Pool contains tw'o apparently unique bioeon- strucdons from South Wales. It is known, that individual Mississippian bioconstructions are primarily contFolled b~ the local lectono-sedimcntary regimes, which, in turn, are embedded in a global fmttern (Webh,. 1994, 20()2). Motto- \or, Aretz (2002a) stressed the importance of the system of multiply interacting intrinsic and extrinsic factors, which control coral-bearing Mississippian bioconstructions. The main controlling fi:tctor in the Blue Pool bioconstruclions was water depth. Thc growth of the tee fmound ceascd below a dohnnilic unit, which originally most probably represented grainy facies, and which in turn is overlain by,' oolitic grainstone. Also in the ovcrlying coral reel, coral growth came to an end durin,g rising hydrodynamic energy, leading to mechanical destruction and erosion at its lop.

Three factors could inl'lucnce water depth. (1), Both bioconslructions grew into shallov,er water and thereli~re reached unfavourable hydrodynamic conditions. (2). Sea- level fall reduced the accommodation space. (3), A lateral shift ~1' ad, iacent hioclastic shelf sands and ooid shoals outpaccd/burJed the bioconsiruclions. All factors could have been in,,olved, and, very likely, did interact. For example. changing sea-level per se will cause the shiti of [acics belts. Small-scaled glacio-eustalic sea-level fluctuations triggered by the beginning glaciation olGondwana arc known to ha~c started in the Asbian (Wright and Vanstonc, 2001). This w, ould correlale with the proposed biostratigraphic datin,~ of the Blue Pool succession (chapter 2) and relate to the Iwo buildup cycles. However, small-scale cyclicily is also documented in Formations of Holkerian age, e.g. in the Belgian l,ives Formation (Poty et el., 2002).

T;,,o intrinsic factors are linked to the hydrodynanfic regilnc: ( I ), the fragility el'the generally delicate CarboniF- erous lasciculate corals, and (2), the fixing of rugosc corals to the substrate. Thus. Aretz (2002a) exemplified lot hish upper Visdan Sil~hOmMemh:m that single corallile branches break, if the coral grows inlo turbulent water. Partial burial of a colony by bioclaslic sands, also seen in Blue Pearl, or cncrt.stations, which increase corallJte diameter, cnloree ils rigidity. The abundance elcoral flagments at the lop of Blue

Fig. 7. M~.~dc] o f the coral reef in unit,, G-I t . F~>~ ',,.'ml-,,.)ls ,,_'~.~mparc Fig. 5. N,.~t to, scale. Mt+di l ied I+ron , Ar,:.,t.,' (20( 2 e l

238

Pool reef and on its flanks demonstrates that fragmentation was prevented only to some extent.

Compared to modern scleractinians, the substrate attachment of rugosa was poor. They were fixed only by a small protocorallite, in some cases supported by talons; in many cases an attachment is not known at all. Spatial encrustations o flarger surfaces are ahnost unknown (Scrutton, 1998). This severely hampered rugose corals in conquering high-energy enviromnents.

In the core of the Blue Pool coral reel', large, cerioid rugosa resisted increasing wave energy, probably mostly due to the weight of the growing colonies. They additionally benefited from microbial encrustation. However, passing a certain hydrodynamic level, their growth came to an end. This might be intimately connected with the impossibility of renewed larval settlement. Characteristically, fasciculate Siphonodendron grew in somewhat lower energetic conditions at the flank of the reef; the size and abundance of the even more delicate Syringopora increased.

8 COMPARISONS

In spite of their general small size, Mississippian bioconstructions are relatively widespread and diverse (e.g. Aretz, 2002a; Webb, 2002). Therelbre, on a regional scale the Blue Pool bioconstructions are only compared with buildups from the British isles, on a global scale with constructional counterparts.

British Isles The only known reel" in the British Isles, where corals

contributed to fi'amework building, is Furness (Adams, 1984). A frame of syringoporid corals and solenoporaceans was encrusted by microbialites/algae. Rugosa, however, were restricted to solitary taxa and belonged to the dweller guild. Contrasting to Blue Pool, rugose corals did not contribute to framework building in the numerous non-Waulsortian bioconstructions of the British Isles (Fig. 1). The supposed coral patch reef of Taylor (1972) could not be confirmed (chapter I). Coral thickets are present in backreef-areas. respectively in the shallow-water bank facies of the apron- reefs and Cracoan buildups (Wolfenden, 1958; Mundy,

1994), but are absent in the proper framework facies (loc. cit., own observation). Rugose corals occurred in most of the other Late Visdan shallow water bioconstructions, but stayed generally inconspicuous (e.g., Jameson, 1987; Wolfenden, 1958). Their occurrence in the top of some bioconstructions could be related to post-constructional stages, respectively to potential capping-bed facies (Jameson, 1987; Somerville et al., 1996).

In the Late VisEan, the importance of rigid non-skeletal framework was documented in the Lancashire peloid cementstone-palaeoberesellid buildup (Horbury, 1992), the Cracoen buildups (Mundy, 1994), and the Irish Kingscourt buildup (Somerville et al., 1996). The Lancashire buildup does not share any characters of the Blue Pool bioconstructions apart from its shallow-water setting and the peloidal matrix. Some of the growth fabrics in the Cracoen buildups are similar to Blue Pool. As mentioned above, they differ by the total absence of colonial rugose corals in the framework, the much more diversified fauna, the non-clustered, but relatively regularly distributed and apparently more abundant skeletal framebuilders in the framework facies, and the abundance of lithistid sponges. A major difference is the large dimensions of the Cracoen buildups. For example, the Stebden Hill buildup (-9 km north of Skipton) measures up to 430 m in width and 120 in in height. This is related to the setting of the Cracoen buildups at the margin ofintraplatform basins, which is another difference to Blue Pool.

France The contribution of rugose corals and microbial

communities to bioconstructions in the Late Vis6an and Serpukhovian of the Montagne Noire, Southern France, reveal stronger similarities with the Blue Pool bioconstructions.

Opposed to the Blue Pool coral reef, the coral-microbial patch-reef at La Serre (Aretz and Herbig, 2003) is neither differentiated into core and flank facies nor into successive ecological stages. Instead, it consists of intergrown clusters of corals and microbial boundstoncs, which, for the main constructional stages are estimated to be of equal importance. The delicate balance of non-skeletal framework-forming microbial communities and coral-dominated skeletal framework controlled its formation. In general, the

239

importance of microbial IEbrics is much greater than in the Blue Pool coral reel'. The sccond bioconstruction al Blue Pool, the microbe-bryozoan-coral reef mound shares the importance of microbial framework lypcs wilh 1,,a SetTe. It differs by its more uniform character, sincc skeletal franlework is less abundant than at La Serte.

Tile small patch reel: at Castelsec (Aretz. 2002b) shows some remarkable similarities to lhc Blue Pool coral reef. An initial microbia l s labi l i sa t ion of mobile sed imcm is documented in both bioconslruct ions; massive corals dominate higher parts. The reef at Caslelsec is smaller than its British counterpart and (therefore?) a differcntia~ion into core and flank facies is lacking.

Australia The importance of rigid non-skeletal framework and co-

occuring coral boundstones wcrc described I'ron~ ,,arious reefs from the upper VisOan of Eastern Australia (Wcbb. 198,9, 1999,2001 ).

The biohcrms of lhc Lion Creek-Forn~ation (Webb. 1989, 2001 ) are embedded in an oolitic sequence. A massive "coral boundstone core" is completely enclosed b? "'algal mudstone facies", which is an "'automicrite facies". [.ike in the Blue Pool coral reel, microbial communities provided a firm substrate for skeletal organisnls during initial stagcs o[ reef formation; lithistid sponges are very abundant, but inconspicuous to absent at Blue Pool. Also, m both core facies microbial boundstoncs contributed to the slabilisalion of the coral framework. A succession from fasciculatc taxa below to cerioid taxa above b; well pronounced in the [,i,an Creek reefs; opposed. Blue Poul shows the same dislincl differentiation laterally between core and flank. An algal mudstone facies is only rudimentarily developed on IOp of the Blue Pool coral reel, and apparently missing at its flanks. According to Ramsay (1987) and our own observations, which are strongly hampered by poor laleral exposures, a transition into oolitic and bioclastic facies is probahlc.

In spite of many superficial sirnilarilies of both coral reefs, relief and water energy seems to be more pronounced in Blue Pool, which displays strong rcworking al the flanks and especially in the top, as evidenced by broken coral heads, the brachiopod tumachclle, and a l]llal exposure. This points to a saw-toollled pattern of a shallowing-upward cycle at Blue Pool. underlined by the Walker-Albersladt succession of the reel', opposed Io a sinuous pattern of tile sea-level curve in the Lion Creek Formation. Fhcrc. coral windows opened only for short time intervals, within a depositional realm favouring scatlered growth ofmicrobialite thickets.

The FC 5 biollenns lkonl Queensland are less di ffcrenti- ated (Webb, 1999). Rigid non skeletal framework domi-- nates, metazoan clusters indicate some constructional sinai- larities to the Blue Pool ree fmound. Main diffc, renccs arc lhc greater diversity of framebuilders and the abundance of lithistid sponges in the flamework of the Qucensland rccfs.

Algeria The Bechm Basin reveals differcnt VisOan and Serpukh-

ovian deeper-water and shallow-water bioconslruclions

(Pareyn, I959: Bourque cl al., 1995). Thc patch-Fecfs described b\. Pare}.n (1959) eonsisl of a coral-dtmfinaled core surrounded b> bioclastic material. If they correlate with the "'rugose coral-nlicrobial lhickets" of Bourque el el. (1995) amJ Madi (1996). main bioconstructors are microbial communi t ies , which form microbia l i tcs and encrust Sipll,mode,dron colonies . Coral eontr ihut ion to the framework was minor. Apparently. the Blue Pool ree fnlouml ix similar, but addilional descriptions of the coral distribution in Ihe Algerian bioconstructions arc required.

Japan Kawalllura 11999) described a coral-reefmot]nd at

Kilakami (N-Japan*. which is incorporated will]in oolilic grainstones, h is subdivided into a lower stage dominaled by brvozoans. Ihrombolites, and snlitarv corals, and an ur~per sta~,e dominated by Iasciculale and ccrioid fLleOsa. The Bluc Pool recfFnound is ~;imilar in ils in t ra-shc l f setting. bioconstructors, and incrcashw coral abundance, h differs by a missing hipartile succession. Jess conmum solitar;' corals and tile existence of syringoporids contributing to framer, ork building. Similarities to the Blue Pool coral rccf are less pronounced. P, oth bioc,,)nstructions share a shallow-- water setting adjacent to ooid shoals and a donfinance stage provided by corals. However. neither a diflerentialion intt~ core and flank nor lhe dc\,elopmenl of a coral franlc is reported from Japan,

Summary of regional deve lopments Tile comparisons demonstrate Ihat the main potcnlial

bioconsI ructors i n upper V isdan shallow-water environlnenls were microbial comnlunities (calcimicrobc~,. thrombolitcs. stromalolilcs), bryozoans, tabulalc corals, rugose corals, and l i th isth l and chaeletkl sponges. Their d i f fc r ing abundanccscharacterise single hioconstructions, which differ in lhcir internal sh-ucture due 1o inanifold mteraclions of mostly cxtnnsic f,'lctors, e.g, general facies selting and sea- level change. Arctz (2002a) proposed acontinuous speclrum 1ram nlicrobia[ mounds to coral-reefs based on file pleSe0Cc- absence elmicrobes-br3 o/oans-algae and corals and showed lhal mosl of lhc smaller upper Visdan bioconstructions can be placed rote il. The Bluc Pool reefmound is from the nliddle parl oF the spcetrtun, where corals contributed on small, localiscd scale to framework building, whereas the Bluc Pool coral reel: is Ihe rare coral-rich end-lllClllbef.

9 EVAI,I.!ATION O!: Mls'TII()DOI,O(;Y

The procedure lo redraw reef surlaces From grids il-'ig. 6) is a modificalion (ff file melhod ushlg transparC[ll plaslic Sheets {Wcidl ich el al., 1993: Bernecker ct al., 1999: Minwegen, 2001 ). Main advanlagcs are: ( l ). the simplici ty of the nlelhod requiring onl) fickl equipment: (2), all excel- lent description and anah~ds of tile spatial (two-dhnen- sional) distri buthm of single organisms and facies types: (3), a detailed description of outlines and easy documentalion of file (twu-dinlensiona[) sizes of biota; (4), tile possihility of lalcr quam i ficaliun, e.g. calculation of distribution quot icn|s i[/ percentage per arc;.l; {5). compared to the plastic sheet

240

Fig. 9. Methodological problems. A. Five thin sections from one sample (BP 7) show remarkable variations in the abundance of components and facies (abundance categories according to Table 1 ). Horizontal lines indicate the variation in abundance deriving from the distribution in the single samples; the cross indicates the mean from the five samples. B. Influence of sample pattern and sample frequency on the distributional pattern of a theoretical succession (abundance categories according to Table 1). 1: curve showing the natural abundances, I1: resulting abundance pattern using regular sample distances, Ilk resulting abundance pattern using randomsample distances. IV comparison of natural abundances (dotted line) and abundances resulting from the two sample methods. Circles: Method 11 yields higher abundances than method III; diagonally ruled: method II sietds lower abundances tahn method I11. Although both patterns will approximate the theoretical frequency in diffemet degrees, underetimation, respectively overestimation is common and the era'or ranges up to 50%.

method, also relatively rough surfaces are easily worked on, and (6), observations are drawn to scale directly in the field, which immediately elucidates large structures and facili- tates comparisons between different reef sectors. Still, problems remain: (I), limitation of suitable surfaces; (2), field- bound identification of organisms and facies types without available microfacies data, which might refine and modify the macroscopic observations, and (3), bias through simpli- fication of observed and down-scaled structures, especially if used for further quantification.

Microfacies analysis is indispensable for the identification of smaller bioclasts and microorganisms like foramini fers and calcareous algae, which, mostly, are especially useful for interpretation of the environment. Moreover, thin-sections are the major tool for taxonomic determinations in endurated limestones. Recognition of the origin of micrite is a must and only possible in thin-sections; especially the recognition of microbial structures and lhc interpretation of their constructional function is essential in reef studies. As pointed out above, many large-sized features of bioconstructions require the incorporation of macroscopic observation. Also, rapid and unpredictable, small- to medium- scaled facies shifts in reefs (cf. Fig. 9A) are a certain limit for the microfacies approach and require standardization as discussed below.

Main advantage of the semi-quantitative approach used herein is that plotted trends are reliable, since all samples are treated, ranked and judged in the same way, and due to fixed abundance categories, results should be reproducible. A problem, however, is the normalisation of a distinct level/ horizon/unit with relatively strong lateral and vertical variations (Fig. 9A). Pattern and intervals of sampling exert strong influence on the recorded distribution of components and facies, respectively (Fig. 9B). As a result, a semiquan- titative approach is most suitable for environments with relatively continuous long-term shifts. Otherwise, each sam- pie has to be normalised through incorpo,-ation of several

thin-sections from an individual sample or sample cluster (see Fig. 9). Another problem is addressing of abundance categories. Errors might be caused, especially because small components generally are underestimated, and, vice versa, large ones overestimated. Also further refinement of the defined component groups would be of advantage, e.g. differentiation of taxa on species level instead on genus level, differentiation of organisms in biomorpha and bioclasts, etc. However, in complex enviromnents, this is mostly not practical. In summary, the discerned categories remain somewhat artificial according to abundance boundaries and choice of components; assignments might shift according to the experience of an author.

In conclusion, the study of the Blue Pool succession exemplified that a combination of different techniques is required to describe the complex structures and interactions of micro-, meso- and macro-scale in fossil bioconstructions. Redrawn surfaces are a very useful tool to elucidate the spatial distribution ofmacroorganisms, skeletal framework, and facies directly in the field. Supplementation - and possibly correction - by detailed microfacies analysis is indispensable for an appropriate description. For example, existence and distribution of non-skeletal microbial framework are only identifiable in thin-sections; polished slabs are not reliable. A senti-quantitative approach improves the recognition of trends on meso- and macroscale, but are not reliable to identify sudden and short-lasting events.

10 CONCLUSIONS

More than one-half of the most probably lower Asbian Blue Pool succession is bioconstructed facies. It can be related to two relatively small, decameter-sized individual bioconstructions: (1), a lower and less thick microbe-bryozoan-coral reefmound; and (2) an upper coral reef. They are separated by a sedimentary unit of dolostone, primarily probably a grainy facies, and oolitic grainstone. Thus, they

241

are related to two distinct sedimentary cycles. Both arc

shal lowing-upward and probably of gtacio-eustat ic origin.

Both bioconstructions display a minor rel ief of about 1.5 m.

Thei r growth stopped due Io higher energy condit ions: the

coral - reef was partly eroded at its top.

The lower microbc-bryoloan-coral rec f e c u n d was domi-

nated by microbial communi t ies and b u o z o a n s throughout

its entire development . Corals contr ibuted locally to framc-

work building. Like in most Palaeozoic buildups, tabulate

corals (syri ngoporids) were pi oneers, respecti vely ubi q u itists.

They contr ibuted to f ramework building from the begmning

of the growth of the reef mound. In its upper part, colonial

rugose corals became increasingly abundant. The develop-

ment of small coral clusters ,~upported the non-skeletal

microbial framework.

The coral reef shows an ecok~gical succession according

to the model of Walker and Alberstadt ( t975). Microbial

conununi t ies are common ill the reel, but their functional

importance, i. e. formation of non-skelctal rigidity, is only

important in the stabilisation and colonisat ion stages. Dar-

ing late diversi f icat ion/dominance stage, the reef was di ffcr- entiated into core and flank facies, both dominated by

rugosa. The primary r igidity of the core racies relied on

closely intergrown cerioid colonies of Lirho,strofion. Sec- ondary stabJlisatJon and porosity reduction was due to encrusting microbial dominated comm unities and bioclastic inf i l l . Abundance of corals decreased towards the llanks, where fasciculate Si])honodetldrotl and SwJngopo,z~ donli- nated.

Both bioconstruct ions are incorporated in a global pat-

tern of Mississippian shal low-watcr bioconstrucfions rang-

ing from microbial Mounds with exclus ively non-skeletal

f ramework to cora l -dominated rcefs with skeletal frame-

work (Aretz, 2002). It has to be stressed, that this pattern is

not related to water depth. This is dcmonslra tcd in the Blue

Pool succession, where both bioconstruct ions formed in

more or less same water depth.

Finally, thc study of the Blue Pool bioconstruclJons

demonstrates that the description of complex construclcd

facies has to be based on various macro- and micro- tcch-

niques.

A c k n o w l e d g e m e n t

This study benefited from extensivc financial support of thc Deutsche Forschungsgemcinschaft (lie 16101111, which is grate- fully acknowledged. Dr. H.M. Weber, K61n and M. Harwood, Cardiff, am thanked for field assistance. Prof. E. Poty, Liege. and Prof. I. Somerville, I)ublin. added valuable comments <m the succession, l)r. G. Webb. Brisbane. is greatly acknowledged for numerous discussions ~m Mississippian reefs. He ann read an earlier draft of the manuscript and improved the English. R. B~umler, M. H/,Scker, M.I,arisch and S. Miiller (Colegnc)arc thanked for the preparation of the thin sections.

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Final version received June 28, 2003

Coral-rich bioconstructions in the Viséan (late Mississippian) of Southern Wales (Gower Peninsula,...

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