Epibionts, their hard-rock substrates, and phosphogenesis during the Cenomanian–Turonian boundary interval (Bohemian Cretaceous Basin, Czech Republic)

Cretaceous Research (1996) 17 , 715 – 739

Epibionts , their hard-rock substrates , and phosphogenesis during the Cenomanian – Turonian boundary interval (Bohemian Cretaceous Basin , Czech Republic)

*Jir ä ı ́ Z ä ı ́ tt and † Olga Nekvasilova ́ * Geological Institute , Czech Academy of Sciences , Rozvojova ́ 1 3 5 , 1 6 5 0 0 Prague 6 , Czech Republic † Ruz ä inovska ́ 6 / 1 1 6 0 , 1 4 2 0 0 Prague 4 , Czech Republic

Revised manuscript accepted 9 April 1 9 9 6

Hard inorganic substrates (rock clasts and rocky bottom) belonging to the Kan ä k Member of the Korycany Formation (upper Cenomanian – ?lower Turonian) and to the Bı ́ la ́ Hora Formation (lower Turonian) have of fered a good opportunity for the study of encrusting faunas of the Cenomanian – Turonian boundary interval . Distributional features of cemented epibionts and the recorded period of phosphogenesis enabled the dif ferentiation of two-phases in the Kan ä k Member conglomerate formation . During the first phase , the rock substrates were occupied by a so-called A-association of encrusters (29 species , with dominance of oysters and bryozoans) . This fauna partly changed during the subsequent phosphogenic period and not only survived the following period of reworking and the second phase of conglomerate formation , but also persisted until the onset of calmer sedimentation in the early Turonian . This changed community is named the Atreta – Bdelloidina community (23 species) . The opportunistic agglutinated foraminifers Bdelloidina cribrosa and / or Acruliammina longa locally dominated the respective communities during several physico-chemically stressed episodes . On sloping substrates , the bivalves Atreta and Spondylus attached themselves in a preferred orientation (so-called slope orientation) . This feature indicates the original position in which some mobile substrates were colonized . Other questions , such as the taphonomy of encrusters and the character of their substrates are also briefly discussed . ÷ 1996 Academic Press Limited

K E Y W O R D S : Cenomanian – Turonian boundary ; Czech Republic ; encrusting fauna ; rock clasts ; rocky bottom ; phosphogenesis ; palaeoenvironment ; opportunism .

1 . Introduction

Studies on ancient and recent epibionts living cemented on hard inorganic substrates are numerous in the literature (e . g ., Palmer & Fu ̈ rsich , 1974 ; Jackson , 1977 ; Fu ̈ rsich , 1979 ; Voigt , 1987 ; Fu ̈ rsich et al . , 1992 ; Johnson , 1992 ; Wilson & Palmer , 1992 , 1994 , Johnson & Hayes , 1993 ; and many others) . In the Bohemian Cretaceous Basin (BCB) , studies on this subject have been carried out mainly during the last few years (e . g ., Nekvasilova ́ & Z ä ı ́ tt , 1988 ; Z ä ı ́ tt & Nekvasilova ́ , 1990 , 1991a , b , 1992 , 1993 and others ; Z ä ı ́ tt , 1992) . In this paper , all basic data on encrusters of the hard-rock substrates , including those previously unpublished , are summarized for the use of other workers and to show the response of Bohemian late Cenomanian – early Turonian en-

This paper is a contribution to the IGCP project 325 .

0195 – 6671 / 96 / 060715 1 25 $25 . 00 / 0 ÷ 1996 Academic Press Limited

J . Z ä ı ́ tt and O . Nekvasilova ́ 716

crusting communities to environmental changes (O 2 depletion , phosphogenesis , reworking) .

Within the exposures of nearshore rocky-coast facies (see Section 2) there exist very wide dif ferences in the possibilities of encrustation studies . At several localities , the rock substrates (conglomerate clasts and the rocky bottom) are imbedded in , and covered by , hard limestones . This lithology was very unfavourable for our studies and did not yield any reliable data . Our conclusions , therefore , are based primarily on the localities with easily accessible clasts , i . e ., on those where the conglomerate matrix is not hard and / or the overlying rocks are clayey . In this lithology , the encrusters and the phosphogenic processes marking the omission of sedimentation are also best recorded . The study , therefore , reflects only the special development of facies at points scattered along the southern margin of the basin (Figure 1) .

2 . Geographical and geological settings

The study area is situated on the southern margin of the central part of the Bohemian Cretaceous Basin (Figure 1) . Respective geological settings belong to the Vltava-Beroun and Kolı ́ n lithofacies developments (according to C ä ech & Valec ä ka , 1994 ; formerly called the Prague and Kolı ́ n lithofacies areas or regions) . Most localities are situated on the rock elevations that formed islands and peninsulas before they were completely flooded during the late Cenomanian – early Turonian sea-level rise . These parts of the Bohemian Massif are geologically highly varied from which follows also the petrographic and structural variation of flooded bedrock . Under the specific conditions defined by the local rock and hydrodynamic parameters , depressions of varying depth and shape were produced by shallow sea erosion of the bottom . Sediments which subsequently covered the rocky bottom relief belong to two lithostratigraphic units . The basal unit , the Kan ä k Member ( sensu Hous ä a , 1991) of the Korycany Formation , is represented by conglomerates , mostly filling the basal parts of the bedrock depressions . They are mainly clast-supported and often very coarse and unsorted . They show features of at least a two-phase origin .

After deposition of the first-phase clastic bodies their matrix was superficially slightly phosphatized and , together with clasts and adjacent rocky bottom surfaces , was coated by phosphatic crusts . During the second phase , the conglomerates were partly reworked , and younger conglomeratic bodies were formed . The phosphatic crusts were preserved in relics only (see Section 4) . Both the phosphogenesis and reworking spanned probably the latest Cenomanian – earliest Turonian . Resulting clastic accumulations were overlain by the lower Turonian siltstone to claystone deposits of the Bı ́ la ́ Hora Formation . Sediments of both this formation and the Kan ä k Member are rich in macrofossils (bivalves , brachiopods , echinoderms , sponges , bryozoans , serpulid worms : see e . g ., Z ä ı ́ tt & Nekvasilova ́ , 1989) . Accumulations of phosphatized invertebrate coprolites and fish remains (vertebrae , teeth) are developed in basal horizons of the Bı ́ la ́ Hora Formation . Foraminifers , both planktic and benthic , and spores and pollen grains are also frequent and provide necessary biostratigraphical data (Hercogova ́ , 1988 ; Svobodova ́ , 1990 ; S ä temprokova ́ -Jı ́ rova ́ , 1991 , and their personal communications) .

Epibionts during the Cenomanian – Turonian boundary interval 717

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Figure 2 . Velim , western part of the quarry with two deep depressions , Va ́ clav (left) and Veronika , filled with Cretaceous sediments . c 5 Kan ä k Member conglomerates , in the left depression dif ferentiated into first and second (dotted) phases ; cl 5 claystones of the Bı ́ la ́ Hora Formation . Arrows indicate rocky bottom parts with epibiont finds ; for those on the now-missing right wall of Veronika (white arrow) , see Figure 11 . Photo by J . Z ä ı ́ tt , 1995 .

3 . Substrates

3 . 1 . The rock clasts At the studied localities , the coarse clasts (pebbles to boulders) form a substantial component of the conglomerate accumulations of the Kan ä k Member . The clasts always show petrographical identity with the rocky bottom on which they were deposited . They must belong to the latest abrasional phases and were most probably entrapped in the depressions close to the sites of their derivation .

The great majority of studied clasts are mechanically worn by sea-abrasion . Clasts from harder rocks (lydite , quartzite , orthogneiss , tonalite) are often well rounded , while those composed of foliated micaceous paragneiss or by shale are flat and have rounded edges . Small lydite clasts sometimes appear as if polished . Boulders are frequently irregular and may bear ledges , crevices , and holes , which could have been important for the encrusting inhabitants .

Lydite clasts of conglomerates occurring in the area of the Vltava-Beroun Development (Figure 1) bear significant post-abrasional traces of dissolution (Z ä ı ́ tt & Nekvasilova ́ , 1990 , 1991b , 1992) . The timing of dissolution is proved by epibionts encrusting the etched surfaces (Figure 6b) . In addition to this corrosive microrelief , dissolution could also have possibly participated in the shaping of larger-scale irregularities of clasts . A large amount of silica had to be released during these chemical processes .

In the area of the Kolı ́ n Development , dissolution is not so apparent . We tentatively suggest that this dif ference primarily reflects the petrographic features of the gneisses that form the clasts at the majority of localities . They were


Figure 3 . (a) Velim , eastern part of the quarry . (b) Kne ä z ä ı ́ vka , site H (Z ä ı ́ tt & Nekvasilova ́ , 1990) . c 5 first-phase conglomerate of Kan ä k Member ; cl 5 claystones of Bı ́ la ́ Hora Formation ; rb 5 rocky bottom . (---) 5 setting of sediments on the bedrock ; ( ??? ) 5 clast surfaces with Atreta – Bdelloidina community ; some clasts have been undisturbed in their position since encrustation . Arrow indicates sample 1 (see Table 2) . Photos by J . Z ä ı ́ tt , 1987 and 1989 .

probably not easily corroded . The only exception in the area of the Kolı ́ n Development was found at Chrtnı ́ ky where diabase clasts (mostly boulders) show surfaces modified into systems of irregular pits for which bioerosion (probably the browsing activity of sea-urchins and gastropods) and , possibly , superficial

Figure 4 . C ä ı ́ c ä ovice . c 5 lydite conglomerate of the Kan ä k Member ; rb 5 rocky bottom ; 1 – 3 5 samples with measured clasts (Table 2) . Photo by J . Z ä ı ́ tt , 1995 .


dissolution are responsible (Z ä ı ́ tt & Nekvasilova ́ , 1991) . Moreover , an alteration of the diabase clasts and rock surfaces was observed . This is similar to ‘chalking’ which followed the etching and bioerosion and preceded the phosphatization and encrustation .

Traces of boring organisms are relatively frequent only on the boulders formed of lydites and shales (Vltava-Beroun Development) . They belong to the Trypanites ichnofacies ( Gastrochaenolites isp . prevails ; see Z ä ı ́ tt & Nekvasilova ́ , 1992) . The borings occurring mainly in the shaly intercalations of lydites are found in various stages of truncation that preceded phosphogenesis (see below) .

3 . 2 . The rocky bottom As noted above , the large-scale shaping of the rocky bottom was produced by sea erosion and abrasion . In this way , depressions formed that were exceptionally more than 10 m deep (Figure 2) , and the smaller-scale relief was frequently developed in some parts of their walls . These features are best developed in the gneiss regions where the rock is foliated and the dip of foliation planes is steep (e . g ., the localities Radim and Velim ; Z ä ı ́ tt , 1992 ; Z ä ı ́ tt & Nekvasilova ́ , 1994) . The elevations on the rocky bottom with crevices and ledges parallel to the foliation , of fering the encrusters ecologically diversified habitats , were , unfortunately , very rarely found intact , being mostly destroyed by exposure to recent weathering .

The lydite rocky bottom in the area of the Vltava-Beroun Development bears the same traces of chemical dissolution and boring as do the clasts (see Section 3 . 1 and Z ä ı ́ tt & Nekvasilova ́ , 1991a , 1992) . To a lesser extent , and combined with the bioerosion , the dissolution occurred also on the diabase bottom at the Na ́ kle and Chrtnı ́ ky localities in the area of the Kolı ́ n Development .

4 . Phosphates

As mentioned above , the first-phase conglomerates and adjacent clast-free rocky bottom were influenced by phosphogenic processes leading to the formation of thin (mostly several mm) phosphatic crusts on their surfaces . For these processes we infer a highly reduced sedimentation rate , depletion in oxygen , and the subsequent impoverishment of the benthos . Under these conditions , microbial populations probably dominated the substrates even though they have not yet been unambiguously proved in our samples (see e . g ., Delamette , 1990 ; Soudry & Lewy , 1990) . Laminae were formed by trapping the clayey calcareous oozes on the microbial coatings followed by their subsequent phosphatization .

Our recent studies show the phosphogenic processes of this episode to be spatially discontinuous on a small scale in the phosphatic areas . Moreover , a close inspection of new , better preserved crust material (by comparison with that already briefly published ; see e . g ., Z ä ı ́ tt & Mikula ́ s ä , 1994 ; Z ä ı ́ tt & Nekvasilova ́ , 1990 , 1991a , b , 1992) revealed more intimate relations of encrusters and phosphatization (Figures 7b , c) .

In the phosphatic areas , supply of soft sediment was in general weak but rather varied in detail . It was most pronounced on the horizontal to slightly inclined substrates . No distinct synsedimentary erosion of individual laminae or their series has yet been found . Encrusting fauna (mainly agglutinated foraminifers ; see below) intercalating the series of laminae is , however , often more or less


destroyed in a very irregular distributional pattern (see Section 7) . The patchy distribution of laminated crusts could have resulted from both the patchy distribution of microbial coatings and the dif ferences in sediment input into the phosphatic areas (modifications influenced by the microtopography of the bottom) .

5 . Encrusting fauna across the C / T boundary

The examination of the distribution of colonized substrates in geological sections revealed two main phases of encrustation separated by the beginning of phosphate crust formation . From this point of view the substrates (clasts and rocky bottom) were divided by Z ä ı ́ tt (1992) into the groups A , B1 , and B2 (in spite of group C denoting the bioclasts) . We are now convinced that the dif ferentiation of only two groups of substrates is suf ficient , as follows :

Substrate group A (A-substrates , i . e ., A-clasts and A-bottom) . This includes the rock clasts of the first conglomerate phase and the rocky bottom buried by these conglomerates .

Substrate group B (B-substrates , i . e ., B-clasts and B-bottom) . This includes the rock clasts of the second conglomerate phase , the rocky bottom which they covered and its adjacent surfaces free of sediment . Dif ferences between the two conglomeratic phases have already been explained . More rarely , no second-phase conglomerates were observed locally and the phosphogenesis , together with the subsequent encrustation phase , are recorded mostly in relics in the uppermost parts of the first-phase conglomerates and on the adjacent free rocky bottom .

Although we have studied only the sections with relatively well preserved encrusters , the record of communities was sometimes so complicated that the determination of their principal features was very dif ficult . This is most obvious in the encrustations of A-substrates . Owing to probable but hardly demonstrable taxonomic dif ferences , and to unreliable species abundance data on the individual clasts , we abandoned the idea of community discrimination of the A-substrates . Because of the prevalence of the oysters we tentatively used the term ‘oyster community’ for all these occurrences (Z ä ı ́ tt & Nekvasilova ́ , 1993 , 1994 ; Z ä ı ́ tt & Mikula ́ s ä , 1994) . It is appreciated that this encompasses several types of communities . We , therefore , tentatively designate the whole association of encrusters of A-substrates as the A - association . We regard this designation as both logical and necessary for emphasizing the dif ferences from the younger , better defined association which can be regarded as the remains of only one community (see below) . We anticipate that further study of the A-association of encrusters might reveal the actual structure of the community .

The association of encrusters of B-substrates is far better defined by its taxonomic composition , the more or less clear association of its first appearance with phosphates , and its wide geographical extent over the basinal margins . This association is named the Atreta - Bdelloidina community (Z ä ı ́ tt & Nekvasilova ́ , 1993 , 1994 ; Z ä ı ́ tt & Mikula ́ s ä , 1994) .

Systematic determinations of encrusters were generally dif ficult . The dominant epibionts are , however , figured in this and / or in our previous papers (for summary , see Table 1) . Moreover , the studied specimens will be deposited in the National Museum in Prague , which will enable their precise systematic identification in the future .


Table 1 . List of encrusters of A-association (1) , and the Atreta – Bdelloidina community during phosphogenic (2a) and post-phosphogenic (2b) periods . Communities of imprecisely known settlement-timing are excluded (for their species , see Section 8) . Papers by Z ä ı ́ tt & Nekvasilova ́

with illustration s of species are noted .

Species Figured 1 2a 2b

Foraminifera Acruliammina longa (Tappan) 1990 1 1 1 A . nekvasilovae Hercogova ́ 2 1 2 1 Bdelloidina cribrosa (Reuss) 1988 , 1990 , 2 1 1

1991a , 1994 ; Figures 7a – c , 10a Bullopora sp . 1992 1 1 1

Porifera Corynella sp . 1988 , 1989 2 2 1 sponges indet . 1988 , 1989 2 2 1 Neuropora sp . 2 1 2 2

Anthozoa Octocoral base type 1 1993 ; 1 1 1

Figures 7a , 10b Octocoral base type2 1993 2 2 1 Octocoral base type 4 1993 2 2 1

Hexacoral base type 1 1991a 2 2 1 Hexacoral base type 2 Figure 5f 1 2 2

Vermes Glomerula solitaria Regenhardt 1991b 2 2 1 Pomatoceros sp . 1991b 2 2 1 Spiroserpula sp . 2 2 2 1 serpulids indet . 2 1 1 1

Bryozoa – Cyclostomata Berenicea sp . 2 1 2 2 Proboscina intermedia Nova ́ k 2 1 2 2 P . suessi Nova ́ k 2 1 2 2 P . sp . Figure 6a 2 2 1 ? Reptoclausa sp . Figure 5d 1 2 2 gen . et sp . indet . 1 Figure 5c 1 2 2 cyclostomes indet . Figure 5e 1 2 2

Bryozoa – Cheilostomata Alderina ? sp . 2 1 2 2 Onychocella michaudiana (d’Orbigny) 2 1 2 2 O . sp . 2 1 2 2 Tyloporella reussi Voigt 2 1 2 2 Stichomicropora sulcata (Reuss) 2 1 2 2 cheilostomes indet . Figure 5e 1 2 2

Brachiopoda Eothecidellina imperfecta (Nekvasilova ́ ) 2 1 2 2 Praelacazella lacazelliformis (Eliott) Figure 7d 2 2 1

Bivalvia Atreta sp . 1 1988 , 1989 , 2 1 1

1990 , 1991a , b , 1992 ; Figures 7c , 8b , c , e Atreta sp . 2 1988 , 1989 , 2 1 1

1991a ; Figures 8a , d Spondylus sp . 1988 , 1989 , 1 1 1

1991 , 1992 , 1994 ; Figure 10d Exogyra haliotoidea (Sowerby) Figures 5a , b 1 2 1 E . reticulata Reuss 2 1 2 2 E . sigmoidea Reuss 1988 , 1989 , 1 2 1

1994 Ostrea ? cf . hippopodium (Nilsson) 1988 , 1989 1 2 1 Ostrea ? operculata Reuss 2 1 2 2 Hyotissa semiplana (Sowerby) 2 1 2 2 pycnodonteine oysters indet . 1988 , 1989 ; 1 1 1

Figure 10c bivalve gen . et sp . indet . Figures 7d , 8f 1 2 1


6 . A-association of encrusters

6 . 1 . Species composition The A-substrates have yielded 29 species of encrusters , among which the oysters and bryozoans dominate both in species (Table 1) and individuals . Most local are the thecideans . Sponges are very rare and , although hardly determinable , they seem to be taxonomically dif ferent from those in the Atreta – Bdelloidina community (Table 1) .

6 . 2 . Distribution on substrates Colonized clasts were studied mainly at the Velim and Radim localities . The encrusted rocky bottom was accessible only at Velim . The relative amount of colonized clasts varies widely within the conglomerate accumulations (Table 2) . Encrusted clasts are distributed irregularly through the accumulations both laterally and vertically . They never form any horizons . Epibionts are often found on several clast faces , but one-sided colonization is common . At Velim , these latter clasts are oriented chaotically in the accumulation .

The exposure of some clasts (especially the boulders at Velim) to encrustation was frequently rather long-lasting so that several spatfalls could colonize the same clast faces . However , the epibionts only rarely reached large dimensions . The pebbles show lower density of epibionts (mostly one or two specimens) , which probably follows from the particularities of this substrate . These clasts were either not attractive for epibionts owing to both their small size and the frequent disturbance of their position , or they simply could not be colonized because they were buried between large clasts . The boulders , being doubtless more stable in their position , often bear richer growths of larger oyster specimens . In this case , the reduced disturbance resulted in a slightly more diverse community (Wilson , 1985 ; Bromley et al . , 1990) . We have not observed any signs of zonation on these boulders (cf . Surlyk & Christensen , 1974) .

Table 2 . Relative abundance of conglomerate clasts bearing at least one epibiont . Communities : 1 5 A-association , 2 5 Atreta – Bdelloidina community in phosphogenic (2a) and post-phosphogenic (2b) development . Samples : 1 for location of samples , see Figure 4 ; 2 locations according to Z ä ı ́ tt & Nekvasilova ́ (1990) ; 3 clasts come from a conglomerate horizon about 0 . 7 m thick (Hradecka ́ et al . , 1994) ; 4 for location of sample 1 , see Figure 3a ; samples 2 and 3 come from the more westerly-lying depressions in the eastern part of the quarry ; 5 all clasts come from the lower part of a conglomerate

horizon about 2m thick (see Z ä ı ́ tt , 1992) .

Locality No . of studied clasts Size in cm % Colonized Community

C ä ı ́ c ä ovice 1

sample 1 22 2 . 0 – 9 . 5 82 . 0 2b? sample 2 30 2 . 5 – 12 . 0 80 . 0 2b? sample 3 81 1 . 8 – 16 . 0 80 . 0 2b?

Kne ä z ä ı ́ vka 2

site J 87 2 . 0 – 22 . 0 13 . 0 1 site P 21 3 . 0 – 8 . 0 62 . 0 1

Odolena Voda 3 82 4 . 0 – 42 . 0 8 . 5 2b Velim 4

sample 1 183 2 . 5 – 25 . 0 19 . 7 1 sample 2 38 1 . 3 – 18 . 5 0 1 sample 3 53 2 . 5 – 12 . 0 43 . 0 2a , 2b

Radim 5 60 3 . 0 – 15 . 0 5 . 0 1


Although several background species are present everywhere (oysters) , some taxa were found on only a very limited numbers of clasts . Thecideans were observed on just two and the same was the case for the massive cyclostomate bryozoan gen . et sp . indet . (Figure 5c) .

Very small areas of the A-bottom adjoining the first-phase conglomerates did not permit any detailed observations of encrusters . The best examples were studied at Velim (lower wall surface of Veronika depression ; Figure 2) . Here oysters were both solitary and in clusters , and Spondylus sp . formed a cluster of roughly radially oriented valves ( sensu Surlyk & Christensen , 1974 ; Z ä ı ́ tt & Nekvasilova ́ , 1994) .

6 . 3 . Taphonomy The great majority of encrusters of A-substrates died before their substrates were buried . Cemented bivalves and thecideans mostly lost their upper valves , and octocorals were disarticulated . Articulated upper valves were found only in several small specimens of Exogyra haliotoidea colonizing the clasts at Velim (Figure 5b) . A wide range of preservation was recognized in epibionts , both of clasts and rocky bottom , which indicates a rather prolonged time of encrustation .

We most frequently observed rounded and broken elevated margins of cemented oyster valves . In small specimens of oysters ( Exogyra haliotoidea , E . reticulata , E . sigmoidea ) we often found that even their flat and thin attachment parts had been destroyed . The same holds true for thin marginal parts of flat valves of pycnodonteine oysters . Major destruction , like the break of large parts of the oyster and Spondylus valves , are not so common (Figure 9) .

Bioerosion was found to be concentrated mostly in the more massive remains of larger specimens of oysters and Spondylus . External surfaces of cemented shells were probably bored during the life of specimens (internal parts are frequently fresh and not bored) . For small specimens we suppose that their shells (and especially the thin attachment parts) were either not so attractive for the borers or that the mortality of juvenile borers was too high (possibly owing to the small amount of carbonate substrate available for successful colonization) . Shell borers belong to the boring sponges and possibly to algae . One large Spondylus valve shows Entobia isp . on the internal surface of shell , hence post-mortem .

6 . 4 . The pre - phosphogenic environment Observed encrustation characteristics exhibit relatively poor colonization of substrates both in terms of numbers of species and individuals . High juvenile mortality suggests frequent disturbances of populations , mainly on smaller clasts . Rare but distinct taxonomic dif ferences between the epibionts of individual clasts might result both from the patchy distribution of the species of a single community , and from the mixing of dif ferent communities during the conglomerate formation . The environment was most likely to have been well oxygenated with normal marine salinity values during the encrustation .

Temporary accumulations of clastic material were probably af fected by episodically increased wave and current energy (storms) until the depth of the transgressing sea exceeded the storm wave base (see also Hous ä a , 1991) . Hence , the process of colonization of clasts may be described as a succession of local colonization and burial events via their transportation to the final repositories . Recolonization of clasts at their final resting places are highly probable but have not yet been proved . However , on the rocky bottom buried by first-phase


Figure 5 . A-association of encrusters on clasts of the first-phase conglomerates at Velim : (b) was found in the Veronika depression , see Figure 2 ; others come from the conglomerate body , a part of which is shown in Figure 3a . (a) Ostrea haliotoidea , various growth stages ; (b) the same species , one specimen with preserved upper valve (arrow) ; (c) three partly damaged specimens of probable cyclostome bryozoan gen . et sp . indet . 1 ; (d) bryozoan Reptoclausa sp ; (e) cyclostome and cheilostome bryozoans var . sp . indet . and young oysters ; (f ) hexacoral base type 2 . Scale bars : a , 10 mm ; b , 6 mm ; c , 5 mm ; d , 2 mm ; e , 3 mm ; f , 2 mm . a – c , e , f , photos by J . Z ä ı ́ tt ; d , photo by J . Broz ä ek .


Figure 6 . (a) a lydite clast from C ä ı ́ c ä ovice colonized by several specimens of the bryozoan Proboscina sp . (b) corroded lydite clast from Kne ä z ä ı ́ vka with attached foraminifer Acruliammina longa (arrows) . (c) Kne ä z ä ı ́ vka , rocky bottom part with quartz-druses in small cavities colonized by A . longa (arrows) . (d) A . longa attached to the lydite clast from C ä ı ́ c ä ovice . Scale bars : a , 4 mm ; b , 2 mm ; c , 4 mm ; d , 3 mm . Photos by J . Z ä ı ́ tt .

clastics , the A-association of encrusters occurred , albeit rarely . Very locally , pre-phosphogenic colonization of rocks by boring bivalves is recorded . Trunca- tion of their borings documents rather pronounced erosion , which possibly prevented the establishment of the A-association of encrusters .

Factors related to rapid sea-level rise caused the final arrangement of the clast repositories and preservation of the A-substrates with epibionts . Until the beginning of phosphogenesis , the bottom sections that have been studied were


exposed only to bioclastic and clay sedimentation which gradually filled the remaining open interstices in the coarse clastic accumulations . There are , however , dif ferences between the localities both in the abundance and taphonomic properties of matrix fauna that are not yet fully evaluated . Ecological features of this fauna that included many brachiopods , bryozoans , echinoderms and bivalves , demonstrate fully marine conditions on the respective bottom areas .

7 . Atreta - Bdelloidina community

7 . 1 . Species composition B-substrates have yielded 23 species of encrusters (Table 1) , among which the bivalves Atreta spp . 1 and 2 ( sensu Nekvasilova ́ & Z ä ı ́ tt , 1988 ; the possible relationship of A . sp . 2 to Dimyodon nilssoni (Hagenow) mentioned by Z ä ı ́ tt & Nekvasilova ́ , 1994 , calls for further detailed evaluation) , and the foraminifer Bdelloidina cribrosa (Reuss) are most conspicuous . Oysters and bryozoans are less common than in the A-association ; sponges are rare but significant because they are identical to those in basal horizons of the Bı ́ la ́ Hora Formation . The foraminifers Acruliammina longa and Bdelloidina cribrosa may form extensive local growths . Thecideans are also local but more frequent than in the A-association . Environmental parameters experienced profound changes after the formation of the first-phase conglomerates . We can clearly distinguish the phosphogenic and post-phosphogenic periods in the development of the sedimentary environment . The fates of B-substrates and their Atreta – Bdelloidina community during these stages are described below .

7 . 2 . Phosphogenic period

Distribution on substrates . Owing to the reworking of the phosphatized horizon , the encrusters of the phosphogenic period were mostly heavily damaged and some may have disappeared completely . Several examples were found only on clasts that were rapidly buried in the second-phase conglomerate bodies or protected from extensive transportation either because of their large dimensions or through sheltering in depressions . A clast from the latter environment (Velim locality , Va ́ clav depression , second phase conglomerates ; Figure 2) was found to be encrusted on all sides , but the richest and best preserved growths were concentrated on the face at present uppermost . The foraminifers Bdelloidina cribrosa and Acruliammina longa dominate this community , together forming dense pavements (Figures 7b , c) cemented to and covered by the thin phosphatic crusts . Rare valves of Atreta are preferentially oriented here (Section 9) , which shows an oblique and temporarily stable position of the colonized substrate .

Examples of well preserved rocky bottom colonization belonging to the phosphogenic period are relatively scarce , having been mostly destroyed by post-phosphogenic exposure . The remains of epibionts were found at Na ́ kle , Chrtnı ́ ky , Lı ́ beznice and Kne ä z ä ı ́ vka . They similarly yield mostly limited informa- tion , as do those found on pebbles and cobbles . An example of the community relics was described from the Na ́ kle locality (Z ä ı ́ tt & Nekvasilova ́ , 1991a) . Among the phosphatized remains of epibionts , the species Bdelloidina cribrosa prevails , showing extensive destruction both during breaks in phosphogenesis and post- phosphatically .


Figure 7 . Post-phosphogenic encrusters . (a) Velim , eastern part of quarry , gneiss cobble with attached octocoral bases type 1 (arrows) and several specimens of the foraminifer Bdelloidina cribrosa . All epibionts are slightly phosphatized and possibly settled during a break in phosphogenesis . (b , c) same locality , Va ́ clav depression in western part of quarry ; epibionts cemented to , and covered by , thin phosphatic crusts and films on a gneiss boulder colonized during several breaks in phosphogenesis : b , well preserved B . cribrosa and A . longa ; c , the same foraminiferal species and a specimen of Atreta sp . (arrow) on a place adjacent to that in Figure b ; note dif ferent modes of preservation . (d) Kutna ́ Hora-Sedlec , post-phosphogenic epibionts on the gneiss rocky bottom . The thecidean Praelacazella lacazelliformis (arrows) ; u , unknown bivalve gen et sp . indet . Scale bars : a , 5 mm ; b , 4 mm ; c , 4 mm ; d , 2 mm . a – c photos by J . Z ä ı ́ tt ; d , photo by J . Broz ä ek .

Taphonomy . The range of preservation is wide in the encrusters of the phosphogenic period . The octocorals (Z ä ı ́ tt & Nekvasilova ́ , 1993) and bivalves are , however , always disarticulated . All remains of epibionts are more or less phosphatized .


Figure 8 . Post-phosphogenic encrusters . (a , b) Velim , eastern part of quarry , gneiss clasts with Atreta sp . 2 (a) and Atreta sp . 1 (b) . (c) Kojetice , bored valve of Atreta sp . 1 on a lydite clast ; valve damaged . (d) Na ́ kle , diabase rocky bottom with Atreta sp . 2 ; note the preferred slope orientation of specimens . (e) Lı ́ beznice , a lydite rocky bottom with a grouping of Atreta sp . 1 in the slope orientation . (f ) Chrtnı ́ ky , a sponge surface with three specimens of bivalves gen . et sp . indet . Scale bars : a , 4 mm ; b , 3 mm ; c , 2 mm ; d , 5 mm ; e , 10 mm ; f , 2 mm . a – c , e , f , photos by J . Z ä ı ́ tt ; d , photo by J . Broz ä ek .

The substrates bear mostly the agglutinated and primarily calcitic types of epibionts . Hercogova ́ (1988) found that the test cements of unphosphatized Acruliammina and Bdelloidina were siliceous and calcareous , respectively . Our recent studies show that the tests of both taxa belonging to the phosphogenic


period may be equally phosphatized . However , more detailed investigations are still necessary .

Chambers of foraminiferal tests are mostly filled with a soft phosphatized sediment similar to that of thicker phosphate laminae on which the foraminifers are attached , and by which they may also be covered . Preservation of foraminifers of any individual substrate may vary widely (Figures 7b , c) . Some parts of the test branches are mechanically worn , and some are in a perfect , nearly fresh , state . The wear is manifested mostly on the upper surfaces of tests where the chamber walls may be uncovered . The otherwise well preserved tests are sometimes fragmented into short sections . Extensively damaged specimens are very frequent on some substrates , resembling the relics of massive rough phosphate crusts (this is best developed at the Na ́ kle and Lı ́ beznice localities ; Z ä ı ́ tt & Nekvasilova ́ , 1991a , 1992) . Comparative taphonomic data on similar types of agglutinated foraminifers preserved attached to their mobile and immobile substrates are , unfortunately , missing from the literature .

In the bivalve Atreta we frequently observed microborings , possibly of algae , phoronids (see e . g ., Bromley , 1970) or other borers that colonized the valves after death (the inner surfaces are equally bored) . Yet in some specimens we observed superficial post-bioerosional etching which exposed the boring traces in valves before the following step of phosphogenesis . It seems that boring and dissolution are generally responsible for some marked changes in shape and reductions in size of Atreta shells both during and after the phosphogenic period .

The phosphogenic environment . From the above data we can conclude that encrusting and phosphogenesis were very closely associated . There existed intervals of phosphogenesis omission during which the epibionts not only colonized the substrates but also subsequently could be damaged or destroyed . These intervals could have been repeated several times during phosphogenesis .

Thin lamination of phosphates and the absence of macrofossil remains were most probably related to significant oxygen depletion of bottom waters (dysoxia – anoxia , see Section 4) . Marked taxonomic impoverishment of the Atreta – Bdelloidina community shows that oxic conditions were not fully restored during the episodes of phosphogenesis omission . The foraminifers Acruliammina longa and Bdelloidina cribrosa were most accentuated in this environment , demonstrating their high ecologic tolerance (see also Section 8 . 1) . Colonization episodes were possibly characterized by stronger currents supplying the encrusters with necessary food . There was also a suf ficient amount of siliceous particles for the agglutination of foraminiferal tests . The duration of each non-phosphogenic episode also involving time necessary for destruction (including bioerosion and partial dissolution) of epibionts , may be approximated as several (5 – 6?) seasons (years?) . Rare valves of Atreta show mostly three to four increments , in which they do not dif fer from those of post-phosphogenic populations (Figure 8c) .

7 . 3 . Post - phosphogenic period

Distribution on substrates . Encrusted B-clasts belonging to this period were found at the Chrtnı ́ ky , Zbyslav , Kutna ́ Hora-Karlov , Velim , Nova ́ Ves , Kojetice , Lı ́ beznice , Kne ä z ä ı ́ vka , and Str ä edokluky localities . The colonized rocky bottom


(B-bottom) was studied at the Rabs ä tejnska ́ Lhota , Na ́ kle , Chrtnı ́ ky , Velim , Lı ́ beznice and Pazderna localities .

Reworking processes af fected the majority of pebbles and cobbles , but the largest boulders were able extraordinarily to maintain their stable positions from the phosphogenic period . The position history of the majority of boulders is , however , mostly shorter . Some boulders found at Chrtnı ́ ky (Z ä ı ́ tt & Nekvasilova ́ , 1991a) and at the Karlov locality (Nekvasilova ́ & Z ä ı ́ tt , 1988 ; Z ä ı ́ tt & Nekvasilova ́ , 1989) , appear to have retained their positions since the post-phosphogenic encrustation . The substrate at Karlov shows very extensive colonization by the Atreta – Bdelloidina community . Ten species are present , among which the bivalves Atreta spp . 1 and 2 , Spondylus sp ., and oysters Ostrea cf . hippopodium and Exogyra sigmoidea are most pronounced . Bdelloidina is rare , but sponges are abundant . While individuals of most species settled solitarily or in small groups (sometimes more generations in overgrowths) , the oyster Ostrea cf . hippopodium formed dense clusters in some parts of the subvertical boulder faces . Atreta and Spondylus are preferentially oriented (see Section 9) . Poorly developed epifaunal zonation suggested by Nekvasilova ́ & Z ä ı ́ tt (1988) was seen mainly in the concentration of sponges on the upper part of the boulder . The same concentration of Atreta is , however , merely a function of avoiding the vertical surfaces of the lower parts of boulder by this bivalve .

Smaller boulders , pebbles and cobbles were mostly post-phosphatically released from the phosphatized horizon and reworked . During this phase , in which the second-phase conglomerate bodies were formed , repeated clast colonization by epibionts occurred . A good example of this process was found at the Velim locality (sample 3 in which a relative abundance of clast colonization was also measured ; see Table 2) . Bdelloidina , Acruliammina and Atreta sp . 2 were most frequent while Spondylus , octocorals (Figure 7a) and oysters were more rare .

Good examples of post-phosphogenic encrustations of clasts are known from the Odolena Voda (Hradecka ́ et al . , 1994 ; here in Table 2) and Lı ́ beznice localities . At the latter , an approximately 0 . 5-m-high cone of clasts piled on the foot of the subvertical rocky bottom wall was buried by the clayey sediments of the Bı ́ la ́ Hora Formation and preserved (Z ä ı ́ tt & Nekvasilova ́ , 1992) . Exposed faces of clasts in the cone formed a surface colonized by Atreta in the preferred slope orientation (Section 9) .

Encrustation of the rocky bottom shows the same features as that of , for example , the Karlov boulder (see above) . At the Lı ́ beznice locality , several settlement phases were distinguished , providing evidence of rather long-term encrustation . It is most probable that the rocky bottom cropping out in the form of an approximatelly 3-m-high , wall-like elevation was colonized during the entire time prior to its burial in sediments . Atreta and Spondylus of all settlement phases are preferentially oriented (Figure 8e) . No where did colonizers prefer any single substrate . They settled both on the phosphate relics and the bare rock .

An interesting example of the Atreta – Bdelloidina community was recently described by Z ä ı ́ tt & Nekvasilova ́ (1994) from the quarry at Velim . Clear avoidance of smooth overhanging rocks by Atreta and Bdelloidina , and the presence of a Spondylus colony on the overhanging roof of a partly sheltered rock cavity were observed here . Spondylus was found attached in a specially modified preferred position (Section 9) . At least three generations of these bivalves settled on the rock , which lacked any phosphate relics .


Figure 9 . Kutna ́ Hora-Sedlec . Distribution of post-phosphogenic epibionts on a fragment of rocky bottom . Specimens of Figure 7d are not visible on this face . Black 5 Bdelloidina cribrosa ; b 5 Atreta sp . 2 ; p 5 fragments of pycnodonteine oysters ; t 5 thecidean brachiopod Praelacazella lacazelliformis ; the others 5 indeterminable young (small arches) and highly fragmented bivalves . Schematic .

Taphonomy . Bivalved and multi-elemental skeletons of post-phosphogenic encrusters are always completely disarticulated . However , the preservation of all encrusters is , in general , better than of those from the phosphogenic period , having been af fected by fewer processes .

On the Karlov boulder , the overall preservational state of encrusters was found to be rather good . The elevated margins of some oyster valves ( Ostrea cf . hippopodium , Exogyra sigmoidea ) and the surface sculpture of several Spondylus valves were sharply defined with only slight fragmentation and wear . These epibionts were probably the last to settle before the burial of their respective substrate parts . Many other bivalve specimens (e . g . , of Atreta spp . 1 . and 2 , and oysters) and sponges , were found here in varying condition , demonstrating a rather prolonged time of substrate encrustation and post-mortem exposure of epibionts . Bioerosion of bivalves was , however , only minor and caused by very small borers , probably algae and phoronids .

Some encrusters of redeposited boulders at the Chrtnı ́ ky locality are exceed- ingly well preserved . Some Bdelloidina specimens from these substrates are actually the best from all substrates at the localities studied (Figure 10a) . The Bdelloidina specimens , even the erect ends of branches , remain intact (Z ä ı ́ tt & Nekvasilova ́ , 1991a) . We suppose they were the latest encrusters and settled during or after the final phase of boulder transportation . Changes of boulder position during transportation are apparent from the distributional pattern of preferentially oriented Atreta (Section 9) .

Preservation of post-phosphogenic encrusters on the rocky bottoms also varies greatly . At Na ́ kle , the Bdelloidina specimens that settled last are very well preserved and approximate those of the Chrtnı ́ ky clasts . Isochronous bivalves (mainly Atreta ) , originally intact , were partly destroyed (dissolved?) in umbonal parts (Figure 8d) . Bioerosion of cemented bivalves is best developed at the localities of the Vltava-Beroun Development (e . g ., at Lı ́ beznice and Kojetice ; Figure 8c) . Borings are of the same type as those of the phosphogenic period age . It seems that microborers , although not precisely distinguishable , participated in


Figure 10 . Post-phosphogenic encrusters . (a) Bdelloidina cribrosa on a diabase boulder at Chrtnı ́ ky . (b) octocoral base type 1 on the Rabs ä tejnska ́ Lhota quartzite rocky bottom . (c , d) pycnodonteine oyster (c) and Spondylus sp . (d) on the rocky bottom gneiss at Velim . Scale bars : a , 4 mm ; b , 3 mm ; c , 7 mm ; d , 7 mm . a , b , photos by J . Broz ä ek ; c , d , photos by J . Z ä ı ́ tt .

the corrasion of many epibiont remains both in the A-association of encrusters and in the Atreta – Bdelloidina community (for corrasion , see Brett & Baird , 1986 ; Parsons & Brett , 1991) .

The post - phosphogenic environment . Post-phosphogenic increase in environmental energy resulted not only in nearly complete reworking of phosphatized superficial parts of the conglomerates but also locally in the varying destruction of the deeper-lying conglomeratic horizons . The fresh , most probably youngest , component of the taphocoenose found in the matrix of reworked conglomerates (abundant echinoderms) indicates the return of oxic conditions . It seems that this holds also for the overlying lowermost siltstone horizons of the Bı ́ la ́ Hora Formation even though there are locally developed grey , organic-carbon and coprolite-rich intercalations (e . g ., at Velim) or glauconite-phosphate-enriched sediments (e . g ., at Karlov) .

Encrusting species colonized the clastics until the final burial of individual substrates . The rate of sedimentation varied locally , resulting in varying residence


times of encrusted clasts on the sea floor and post-mortem ef fects on cemented epibionts . In the depression fillings , the Atreta – Bdelloidina community also colonized the relatively small bioclasts and phosphatic intraclasts found in the siltstones of the Bı ́ la ́ Hora Formation and the large sponges in sponge accumulations of the topmost horizons (Figure 7d) .

8 . Populations of uncertain settlement timing

8 . 1 . Foraminiferal associations As mentioned previously , the agglutinated foraminifers sometimes predominate in the encrusting communities or may even form monospecific growths . The occurrences of nearly monospecific encrustations ( Acruliammina longa ) may only be correlated approximately with either the A-association of encrusters or with the Atreta – Bdelloidina community . The geological position of the rocky bottom and overlying conglomerates at the Kne ä z ä ı ́ vka locality , both of which were colonized by Acruliammina , only possibly indicates that they could belong to the A-substrates (i . e ., to the pre-phosphogenic period ; Z ä ı ́ tt & Nekvasilova ́ , 1990) . The same age may be assumed for rarely colonized clasts of the Kojetice locality (Z ä ı ́ tt & Nekvasilova ́ , 1991b) . At the C ä ı ́ c ä ovice locality , the relative age of clast encrustation by A . longa and its rare faunal associates is still more puzzling because of the unclear record of phosphogenesis .

Distribution on substrates . The specimens of Acruliammina longa settled both solitarily and in clusters on the conglomerate clasts (Z ä ı ́ tt & Nekvasilova ́ , 1990 ; Figure 6d) where they often grew over each other . The clasts were colonized on all sides . Many bear dif fering growth stages of Acruliammina tests . There are frequent tests of juveniles (even smaller than 1 mm) scattered among , and on , the large specimens . At C ä ı ́ c ä ovice , the colonized clasts are more or less regularly dispersed within the conglomerate body . Their distribution at Kne ä z ä ı ́ vka is irregular (Table 2) . At this locality , some more or less abraded lydite clasts were found to have been slightly corroded chemically (Section 3 and Figure 6b) and subsequently re-abraded before encrustation . Except for A . longa , the clasts of Kne ä z ä ı ́ vka very rarely bear the species Acruliammina ? sp . and Exogyra sp . (Z ä ı ́ tt & Nekvasilova ́ , 1990) . At C ä ı ́ c ä ovice , the frequency and density of clast colonization by A . longa are higher than at Kne ä z ä ı ́ vka , and 14% of the clasts yielded yet other colonizers (unidentified serpulid worms , bryozoans Marginalia cf . ostiolata Reuss , Onychocella sp ., Proboscina sp ., Proboscina cf . simplicissima Nova ́ k , Berenicea sp ., and an oyster gen . et sp . indet . ) . These epibionts are mostly rare on individual clasts and A . longa always clearly predominates . Only the bryozoan Proboscina sp . may form more extensive monospecific growths (Figure 6a) .

At Kne ä z ä ı ́ vka , Acruliammina is also very rich on the rocky bottom (Hercogova ́ , 1988 ; Z ä ı ́ tt & Nekvasilova ́ , 1990 ; Figure 3b) , colonizing both its depressed and elevated surfaces regardless their dip angle . The foraminifers settled here also on the quartz crystal druses in the cavities of lydite rock cut by the abrasion of the sea (Z ä ı ́ tt & Nekvasilova ́ , 1990 ; Figure 6c) .

Taphonomy . The preservation of Acruliammina from the Kne ä z ä ı ́ vka , C ä ı ́ c ä ovice , and Kojetice localities is rather variable , even on the same clast or a small area of the rocky bottom . Careful preparation preventing secondary destruction has shown that some specimens are entire whereas others are damaged (Figure 6d) . In extreme cases the tests may be reduced to chainlets of basal parts of chambers .


There is no apparent system to the distribution of damage (e . g ., a concentration of similarly preserved specimens on some substrate parts) . No overgrown damaged specimens were found . The test chambers are completely or partially free of sedimentary infilling . Traces of other processes (e . g ., dissolution) were not found , which is probably related to the siliceous character of the test cement (Hercogova ́ , 1988) .

The environments . Preservation of fragile foraminiferal tests at Kne ä z ä ı ́ vka , Kojetice and C ä ı ́ c ä ovice clearly indicates the absence of rough abrasion . We suppose that colonization of individual clasts was very short-term , including some small overturning events that ensured colonization of the opposite faces of the clasts . However , a similar ef fect could have come about by settling in the sediment-free interstices of clastics . This possibility is suggested by frequent finds of successfully growing Acruliammina in narrow , relatively deep rock crevices .

We suggest that mass occurrences of otherwise ubiquitous A . longa reflect the opportunistic character of this species . It is apparent that under suitable conditions , it was the high reproduction rate and rapid growth of tests which enabled Acruliammina to dominate the community locally (e . g ., Levinton , 1970 ; Boucot , 1981 ; Lipps , 1983 ; Harris , 1993) . The dominance of this foraminifer and the minor representation of other faunal elements , some of which (mainly the bryozoan Proboscina sp . ) may also be opportunists , indicates that a physico- chemical control of the environment could be more emphasized here . The possibility of a negative ef fect by factors that were responsible for the pre- settlement corrosion of lydite substrates of epibionts (Section 3) seems , in particular , to be suggested .

Acruliammina , together with Bdelloidina , also successfully invaded the environments of the phosphogenesis breaks . The dominance of these foraminifers within these intervals may be explained by their tolerance of dysoxic conditions which smothered most other epibionts .

8 . 2 . Populations at Radim At this locality , the community with Atreta probably belongs to two periods . During the first period , only the rocky bottom was colonized , and encrusters were preserved owing to subsequent burial by the accumulation of conglomerates . Under these conditions , the community was protected from the phosphogenesis that later af fected only the upper surface of clastic accumulation . During the second , post-phosphogenic , period the topmost clasts were recolon- ized by the same community .

The clasts of the transported conglomerate bear encrusters of the A-association and contain the following species : Acruliammina sp ., Exogyra sigmoidea , Serpula sp ., Spiraserpula cf . spirographis (Goldfuss) , Spondylus sp ., pycnodonteine oyster gen . et sp . indet ., and the thin-walled bivalve gen . et sp . indet .

The Atreta – Bdelloidina community included Atreta spp . 1 and 2 , Bullopora sp ., Exogyra sigmoidea , Ostrea cf . hippopodium , Serpula sp ., Spondylus sp ., and the pycnodonteine oyster gen . et sp . indet .

9 . Preferred orientations of bivalves

Data on preferred orientation of Atreta and Spondylus cemented to the hard rock substrates have been discussed several times (e . g ., Z ä ı ́ tt & Nekvasilova ́ , 1988 ,


Figure 11 . Velim , post-phosphogenic encrusters on the subvertical gneiss rock wall of the Veronika depression (after photos taken by O . Nekvasilova ́ during quarrying in the 1960s ; location in Figure 2) . Black 5 Bdelloidina cribrosa ; a 5 Atreta sp . 1 ; p 5 pycnodonteine oysters ; w 5 fragment of worm ( Pomatoceros sp . ) tube , the other epibionts 5 Atreta sp . 2 . Arrow indicates dip direction of the sloping surface . Note the prevailing slope orientation of Atreta .

1991a , 1994) . Specimens of both taxa orientated themselves chaotically on horizontal surfaces but preferentially on oblique ones . Because the true causes of this orientation on slopes are not apparent , it is denoted here as the slope orientation (in accordance with Seilacher , 1960) . We found that if the angle of substrate dip exceeded about 10 o , the bivalves oriented their valves by their umbonal parts to the upper left quadrant (see Atreta in Figures 8d , e , 11) . This position originates as a response only to the strictly local and not to the general substrate dip . The slope orientation on inclined substrates becomes more uniform with increasing smoothness .

While Atreta was never found on overhanging substrates , Spondylus commonly colonized them in the uniform slope orientation (see above) . Under special conditions , this slope orientation may , however , be distinctly modified . Z ä ı ́ tt & Nekvasilova ́ (1994) found Spondylus umbos directed to the lower left quadrant or horizontally to the left as a probable result of hydrologically modified conditions in a small sheltered rock cavity .

In several cases pycnodonteine oysters were found oriented with their umbos up-slope (Z ä ı ́ tt , 1992) , but the material is not representative . As noted earlier , specimens of Ostrea cf . hippopodium have been found oriented by their umbos upwards or to the upper right on sloping surfaces of the Karlov boulder (Nekvasilova ́ & Z ä ı ́ tt , 1988 ; Z ä ı ́ tt & Nekvasilova ́ , 1989) and on the inclined rocky bottom at Radim (Z ä ı ́ tt , 1992) .

The presence of slope orientations , mainly of Atreta , is a very useful tool for recognizing substrate position during encrustation (Schmid , 1949 ; Seilacher , 1960 ; Fu ̈ rsich , 1979 ; Z ä ı ́ tt & Nekvasilova ́ , 1994) and eventual later changes in this position (e . g ., Z ä ı ́ tt & Nekvasilova ́ , 1991a) .


10 . Conclusion

The results of our study show that analysis of fauna encrusting hard-rock substrates may contribute significantly to knowledge of sedimentary environments (see Sections 6 . 4 , 7 . 2 , 7 . 3 , and 8 . 1) . Unlike the epibionts of smaller mobile clasts , those that settled on stable substrates (rocky bottom and rarely the large boulders) are the only components of the original faunal communities that reflect in situ environmental conditions . On the other hand , encrusters of mobile substrates (majority of clasts) are excellent for indicating the fate of clasts between their colonization and final burial . The nearshore conglomerates studied (Kan ä k Member) could only have been genetically dif ferentiated on the basis of their encrusters supplemented by the data on phosphates . Palaeobiologic features of epibionts (high estimated reproduction and growth rates of opportunists ; slope orientation of some bivalves as a type of preferred growth) enabled the recognition of otherwise hidden environmental fluctuations and , in mobile substrates , the degree of their disturbance .

Major changes in encrusting communities occurred during the episode of phosphogenesis . Nevertheless , immigration of new taxa was not strictly depen- dent on the phosphogenic conditions as they very successfully survived the ending of phosphogenesis . We suppose that only strong opportunistic features of new taxa enabled their establishment , even under the relatively extreme conditions of short breaks in phosphogenesis .

Knowledge of relationships between various palaeoenvironmental processes and events (phosphogenesis , organic carbon-enrichment , corrosion of substrates , reworking) as they are recorded in the rocky coast facies (see e . g ., Z ä ı ́ tt & Nekvasilova ́ , 1990 ; Z ä ı ́ tt et al . , in press) and reflected by encrusting communities is , for the present , rather unsatisfactory . However , recently initiated studies of these phenomena and their relationships to basinal development (Ulic ä ny ́ et al . 1991 , 1993) may significantly contribute to a better understanding of the problem and assist in the correlation of global Cenomanian – Turonian boundary events with their reflections in the Bohemian Cretaceous Basin .

Acknowledgements

The authors thank Markes E . Johnson from the Williams College , Massachusetts , and Annie V . Dhondt from the Institut Royal des Sciences Naturelles de Belgique , Brussel , for stimulating discussions . The identification of some species (mainly bryozoans) was discussed with E . Voigt , Hamburg . Our oyster determinations were kindly revised by B . Za ́ ruba of the National Museum , Prague . The work was financially supported by the Grant Agency of the Czech Academy of Sciences CR , grant project No . 31307 .

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Documents

Epibionts, their hard-rock substrates, and phosphogenesis during the Cenomanian–Turonian boundary interval (Bohemian Cretaceous Basin, Czech Republic)