Facies from Palaeozoic reefs and bioaccumulations

Facies from Palaeozoic reefs and bioaccumulations

Memoires du Museum national d'Histoire naturelle Tome 195

edited by

Emmanuelle VENNIN o>, Markus ARETZ (2), Frederic BOULVAIN (3) & Axel MUNNECKE w

• U M R 5 5 6 1 CNRS Biogeosciences, Universite d e B o u r g o g n e 2, b o u l e v a r d Gabriel 21 0 0 0 D i j o n - France

2

Institut fur Geologie u n d Mineralogie

Universitat z u K b l n - Z u l p i c h e r strasse 4 9 a 50 6 7 4 Kbln - Germany

3

Petrologie sedimentaire

B 2 0 , Sart T i l m a n , U n i v e r s i t e d e L i e g e 4 0 0 0 Liege - B e l g i u m •» I n s t i t u t f u r P a l a e o n t o g i e Universitat z u Erlangen, Loewenichstrasse 2 8 91 0 5 4 Erlangen - G e r m a n y

Publications Scientifiques du Museum Paris

2007

CAMBRIAN

EARLY-MIDDLE CAMBRIAN (TOYON1AN) MICROBIAL-ARCHAEOCYATHAN-RADIOCYATHAN BIOHERMS, SOUTH AUSTRALIA LOCALITY. - Late Early Cambrian (Toyonian-equivalent) tropical rift-shelf, Flinders Ranges, Adelaide Rift, South Australia. Mounds k n o w n from vicinity of Ten Mile or M o u n t Billy Creek and Balcoracana Creek (Fig. 48). STRATIGRAPHY. - W i r r e a l p a Limestone, in medial (depositionally deepest) portion of this 140 m-thick transgressiveregressive unit. FACIES A N D MICROFACIES (Figs 50-57). - The formation was deposited i n low-energy, open marine lagoons on a broad shelf protected by ooid shoals at the seaward margin (Youngs 1977). Bioherms are within thin-bedded to nodular silty bioclast limestone, with flexure of peribiohermal beds due to draping of peribiohermal sediments against the b i o h e r m m a r g i n a n d their subsequent differential compaction. The largest studied m o u n d is 36 m long and 3 m thick (Fig. 49; Kruse 1991). It rests directly on a substrate of bioclast packstone-floatstone-rudstone,

1 m thick, i n c l u d i n g

fragments of chancelloriids, trilobites, h y o l i t h s , brachiopods, molluscs, echinoderms, radiocyaths and archaeocyaths and minor peloids and intraclasts (Fig. 50). Associated o n c o i d rudstone (Fig. 51) comprises centimetre-scale flat Girvanella-hch

intraclasts

and oncoids in a matrix rich i n Epiphyton clots and fragmentary bioclasts. Gin>anella occurs i n the oncoids as crusts, tussocks and m i n i c o l u m n s (Figs 52-53). The bioherm itself is a thrombolitic stromatolite framestone of the calcimicrobes Epiphyton and minor Renalcis and Giiyanella, but presumably dominantly of automicrite generated by non-calcified microbes. This framestone is colonised by archaeocyaths, radiocyaths, brachiopods, hyoliths and macroborers (Figs 54-55). Intercolumn matrix is micrite with minor hyolith and brachiopod debris.

F I G . 4 8 . O u t c r o p area o f W i r r e a l p a L i m e s t o n e (shaded). Flinders Ranges, S o u t h Australia. A = B a l c o r a c a n a C r e e k s e c t i o n . B = Ten M i l e o r M o u n t Billy C r e e k s e c t i o n ; f r o m K r u s e ( 1 9 9 1 ) .

53

PETER D.

KRUSE

5m

| thin-bedded to nodular silty bioclastic limestone

I ]

bioclastic packstone, floatstone, rudstone

I

oncoid rudstone

-\

§ ^ Girphanovella gondwana @

Archaeocyathus abacus

thrombolitic stromatolite framestone

F I G . 4 9 . S c h e m a t i c s e c t i o n t h r o u g h m i c r o b i a l - a r c h a e o c y a t h a n - r a d i o c y a t h a n b i o h e r m . Ten M i l e o r M o u n t Billy c r e e k s e c t i o n .

BIODIVERSITY A N D TAPHONOMY. - Low-diversity biota of calcimicrobes Girvanella,

Epiphyton and Renalcis; non-

calcified microbes; archaeocyaths Archaeocyathus abacus (Fig. 56) and very rare Ajacicyathus sp.; radiocyath Giiyhanovella gondwana (Fig. 57); other skeletal invertebrates; and macroborers (Kruse 1991). Archaeocyaths and radiocyaths are rarely or never preserved in growth position; they are generally toppled and immured by the thrombolitic stromatolite framestone. Macroborings penetrate stromatolites, calcimicrobes, bioclasts and lime m u d . DISCUSSION. - These bioherms represent the pronounced archaeocyathan decline toward the end of the Early Cambrian reef-building phase - the first widespread reef-building episode of the Phanerozoic.

REFERENCES KRUSE P. D. 1 9 9 1 . — C y a n o b a c t e r i a l - a r c h a e o c y a t h a n - r a d i o c y a t h a n b i o h e r m s i n t h e W i r r e a l p a L i m e s t o n e o f S o u t h A u s t r a l i a . Canadian Sciences

28: 601-615.

Journal

of Earth

Y O U N G S B. C. 1 9 7 7 . — T h e s e d i m e n t o l o g y o f t h e C a m b r i a n W i r r e a l p a a n d A r o o n a C r e e k L i m e s t o n e s . Geological tin 4 7 : 1-73.

Survey

of South

Australia,

Bulle-

F I G S 5 0 - 5 7 . B i o h e r m a l s u b s t r a t e , Ten M i l e o r M o u n t Billy C r e e k s e c t i o n : 5 0 , B i o c l a s t p a c k s t o n e - f l o a t s t o n e - r u d s t o n e ; s c a l e = 0 , 5 c m O n c o i d r u d s t o n e , s u b s t r a t e t o t h i c k e s t p a r t o f b i o h e r m ; scale = 0 , 5 c m . 5 2 , D e t a i l o f o n c o i d c o r t e x in Fig. 5 1 s h o w i n g Girvanella a n d o p e n n e t w o r k ( a b o v e ) ; s c a l e = 0 . 5 m m . 5 3 , D e t a i l o f o n c o i d c o r t e x in Fig. 5 1 s h o w i n g Girvanella m a l f a b r i c a n d b i o t a , Ten M i l e o r M o u n t Billy C r e e k s e c t i o n : 5 4 , Epiphyton

r a d i o c y a t h Girphanovella

abacus

m i m c o l u m n s ; scale = 2 5 0 u r n . B i o h e r -

thrombolitic stromatolite framestone. A r r o w h e a d denotes macro-

b o r i n g s ; s c a l e = 0 , 5 c m . 5 5 , T h r o m b o l i t i c s t r o m a t o l i t e f r a m e s t o n e e n c l o s i n g Archaeocyathus N a r r o w l y c o n i c a l a r c h a e o c y a t h Archaeocyathus

51,

crust (below)

a b a c u s a n d h y o l i t h c o n c h s ; scale = 0 , 5 c m . 5 6 ,

in l o n g i t u d i n a l s e c t i o n . A r r o w h e a d d e n o t e s m a c r o b o n n g ; s c a l e = 0 , 5 c m

g o n d w a n a in t r a n s v e r s e ( a b o v e ) a n d l o n g i t u d i n a l s e c t i o n ( b e l o w ) ; scale = 0 , 5 c m .

5 7 , Conical