Earth Sciences & Resources Institute. Lithofacies, biofacies, and ichnoassemblage evolution of a shallow submarine volcaniclastic fan-shelf depositional system ...
Journal of South American Earth Sciences, Vol. 4, No. 3, pp. 239-260, 1991 Printed in Great Britain
0895-9811/91 $3.00 + 0.00 © 1991 Pergamon Press plc & Earth Sciences & Resources Institute
Lithofacies, biofacies, and ichnoassemblage evolution of a shallow submarine volcaniclastic fan-shelf depositional system (Upper Cretaceous, James Ross Island, Antarctica) R. A. SCASSO1, E. B. OLIVERO 1, a n d L. A. BUATOIS2 1Centro de Investigaciones en Recursos Geol6gicos;R. de Velasco 847, (1414) Buenos Aires, Argentina; 2CONICET, Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucum~n, C.C. 1 (4000), San Miguel de Tucuman, Argentina (Received December 1990; Accepted May 1991) Abstract--The Upper Cretaceous (Santenian-Campaniarglower Maastrichtian) Santa Marta Formation on James Ross Island, Antarctica, represents voicaniclastie shallow marine fan and shelf sedimentation adjacent to an active volcanic arc. A combined analysis of sedimentologic, paleoecologic, and ichnologic data allows for the recognition in this unit of six lithofacies associations, eight hiofacies, and five trace fossils assemblages. Lithofacies are dominated by fine, massive, tuffaceous rocks; graded, turbidite-like tuffaceous sandstones; carbonaceous mudstones; resedimented conglomerates; coquinas; sandstones; silty sandstones; and minor stromatelite beds. Biofaeies are defined by different composition and relative abundance of elements of the benthic fauna, mainly bivalves, gastropods, and serpulids, with minor elements represented by scaphopods, corals, brachiopods, and echinoids. Trace fossil assemblages include the most common elements of the Skolithos and Cruz/ann ichnofacies. A striking result of the analysis is that lithofacies, biofacies, and trace fossil assemblages form distinct, non-repetitive, vertically successive horizons, with their distribution boundaries roughly coincident. On this basis, seven major facies groups, showing a distinct combination of lithofacies, biofacies, and trace fossils, are distinguished in the Santa Marta Formation. These non-repetitive, vertically stacked facies groups reveal a one-way evolution of the depositional system during a transgressive-regressive cycle, with a new transgression atthe top of the unit. The lower facies groups represent shallow marine settings with a very high rate of volcaniclastie sedimentation within subsiding basin. Shallow, volcanielastic fan systems were probably formed at the base of delta slope and grew rapidly as a consequence of high sedimentary supply in equilibrium with basin subsidence. The upper facies groups probably represent sedimentation within the marine part of the envisaged deltaic system on a more stable shelf with diminished voleaniclastic sedimentary input. R e s u m e n - - s e e p. 257.
INTRODUCTION MARINE CRETACEOUS d e p o s i t s on n o r t h w e s t e r n J a m e s Ross I s l a n d f o r m a thick, continuous sequence divided into two m a j o r s t r a t i g r a p h i c units. T h e oldest d e p o s i t s c o m p r i s e the G u s t a v G r o u p of B a r r e m i a n S a n t o n i a n a g e ( I n e s o n et aI., 1986; M e d i n a et al., 1982; O l i v e r o a n d P a l a m a r c z u c k , 1987). The y o u n g e s t C r e t a c e o u s deposits, m a i n l y of S a n t o n i a n C a m p a n i a n age, a r e i n c l u d e d in t h e S a n t a M a r t a F o r m a t i o n (Olivero et aI., 1986; Olivero, 1984; 1988) which f o r m s the b a s a l u n i t of the M a r a m b i o G r o u p (Rinaldi, 1982). T h e s e two g r o u p s a r e t h o u g h t to r e p r e s e n t the filling of a b a c k - a r c b a s i n t h a t d e v e loped to the e a s t of a volcanic arc active d u r i n g L a t e J u r a s s i c - T e r t i a r y t i m e s a n d located a l o n g the A n t a r c t i c P e n i n s u l a (MacDonald et al., 1988; Medina et al., 1990). T h e G u s t a v G r o u p is considered to repres e n t d e e p - s e a e n v i r o n m e n t s - - e.g., slope a p r o n and s u b m a r i n e fans, followed by shelfal s e d i m e n t s within a f a n - d e l t a s e t t i n g (Ineson, 1989), w h e r e a s the u p p e r Address all correspondence and reprint requests to: Dr. R. A. Scasso (telephone: 1541( l ) 772-9729; telefax: [54] (1) 812-2039) 239
M a r a m b i o Group, on S e y m o u r Island, consists of s h e l f s e d i m e n t s (Macellari, 1988). The S a n t a M a r t a F o r m a t i o n , t h e focus o f o u r study, consists of a p p r o x i m a t e l y 1200 m e t e r s of m a r ine volcaniclastic and epiclastic s h e l f s e d i m e n t s . It is divided into t h r e e m e m b e r s : A l p h a , B e t a a n d G a m m a (Olivero et al., 1986; Fig. 1). T h e r e c o r d e d a m m o n i t e f a u n a indicate a S a n t o n i a n - e a r l y C a m p a n i a n age for the A l p h a M e m b e r , a n e a r l y C a m p a n i a n to e a r l i e s t late C a m p a n i a n age for the B e t a M e m b e r , and a late C a m p a n i a n - e a r l y M a a s t r i c h t i a n a g e for the G a m m a M e m b e r (Olivero, 1984, 1988, in prep.). Despite the fact t h a t p a r t of the lower two m e m b e r s , A l p h a a n d Beta, c o m p r i s e t u r b i d i t e - l i k e g r a d e d beds a n d coarse r e s e d i m e n t e d d e b r i s flow deposits, a v a i l able evidence points to shallow m a r i n e e n v i r o n m e n t s . D o m i n a n t t r a c e fossils belong to the S k o l i t h o s a n d C r u z i a n a ichnofacies, i n d i c a t i v e of c o a s t a l to s u b littoral settings, r e s p e c t i v e l y ( S c a s s o a n d B u a t o i s , 1987). C o m m o n p r e s e r v a t i o n of v e r t i c a l l y o r i e n t e d a m m o n i t e s , with the p l a n e of s y m m e t r y in a n u p r i g h t position, in s e v e r a l horizons of the A l p h a a n d B e t a m e m b e r s also s u g g e s t s shallow w a t e r s e t t i n g s (Olivero and Scasso, 1988; Olivero et al., 1989). C o a s t a l and shallow shelf environments for part of the
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