New dredge samples from the continental margin bordering Rockall Trough

Marine Geology, 16 (1974): M57--M62 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

Letter Section N e w dredge s a m p l e s f r o m t h e c o n t i n e n t a l m a r g i n b o r d e r i n g R o c k a l l T r o u g h

R.J. BAILEY and J.R. HAYNES U.C.N. W. Marine Science Laboratories, Menai Bridge, Anglesey (Great Britain) Geology Departmen t, University College of Wales, A berystwy th, Cardiganshire (Great Britain) (Accepted for publication March 19, 1974)

ABSTRACT Bailey, R.J. and Haynes, J.R., 1974. New dredge samples from the continental margin bordering Rockall Trough. Mar. Geol., 16: M57--M62. Rockall Trough may represent the oldest part of the Atlantic Ocean north of latitude 51 ° N. Stratigraphical evaluation of this hypothesis has been hindered by the lack of dateable samples of the Trough's sedimentary succession. A recent programme of dredging, aimed at sampling the sediment prism along the Trough's upper continental slope, yielded five hauls of in°situ carbonate rocks. Microfaunal analysis of these rocks suggests that the deposition of the marginal sediment prism spans the Late Cretaceous-Recent interval, tending to confirm that Rockall Trough was already in being in Late Cretaceous times.

INTRODUCTION The floor of Rockall Trough may constitute the oldest Atlantic Ocean c r u s t n o r t h o f l a t i t u d e 5 1 ° N (Vine, 1 9 6 6 ; Bailey et al., 1 9 7 1 , 1 9 7 4 ; L a u g h t o n , 1 9 7 2 ) . H o w e v e r , t h e e v a l u a t i o n o f the a n t i q u i t y o f t h e T r o u g h has b e e n h i n d e r e d b y t h e lack o f s t r a t i g r a p h i c a l data, n o t a b l y b y t h e lack o f d a t e a b l e in-situ samples. T h e m o s t readily accessible successions are t h o s e c o m p r i s i n g t h e m a r g i n a l s e d i m e n t p r i s m e x p o s e d a l o n g t h e u p p e r c o n t i n e n t a l slope b o r d e r i n g t h e T r o u g h . H e r e seismic r e f l e c t i o n profiles p r o v i d e a generalised s t r a t i g r a p h y e m b r a c i n g t w o series o f s e d i m e n t s w i t h an i n t e r v e n i n g u n c o n f o r m i t y (Bailey et al., 1 9 7 4 ) . A r e c e n t p r o g r a m m e o f d r e d g e s a m p l i n g was a i m e d at s a m p l i n g these t w o s e d i m e n t a r y series w i t h a view t o assigning a m i n i m u m s t r a t i g r a p h ical age to R o c k a l l T r o u g h . W h e r e v e r possible, t h e r e f o r e , d r e d g i n g r u n s w e r e m a d e b y r e f e r e n c e t o p r e l i m i n a r y seismic r e f l e c t i o n profiles.

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RESULTS Five dredge hauls (Figs.1 and 2) yielded material which seemed likely to be representative of the "target" strata. The results of the analysis of these samples are summarised in Table I. The oldest sample recovered, JM/1, comes from the flanks of a submarine canyon which deeply incises the marginal sediment prism and almost certainly exposes the lower of the t w o sedimentary series recognised. The washed residue of this sample shows a dominantly Upper Cretaceous (Senonian) microfauna with what are regarded as Quaternary contaminants.

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Fig.1. The continental margin bordering Rockall Trough, showing the location of the preliminary seismic reflection profiles and the dredge stations. The numbered subdivisions of the profile lines represent half-hourly navigational fixes, and correspond to the horizontal scales of the profiles in Fig.2. Bathymetric contours in metres.

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3'0 Fig.2. The preliminary seismic reflection profiles, showing the location of the dredging lines with respect to the bathymetry and the stratigraphy of the marginal sediment prism. The uppermost of the two marginal sedimentary series recognised is lightly stippled. In 2D acoustic basement is shaded. Horizontal scales represent ship's time in hours. Vertical scales denote the reflection time in seconds.

Samples JM/9, J M / l l , PM/3 and JM/2 all appear to be carbonate sediments of Neogene or Quaternary age, and were dredged in the vicinity of the unconformity between the two marginal sedimentary series (Fig.2). The Neogene fauna of PM/3 includes Korobkovella grosserugosa (= Anomalina grosserugosa in part of authors) which has recently been redefined on the basis of Upper Eocene (Bavarian) type material (Hagn and Ohmert, 1972). Its full range is unknown; though Recent records of the species are probably in error. JM/9,

Position

54 ° 23'N, 11°21 'W

54 ° 03 'N, 12°02'W

53 °4 3' N 14°11'W

51 ° 51 'N 15° 02'W

51 ° 5 4 ' N 15 ° 06'W

Station

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1260--780

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Depth (m)

Fig.2D

Fig.2D

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Fig.2A

Stratigraphy

well indurated, pale-grey, porcellaneous micritic limestone

poorly indurated, brownish argillaceous limestone

careous buff-coloured calcarenite with ferruginous layers containing rolled forams

tough, creamy coloured, calcarenite

soft, creamy coloured bryozoanforam, calcarenite

Lithology

Nature and microfaunal content of the dredge samples

TABLE I

Globigerina bulloides, Globorotalia inflata, G. truncatulinoides, G. cf. tumida, Cibicides lobatulus, Pyrgo williamsoni with possible Spirillina sp., Globorotalia scitula, and Globobulimina sp.

Gavellinella sp., Globorotalites michelinianus, Globotruncana cf. ventricosa, Valvulinaria allomorphinoides, with Quaternary contaminants: Globigerina bulloides, Globorotalia inflata, G. scitula

similar to JM/9,eroded Asterocyclina sp. again present

"Orbitoids" -- apparently eroded Asterocyclina sp., Sigmoidella sp., rotalids, and globigerines

Korobkovella grosserugosa, Rotalia sp., Lenticulina sp., Cibicides mexicanus, Globigerinoides trilobus, Globorotalia inflata, Globigerina ?euapertura, plus Quaternary contaminants

Microfauna

o

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and the similar J M / l l , are sediments of Neogene or Quaternary data; but both contain eroded "Orbitoids" which, on sectioning, show a eulepidine juvenarium with rayed equatorial layer. These appear to belong to the MiddleUpper Eocene genus Asterocyclina (Discocyclinidae); and Dr. C.G. Adams of the British Museum (Natural History), who kindly examined the material, is inclined to refer it to A. cf. stellata (d'Anchiac). This would be the most northerly occurrence of the genus yet noted outside the Pacific; and the presence of this derived (shelf) fauna, which seems unlikely to be far-travelled, implies the erosion of Middle to Upper Eocene sediments in the vicinity of the sample site. Surprisingly, in view of its advanced state of induration, the micrite JM/2 contains a wholly Quaternary fauna. CONCLUSIONS

The dredging failed to achieve the precision necessary for direct correlation between the in-situ samples and the stratigraphical sequence established by seismic reflection profiling (Fig.2). In consequence, these first dated samples from the continental margin bordering Rockall Trough cannot be used to directly test the tentative Eocene--Recent and Late Cretaceous ages inferred for the two series of strata which constitute the marginal sediment prism (Bailey et al., 1974). Nonetheless, they do suggest that the deposition of the lower of the t w o series was in progress in Late Cretaceous times; and that the minimum age range of the sequence comprising the higher strata o f this series and the overlying upper sedimentary series is Neogene--Recent. Alternatively, assuming that the samples of Neogene age all represent the upper sedimentary series, the lower series acquires a minimum Late Cretaceous to Late Eocene age range, since JM/9 and J M / l l contain a derived Mid to Late Eocene fauna of local origin. Thus, although the precision of the sampling proved unsatisfactory, the samples recovered tend to confirm the Late Cretaceous to Recent age range assigned to the marginal sediment prism, and to support the inference that Rockall Trough is a feature of possible Mesozoic date. ACKNOWLEDGEMENTS

We give our warmest thanks to Captain Peter Maw, the Officers and Crew of the R R S " J o h n Murray" and to Captain T o m Donovan, the Officers and Crew of the RV "Prince Madog" for their unstinting cooperation in the dredging programme. R.J.B. acknowledges the award of an N.E.R.C. Research Grant in support of this work.

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REFERENCES Bailey, R.J., Buckley, J.S. and Clarke, R.H., 1971. A model for the early evolution of the Irish continental margin. Earth Planet. Sci. Lett., 13: 79--84. Bailey, R.J., Grzywacz, J.M. and Buckley, J.S., 1974. Seismic reflection profiles of the continental margin bordering Rockall Trough. J. Geol. Soc. (Lond.), 130: 55--69. Hagn, H. and Ohmert, W., 1972. R~vision de "Truncatulina" grosserugosa Giimbel et de "Truncatulina" sublobatula G~imbel (Foraminiferes) de l'Eocene des Pr~alpes Bavaroises. Rev. Micropal~ontol., 14: 131--144. Laughton, A.S., 1972. The southern Labrador Sea -- a key to the Mesozoic and early Tertiary evolution of the North Atlantic. In: A.S. Laughton and co-workers, Initial Reports of the Deep Sea Drilling Project, XII: 1155--1179. Vine, F.J., 1966. Spreading of the ocean floor: new evidence. Science 154: 1405--1415.