•

8 Deep marine slide and channel deposits from the Jurassic-Cretaceous Fossil Bluff Group, Alexander Island, Antarctica · D.I.M. Macdonald\ P.J. Butterworth2 andJ.A. Crame1

ßritish Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 OET, UK

'Current address: Cambridge Arctic Shelf Programme, West Building, Gravel Hili, Huntingdon Road, Garnbridge CB3 ODJ, UK 2Current address: P.T. Robenson Utama Indonesia, Sirnon Petroleum Technology, Cilandak Industrial Estate, Jakarta Selatan, Indonesia

Setting The Fossil Bluff Group (FBG: ?Bathonian-Aibian) is the fill of a forearc basin which lay between the Antarctic Peninsula volcanic arc and an accretionary complex (LeMay Group) to the west (Figs 8.1 and 8.2). Volcanism in the arc, and accretion and deformation in the LeMay Group, were contemporaneous with Sedimentation in the forearc basin. The basin was inverted during Late Cretaceous or Tertiary times during an episode of dextral transpressive deformation (Storey and Nell 1988). Tertiary igneous rocks in centrat and northern Alexander Island reflect arc migration, followed by Neogene extension.

Most of the FBG is made up of five formations , with an aggregate thickness of 7. 2 km (Table 8.1; Butterworth et al. 1988; Moncrieff and Kelly 1993). These range in age from Kimmeridgian to Albian, are entirely arc-derived (Butterworth 1991) and form a broad shallowing-upward succession from deep marine to terrestrial (Butterworth and Macdonald 1991). This paper deals with !arge slide deposits (Ablation Point Formation) and submarine channel deposits (part of the Himalia Ridge Formation), which are particularly weil exposed. Both formations have a consistent west or WNW palaeoslope.

Methodology The FBG forms cliffs up to 600 m high and access is possible only in very few places. Most of the !arge cliffs have been photographed from the air using a hand-held 35 mm camera from a De Ha viiland Canada

Twin Otter. Flying distance varied due to operating conditions, but was usually c. 500-1000m at a speed of 80-90 knots. There was at least 60% overlap between frames which have been made into panoramic montages. Most of the oblique air photo montages are curved; this is due to changes in heading, or to unavoidable changes in aircraft attitude. Interpretive line drawings of all panoramas have been made, attempting to smooth out the warst of the curvature.

The interpretation is aided by the restricted number of lithologies and their distinctive appearance. Mudstones are silty and weatherto black, smooth slopes. Sandstones of all grades weather white-cream, due to their high diagenetic laumontite content. Conglomerates tend to be brown-weathering and form distinctive rugged cliffs.

Ablation Point Formation The Ablation Point Formation is defined on the presence of !arge syn-sedimentary slide deposits and consists of rafts and blocks of turbidite sandstone, with variable amounts of chaotic, mudstone-rich matrix. The formation is one of the largest ancient slide deposits recognized. The Ablation Point Formation is particularly well-exposed at its type locality at Ablation Point and at Belemnite Point, 20 km to the north. At neither locality is the base exposed.

The Himalia Ridge Formation The HirnaHa Ridge Formation is a major unit of turbidite mudstone, sandstone and conglomerate. lt is 2.2 km thick at its type locality, which is just inland of Ablation Point, and contains four major conglomerate complexes with WNW palaeocurrents. The conglomerate complexes are from 15 km to more than 22 km wide and 100-250 m thick. They represent inner fan channels incised into inter-channel deposits (Butterworth 1985, 1991). The Hirnaha Ridge

Atlas of Deep Water Environments: Architectural style in turbidite systems. Edited by K.T. Pickering, R.N. Hiscott, N.H. Kenyon, F. Ricci Lucchi and R.D.A. Smith. Published in 1995 by Chapman & Hall, London. ISBN 0 412 56110 7.

Formation thins to both north and south from its type locality and the proportion of conglomerate declines. In the northern part of Planet Heights, which lie 25 km south of the type locality, the Himalia Ridge Formation is 1 km thick and comprises mainly mudstone. South from this is another channel complex which can be traced along strike for more than 17 km.

Discussion The scale of these features is comparable to those seen on seismic sections. The exposure of the conglomerate complexes of the Himalia Ridge Formation permits detailed analysis of changes of depocentre with time (e.g. Butterworth 1991). The slide deposits of the Ablation Point Formation are the largest ever recognized in outcrop (Macdonald et al. 1993). Similar­scale features have been recognized on the Labrador Slope (Hesse, Ch. 2).

Alexonder

lslond

60'W 45'W

, 60'S

A ntorctic Peninsula

70'S

Fig. 8.1. Location of Alexander Island on the west side of the Antarctic Peninsula.

f .. : .. :•:•~ Neogene alkoline volcanics

ffiTiill Tertiary calc-olkoline volcanics

c::J Tertiory colc·alkaline plutons

D fossil Bluff Group - Mesozoic fore-orc bosin

~ LeMoy Group - Mesozoic occretionory prism

C=:J LeMoy Group - occreted oceonic material

Fig. 8.2. Geological sketch map of Alexander Island. The position of the three localities figured in this paper are shown: AP: Ablation Point; BP: Belemnite Point; PH: Planet Heights.

Fig. 8.4. omact berween t \\'O !arge rafts of turbidite

sand~ton and mudsrone. 1ote figur for scale (arrow<:d .

Abla11on VoiiV1

0....._

Pholographic ( C)

I 3

Fig. 8.3. The blarion Poinr F rrnation at bhtion Point.

Photo-mosaic (a) and imerpr tiv sk tch (b) of the ·Iide deposir: of the

blarion Point Formation at blmion Point. The precise location of the

photographic panorama i: sho\\'n in (c). and the key u. ed in thi: and all

·ubsequent diagram . . 1 01e that the rop of blation Point (th prominem

peak on rhe left) is c. --om ab ve s a le,· I (ma ·I); rhe ha e of the uee·

vari s between r and 50 ma I. 1 this localir . rhe blarion Point Formation i at lea ·t -10m thick. The

lmver part c n ·i ·ts of a high proporrion of marrix wirb hl k. and ·heets

of int rhedded rurl idire and ·r ne and mud t ne. Th he r are up ro

1-oom I ng and - m thick and ar only d f rrn d at rheir margin. : where

adja nt he t. touch the conract is. harp (fig. . ). heet defin a · ri

of ·Iide plane · \Vith an ea. t-dipping irnbricare panern. ear the top of rhe

cliff at 3. - km. a prominent area of chaotic marrix ourl ines rhe nos of l\\"0

sheer:. The proponion of matrix decrea e · and the ·ize of th sheet ·

in r a •• d wn the inferr d tran pon direcrion to the WN ' . t 1- 1.- km

one ·Iide plan i · O\ ' rlain by bl k. of pebbly and tone up ro 10 m

long by - m thick. The upper p·•n of th I lati n Point Formation is

formed of ·everal :heet · (30(}-1 0 0 m long). The number of ·h et. i

uncenain, as comacts are harp and ub-parallel to hedding. trata in rhe e

heets downlap . harply onro the slid plane below. The upper h undary

c m only he recognized \Yhere undi ·rurb d • trata nlap local relief.

\ Skm

Conglomerate

Pebbly sandstone

Sandstone

Chaotic matri x

o-

1-

I 2

Fig. 8.5. The Ablation Point Formation at Belemnite Point.

Tertiary

Dyke

I 3

Photo-mosaic (a) and interpretive sketch (h) of the slide deposits of the Ablation Point Formation at Belemnite Point. The precise location of the photographic panorama is shown in (c). Note that the top of Belemnite Point (the prominent peak on the left) is 558 masl; the base of the cliff climbs from 40 to c. 100 masl. Points A (lowest exposed point of the Tertiary dyke) and B (the prominent fold nose) relate the aerial panorama to the detailed photograph shown in Fig. 8.6.

The Ablation Point Formation is at least 300m thick at Belemnite Point, where it is exposed in a 6 km lang cliff. The lowest exposed unit consists of east-dipping imbricate blocks, all formed of well-bedded sandstone turbidites. These blocks have been over-ridden by a mudstone-rich sheet which drapes over their crests, with repetition of a sandstone marker bed by low-angle slides in the troughs between blocks. The mudstone sheet is cut by a majorslideplane below a 100-160m thick thrust stack which

\ 4

\ s

\. 6km

has the morphology of an emergent imbricate fan. Individual horses are up to 800 m lang and dip east at c. 25°. They comprise interbedded sandstone and mudstone turbidites. Stratigraphy can be matched between them, implying that they all belonged to a single sheet. The upper boundary of this member is sharp, following the crests and troughs of the horses. When traced to the west this unit becomes more chaotically deformed. It is overlain by 0-60 m of chaotic matrix, thickest in the troughs between horses. The overlying unit of well-bedded turbidites may either downlap or onlap at the east end of the exposure; it is not certain whether this is a single !arge slide block or in situ sedimentation. This is capped by a 20 m thick bed of very coarse sandstone containing blocks of pebbly sandstone. The blocks are largest at the eastern end of the outcrop and protrude into overlying strata, which onlap onto the crests of the blocks.

Photographie

Fig. 8.6. Photographie panorama (taken from the ground) showing the eentral portion of the Belemnite Point cliff; points A and B relate to Fig. 8.5. The triangular peak at the far left is 378m above the view point.

Table 8.1. Summary of the stratigraphy of the main part of the FBG (after Butterworth et ai. 1988; Monerieff and Kelly, 1993)

Formation Age Thiekness (km) Lithology Depositional environment

Neptune Glacier Albian 2.4 Sandstone Shelf-fluvial

Pluto Glacier Aptian-Aibian 1.0 Sandstone and Shelf mudstone

Spartao Glacier Val.- ?Barrem. 1.0 Mudstone Slope- shelf

Himalia Ridge Berr.- Tith. 2.2 Mudstone and Submarinefan conglomerate

Ablation Point Kimm. >0.44 Mudstone and Submarine slide sandstone

Acknowledgements The authors are grateful to Dr C.W.M. Swithinbank for taking the air photographs used in these montages, to John Hall and the BAS Air Unit for their invaluable support, and to Dr G.]. Niehals for reading an early draft of the manuseript. We are indebted to Peter Buektrout for printing the photos and above all ro Roger Missing for the skilful way he made the montages.

Heferences Butterworth, P.J. 1985. Seclimentology of Ablation Valley,

Alexancler Island. British Antarctic Suruey Bulletin, 66, 73-82.

Butterworth, P.J. 1991. The roJe of eustasy in the development of a regional shallowing event in a tectonically active basin: Fossil Bluff Group Qurassic-Cretaceous), Alexander Island, Antarctica. In: Macdonald, D.I.M. (ed.) Sedimentation, Teetonicsand Eustasy: sea-level changes at active margins. Special

Publications of the International Association of Sedimentologists, 12, 307- 329.

Butterworth , P.]., Crame, J.A., Howlett, P.J. and Macdonald, D.I.M. 1988. Lithostratigraphy of Upper Jurassic- Lower Cretaceous strata of eastern Alexander Island, Antarctica. Cretaceous Research, 9, 249- 264.

Butterworth, J.P. and Macdonald, D.I.M. 1991. Basin shallowing from the Mesozoic Fossil Bluff Group of Alexander Island and its regional tectonic significance. In: Thomson, M.R.A., Crame, J.A. and Thomson, J.W. (eds) Geological Evolution of Antarctica. Cambridge University Press, Cambridge, pp. 449--453.

Macdonald, D.I.M., Moncrieff, A.C.M. and Butterworth, P.J. 1993. Giant slide deposits from a Mesozoic fore-arc basin. Alexander Island, Antarctica. Geology, 21, 1047- 1050.

Moncrieff, A.C.M. and Kelly, S.R.A. 1993. Lithostratigraphy of the uppermost Fossil Bluff Group (Early Cretaceous) of Alexander Island, Antarctica: history of an Albian regression. Cretaceous Research, 14, 1-15.

Storey, B.C. and Nell, P.A.R. 1988. RoJe of strike-slip faulting in the tectonic evolution of the Antarctic Peninsula. Journal oj the Geological Society, London, 145, 333-337.

Fig. 8. 7. The Himalia Ridge Formation ar PI-met Height ·. Photo-mosaic (a) and im rpr tiv ·ketch (b) of rh Himalia Ridge Formation in northernmos1 Planet Heights.

Hea\ line. mark major r ion surfa e ; bed drawn in bla k L thin congl m rate unit. The preci ·e location of the photographic panorama i sho n in (c).

The xp ur of rh Himalia Ridg Formation at Planer H ighr- rr tch s 1 km from nonh to . outh; it an be di ided int ~ ur portion . In th fir t 3 km [(a)-(c)] 1he · cti n i · alm ·t entirely mudstone. II is ·ut by a numl er of !arge- and ·mall-scale ero ion ·urfa , and th re L a marked Iack of b dding ·ominuit . T\vo of the ero. ion ·urfaces ar overlain I y unit of cha rically bedded mucl wne, which contain bio ks of bedded and. tone. These appear t ' be d bri · flow ·. Th onl conglom rate in thi. pan of the compl x is a rhin unit (0-lOm) underlying rhe more outherly debri flow ancl mantling rhe ro ion . urface.

(b) I 2

(a)

(b

Fi . 8.8. Photo-mosaic (a) and interpretati,·e sketch (b) of th expo:ure of the Himalia Ridge Fonnation bem·een __ ~ and -km in nonhern Phnet Heigh1s. Note the black bed continue. from Fig. . . Figure 8. - (c) sho"'·s the preci. e lo ·ation of Ihe photographi · panorama. ßem·een 3 and ')km l<al and (b)l. there i:-. a serie · of mall c nglomeratc-fill d ·hannels. Thcre are at lea. t nin of the · channels. mo~tly a few hundred merre. acro. :-, and tens of metres thi ·k. They seem lO

define two :eparme le,·el. and become more amalgamated sourhward.

S~erch~~ J? ,, (!~ I Pd lunar

Crcg f .n lia~~· \(q \) • ff' ~

10m Q

(c)

\ S km

/ 5

Planet He ights I 6

Fig. 8.10. lea. ur d tion thr ugh th imalia Ridge Formation in northern Planet Height .

Fig. 8.9. Photo-mo ai (a) and interpretati e k tch (b) f th c nglomerate

channel complexe of th Himalia Ridge Formation bet\ e n ~ and 12 km in northern Plan t H ight . Figure . (c) ho'ii . the pr i lo ation of th

photographic panorama. Fr m ~ to 12 km [(a) and (b)) congl merat i continu u , thickening

uth . ard to c. 250m by progr • i e in ertion f channel at th ba ·e of the unit ( .. feather dge at 5 9 and 10 km). There i · a I w r unit " here indi idual channel ar r co nizabl and a conlinu u upp r unit wh r the are les obvi u . In a m a ur d ction Fi . .10 = locality HR f Bun rworth el a/. 19 ) the lower unit m a ured om 10~ m in thickne and

th upper one m.

I 7

I 8

-

---I 9

' \2 km

\ \0

Fig. 8.11. Interpretive k tch of th w t fac of Lunar Crag, northern Planet

Height . Ora\ n from phOto raph of th am · ale a the r t of th panor ma. There i an expo ure gap b rween 12 and 13km th n a furth r 3 km of ominuou exp ur ( =locality HRS f Bunerworth el al. 19 her the c nglomerate hann 1-fill are in ol ed in a major Iid depo it. In th

c mre of th xpo ure a !arg bl ·k of medium-b dd d and tone turbidire dip E E at c. 35° cutting into the underlying mud ton . Th er t of th bl ck ha · be n planed ff and i draped by mud t n , " hile it clip lop i o rlain by m tr - cale and tone bl ck · and onlapped by mucl ton . Th r t of the bio k i fl nk d by channeL (up to 0 m thick) of the low r nglomerat unit, whi h are overlain in LUrn by a unit of lump d ·heet of thin-bedd d mud tone and turbidite . and rone. Indi idual h et are up to ~o m long. The t p f the ection i formecl of a 200m thick c nglomerat unit whi h can I tra ecl thr ugh di. ntinuou exp ·ure for a furth r km ·outh ard .