Eocene sed i men tary fa cies in a volcanogenic suc ces sion on King George Is land, South Shet land Is lands: a re cord of pre-ice sheet ter res trial
en vi ron ments in West Antarctica
Anna MOZER1, *
1 In sti tute of Geo log i cal Sci ences, Pol ish Acad emy of Sci ences, Twarda 51/55, 00-818 Warszawa, Po land
Mozer A. (2013) Eocene sed i men tary fa cies in a volcanogenic suc ces sion on King George Is land, South Shet land Is lands: a re cord of pre-ice sheet ter res trial en vi ron ments in West Antarctica. Geo log i cal Quar terly, 57 (3): 385–394, doi:
10.7306/gq.1100
About 34 Ma ago there was a rad i cal change of cli mate that led to the for ma tion of Ant arc tic ice sheet. King George Is land, lo - cated in the South Shet land Is lands vol ca nic arc (north ern Ant arc tic Pen in sula re gion), is one of a few places in West Antarctica which shows a geo log i cal re cord of sed i men tary en vi ron ments pre ced ing the de vel op ment of the ice sheet. The Eocene sed i men tary fa cies oc cur in the dom i nantly volcanogenic suc ces sion of King George Is land. They have been rec og - nized in the Arctowski Cove and Point Thomas for ma tions (Ezcurra In let Group) and in the Mount Wawel For ma tion (Point Hennequin Group) in Ad mi ralty Bay, and in the Mazurek Point For ma tion (Cho pin Ridge Group) and Lions Cove For ma tion (Polonia Gla cier group) in King George Bay. They re cord a cool ing trend in ter res trial en vi ron ments that be gan at ter mi na tion of the Early/Mid dle Eocene Cli ma tic Op ti mum, and was fol lowed by a sig nif i cant de te ri o ra tion of cli mate dur ing Late Eocene and ear li est Oligocene, di rectly pre ced ing gla cial con di tions in the north ern Ant arc tic Pen in sula re gion. The on go ing re - search con firms the ex is tence of three preglacial cli ma tic stages (PGS-1–PGS-3) dur ing Eocene–ear li est Oligocene, from hu mid, warm to mod er ate cli mate (PGS-1), through cool and dry cli mate (PGS-2), up to cold and hu mid con di tions (PGS-3).
Stud ies were car ried out on usu ally fine-grained volcanoclastic sed i ments, con tain ing Podocarpaceae–Arau ca - ria–Nothofagus plant fos sil as sem blies. Beds of re worked pyroclastic ma te rial al ter nate with lava flows or vol ca nic ag glom er - ates, as well as ex situ blocks of Eocene volcanogenic sed i ments on a mo raine. Cal cu lated geo chem i cal in di ces of weath er - ing (CIA, PIA and CIW) con firm mod er ate to high chem i cal weath er ing un der warm and hu mid cli mate con di tions at the be gin ning and de te ri o ra tion of con di tions in the end of Eocene.
Key words: Antarctica, King George Is land, Eocene, preglacial en vi ron ments, plant fos sils.
INTRODUCTION
Ac tual pat terns of cli mate cool ing dur ing the Late Eocene and the causes of per ma nent and sta ble gla ci ation are still far from be ing un der stood, but there are clues from the sed i men - tary re cord of Antarctica that help in ter pret this green house-ice - house tran si tion (Zachos et al., 2001, 2003). Geo log i cal ev i - dence from rocks and fos sils from the Ant arc tic Pen in sula re - gion pro vide sparse clues about cool ing Ce no zoic cli mates in the ter res trial realm (Askin and Spicer, 1995; Dutra, 2001;
Fran cis and Poole, 2002; Fran cis et al., 2008). The strati - graphic se quence ex posed on King George Is land (KGI), South Shet land Is lands, West Antarctica (Fig. 1A) spans the Late Cre - ta ceous to Early Mio cene. It is com posed of ter res trial lavas, pyroclastic and volcaniclastic sed i ments of a va ri ety of grain sizes, of ten with plant macrofossils, cut by youn ger in tru sions and dykes. The high est part of the lithostratigraphic col umn of KGI is com posed of ma rine and glaciomarine sed i ments. This
se quence pro vides im por tant de tails that re cord en vi ron men tal con di tions on land dur ing the Early and Mid dle Paleogene, di - rectly pre ced ing ice sheet de vel op ment on the Ant arc tic con ti - nent. The cli ma tic trend that the suc ces sion re veals can be cor - re lated with an oce anic re cord of Ant arc tic cli mate evo lu tion based on a com pi la tion of ox y gen iso tope anal y ses and ben thic foraminifera (Zachos et al., 2001, 2003, 2008).
This pa per anal y ses sed i men tary se quences bear ing plant macrofossils that crop out in sev eral sites lo cated on the south - ern part of KGI that are con sid ered Eocene in age. They are ev i - dence for the pres ence, di ver sity and ex tinc tion of ter res trial veg e ta tion in the north ern Ant arc tic Pen in sula re gion dur ing this time. Palaeo eco logi cal in ter pre ta tions, from fa cies and palaeo - botanical anal y ses sug gest far-reach ing cli mate change in clud - ing rec og ni tion of three preglacial palaeoclimatic stages.
EVOLUTION OF EOCENE CLIMATE IN ANTARCTIC PENINSULA AREA
The Ce no zoic cool ing trend fol lowed the Paleocene/Eo - cene Ther mal Max i mum (PETM) and in cluded a short-lived warm pe riod dur ing the Early/Mid dle Eocene Cli ma tic Op ti mum.
Fur ther sig nif i cant cool ing dur ing the Late Eocene and ear li est
* E-mail: amozer@twarda.pan.pl
Received: July 3, 2012; accepted: February 25, 2013; first published online: May 23, 2013
Oligocene, di rectly pre ceded gla cial con di tions in Antarctica (Zachos et al., 2001, 2003). Ce no zoic global cli mates were gen er ally con trolled by tectonism, that oc curred in south ern mid- to high lat i tudes, cul mi nat ing in the iso la tion of Antarctica, in cep tion of the Ant arc tic Cir cum po lar Cur rent, de vel op ment of the mod ern South ern Ocean, build-up of Ant arc tic ice and de te - ri o ra tion into ice house con di tions (Ken nett and Baker, 1990 and ref er ences therein; Fran cis et al., 2009; Sijp et al., 2009).
The geo log i cal re cord ex posed on land is con fined to a few out - crops lo cated in the north ern Ant arc tic Pen in sula re gion. Rich ter res trial flo ras pre served in Eocene sed i ments of this re gion pro vide sig nif i cant clues about Ce no zoic cli mate change. Sev - eral au thors have used var i ous bi otic com po nents to re con - struct depositional and cli ma tic con di tions dur ing the ac cu mu la - tion of the sed i men tary se quences: wood (Lucas and Lacey, 1981; Fran cis and Poole, 2002), leaves im print (Zastawniak, 1981, 1990, 1998; Zasta wniak et al., 1985; Birkenmajer and Zastawniak, 1986, 1989a, b; Li, 1992; Dutra, 2001, 2004; Hunt and Poole, 2003; Kellner et al., 2007; Fontes and Dutra, 2010), plant and fish as sem blage (Doktor et al., 1996), wood, tree-ring and palynomorph set to gether (Askin and Spicer, 1995; Duan and Cao, 1998; Fran cis et al., 2008; Panti et al., 2012). The ter - res trial flo ras con firmed a warm (i.e., frost-free) Eocene cli mate with a gen eral cool ing trend up the suc ces sion, cul mi nat ing in a sig nif i cant cool ing dur ing the Late Eocene.
METHODS
The ma te rial ana lysed in this pa per (vol ca nic and sed i men - tary rocks and plant macrofossils) were col lected from a thick vol ca nic and volcaniclastic suc ces sion ex posed within the Ad - mi ralty Bay and King George Bay ar eas, on King George Is land (Fig. 1B).
Petrographic ob ser va tions of vol ca nic and sed i men tary rocks were made us ing thin sec tions un der trans mit ted and re - flected light mi cros copy (TLM and RLM re spec tively) and scan -
ning elec tron mi cros copy (SEM). A JEOL JXA 840A scan ning elec tron mi cro scope op er at ing at a 15 kV ac cel er at ing volt age was used. Quan ti ta tive En ergy-Dispersive Spec tros copy (EDS) anal y ses of py rite were ob tained us ing the same mi cro scope, equipped with a Thermo Noran Van tage EDS sys tem. Op er at ing con di tions were a 15 kV ac cel er a tion volt age, 1 to 5 mm beam di - am e ter, and 100 s count ing time. De tec tion lim its of the ana lysed el e ments (S, Fe, Co, Ni, Cu, Zn) were better than 0.05 wt.%.
The X-ray dif frac tion anal y sis (XRD) was un der taken on pow dered whole-rock sam ples. The rock sam ples were crus - hed and pow dered to the 5–10 mm frac tion. X-ray dif frac tion pat - terns were re corded on a SIGMA 2070 diffractometer us ing a curved po si tion sen si tive de tec tor in the range 2–120° 2Q with CoKa ra di a tion and 20 hour anal y sis time. Diffractionel soft ware v. 03/93 was used to pro cess the data ob tained.
Geo chem i cal anal y sis was un der taken on the same rock sam ples. The ba sis for these anal y ses were In duc tively Cou - pled Plasma Emis sion Spec trom e try (ICP-ES) and In duc tively Cou pled Plasma Mass Spec trom e try (ICP-MS) re sults pro - vided by Acme Labs, Can ada. Re sults ob tained were sub ject to math e mat i cal pro cess ing which en abled de tail ing data col lec - tion of the mo lar con tent of el e ments and en rich ment fac tors, re lated to vol ca nic bed rock. These data were used to cal cu late weath er ing in di ces, such as the Chem i cal In dex of Al ter na tion (CIA), Chem i cal In dex of Weath er ing (CIW) and Plagioclase In - dex of Al ter na tion (PIA). The most pop u lar chem i cal in dex to quan tify the de gree of source-area weath er ing is CIA, pro posed by Nesbitt and Young (1982). This in dex is de fined as: CIA = [Al2O3/(Al2O3 + CaO*+ Na2O + K2O)] × 100, in mo lec u lar pro - por tions, where CaO* is the amount of CaO in cor po rated in sil i - cates. This in dex gives a prac ti cal ap proach to the mea sure - ment of feld spar trans for ma tion to clay min er als and the in flu - ence of weath er ing in creases with in creas ing val ues of CIA. An al ter na tive chem i cal in dex to the CIA was pro posed by Harnois (1988) and termed the chem i cal in dex of weath er ing (CIW). It elim i nates K2O from the equa tion, which is de fined as fol lows:
CIW = [Al2O3/(Al2O3 + CaO* + Na2O)] × 100, where the ox ides
KGI
A
0 10 20 km
1
2
3
Fig. 1A – lo ca tion of King George Is land in Antarctica; B – top o graphic map of King George Is land, with lo ca tion of de scribed out crops: 1 – Mount Wawel – Dragon Gla cier Mo raine, 2 – Cytadela,
3 – Lions Rump – Con glom er ate Bluff (based on Top o graphic Map, 2001)
cates. The third in dex used to de scribe the de gree of chem i cal weath er ing is PIA, de fined by Fedo et al. (1995) as:
PIA = [(Al2O3-K2O)/(Al2O3 + CaO* + Na2O-K2O)] × 100, where CaO* is the CaO re sid ing only in the sil i cate frac tion.
More over, all the col lected macrofossils flora, which pre - served at least ba sic an a tom i cal fea tures, have been the sub - ject of qual i ta tive iden ti fi ca tion. Plant re mains were col lected from lose blocks (Mount Wawel For ma tion) and in situ sites (Point Thomas For ma tion and Lions Cove For ma tion). Leaf iden ti fi ca tion was carried out based on avail able palaeo - botanical lit er a ture that con cern ing Ce no zoic plant re mains known from Antarctica, South Amer ica and Aus tra lia, and with as sis tance of Pro fes sor E. Zastawniak-Birkenmajer. All macro - fossils flora sam ples ana lysed in this pa per have rich pho to - graphic doc u men ta tion and are housed at the In sti tute of Geo - log i cal Sci ences, Pol ish Acad emy of Sci ences, in Warszawa.
RESULTS
Eocene vol ca nic and volcaniclastic suc ces sions, which were de pos ited in a fore-arc set ting, and are ex posed in the Ad - mi ralty Bay and King George Bay ar eas (Elliot, 1988), were ex - plored. These se quences are dom i nated by al ter nat ing lava flows and pyroclastic ma te rial of dif fer ent grain size (from blocks to very fine sand) and lev els of re worked sed i ments (clasts and tuffites) con tain ing plant de tri tus. The ma jor ity of in - for ma tion con cern ing preglacial cli mate and palaeo environ - mental con di tions are de rived from sed i ments de pos ited dur ing breaks in vol ca nic ac tiv ity, i.e. be tween ho ri zons of lavas.
ADMIRALTY BAY AREA – MOUNT WAWEL LOCALITY
In the Ad mi ralty Bay area, the Mount Wawel For ma tion (Point Hennequin Group) con tains a very rich as sem blage of plant macrofossils, which pro vides in for ma tion about prob a ble Mid dle Eocene en vi ron ments. The clastic fa cies of the Mount Wawel For ma tion is en riched in authigenic py rite (Fig. 2A), which oc curs mostly in the form of framboids as so ci ated with pyritized plant de bris (Mozer, 2010). The framboids are sphaeroidal to sub-sphaeroidal in shape and are com posed of densely packed microcrystals. Py ritic microcapsules as so ci - ated with py rite framboids show ing empty in te ri ors and ap er - tures open to wards the pore space (Fig. 2B). Plant re mains and the as so ci ated authigenic py rite oc cur in fine-grained rocks, mostly in grey and grey ish-brown mudstones, siltstones and claystones, which con tain small-sized vol ca nic bombs (up to a few centi metres in size) and scat tered finer volcanigenic clasts (Fig. 2C). Al most all loose blocks ob served at the mo raine re - veal strat i fi ca tion em pha sized by ver ti cal changes in grain size, as well as by the hor i zon tal align ment of plant re mains. Sed i - ments also con tain lev els of re worked pyroclastic ma te rial which builds sed i men tary struc tures like small-scale cross bed - ding (Fig. 2D). Sur faces with asym met ri cal wave rip ples were found on bed ding planes of the loose blocks on the mo raine also (Fig. 2E, F).
In most cases the clastic ma te rial is dom i nated by recrystal - lized brown and green vol ca nic glass, lithoclasts, some plagio - clases and rel a tively large pyroxene crys tals (Fig. 3). X-ray
xene, which sug gests only short trans port of clastic ma te rial.
Macroflora de pos its that had orig i nated from an in di vid ual lo cal ity named Dragon Gla cier Mo raine, were known and had been de scribed (Zastawniak, 1981; Zastawniak et al., 1985;
Fontes and Dutra, 2010). Leaf im prints are the pre dom i nant plant re mains vis i ble in tuffaceous rocks. The plant as sem blage is com posed of Equisetum (horse tail), ferns, sev eral Notho - fagus spe cies and other an gio sperms and Podocarpaceae (Fig. 4A–I). Nothofagus leaves are eas ily rec og niz able due to their rel a tively thick pri mary and sec ond ary veins, in spite of the leaf mar gins be ing nearly al ways dam aged. Some of the leaves re veal toothed mar gins. Co ni fers are rep re sented by nu mer ous frag ments of leaves. Frag ments of branches of Dacrydium vel Halocarpus (de scrip tion af ter Fontes and Dutra 2010, orig i nally de scribed as Dacrydium vel Dacrycarpus by Zastawniak, 1981) are usu ally long, with di a mond-shaped twigs (Fig. 4J). Only one poorly pre served leaf im print of Dicksonia sp. (Fig. 4K) was found (Cunha et al., 2008).
The chem i cal weath er ing in di ces cal cu lated and used in this study are pre sented in Ap pen dix 1*. The CIA, CIW and PIA are based on the ra tio of a group of mo bile ox ides to im mo bile ones (these in di ces as sume that Al is im mo bile) and they are in ter - preted as a mea sure of the ex tent of con ver sion of feld spars to clays. Al though CIA ra tios cal cu lated for Mount Wawel sed i - ments are typ i cal for low chem i cal weath er ing un der frosty con - di tions (CIA: 0.52–0.65) ac com pa ny ing low ma tu rity val ues sug gest that the CIA val ues are likely to re flect the high con tent of fresh, con tem po rary pyroclastic ma te rial sup ply. Ev i dences for pres ence of veg e ta tion in di cate warm, non-gla cial con di - tions. Syndepositional vol ca nism have sim i lar in flu ence on PIA (0.52–0.64) and CIW (0.54–0.65) in di ces, as it de creases those val ues. Anal y sis car ried out on vol ca nic bombs ex plain and con firm the rea son for low val ues of these in di ces (CIA:
0.42–0.44; PIA: 0.42–0.44; CIW: 0.43–0.45).
X-ray dif frac tion stud ies, as well as mi cro scope ex am i na tion and SEM anal y ses sug gest that laumontite, gismondine and other zeolites are pres ent as weath er ing prod ucts of plagio - clase in the up per part of the Mount Wawel For ma tion.
ADMIRALTY BAY AREA – CYTADELA LOCALITY
The Mid dle Eocene Pet ri fied For est Creek flora (Arctowski Cove For ma tion) and the Mid dle–Late Eocene Cytadela flora (Point Thomas For ma tion) both be long to the Ezcurra In let Group (Birkenmajer, 1980, 2003). They oc cur in a vol ca nic and volcaniclastic suc ces sion with plant de tri tus and pet ri fied wood (Stuchlik, 1981).
The Cytadela ex po sure rep re sents the up per part of the Point Thomas For ma tion. The ex po sure is com posed of two mem bers and dem on strate dis tinct in for mal units. The lower mem ber is com posed of a com plex of andesitic lavas, con glom - er ates and volcanoclastic sed i ments with abun dant fos sil plants, whereas the up per mem ber is com posed mainly of andesitic lavas and con glom er ates (Mozer, 2012; Fig. 5A).
The fos sils oc cur in volcaniclastic fine-grained sed i ments, tuffs and tuffites, usu ally of re worked ma te rial of vary ing thick - ness. Above the fine-grained sed i ments with beds and abun - dant leaves, there are lavas with pil low struc tures at the base, most likely formed as a re sult of lava cool ing in shal low lakes or ponds with stag nant wa ter (Fig. 5B). The Cytadela leaf flora con tains mostly small Nothofagus-type leaves, and pin nately
* Supplementary data associated with this article can be found, in the online version, at doi: 10.7306/gq.1100
veined leaves of other dicotyledonous types. Among the ferns, a Blechnum-re lated form is re corded. Poorly pre served frag - ments of Podocarpaceae are also pres ent (Birkenmajer and Zastawniak, 1989a; Mozer, 2012). This plant fos sil as sem blage is sim i lar to those de scribed by Askin (1992) and Fran cis et al.
(2009) from other lo cal i ties of plant beds on KGI. More over the fos sil plant as sem blage from Cytadela is sim i lar to the Dragon
Gla cier Mo raine flora; the an gio sperm leaves be ing dom i nated by Nothofagus and the co ni fers in clud ing Araucariaceae (Fig.
5C, D) in ad di tion to Podocarpaceae (Fig. 5E, F). Trunk trees in - cor po rated in lava and tuffs were found (Fig. 6). Im pres sions of leaves are of ten dam aged, es pe cially at the mar gins, but usu - ally show dis tinct pri mary and sec ond ary veins, which make them easy to rec og nize. More over, sed i men tary beds con tain - Fig. 2A – framboidal py rite in sed i men tary fa cies of the Mount Wawel For ma tion, TLM and RLM im ages of polyframboidal ag gre gates; B – py ritic struc tures as so ci ated with framboids in sed i men tary fa cies of the Mount Wawel For ma tion; py - ritic microcapsules show ing empty in te ri ors and ap er tures open to wards the pore space (ar row); SEM im age; C – typ i cal block of volcanoclastic sed i men tary fa cies from Dragon Gla cier Mo raine, Mount Wawel For ma tion, con tain ing ho ri zons of plant de tri tus; note high con tent of vol ca nic bombs; D – de tails of the ver ti cal ac cre tion strata: small-scale cross bed - ding and streaky strat i fi ca tion in block on Dragon Gla cier Mo raine; E – sed i men tary struc tures in the Dragon Gla cier Mo - raine, Mount Wawel For ma tion – sur face of mo raine block with asym met ri cal wave rip ples (the ham mer is 32 cm long); F – highly frag mented plant fos sil re mains ex hib it ing an uni di rec tional ori en ta tion and ho ri zon of rip ples on a block from the Dragon Gla cier Mo raine, Mount Wawel For ma tion
ing plant de tri tus ex pe ri ence di rect con tact with over ly ing lava flows, caus ing ther mal al ter ation and dam age (burn or char) of plant ma te rial mak ing their iden ti fi ca tion dif fi cult.
The CIA val ues ob tained from Cytadela from the lower mem ber of the volcaniclastic in ter ca la tions within volcanites with abun dant plant fos sils range from 0.75 to 0.85. The unweathered ba saltic bed rock (lava flows) was used as the ref - er ence (<0.60). PIA and CIW also re veal high val ues, con firm - ing that dur ing this preglacial stage (PGS-1) warm, non-sea - son ally wet con di tions pre vailed (PIA val ues >0.75; CIW val ues
>0.80). The up per mem ber of the Cytadela ex po sure does not con tain plant fos sils. It is com posed of con glom er ates and andesitic lavas with regoliths on their sur faces. Geo chem i cal weath er ing in di ces are fre quently used in char ac ter iz ing regoliths by quan ti fy ing the in ten sity of chem i cal al ter ation ver - sus stra tig ra phy (StrÝmsÝe and Paasche, 2011). Lack of vari a - tion in the in di ces among vol ca nic sub stra tum and sed i ments in the up per mem ber and a rel a tively nar row range of low val ues were re corded. Dur ing the sec ond preglacial stage (PGS-2), a cli ma tic change oc curs to wards de te ri o ra tion and rel a tively dry re gime (CIA val ues ap prox i mate to 0.60). The CIA, PIA, CIW val ues of regolith sam ples are show in Ap pen dix 1.
Pri mary min er als were iden ti fied in the thin sec tions by op ti - cal mi cros copy; sec ond ary min er als in the weath er ing prod ucts were iden ti fied by means of ran dom pow der X-ray dif frac tion.
X-ray dif frac tion anal y sis sug gests that clinoptilolite, heulandite and other zeolites are pres ent as weath er ing prod ucts in the up - per part of Point Thomas For ma tion.
The lower mem ber cor re sponds with an Early–Mid dle Eocene preglacial phase (PGS-1). It was de vel oped in a ter res - trial en vi ron ment with high vol ca nic ac tiv ity, and is char ac ter - ized by abun dant veg e ta tion sim i lar to mod ern Valdivian for ests with dom i na tion of Nothofagus and Podocarpaceae. The up per mem ber cor re sponds with the Mid dle–Late Eocene preglacial phase (PGS-2), de vel oped dur ing con tin ued vol ca nic ac tiv ity in the ter res trial en vi ron ment and rel a tively dry and cool cli mate con di tions.
KING GEORGE BAY AREA – LIONS RUMP LOCALITY
In the King George Bay area, the up per part of the Mazurek Point For ma tion (Cho pin Ridge Group) and Lions Cove For ma - tion (Polonia Gla cier Group) were examinated. These units are ex posed on Con glom er ate Bluff out crop. The Con glom er ate Bluff out crop is com posed pre dom i nantly of lapilli tuff and pyroclastic brec cia, pyroxene an de site and ashes. Tuffaceous
mudstone and sand stone con tain plant fos sils, as well as coal seams and coaly shales. Un for tu nately the ma jor ity of the flora is rep re sented by non-di ag nos tic leaf frag ments that pre vent even ap prox i mate iden ti fi ca tion. Nothofagus dom i nated among as sem blage of plant macrofossils. Most of the or ganic mat ter is dif fi cult to de ter mine and poorly pre served. It is usu ally found as plant de tri tus or coal beds and pet ri fied wood. Moun tain ous land scapes un der in ten sive wa ter ero sion (prob a bly caused by the low er ing of the ero sive base in the Ant arc tic Pen in sula re - gion) formed river can yons filled with con glom er ate de pos its.
An es sen tial part of the Con glom er ate Bluff pro file rep re sents a large val ley eroded in sand stones and mudstones. Con glom er - ates fill ing the val ley are rep re sented by lo cal clasts de pos ited un der ter res trial con di tions by rivers and streams (Fig. 7A) and rep re sent the third preglacial stage (PGS-3). The sed i men tary suc ces sion un der ly ing the con glom er ate is com posed mostly of brown tuff-mudrock, green tuff and brec cias, and mudrock with coal and wood. These sed i ments, rich in scat tered or ganic mat - ter and coal are ther mally al tered on con tact with the andesitic lava flow (Fig. 7B). The bot tom of the lava flow shows a pil low struc ture, which sug gests cool ing of lava in a shal low lake. Thin sec tions of mudrock as so ci ated with the coal, re vealed crushed plagioclase crys tals with in clu sions, sur rounded by opaque min - er als and or ganic mat ter (Fig. 7C).
Weath er ing in di ces cal cu lated for the Lions Rump pro file, CIA, CIW and PIA, show rather di verse val ues. High val ues of in di ces (> 0.70) in the lower part of sec tion sug gest rel a tively ad vanced chem i cal weath er ing. Low and rather nar row ranges of val ues (around 0.50–0.60) in the up per part of pro file con firm the main te nance of vol ca nic ac tiv ity and the sup ply of pyro - clastic ma te rial and ashes, as well as dis tinct changes in cli - mate, with de te ri o ra tion of cli mate con di tions.
The Lions Cove For ma tion in cludes the re cord of sed i ments di rectly prior to the on set of gla ci ation. Con glom er ate de pos its are cov ered di rectly by mas sive and/or bed ded diamictite con - tain ing lo cal and ex otic clasts re lated to a new type of ac tiv ity in the area. Gla cial ac tiv ity re veals ice rafted de pos its and diamictites in the Krakowiak Gla cial Mem ber of Polonez Cove For ma tion in King George Bay (Birkenmajer, 2001). Dur ing this time the ice sheet reached the edges of the con ti nent and was grounded on a shal low ma rine shelf, pro duc ing ice bergs, which sup plied ex otic ma te rial in the form of boul ders, gravel and sand.
Ex otic clasts, mostly non-mafic, seems to have been de liv ered by the rocks from the Ellsworth and Transantarctic Moun tains.
The KGM (Krakowiak Gla cier Mem ber) con sists largely of diamictite and grav elly sand stone, with mi nor interbeds and lenses of gran ule to peb ble con glom er ate and me dium sand - Fig. 3. Thin sec tion of plant-bear ing tuffite from Mount Wawel For ma tion, Dragon Gla cier Mo raine
(A – TLM im age; B – RLM im age, nor mal light)
Note con tent of lithoclasts, al tered vol ca nic glass, opaque min er als, pyroxene crys tals and al tered plagioclases
stone. Peb ble to boul der-sized clasts are most com monly subangular or subrounded (Fig. 7D), ma rine fos sils lo cally are sparse but widely dis trib uted (Troedson and Smellie, 2002).
There is gen eral agree ment among re search ers that age of the KGM should be con sid ered to be at least mid-Oligocene (Troedson and Smellie, 2002), al though the lat est study on Sr iso topes sug gest youn ger ages for this mem ber (K. Krajewski, pers. comm., 2011). In con nec tion with this Sr iso tope data the high est part of preglacial seems to be not youn ger than 32 ± 0.5 Ma (Early Oligocene).
DISCUSSION
A Mid dle Eocene to Late Oligocene age for the Point Hennequin Group has been sug gested by many au thors (Pankhurst and Smellie, 1983; Smellie et al., 1984; Zastawniak et al., 1985; Birkenmajer, 1989a, 2003; Dutra, 2004; Fontes and Dutra, 2010), how ever, a new se ries of K-Ar datings of the Mount Wawel For ma tion points to its Eocene age (Z. Pécskay, pers. comm., 2011). A Mid dle Eocene age for the Mount Wawel Fig. 4. Se lec tion of plant fos sils from Dragon Gla cier Mo raine, Mount Wawel For ma tion
A–H – va ri ety of Nothofagus leaf im pres sions: gen eral view of the leaf and ve na tion pat tern; I – im pres sions of frag men tary fern frond; J – frag ments of twigs of Dacrydium vel Halocarpus (Podocarpaceae) on block sur face; K – poorly pre served
im print of Dicksonia sp.; scale bars in all pho tos are 1 cm
For ma tion was pro posed by Nawrocki et al. (2011) as a re sult of U-Pb and 40Ar-39Ar anal y ses. Tak ing into ac count that sim i lar plant beds were found in the Fildes For ma tion on Fildes Pen in - sula and dated to be Eocene in age (Poole et al., 2001; Fontes and Dutra, 2010), a sim i lar age of the Point Thomas For ma tion in Ezcurra In let is highly prob a ble (Mozer, 2012). A Mid dle Eocene age was sug gested for the Lions Cove For ma tion (Z.
Pécskay, pers. comm., 2011).
The re sults show that KGI ex pe ri enced a warm, wet, non-sea sonal cli mate dur ing the Paleogene cli ma tic op ti mum, which per sisted un til early Mid dle Eocene time. The lat est Late Eocene cli mate on KGI was cold and rel a tively dry, pos si bly with a sea sonal snow cover. There is no di rect ev i dence for gla - cial de po si tion on KGI dur ing the lat est Late Eocene (Up per Mem ber of Point Thomas For ma tion).
The clastic fa cies of the Mount Wawel and Point Thomas For ma tions re cord sed i men ta tion in lo cal de pres sions and coa - stal ar eas on a young mag matic is land arc, with re cur rent events of en hanced sup ply of plant de tri tus from a for est-cov ered moun - tain ous land scape. Mid dle Eocene age flo ras from King George Is land sug gest warm to cool tem per ate cli mates, gen er ally moist and prob a bly frost-free, al though in gen eral the cli mate trend was to wards cool ing. Af ter the Early Eocene Cli ma tic Op ti mum the cli mate was cold, but veg e ta tion was able to per sist, al though higher di ver sity as sem blages and thermo phytes had dis ap - peared. They were re placed by veg e ta tion that was dom i nated
by sev eral spe cies of the south ern beech, Nothofagus in Early and Mid dle Eocene for ests. Along with few ferns and some podocarp co ni fers, south ern beech trees prob a bly grew as shrubby veg e ta tion in most ar eas. Ge nus Dacry carpus (co ni fers be long ing to the Podocarp fam ily) oc curs in moun tain for ests up to 3600 m a.s.l. in south-east Asia, is lands of Mel a ne sia and New Zea land, and only one New Zea land spe cies Dacrycarpus dacrydioides grows in mixed swamp for ests (Zastawniak, 1981), whereas Nothofagus for ests grow to day usu ally in very hu mid hab i tats, mainly in tem per ate and tem per ate-cool cli mates (Zastawniak, 1981; Fontes and Dutra, 2010). The pres ence of leaves from Arau ca ria and Nothofagus trees in sed i ments of Point Thomas For ma tion sug gest that the palaeoenvironmental con di tions were not so ex treme. The Cyta dela fos sil plant as - sem blage in di cates a gen er ally warm cli mate dur ing the Early Eocene. Moist, cool tem per ate rain for ests were pres ent, sim i lar to mod ern low to mid-al ti tude Valdivian for ests in South ern Chile (Poole et al., 2001; Fran cis et al., 2009). As sem blages of Nothofagus-podocarpaceous veg e ta tion and other an gio sperms grow in tem per ate cli mate con di tions (Askin, 2000). The pres - ence of Nothofagus spe cies in sed i ments of the Lions Cove For - ma tion sug gest a rather cool ing of cli mate.
Sed i men tary struc tures like asym met ri cal wave rip ples, small- scale cross bed ding and strat i fi ca tion ob served in volcani - clastic suc ces sions ex posed in all out crops dis cussed here car - ried im por tant in for ma tion about pre dom i nant con di tions dur ing Fig. 5A – gen eral view of Up per Mem ber of Point Thomas For ma tion at Cytadela; ba salt to ba saltic an de site lavas with de vel oped regoliths sur face; wall is about 150 m high; B – con tact of andesitic lava flow with un der ly ing mudstone/siltstone con tain ing fos sil plants, Lower Mem ber of Point Thomas For ma tion at Cytadela; note pil low struc - ture in the bot tom of lava; C, D – frag ments of leaf im pres sion of Arau ca ria, note wide par al lel densely-ar ranged ve na tion (C – spec i men from Point Thomas Formation, D – spec i men from Mount Wawel For ma tion), scale bars are 1 cm; E, F – lan ceo late and elon gate podocarp leaves with par al lel mar gins from plant-bear ing ho ri zon of Lower Mem ber of Point Thomas For ma tion at Cytadela, scale bars are 0.5 cm
sed i men ta tion. These struc tures are the re sult of de po si tion in stag nant and flow ing wa ter en vi ron ments with re cur rent pulses of in creased sup ply of plant ma te rial. The fos sils from Cytadela ex po sure oc cur in sed i ments that rep re sent lac us trine – bog and marsh fa cies, in di cat ing a low en ergy en vi ron ment.
Among clastic fa cies of Mount Wawel For ma tion un com - mon form of py rite is pres ent. Tex tural and chem i cal fea tures of py rite framboids sug gest that their for ma tion in volved in ter me - di ate pre cip i tates of amor phous or poorly or dered iron monosulphides, fol lowed by crys tal li za tion of the min eral phase (Mozer, 2010). In ad di tion, the abun dance of authigenic py rite in clastic fa cies sug gests that there was a sup ply of ma rine or mixed ma rine-vol ca nic sulphates to the sed i men tary en vi ron - ments of the aris ing arc is lands.
Weath er ing in di ces of sed i men tary rocks were used to pro - vide in for ma tion about vol ca nic ac tiv ity and cli ma tic con di tions of the area. High av er age CIA, PIA and CIW val ues of Lower Mem - ber from Cytadela dem on strate the pre dom i nance of chem i cal weath er ing un der hu mid and warm con di tions. It seems to re flect in ten sive and ad vanced weath er ing pro cesses sug gest ing warm and ice-free con di tions dur ing Early to Mid dle Eocene. Lack of vari a tion in the in di ces among vol ca nic sub stra tum and sed i - ments in up per mem ber sug gests a mod i fi ca tion and de te ri o ra - tion of palaeoenvironmental con di tions dur ing Mid dle–Late Eo - cene. The rel a tively nar row range of low val ues points to a pre - dom i nance of phys i cal weath er ing un der pro gres sively colder con di tions. Dur ing the sec ond preglacial stage (PGS-2), a cli ma - tic change oc curs to wards a frost-prone and rel a tively dry re - gime. Low val ues of CIA from Wawel sed i ments re flect the high con tents of fresh, con tem po rary vol ca nic de bris/pyroclastic ma - te rial sup ply. Sed i ment ma tu rity was low, sug gest ing lit tle sed i - ment re cy cling and de po si tion rel a tively close to source. Flo ral ev i dence in di cates that this pe riod of warmth also co in cided with Fig. 6. Par tially pet ri fied and charred wood in cor po rated
in the lava flow, Lower Mem ber of Point Thomas For ma tion at Cytadela
Ham mer is 32 cm long
Fig. 7. De tails of Lions Rump – Con glom er ate Bluff ex po sure
A – val ley eroded in sand stone and mudstone, filled by con glom er ate; note the de gree of clasts round ness; B – con tact of andesitic lava flow with un der ly ing mudstone/siltstone con tain ing fos sil plants and coal; note pil low struc ture in the bot tom of lava;
C – thin sec tion of ba saltic tephras con tain ing plant de tri tus and coal seams; note crushed plagioclases crys tals with in clu sions;
TLM and RLM im age; D – ex otic ice-rafted clasts in coarse sed i ments of the Krakowiak Gla cier Mem ber, Polonez Cove For ma - tion; the ham mer is 32 cm long
val ues of weath er ing in di ces of the Up per Mem ber from Cyta - dela point at the de te ri o ra tion of cli mate con di tions and dom i na - tion of phys i cal weath er ing. Val ues of weath er ing in di ces from Lions Rump are di verse; be ing rather high in the lower part of the sec tion, point ing at rel a tive chem i cal weath er ing (low val ues are caused by sup ply of vol ca nic ma te rial), and are rather low in the up per part of the sec tion, point ing at the cool ing of the cli mate. By ear li est Oligocene times, there is no doubt that ice was pres ent on Antarctica with sed i ments in clud ing ex otic clasts in dic a tive of ice berg raft ing. The un ques tion able re cord of gla cial ac tiv ity is reg is tered in sed i men tary rocks in the Krakowiak Gla cier Mem - ber (Polonez Cove For ma tion) of at least mid-Oligo cene age.
Glacio-ma rine de pos its of the Polonez Cove For ma tion re cord ice sheet fluc tu a tion and ice-raft ing in a shal low ma rine en vi ron - ment. Ex otic clasts were trans ported from the cen tre of the Ant - arc tic con ti nent (Ellsworth Moun tains and Trans antarctic Moun - tains).
The Eocene of KGI, like the en tire Ant arc tic Pen in sula is marked by di verse flo ras and a gen eral shift from co ni fer-dom i - nated for est to Nothofagus-dom i nated veg e ta tion in the coastal high lat i tudes. Cal cu lated val ues of dif fer ent weath er ing in di - ces, petrographic anal y sis of thin sec tions as well as XRD anal - y ses from the KGI suc ces sion pro vide valu able in for ma tion con cern ing Eocene cli ma tic con di tions.
CONCLUSIONS
Ev i dence from both the rocks and the fos sil plants from KGI area play a sig nif i cant role in un der stand ing the biodiversity and palaeo ec ol ogy, in clud ing palaeoclimate, at south ern high lat i - tudes dur ing the tran si tional Ce no zoic pe riod. That time was a fi nal phases of global tec tonic and cli ma tic re or ga ni za tion for the en tire Earth. Veg e ta tion types re corded from KGI is sim i lar to that of the pres ent Valdivian rain forests of south ern Chile.
The grad ual de crease of biodiversity and tem po rary ex tinc tion at the end of Eocene might be ob served in the palaeobotanical re cord. To gether with re sults of sedimentological and pet ro log i - cal ob ser va tions, as well as geo chem i cal anal y sis of sed i men - tary rocks suc ces sion, it per mits to iden tify three preglacial stages that di rectly pref ace de vel op ment of gla cial con di tions on Ant arc tic con ti nent. These pre-gla cial palaeoclimatic stages (PGS-1–PGS-3) doc u mented cli mate cool ing trends, re corded
green house–ice house tran si tion, known from ma rine re cord (Zachos et al., 2001, 2003, 2008).
The first des ig nated unit of the preglacial stage (PGS-1) is char ac ter ized by Valdivian-type for ests de vel oped in ter res trial en vi ron ment. Ev i dence of this stage can be found in the Arcto - wski Cove For ma tion, in the lower part of the Point Thomas For - ma tion at Cytadela ex po sure and in the Mount Wawel For ma - tion. It cor re lates with the Early Eocene Cli ma tic Op ti mum, known from the oce anic re cord and the Early Eocene warm phase ob served in La Meseta For ma tion on Sey mour Is land (Doktor et al., 1996; Din gle and Lavelle, 1998; Din gle et al., 1998; Poole et al., 2003; Fran cis et al., 2008).
The sec ond preglacial stage (PGS-2) is a con tin u a tion of vol - ca nic ac tiv ity of the mixed ef fu sive-ex plo sive style un der con di - tions of a dryer cli mate. Vol ca nic land forms com posed of ag - glom er ates and tuffaceous de pos its in ter ca lated with lava flows tipped by weath er ing sur faces and regoliths formed as a re sult of these con di tions. The re cord of this stage can be found in the up - per part of Point Thomas For ma tion at Cytadela out crop.
Third preglacial stage (PGS-3) is char ac ter ized by the cool - ing of cli mate and hu mid con di tions in an ac tive vol ca nic set ting and con trib uted to wa ter ero sion of moun tain ous land scape, for ma tion of river can yons and val leys, and their in fill ing with con glom er atic de pos its. Ev i dence of this stage can be found in the up per part of Lions Cove For ma tion at Lions Rump.
The next stage de fined in this palaeoclimatic scheme is a re sult of rapid progradation of Ant arc tic ice-sheet on the vol ca - nic arc and par tial flood ing of moun tain land scape as so ci ated with a ma rine trans gres sion. It no lon ger be longs to preglacial and rep re sent a mem ber of gla cial stage (GS-1). The on set of gla ci ation is dated at 32 ± 0.5 Ma and was found in a lower part of the Polonez Cove For ma tion and de ter mines mo ment when ice-sheet reach edge of Ant arc tic con ti nent. Sed i ments of this stage cover preglacial units and pres ent a re cord of the be gin - ning of gla cial his tory on KGI.
Ac knowl edg ments. Pa per car ried out with lo gis tic sup port of Pol ish Ant arc tic Sta tion Henryk Arctowski within ACE pro - gram (Ant arc tic Cli mate Evo lu tion). I am grate ful to the re view - ers Pro fes sor T.L. Dutra, Dr V. Bow man, and anon y mous re - viewer for con struc tive re views and crit i cal re marks. I also thank Pro fes sor E. Zastawniak-Birkenmajer (W. Szafer In sti tute of Bot any PAS) for kind help in flora anal y sis.
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