Figure 4. Date distribution plot of analyzed zircons from the
interstratified tuffaceous samples of Mount Flora Formation. They show206Pb/238U dates of individual
zircons with 2σ analytical uncertainties (vertical bars) and their
calculated weighted mean (horizontal line) with its 95% confidence
error envelope (shaded band) for each sample. Grey bars are outliers
excluded from date calculation.
The Mount Flora succession has produced one of the better understood
fossil plant assemblages known for the Mesozoic of Gondwana (Halle 1913;
Gee 1989; Rees and Cleal, 2004;Birkenmajer and Ociepa 2008). The new
collection includes a little over 1400 specimens, collected at several
points in the Mount Flora area, both from drifted blocks and in
situ levels (Fig. 2).
The taphoflora is characterized by the coexistence of numerous linages,
including horsetails, ferns, cycads, bennettitales, conifers and seed
ferns (e.g., Gee, 1989), constituting a highly diverse plant community.
Horsetails are represented by vegetative remains, including axes, leaf
whorls and isolated nodal diaphragms presenting no major morphological
differences respect to extant Equisetum species, and therefore
are included in this genus. Fern diversity at Mount Flora includes
representatives of the families Dipteridaceae (Hausmannia andGoeppertella ), Dicksoniaceae (Coniopteris ), and
Osmundaceae (Todites ), together with taxa of uncertain systematic
affiliation, such as the sterile fronds Sphenopteris andCladophlebis . The seedferns are represented by Caytoniales, with
leaves of the Sagenoperis genus being present as isolated
leaflets, and hitherto unknown polliniferous organs of theCaytonanthus genus, with both genera thought to belong to the
same plant. The presence of other seedfern families (e.g. Umkomasiaceae)
is suggested by the occurrence of vegetative remains assigned toArchangelskya Herbst, in addition to possible seedfern taxa with
uncertain affinities (e.g. Pachypteris , Komlopteris ).
Fronds of possible cycadales (e.g., Ctenis , Pseudoctenis )
and bennettitales (e.g., Zamites , Otozamites ,Taeniopteris ) are also common in the plant associations, isolated
scale leaves assigned to Cycadolepis are rare whereas probable
megasporophylls of the Cycadospadix type are even rarer. Remains
of the polliniferous organs of bennettitaleans such as Weltrichiapreviously mentioned by Gee (1989) weren’t found, whereas the
ovuliferous organ Williamsonia is represented by numerous newly
collected specimens. Numerous conifer leaves and leafy twigs have been
also described (e.g., Brachyphyllum, Pagiophyllum ,Elatocladus ), which are generalized morphologies that can
potentially belong to different families during the Mesozoic (e.g.,
Araucariaceae, Podocarpaceae, Cheirolepidiaceae, Cupressaceae). Isolated
ovuliferous complexes of various morphologies, all of them apparently
bearing a single central seed covered by the tissues of the complex
(i.e., Araucarites ) strongly suggest the presence of
Araucariaceae, although other families are still in debate. Reproductive
organs subtended by branches with conifer-like leaves, previously
assigned to Schizolepidella (Halle 1913) and Sphenolepis(Gee 1989) are also present on the new collections, but whether their
affinities lie with modern or extinct conifer is still uncertain. Plant
diversity at Mount Flora is completed by other genera of unknown
systematic affiliation pending of a more detailed examination (e.g.,Scleropteris, Stachyotaxus ).
5 Discussion
5.1 Age and depositional history of the Mount Flora Formation
Elliot and Gracanin (1982) were the first to point out the presence of
tuff interbeds in the Mount Flora Formation at Hope Bay. Their tuff
dominated Unit 8 (Unit 4a of Montes et al., 2005; 2019) with a total
thickness of 36 m matches the tuffaceous interval reported here.
Correlative tuff intervals have been reported from the Tower Peak
Formation, as well (Farquharson, 1984). Birkenmajer (1993) and
Birkenmajer and Ociepa (2008, see their figs. 5 and 6) considered the
interval as a sill. Dikes and sills intruding the Mount Flora Formation
(Fig. 3b) differ from the tuffaceous beds in their petrography and field
relationships.
The depositional facies of the tuffaceous interval of Mount Flora Fm.
(Text S1; Fig. S1) suggest widespread explosive volcanism during active
alluvial (fan) deposition. The lower part of the tuffaceous interval
(Fig. 3a) contain tuffs, lapilli tuffs and breccias. Coarser-grained
breccias are interpreted as the coarse facies of ignimbrites, deposited
in energetic conditions through the lower flow boundary of a pyroclastic
density current (Branney and Kokelaar, 2004). The thick ash-lapilli
intervals with lensoidal beds suggest accumulation in fluctuating
granular regime of pyroclastic density currents (Sulpizio et al., 2014).
The middle part of the tuffaceous interval represents the typical
background sedimentation of the alluvial fan combined with volcanic
clasts and blocks eroded from loose or consolidated beds product of
explosive volcanism exposed elsewhere in the highlands. Beds with
tuffaceous matrix and volcanic clasts are interpreted as lahar deposits
formed after the deposition of the pyroclastic density currents, when
fresh, loose particles from the eruption were mixed with rainfall water
and entrained in a lahar flow together with clasts from the TPG.
Incipient rounding of the volcanic clasts pointing to reworking of the
primary volcanic products in the sedimentary environment The log remains
within these conglomerates suggest well-vegetated highland and/or
alluvial fan areas.
The lithic-rich, coarser grained breccias at the base of the upper
interval (Figs. 3a; S1) are interpreted as deposited in energetic
conditions through the lower flow boundary of a pyroclastic density
current (Branney and Kokelaar, 2004). The 14 m thick lapillitic interval
above capped by a 2 m thick, reverse-graded upper breccia (Fig 3a, b)
constitute the main body of a pyroclastic flow deposit formed under a
granular flow regime (Sulpizio et al., 2014). Wood pieces in the
overlying sandstone and large logs concentrated at the base of the
conglomerate bed above (Figs. 3; S1) reveal that explosive volcanism
devastated a forested area and sourced abundant trunks of dead trees to
the depositional system. Extensive reworking of volcanic material into
the upper member (Miembro Areniscas) conglomerates further substantiates
the syneruptive sedimentation. In addition, petrified wood indicates
dense upstream vegetation in the form of a forested landscape.
The depositional facies of the Mount Flora Formation and its tuffaceous
interval suggest at least two main episodes of explosive volcanism with
associated pyroclastic density currents during active alluvial (fan)
deposition heralding the widespread volcanism represented in the
overlying Kenney Glacier Formation. Coarse-grained ignimbritic breccias
suggest proximal to medial location with respect to the sources and
broad lens shape of the tuffaceoous interval topographically controlled
sedimentation in valleys typical features of this kind of ignimbrites
(Branney and Kokelaar, 2004)
Our new U-Pb geochronology constrains the peak volcanism of the Mount
Flora Formation tuffaceous interval at 163.55 ± 0.10 Ma (2σ including
tracer calibrations uncertainty). Hunter et al. (2005) reported tuff
U-Pb SIMS ages (2σ excluding any systematic uncertainties) of 167.1 ±
1.1 Ma from the Camp Hill Formation at Botany Bay and 168.9 ± 1.3 Ma
from the Tower Peak Formation at Tower Peak. The volcanic rocks of the
Antarctic Peninsula Volcanic Group yielded U-Pb SIMS age of 162.2 ± 1.1
Ma from Mount Flora and 166.9 ± 1.6 Ma from Camp Hill (Pankhurst et al.
(2000). In the absence of any reported accompanying zircon standard
measurements, the accuracy of the above in situ U-Pb dates cannot
be directly evaluated. However, the reproducibility of the U-Pb dates
measured by Sensitive High-Resolution Ion Microprobe (SHRIMP), in
general, has been estimated to be no better than 1% at 2σ (Ireland and
Williams, 2003; Stern and Amelin, 2003). This translates to minimum
uncertainties of ± 1.7 m.y. for the above SIMS ages. If the previously
reported U-Pb SIMS age of 162.2 ± 1.1 Ma from volcanic rocks overlying
the Mount Flora Formation (presumably a Kenney Glacier Formation sample)
is accurate, it implies that the upper half of the Mount Flora Formation
with a thickness of nearly 200 m and including all of its plant remain
was deposited in a relatively short time period of 1.4 ± 1.1 m.y. in the
Callovian.
The succession of the Mount Flora indicates that basin formation started
with active, amagmatic tectonics triggering the formation of fault
scarps and coarse-grained alluvial fans associated to them. Silicic to
intermediate volcanism started shortly after the tectonic processes as
is typical in many Jurassic-Cretaceous small basins from rift, back-arc
or trastentional settings all over Patagonia (e.g. Uliana and Biddle,
1987; Figari et al., 2015; Di Capua and Scasso, 2020). Explosive
volcanism leading to the deposition of tuffs, ignimbrites and other
volcaniclastic products initiated when the alluvial fans were still in
development, as these volcanic products are interbedded within the
coarse-grained conglomerate successions in the northern part of the
Antarctic Peninsula. The irregular relief product of formation of fault
scarps and volcanic edifices led to the extended formation of endorreic
fluvial networks in actively subsiding basins. Subsequently, small,
sometimes deep, lakes were formed in humid climates (e,g, Di Capua and
Scasso, 2020). These were rapidly filled with epiclastic and
volcanoclastic sediments, producing thick columns that may thin out
laterally in few kilometers. The primary or secondary (reworked by the
sedimentary agents) products of the increasingly extensive volcanism
finally buried the landscape and gave place to a succession with
coarse-grained epiclastic beds underlying a thick volcaniclastic column.
On the other hand these products are widely present as primary or
reworked tuffs intercalated in the younger (Kimmeridgian onwards) of the
Ameghino (Nordenskjöld) Fm. marine deposits formed after major
subsidence in the northern Antarctic Peninsula (Kiessling et al., 1999;
Scasso, 2001; Kietzmann and Scasso, 2019). Initiation of foreland basins
in the Early Cretaceous also led to the formation of flexural marine
basins showing similar successions elsewhere in Patagonia (Fosdick et
al., 2014;.Malkowski et al., 2016).
Both, the coarse-grained, terrestrial, Callovian Botany Bay Group and
the radiolarian-rich, marine, Kimmeridgian-Early Berriasian Ameghino
(Nordenskjöld) Fm. (Kiessling et al., 1999; Kietzmann and Scasso, 2019)
crop out in several isolated localities along the northeastern part of
the Antarctic Peninsula. Although these localities are, in some cases,
few kilometres apart (Fig. 1b) both units have not yet found in contact.
The lack of good radiometric ages lead some authors in the 80‘s to
consider both units coeval, representing different sedimentary
environments in the Larsen Basin, a back-arc basin developed behind an
Early Cretaceous arc emerging on the Antarctic Peninsula. While favoring
a Lower Jurassic age for the Botany Bay Group, Hathway (2000) linked its
deposition to localized, volcanically active, rift basins that developed
across Patagonia and Antarctic Peninsula in response to lithospheric
extension and crustal anataxis associated with the early stages of
Gondwana breakup. The coarse-grained terrestrial sediments where buried
by thick volcanic piles made of lavas and volcaniclastic sediments in
areas close to the volcanoes (e.g. the Kenney Glacier Formation in Hope
Bay). This ‘syn-rift megasequence’, which incorporated widespread
ignimbrite-dominated volcanism (presumably mid-Jurassic) was followed in
the northern Antarctic Peninsula by the Kimmeridgian-Early Berriasian,
radiolarian-rich, mudstones of the Ameghino (Nordenskjöld) Formation and
its marine equivalents (‘post-rift transgressive megasequence’),
although the contact relationships of the latter remain unclear. The
Ameghino Formation represents deposition in a deep anoxic marine
environment, with periodic fallout tuffs (Scasso 2001) on the inboard
flank of an emerging magmatic arc, with the latter forming a barrier to
ocean circulation (Hathway, 2000).
An unequivocal Callovian age for the Mount Flora Formation based on our
U-Pb geochronology brings the non-marine Hope Bay Group and the
Antarctic Peninsula Volcanic Group much closer in age to the marine
Ameghino (Nordenskjöld) Formation and its correlatives throughout the
Larsen Basin. In addition, this age proximity obscures the stated
distinction between the syn-rift and magmatic arc-related depositional
sequences associated with Gondwana break up and formation of the
Antarctic Peninsula magmatic arc, respectively. High-precision
radioisotopic ages from the Nordenskjöld Formation tuff interlayers will
be necessary to untangle the regional tectonics and basin evolution
histories.
Comparison with the high-resolution chronostratigraphy of the Jurassic
basin fill in the Chubut River Valley region of Patagonia (Cúneo et al.,
2013; Pol et al., 2020) indicates that the Mount Flora Formation was
deposited subsequent to the Cañadón Asfalto Formation and prior to the
deposition of the Cañadón Calcáreo Formation. Both of these formations
are non-marine successions with tuff interbeds and abundant vertebrate
and plant fossils (see floral comparisons below); their contact is
characterized by an unconformity. It has been postulated that a
post-Aalenian (Middle Jurassic) tectonic event resulted in down-cutting
into the Cañadón Asfalto and older formations, followed by the onset of
deposition of the predominantly fluvial and lacustrine Cañadón Calcáreo
Formation prior to ca. 158 Ma (Cúneo et al., 2013). This suggests that
the Mount Flora formation either coincides with a period of Middle
Jurassic non-deposition or correlates with the basal conglomerates of
the Cañadón Calcáreo Formation, in central Patagonia.
5.2 Middle Jurassic Flora of
Mount Flora and biostratigraphic implications
The Mount Flora succession has been assigned a broad range of ages, from
the Early Jurassic to the Early Cretaceous, with the same floristic
elements -at generic and specific level- being used to support
alternative age assignments. Taking into consideration its overall
paleobotanical content, the plant association in Mount Flora can
undoubtedly be assigned to the Jurassic, and this was observed as early
as the flora was initially studied (Nathorst 1904; Halle 1913). In this
sense, most recent paleobotanical studies have constrained the
taphofloras of Mount Flora and Botany Bay to the Early Jurassic (e.g.,
Rees, 1993a, b; Rees and Cleal, 2004) based on the presence of an
association of taxa related to this age. Those taxa include the
dipteridaceous ferns Goeppertella andDictyophyllum /Clathropteris , seed ferns likeSagenopteris nilssoniana and Dicroidiumfeistmantelli, and other possible seed ferns such asArchangelskya furcata and Pachypteris indica . The
utility of these taxa as biostratigraphic indicators of Lower Jurassic
are discussed in detail below.
Goeppertella woodii and G . jeffersonii are present
at Mt. Flora and Botany Bay plant assemblages, and were regarded as
being most similar to Early Jurassic species from South America (Rees
1993a; Rees & Cleal 2004). Species of this genus from the Northern
Hemisphere are mostly Late Triassic, while those of the Southern
Hemisphere are mostly Late Triassic-Early Jurassic, suggesting the same
age for Mt. Flora. However, Rees (1993b) also reported fossils ofG . cf. woodii from Clent Hills, New Zealand, which are
almost indistinguishable from the Antarctic remains. Since the New
Zealand fossils come from an assemblage now regarded as Middle-Late
Jurassic (Kamp 2001; Pole 2009), then although the majority of records
from South America suggest that Goeppertella is a good index
fossil for Late Triassic-Lower Jurassic, the records from New Zealand
point to a younger age.
Dictyophyllum is often found in localities dated as Lower to
middle Jurassic from South America (Herbst 1975). However,Dictyophyllum is not present at Mt. Flora, but Rees and Cleal
(2004) argued that fragmentary fronds of the genus can’t be
distinguished from Goeppertella fragments, which is present at
Mt. Flora. Nevertheless, since despite intensive field work done at Mt.
Flora there still aren’t fossils that can be confidently assigned toDictyophyllum (Halle 1913; Morel et al., 1994; Rees & Cleal
2004; Birkenmajer & Ociepa 2008; this study), its use as an argument in
favour of a Lower Jurassic age is not supported, and as they are not
readily identifiable when fragmentary, their usefulness as a marker for
Lower Jurassic results quite limited.
Sagenopteris nilssoniana (and its synonym S .rhoifolia ) is present on numerous Lower Jurassic localities from
South America and Europe (Halle 1913; Bonetti 1963; Quattrocchio et al.,
2007), and its presence on Mt. Flora was used to argue for a Lower
Jurassic age for the plant assemblage (Rees and Cleal 2004). However, it
has been argued that S . nilssoniana display a vast amount
of contrasting features that suggest that its records may be
attributable to more than a single species, and that they should be
carefully revised (Elgorriaga et al., 2019). More importantly, some of
the specimens from Mt. Flora and Botany Bay display at least one feature
that do not conform with neither S . nilssoniana nor any
species of the genus (i.e. leaflets with numerous deep lobes on their
margins), casting doubts on the specific and perhaps even generic
attribution of those specimens (Pattemore et al., 2015).
Archangelskya furcata is one of the two species of the genus, the
other one being A . proto -loxsoma from the Lower
Jurassic of Mendoza, Argentina (Herbst 1964). Since A .furcata was also reported from the Lower Jurassic of Patagonia
(Herbst 1964 and Harbst and Anzoátegui 1968), it was suggested that the
presence of the genus could be used as a good indicator for a Lower
Jurassic age (Rees and Cleal 2004). However, since A. furcataremains were also found on Cretaceous rocks from Livingston Island
(Parica et al., 2007) then the species has a longer biochron than
initially thought, and hence it is no longer advisable to use its
presence as an indicator for Lower Jurassic. .
Three specimens assigned to Dicroidium feistmantelli were found
on Botany Bay (Rees & Cleal 2004), and since the genus rarely surpasses
the Late Triassic (although see Bomfleur et al., 201), its presence
suggested a Late Triassic/Lower Jurassic age for Mt. Flora plants. But,
as Rees (1990) argues, the remains from Botany Bay lack cuticular
features and are too fragmentary to confidently place them on theDicroidium genus, lacking one of the features that characterizes
the genus (i.e.. basal dichotomy), and they may be better placed in
other genera that possess similar morphology (e.g. Thinnfeldia ).
The remains of Pachypteris indica from Mt. Flora resemble those
of India, which are of probable Lower Cretaceous age (Bose and Banerji
1984). Additionally, as noted by Rees & Cleal (2004), they also
resemble the fronds described as Sphenopteris bagualensis by
Menendez (1956) from Bajo de los Baguales, Argentina, that is dated as
Middle Jurassic. Therefore, the presence of P . indicasuggests a Middle Jurassic minimum age for the Mt. Flora assemblage,
which agrees with the late Middle Jurassic age determination of this
study. In summary, the taxa that were previously used to estimate the
age of Mt. Flora and the Botany Bay Group do not provide unequivocal
support for a Lower Jurassic age for the plant assemblage as a whole,
and as will be discussed below, their presence in the terminal Middle
Jurassic of Antarctica may have a paleoecological explanation instead.
Our new U-Pb CA-ID-TIMS geochronology, along with the U-Pb SIMS data of
Pankhurst et al. (2000) and Hunter et al. (2005), strongly support a
Middle Jurassic (Callovian) age for the entire plant associations at
Hope Bay. By direct correlation, this age assignment can be extended to
the floral associations at Botany Bay and Tower Peak (Fig. 1), as well.
The calibrated age of this paleoflora has important biostratigraphic
implications. It has been demonstrated that some of the genera used to
date Mt. Flora have a broad biochron, including several stages within
the Jurassic and, in some cases, reaching the Triassic or the Cretaceous
(e.g. Goepertella , Pachypteris ). Also, in the absence of
cuticular or anatomical information, many species-level taxa are defined
by characters that are highly homoplastic and, therefore, their
biostratigraphic or biogeographic relevance is questionable. The Middle
Jurassic floras of the Antarctic Peninsula are remarkably similar to the
floras in some localities in Patagonia (Argentina) in terms of their
most characteristic elements (e.g., Sagenopteris ,Geoppertella , Archangelskya ) and some other floristic
components (Escapa 2009).The Cerro Taquetrén locality in the Jurassic
Cañadón Asfalto Basin (Chubut River Valley, Chubut Province, Argentine
Patagonia), for instance, shows a high degree of similarity in plant
composition with respect to the Hope Bay floras (Bonetti, 1963; Herbst
and Anzótegui, 1968; Escapa et al., 2008; Escapa, 2009). However, the
Cerro Taquetrén flora occur in the basal Lonco Trapial Formation, which
has an Early Jurassic (Pliensbachian-Toarcian) age on the basis of
CA-ID-TIMS geochronology (Cúneo et al., 2013; Pol et al., 2020),
indicating at least a 17 m.y. age difference between the two. The fossil
flora of Piedra Pintada in the province of Neuquen is another Patagonian
assemblage with a Lower Jurassic age (based on ammonite
biostratigraphy), which contains Sagenopteris andGoeppertella among other elements also present at Mt. Flora
(Herbst 1966). In addition to these key elements, the Patagonian Early
Jurassic floras are similar to their Antarctic Middle Jurassic
counterparts in the presence of Equisetum-like equisetales,
bennetttitales, and other conifer and fern remains (e.g. Elatocladus,
Osmundopsis).
In the Cañadon Asfalto Basin, the diverse Pliensbachian-Toarcian floras
were replaced by relatively less diverse ones by the end of the Toarcian
(i.e., Cañadón Asfalto Fm.), which are dominated by conifers (e.g.,Brachyphyllum ) and small leaf ferns (e.g., Cladophlebis )
(Escapa 2009). The permineralized flora of the overlying Cañadón
Calcáreo Formation with an unambiguous Late Jurassic based on U-Pb
geochronology (Cúneo et al., 2013) are dominated by conifer seed cones
of the families Araucariaceae and Cheirolepidiaceae. Thus, Mount Flora
exhibits stark differences with its coeval successions in South America
in terms of paleofloral associations, indicating the low value of
macrofossil plant associations in establishing biochronologic stages for
the Jurassic.
The large age disparity between near-identical South American and
Antarctic flora is of particular paleoecologic significance. In the case
of Dipteridaceae, one of the most conspicuous taxa of the Gondwanic
floras mentioned here, a similar pattern of distribution in the Northern
Hemisphere has been interpreted to reflect migration from Southeast Asia
(Late Triassic) to Europe (Early Jurassic) as a function of climate
change (Barale, 1990; van-Konijnenburg-van Cittert, 2002). The response
of the vegetation to changes in the Jurassic climate has been also
documented at a finer scale by measuring the effect of the Toarcian
Anoxic Event on the continental environments of Yorkshire (Slater et
al., 2019). The scheme seems to be similar: the same basic lineages,
without major changes in composition throughout the Jurassic, comprised
associations that were strongly correlated with temperature and
humidity. In contrast with the Northern Hemisphere, well-described
Gondwanic Jurassic associations with well-documented ages are scarce.
However, the available information suggests that a similar ecologic
plant mosaic could have been present in the Jurassic of Gondwana. It is
also possible that tectonically controlled inception of foreland basins
established the paleoecological conditions for similar plant
associations to flourish at separate locations and at different times
during the Jurassic. More complete plant concepts with broader
geographic distributions and precise ages are needed in order to
decrease the probability of homoplasty biases in the biotic comparisons
and to better understand complex patterns of vegetation change in time
and space.
5.3 Mount Flora and the Gondwana extensional history
The tectonic evolution of the Antarctic Peninsula has been traditionally
interpreted by correlation of its magmatic and sedimentary rocks to
presumed equivalent units throughout the South American Patagonia.
Pankhurst et al. (2000) divided the ca. 35 m.y. regional Jurassic
silicic volcanism into three episodes (V1-V3), with the Early Jurassic
V1 (188-178 Ma) influenced largely by the Karoo-Ferrar magma plume and
the Middle Jurassic V2 (172-162 Ma) reflecting a progressive trend
towards rifting and break-up along the proto-Pacific margin of Gondwana.
The Late Jurassic V3 volcanism (157-153 Ma) and associated granitoids
were interpreted to have an active-margin affinity and to represent a
precursor to the intrusion of the Andean and Antarctic Peninsula arc
batholiths. In this scheme, the volcanic rocks at Mount Flora would
correlate with those of the Chon Aike Formation of central Patagonia and
fall within the V2 episode of Pankhurst et al. (2000). Riley et al.
(2017) recognized a ca. 185-181 Ma episode of granitoid emplacement
throughout Patagonia and (southern) Antarctic Peninsula, which although
coincided with the V1 episode, but had a subduction genetic affinity.
This shifted the onset of subduction along the proto-Pacific margin of
Gondwana to Early Jurassic, in contrast to the earlier interpretation of
Pankhurst et al. (2000). Finally, Bastias et al. (2021) added a yet
earlier episode of active margin magmatism of Triassic age (V0) to the
geologic history of Antarctic Peninsula and Patagonia and concluded,
based on a compilation of age, geochemical and isotopic data, that the
entire Triassic-Jurassic magmatic evolution can be explained by
subduction-related processes involving a westward migrating slab. In
their view, a mantle plume need not be invoked and the disassembly of
southern Gondwana followed the Late Jurassic episode (V3) of back-arc
extension and rifting (Bastias et al., 2021).
The above interpretations have been derived to a large extent from
temporal correlations of magmatic rocks throughout the Antarctic
Peninsula and Patagonia based on in situ U-Pb geochronology using
SIMS (Pankhurst et al., 2000; Riley et al., 2017) or LA-ICPMS (Bastias
et al., 2021) methods with limited precision and accuracy. Recent
high-precision U-Pb geochronology by the CA-ID-TIMS technique has
demonstrated distinct and short-lived, Early Jurassic magmatic episodes
for the Karoo (e.g., Sell et al., 2014) and Ferrar (e.g., Burgess et
al., 2015) large igneous provinces of South Africa and Antarctica, as
well as for the Lonco Trapial Formation of northern Patagonia (Pol, et
al., 2020), all of which were once considered to be contemporaneous and
associated with rifting. Such high resolution temporal constraints
nevertheless remain sparse in the region. Untangling the complex
geologic evolution of southern Gondwana driven by protracted magmatic
arc-rift interactions requires an integrated approach that would include
detailed depositional histories of its sedimentary successions, in
addition to precise age calibration of its magmatic episodes.
The chronostratigraphy and depositional facies of the Mount Flora
succession indicates rapid deposition in a continental landscape
characterized by high topographic relief and dense vegetation proximal
to centers of active felsic volcanism near the end of the Middle
Jurassic. The high relief may have been the result of a regional uplift
event during the development of the Jurassic arc, which was also
responsible for a regional unconformity in the northern Patagonian
foreland basin(s).
5 Conclusions
Two high-precision U-Pb zircon ages (CA-ID-TIMS method) from a distinct
tuffaceous interval in the lower conglomerate member of the Mount Flora
Formation at Hope Bay produced statistically coherent clusters of206Pb/238U dates of 163.541 ±
0.052/0.090/0.20 Ma (MSWD = 0.66) for crystal lithic lapilli tuff and
163.555 ± 0.071/0.10/0.20 Ma (MSWD = 1.5) for an ignimbritic lapilli
tuff indicate a Middle Jurassic (Callovian) age
The succession of the Mount Flora indicates that basin formation started
with active, amagmatic tectonics triggering the formation of fault
scarps and coarse-grained alluvial fans associated to them. In this
setting, silicic to intermediate volcanism started shortly after the
tectonic processes, as is typical in many Jurassic-Cretaceous small
basins from rift, back-arc or transtensional settings all over
Patagonia.
The depositional facies of the Mount Flora Formation, its age proximity
to the marine Ameghino (Nordenskjöld) Formation in the Antarctic Larsen
Basin, and its coincidence with a regional unconformity in the northern
Patagonia point out to a complex interplay among magmatic arc
development, tectonic extension and continental break up that dominated
the geologic and paleoenvironmental evolution of southern Gondwana near
the end of the Middle Jurassic.
The highly diverse Jurassic plant association that dominated the
Antarctic Peninsula nearly 17 million years after its disappearance from
northern Patagonia suggests similar paleoecological conditions were
established diachronously throughout foreland basins of southern
Gondwana, possibly facilitating floral migrations in response to local
climate change.
Acknowledgments, Samples, and Data
We acknowledge the Instituto Antártico Argentino for logistic support of
field work in Antarctica. Rodolfo del Valle provided valuable geological
information and logistic advice for developing our project. We thank
Juan Manuel Lirio for his encouragement, as well as support and
cooperation during our field trip. Pablo Puerta was of great help in
plant collection and fieldwork. U-Pb geochronology was supported by the
MIT Isotope Lab funds.
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