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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