Paleoenvironment of the Cerro Negro Formation (Aptian, Early Cretaceous) of Snow Island, Antarctic Peninsula

Abstract

A study of macro and microfacies, palynoflora and palynofacies of the non-marine Cerro Negro Formation at President Head Peninsula, Snow Island, northwest of the Antarctic Peninsula, was developed. Two assemblages were recognized: Palynofacies assemblage 1 (P1) at the base of the section with a dominance of fern spores and conifer pollen grains, and facies association consisting of a clastic layer, with the predominance of mudstones; and Palynofacies assemblage 2 (P2) at the top of the section, with remarkable abundance of AOM/Pseudoamorphous particles, associated with facies that includes tuffs. The complete section shows in some levels the presence of freshwater algae and translucent phytoclasts. The integrated data characterizes a fluvial-lacustrine environment, what is reinforced by the occurrence of freshwater algae (Botryococcus) in some levels of P1 and P2. We could verify an increase in volcanic activity towards the top of the section that apparently has played an important role in the collapse of the palynoflora. The occurrence of the spore species Muricingulisporis annulatus, Sotasporites elegans, S. triangularis, Foraminisporis wonthaggiensis, and F. asymmetricus in the Cerro Negro Formation allows the correlation with sections in South America and Australia, suggesting an Aptian age for these deposits.

Key words
Antarctica; Cretaceous; palynology; sedimentology; geochemistry

INTRODUCTION

Snow Island is part of the South Shetland Archipelago, with a northeast orientation along the Pacific margin of the Antarctic Peninsula, and is largely covered by permanent ice. Outcrops of this island are mainly restricted to the President Head Peninsula. The geology of this area has been described as a volcano-sedimentary sequence that comprises thin sandstone interlayers and thick layers of conglomerates, and calcareous concretions. Two sedimentary units can be recognized, albeit the contact between them does not outcrop: a lower marine unit, overlain by upper volcanic-influenced continental strata (Israel 2015). The lower marine sedimentary unit is restricted to a small-exposed area at the central portion to the western of the Peninsula, near the permanent glacier. This marine deposit consists of ammonite-bearing mudstone, shale, sandstone and breccia strata, which have been litho- and paleontologically correlated with the Sealer Hill Member of Chester Cone Formation at Byers Peninsula, Livingston Island (Crame et al. 1993, Philippe et al. 1995, Duane 1996, Hathway & Lomas 1998). The upper non-marine volcaniclastic unit is more extensive across President Head Peninsula, and is composed of plant-bearing mudstones, shales, sandstones, conglomerates, and tuffs (Israel 2015). Based on the flora content, these non-marine strata have been correlated with the volcaniclastic Cerro Negro Formation from Byers Peninsula at Livingston Island (Torres et al. 1997a, b, Cantrill 1998). Several hypabyssal, mostly doleritic, intrusive bodies such as sills and stocks outcrop in the Peninsula (Israel 2015), and their age based on whole-rock dating is Eocene (Pankhurst & Smellie 1983, Watts et al. 1984). The Cerro Negro Formation at Byers Peninsula is predominantly composed of plant-bearing pyroclastic and sedimentary rocks that were deposited in a fluvio-lacustrine environment.

The Early Cretaceous paleoflora of Snow Island is dominated by ferns and conifers (Torres et al. 1995, 1997a, Cantrill 2000) and other gymnosperms as Bennetitales (Falcon-Lang & Cantrill 2002). The palynology data from President Head (Snow Island) allows a correlation with sediments from the Byers Peninsula, in Livingston Island, and with palynological assemblages from Western Australia and South America (Duane 1996, Torres et al. 1997a).

Although the macroflora and palynoflora from President Head Peninsula are fairly well known (e.g., Philippe et al. 1995, Duane 1996, Torres et al. 1997a,b, Cantrill 1998, 2000, Césari et al. 1998, 1999, Hathway et al. 1999), there is no study of the palynofacies nor the geochemical features of the volcaniclastic rocks. Also, the petrologic processes in these deposits have not been examined in detail. Aiming a paleoenvironmental reconstitution of the Aptian deposits of President Head Peninsula at Snow Island, in the present study we analyze the palynological and palynofacies information and integrate it with the sedimentology and geochemistry data.

Geology and stratigraphy background

The South Shetland Islands is an ENE–WSW oriented archipelago of ca. 450 km in length (Fig. 1), which constitute part of the geological Western Domain of the Antarctic Peninsula (Vaughan & Storey 2000). It is a crustal block delimited by the South Shetland Trench to the northwest and by an axis of spreading ridge in the Bransfield Strait to the southeast (Galindo-Zaldívar et al. 1996). According to Birkenmajer (1994), the geological history of the Pacific margin of the northern Antarctic Peninsula includes a first stage of marginal basin deposition (?Permian–Triassic), the Gondwanian orogeny (Late Triassic?), a subduction stage (Middle Jurassic–Miocene) during which the inner Antarctic Peninsula and the outer South Shetland Islands magmatic arcs were formed, and the opening of the Bransfield Strait that separates these two magmatic arcs. The Bransfield Strait is a back-arc basin that evolved through rifting and spreading and probably began to open due to significant transtensional effects after the activity of the West Scotia Ridge ceased at ca. 7 Ma. This is thought to be due to Phoenix Plate roll-back under the South Shetland Islands after cessation of the spreading activity of the Phoenix Ridge at ca. 3.3 Ma (Solari et al. 2008).

Figure 1
Location of President Head Peninsula, Snow Island. a) Plate tectonic position of the northern Antarctic Peninsula at the present day (adapted from Birkenmajer 1994); AR: Atlantic Ridge, BS: Bransfield Strait, SSa: South Sandwich Islands, SG: South Georgia Islands, SSh: South Shetland Islands, SO: South Orkneys Islands. b) Geographic map positioning the President Head Peninsula, Snow Island at the South Shetland Islands Archipelago, metadata obtained from the Antarctica Digital Database Mapping Services SCAR https://www.add.scar.org/. c) Geological map of President Head Peninsula, indicating the studied section of the Cerro Negro Formation (adapted from Israel 2015).

The South Shetland Islands magmatic arc is founded on a sialic basement of schists and metasedimentary rocks (Smellie et al. 1984). In the western South Shetland Islands the Byers Group is exposed and comprises a thick succession recording Late Jurassic–Early Cretaceous sedimentation and volcanism (Hathway & Lomas 1998). The Byers Group on Byers Peninsula (Livingston Island) is composed of at least 1.3 km of marine clastic rocks, overlain by about 1.4 km of Lower Cretaceous non-marine volcaniclastic strata which record the expansion of the continental-arc facies into a marine intra-arc basin (Hathway 1997). Nevertheless, a fore-arc configuration has also been proposed as an alternative of the depositional environment (Crame et al. 1993, Bastias et al. 2020). Rocks of the Byers Group are also exposed on the nearby Rugged Island, at President Head Peninsula in Snow Island (Smellie et al. 1984, Crame et al. 1993, Hathway & Lomas 1998), and to the most southwestern South Shetland Islands at Cape Wallace in Low Island (Bastias et al. 2020). At the base of the Byers Group, deep-marine strata of the Kimmeridgian–Tithonian Anchorage Formation and the Berriasian President Beaches Formation are overlain by Berriasian–Valanginian shallower-marine deposits of the Chester Cone Formation and the volcanic breccias of the Start Hill Formation. These deposits are separated from the Aptian non-marine volcaniclastic rocks of the Cerro Negro Formation at the top by an unconformity (Hathway & Lomas 1998).

The non-marine volcaniclastic Cerro Negro Formation was formally defined by Hathway (1997) and is exposed on the eastern Byers Peninsula (Livingston Island), where it is broadly equivalent to the Volcanic Member of Smellie et al. (1980). This formation dips gently and becomes younger to the ENE. Hathway (1997) divided the non-marine volcaniclastic Cerro Formation strata into two informal units. The lower division consists mainly of welded and non-welded ignimbrites, intercalated with subordinate reworked silicic pyroclastic and epiclastic strata. The upper division consists mainly of poorly sorted, basaltic lapilli-tuffs and tuffaceous breccias. The unit consists largely of primary pyroclastic strata and syn-eruption debris- and flood-flow deposits, but the subordinate conglomerates, sandstones and mudstones record inter-eruption fluvial-lacustrine deposition (Hathway & Lomas 1998).

The main controls on deposition of the Cerro Negro Formation were volcanism and perpendicular extension to the trend of the Antarctic Peninsula arc manifested in the emplacement of a dike swarm, and in syn-sedimentary faulting resulting from differential subsidence, suggesting that the Cerro Negro Formation depocenter was probably fault-bounded (Hathway 1997). Plagioclase and biotite ⁴⁰Ar/³⁹Ar geochronology from lower silicic pyroclast beds and from a welded ignimbrite close to the topmost indicates an Aptian age (between 120.3 ± 2.2 and 119.4 ± 0.6 Ma) for the Cerro Negro Formation, agreeing with the palynomorph taxa relative dating (Hathway et al. 1999).

MATERIALS AND METHODS

The studied material was collected by the PALEOANTAR team from President Head Peninsula, Snow Island in the austral summer of 2017, during the 35th Antarctic Operation (OPERANTAR XXXV), supported by the Brazilian Antarctic Program (PROANTAR). The PALEOANTAR project that has been working in the Antarctic Peninsula for the last years (e.g., Kellner et al. 2019).

Due to the glacier deposits, snow cover and regolith, outcrops at the President Head Peninsula (Fig. 2) are scarce and discontinuous, impeding the establishment of a continuous stratigraphic profile. Three stratigraphic profile intervals were measured, described and joined to build up a stratigraphic composite profile (Fig. 3). Section A (0 to 5.5 m) and section B (7.7 to 9.13 m) were separated by 6 m, and section B and section C (19.11 to 29.64 m) by 29 m. The strata bedding displays NNE-SSW trending strike and a dip of 19° to SE. A total of 27 samples were collected for palynology, microfacies, and whole-rock geochemistry. Additional data is available on-line (https://doi.org/10.5281/zenodo.5140569).

Figure 2
Outcrop photographs. a) General view of the studied area that is mostly covered by regolith. b) Stratigraphic profiling process.

Figure 3
Composite lithostratigraphic profile of the Cerro Negro Formation in the north-central portion of the President Head Peninsula, Snow Island.

Major oxides and some trace elements of collected samples were determined using X-ray fluorescence (XRF). Each sample was oven dried at 100°C. A portion of the dried sample was taken to a muffle at 1000°C for two hours to determine the loss on fire. Another portion of the dried sample was melted using lithium tetraborate. Chemical analyses of fused beads were performed using a Rigaku X-Ray Fluorescence Spectrometer model ZSX Primus II equip with a Rh tube and seven analyzer crystals by the calibration curve method based on international references material at the Nucleus for Geochemical Studies and Stable Isotopes Laboratory (NEG-LABISE), Federal University of Pernambuco (UFPE), Recife, Brazil. The petrographic data are based on 12 standard thin sections prepared by GEOLAB Geology Solutions Company, Olinda, Brazil.

Table I
Bulk chemical composition of the primary volcaniclastic rocks from Cerro Negro Formation at the President Head Peninsula, Snow Island obtained by X-ray fluorescence (XRF). Major element compositions were recalculated on volatile-free basis and reported in weight percentage (wt. %).

Sample	Rock type	SiO2	Al2O3	TiO2	Fe2O3T	MgO	MnO	CaO	K2O	Na2O	P2O5
M.27	Tuff. litharenite	67.01	15.83	0.62	4.86	1.70	0.17	1.76	5.56	2.41	0.09
M.26	Tuff	68.72	16.55	0.54	3.56	1.26	0.11	1.49	2.04	5.60	0.13
M.25	Tuff	71.36	14.22	0.63	4.21	0.99	0.11	1.40	1.87	5.11	0.09
M.24	Tuff	74.23	13.83	0.51	3.45	1.51	0.09	2.74	1.45	2.09	0.10
M.23	Tuff	70.95	14.11	0.56	3.57	1.17	0.08	1.24	6.51	1.72	0.08
M.22	Tuff	70.00	13.70	0.72	5.01	1.65	0.17	1.14	5.60	1.88	0.13
M.21	Tuff	64.71	14.86	0.70	7.06	2.41	0.27	0.78	7.62	1.44	0.14
M.20	Non-volcanic	62.90	15.20	0.91	8.39	2.90	0.24	1.53	5.61	2.16	0.15
M.19	Non-volcanic	67.20	13.31	0.71	6.62	2.17	0.21	1.66	5.77	1.89	0.46
M.18	Tuff	76.70	12.36	0.31	2.43	0.62	0.08	0.30	0.97	6.19	0.04
M.17	Tuff	68.63	13.93	0.84	6.03	1.89	0.20	1.12	5.29	1.96	0.11
M.16	Non-volcanic	63.39	15.51	0.78	7.94	2.01	0.24	1.04	7.05	1.91	0.14
M.15	Non-volcanic	64.31	14.78	0.70	7.67	1.99	0.20	0.86	8.00	1.33	0.16
M.14	Tuff	58.62	16.45	1.05	10.71	3.63	0.35	1.32	5.19	2.54	0.15
M.13	Tuff	66.40	14.29	0.79	6.37	2.07	0.18	1.74	5.87	1.91	0.37
M.12	Non-volcanic	66.71	16.23	0.45	5.41	2.14	0.15	1.35	4.55	2.91	0.09
M.11	Non-volcanic	64.92	16.45	0.56	6.22	1.93	0.16	1.01	3.55	5.14	0.06
M.10	Non-volcanic	69.75	13.31	0.58	5.52	1.80	0.16	0.81	6.50	1.49	0.09
M.9	Non-volcanic	66.20	14.36	0.68	7.60	2.31	0.23	0.87	5.10	2.54	0.12
M.8	Non-volcanic	60.80	17.11	0.92	8.51	2.68	0.18	1.22	5.24	3.14	0.20
M.7	Non-volcanic	64.63	17.11	0.49	5.64	2.02	0.16	0.93	5.72	3.23	0.06
M.6	Non-volcanic	62.59	16.88	0.78	7.10	2.56	0.21	1.19	4.37	4.18	0.14
M.5	Greywacke	63.61	14.69	0.90	8.63	3.10	0.26	2.76	3.11	2.25	0.70
M.4	Non-volcanic	61.72	16.41	1.28	8.37	2.19	0.26	1.55	3.42	4.56	0.23
M.3	Non-volcanic	64.83	15.88	0.64	6.96	1.94	0.18	0.81	4.27	4.39	0.10
M.2	Non-volcanic	70.06	13.54	0.55	5.60	1.67	0.15	1.50	3.93	2.92	0.07
M.1	Non-volcanic	68.48	14.19	0.74	6.35	1.96	0.19	1.57	3.74	2.68	0.09

A total of 26 samples from an outcrop on Snow Island were processed (~40 g) using standard palynological techniques to dissolve the mineral constituents at the Instituto tecnológico de Paleoceanografia e Mudanças Climáticas (itt Oceaneon), UNISINOS University. This approach was compiled by Wood et al. (1996) and non-oxidative procedures described by Tyson (1995) and Mendonça Filho et al. (2011). The palynological slides are deposited in the LMA (Laboratório de Micropaleontologia Aplicada), at the Federal University of Pernambuco, Brazil, under the collection numbers LMA-P00001–P00026.

The palynological slides were analyzed using transmitted white light and incident blue light/UV fluorescence mode, under a Zeiss-Imager A2 optical microscope, 200X, 630X and 1000X magnification. In each sample, ~300 palynomorphs were counted for palynology and 500 particles for palynofacies (when possible). For a statistical treatment, only samples with more than 400 particles for the palynofacies were used, as well as samples with more than 100 palynomorphs for the palynoflora (SM-1 and SM-2). The raw, percentage and normalized data are presented in the Supplementary Material (SM-1 and SM-2, https://doi.org/10.5281/zenodo.5140569).

The material consists of 5504 palynomorphs (see SM-1). A rarefaction curve was constructed using the PAST software (Hammer et al. 2001) to determine whether most species recovered were enough by combining data from the samples (Gotelli & Colwell 2001).

The palynofacies classification was based on the composition and abundance percentages of the sedimentary organic matter (SOM) (Tyson 1995, Mendonça Filho et al. 2010), The paleoenvironmental inference was based on the application of multivariate statistics and ecological indexes in the studied samples. Cluster analysis was used to identify compositional similarities between SOM (phytoclasts, Amorphous Organic Matter (AOM) and palynomorphs) from different depositional settings. This analysis was employed using agglomerative, hierarchical clustering and stratigraphically constrained cluster analysis (CONISS) to establish groupings of samples (Grimm 1987), displayed in a dendrogram. The kerogen was represented in Ternary graphic (Tyson 1993, 1995), plotted using the PAST software (Hammer et al. 2001). The principal component analysis (PCA) with varimax rotation was performed using the statistical software PAST (Hammer et al. 2001). For this analysis, the raw data were standardized, and each variable was transformed, according to Gotelli & Ellison (2011) (see SM-1 and SM-2). In this study, were the PC loadings >0.2 to palynoflora, and >0.4 to palynofacies. PC scores >5 to palynoflora were assigned to dominant taxa and PC scores between 1 to 5 to associated flora. As for palynofacies, PC scores >1 are dominant elements and 0,5 to 1 significant associated (SM-2).

Diversity (Shannon-Wiener), dominance (Simpson) and equity (Eveness) indexes for palynoflora were calculated using the PAST software (Hammer et al. 2001). An x² -test was used for sedimentary organic matter to determine which paleoenvironment was preferred, calculated on the MedCalc software.

RESULTS

Sedimentological and geochemical characterization

The composite stratigraphic profile at President Head Peninsula is ~ 30 m thick, constitutes a fine to medium-grained clastic and volcaniclastic deposit with abundant plant fossils (Fig. 3). Six lithofacies were grouped into two associations, one of clearly clastic origin and the second with volcanic influence.

Within the first 4 m from the base to the top, a clastic layer without volcanic influence constitutes the first facies association, where mudstones without volcanic components predominate. Organic-rich claystone are interspersed in a planar-bedding pattern (Figs. 4a–b) with laminated siltstone to fine-grained quartzarenite, abundant kerogen clasts are observed in these facies (Figs. 5a–b, e–f). To the middle of the pelitic interval, a medium- to coarse-grained graywacke with low angle cross-lamination and abundant subangular to subrounded rhyodacite lithic fragments (Figs. 4a; 5c–d) is intercalated. Massive siltstones are found to the base and to the top of this interval (Fig. 4e).

Figure 4
Samples macrophotographs showing the different lithofacies. a) Sample M.5, medium to coarse-grained graywacke (Gw) with interbedded planar laminated claystone (Cy) and siltstone (Sl) levels that are crossing by two microfractures (mf) and to the top an overload structure (Ls) is observed lying on the finer sediments. b) Sample M.7, claystone, siltstone and very fine-grained sandstone interspersed with flame structure (Fs) and Ls. c) Sample M.14, fine (fT) and coarse-grained (cT) ash tuff laminae crossed by a volcanic glass vein (vgv). d) Sample M.14, matrix-supported lapilli tuff. e) Sample M.15, massive siltstone. f) Sample M.17, siltstone laminae with interbedded fine and coarse-grained ash tuff, several volcanic glass veins are crossing these. g) Sample M.23, fine to medium-grained ash tuff interbedded with few siltstone fine laminae. h) Sample M.25, matrix-supported lapilli tuff with abundant black organic fragments (Of). i) Sample M.27, clast-supported tuffaceous litharenite.

The second association is characterized by volcaniclastic facies. From ~ 3.9 m upward to ~ 30 m, excluding the regolith-covered intervals which were not recovered, welded fine- to coarse-grained ash tuff, siltstone and lapilli tuff beds are intercalated (Figs. 4c–d, f–i; 5f-p). The tuff levels display well-preserved volcanic components including welded pyrogenic ash forming the matrix, pumice and angular to subangular plagioclase and quartz crystals up to > 3 mm sized, vesicles and oriented volcanic glass shards. Palagonite is observed filling amygdales and as alteration of the matrix glass. Most of the tuff levels are matrix-supported but to the top of the section contains a clast-supported tuffaceous litharenite with pyrogenic crystals as well as subrounded to rounded rhyodacite lithic fragments. In these facies are also present kerogen clasts, as amorphous and as structured grains.

Twenty-seven samples were classified by combining information from the major element whole rock geochemistry (Table I) together with macroscopic and petrographic information. There are clastic rocks without volcanic influence and volcaniclastic rocks. Sixteen samples correspond to clastic rocks, one sample was classified as a graywacke and is a secondary volcaniclastic rock, and ten samples were identified as primary volcaniclastic rocks. The volatile free-base composition of all the rocks shows they are moderate to highly evolved silicic rocks with SiO₂ values ranging between 58 and 77 wt. %. The igneous classification diagram, based on Irvine & Baragar (1971) and Middlemost (1994), includes just the volcaniclastic rocks, this displays a main group of samples lying within the subalkaline rhyolitic composition with SiO₂ > 66 wt. % and total alkali content (K₂O+Na₂O) between 3.54 and 8.23 wt. % (Fig. 6a). With SiO₂ < 65 wt. %, the subalkaline dacitic graywacke sample M.5 and the tuffs samples M.14 and M.21 are on the alkaline limit of trachy/andesite composition, respectively with a total alkali content of 3.54, 7.73 and 9.06 wt. %. The immobile elements have generally high Al₂O₃ values ranging between 12.36 and 17.11 wt. % and moderate Fe₂O₃ values between 2.43 and 8.63 wt. % (and the anomalous tuff sample M.14 that has 10.71 wt. %). MgO content is < 3 wt. % in all rocks except the graywacke sample M.5 and the Fe₂O₃-rich tuff sample M.14 (3.10 and 3.63, respectively). CaO content is ~ 2.75 wt. % for graywacke sample M.5 and tuff sample M.24, for the rest of the samples is < 1.76 wt. %. All the rocks present TiO₂ and P₂O₅ values < 1.28 and 0.70 wt.%, respectively.

Figure 5
Petrographic photographs showing some details of the microfacies. a–b) Sample M.3, claystone-siltstone laminae under cross polarized light XPL. c–d) Sample M.5, Medium- to coarse-grained grauwacke with abundant rhyodacite lithic fragments, parallel polarized light PPL and XPL, respectively. e) Sample M.7, claystone-siltstone laminae under XPL. f–h) Sample M.13, tuff under XPL, ash matrix with abundant brown kerogen (f), phenocrystals of < 1 mm of plagioclase in pumice (g) showing vesicles (h). i–j) Sample M.14, lapilli tuff, glass, and ash matrix with phenocrystals of > 3 mm of plagioclase under PPL and XPL, respectively. k–l) Sample M.17, siltstone-tuff laminae under PPL and XPL, respectively. m–n) Sample M.25, lapilli tuff under XPL, matrix compound of ash and clastic fine grains with phenocrystals of quartz, plagioclase and oxides, abundant amorphous (Am) kerogen and structured (St) kerogen, and accretionary lapilli filled with palagonite (Pgte) are observed. o–p) Sample M.27, medium- to coarse-grained tuffaceous litharenite under XPL, poor selected and grain supported matrix with sub-rounded to rounded quartz, plagioclase, lithic fragments in welded ash altered to clay minerals (o), structured kerogen (p) grains are abundant as well as amorphous.

Figure 6
Geochemistry plots a) Total alkali vs. silica (TAS) compositional plot (based on Irvine & Baragar 1971, Middlemost 1994). b) Tectonic setting plot, where X = 0.303 − 0.0447SiO2 − 0.972TiO2 + 0.008Al2O3 − 0.267Fe2O3 + 0.208FeO − 3.082MnO + 0.14MgO + 0.195CaO + 0.719Na2O − 0.032K2O + 7.51P2O5 and Y = 43.57 − 0.421SiO2 + 1.988TiO2 − 0.526Al2O3 − 0.551Fe2O3 − 1.61FeO + 2.72MnO + 0.881MgO − 0.907CaO − 0.177Na2O − 1.84K2O + 7.244P2O5 (based on Bhatia 1983).

The tectonic setting plot modified from Bhatia (1983) (Fig. 6b) considers both primary and secondary volcaniclastic and the clearly clastic rocks, it shows a provenance characteristic of arc magmatism, corresponding to a sialic continental arc. Most of the samples fall into these fields, but the graywacke sample M.5 with secondary volcaniclastic grains and the Fe₂O₃-rich tuff sample M.14 fall into the passive margin and oceanic island arc fields.

Palynofacies and Palynology

Palynofacies analysis

Three main groups have been identified along the section, palynomorphs (54.2%), AOM/pseudoamorphous (24.8%), and phytoclasts (21.0%). The Cerro Negro Formation section is dominated by palynomorphs, mainly spores (41.6%), followed by phytoclasts translucent (20.4%), and pseudoamorphous (14.7%) components (Fig. 7, SM-2).

Figure 7
Stratigraphical distribution of palynofacies ratio, Principal Component Analysis (palynofacies) (PCA), palynoflora ecological indexes ratio, and paleoenvironmental intervals (Q-mode cluster).

The Principal Component Analysis (PCA) reveals the three palynofacies assemblages in the Cerro Negro Formation that are composed of spores (Principal component 1, PC1), pollen grains (PC2), and freshwater algae/AOM (PC3) (Fig. 7). They represent the dominant organic elements; the multivariate model explains 76% of the total variance of the dataset. The vertical distribution of palynofacies assemblages and PCA in the section is presented in Fig. 7. Supporting data on loading, scores, and summary PCA are given in SM-2. The PCA data illustrates the variability of each organic group in the samples analyzed. As revealed in a biplot scatter plot, the samples represent two different groups (square and circle) according to their location in the section (Fig. 8). We observed a separation between the group of palynomorphs and phytoclasts, mostly in the quadrants on the right, associated with the samples M.1 to M.14, and the group of AOM (quadrants on the left) (samples M.15 to M.27) (Fig. 8). The variability and depositional distinction of the groups are confirmed by the chi-square test, being significant for spores (X² = 194.9; p<0.0001), pollen grains (X² = 15.2; p<0.0001) and AOM + pseudoamorphous (X² = 2046.2; p<0.0001).

’ 8
PCA Biplot scatter plot for palynofacies and samples from the Cerro Negro Formation. In squares (samples M.1 to M.14), and circles (samples M.15 to M.27).

According to the application of the Q-mode cluster for the palynofacies distribution, and corroborated with PCA data and ecological indexes, it was possible to recognize two palynofacies assemblages (P1 and P2, Fig. 7) which represent different depositional paleoenvironments for the Cerro Negro Formation.

The Palynofacies assemblage 1 (P1) is recognized in the lower-middle portion of the studied section, between samples 1 to 14 (Fig. 7). This palynofacies is dominated by the palynomorphs (66%), with spores (52%; p<0.0001) and pollen grains (14%; p<0.0001); and the phytoclasts (19%) (Fig. 7; SM-2). There is an increase of Botryococcus (freshwater algae) to the top, samples 8 to 13 (8.4% to 11.4%), with the exception of sample 11 (3,4%). Pseudoamorphous particles occurs in low percentages in the P1, except for sample 13 (11.45%), and AOM has a high percentage value in sample 4 (44.1%).

The Palynofacies assemblage 2 (P2) is identified among samples 15 to 27, in the middle-upper portion of the Cerro Negro Formation section. In this interval, AOM/Pseudoamorphous particles (43.1%) are dominant (p<0.0001), the palynomorphs decrease significantly (spores with 29.6%; pollen grains with 2.9%; p<0.0001). The phytoclast group is 21.6%, similarly to Palynofacies assemblage 1, where it is 19%. (Fig. 7), and Botryococcus occurs in samples 15 to 20 (2% to 8.4%) in the base of the P2. The chi-square test, in comparison of proportion between intervals, is not significant for phytoclasts (X² = 0.018; p = 0.89) or freshwater algae (X² = 1.53; p = 0.21). In this interval, the maximum distribution of the PC3 (Freshwater algae/AOM) is inversely correlated with those of the assemblage of spores and pollen grains (PC1 and PC2) (Fig. 7).

Palynological analysis

A total of 43 taxa were identified in Snow Island, including spores, pollen grains, freshwater algae and fungi (Figs. 9–11; SM-1 and SM-3). Ferns, lycophytes and bryophytes are the most abundant with 33 species, followed by conifers with seven species, and others palynomorphs with three elements (algae, fungi and indeterminate palynomorph) (SM-1). Rarefaction curve indicates that several palynomorphs species were recovered and that the sampling was enough (Fig. 12).

Figure 9
Spores recovered from the Cerro Negro Formation, President Head Peninsula, Snow Island: a–b) Aequitriradites superspinulosus, M.12, EF (R27/3), b) fluorescence mode, c) Antulsporites baculatus. M.2, EF (Z32/3), d–e) Baculatisporites comaumensis, M.7, EF (W28/1), e) fluorescence mode, f) Biretisporites sp., M.2, EF (F36/3), g) Camarozonosporites insignis, M.14, EF (U29/2), h) Ceratosporites equalis, M.1, EF (U28/2), i) Cibotiumspora jurienensis, M.3, EF (R27/2), j) Cicatricosisporites sp., M.7, EF (G29/3), k) Converrucosisporites sp., M.6, EF (X33/2), l) Cyathidites australis, M.14, EF (B29/1). Scale bar 20 µm. England Finder (EF).

Figure 10
Spores recovered from the Cerro Negro Formation, President Head Peninsula, Snow Island: a) Deltoidospora hallii, M.2, EF (L32/2), b) Densoisporites microrugulatus, M.6, EF (C37/2), c) Foraminisporis asymmetricus, M.1, EF (E33/3), d) F. wonthaggiensis, M.1, EF (X27/2), e) Gleicheniidites senonicus, M.1, EF (T26/4), f) Ischyosporites sp., M.1, EF (B27/1), g) Leiotriletes sp., M.14, EF (O29/2), h-i) Muricingulisporis annulatus, M.5, EF (W32/1), i) fluorescence mode, j) Ornamentifera sp., M.1, EF (H36/1), k) Polycingulatisporites sp., M.2, EF (F25/3), l) Psilatriletes radiatus, M.1, EF (P32/4). Scale bar 20 µm.

Figure 11
Spores, pollen grains, algae, and SOM recovered from the Cerro Negro Formation, President Head Peninsula, Snow Island. Spores: a) Sotasporites elegans, M.14, EF (S41/3), b) S. triangularis, M.2, EF (O32/3), c) Undulatisporites pannuceus, M.14, EF (S29/4). Pollen grains: d) Alisporites bilateralis, M.1, EF (A36/3), e) Araucariacites sp., M.1, EF (T26/2), f) Bisaccate, M.14, EF (N29/2), g) Podocarpidites sp., M.14, EF (O38/1), h) Vitreisporites sp., M.14, EF (Q34/1). Algae: i) Botryococcus sp., M.14, EF (H33/1). SOM: j) AOM, M.15, EF (X33/2), k) Phytoclast translucent, M.15, EF (V36/4), l) Pseudoamorphous, M.15, EF (C37/2). Scale bar 20 µm.

Figure 12
Rarefaction curve with 95% confidence intervals of the palynofloristic species from the Snow Island.

Smooth spores included within the suprageneric classification Triletes are the most abundant (47%), and display a high value of standard deviation (sd=23.7), indicating that there is a large variation in the whole section. Despite this, they are dominant in samples 24 to 26. As Cantrill (2000) recorded, in our study many of the smooth spores (Triletes group) specimens could belong to Cyathidites, furthemore, bisaccate pollen grains occurs in great percentage but are poorly preserved.

Based on the Principal Component Analysis (PCA) for the palynoflora, it was possible to distinguish three assemblages for Snow Island: Ischyosporites (principal component 1, PC1), Podocarpidites spp. (PC2) and Deltoidospora hallii (PC3). The multivariate model explains 51.45% of the total variance of the dataset (Table II).

Table II
Principal Component (PC) number, dominant palynomorphs, and associated species with loadings and explained variance in percentage of total variance are given.

PCA	Dominant palynomorphs	Loadings (Load)	Variance %	Associated species	Load
PC1	Ischyosporites spp.	0.32	23.97	Foraminisporis wontahggiensisCyathidites australisCeratosporites equalisBiretisporites spp. Foveotriletes spp. Araucariacites spp. Gleicheniidites senonicus	0.31 0.30 0.27 0.27 0.26 0.26 0.25
PC2	Podocarpidites spp	0.28	15.75	Camarozonosporites insignisCallialasporites segmentatus Bisaccates pollen grains Leiotriletes spp. Baculatisporites comaumensis Alisporites bilaterales Vitreisporites spp.	0.26 0.26 0.26 0.25 0.25 0.25 0.23
PC3	Deltoidospora hallii	0.39	11.73	Sotasporites triangularis Antusporites baculatus Cyathidites minor	0.37 0.32 0.28

The Ischyosporites assemblage (PC1) accounts for 23.97% of the total variance of the dataset. This assemblage includes Foraminisporis wonthaggiensis, Cyathidites australis, Ceratosporites equalis, Biretisporites spp., Foveotriletes spp., Araucariacites spp., and Gleicheniidites senonicus (Table II). Notably, there is a dominance of spores.

The Podocarpidites spp. assemblage dominance (PC2) explains 15.75% of the total variance, also including Camarozonosporites insignis, Callialasporites segmentatus, Bisaccates pollen grains, Leiotriletes spp., Baculatisporites comaumensis, Alisporites biateralis and Vitreisporites spp. (Table II). In PC2 there is a predominance of conifer pollen grains associated with spores. The palynoflora assemblages (PC1 and PC2) together explain 39.72% of the total variance in the section, occurring mainly in interval P1 of Snow Island (Table II). The PC3 is less representative, showing a variance of 11.73% (Table II).

The palynoflora of the Snow Island has mean values of 0.35 for dominance (D), 1.63 for diversity H(S), and 0.65 for equity (E). The lowest mean values calculated for D (0.11 to 0.29) appear in interval P1, and the maximum values above the mean occur in samples 24 to 26 for interval P2, as 0.59 to 0.77 (e.g., Triletes) (Fig. 7; SM-1). The diversity H(S) and equity E both have a general increase of mean values for interval P1, as H = 1.64 to 2.59 and E = 0.70 to 0.88, respectively. These results are positively correlated with the highest record of spores (PC1) and pollen grains (PC2) for interval P1. The H(S) and E generally have low values in the interval P2 (Fig. 7).

DISCUSSION

Paleoenvironmental inferences

The paleoflora of South Gondwana contains widely distributed taxa during the Cretaceous, with a dominance of ferns, conifers and the invasion of angiosperms (Cantrill & Poole 2012). Torres et al. (1995) and Césari et al. (2001) suggest for the Cerro Negro Formation on Livingston Island (South Shetland Islands, Antarctica) a diversity of ferns as indicated by the macroflora compressions, and that the vegetation was influenced by different local habitats. Also, Falcon-Lang & Cantrill (2002) based on megaflora taphonocoenoses analyses defined three dominant-plants communities: podocarp-araucarian-fern forests at lowland floodplains; podocarp-bennettite forests of mid-altitude volcanic cones, and shrubby bennettite-fern, developed during sustained periods of high frequency basaltic ash falls.

The dispersion of organic matter has been used as a proxy for environmental inferences (e.g., Oboh-Ikuenobe et al. 2005, Quattrocchio et al. 2006, Mendonça Filho et al. 2010, Carvalho et al. 2013, Santos et al. 2013). In our study, a varied distribution of constituents of kerogen were observed, and two palynofacies were recognized according to the depositional environment. For this purpose, a ternary plot of the main palynofacies elements registered in each sample studied is given in Fig. 13.

Figure 13
Ternary plot AOM-Phytoclast-Palynomorph based on relative frequency kerogen from Snow Island (Adapted from Tyson 1993, 1995).

The variations in the stratigraphic distribution of the two palynofacies assemblages reflect changes during deposition of the Cerro Negro Formation on Snow Island (Figs. 7; 13). The paleoenvironment recognized by palynofacies assemblage 1 indicates a predominance of palynomorphs, represented by sporomorphs, mainly spores of ferns (Figs. 9–11). This assemblage is similar to the macroflora recorded in the Early Cretaceous of the South Shetland Island, Antarctica, for example Osmundaceae and Gleicheniaceae families (Torres et al. 1995, Cantrill 2000, Césari et al. 2001). Fern spores (e.g., Ischyosporites, Cyathidites and Deltoidospora) and bryophyte spores (e.g., Aequitriradites and Foraminisporis) observed in the Cerro Negro Formation at Snow Island are very common genera in humid climates, because the fertilization of this plants requires water availability (Tyson 1993, Mendonça Filho et al. 2011, Carvalho et al. 2017) and they are mainly dispersed by water. Gymnosperms (e.g., Podocarpidites) occur secondarily in this assemblage suggesting the presence of highlands in the Snow Island region during the Early Cretaceous. Regions with a high percentage of plant spores from total kerogen, suggests an oxidized environment, close to the source of production and redeposition in areas of active fluvio-deltaic origin (Tyson 1993, Mendonça Filho et al. 2011, Carvalho et al. 2013). The oxidized environment is also confirmed in our material (Palynofacies Assemblage 1) by the low occurrence of AOM. The sedimentological and geochemical data support a flooded plain to lacustrine depositional environment associated with Palynofacies 1 interval. Medium- to coarse-grained, moderately sorted graywacke with low angle cross-lamination indicate deposition of reworked volcanoclastic material and thus is not considered a primary volcaniclastic deposit. According to Miall (2006), this type of psammitic facies is deposited within moderate energy fluvial channels that could represent crevasse splays within a flooded overbank transitory to full lacustrine conditions interpreted from fine-grained quartzarenite intercalated with the organic-rich and laminated siltstone to claystone facies. The presence of desiccation cracks suggests that the floodplain was exposed to subaerial conditions. This facies association can be correlated to those from Cerro Negro Formation at Byers Peninsula in Livingston Island described by Hathway (1997) as sandstones, granule-pebble conglomerates and mudstones grouped in the basaltic sandstone facies that indicates deposition from suspension and initially high-energy, waning flows in bodies of standing water and lacustrine turbidite successions. Falcon-Lang & Cantrill (2002) interpreted their sandstone-dominated and laminated mudstone facies as having been deposited on low-land fluvial floodplains within half-graben and deposits of large permanent lakes formed during periods when basin subsidence outpaced volcanic sedimentation.

From the ~ 4 m upwards the section, the mudstone-siltstone predominant facies are in part substituted by abundant volcaniclastic content. Primary volcaniclastic materials are directly emitted from a vent and transported through air, water, granular debris or the combination these (White & Houghton 2006). The petrographic investigations of the rocks evidence a high content of pyroclastic fragments that mainly resulted as a direct action of volcanic activity without reworking by sedimentary processes. Moreover, the high percentage of pyroclasts and the rhyodacite content indicate an explosive volcanism. Modern silicic, explosive, steep-sided volcanoes are characterized by ephemeral, high-magnitude eruptions every few hundred years, followed by long periods of quiescence (Cas & Wright 1987). Welded ash and lapilli tuffs recognized in the upper interval of the studied section, are considered as primary pyroclastic fall out into standing ponds between mudstone-siltstone accumulations on an overbank. At Byers Peninsula, Livingston Island, the Cerro Negro Formation follows a middle Valanginian to earliest Aptian erosional hiatus considered to record tectonic or volcano-tectonic uplift, perhaps related to thickening of arc crust by magmatic processes Hathway (1997). At Byers Peninsula, punctuated silicic eruptions in the lower unit of Cerro Negro Formation are suggested by the intercalation of fluvial-lacustrine strata with cool fall/flow pyroclastic deposits (Hathway 1997, Falcon-Lang & Cantrill 2002). The cumulative synsedimentary volcanism was preserved by inter-eruption fluvial-lacustrine deposition favored by differential subsidence within a fault-bounded intra-arc basin (Hathway 1997). The presence of tuffaceous litharenite at the top of our studied section is suggestive of redeposition of unconsolidated tephra by debris flows and flood flows on the volcaniclastic apron, as interpreted by Hathway (1997) at Byers Peninsula, deposition occurred from high-energy, subaerial, waning flows, probably representing overbank flood deposits from adjacent sediment-charged streams.

The ternary graph (Fig. 13) clearly shows the change in paleoenvironment (palynofacies assemblage 2, P2) indicated by high values of AOM/pseudoamorphous material and a decrease of palynomorphs, specially represented by sporomorphs. AOM suggests, even temporarily, conditions of environments with low oxygenation (Mendonça Filho et al. 2011), and environments with low energy (Carvalho et al. 2013). The amorphized material from Snow Island appears to come from aquatic algae, as Botryococcus species. This palynofacies presents the pseudo-amorphized material from the biodegradation of plant fragments (Tyson 1995), easily recognized by the diffuse outline of phytoclasts and absence of fluorescence (Fig. 11). According to Carvalho et al. (2013), this pseudo-amorphization may be related to the oxidation process during particle transport.

In both palynofacies assemblages we observed the occurrence, even non-dominant, of freshwater algae and fungi spores mainly in the transition from palynofacies assemblage 1 to palynofacies assemblage 2. This type of microplankton (Botryococcus), according to Mendonça Filho et al. (2011), has been found in lagoon, lake, fluvial and delta facies, or in a freshwater input redeposition area (Tyson 1993, 1995). Therefore, the paleoenvironmental conditions for the Cerro Negro Formation, in the Snow Island, are associated with a system influenced by water bodies. Additionally, we recorded the palynomorphs and elements of SOM are very damaged, possibly due to volcanic influence, as previously pointed out by Cantrill (2000), analyzing cuticular material in the same region.

The whole-rock compositions from the pyroclastic rocks mostly plot in the subalkaline rhyodacite fields of the Total alkali vs. silica (TAS) diagram (Fig. 6a based on Irvine & Baragar 1971, Middlemost 1994), thus corroborating our petrographic interpretation as originated from silicic magmatism and the correlation of the President Head studied interval with the lower unit of Cerro Negro Formation at Byers Peninsula. As with the volcanic rocks, the major element geochemistry of sediments can be used to infer the provenance differences that depend upon the tectonic setting of the volcanism and of the ancient sedimentary basins (e.g., Bhatia 1983). Tectonic plate interactions govern the isostatic movements and composition of source areas, as well as the position of the basin within the plate or the plate boundary and four broad tectonic settings for continental and oceanic basins are recognized: oceanic island arc, continental island arc, active continental margin and passive margins (Bhatia & Crook 1986). The melts that rise to feed the volcanism typical of island arcs and active continental margins are generated by the interaction of the subduction zone and the asthenosphere of the mantle (Frisch et al. 2011). Continental island arc and the active continental margin correspond to tectonic settings where oceanic lithosphere is subducted beneath continental lithosphere with the difference that the island arc type is separated from the continental lithosphere by a marine basin behind the volcanic arc; rather, the arc of the active margin is built directly on the adjacent continent (Frisch et al. 2011). Based on this information, we modified the tectonic setting plot of Bhatia (1983) (Fig. 6b) joining the continental island arc and active continental margin fields in one field called continental arc. Most of our samples, including both clastic and volcaniclastic are plotted within the continental arc field, leading to corroborate that the non-marine volcaniclastic Cerro Negro Formation at President Head Peninsula have their provenance of magmatism founded on a sialic basement as proposed by Smellie et al. (1984) for the South Shetland Islands.

The integrated palynological, sedimentological and geochemical data suggests an initial trend deposition in proximal oxide fluvial-lacustrine environment for Cerro Negro Formation in Snow Island, followed by volcaniclastic strata. According to the paleoenvironmental change observed in our data, the intense volcanism on Snow Island played a significant role in the biological history, and the collapse of the flora.

Age based on palynology

The paleoflora of the Byers Group, recorded at the Byers Peninsula, Rugged Island, and at President Head, Snow Island, Antarctica, had previously been suggested to be from the Middle Jurassic (Fuenzalida et al. 1972) to the Early Cretaceous (Philippe et al. 1995, Torres et al., 1995, 1997a, b, Cantrill 1998, 2000, Césari et al. 1998, 1999). Palynological studies of the Jurassic marine sediments (Duane 1997) and those from the Cretaceous continental deposits (Torres et al. 1997a) are supported by radiometric studies from Byers Peninsula (Hathway et al. 1999) indicating an Aptian age for this sequence.

The palynoflora from Snow and Livingston islands, and President Head was correlated with Australia and South American sections, mainly with ones from the Baqueró Formation of Santa Cruz Province, Argentina, in strata no older than Valanginian (Duane 1996, 1997, Torres et al. 1997a). Interulobites pseudoreticulatus, Appendicisporites, Foraminisporis wonthaggiensis, F. asymmetricus allow correlation with the Interulobites–Foraminisporis Zone and the lower part of the tectifera–corrugatus Zone in South America and suggests the correlation with the early–late Aptian Cyclosporites hughesii Interval Zone of Australia (Hathway et al. 1999).

The age of the Cerro Negro Formation in Livingston Island was estimated to be 120.3 ±2.2 Ma, 119.4 ±0.6 Ma and 119.1±0.8 Ma using the ⁴⁰Ar/³⁹Ar method (Hathway 1997, Hathway et al. 1999), early Aptian. According to Ugalde et al. (2013), in a study of two groups of U-Pb zircon indicates an absolute age of 116.53 ±0.79 Ma and 109.0 ±1.1 Ma, suggesting deposition on President Head during the late Aptian.

In our study, the palynoflora demonstrates richness in ferns and conifers, that matches with the palynological record of Duane (1996), Torres et al. (1997b), Cantrill (2000), and also shows similarities with Antarctic macroflora (Philippe et al. 1995, Torres et al. 1997b, Cantrill 2000).

The Early Cretaceous geographic distribution of Sotasporites and Muricingulisporis is restricted to Patagonia and surrounding basins, including Antarctica (Archangelsky & Archangelsky 2006). Therefore, the co-occurrence of spores in this study, such as Muricingulisporis annulatus, Sotasporites elegans, S. triangularis, can be correlated with those occurring in the Austral Basin in Patagonia, Argentina (Archangelsky & Archangelsky 2006). Other taxa, such as Foraminisporis wonthaggiensis e F. asymmetricus can be compared to the Australian biozones F. wonthaggiensis and Cyclosporites hughesii (Helby et al. 1987, Hathway et al. 1999), suggesting that the deposition of the Cerro Negro Formation in Snow Island occurred during the Aptian.

Supplementary material

Dataset is available on Zenodo repository (https://zenodo.org/) under the DOI https://doi.org/10.5281/zenodo.5140569.

ACKNOWLEDGMENTS

This study was supported by the Programa Antártico Brasileiro - PROANTAR (CNPq # 407670/2013, 442677/2018-9 to AWAK). The team of the PALEOANTAR Project wants to thank the NaPo Almirante Maximiano and HU-1 helicopter squadron military group for safely transporting personal and equipment from Brazil to Antarctica. The alpinists Ricardo Leizer and Luiz Carlos assisted in the deployment of the PALEOANTAR research team as well as in the maintenance of the 28-day camp at the XXXV Antarctic Operation. Ricardo Leizer is the author of the photographs used in figure 2. Michele Moraes of the MEDIANTAR Project shared a camping with our team and assisted in the collection and transportation of the material used here. AS is grateful to Cecília Amenábar for the discussions on the identifications of the palynoflora. We would like to thank additional support from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) to JMS (#311715/2017-6) and AWAK (#313461/2018-0); Fundação de Desenvolvimento Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) to AWAK (#E-26/202.905/2018) and Pró- Reitoria de Pesquisa e Pós-Graduação of Universidade Federal de Pernambuco PROPESQ-UFPE for fellowship to JMS and EKP.

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Publication Dates

• Publication in this collection
  18 Feb 2022
• Date of issue
  2022

History

• Received
  20 Dec 2020
• Accepted
  14 Aug 2021