Figure 1. a, Location map of the Antarctic Peninsula.b, Main outcrop localities of the Jurassic marine (Ameghino/Nordenskjöld Formation) and terrestrial beds (Mount Flora Formation /Botany Bay Group) in the northern part of the Antarctic Peninsula.
2 Stratigraphy and geological setting
The Mount Flora Formation, together with its correlative Camp Hill and Tower Peak formations, are grouped under the non-marine Botany Bay Group, which is predominantly composed of basement-derived conglomerates exposed in discontinuous outcrops along the Antarctic Peninsula (Farquharson, 1984; Hathway 2000). The conglomeratic deposits are correlated on the basis of their lithology and stratigraphy; they were presumably accumulated in small, fault-bounded grabens as a product of debris flow and braided stream deposition in alluvial fans (Farquharson 1984). The Botany Bay Group unconformably overlies the (meta-) clastic sediments of the Trinity Peninsula Group (e.g., Hope Bay Formation) and is overlain by the volcanic and pyroclastic rocks of the Antarctic Peninsula Volcanic Group (e.g., Kenney Glacier and Mapple formations) (Fig. 2). Although early paleobotanical investigations suggested a Middle Jurassic age for the Botany Bay Group, a Late Jurassic to Early Cretaceous age was later adopted by many based on stratigraphic arguments and correlation to marine fossil-bearing strata (see Farquharson 1984). Limited U–Pb zircon geochronology by the in situ secondary ion mass spectroscopy (SIMS) technique on the interbedded tuffs of the Botany Bay Group and the overlying volcanics from the Hope Bay, Botany Bay and Tower Peak outcrops (Fig. 1b) have yielded Middle to Late Jurassic ages ranging from 168.9 ± 1.3 Ma (2σ internal uncertainties) to 162.2 ± 1.1 Ma (Pankhurst et al., 2000; Hunter et al., 2005). These radioisotopic dates have formed the basis of correlation of the Botany Bay and Antarctic Peninsula Volcanic groups to the presumed equivalent units throughout the Patagonian region of South America (Pankhurst et al., 2000).
The Mount Flora Formation (Caminos and Massabie, 1980) was originally studied by Andersson (1906), and later by Bibby (1966), Elliot and Gracanin (1983), Farquharson (1984), Birkenmajer (1993), Montes et al. (2005) and Montes et al. (2019), with its fossil flora described originally by Halle (1913) and more recently by Birkenmajer and Ociepa (2008). It is a terrestrial clastic sedimentary unit in excess of 300 m in thickness, exposed to the SE of Hope Bay near the tip of the Antarctic Peninsula (Fig. 1a, b). The Mount Flora Formation is separated by an angular unconformity from the underlying Hope Bay Formation of Late Paleozoic/Triassic age and is overlain by the Kenney Glacier Formation (Birkenmajer, 1993), which is equivalent to the Mapple Formation of Riley and Leat (1999). A Late Jurassic U-Pb SIMS age of 162.2 ± 1.1 Ma for a volcanic rock at Hope Bay (Pankhurst et al., 2000), whose exact location has not been documented, most likely belongs to the several-hundred-meter-thick Kenney Glacier Formation.
The Mount Flora succession is a fining-upward terrestrial clastic succession initially deposited as alluvial fans on a fault-controlled morphology. We follow here the two-fold lithostratigraphic subdivision of the Mount Flora Formation proposed by Montes et al. (2019). The stratigraphically lower member (Miembro Conglomerados), about 260 m thick, is composed of clast-supported conglomerates with large, rounded blocks of the Trinity Peninsula Group. Most beds show normal grading and may transition upward to thin, discontinuous sandstone beds. Towards the upper part of this member (140 – 190 meters above base) a ca. 50 m thick tuffaceous interval composed of light-coloured, hard beds is readily recognized. This interval is predominated by ash tuffs, lapilli tuffs, ignimbrites, and volcanic breccias. The upper member (Miembro Areniscas) consists of about 100 m of sandstones, thin-bedded conglomerates, siltstones and shales in beds up to 3 m thick. The conglomerates contain variable quantities of volcanic clasts, which are absent from those in the lower member below the tuffaceous interval. Sandstones may show cross-bedding and shales are well-laminated and may contain numerous, well-preserved, plant remains. Detailed facies analysis (Elliot and Gracanin 1983; Birkenmajer 1993) suggest the succession represents a retrograding alluvial fan, where fluvio-deltaic and lacustrine beds overlie the coarse alluvial fan facies. The whole Mount Flora Formation contains abundant plant remains. Large logs are common at the top of the conglomerate beds, whereas small branches and leaf fragments in the sandstones. The well-preserved plants, together with rare fossil bivalves, beetles and fish remains, were recovered from the lacustrine shales in the uppermost part of the unit and in blocks of recent moraine deposits (Andersson, 1906; Halle 1913; Rees and Cleal, 2004; Birkenmajer and Ociepa, 2008, Martínez et al., 2019). The latter probably eroded from the shale horizon and scattered along the NE pathway of the Flora Glacier (Martínez et al., 2019; Fig. 1).