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).