OR/14/040 Lafonia Group

Stone, P. 2014. Recent contributions on Falkland Islands bedrock geology, with an inventory of representative lithostratigraphical specimens held by the British Geological Survey. British Geological Survey Internal Report, OR/14/040.

Lithostratigraphy and palaeoenvironment

The Lafonia Group was described in detail by Trewin and others (2002) in a publication that incorporated work carried out as part of a PhD project by Thomas (2001) at Aberdeen University. The contrary view of Meadows (1999) notwithstanding, a very close comparison was drawn between the Lafonia Group and part of the South African, Karoo Supergroup succession which it was felt strengthened the rotational model of the Falkland Islands’ origins at the eastern margin of the Karoo Basin. Six points of close lithostratigraphical similarity were stressed.

• The glacigenic diamictities (Fitzroy and Dwyka tillites) are of identical appearance and lithofacies, and have compatible palaeocurrents.
• The glacial beds are overlain by organic-rich black mudstone.
• K-bentonites occur in the basin-floor successions.
• Interbedded basin-floor turbidite sandstone and rhythmite have a distinctive ichnofauna (see Biostratigraphy and palaeontology).
• Coarsening-up delta-front sequences are overlain by fining-up channel sandstone units.
• Sandstones in both areas have a common provenance in a contemporaneous volcanic arc. Trewin and others (2002) stress that in their reconstruction marine conditions would not be expected to occur above the level of the Fitzroy Tillite Formation, and that post-glacial sedimentation was essentially lacustrine.

The glacigenic Fitzroy Tillite Formation and the overlying Hells Kitchen Member at the base of the Port Sussex Formation were fortuitously sampled during mineral exploration drilling in East Falkland between 2005 and 2007. Borehole core illustrating the transitional ‘deglaciation’ succession was recovered back to UK (Stone 2011) and formed the basis of a M.Sc. research project at Cambridge University (Horan 2013), which was subsequently developed for publication (Horan in press). The research demonstrated that the Late Carboniferous to Early Permian ‘Icehouse to Greenhouse’ transition in the Falkland Islands was accompanied by cyclical waxing and waning of the Gondwanan, south polar ice sheet. The lithologies and sedimentary fabrics record a switch from deposition under a grounded ice sheet to glacio-lacustrine or glacio-marine deposition punctuated by minor episodes of ice advance and retreat during a period of net ice sheet retreat. X-Ray Fluorescence and reflectance data were used to quantify the change in terms of geochemical and geophysical properties respectively. Elements including zirconium, manganese, chromium, iron and titanium helped to constrain the cyclicity. Wavelet and spectral analyses demonstrated periodicities in the data that were suggestive of orbitally forced oscillations within the transition. This permitted the age modelling of a hypothetical time framework for the series spanning approximately 1.2 million years. The integrated approach of this research, which combined sedimentological data with geochemistry, gave a robust insight into this past climatic transition which may help to evaluate and inform predictions of future climate change.

Geochemical evidence for the non-marine deposition of the Black Rock Member (Port Sussex Formation) which conformably succeeds the Hells Kitchen Member was illustrated by López-Gamundí (2010, figure 8b). Two specimens were shown to have high Total Organic Carbon (13–15%) and low Sulphur content (<2%), a combination thought to be indicative of fresh-water salinity in the depositional environment. López-Gamundí interpreted the boundary between the Fitzroy Tillite Formation and the Port Sussex Formation in sequence stratigraphy terms as a transgressive ravinement surface that initiated a postglacial transgressive systems tract.

Horan’s (in press) comparisons with other Gondwanan glacigenic successions suggest a Sakmarian (Early Permian) age for the transitional Hells Kitchen Member of the Port Sussex Formation. This is compatible with the Late Carboniferous or younger age for the Bluff Cove Formation, the unit immediately below the Fitzroy Tillite, reported by Aldiss and Edwards (1999) from the presence of sparse bisaccate pollen. However, Meadows (1999) refers to Devonian palynomorphs having been discovered during ‘proprietary palynological dating’of the Bluff Cove Formation; no further details are given and reworking might be a possibilty.

Correlation of the Fitzroy Tillite Formation with the tillites of the South African Dwyka Group was further strengthened by the U-Pb dating of detrital zircons. The results were presented by Craddock and Thomas (2011) at a symposium in South Africa, but have not yet been formally published. The Fitzroy Tillite sample was described as ‘massive tillite’ collected ‘west of Port Stanley’. Its zircon population (96 analysed zircons) showed major age peaks at around 550 Ma and 1100 Ma with minor contributions in the ranges 600 to 900 Ma and 2000 to 2700 Ma. These results were very closely matched by those from the Dwyka tillites of the Eastern Cape Province, South Africa.

Biostratigraphy and palaeontology

An exotic contribution to Falkland Islands palaeontology followed the discovery of clasts of fossiliferous limestone (ranging up to 45 cm in maximum dimension) in the uppermost Carboniferous to Lower Permian Fitzroy Tillite Formation at Port Purvis and east of Hill Cove, both West Falkland, with smaller and rarer clasts found at Frying Pan Quarry, East Falkland. The limestone proved to contain a rich Early Cambrian (probably Botomian) fauna of archaeocyaths (Figure 3c), together with a radiocyath and a few trilobites, which has been described by Stone and Thomson (2005) and Stone and others (2012). Neither Cambrian strata nor limestone are present in the Falklands rock succession and the clasts are regarded as exotic erratics, introduced during the Permo-Carboniferous glaciation of southern Gondwana, prior to its Mesozoic break-up. Nineteen archaeocyath taxa have been identified, with seven (plus a radiocyath) occurring in a single clast. Trilobite identifications are less definitive but they are compared to Yorkella and the Siberian genera Edelsteinaspis, Namanoia and Chondrinouyina. The archaeocyath fauna has an Australo-Antarctic character and the Transantarctic Mountains seem the most likely source for these unusual erratics despite recent, tantalising archaeocyath discoveries in Argentina (González and others 2012), the first records from South America. In those reconstructions of Gondwana that rotate a Falklands microplate, it is moved into a position between South Africa and East Antarctica, in proximity with the Eastern Cape Province and the ‘Atlantic’ end of the Transantarctic Mountains. Tillites within the Permo-Carboniferous Dwyka Group of the Eastern Cape Province are correlatives of the Fitzroy Tillite Formation and also contain rare clasts of archaeocyathan limestone with a fauna similar to that seen in the Falkland Islands examples. The rotational reconstruction also produces a continuity of the apparent ice-flow directions in South Africa and the Falkland Islands.

Figure 3 Some recent fossil discoveries from the Falkland Islands:
a.  Bryozoa encrusting a snail shell from the Devonian Fox Bay Formation, Pebble Island, West Falkland. BGS image number P511902.
b.  A starfish-like ophiuroid from the Devonian Fox Bay Formation, Fish Creek, Berkeley Sound, East Falkland. BGS image number P727089.
c.  Early Cambrian archaeocyaths shown in a thin section from an erratic clast of limestone in the Early Permian Fitzroy Tillite Formation, Hill Cove, West Falkland. BGS image number P537735.
d.  Bivalve shells resembling Palaeanodonta sp. aff. P dubia from the Brenton Loch Formation at Rory’s Creek, Victoria Harbour, East Falkland. BGS image number P573118.

The figured material from Stone and others (2012) has been lodged in The Natural History Museum, London. Additional samples of archaeocyathan limestone from the same localities have been incorporated into the collections of the British Geological Survey with registration details listed below. The numbers prefixed LX are the BGS accession numbers. The numbers prefixed PS are the working, field numbers that allow correlation with the clasts from which the figured specimens were derived and which contain the formally described faunas as held by The Natural History Museum. For ease of comparison, the details of the NHM specimens, as listed in Stone (2011) and in Stone and others (2012) are also included in this report. (see Appendix 2 – Correlation of working, image and NHM catalogue numbers for the archaeocyath-bearing limestone clasts).

Frying Pan Quarry, East Falkland: [Lat. 51º 49’ South; Long. 58º 20’ West].
LX 1004-1 to 5 (= PS 303-1 to 5). Note that archeaocyaths have only been positively identified in LX1004-1 and 2; the limestone clasts numbered LX1004-3 to 5 may not be fossiliferous.

Old House Rocks, East Falkland: [Lat. 51º 45’ South; Long. 58º 53’ West].
LX1006-19. Borehole core specimen containing a clast of cleaved and deformed limestone with possible traces of archaeocyaths.

Fox Point West (east of Hill Cove), West Falkland: [Lat. 51º 29’ South; Long. 60º 04’ West].
LX1008-1 to 5 (= representative pieces from the five clasts PS304-4 to 304–8).
LX1073a, b (= two pieces from clast PS304-2)
LX1074a to e (= five pieces from clast PS304-3)
LX1075a to e (= five pieces from clast PS304-5)
LX1076 (= one piece from clast PS304-6)
LX1077a to f (= six pieces from clast PS304-7)
LX1078a to s (= nineteen pieces from clast PS304-8)

The fish-generated trace fossil Undichna had been noted from several localities within the outcrop of the Lafonia Group by Aldiss and Edwards (1999), but was the subject of an in-depth study by Trewin (2000). Several different species were described from the Brenton Loch Formation at Camilla Creek, 5 km north of Darwin (Figure 4), and from Sea Lion Island, both East Falkland, with some representative specimens now held by the Department of Geology and Petroleum Geology, University of Aberdeen. Trewin considered the environment of the traces’ creation to be non-marine, and noted the close similarity of the Falklands ichnofauna to that of the Permian succession in the Eastern Cape Province, South Africa.

Figure 4 The trace fossil Undichna from the Brenton Loch Formation, Camilla Creek, East Falkland.

A non-marine depositional environment for the Brenton Loch Formation, and its South African association, was also supported by the discovery of a sparse bivalve fauna at Rory’s Creek, on the southern shore of Choiseul Sound, East Falkland. The first specimen was recorded by Stone and Rushton (2003), and was described jointly with two subsequent small collections by Simões and others (2012) . Prior to the discovery of the bivalves the only animal fossil known from the entire Lafonia Group was a remarkably preserved damselfly wing, found amidst the widespread Glossopteris flora by Halle during the Swedish Magellanic Expedition and described by Tillyard (1928); it is now held by the Natur Historiska Riksmuseet in Stockholm. The bivalves are associated with trace fossils and were collected from a bed in the upper part of the Brenton Loch Formation, within a 25 cm thick interval of dark siltstones and mudstones with planar lamination, overlain by more massive sandstones.The shells are commonly articulated, with the valves either splayed open (Figure 3d) or closed. At the top of the succession, mudstone beds nearly 1.5 m above the bivalve-bearing layers contain well-preserved Glossopteris sp. cf. G. communis leaf fossils. The closed articulated condition of some shells indicates preservation under high sedimentation rates with low residence time of bioclasts at the sediment/water interface. However, the presence of specimens with splayed shells is usually correlated to the slow decay of the shell ligament in oxygen-deficient bottom waters. The presence of complete carbonized leaves of Glossopteris close to the bivalve-bearing bed also suggests a possibly dysoxic-anoxic bottom environment with the bivalves preserved by abrupt burial, possibly by distal sediment flows into a lake.They are most probably autochthonous to parautochthonous fossil accumulations. The shells resemble those of anthracosiids and were assigned by Simões and others (2012) to Palaeanodonta sp. aff. P. dubia, a non-marine species also found in the Permian succession of the Karoo Basin, South Africa and thought to be indicative of a Middle Permian (Capitanian) age. Specimens are held by the British Geological Survey with the registered numbers LX1010-1 to LX1010-6; other specimens are held by the University of São Paulo, Brazil, under the prefix GP.