Calcimicrobial-archaeocyath-bearing clasts from marine slope deposits of the Cambrian Mount Wegener Formation, Coats Land, Shackleton Range, Antarctica
Clastos con calcimicrobios y arqueociatos procedentes de depósitos marinos del talud de la Formación cámbrica del Monte Wegener, Coats Land, Cordillera de Shackleton Antártida

M. Rodríguez-Martínez¹, A. Perejón¹, E. Moreno-Eiris¹, S. Menéndez², W. Buggisch³

¹Universidad Complutense de Madrid, Departamento de Geodinámica, Estratigrafía y Paleontología, Madrid, Spain. Email: martarm@geo.ucm.es, aparqueo@ucm.es, eiris@geo.ucm.es; ORCID ID: http://orcid.org/0000-0002-4363-5562, http://orcid.org/0000-0002-6552-0416, http://orcid.org/0000-0003-2250-4093

²Museo Geominero, Instituto Geológico y Minero de España (IGME), Ríos Rosas, 23, 28003 Madrid, Spain. Email: s.menendez@igme.es; ORCID ID: Silvia Menéndez: http://orcid.org/0000-0001-6074-9601

³GeoZentrum Nordbayern. Friedrich-Alexander-University of Erlangen-Nürnberg (FAU). Schlossgarten 5, 91054 Erlangen, Germany.

Introduction and geological settingTOP

The Bruce’s Scottish National Antarctic Expedition 1902-1904 sampled the deep sea sediments of the Weddell Sea, Antarctica. Later on, Pirie (1913) first mentioned about the presence of “Archaeocyathinae” from these erratic Cambrian samples. Throughout the twentieth century, different expeditions collected archaeocyaths in the dispersed Cambrian record of the Antarctica continent. The studies of the autochthonous archaeocyathan assemblages from the lower Cambrian Shackleton Limestone (Transantarctic Mountains, Hill, 1964a; Debrenne & Kruse, 1986, 1989), Miaolingian Nelson Limestone (Pensacola Mountains, Debrenne & Kruse, 1989; Wood et al., 1992) and Furongian Minaret Formation (Ellsworth Mountains, Debrenne et al., 1984) have been essential to infer the likely sources of the abundant and dispersed record of allochthonous archaeocyathan assemblages within Palaeozoic conglomerates as the Whiteout Conglomerates (Ellsworth Mountains, Debrenne, 1992) to Cenozoic deposits as the Polonez Cave and Cape Melville Formations (King George Island, Morycowa et al., 1982; Wrona & Zhuravlev, 1996), deep sea gravels (Weddell Sea, Gordon, 1920), and glacial tills and moraines from the Whichaway Nunataks (Hill 1965; Debrenne & Kruse, 1989), Shackleton Range (Stephenson Bastion and Du Toit Nunataks, Höfle & Buggisch, 1995) and the Transantarctic Mountains (Hill, 1964b). In fact, the presence of archaeocyath-bearing clasts in other Palaeozoic records, with unknown Cambrian carbonate platforms, has allowed palaeobiogeographic comparisons between the Antarctica assemblages and others exotic archaeocyaths from South Gondwana localities (Fig. 1), such as the Falkland Islands (Fitzroy Tillite Formation, Stone et al., 2012), South Africa (Dwyka Group, Debrenne, 1975), Namibia (Dwyka Group, Perejón et al., 2019), and Argentina (El Jagüelito and Sauce Grande Formations González et al., 2011, 2013).

The occurrence of archaeocyath-bearing clasts as Cenozoic glacial erratics and as Cambrian marine slope conglomerates of the Cambrian Mount Wegener Formation at the Shackleton Range (Fig. 2) was firstly reported by Buggisch et al. (1994a), Höfle & Buggisch (1995) and Buggisch & Henjes-Kunst (1999). The Shackleton Range at the Coats Land region of Antarctica is bounded by the Slessor and Recovery glaciers which drain the East Antarctic Ice Sheet into the Weddell Sea (Fig. 2B). The lithostratigraphic framework of the Shackleton Range has been established after the outstanding work done by a number of geological field expeditions (see Kleinschmidt, 2007) in spite of the great disconnection between outcrops and its complex tectonic structure. In the Shackleton Range, the dispersed rock outcrops mainly consist of medium-high grade amphibolite facies of the Proterozoic Shackleton Range Metamorphic Complex (Clarkson, 1972) and diverse Proterozoic-Cambrian metasedimentary and metavolcanic rocks, where infracrustal and supracrustal rocks are tectonically interleaved. The remaining stratigraphic record, Ordovician red beds and molasses, Permian tillites, Jurassic tuffites and dolerites and Cenozoic tills and moraines occur more or less disconnected into the different tectonostratigraphic units developed during the Ross Orogeny (Buggisch et al. 1994b; Buggisch & Kleinschmidt, 1999). In general, the Shackleton Range is interpreted as a collisional orogen as a result of the final amalgamation between East and West Gondwana during late Precambrian-Cambrian times (Tessensohn et al., 1999). The Cambrian Mount Wegener Formation is forming part of the Mount Wegener Nappe (Fig. 2C), whose transport to the south and the low-grade metamorphic overprint were around 490 Ma as a result of Ross Orogeny (Buggisch et al., 1994b).

Microfacies analysis of carbonate clastsTOP

The analyzed carbonate clasts from the Cambrian marine slope deposits were collected from metaconglomerates and boulders of the Mount Wegener Formation at the southern part of the Read Mountains, from the Swinnerton Ledge, Oldhamia and Trueman Terraces (Fig. 2D) during the EUROSHACK expedition (1993-1994). A total number of 42 thin sections have been analyzed with sedimentological, palaeontological and diagenetic purposes to reconstruct the development and destruction of calcimicrobial-archaeocyath boundstones. The carbonate clasts are accessory to primary components in polymict sandy metaconglomerates and polymict breccias from the Mount Wegener Formation which was deposited in slope to basin environments. The carbonate clasts correspond to dolostones and limestones with varied microfacies as silty/sandy mudstone, bioclastic to intraclastic wackestone/packstone, common algal and mud peloidal silty packstone/grainstone, rare ooidal and oncolitic grainstone. The bioclastic to intraclastic wackestone/packstone shows a rich fossil assemblage with trilobites, brachiopods, echinoderms, archaeocyaths and undetermined skeletal remains. Calcimicrobes belonging to Epiphyton, Renalcis and Girvanella groups and archaeocyaths are conspicuous constituents in limestone clasts, forming calcimicrobial boundstone, calcimicrobial-archaeocyathan boundstone and archaeocyathan cementstone. The initial study of pore-filling and fracture cements points out to meteoric to marine-phreatic up to burial diagenesis. The observed low grade tectonically induced fabrics (achaeocyaths tectonically elongated, mechanical twinning in calcite cements, cataclasite fabrics) were produced during the final emplacement and southwards transport of the Mount Wegener Nappe. The preliminary data from the palaeontological study shows a rich and diverse archaeocyathan assemblage with more than twenty-one genera identified. The 74% of genera belong to the Ajacicyathina Suborder, 9% to Archaeocyathina, 6% to Erismacoscinina, 5% to Loculicyathina, 4% to Dokidocyathina, and 1% to Monocyathida and Putapacyathida Orders.

Concluding remarksTOP

Prof. Dr. Werner Buggisch deceased on 6.04.2019 after a long illness. In the year 2016 we asked Professor Buggisch about the material from the Shackleton Range and about the possibility of studying the calcimicrobial-archaeocyath-bearing clasts. Professor Buggisch responded enthusiastically and sent us all the material he had. Unfortunately, although he was aware of our advances, he died before seeing the first results published. Between 1979 and 1995 he participated in four Antarctic expeditions, thus this work and future publications happen thanks to his tremendous work in Shackleton Range. A brief profile of his work can be seen on the website of the GeoZentrum Nordbayern (https://www.gzn.nat.fau.de/2019/04/09/das-geozentrum-trauert-um-prof-dr-w-buggisch-verstorben-am-6-04-2019/)

ReferencesTOP