Carnets de Géologie / Notebooks on Geology - Memoir 2005/02, Abstract 09 (CG2005_M02/09) Ordovician to Lower Silurian palynomorphs from the Sierras subandinas (Subandean ranges), northwestern Argentina: a preliminary report. [Palynomorphes de l'Ordovicien et du Silurien inférieur des Sierras subandinas ("Chaînes sub-andines"), nord-ouest de l'Argentine : rapport préliminaire] Claudia Viviana RUBINSTEIN 1 Key Words: Ordovician; Silurian; palynomorphs; biostratigraphy; palaeogeography; palaeoenvironment RUBINSTEIN C.V. (2005).- Ordovician to Lower Silurian palynomorphs from the Sierras subandinas (Subandean ranges), northwestern Argentina: a preliminary report. In: STEEMANS P. & JAVAUX E. (eds.), Pre-Cambrian to Palaeozoic Palaeopalynology and Palaeobotany.- Carnets de Géologie / Notebooks on Geology, Brest, Memoir 2005/02, Abstract 09 (CG2005_M02/09) Mots-Clefs : Ordovicien ; Silurien ; palynomorphes ; biostratigraphie ; paléogéographie ; paléoenvironnement Introduction Member and the early-middle Llanvirn Lagunillas Member; the Capillas Formation, of Ordovician clastic sediments of the Central late Llanvirn-basal Caradoc age; the Centinela Andean Basin, northwestern Argentina, were Formation, considered to be not older than late deposited in a proto-Andean foreland basin on Caradoc, and the Zapla Formation of Hirnantian the western margin of Gondwana (ASTINI, 2003; age. The uppermost Ordovician is overlain by ASTINI & MARENGO, 2003). The Sierras the Silurian Lipeón Formation that initiates a Subandinas (Subandean Ranges) represent the separate supercycle (ASTINI & MARENGO, 2003; outermost exposures of this Ordovician basin, ASTINI et alii, 2003). where proximal environmental settings and This work analyses the succession of their related palynological content are currently palynological assemblages throughout the being analysed. These platform facies are Ordovician and lower Silurian, and attempts to exposed in the Eastern Cordillera, where determine the biostratigraphic control of the palynological knowledge has increased units, their palaeogeographic affinities, and the remarkably in recent years (RUBINSTEIN & TORO, palaeoenvironmental changes related to 2001; RUBINSTEIN, 2003). On the other hand recurrent fluctuations in sea level. palynological information on the deep marine facies exposed in the Puna is still meager and Palynological results involves only the Ordovician-Silurian boundary (RUBINSTEIN & VACCARI, 2004) (Pl. 1). All units involved in this study yielded palynomorph assemblages that include The outermost portion of the foreland basin, acritarchs and related marine forms, the Sierras Subandinas, is characterized by chitinozoans and cryptospores (Pl. 1). Their alternating shallow-marine deltaic systems and abundance, diversity and preservation range estuarine environments, which indicate widely in accordance with stratigraphic level repeated changes in the coast line caused by and sedimentary facies. fluctuations in relative sea-level (ASTINI & MARENGO, 2003) (Pl. 1). These authors Detailed descriptions and illustrations of the undertook a detailed study of sequence palynological assemblages from all units stratigraphy in the Ordovician-earliest Silurian discussed herein are currently in progress. rocks of this region. However, determination of their age is difficult because of the scarcity in The Zanjón Formation is characterized by some units of biostratigraphically useful fossils. repeated tidal parasequences that shallow- upward into heterolithic and muddy intervals. It Ordovician and earliest Silurian rocks are yielded a rich acritarch assemblage containing well exposed along the Río Capillas, in the Striatotheca principalis parva BURMANN 1970, Sierra de Zapla, Jujuy Province (Fig. 1). The Arbusculidium filamentosum (VAVRDOVÁ) lowest unit is the Zanjón Formation, considered VAVRDOVÁ 1972 emend. FATKA et BROCKE 1999, as middle-late Arenig. It is followed in Aureotesta clathrata var. simplex (CRAMER et succession by the Labrado Formation with its alii) emend BROCKE et alii 1998, Cymatiogalea two sub-units, the late Arenig Laja Morada 1 CONICET, Unidad de Paleopalinología, IANIGLA, CRICYT, C.C. 131, 5500 Mendoza (Argentina) [email protected] 51 Carnets de Géologie / Notebooks on Geology - Memoir 2005/02, Abstract 09 (CG2005_M02/09) Figure 1: Location map of the study area, in the Sierras Subandinas, north-west Argentina. Ordovician and Lower Silurian stratigraphy of the Sierra de Zapla, recognized sequences, lithologies and sea-level curve from ASTINI & MARENGO, 2003. granulata VAVRDOVÁ 1966, Cymatiogalea middle Arenig, although stratigraphic messaoudensis JARDINÉ et alii 1974 var. correlation suggests that it includes the late messaoudensis autonym, Rhopaliophora Arenig. palmata (COMBAZ et PENIGUEL) PLAYFORD et MARTIN The lower subdivision of the Labrado 1984, Dactylofusa velifera COCCHIO forma brevis Formation, the Laja Morada Member, records ALBANI 1989, Eisenackidium orientalis RUBINSTEIN exposure during a lowering of relative sea level in RUBINSTEIN et alii, 1999, Coryphidium sp., and indicated by subaerial features and a gradual Vogtlandia sp.. This assemblage can be change toward purple-red colours near the top. correlated with acritarchs of the Acoite Consequently, the only palynomorphs present Formation in the Cordillera Oriental. are leiospheres and other prasinophycean algae Consequently, its age is not younger than such as Cymatiosphaera, all devoid of 52 Carnets de Géologie / Notebooks on Geology - Memoir 2005/02, Abstract 09 (CG2005_M02/09) stratigraphic value. The upper Lagunillas contain terrestrial cryptospores (mainly Member, representing restricted estuarine tetrads). Stratigraphically, the Zapla Formation facies, yielded a poorly diversified assemblage is dated latest Ordovician, probably with some acritarchs such as ? Aremoricanium corresponding to the Hirnantian Glaciation. simplex LOEBLICH & MCADAM 1971, in addition to However, chitinozoans indicate an Aeronian s.l. leiospheres. to early Telychian age (GRAHN & GUTIÉRREZ, 2001). Acritarch assemblages are meager and Transition to the open marine deposits of the not well preserved so they do not provide Capilla Formation is indicated by a maximum accurate stratigraphic control. In addition, flooding surface. This formation yielded a rich transgressive-regressive events frequently and well-preserved acritarch assemblage caused reworking of fossils. Nevertheless, it containing such taxa as Arbusculidium should be noted that no typical Silurian forms filamentosum, Striatotheca spp., Arkonia sp., were found among acritarchs or cryptospores. Dactylofusa cf. D. striatogranulata JARDINÉ et alii Further investigations, particularly on 1974, Ericanthea pollicipes CRAMER et DÍEZ 1977, chitinozoans from the levels under discusssion, Liliosphaeridium cf. L. intermedium (EISENACK) may shed some light on this controversy. PLAYFORD et alii 1995 and Leprotolypa evexa COLBATH 1979. It also contains cryptospores of The Lipeón Formation initiates a new the morphon Dyadospora murusattenuata supercycle with the deposition of oolitic STROTHER et TRAVERSE 1979 sensu STEEMANS et ironstones during the transgression that alii 1996, and Sphaerasaccus glabellus STEEMANS followed the Hirnantian glaciation. These et alii 2000. The latest record of Arbusculidium Silurian deposits correspond to a series of high- filamentosum, independently dated, is probably frequency sea-level fluctuations. In the area lowermost Llanvirn. But other less well age- studied the lowermost levels of the Lipeón constrained records of its occurrence attain the Formation have yielded abundant, diverse and Llanvirn and may reach the Caradoc strata well-preserved palynomorphs. The assemblage (TONGIORGI et alii, 2003). The genus is dominated by acritarchs and prasinophyte Striatotheca may also extend up to the Caradoc algae, but also contains cryptospores and (VECOLI & LE HÉRISSÉ, 2004). The genus Arkonia, chitinozoans. Acritarch species include Domasia and the species Ericanthea pollicipes and trispinosa DOWNIE 1960, Domasia elongata Liliosphaeridium cf. L. intermedium were DOWNIE 1960, Dactylofusa estillis CRAMER et DIEZ recorded from Llanvirn strata (TONGIORGI et alii, 1972, Dactylofusa maranhensis BRITO et SANTOS 2003; VECOLI & LE HÉRISSÉ, 2004) and 1965, Beromia rexroadii WOOD 1996, Dactylofusa striatogranulata, previously known Baiomeniscus camurus LOEBLICH 1970, only in the Ashgill has recently been found in Carminella maplewoodensis CRAMER 1968, the middle Arenig (uppermost Lower Eupoikilofusa striatifera (CRAMER) CRAMER 1970, Ordovician) of Argentina (ACHAB et alii, in among others. Cryptospores are represented by press). Furthermore, Leprotolypa evexa is tetrads and also Imperfectotriletes vavrdovae unknown below the base of the Caradoc (VECOLI (RICHARDSON) STEEMANS et alii 2000 and & LE HÉRISSÉ, 2004). Although there is some Laevolancis chibrikovae STEEMANS et alii 2000. dispersion in the stratigraphic ranges of the The acritarch assemblage shares some species acritarch species, a late Llanvirn-earliest with those described by BULTYNCK & MARTIN Caradoc age assignment for the Capilla (1982) from the lower part of the unit, dated Formation based on the presence of late Llandovery-early Wenlock, and also has Sacabambaspis janvieri (Vertebrata), roughly species in common with acritarch assemblages agrees with the age inferred from the from the Vargas Peña Formation, in Paraguay palynomorph assemblages. (WOOD & MILLER, 1997). The palynomorphs of the Lipeón Formation have particularly