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The Ediacaran-Early Cambrian Fossil Record in Southwest Gondwana 20 The Ediacaran-Early Cambrian Fossil Record in Southwest Gondwana 20 Claudio Gaucher Abstract 20.1 Introduction A review of the Ediacaran-Early Cambrian fossil record in SW Gondwana is presented. Organic-walled microfossils SW-Gondwana includes several sedimentary successions of (acritarchs) reported from the region can be assigned for predominantly Late Ediacaran age (Figs. 20.1 and 20.2), fl the most part to the Late Ediacaran Leiosphere Palyno ora which preserve a rich fossil record. Early studies of these (LELP), with only a few occurrences of lower Ediacaran fossils include in southern Africa the reports by Gürich acritarchs. So far, complex acanthomorphs (ECAP assem- (1930, 1933) of vendobionts (Pteridinium, Rangea), which blage) are absent in fossil-bearing units of lower to were some of the first descriptions of Ediacaran soft-bodied mid-Ediacaran age, which may be the result of an adverse fossils. paleoclimate. The Ediacara soft-bodied biota is best In South America, Beurlen and Sommer (1957) described recorded in the Nama Group, with occurrences recently the first skeletal fossils from Precambrian rocks, although reported in South America. Biostratigraphically, they were assigned a Cambrian age. Later, Germs (1972a, b) soft-bodied fossils are best assigned to the latest Ediacaran erected the genus Cloudina and accepted a Neoproterozoic Nama Association. Ediacaran shelly fossils are wide- age for the fossils, representing the first Precambrian skeletal spread in SW Gondwana, including biostratigraphically fossils to be described worldwide. relevant genera such as Cloudina, Namacalathus, Corum- This chapter deals with the latest advances in Ediacaran bella and Titanotheca. The only group with a record paleontology and biostratigraphy in southwestern Gond- – across the Ediacaran Cambrian boundary are trace fossils. wana. Four main fossil groups will be dealt with: acritarchs, Whereas simple, mostly bedding-parallel burrows and the Ediacara soft-bodied biota, skeletonized metazoans and “ ” biomat structures occur in Ediacaran matgrounds, protists, and trace fossils. Ediacaran-Early Cambrian meta- diverse vertical bioturbations characterize Early Cambrian phytes (vendotaenids) also occur in SW Gondwana, but “ ” mixgrounds. Ediacaran fossils can be useful for bios- because of the paucity of their record the reader is directed to tratigraphy. The early and Mid-Ediacaran are best zoned the few studies dealing with these fossils (Grant et al. 1991; using acritarchs, the Late Ediacaran with shelly fossils and Gaucher et al. 2003; Cohen et al. 2009). the Ediacaran–Cambrian boundary can be determined using trace fossils. Biostratigraphy, in turn, enables the correlation of distant sedimentary successions, which may 20.2 Acritarchs have paleogeographic implications. Ediacaran paleobio- geography, on the other hand, is hampered by the Ediacaran acritarchs (i.e., organic-walled microfossils of cosmopolitan nature of most taxa found in SW Gondwana. uncertain affinity) were first described in SW Gondwana from the Nama Group in Namibia (Germs 1972b; Germs et al. Keywords 1986) and from the Sierras Bayas Group in Argentina (Pothe Acritarchs Á Shelly fossils Á Ediacara biota de Baldis et al. 1983; Cingolani et al. 1991; Fig. 20.2). Cambrian explosion Á Biostratigraphy Acritarchs were thereafter reported for other units in the region (Figs. 20.1 and 20.2), such as the Corumbá Group in C. Gaucher (&) í Facultad de Ciencias, Instituto de Ciencias Geológicas, SW Brazil (Zaine 1991; Gaucher et al. 2003), the Bambu Universidad de La República, Iguá 4225, 11400 Montevideo, Group in central Brazil (Fairchild et al. 1996;Sánchez, Uruguay 2010), the Arroyo del Soldado Group in Uruguay (Gaucher e-mail: [email protected] © Springer International Publishing AG, part of Springer Nature 2018 543 S. Siegesmund et al. (eds.), Geology of Southwest Gondwana, Regional Geology Reviews, https://doi.org/10.1007/978-3-319-68920-3_20 544 C. Gaucher Fig. 20.1 Fossiliferous Ediacaran units indicated on a pre-drift Belt. Ediacaran fossiliferous units are indicated in italics: ASG Arroyo reassembly of Gondwana, modified after Porada (1989). PP Parana- del Soldado Group (and Las Ventanas Formation); BG Bambuí Group; panema Craton; RPC Río de la Plata Craton; SFC São Francisco CCG Cango Caves and Gamtoos groups; CG Corumbá Group; IG Craton. Red areas: Neoproterozoic to Early Paleozoic belts. Itapucumí Group; MG Mulden Group; NG Nama Group; OG Otavi (1) Western Sierras Pampeanas, (2) Eastern Sierras Pampeanas, Group; PF Puncoviscana Formation; SBG Sierras Bayas and La (3) Southern Paraguay Belt, (4) Northern Paraguay Belt, (5) Tandilia Providencia groups; U&JB Ubajara and Jaibaras groups. Rectangle: System, (6) Saldania Belt, (7) Gariep Belt, (8) Damara Belt, (9) Kaoko see detail in Fig. 20.2 Belt, (10) Ribeira Belt, (11) Aracuaí Belt, (12) Lufilian Belt, (13) Zambezi et al. 1996, 1998, 2003, 2004; Gaucher 2000, 2014; Gaucher Early Cambrian acritarchs have not been found in SW and Poiré 2009), the Eleutério-Pico de Itapeva and Pouso Gondwana because the sparse sedimentary record of that age Alegre basins (SE Brazil; Teixeira and Gaucher 2004), the is either continental or very shallow marine, such as the Holgat Formation in Namibia (Gaucher et al. 2005b) and the upper Nama Group (Germs 1995) and the upper Arroyo del Cango Caves and Gamtoos groups in the Saldania Belt, Soldado Group (Sprechmann et al. 2004). Early Cambrian South Africa (Gaucher and Germs 2006). shales of the Puncoviscana Formation in NW Argentina More recently, acritarchs were described for the Las were deposited in deeper settings, but so far no acritarchs Ventanas Formation in Uruguay (Blanco and Gaucher 2005; have been found in these strata. Gaucher et al. 2008a), the Otavi and Mulden groups in Biostratigraphically, almost all of the known occurrences Namibia (Germs et al. 2009), the Alicia Formation of the La belong to the “Kotlin-Rovno” assemblage of Vidal and Providencia Group in Argentina (Poiré et al. 2010; Arrouy Moczydłowska-Vidal (1997), synonymous with the Late et al. 2015) and the Frecheirinha Formation of the Ubajara Ediacaran Leiosphere Palynoflora (LELP) of Gaucher and Group, NE Brazil (Chiglino et al. 2015; Fig. 20.1). Sprechmann (2009). 20 The Ediacaran-Early Cambrian Fossil Record … 545 Fig. 20.2 Map showing the location of fossiliferous Ediacaran units dealt with in this work, modified after Gaucher et al. (2005b) 546 C. Gaucher The low-diversity LELP assemblage is dominated by Interestingly, no occurrences of morphologically complex small (<150 lm) and predominantly thin-walled Leio- acanthomorphs belonging to the ECAP have been found so sphaeridia (Fig. 20.3c), Bavlinella faveolata (Fig. 20.3d, e), far in SW Gondwana. The ECAP is recorded in Australia colonial microfossils of the genus Soldadophycus (Grey 2005), Baltica (Vorob’eva et al. 2009), in the upper (Fig. 20.3f), small (<20 lm) acanthomorphs of the genus Doushantuo Formation in China (McFadden et al. 2008; Liu Asteridium (=Micrhystridium) and large sphaeromorphs of et al. 2013; Fig. 20.4) and in Siberia (Moczydłowska 2005), the genus Chuaria (Germs et al. 1986; Steiner 1994). and it probably represents a relatively short period, possibly The LELP represents a depauperate assemblage which between the Gaskiers Glaciation (580 Ma; Pu et al. 2016) characterizes the last 20 myr of the Neoproterozoic (c. 560– and c. 560 Ma. This entire period is characterized by posi- 541 Ma), just before the Cambrian Explosion. It follows an tive d13C values known in the Doushantuo Formation as EP2 extinction event that wiped out the diverse acritarchs of the excursion (Liu et al. 2013; Fig. 20.4). Ediacaran Complex Acanthomorph Palynoflora (ECAP; Deposition of the rift succession of the Las Ventanas Grey et al. 2003; Grey 2005). Between 75 and 90% of Formation in Uruguay is radiometrically constrained by the acritarch species became extinct during this event (Vidal and U–Pb SHRIMP method between 590 ± 2 Ma for basalts at Moczydłowska-Vidal 1997; Knoll et al. 2006), which may its base (Mallmann et al. 2007) and 573 ± 11 Ma for vol- have been caused by ocean eutrophication and the exclusion canoclastics at its top (Oyhantçabal et al. 2009). Glaciogenic of eukaryotic plankton by massive cyanobacterial blooms diamictites occur near the base of the unit and were assigned (e.g., Bavlinella faveolata blooms; Gaucher and Sprechmann to the Gaskiers Glaciation (Gaucher et al. 2008a). Thus the 2009). Las Ventanas Formation encompasses the time period in Fig. 20.3 Ediacaran acritarchs from SW Gondwana. a Leiosphaeridia Yerbal Formation, Arroyo del Soldado Group (Gaucher 2000). tenuissima, large specimen from the Villa Mónica Formation, Sierras e Bavlinella faveolata, Kombuis Member, Cango Caves Group Bayas Group (Gaucher et al. 2005a). b Leiosphaeridia tenuissima, Las (Gaucher and Germs 2006). f Soldadophycus bossii,a Ventanas Formation (Gaucher et al. 2008a). c Leiosphaeridia minutis- saucer-shaped colony attached to a smaller, sphaeroidal colony. sima, two attached carbonized specimens from the Alicia Formation, Yerbal Formation (Arroyo del Soldado Group; Gaucher et al. 2004). La Providencia Group. d Bavlinella faveolata, thin section from the White scale bars represent 10 lm and black scale bars 50 lm 20 The Ediacaran-Early Cambrian Fossil Record … 547 Fig. 20.4 Correlation chart between different Ediacaran-Cambrian Tandilia: C. Lar Cerro Largo; Olav. Olavarría; L. Negra Loma Negra. successions in SW Gonwdana with South China as a reference, Arroyo del
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