An in situ shelly fauna from the lower Paleozoic Zapla diamictite of northwestern Argentina: Implications for the age of glacial events across Gondwana

Abstract

A shelly fauna from the upper part of the Zapla glacial diamictite includes the lingulate brachiopod Orbiculoidea radiata Troedsson, the rhynchonelliforms Dalmanella cf. testudinaria (Dalman) and Paromalomena sp., the bivalve Modiolopsis? sp., and the trilobite Dalmanitina subandina Monaldi and Boso. Both taphonomic and paleoecologic data indicate a lack of transport reflecting the original community. The assemblage is closely comparable to the widespread latest Ordovician Hirnantia-Dalmanitina fauna. The Hirnantian age of the Zapla diamictite is further corroborated by the record of the northern Gondwana chitinozoans Spinachitina cf. oulebsiri Paris and Desmochitina minor typica Eisenack. The graptolites and chitinozoans from the overlying Lipe on Formation indicate that the postglacial transgression took place in the earliest Llandovery (Parakidograptus acuminatus Biozone). According to the tectonosedimentary evidence, the Early Silurian age of the Cancañiri and San Gab an diamictites of north ecentral Bolivia and south Peru based on their palynological record is more likely the age of posglacial gravity flows and not that of the glaciation. We support the hypothesis that the weakly lithified glacigenic deposits of Hirnantian age were reworked and redistributed by high-energy marine processes during the postglacial transgression and then transported to the adjacent deep-marine trough. Iron-rich horizons have been recognized in many basins of southern South America reflecting eustatic and paleoclimatic fluctuations. Most of them formed during the early stages of the postglacial transgression at the Ordovician/Silurian transition and are associated with low sedimentation rates and condensed intervals. The mild maritime postglacial climate, the increasing atmospheric CO 2 , and possibly the presence of incipient vegetated areas led to extensive weathering of glacigenic sediments supplying iron into the marine system to form ferruginous deposits. The sea level fall related to the peak of glaciation is recorded by both paleovalley incision and a sharp subaerial to subglacial unconformity. The transgressive systems tract starts with fluvio-estuarine deposits within incised valleys followed by widespread deposition of subtidal to open marine organic-rich shales onlapping regionally the basement rocks. The recognition of key stratigraphic markers (e.g. sequence boundary, flooding surface, ferruginous beds), alongside reliable micro and macropaleontological evidence allow a more accurate correlation between the Central Andean Basin of Peru, Bolivia and NW Argentina, the W Puna region, the Paraguayan and Brazilian sectors of the Paran a Basin, the Precordillera Basin of W Argentina, and the Cape Basin of South Africa.

Latest Ordovician-earliest Silurian chitinozoans from the Puna region, north-western Argentina (Western Gondwana)

“…It is overlain by transgressive deposits of the Silurian Lipeón Formation (Turner 1960). In the eastern part of the basin, chitinozoan assemblages document a late Hirnantian age for the glacial deposits and support a basal Silurian age assignment for the strata representing the postglacial sequences in a condensed section (de la Puente et al 2012, de la Puente & Rubinstein 2013, Benedetto et al 2015). In the central part of the basin, chitinozoan assemblages document a Hirnantian age for the glacial deposits and a Telychian age for the postglacial sequences (Rubinstein et al 2016).…”

“…The southern end of this basin is located in north-western Argentina and its outcrops are exposed in the geological provinces of Puna, Cordillera Oriental, Sierras Subandinas and Sistema de Santa Bárbara (Fig. 1A) (Ramos 1999). In the eastern and central part of the basin, the Upper Ordovician is classically represented by the "Zapla Glacial Horizon" (Schlagintweit 1943) of late Hirnantian age (Monaldi & Boso 1987, de la Puente & Rubinstein 2013, Benedetto et al 2015, which is locally called the Mecoyita Formation (Turner 1960) or Caspalá Formation (Starck 1995). This glacial horizon is deposited above the Ocloyic regional discordance, which separates it from the Lower and Middle Ordovician strata.…”

Stratigraphy and facies analysis of the La Chilca Formation, Central Precordillera: Insights on the postglacial Ordovician–Silurian boundary and Early Silurian deposits from Argentina

“…Similar deposits corresponding to the Ordovician–Silurian (O‐S) boundary have been recognized on many continents from Gondwana, such as Africa, Europe, and Asia (Arnaud & Etienne, ; Benedetto et al, ; Chatalov, ; Hambrey, ; El‐ghali, ; Young, Minter, & Theron, ) related to the O/S boundary. However, until now, there has been no mention of this deposit association from the Central Precordillera.…”

“…(b) Palaeogeographical map of the latest Ordovician (Cocks & Torsvik, ). The Precordillera Terrane of north‐western Argentina has Cambrian and Earliest Ordovician faunas of Laurentian affinity, as reviewed by Benedetto, Sánchez, Carrera, Brussa, and Salas () and Benedetto, Halpern, de la Puente, and Monaldi (). Whether it then formed an integral part of Laurentia or was peri‐Laurentian is controversial, but before the Middle Ordovician, the endemic faunas indicate that it had become a separate terrane within the Iapetus Ocean.…”

Tracking the isostatic and eustatic signals in an end Ordovician–early Silurian deglacial sedimentary record (Algeria – Libya border)

“…H) known from Hirnantia fauna of the Late Hirnantian of Czech Republic and Argentina (Křiž & Steinov, ; Benedetto et al ., ) with subordinate brachiopods (10%) represented by the rhynchonellid Plectothyrella aff. libyca (Hirnantia fauna, Hirnantian in age – Massa, ; Benedetto et al ., ). Firmgrounds show a variety of bioturbations, some of which were identified as Taedinium, Scolicia and Tomaculuum .…”