Preglacial palaeoenvironmental evolution of the Ediacaran Loma Negra Formation, far southwestern Gondwana, Argentina

GÓMEZ-PERAL, LUCÍA E.; KAUFMAN, ALAN J.; ARROUY, MARÍA JULIA; RICHIANO, SEBASTIÁN; SIAL, ALCIDES N.; POIRÉ, DANIEL G.; FERREIRA, VALDEREZ P.

PRECAMBRIAN RESEARCH

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2018 vol. 315 p. 120 - 137

0301-9268

The Ediacaran Loma Negra Formation may be deposited in a platform environment related to the Clymene Ocean. This unit is exposed in the Olavarría and Barkerareas within the Tandilia Basin, and composed entirely of lime mudstones (∼40m thick). The age of the Loma Negra Formation (LNF), the uppermost unit of theSierras Bayas Group, has been debated, but it should be older than 560 Ma (age of the overlying succession) and younger than 600 Ma (age of the underlying iron-richmudstone of the Colombo Formation). This extended carbonate platform (> 100 km) is divided into two sedimentary facies: one related to the basal section, in whichlime mud originated by suspension fall-out in an open-marine ramp (reddish or greenish limestones), and the other facies represented by black to dark greylimestones from the middle and upper sections. The LNF limestones studied, obtained from five sections near the Olavarría and Barker areas, reveal constant curves ofpositive δ13C values (∼+2 to +4%), similar to those recorded in other middle-Ediacaran marine proxies worldwide. Our sedimentological and geochemical study ofthe LNF sections shows extraordinarily consistent carbon and oxygen isotope trends that may be useful for detailed intrabasinal correlations. Samples of the LNFfabric-retentive micrite preserve an unusually narrow range of oxygen isotope compositions, which is consistent with potential climatic variation, considering themulti-proxies retained. A subsequent decrease in carbonate primary productivity and an increase in siliciclastic supply are associated with the negative δ13C trend inthe overlying unit, at the base of the La Providencia Group. This negative excursion in δ13C from +2.8 to−3.6% is also regarded as a response to the cooling and sealevelfall probably linked to the Gaskiers glaciation. Furthermore, such data are consistent with late-Ediacaran carbonates from other Neoproterozoic successions.Strontium isotope ratios in limestones (0.7060?0.7070) should reflect the near-primary strontium isotope composition of the original seawater, particularlyregarding the strontium content (∼300 ppm) and the Rb/Sr ratios (≤0.001) of primary micrite, consistent with a seawater origin. Strontium isotope data reaches0.7082 with an Rb/Sr ratio of 0.001?0.006.