Marinoan glaciation in east central Brazil

Abstract

Remnants of a Neoproterozoic glaciation in east central Brazil are represented by thin diamictite layers (Jequitaí Formation and correlative units), locally overlying striated pavements on the São Francisco craton. The diamictites are covered by the Sete Lagoas Formation of the basal Bambuí Group, which is generally accepted to be a typical cap carbonate sequence. Although most authors have preferred a mid-Cryogenian (post-Sturtian) age for it, based mainly on Pb-Pb whole rock data, the Sete Lagoas Formation bears lithostratigraphic and isotopic characteristics that are identical to early Ediacaran cap carbonates worldwide, including a basal thin (0–10m) pale and flinty cap dolostone, preserving a drop in δ13C values from around −3.2‰ to −4.5‰ with associated δ18O around −5‰, and crystal-fan facies interpreted as aragonite pseudomorphs. Ediacaran zircons have been recovered from the middle of the Sete Lagoas Formation, constraining the deposition of its upper half to be younger than 610Ma (Rodrigues, 2008). Although there is an unconformity below the point where the zircons were collected, it is short-lived, as suggested by the identical, typically Ediacaran 87Sr/86Sr values above and below (0.7074–0.7076). Carbonate clasts from the Jequitaí Formation and correlative diamictite-bearing units in the fold belts that surround the São Francisco craton (Canabravinha and Serra do Catuni formations) display similar ranges in δ13C (−6.7 to +2.6‰), suggesting the erosion of a pre-glacial carbonate platform with negative δ13C values (i.e. the Islay and/or Trezona anomalies). The cratonic Carrancas Formation, on the other hand, yielded pale dolostone clasts with δ13C in a small range between −4.2 and −3.4‰, and δ18O values around −6.5‰. These clasts could be derived from the cap dolostone unit itself, in which case the Carrancas Formation would represent resedimented basal Sete Lagoas Formation and imply that sections of the Sete Lagoas Formation sitting atop the Carrancas Formation are incomplete. The base-truncated sections have confused previous attempts to correlate the Sete Lagoas Formation with other cap carbonate successions. In light of the available lithostratigraphic, isotopic and U-Pb zircon data, we propose that the Sete Lagoas Formation represents a basal Ediacaran cap carbonate sequence (∼635–610Ma) deposited after the Marinoan glaciation in east central Brazil.