Marinoan glaciation in east central Brazil

作者：Caxito Fabricio de Andrade^*; Halverson Galen P; Uhlein Alexandre; Stevenson Ross; Dias Tatiana Goncalves; Uhlein Gabriel J

来源：Precambrian Research, 2012, 200: 38-58.

DOI：10.1016/j.precamres.2012.01.005

• Neoproterozoic
• Isotope stratigraphy
• Marinoan glaciation
• Sao Francisco craton
• Bambui Group
• Cap carbonate

Remnants of a Neoproterozoic glaciation in east central Brazil are represented by thin diamictite layers (Jequitai Formation and correlative units), locally overlying striated pavements on the Sao Francisco craton. The diamictites are covered by the Sete Lagoas Formation of the basal Bambui 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 (0-10 m) pale and flinty cap dolostone, preserving a drop in delta C-13 values from around -3.2 parts per thousand to -4.5 parts per thousand with associated delta O-18 around -5 parts per thousand, 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 610 Ma (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 Sr-87/Sr-86 values above and below (0.7074-0.7076). Carbonate clasts from the Jequitai Formation and correlative diamictite-bearing units in the fold belts that surround the Sao Francisco craton (Canabravinha and Serra do Catuni formations) display similar ranges in delta C-13 (-6.7 to +2.6 parts per thousand), suggesting the erosion of a pre-glacial carbonate platform with negative delta C-13 values (i.e. the Islay and/or Trezona anomalies). The cratonic Carrancas Formation, on the other hand, yielded pale dolostone clasts with delta C-13 in a small range between -4.2 and -3.4 parts per thousand, and delta O-18 values around -6.5 parts per thousand. 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 (similar to 635-610 Ma) deposited after the Marinoan glaciation in east central Brazil.

• 出版日期2012-4