Stromatolites and associated structures in the cap carbonate from Mirassol D\'Oeste Formation, Araras Group, Paraguay Belt (Neoproterozoic, MT)

Author(s)

Guilherme Raffaeli Romero

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Instituto de Geociências (IG/BT)
• Defense date: 2010-09-22

Examining board members

Setembrino Petri; Paulo Cesar Boggiani; Afonso Cesar Rodrigues Nogueira

• Advisor: Setembrino Petri

Abstract

Neoproterozoic cap carbonates are of great importance because they formed during a period of singular paleoclimatic and evolutionary changes, whose origin and influences upon subsequent geological and evolutionary history have yet to be unraveled. This dissertation sought to comprehend part of this story through the study of stromatolitic sedimentation associated with the cap carbonate represented by the Mirassol DOeste Formation (base of the Araras Group), deposited about 635 Ma ago, immediately following the Marinoan glaciation, represented by the Puga Formation. This research was carried out at Mirassol DOeste, Mato Grosso, on the southwest border of the Amazon craton next to the Paraguai fold belt. Meso and macroscopic characteristics of stromatolites and associated sedimentological features (tubular structures, megaripples, megapeloids) were studied in outcrop, cut specimens and petrographic thin sections. Two meters above the base of the formation a 10 m-thick succession of dolomitic microbial boundstones begins, characterized throughout by alternating thin and thick laminae made up of peloids (interpreted as the micritized remains of colonial microorganisms), subordinate dolomicrite, and fenestrae. They make up laterally continuous and morphologically simple stromatolites. Stratiform stromatolites occur throughout the succession, with irregularly scattered meter-sized domes till about two to three meters from the top. Above this point, very irregularly undulated, commonly asymmetric, decimeter-sized stromatolites predominate, and these, in turn, are covered by megaripple-marked dolomitic peloidal grainstones-packstones with millimetric megapeloids. Stromatolitic sedimentation ceased in the previously calm platform environment of the Mirassol DOeste Formation when wave action began to rework bottom sediments as extremely intense storms reached the locale with the rise in sea level. Narrow (<3 cm), vertical tubular structures of decimetric length and filled by massive dolosparite cut stromatolitic sediments, principally in the central portions of domal forms. These structures appear to have formed by the percolation of gas and/or liquids derived from the decomposition of organic material in the microbial mats. (AU)