Fácies e estratigrafia da formação sepotuba: registro da última incursão marinha na transição neoproterozóico-cambriano da faixa Paraguai Norte, Mato Grosso

Abstract

Anomalous events of terminal Neoproterozoic have been mostly reported in the cap carbonates exposed in several cratonic regions worldwide. The records of these events in siliciclastics deposits are still incipient. In this study a siliciclastic succession was investigated that overlies carbonates deposited after Marinoan glaciation (630 Ma) in the southern Amazon Craton and Paraguai Belt, state of Mato Grosso. In this context, part of the sedimentary history contained in the shales and sandstones of the Sepotuba Formation was suppressed from stratigraphic proposals for that area. The Sepotuba Formation is rescued here a stratigraphic which is fundamental to understand this evolutionary history, consequent of the facies and stratigraphic analysis carried out in this work. The Sepotuba Formation unconformably overlies the Raizama Formation, and is gradually overlaid by Diamantino Formation, composing the Alto Paraguai Group. The upper part of the Raizama Formation is composed of: 1) amalgamated beds of fine to medium-grained sandstone, with swaley and hummocky cross-stratifications, wave truncated and even parallel laminations, interpreted as storm influenced littoral deposits; 2) Coarsening upward cycles composed of wavy-bedded rithymite and concave cross-bedded sandstone, interpreted as tidal bars; and 3) siltstone, medium to coarse-grained sandstone with dissecation cracks, ripples marks and evaporites molds related to intertidal deposits. The Sepotuba Formation consists in: 1) pelite and fine to medium-grained sandstone with ripple marks, even parallel lamination, hummocky and swaley stratifications and tidal bundles, interpreted as lower shoreface deposits; e 2) siltstone, pelite and fine to medium-grained sandstone with ripple marks, hummocky cross stratification, even parallel and low-angle cross laminations and climbing ripple-cross lamination that represent offshore deposits. The base of Diamantino Formation comprises: 1) siltstone and claystone with wavy bedding, convolute and even parallel laminations related to lacustrine and/or seaway environment; and 2) clast-supported conglomerate and medium to coarse-grained sandstone with even parallel stratification, through, planar and concave cross stratification, considered as braided river deposits. The Sepotuba Formation records a transgression that reworked littoral deposits of Raizama Formation. This event is the last marine incursion after the Marinoan global glaciation (630 Ma) that preceded the oceanic enclosure associated with the collision of Amazonia and Rio de La Plata blocks during the Pampean-Araguaia orogeny (520-500 Ma). The continuity of the collisional processes resulted in the individualization of foredeep sub-basins, confining the Sepotuba sea and leading to lakes and/or seaways formations that were supplied by fluvial to deltaic sediments (Diamantino Formation), derived from uplifted areas in the collisional blocks, during the final stages of foreland evolution of Paraguai belt.