Siliciclastic rocks from the Raizama Formation, a basal unit of the Alto Paraguai Group, from the Ediacaran-Cambrian interval (635-541 Ma), is discontinuously occur distributed along the southern margin of the Amazonian Craton within the Paraguay Fold Belt northern segment, west-central of Brazil, Mato Grosso state. This Group unconformably overlies carbonate shelf deposits of the Araras Group, where evidence of Marinoan glacial event (635 Ma) was recorded. The Alto Paraguai Group represents the final stages of the collision between the Paranapanema and Amazonian blocks, leading to the closure of the Clymene Ocean (540-520 Ma). The Raizama Formation is approximately 570 m of thickness and is composed by mudstone, fine to coarse sandstones, and sandstones with dolomitic cement previously interpreted as fluvial-coastal deposits distributed in the lower member (270 m) and upper member (300 m). The facies and stratigraphic studies of this unit in the Nobres region, Mato Grosso state, were mainly focused on the outcropping section of 600 m in the bed of Rio Serragem II, which includes the Serra do Tombador waterfall. In this stratigraphic section, 17 sedimentary facies were described and grouped into five facies association (AF), representative of a progradational coastal sequence beginning with lower shoreface deposits, overlying in correlative conformity the shelf carbonate deposits of the Serra do Quilombo Formation (Araras Group). The AF1 facies consists of sandstones with planar lamination and wave-ripple cross-lamination (microhummocky), individualized by layers of laminated pelite interpreted as lower shoreface deposits. It stands out in the AF1 the first occurrence of centimetric bioturbed levels of Skolithos in Neoproterozoic-Cambrian deposits in the Paraguay Belt. The AF2 facies is composed by sandstones with swaley cross-stratification and plane bedding interpreted as upper shoreface deposits. The AF3 facies is composed by sandstones with tangential and trough cross-stratification with drapes of siltstone/very fine sandstone representative of channel and subtidal bars deposits. The AF4 facies is characterized by sandstones with tangential and sigmoidal cross-stratification, planar to low angle cross-lamination, rhythmites very fine sandstone/siltstone with flaser bedding and mudcracks, organized in metric tidal flat shallowing upward cycles. The AF5 facies is comprised of sandstone with trough cross-bedding characterized by common lags at the base of the association, sandstone with planar to low-angle cross-stratification, interpreted as distal braided rivers, in part reworked by waves. Detrital zircon grains were obtained from AF3 and dated by U-Pb method, resulting in an age 1001±9 Ma interpreted as the age of the maximum deposition of Raizama Formation. Combined with this analysis, the NE-SE paleocurrents show that source area of these sediments would be the Sunsas Fold Belt, SW of the Amazonian Craton not being discarded contributions coming from the NW part of this Craton. The obtained Mesoproterozoic age has predominantly served to unravel the provenance of Raizama Formation. Whereas dating from the base of Araras Group, around 627-622 Ma, associated with the clear presence of the ichnogenus Skolithos, suggests that the age of this unit is closer to the limit with the Lower Cambrian. Trace fossils from the Proterozoic are characterized almost exclusively by horizontal traces, while vertical bioturbation are virtually absent throughout the Neoproterozoic. This inference is confirmed by the maximum age of 541 Ma obtained for Diamantino Formation, which overlies the studied unit. The radiometric data combined with paleoenvironmental interpretation, including the record of the first burrowing activities in Paraguai Fold Belt, opens up perspectives to understand in greater detail the sequence of events that typify the Ediacaran-Cambriam boundary strata of Brazil, still poorly known.
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