Gondwana coals of the Rio Bonito Formation (Parani Basin) in Southern Brazil have generally large ash yields, so they could be better called coaly siltstones than coal. In addition, hummocky cross stratification (HCS) was found in several coal beds of the Rio Bonito Formation throughout the basin. In this formation, the frequent and close relationship between facies involving rocks generated by subaqueous gravity flows (diamictites) and coal itself provides an excellent depositional model based on resedimentary processes acting during deposition, as well as a stratigraphic rearrangement of the present units. In the State of Rio Grande do Sul (southern part of Parana Basin), coals are actually prodelta deposits related to delta-front diamictite and conglomeratic sandstone with sigimoidal bedding. Coal-forming organic sediments would come from trees plucked by the floods, as indicated by the wood logs floating in the diamictite, and reworking of previous peat accumulations. Every coal layer is covered generally by paleosoil siltstones, which represent colonization at the top of the catastrophic flood deposit, ending a sedimentary cycle. In case of Brazilian coal settings, several authors recognized deltas (fan deltas or braid deltas). Here is particularly considered the general environment as a salted interior sea (lago mare, Hsu et al. sense). The present study will refer to three important lithostratigraphic units in the Carboniferous-Early Triassic cycle: the Itarare Group, the Rio Bonito Formation, and the Palermo Formation. Although the preferential mode of occurrence of HCS in shallow marine environments indicates a genesis attributed to storm action, other causes, such as catastrophic flooding, have been advanced. Mutti et al. [Mem. Sci. Geol. 48 (1996) 233] described flood-dominated deltaic systems with thick conglomerate, sandstone, and pelitic deposits, derived from small- to medium-scale fluvial systems and mountain-bordered drainage basins adjacent to the sea. In such settings, seaward sediment flow can increase dramatically when weather conditions can supply water in such amounts to produce catastrophic floods. Thick and laterally extensive sandstone lobes with HCS are the fundamental depositional elements of fan deltas and other river-dominated delta systems.
