GENESIS OF COAL LINKED TO CATASTROPHIC FLOODS, EARLY PERMIAN, PARANA BASIN, BRAZIL

Gondwanic coals of the Rio Bonito Formation (Paraná Basin) in Southern Brazil have generally large ash yelds, 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 claims for the proposition of a new depositional model based on resedimentary processes acting during deposition, as well as a stratigraphic rearrangement of the present units.

Although the preferential mode of occurrence of HCS in shallow marine environments indicates a genesis attributed to storm action, other causes as catastrophic flooding have been pointed recently. Mutti et al. described in 1996 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 sea. In such settings, seaward sediment flow can increase dramactically when weather conditions can supply water in such amounts that produce catastrophic floods.

The present study will refer to three important lithostratigraphic units in the Carboniferous-Early Triassic cycle: the Itararé Group, the Rio Bonito Formation and the Palermo Formation.

In the State of Rio Grande do Sul (southern part of Paraná Basin), coals are actually prodelta deposits related to delta front diamictite and conglomeratic sandstone with sigmoidal bedding. Vegetation forming coal would come from trees plucked by the floods, indicated by the wood logs floating in the diamictite, or for reworking of previous peat accumulations. Every coal layer is covered generally by paleosoil siltstones or tonsteins, which represent plant colonization in the top of the catastrophic flood deposit, ending a sedimentary cycle.

The proximal facies in this setting are related presumably to residual mountain glacier melting, forming deposits assigned as Jökulhaupts, similar to the Scablands, in the Columbia River valley (15.000 BP) and in Iceland modern examples.