Palynology and sedimentology of laminites and tillites from the latest Famennian of the Parnaiba Basin, Brazil

Abstract

Varvelike rhythmites, usually laminated siltstones and shales with scattered clasts, are known from outcrops and well cores of the upper Cabeças Formation in the Parnaíba (Maranhão) Basin, Brazil, with sediments laid down under glacial and periglacial conditions. Palynological study from different lithologies, indicates a latest Famennian age (LN Zone). Sedimentological approach of one rhythmite indicates that the grain-size contrast clearly separate between silt and sand layers but that all layers, even the dark silty ones, show features characteristic of sediment-gravity flows. Lateral influx was permanent during the deposition of the sediment. Clay analyses reveal the predominance of kaolinite probably originated from Middle Devonian rocks. Latest Famennian miospores and acritarchs are present and confirm the marine character of the depositionnal environment but a large part (70%) of the palynological material is reworked from Givetian / Frasnian rocks. No reworked miospores from early to late Famennian can be demonstrated. At least two distinct source-areas of the reworked material, Givetian (or older) and Frasnian, can be recognized. Contemporaneous miospores are significally less present in the tillites and associated shale than in the laminites which suggest that they are produced locally, the glacial tongue, carrying the reworked part of the material, only partially overlapping adjacent environments. The rythmites are presumed to be true varves, the sandy layers being first settled after the local seasonal melting of the ice cover and the rush of fresh water supply into the sea, the silty layers being deposited when the spring water run-off decreases. The Vallatisporites mother-plant, believed to live in a swamp margin environment, might have been the first to produce spores immediately after the melting of the ice cover