Abstract

Devonian sandstones of the Parana and Parnaiba Basins of Brazil were studied with special attention to their primary sedimentary structures, as part of a larger project on the palaeogeography of the South American Devonian. Palaeocurrent trends were deduced from a total of 4,773 measurements of cross-bedding and related features. Texture analyses provided supplementary information concerning the palaeoclimatological and environmental conditions in the basins. The Pre-Devonian depositional surface on which these strata were laid down was at that time almost flat with practically no relief.

The results so far obtained refer specifically to three major areas of outcrops:

a) Exposures of Lower Devonian Furnas Sandstone in the southern part of Parana Basin, where 2,839 crossbedding measurements were made at 41 localities from Itapeva (State of Sao Paulo) to Engenheiro Bley (State of Parana). The average current trend deduced from these readings is S 55° W. However, the properties of the lower Furnas Sandstone show it was transported from east to west, a direction coincident with the orientation of its constituent pebbles, and reflecting currents originating in the former eastern borderland and flowing basinward. In contrast, the middle and upper layers of the formation show an average palaeocurrent trend of S 31° W, indicating currents from the NNE parallel to the Lower Devonian coastline.

b) In the northernmost outcrops of the Furnas Sandstone, in the States of Mato Grosso and Goias, two separate areas of cross-strata were studied. In the eastern area 442 measurements yielded an average transportation direction of N 32° W and average pebble direction of N 39° W. In the western area 162 measurements were made and gave an average direction of N 62° W. The latter measurements are considered insufficient in number to provide a definite palaeogeographical picture, because the results show discrepancies which are not quite understood.

c) Sandstones of Serra Grande Formation (895 cross-strata measurements) and Pimenteira Formation (388 measurements) of the Parnaiba Basin, northeastern Brazil, were examined in the field. Palaeocurrent directions during deposition of the lower Serra Grande Formation were to the WSW, but its upper beds, as well as Pimenteira Formation sediments, originated from prevailing currents flowing northwestward. The former palaeocurrents were derived from the land, while the latter are believed to have flowed parallel to the coast.

Comparison of the data from the southern Furnas Sandstone with information on its northern outcrops demands a revised palaeogeographical interpretation of Parana Basin different from that commonly accepted for the Devonian. There is good evidence that this basin had a greater extension to the northward and northwestward than thought formerly, and was connected with the Parnaiba Basin as well as with the Amazonas Basin.