ORIGIN AND TRANSPORT OF FLUVIAL MUDDY OUTWASH SEDIMENTS OF THE PLIO-PLEISTOCENE RAIGON FORMATION, SOUTHWESTERN URUGUAY

The Raigon Formation of southwestern Uruguay is believed to be of Plio-Pleistocene in age based on examination of vertebrate and invertebrate fossils and some paleo-magnetic data. Petrographic examination of the sand fraction of sediment samples suggests two sources. Some well-rounded quartz grains likely originated from the Precambrian shield of Brazil and Argentina and were transported to the area via the regional drainage system (a large river predating the Uruguay River). The finer sand fraction of the sediment contains significant concentrations of potassium feldspar, suggesting a more local source. Mud constitutes between 15 and 28% of the sediment.

The coarse fraction was separated into four sizes, granular (greater than sand-size, coarse, medium and fine-very -fine sand sizes. The feldspar content ranged between 10 and 25%. The quartz component contains a varying fraction of well-rounded to subrounded grains and the feldspar fraction is significantly more angular. The well-rounded quartz grains suggest long-distance transport. However, the high percentage and angularity of the potassium feldspar fraction of the sediment is much greater than occurs in sediment samples collected from the modern Uruguay River at Paysandu, which suggests a more local source of the Raigon Formation sediments.

Erosion of the Precambian Shield within southern Uruguay and transport to the coast via smaller river systems is suggested for the metastable fraction and some of the quartz sand. The local Uruguayan component of the sands was transported and buried rapidly, allowing greater preservation of the metastable components, particularly the feldspars and sandstone rock fragments. The sandstone fragments likely formed at or near the coastline based on cementation with a clay cement.

Cementation appears to have occurred in at least two phases with a fine-grained clay cement occurring at grain contact points and isopachous clay cement surrounding some grains and infilling occurring within large pores. Only a few skeletal marine carbonate grains were found in the sediment, which suggests that depositional environment was primary terrestrial, but with a minor marine component.