1. ORIGIN AND TRANSPORT OF FLUVIAL MUDDY OUTWASH SEDIMENTS OF THE PLIO-PLEISTOCENE RAIGON FORMATION, SOUTHWESTERN URUGUAY
Thomas M. Missimer, Robert G. Maliva, Daniel Perea, Ana Clara Badin
Geological Society of America Annual Meeting
Seattle, Washington, USA
October 21-25, 2017

2. Research Objectives
●Little is known concerning the origin of the sediments constituting the Plio-Pleistocene continental outwash sediments of southern Uruguay
●Did they originate from the primary drainage of the region via the regional drainage system provided by the Uruguay River (Argentina and Brazil) or were they carried to the coast by more localized streams draining the Precambrian craton exposed in Uruguay?

3. Methods
●Sediment samples were wet sieved and the gravel and sand-sized fractions separated
●The coarse sediment was separated into four size fractions-granule, coarse sand, medium sand, and fine sand.
●Standard grain thin sections were constructed with staining for k-feldspar and plagioclase.
●500 grains were point-counted on each thin section using the QFL method.
●The QFL method was used because the samples could be compared to other Holocene samples (Potter)

4. Three investigations being conducted.
Uruguay is located in southern South American between Argentina and Brazil.
Major drainage is provided by the Uruguay and the Paraná Rivers which drain part of the Brazilian Shield (Precambrian) and the Pampas region of Argentina. The rivers merge at the head of the Rio del la Plata which is a large estuary.
Three investigations being conducted.
Location

5. Geologic Map of Uruguay (from Weaver and Ward, 1994; Schenk, 1999)
The Precambrian Shield occurs in the southern part of the country with Quaternary-age and some Pliocene-age sediments lying along the southern coastal area.
Rio Uruguay forms the western boundary of the country and small streams drain the southern region.

6. Area of Investigation N
The stratigraphic unit of interest is the Plio-Pleistocene Raigon Formation.
The southernmost red arrow is the location of the Playa Balneario Arazati section and the second red arrow shows the Playa Barrancas de San Pedro site.
The red circle is the Rio Uruguay sample (Caridad Island).

7. Stratigraphic Section at Playa Balneario Arazati
Libertad Formation
The Camacho Formation is a Late Miocene age marine unit that contains some siliciclastic sediments.
The Raigon Formation is a Plio-Pleistocene unit that contains continental outwash sediments.
The Libertad Formation in an Early Pleistocene unit that is also composed of continental outwash sediments.
Triangles mark sample sites.
Raigon Formation
Camacho Formation

8. Stratigraphic Section at Playa Barrancas de San Pedro near Colonia
Raigon Formation
Samples were collected at the location of the three triangles and at the sandstone unit found at the base of the unit.
Top of Miocene
Camacho Formation

9. Comparison of the Composition of the Raigon Formation with the Caridad Island Sand Composition (Granular and Coarse Sand Fractions)
Note that there are no grains coarser than sand size in the Caridad Island sediment. In the coarse sand fraction the Caridad Island sediment is a quartz arenite, while the Raigon Formation sediments are classified as litharenite or feldspathic litharenite.

10. Comparison of the Composition of the Raigon Formation with the Caridad Island Sand Composition (Medium and Fine Sand Fractions)
The medium sand fraction shows the Caridad Island sediment is a quartz arenite with a metamorphic origin and transported in a humid environment. The Raigon Formation sediments are from a plutonic environment and transported in a humid climate.

11. Comparison of the Roundness of the Quartz and Feldspar Grains
Within the granule and coarse sand fractions, well-rounded quartz grains occur.
In the medium-grain sand fraction, very angular feldspar grains occur (both k-feldspar and plagioclase).

12. View of the Sediment Texture of the Raigon Formation
In the fine-grain fraction, the feldspar fraction is higher and the overall angularity of the sediment is greater.
Sandstone fragments occur throughout the deposit. The base of the formation is a sandstone unit cemented by chamosite.

13. Conclusions
●The Caridad River contains sand with a high degree of maturity with a low percentage of K-feldspar and high quartz content (>90%).
●The sediments of the Raigon Formation are immature with high content of angular K-feldspar and some plagioclase in the coarse and medium sand size fraction.
●There are sandstones within the Raigon Formation that are cemented with unusual clay cements.
●The Raigon Formation sediments contain between 25 and 60% clay of variable composition.

14. Conclusions
●Based on the current composition of the major river sediments with greater maturity and the less mature Raigon Formation sediments, it is likely that the sediments were transported to the coast in small streams that were eroding the local exposure of the Precambrian shield at the time.
●This suggests that the region climate was wet with localized flood events to move the outwash sediments to the coast.

15. Questions?
Questions?