SEDIMENTARY ENVIRONMENTS OF THE MIOCENEFILARET FORMATION

Toshihiro Yamada1, and Kazuhiko Uemura1

1Department of Geology and Paleontology, National Science Museum, Tokyo 169-0073, JapanE-mail: tyamada@kahaku.go.jp

Introduction

Tierra del Fuego Island is located in the southernmost part of South America (Fig. 1) and paleovegetationalhistory in the island might play critical role for elucidating migration process of South American plantlineages because some lineages are migrated via Antarctica. For better understanding of paleovegetationin the island during mid-Cenozoic, we conducted excavation of fossils twice at Tierra del Fuego, where theMiocene Filaret Formation is distributed.

It is well known that a plant fossil assemblage uncovered from the ancient sediments is subject to bebiased, such as transportation bias. This means that plant fossil assemblage might not reflect the originalvegetation, suggesting that changes of plant assemblages observed in a series of sediments must be carefullyexamined before discussing the vegetational history.

To clarify sedimentary facies of a plant fossil assemblage before their burial is the first step to understandtheir history, because the taphonomic bias depends on where plant remnants were trapped. As preliminarydata for understanding the vegetational history in Tierra del Fuego Island, we report the sedimentary faciesof plant-bearing horizons of the Filaret Formation.

Localities

Plant fossils were collected in trenches (ENAP Cisne pozo #6) formed in a pasture at 20 km northwest ofSan Sebastian along the road to Cullen (Fig. 1), where the lower part of the Filaret Formation outcrops(ENAP, 1974; Hromic, 1991, 1995). This site exposes almost the same horizon as that referred to byTanai (1981), as the plant bearing beds of the Brush Lake Formation.

Silicified woods were collected along a pasture at Filaret, where a road to Porvenir is branched offfrom the main road from San Sebastian to Cullen. The area includes the type section of the Filaret

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Fig. 1. Index map of Tierra del Fuego Island.

Formation (ENAP, 1974). According to more recentstudies (Hromic, 1991, 1995), the beds bearing silicifiedwoods occupy the upper part of the Filaret Formation.

Geology

Along the main route from Filaret to Sansebastian, theFilaret Formation is distributed, roughly with NW-SEstrike and dips NE at 5 to 20 degrees. The Filaret For-mation conformably overlies the Brush Lake Formation,which is composed mainly of mudstone containing ma-rine molluscs and foraminifers (ENAP, 1974; Hromic,1991).

Reprinted from: Nishida, H. (ed.) Post-Cretaceous Floristic Changesin Southern Patagonia, Chile, ChuoUniv., Tokyo, p. 103-107, (2006)

Yamada et al.: Miocene sedimentary environment in south Chile104

Fig. 2. The Filaret Formation outcropped at test trench (ENAP Cisne pozo #6). A. Alternating beds of sandstone

and siltstone. In the sandstone, wave-ripples are often observed. B. Nothofagus sp. contained in the laminated

mudstone (right to the pencil). C. Close-up of wave-ripple laminae observed in the sandstone.

The Filaret Formation

A. Lithofacies in general

The lower part of Filaret Formation consists mainly of fine-grained sandstone and mudstone in the lower

part (Fig. 2). Intercalation of coarse- to very-coarse-sandstone beds is common in the upper part. The

age of the Filaret Formation is assigned to the Miocene (ENAP, 1974, Natland et al., 1974, Hromic,

1995).

B. Lithofacies at test trenches (ENAP Cisne pozo #6)

The Filaret Formation exhibits five lithofacies assemblages (LA) at these trenches (Figs. 2, 3)

LA1. Alternating beds of very fine- to medium grained sandstone and mudstone with almost same

thickness (30 to 50 cm). Wave ripple laminae with chevron-build-up structure are often developed in the

sandstone (Fig. 2C). The mudstone is well-laminated. Sandstone bed with planar cross-stratification (<

Yamada et al.: Miocene sedimentary environment in south Chile 105

Fig. 3. Columnar section of the Filaret Formation outcropped at test trench (ENAP Cisne pozo #6). Horizons

bearing plant fossils are indicated by leaf mark.

1m) is sometimes intercalated (Fig. 2A). Plant fossils are contained in the mudstone and sandstone (Fig.

2B).

LA2. Alternating beds of very fine- to medium grained sandstone with symmetrical ripple laminae and

mudstone with almost same thickness (30 to 50 cm). Plant fossils are contained in the mudstone and

sandstone.

LA3. Medium-grained sandstone with Swaley Cross-stratification (SCS). It bedded to 1 m thickness.

LA4. Alternating beds of sandy mudstone and very-fine-grained sandstone containing climbing ripple

laminae with symmetrical crest. Plant fossils are contained in the mudstone (Fig. 4).

LA5. Very-coarse-grained sandstone with trough cross-stratification. The bed erodes the underlain

sediments. It attains 2 m at maximum, but pinches out laterally.

C. Sedimentary environments

LA1 to 4 indicate that the sedimentary environments of the deposits were the transition zone between the

shoreface to shelf and LA5 deposited in shoreface (Reineck and Singh,1980).

D. The Filaret Formation at type section

The Filaret formation at Filaret occupies its upper part and consists mainly of medium-grained sandstone

of whitish yellow or of whitish grey (Fig. 5A). Sometimes silicious nodules are formed and silicified woods

are contained in the nodules (Fig. 5B).

In the sandstone outcropped at a pasture of Filaret, cross-stratifications with bimodal orientation are

developed (Fig. 5A). These cross-stratifications show that the paleocurrent from SE to NW is dominative

over those from NW to SE. Boundaries between sets of the cross-stratification are demarcated by sharp

Yamada et al.: Miocene sedimentary environment in south Chile106

Fig. 4. Mode of occurrence of plant megafossils from the Filaret Formation at test treruch

(ENAP Cisne Pozo #6 . A. Plant debris in mudstone. B. Nothofagus simplicidens Dúsen and

other leaf remains in fine sandstone. Both from LA-4 of text-fig. 3. Scale 10 mm.

erosional surface. Among each set of the cross-stratification, there are bimodal distributions of set thickness

and dip angles (Fig. 5A). These features found in the cross-stratifications suggest that the formation partly

formed under the tidal currents (Reineck and Singh, 1980). However, the sedimentary environments of the

formation as a whole could not be examined because of poor outcrop conditions.

References

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Hromic T. 1991. Foraminíferos de la Formación Brush Lake: Cuenca Austral, Chile. Anales Instituto Patagonia, Ser. Cs.

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Hromic T. 1995. Catalogo de foraminíniferos (Protozoa: Foraminiferida) del Terciario de Chile. Anales Instituto Patagonia,

Ser. C. Nats., Punta Arenas (Chile), 23: 51-94.

Yamada et al.: Miocene sedimentary environment in south Chile 107

Fig. 5. The Filaret Formation outcropped at Filaret. A. Close-up of the cross-stratified sandstone. The cross-

stratification indicates bidirectional paleocurrents. B. Silicate nodule contained in the sandstone. Silicified woods

were found from the nodule. C. Molluscan fossils uncovered from the Filaret Formation.

Natland M L, Gonzales E. P, Canon A, Ernst M. 1974. A system of stages for correlation of Magallanes basin sediments.

Geological Society of America, Memoir 139: 9-126.

Reineck H E and Singh I B. 1980. Depositional sedimentary environments with reference to terrigenous clastics. Springer-

Verlag, Berlin.

Tanai T. 1981. Geological notes on the fossil localities. In: M. Nishida (ed.). A report of the paleobotanical survey to

southern Chile by a Grant-in-Aid for Overseas Scientific Survey, 1979. Chiba University, Chiba, 10-30.