SEDIMENTARY ENVIRONMENTS OF THE MIOCENE FILARET FORMATION

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1 Reprinted from: Nishida, H. (ed.) Post-Cretaceous Floristic Changes in Southern Patagonia, Chile, ChuoUniv., Tokyo, p , (2006) 103 SEDIMENTARY ENVIRONMENTS OF THE MIOCENE FILARET FORMATION Toshihiro Yamada 1, and Kazuhiko Uemura 1 1 Department of Geology and Paleontology, National Science Museum, Tokyo , Japan tyamada@kahaku.go.jp Introduction Tierra del Fuego Island is located in the southernmost part of South America (Fig. 1) and paleovegetational history in the island might play critical role for elucidating migration process of South American plant lineages because some lineages are migrated via Antarctica. For better understanding of paleovegetation in the island during mid-cenozoic, we conducted excavation of fossils twice at Tierra del Fuego, where the Miocene Filaret Formation is distributed. It is well known that a plant fossil assemblage uncovered from the ancient sediments is subject to be biased, such as transportation bias. This means that plant fossil assemblage might not reflect the original vegetation, suggesting that changes of plant assemblages observed in a series of sediments must be carefully examined before discussing the vegetational history. To clarify sedimentary facies of a plant fossil assemblage before their burial is the first step to understand their history, because the taphonomic bias depends on where plant remnants were trapped. As preliminary data for understanding the vegetational history in Tierra del Fuego Island, we report the sedimentary facies of 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 of San 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 by Tanai (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 off from the main road from San Sebastian to Cullen. The area includes the type section of the Filaret Formation (ENAP, 1974). According to more recent studies (Hromic, 1991, 1995), the beds bearing silicified woods occupy the upper part of the Filaret Formation. Geology Along the main route from Filaret to Sansebastian, the Filaret Formation is distributed, roughly with NW-SE strike and dips NE at 5 to 20 degrees. The Filaret Formation conformably overlies the Brush Lake Formation, which is composed mainly of mudstone containing marine molluscs and foraminifers (ENAP, 1974; Hromic, 1991). Fig. 1. Index map of Tierra del Fuego Island.

104 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).

2 104 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 (< 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.

3 105 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

4 106 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 ENAP (Empresa Nacional del Petroleo) Nota explicativa del mapa geologico de la XII region - Magallanes. Escala 1:500,000. Hromic T Foraminíferos de la Formación Brush Lake: Cuenca Austral, Chile. Anales Instituto Patagonia, Ser. Cs. Nats., Punta Arenas (Chile), 20: Hromic T Catalogo de foraminíniferos (Protozoa: Foraminiferida) del Terciario de Chile. Anales Instituto Patagonia, Ser. C. Nats., Punta Arenas (Chile), 23:

107 Fig. 5. The Filaret Formation outcropped at Filaret. A. Close-up of the cross-stratified sandstone. The crossstratification indicates bidirectional paleocurrents. B.

5 107 Fig. 5. The Filaret Formation outcropped at Filaret. A. Close-up of the cross-stratified sandstone. The crossstratification 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 A system of stages for correlation of Magallanes basin sediments. Geological Society of America, Memoir 139: Reineck H E and Singh I B Depositional sedimentary environments with reference to terrigenous clastics. SpringerVerlag, Berlin. Tanai T 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, Chiba University, Chiba,

GEOLOGICAL NOTES ON PLANT FOSSIL LOCALITIES OF THE LIGORIO MÁRQUEZ FORMATION, CENTRAL PATAGONIA, CHILE

Reprinted from: Nishida, H. (ed.) Post-Cretaceous Floristic Changes in Southern Patagonia, Chile, ChuoUniv., Tokyo, p. 29-35, (2006) 29 GEOLOGICAL NOTES ON PLANT FOSSIL LOCALITIES OF THE LIGORIO MÁRQUEZ