A stratigraphic framework for the Miocene from the lower Tagus Basin (Lisbon, Setúbal Península, Portugal) Depositional sequences, biostratigraphy and isotopic ages

Abstract

Lithostratigraphy, main biostratigraphic events (first and last ocurrence), 87Sr/86Sr as well as KlAr isotopic ages, are presented for the Lower Tagus basin (LTB) Miocene (Lisboa and Peninsula de Setubal regions). Eight depositional sequences (DS) delimited by regional disconformities related with transgressive surfaces are defined. The main features of the Depositional Sequences are as follows: Depositional sequence (DS) A (Aquitanian). Biostratigraphy (Biost.): Small mammals from Km 10: MN2 (MN3?); ostracoda: Aquitanian fauna. Isotopic age (IA): glauconites (KlAr) values between 19 and 24 Ma. 87Sr/86Sr ages from the glauconite levels: 22.3 (+0.4-0.7) Ma. Lower and upper boundaries not well established. DS BO (Lower Burdigalian). Biost.: foraminifera - Fa Globigerinoides altiaperturus (NS); mammals - Fa Brachyodus intermedius (MN3); ostracoda - La Hemicyprideis helvetica and Pokomyella lusitanica at the LTB. IA (87Sr/86Sr): Foz da Fonte -19.S (±0.2) Ma, Penedo Sui (base) - 20.0(±0.4)Ma; Almada (top) - 18.3 (+0.-0.2) Ma. Mammal sites: Av. do Uruguai - 21.S( +O.S- 0.3) Ma; Univ. Cat6lica - 20.S (+0.3-0.2) Ma. DS Bl (Middle Burdigalian). Biost.: foraminifera - Catapsydrax unicavus and Globigerinoides altiaperturus (N6); mammals - Fa Gomphotherium, La of Brachyodus (MN4); ostracoda -La Ruggieria (R.) micheliniana, Cnestocythere truncata, Pokomiella minor and Triebelina raripila. fA (87Sr/86Sr) Foz da Fonte - 19.7 (+0.3-0.2) to 18.S (+0.2-0.S) Ma. DS B2 (Upper Burdigalian). Biost.: mammals -Fa Bunolistriodon and Megacricetodon primitivus (MN4); ostracoda - La Miocyprideisfortisensis. IA (87Sr/86Sr): Penedo Norte 17.7 (+0.7-0.S) Ma; 17.8 (+0.7- O.S) Ma. DS Ll (Upper Burdigalian and Langhian). Biost.: foraminifera - Fa Praeorbulina (N8); mammals - Fa (upper part) Hispanotherium and Megacricetodon collongensis, La M. primitivus (MNS); ostracoda - Fa Pterigocythereis (P.) siveteri and Loxoconcha (L.) ducasseae. IA (87Sr/86Sr): Penedo Norte 17.3 (+0.6-0.5) Ma. DS SI (Upper Langhian and Serravallian). Biost.: foraminiferaFa Orbulina suturalis, O. universa and Globorotalia cf. menardii; ostracoda -Fa Aurila (u.) oblonga, Ruggieria (R.) nuda, R. tetraptera tetraptera, Nomurocythereis seminulum, Pterygocythereis (P.) jonesi y Olinfalunia costata. IA (87Sr/86Sr): Chelas 14.7 (+ 1.S-0.S) Ma, Almada (Brielas ) (lower part) - 14.0 (±0.4 ) Ma, Penedo (upper part) 12.S (+ 1.0-2.0) Ma. DS S2? (Upper Serravallian) is poorly characterized due to strong condensation (Foz da Fonte-Rib. Lage) and poor outcrops (Lisbon region). Biost.: foraminifera - La Globigerinoides subquadratus (top). DS Tl (Tortonian). Biost.: foraminifera - La Globorotalia mayeri (lower part), Fa Neogloboquadrina acostaensis (upper part); ostracoda - Fa Aurila (U.) zbyszewskii, Celtia quadridentada and Cyheretta (C.) simplex. IA: Penedo Norte: KlAr- 1O.9S±0.2S Ma; 87Sr/86Sr - 11.3 (+ 1.7-2.8) Ma; Almada (Foz de Rego) - 8.3 (+ 1.9-3.3) Ma; Fonte da Telha S.2 (+3.1-1.2) Ma.