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The Age of the Tojeira Formation (Late Jurassic, Early Kimmeridgian), of Montejunto, West-Central Portugal

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The Age of the Tojeira Formation (Late Jurassic, Early Kimmeridgian), of Montejunto, West-Central Portugal Swiss J Palaeontol (2017) 136:287–299 DOI 10.1007/s13358-017-0137-6 The age of the Tojeira Formation (Late Jurassic, Early Kimmeridgian), of Montejunto, west-central Portugal 1 1,2 1 3 Holly E. Turner • Felix M. Gradstein • Andy S. Gale • David K. Watkins Received: 9 May 2017 / Accepted: 27 June 2017 / Published online: 12 July 2017 Ó The Author(s) 2017. This article is an open access publication Abstract Precise biostratigraphic dating of the Tojeira Introduction Formation (Late Jurassic, Early Kimmeridgian) of the Montejunto section of west-central Portugal, which has Several studies have been published on the micropalaeon- yielded important planktonic foraminiferal assemblages, is tology of the dominantly clay, marine Tojeira Formation, hindered by poor preservation in the upper part of the section west-central Portugal. The formation has received attention as the lithology shifts from shale to coarser clastics. because of its content with six or more taxa of common to Assignment was previously made to the Idoceras planula frequent and relatively well-preserved planktonic for- and Sutneria platynota zones based on ammonites. Coccolith aminifera of Kimmeridgian age (Stam 1986; Agterberg and dinoflagellate assemblages described here concur with et al. 1989; Gradstein 2017; Gradstein et al. 2017). How- the Early Kimmeridgian, yet, a finer age constraint is pro- ever, the age of the formation as it outcrops in the Mon- posed by cyclo- and chemostratigraphical correlation. Peaks tejunto area has received limited attention. Age assignment 13 in d Corg and TOC, if equivalent to maxima in the envelope to the I. planula–S. platynota ammonite zones of early of clay/carbonate cycles in SE France, imply that the c. Kimmeridgian age is explained in Atrops and Marques 50 m-thick section spans a 0.8-myr interval of the S. platy- (1986) and Stam (1986). Yet, whilst the lower part of the nota through upper Ataxioceras hypselocyclum ammonite Tojeira Formation and much of the underlying Montejunto zones, with the approximate base of the A. hypselocyclum Formation yield an abundance of ammonites, they become Zone at c. 15.4 m (level 13). Such stratigraphy provides new rare or absent towards the upper part of the Tojeira For- insights into the upper part of the formation by interbasinal mation. Moreover, macrofossil preservation is poor and correlation with other Tethyan records. An extended first many specimens are fragmentary. High-resolution bios- occurrence of the dinoflagellate species Dichadogonyaulax? tratigraphy is hampered by lack of internal biozonation or pannea in the S. platynota Zone is also proposed. physical events stratigraphy in the Tojeira Formation. The foraminiferal studies quoted report on few age determinate Keywords Portugal Á Jurassic Á Kimmeridgian Á species which also provide no additional stratigraphic Correlation Á Biostratigraphy Á Stable isotope stratigraphy information within the Kimmeridgian. The Tethyan ammonite zonal scheme (Sub-Mediter- Editorial handling: D. Marty. ranean) has been calibrated to those of the Sub-boreal (Britain) and Boreal (Russian Platform–Arctic) (Gradstein & Holly E. Turner et al. 2012 and references therein). However, due to the [email protected] provincial distribution of ammonite faunas in the Late 1 School of Earth and Environmental Sciences, University of Jurassic Boreal seaways and marine shelf margins of the Portsmouth, Portsmouth PO1 3QL, UK Tethys Ocean, exact interregional correlation is uncertain 2 Natural History Museum, Oslo, University of Oslo, and an ongoing topic for debate (Wimbledon 2008). The POB 1172, Blindern, 0318 Oslo, Norway international base of the Kimmeridgian occurs at the base 3 Department of Earth and Atmospheric Sciences, University of the Sub-boreal Pictonia baylei Zone and, as now of Nebraska-Lincoln, Lincoln, NE 68588-0340, USA defined, corresponds to the middle of the Epipeltoceras 288 H. E. Turner et al. bimammatum Zone in the Tethyan realm or, according to which preceded influx of over 1 km thick marine silici- Wierzbowski and Matyja (2014), at the boundary between clastics of the Abadia Formation, caused break-up of the the Aspidoceras hypselum and E. bimammatum zones. The carbonate platform. A transect sketch (Fig. 1b) shows the Tethyan Kimmeridgian base is no longer at the S. platynota postulated tectonic-sedimentary setting. Upper Jurassic Zone, as used by previous studies of the Tojeira-1 section strata include, from older to younger, the Cabacos, Mon- (e.g., Atrops and Marques 1986). tejunto, Tojeira, Cabrito, Abadia and Amaral formations, Stratigraphic resolution is increased by calibrating with a total thickness of over 1500 m. ammonite zones with the coccolith zonation scheme of Above the disconformity between the Upper Callovian Casselato (2010) and the dinoflagellate zonation scheme of and Lower Oxfordian, the Cabacos Formation comprises Riding and Thomas (1988, 1992) and Poulsen and Riding about 250 m of platy, thin-bedded grey limestones. The (2003) which have finer subdivisions than either regional Montejunto Formation comprises thick-bedded, white to ammonite scheme. Considerable literature exists on the grey micritic limestone which alternate with bluish-grey ranges of dinoflagellate index species in the Late Jurassic; limestones, and grey hard shales, with thicker shales near however, the provincialism of those index species between the top. Several metres wide crinoid-brachiopod bios- the north-western Europe faunal realms is less reported on. tromes occur in the middle part of the unit. The formation Ammonite and dinoflagellate assemblages intermediate is over 200 m thick with ammonites common to frequent. between the Boreal and Sub-boreal biomes apparently The Tojeira Formation is over 70 m thick consisting almost show comparable ranges (Wierzbowski et al. 2002). entirely of dark grey shales with limonitic concretions and Poulsen and Riding (2003) also note that, despite marked is generally brownish-red towards the middle. Pyritized provincialism, many index species remain of value for ammonites are common in the lower part of the unit, and local correlation. silt content increases considerably near the top; belemnites Carbon isotope chemostratigraphy is a reliable correla- are rare. The overlying Cabrito, Abadia and Amaral for- tion tool when integrated with biostratigraphy (e.g., Gale mations together are over 1 km thick with marine sands, et al. 1993; Jarvis et al. 2006; and many others). Reliability, silts and conglomerates. Olistolithic limestone blocks and which also may be called stratigraphic fidelity in marking reworked coralline bodies occur, along with sand channel specific levels, is dependent on multiple factors, however. deposits with abundant ripple marks and low-angle cross- This includes whether there was restricted water mass bedding. exchange at the time of deposition which would cause an The Tojeira sections of Stam (1986) and Agterberg et al. imprint over the original d13C signal. Nevertheless, mul- (1989) were revisited in 2016 to resample the Tojeira-1 13 tiple d Ccarb records exist for the Tethyan Late Jurassic section approximately every metre (Fig. 2). Our 2016 which, when stacked, show regional carbon cycle pertur- samples M1-1–M1-3 are from a dirt path outcrop of the bations over localised records (e.g., Price et al. 2016). Montejunto section (Fig. 1), but are not reported on here. Additionally, where relative sedimentation rates are The lowest sampling level from the Tojeira-1 section is assumed to be steady, cyclostratigraphic correlation with approximately at the base of the stratigraphic column in an orbitally tuned coeval section improves age resolution. Fig. 2—see Gradstein (2017) and Gradstein et al. (2017) Comparison of some Kimmeridgian d13C records with the for full locality description. Our investigations reveal that 13 Tojeira section d Corg profile is applied as a support in age the Tojeira-2 section in cross section 2 of Stam (1986), determination. New geochemical and biostratigraphical between the village of Tojeira and Pereiro, had largely data increase stratigraphic resolution of the Tojeira section, vanished due to domestic construction and agriculture. A as reported on here. new outcrop, Tojeira-3 section (Fig. 1a), is exposed half- way along the road from Montejunto to Vila Verde dos Francos. Sandy siliclastics of the upper part of the outcrop Geological setting probably indicate upper Tojeira or lower Cabrito Forma- tion, but is not reported on here (see Gradstein 2017). An almost complete, but folded and faulted, sequence of Bathonian through Kimmeridgian marine strata is exposed in the Montejunto area, c. 50 km north of Lisbon. The Biostratigraphy strata are well exposed on the flanks of Montejunto, a 664-m high diapiric structure, c. 7 km NE of Vila Verde The Tojeira Formation was assigned to the I. planula–S. (Fig. 1a). Our stratigraphic account follows that of Stam platynota ammonite zones by several authors (Mouterde (1986). From Bathonian through Oxfordian times, a car- et al. 1971, 1973, 1979; Atrops and Marques 1986; Stam bonate platform facies persisted, locally of lagoonal facies. 1986). The boundary between the two zones can be iden- An early Kimmeridgian rifting (tectonic subsidence) event, tified by the first occurrence datum (FAD) of S. platynota The age of the Tojeira Formation, Jurassic of Montejunto, Portugal 289 Fig. 1 Location of the sections involved (a) and a schematic of the southern slope of Montejunto (b). Numbers 1–7 correspond to the ammonite successions of Atrops and Marques (1986) (Moliner and Olo´riz 2009). However, in general, occur- fairly abundant. Atrops and Marques (1986) suggest the rences of S. platynota reduce towards the upper part of the Oxfordian–Kimmeridgian boundary (which is now equiv- S. platynota Zone. The base of the following A. hypselo- alent to the I. planula–S. platynota zonal boundary) occurs cyclum Zone is defined by the FAD of Ataxioceras and last at this horizon at the top of succession 4. Succession 5 is occurrence (LAD) of S. platynota. Age controls for the attributed to the S. platynota Zone, sub-zone O. polygy- section are reviewed with consideration of new material ratus.
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