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Candidate sections for the GSSP of the base of the Bathonian Stage (Middle Jurassic)

Fernandez-Lopez S.

Volumina Jurassica

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2009

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Vol. 7, no. 1

77-88

EN

In accordance with the recommendations of the International Commission on Stratigraphy (ICS), the leading candidate for the Global Boundary Stratotype Section and Point (GSSP) of the base of the Bathonian Stage is located in the Ravin du B?s (Bas Auran area, SE France). This section was formally proposed as candidate nineteen years ago. A second potential candidate section is located in the Cabo Mondego area (Portugal). This Portuguese section, however, has not been formally proposed as a candidate for the Bathonian basal boundary stratotype. The formal selection and proposal of a GSSP for the Bathonian Stage is the responsibility of the Bathonian Working Group (BtWG) and is expected by September 2007. In accordance with the procedures to ratify GSSPs, a formal ballot on the selection and proposal of a GSSP for the Bathonian Stage, by post or email, by all members of the BtWG is the responsibility of the convenor and the International Subcommission on Jurassic Stratigraphy Executive, and will be carried out within this time scale.

2

Depositional sequences and ammonite fossilization on deep carbonate platform environments (Upper Oxfordian, Iberian Range, Spain)

Fernandez-Lopez S., Melendez G.

Volumina Jurassica

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2006

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Vol. 4, no. 1

87-88

EN

The uppermost deposits of the Yatova Formation in the Ricla area represent a condensed section, 1.5 m thick, developed during the Semimammatum and Berrense subchronozones. This interval is composed of grey-reddish wackestone to packstone and boundstone beds alternating with marly intervals, bearing common sponges, ammonites and bioturbation textures. Terebratulid and rhynchonellid brachiopods, belemnites, bivalves, gastropods, serpulids, bryozoans, crinoids and echinoids are very scarce. Small sponge mud mounds, some few metres wide and less than 50 cm high, are locally developed. Limestone beds, 10 to 40 cm thick, show sharp base, but gradual size-increase or inverse grading of fossils and gradational upper boundary. Marly intervals, under 40 cm thick, display planar fabric, being normal grading of fossils more common than inverse grading. Hardground surfaces, ferruginous crusts and glauconite grains are common on the limestone beds. In contrast, hardground surfaces are not developed within marly intervals, although reworked concretions and remobilization surfaces are common, often capping the underlying argillaceous sediments. Parasequences show a less distinct development than in underlying intervals. Thickening and coarsening upwards sequences of metric thickness are common. Thinning and fining upwards sequences are scarce, generally developed between the last sponge mounds and associated with the thickest intervals of condensed deposits. This condensed interval is interpreted as formed in an open marine, moderately deep carbonate platform, below effective wave base, showing generally low-energy conditions with extremely low rates of carbonate and terrigenous sedimentation. Marly deposits represent background sedimentation, with very low rates of sediment accumulation, which may be due to sedimentary starving as well as to winnowing action on the seafloor. In contrast, limestone beds correspond to event sedimentation, with relatively high rates of sediment accumulation, probably distal tempestites. Lasting episodes of background sedimentation give rise to clay deposits showing no evidence of basal discontinuity, whereas brief events of turbulence lead to carbonate deposits with sharp base. The low diversity of the benthic fauna, scarce development of sponge bioherms and microbial crusts, as well as ammonite populations inhabiting the platform are all palaeobiological criteria confirming these deep and distal palaeoenvironments. Taphonomic features of ammonite assemblages indicative of sedimentary starving are the occurrence of: 1 - high concentrations of reelaborated ammonites showing very low values of taphonomic condensation; 2 - taphonic populations of type-2; 3 - predominant internal moulds of phragmocones completely filled with homogeneous sediment up to the innermost whorls; and 4 - reworked fossils bearing no signs of abrasion, bioerosion or dense encrusting. These condensed deposits characterize the last phase of a deepening half-cycle, attaining the maximum deepening environments during Upper Jurassic, within a 3rd order deepening/shallowing cycle developed in the Iberian platform system.

3

Ammonite horizons at the basal Bathonian zone (Parvum Subzone) in Cabo Mondego, Portugal

Fernandez-Lopez S., Henriques M., Manglod Ch.

Volumina Jurassica

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2006

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Vol. 4, no. 1

161-161

EN

The ammonite succession at the Bajocian/Bathonian boundary in the Cabo Mondego region provides one of the most complete biostratigraphical records so far recognized in the Iberian Plate. Lower Bathonian ammonite fossil assemblages are composed of Submediterranean taxa. Parkinsonids characterizing the Northwest European Province, as well as phylloceratids and lytoceratids characterizing the Mediterranean Province, are very scarce. The basal Bathonian zone (Zigzag Zone) established for NW Europe areas, belonging to the Northwest European Province, can be identified in the Lusitanian Basin. The Lower Bathonian boundary may be established by the lowest occurrence of the dimorphic group Morphoceras (M) + Ebrayiceras (m), although morphoceratids are scarce. The Zigzag Zone can be characterized as composed of two subunits (Parvum and Macrescens subzones) represented in diverse European basins of the Submediterranean Province. The revision of previous collections from the classical section and new field samplings of two other separate sections on Cabo Mondego allow to distinguish the lowest subzone of Bathonian (Parvum Subzone, Zigzag Zone). Along up to ten metres of thickness, over forty successive assemblages have been recognized in the Parvum Subzone. The lowermost subzone of the Bathonian yields common perisphinctids (40%), oppeliids (25%) and hecticoceratids (20%), being the most abundant genera: Planisphinctes (m) + Lobosphictes (M), Oxycerites (M) + Paroecotraustes (m) and Nodiferites (m) + Zeissoceras (M). Ammonite fossil assemblages of the Parvum Subzone may be grouped into two successive biohorizons. The lower biohorizon, beginning with the lowest occurrence of Morphoceras (M) + Ebrayiceras (m), has been characterized by perisphinctids of the dimorphic couple Bigotites gr. diniensis Sturani (M) + “Bigotites” acurvatus (Wetzel) in Torrens (m), although they are scarce. The upper biohorizon, through 1.5-2 m of thickening upwards beds, has been identified in the stratigraphic interval beginning with the lowest occurrence of Zigzagiceras (m) + Procerozigzag (M) and underlying the lowest occurrence of Morphoceras macrescens (Buckman). These two successive ammonite horizons are biochronostratigraphically equivalent to the subdivisions of the Convergens Subzone distinguished on the Digne-Barr˘me area (France). The occurrence of Bigotites gr. diniensis (M+m) in Cabo Mondego in the Parvum Subzone represents a new criterion for chronostratigraphical subdivision and chronocorrelation with the Digne-Barr˘me area, useful in understanding the evolution of the West Tethyan Perisphinctidae during earliest Bathonian.