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

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.