The Middle-Upper Jurassic ooliticironstone level in the Iberian range (Spain). Eustatic implications

Abstract

The oolitic ironstone level occurring at the Callovian-Oxfordian boundary across wide areas in the central Iberian Range (Eastern Spain) is interpreted here as formed on an extended, very shallow to temporarily emerged, uniform carbonate platform. Main evidence to support this interpretation comes from both sedimentological analysis of facies and taphonomic analysis of the ammonites. The sedimentological analysis gives support to the idea of iron oolites being formed on, or in the surroundings of, emerged areas. On the other hand, the taphonomic analysis shows that some inner moulds of ammonites from this level display evidence of taphonomic reworking, such as the presence of ellipsoidal abrasion facets on the final part of the last preserved whorl, or annular abrasion furrows carved on the external region. These features would have developed by the action of directional currents under extremely shallow conditions. Bathymetric implications are relevant for the interpretation of the sea level fluctuations at this stratigraphic interval: A relative lowstand of sea level is proposed for the Upper Callovian-Lower Oxfordian interval in the studied area. KEY-WORDS: FACIES ANALYSIS, TAPHONOMIC ANALYSIS, EUSTATIC CHANGES, CALLOVIAN-OXFORDIAN BOUND-ARY, IBERIAN RANGE. RESUMELe niveau d'oolites ferrugineuses situe it la limite Jurassique moyen-superieur, rencontre sur une grand partie de la Chame lberique (Est de l'Espagne), est interprete comme ayant ete forme sur une plate-forme carbonatee uniforme, tres etendue et peu profonde, voire localement emergee. Cette interpretation est basee sur l'etude sedimentologique des facies et l'analyse taphonomique des associations d'ammonites. L'etude sedimentologique etaye bien l'hypothese de la formation du niveau oolitique sur, ou proches, des aires emergees. D'autre part, l'analyse taphonomique a montre la presence dans ce niveau de moules internes d'ammonites qui presentent notamment des facettes ellipsoldales d'usure recoupant le dernier tiers de spire ou des sillons annulaires d'usure, interpretes comme preuves de remaniement des moules internes, et probablement dus it l'action des courants directionnels en milieux extremement peu profonds. Ces implications sur la profondeur des eaux au-dessus de la plate-forme et les variations de la bathymetrie sont d'importance primordiale pour l'interpretation des fluctuations du niveau marin pendant cet intervalle stratigraphique. Une phase de bas niveau relatif est proposee pour l'intervalle Callovien superieur-Oxfordien inferieur dans la region etudiee. MOTS-CLES : ANALYSE DE FACIES, ANALYSE TAPHONOMIQUE, CHANGEMENTS EUSTATIQUES, LIMITE CALLO-VIEN-OXFORDIEN, CHAINES IBERIQUES.

From platform to basin to swell: orbital control on sedimentary sequences in the Oxfordian, Spain

“…At the base of the studied interval, the presence of iron ooids and reworked ammonites points to important condensation at the Callovian-Oxfordian boundary (Aurell et al, 1994 Aurell et al, 2003), and positioning of the three sections presented in this study. Fig.…”

“…The Ricla section commences with a condensed interval including frequent stratigraphic gaps and a condensed ammonite association including reelaborated specimens ranging in age from the latest Callovian (Lamberti Zone) to the early Middle Oxfordian (lower Plicatilis Zone; Aurell et al, 1994). Then follow marl-limestone alternations that contain abundant sponges, tuberoids and microbial crusts (up to bed 70a, Fig.…”

Abrasion surfaces on internal moulds of ammonites as palaeobathymetric indicators

“…Internal moulds of ammonites showing so-called ellipsoidal abrasion facets or annular abrasion furrows are quite common at some levels of the Middle Jurassic of the Iberian Range, and they become even more abundant near the Middle-Upper Jurassic boundary. This phenomenon has been mentioned by several authors (Fernández-López, 1985a,b;Aurell, 1990;Aurell and Meléndez, 1990;Fernández-López and Gómez, 1990b;Meléndez et al, 1990;Aurell et al, 1992).…”

“…These surfaces show a distribution of over tens of kilometers and locally display mud cracks. These facies have been interpreted to represent very shallow subtidal environments of an extended carbonate marine platform locally at times exposed to subaerial conditions (Fernández-López, 1985b,c;Fernández-López and Gómez, 1990a;Aurell and Meléndez, 1990;Aurell et al, 1992).…”

Phylloceratina ammonoids in the Iberian Basin during the Middle Jurassic: a model of biogeographical and taphonomic dispersal related to relative sea-level changes

“…They are either accumulated or else resedimented elements (sensu Fernandez L6pez, 1991) showing no signs of colonization by epizoans. They form ammonoid fossil assemblages characterized by their low specifi c diversity and equitability, composed of taphonic populations of the type 1 as categorized by Fernandez-L6pez (1985cFernandez-L6pez ( , 1987Fernandez-L6pez ( , 1995. These associations are domi nated by juvenile individuals and characterized by the occurrence of both dimorphs, macro-and microconch.…”