last changes 29.04.2005

The Yátova Formation

The Formación Calizas con Esponjas de Yátova or short Yátova Formation (Middle to Late Oxfordian) outcrops in eastern Spain where it reaches a maximum thickness of about 45 m. In general it is only 10-15 m thick and ranges stratigraphically from the biozone of Perisphinctes plicatilis to the biozone of Idoceras planula - at least in certain sections.



Lithostratigraphic units of the late Middle Jurassic to Early
Cretaceous of the Iberian Chain (from Krautter, 1998).




Isopach map of the Yátova Formation in eastern Spain.
Small black dots are sections (from Krautter, 1997).
Click thumb to enlarge

 



Thickness and stratigraphic range of the Yátova Formation.
(from Krautter, 1997).
Click thumb to enlarge

In the central part of the Iberian Chains - in the Montes Universales de Albarraçín - the sponge facies shows its most characteristic development (see pictures below).

The sponge-bearing limestones show greyisch to yellow, sometimes reddish or even black colours. It can be described generally as intraclast-bearing micrites. So the microfacies oscillates between pure mud-, wackes-, pack-, float- and boundstones. The dominance of a micritic matrix mirrors the overall low-energy situation which are a prerequisite for an lushy siliceous sponge growth. In the upper part of the sections an alternation between mudstones beds and marly intercalations is obvious. The limestone beds are mostly 0,1 to 0,5 m thick with a maximum thickness between 0,1 and 0,3 m. In the central Iberian Basin a characteristic 2 m thick limestione bed occurs at the base of the sections.

Siliceous sponges are very abundant, sometimes rock-forming and hexactinosidan sponges dominate by far.

The siliceous sponge fauna of the Oxfordian spongiolites (Yátova-Formation) of eastern Spain clearly demonstrates that the shape of morphovariable sponges as well as the taxonomic composition of morphoconstant sponges precisely mirrors external environmental factors. Richness of hardgrounds, automicrites and glauconite, very reduced sediment thicknesses as well as overall character, abundance, low diversity and uniformity of benthic fauna over a minimum area of 75000 qkm suggest a moderately deep, uniform low-energy ramp setting with extremely reduced carbonate and terrigeneous background sedimentation. The very reduced influx is interpreted to have resulted in a very low nutrient level, which is reflected by a strong reduction in filter-feeding organisms such as bivalves, crinoids, brachiopods or serpulids frequent in other Late Jurassic sponge settings. The sponge fauna itself is characterized by the almost exclusive occurrence of a uniform low-diversity, specimen rich fauna of hexactinosidan dish-shaped sponges, uncommon in most other Late Jurassic sponge faunas.
These observations indicate that the major factors controlling the morphological and taxonomic composition of siliceous sponge faunas are sedimentation rate and food supply.

When food is available filter-feeding sponges live on free bacteria. The mesohyl of these sponges shelters a rich microflora of symbiotic bacteria. At low nutrition rates two different strategies are developed. Demosponges enlarge their mesohyl so that more bacteria can be stored during times of reduced food supply. In morphoconstant demosponges, this strategy results in a thickening of the sponge walls in order to increase the available volume. Morphovariable sponges are able to completely change their form and will preferably develop thick knob-like or tube-like morphologies.
The tissue organisation of hexactinellids allows them to absorb colloidal organic matter or dissolved amino acids, which represents their predominant feeding strategy. If even this type of food is scarce, hexactinosidans tend to reduce wall thickness which improves the intense overall contact with sea water. Moreover, morphovariable taxa enlarge their surface by developing thin plate- or dish-shaped forms. Among morphoconstant taxa dish-shaped forms have an adaptional advantage in these settings and will outcompete other forms.

Reduced sedimentation rate is a general precondition for the establishment of sponge communities, although sponges can adapt to a certain degree of sedimentation. Elevated sedimentation favours tube-shaped sponges, since a narrowed osculum produces bundled exhalant water current which shelters the animal from the settlement of sediment particles. Since changes in allochthonous sedimentation are accompanied by changes in the influx of nutrients, sedimentation rate directly or indirectly influences the sponges. Sedimentation therefore represents a primordial factor controlling morphological as well as taxonomic dominance and diversity of siliceous sponge faunas.




Pictures (click the thumbs to enlarge)

 

	Typical view of the Yátova Formation in the central Iberian Chains near Frías de Albarraçín
	Frías de Albarraçín section. Red arrow marks the base of the Yátova Fromation.
	The little village of Moscardón is built on top of the Yátova Formation. Central Iberian Chains, in the vicinity of Albarraçín.
	El Vallecillo section (central Iberian Chains).
	Pozuel del Campo section (north central Iberian Chains).
	Añón section (northern Iberian Chains). Note the dark colour of the Yátova Formation.
	Gallipuén section (northern Iberian Chains). Yátova Formation is reduced in thickness.
	Puebla de San Miguel section (southern Iberian Chains). Condensed top of the section with ammonites, belemnites and crinoid ossicles.
	Enguidanos section (southern Iberian Chains). Picture shows central part of the section.
	Part of the Pozo Cañada section in the Prebetic Chains. Note the small sponge mud mound. Scale: 1 m.

 

 

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