last changes 15.04.2005

In the Lusitanian Basin (south of Coimbra), the marl-limestone alternations of the middle Toarcian/lower Aalenian show the occurrence of small- -scale siliceous sponge mud mounds.

These facies are particularly well documented and preserved, belonging to the following two units: Marls and marly limestones with sponge reefs (S. Gião Fm) and to the lower part of the Póvoa da Lomba Fm. Both units correspond to very bioturbated (Chondrites, Zoophycos, Planolites and Thalassinoides) marl-limestone successions. Between these two units exists the Marls and marly limestone with brachiopods (top of S. Gião Fm) wich corresponds to a marly unit, with rare limestone levels and are very poor in siliceous sponge reefs. This formation is dated of the uppermost Bifrons Zone through the base of the Bonarellii Zone. The majority of the siliceous sponge mud mounds occur within this time slice. These mounds are characterized by a great abundance and diversity of benthic macrofauna mainly composed of brachiopods (rhynchonellids and terebratulids), crinoids and bivalves. The base (Meneghini-Opalinum interval) of the Póvoa da Lomba Fm is composed by bioturbated marl/limestone alternations, with an upward increase of calcareous (biomicrite/wackestone, locally packstone) facies. The rhythmicity of the marl/limestone couplets is an important feature, defining elementary sequences (parasequences). The vertical succession of the parasequences is organised into parasequence groups showing a clear asymmetry, with the marls typically thicker at the base. Besides, the tops of these fourth-order sequences are marked by a thickening of the limestone beds and, sometimes, by a large density of Zoophycos. The siliceous sponge mud mound occurrences are particularly associated with these levels.

 

Map of Portugal. The siliceous sponge mud mounds are located just south of Coimbra (asterisk).

Litho- and biostratigraphy of the Toarcian- Lower Aalenian from the Lusitanian Basin.

 

The Toarcian mud mounds of the Lusitanian Basin are usually only a few decimetres thick and most display irregular knob -like to flat lenticular morphologies. Some build-ups are round and can reach 1.5 metres in thickness and ten metres in diameter. The upper mound surface is normally rough and uneven. In both sequences they are always related laterally with carbonate beds, which correspond preferentially to the tops of fourth-order sequences. The mud mounds consist of mostly brownish, iron-rich calcified siliceous sponges and a greyish, sometimes peloidal allochthonous micritic matrix. In general, the sponges themselves consist of dense leiolitic microbolites (automicrites sensu Reitner & Neuweiler, 1993). The sponge spicules are diagenetically transformed into calcite. The great majority of the sponge specimens belongs to the Hexactinosida (Class Hexactinellida) and are unknown and undescribed to date. “Lithistides” (polyphyletic desma-bearing demosponges) are very rare and only occur as forms encrusting hexactinosidan sponges. The benthic macrofauna is abundant and consists of monospecific crinoids, rhynchonellids, terebratulids and bivalves (mainly pectinids and ostreids). Encrusting organisms are serpulids, bryozoans and foraminifera, as well as “Lithistids”, mentioned above. They are entirely restricted to the stratal surfaces of the siliceous sponges. The sponge reefs consist of several microfacies types (wackestones, packstones, floatstones and boundstones). All of them are micrite dominated and represent low energy environments. They differ mainly in the amount of siliceous sponges, micrite, microbialites, encrusting organisms and the accompanying fauna.

Palaeoenvironmental Significance

The amount of microbially induced carbonate clearly emphasizes the importance of microbial activity in respect of the reef building potential. Furthermore, three other controlling factors played an important role in the initiation of the siliceous sponge mud mounds of the Lusitanian Basin: bathymetry, sea-floor morphology and sedimentation rate. The role of the first two factors is evident because the siliceous sponge mud mounds are particularly important (abundance and volumetric expression) in the eastern part of the basin (Rabaçal-Alvaiázere region). They are practically absent towards the west (essentially in the base of Póvoa da Lomba Fm) where the series shows hemipelagic features. Reduced sedimentation rate was a precondition for the settlement of siliceous sponges and Hexactinosida in particular. The preferential occurrence of these bioconstructions at the top of fourth order sequences seems to be related with sediment-starved intervals.

 

Pictures (click the thumbs to enlarge)

 

	typical small-scaled knob-like sponge mud mound in the Lusitanian Basin of Portugal
	small-scaled lentille-shaped sponge mud mound in the Lusitanian Basin of Portugal
	small knob-like sponge mud mound in the Lusitanian Basin of Portugal
	undiscribed hexactinosidan sponge
	Bioclast-rich sponge floatstone with various sponge specimens partly stained with iron-oxide
	"Baby"-sponge reef, formed by several small hexactinosidan sponges which grow upon each other

 

 

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