Abstract
back to Publications Duarte, L. V., Krautter, M. & Soares, A.F. (2001): Siliceous sponge buildups in the late Liassic of the Lusitanien basin (Portugal): stratigraphy, sedimentology and palaeogeographic significance. - Bull. Soc. géol. France 172/5: 637-647, 8 figs.; Paris.
Siliceous sponge buildups in the late Liassic of the Lusitanien basin (Portugal): stratigraphy, sedimentology and palaeogeographic significance
Abstract: The Upper Liassic series in the western border
of Iberia (Lusitanian Basin, Portugal), show an important lutitic sedimentation,
characterized generaly by a monotonous marl/limestone alternation. Small
scale siliceous sponge mudmounds occur in these deposits from Middle Toarcian
to Lower Aalenian age. The scope of this work is to pinpoint the stratigraphical
and sedimentological context and to characterize controlling factors of
the spongioliths.
Stratigraphic and Facies Analysis: Relevant sections were observed and
investigated in different locations of the Lusitanian Basin (e. g. Alvaiázere,
Porto de Mós, Rabaçal, Coimbra and Cantanhede) (Fig. 1).
The siliceous sponge facies correspond to the upper part of the S. Gião
Unit and to the lower part of the Póvoa da Lomba Unit (Fig. 2).
Considering the sequential scheme of Duarte (1997), the sediments correspond
to groups of third-order depositional sequences MST3 and MST4 (mainly
in the upper part of this sequence: MST4B). The sedimentary evolution
of these units shows a stacking pattern composed of shallowing upward
sequences deposited in an outer homoclinal ramp setting, dipping northwestwards.
Both units increase in thickness from south to north (Fig. 3) and their
vertical facies associations correspond to a very bioturbated (Chondrites,
Zoophycos, Planolites and Thalassinoides) marl/limestone succession (Figs.
4 and 5). MST3 is demonstrably more marly than MST4B. The base of MST4
(MST4A in Duarte, 1997) corresponds to a marl/marly limestone alternation,
very poor in siliceous sponge mudmounds.
The first unit (MST3) which includes sponge mudmounds is dated as uppermost
Bifrons zone through the base of the Bonarellii zone. The majority of
the siliceous sponge mudmounds occur within this time slice. These mounds
are characterized by a great diversity of accompanying fauna mainly composed
of brachiopods (rhynchonellids and terebratulids), crinoids and bivalves).
The initial growth of the sponge build-ups can be correlated basin-wide
to the intra Bifrons regional flooding surface (MST2/MST3 boundary).
The second unit (MST4), particularly its upper part (MST4B), corresponds
to the top of the Meneghinni-Opalinum interval and is related to a carbonate
progradational phase. In the eastern part of the basin, the calcareous
facies of MST4B are more bioclastic.
Siliceous Sponge Mudmounds: The Toarcian mudmounds 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 diametre. Also worth mentioning
is a siliceous sponge biostrome developed at the base of MST3 in the Porto
de Mós section (Fig. 6). The upper mound surface is normally rough
and uneven. In both sequences they are always related lateraly with carbonate
beds, which corresponds to the top of fourth order sequences.
The mudmounds 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 belong to the Hexactinosa (Class Hexactinellida) and
are unknown and undescibed to date. "Lithistides" (polyphyletic
desma-bearing demosponges) are very rare and only occur as forms encrusting
Hexactinosan 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 stratinomical surfaces of the siliceous
sponges.
The sponge bioherms consist of several microfacies types (wackestones,
packstones, floatstones and boundstones). All of them are micrite dominated
and represent low energy environnements. They differ mainly in the amount
of siliceous sponges, micrite, microbialites and the accompanying fauna.
Palaeoenvironmental Significance: The amount of microbial induced carbonate
clearly mirrors the importance of microbial activity in respect of the
reef building potential. Furthermore, three other controling factors played
an important role in the initiation of the siliceous sponge mudmounds
of the Lusitanian Basin: bathymetry, sea-floor morphology and sedimentation
rate. The role of the first two factors is evident because the siliceous
sponge mudmounds are particularly important (abundance and volumetric
expression) in the eastern part of the basin (Rabaçal-Alvaiázere
region). They are pratically absent towards the west (essentially in MST4B)
where the series show hemipelagic sedimentologic features (Figs. 7 and
8).
Reduced sedimentation rate is a precondition for the settlement of siliceous
sponges and Hexactinosa in particular. Compared to of all other toarcian
sequential units, MST3 and MST4B are the thinnest and therefore reflect
the lowest sedimentation rates (Fig. 8).