Siliceous sponge facies from the Upper Jurassic of Portugal.-
Memorial Vol. Prof. Gwinner, N. Jahrb. Geol. Paläont., Abh., 189, 199-254, Stuttgart, 1993
Siliceous sponge facies is widely developed in the Upper Jurassic sedimentary basins of Portugal. Most occurrences are situated in the eastern Algarve basin, and can be considered as part of the Upper Jurassic sponge facies belt extending across Europe along the northern Tethyan shelf. The Portuguese occurrences of siliceous sponges range from Middle Oxfordian to Middle Kimmeridgian in age. They characterize a variety of facies, namely (1) sponge meadows and biostromes, (2) sponge/microbial crust mud mounds, (3) microbial thrombolites containing siliceous sponges, (4) thrombolites with corals and siliceous sponges, and (5) coral/siliceous sponge debris facies and muddy boundstones. Siliceous sponge groups comprise dictyids, lychniscids, tetractinellids and "lithistids". These groups vary in their proportions depending on the sponge facies type. The sponge genera and species are characteristic of the Tethyan realm.
The major factors controlling the occurrence and composition of sponge facies are bathymetry, sedimentation rate and fluctuations in oxygen content. Bathymetric interpretation is based on position of sponge facies within shallowing-upwards successions and on faunal gradients established by semiquantitative palaeoecological analysis. Low sedimentation rate is a prerequisite for the occurrence of sponge facies, particularly the crust-rich types. Indicative for low sedimentation rates are glauconite content, faunal incrustations and association with distinct condensation levels. Glauconite enrichments, the occurrence of the dysaerobic bivalve Aulacomyella and strong impoverishment of fauna in some levels indicate low to fluctuating bottom water oxygenation responsible for the growth of sponge-bearing thrombolites and for rapid transitions from thrombolite facies to coral facies.
The controlling factors are directly linked to sea level fluctuations and their influence on climate and ocean water circulation. Lowering of sedimentation rate during sea level rise enabled establishment of reef facies across a wide bathymetric range. From shallow to deep, the coral facies is followed by a mixed coral-siliceous sponge facies, by siliceous sponge facies and, finally, by oxygen-controlled sponge thrombolites. In the balanced greenhouse climate of the Late Jurassic, larger sea level rises resulted in additional climatic equilibration and hence slacking of oceanic circulation. This induced major rises of oxygen deficient waters, which explains the occurrence of shallow-water sponge-bearing thrombolites and rapid transitions from sponge thrombolites to coral facies.
Last changes Nov. 2004 by Reinhold Leinfelder