Benthonic microfaunal associations from the liassic (lower jurassic) of Zambujal, West Central Portugal


Exton, John.




Thirty-one surface samples, collected from sediments belonging to the Pliensbachian and Toarcian stages (Lower Jurassic) exposed near Zambujal in west central Portugal, have been analysed for foraminifera and ostracods. A total of 112 species of foraminifera and 35 species of ostracods have been identified and systematically described.

The boundary between the Pliensbachian and Toarcian stages is marked by a mass extinction of species; only 8 species of foraminifera and 2 species of ostracods are common to both stages, these are:- Ammobaculites alaskensis Tappan, Trochammina sp. 1, Lenticulina gottingensis (Bornemann), Astacolus varians (Bornemann), Dentalina bartensteini Tappan, Dentalina sinemuriensis Terquem, Dentalina pseudocommunis Franke, Eoguttulina liassica (Strickland), Polycope sp. 1, and Cytheropteron sp.

Six microfaunal associations have been recognised based upon percentage abundances of species, taxa of restricted occurrence, and recurring combinations of species and morphological trends. Pliensbachian microfaunas define 3 associations as follows:

Association 1 - characterised by the restricted occurrence and recurring combinations of the following species, Ammobaculites alaskensis, Verneuilinoides mauritii, Ophthalmidium sp. 1, Berthelinella involuta, Nodosaria columnaris, Citharina eugenii, Citharina suturalis, Pseudonodosaria multicostata, Pseudonodosaria guinguecostata, Ogmoconcha spp., Bairdia sp. l, Bairdia sp. 2, and Ostracod sp. 6.

Association 2 - characterised by faunas dominated by either Marginulina prima or Lingulina tenera, together with abundant specimens of species of Polycope, and Cytheropteron sp.

Association 3 - characterised by faunas low in numbers of specimens, and showing little similarity to other Pliensbachian assemblages; Spirillina numismalis and Brizalina liasica are common only in this association.

Toarcian assemblages define three associations as follows:

Association 4 - characterised by faunas dominated by the genus Dentalina and Ostracod sp. 6.

Association 5 - characterised by faunas dominated by Ophthalmidium liasicum and Ostracod sp. 6.

Association 6 - characterised by the following combination of species: Ammobaculites fontinensis, Ammodiscus tenuissimus, Citharinella deslongchampsi, Lenticulina d'orbignyi, Kinkelinella sp. 1 and Cytherella toarcensis.

The distribution of these associations throughout the section is shown to be related to changes in palaeoenvironment.

The following criteria were used in the interpretation of environments:- a) comparison of the fossil microfaunas with known microfaunal distribution and ecology on the present day continental shelves; b) the distribution of microscopic remains of ammonites and belemnites in the Zambujal section; c) variations in sample lithology; d) comparison of the lithological changes seen in the Zambujal section with similar changes occurring at the same stratigraphic level throughout the Tagus Basin.

Microfaunal Associations 1 and 6 occurred in the shallow water, nearshore subtidal environments found in the Jamesoni, lbex and Spinatum Zones of the Pliensbachian, and the Levesquei Zone of the Upper Toarcian.

Associations 2, 3, 4 and 5 occurred in deeper water, nearshore shelf environments found in the Davoei and Margaritatus Zones of the Pliensbachian and in the Lower Toarcian.

The sediment types and microfaunas show a cyclic distribution which correlates well with the stratigraphical distribution of widespread sedimentary cycles known from other parts of Europe. An examination of literature concerning the Liassic sediments of the Tagus basin suggests that the two major sedimentary cycles of the Lias, corresponding to the Pliensbachian and Toarcian stages, are present in Portugal.

Microfaunas of comparable composition to those found at Zambujal are of widespread occurrence in the Lower Jurassic, and are known from Alaska, other parts of Europe, the Grand Banks of Newfoundland, and Sicily in the heart of the Tethyan faunal realm.






Carleton University

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