Quantitative analysis of middle and late Jurassic foraminifera from Portugal and its implications for the Grand Banks of Newfoundland

Abstract

Quantitative analyses were made of the foraminiferal assemblages in 137 samples from five Portuguese sections (Murtinheira, Brenha, Tojeira 1, Tojeira 2 and Mareta Beach) of Middle and Late Jurassic age. Trends in depositional environment (for example from deep to shallow water sediments) together with R- and Q-mode analyses determined three different groups of foraminifera, each group having its own bathymetric preference. Spirillina tenuissima, Spirillina elongata, Spirillina infima, Ophthalmidium carinatum and Paalzowellalei/eli preferred relatively shallow water, up to about 50 metres. Epistomina mosquensis, Pseudolamarckina rjasanensis, Ophthalmidium strumosum and the agglutinated foraminifera preferred relatively deep water, from about 200 to 250 metres or possibly more. Lenticulina muensteri, Lenticulina spp., Discorbis spp., Eoguttulina spp. and Nodosaria/Dentalina spp. filled the depth niche in between, from about 50 to over 150 metres. Eoguttulina spp. and Nodosaria/Dentalina spp. probably preferred depths closer to 150 than to 50 metres. Coeval foraminiferal assemblages from 18 wells on the Grand Banks off Newfoundland proved to be closely similar to those of Portugal, allowing for a more precise estimate of the paleowaterdepths of Middle and Upper Jurassic sediments of the Grand Banks. Middle Jurassic representatives of the genera Garantella, Reinholdella and Epistomina occur in three wells on the Grand Banks, but are not known from the coeval parts of any other wells and any of the Portuguese sections. This is explained as being the result of local differences in depositional environment; the epistominids preferring relatively deep water. Two new biostratigraphic zones are defined: (1) Reinholdella spp. Zone of (Late) Bathonian age, and (2) Epistomina regularis Zone of Callovian age. A range chart shows the stratigraphic distribution of taxa known from the Grand Banks and Portugal. The burial history of five wells, using the specially developed Fortran 77 programs BURSUB and DEPOR, illustrates that up to 50% of the (de)compacted burial is due to tectonic subsidence. Decompacted sedimentation rates, especially during the Late Jurassic, are higher than the decompacted burial rates, resulting in a shallowing. This Late Jurassic shallowing can be recognized in all 18 wells. Of the 14 Jurassic planktonic species described to date only three can be recognized in the Portuguese and Grand Banks strata, using type material from Poland, France and Switzerland. These three taxa have been re-defined such that they encompass the variation previously assigned to the 14 taxa listed in the literature. The three taxa are: (1) Globuligerina bathoniana, ranging from ?Late Bajocian through Early Valanginian (2) Globuligerina oxfordiana, ranging from Middle-Late Bajocian through Early Valanginian, and (3) Globuligerina balakhmatovae, ranging from Late Bajocian through Late Bathonian, possibly through Early Kimmeridgian. The occurrences of these three species, which are among the oldest planktonic foraminifera known, may be ecologically controlled.