Facies, Stratigraphy and Paleogeographic Analysis of Upper? Kimmeridgian to Upper Portlandian Sediments in the Environs of Arruda dos Vinhos, Estremadura, Portugal.-
Münchner Geowissenschaftliche Abhandlungen, A, 7, 1-215, München.
Late Upper Kimmeridgian to final Portlandian sediments in the environs of Arruda dos Vinhos, Portugal, consist of deltaic siliciclastics, the Sobral formation, and lagoonal deposits, the limestone-marl sequence of the "Pteroceriano" formation and the limestone-marl-sandstone sequence of the Freixial formation. The lagoonal sediments grade into coastal and terrestrial clastics in northward and westward direction.
Formal definition according to the ISSC norms is suggestively given for all lithological units and tentatively applied to further regions by basing on published studies in order to give a lithostratigraphic framework for the highly intertonguing sequence of the study area and the entire Lusitanian Basin. This approach is of special importance, since available stratigraphic data are scarce and sometimes even contradictory. The examined sequence lies within the Virguliana biozone and the Lusitanica biozone, based on benthic foraminifers. A further subdivision of the first is possible by the appearance of the alga Permocalculus n.sp. which is preliminarily described. The lack of a refined biostratigraphic classification can be partly balanced by considerations on subsidence and depositional rates, global sea level changes, diachronism, sedimentary processes and depositional models, so that a sketch-like paleogeographic reconstruction can be concluded.
The Upper to late Upper Kimmeridgian Sobral formation consists of three main sediment types: Sandstones exhibiting the brackish water bivalve Eomiodon securiformis are attributed to delta front environments, thin oolitic horizons are interpreted as distal/tidal bars, and silty-marly sediments, often with bivalve meadows of Gervillia sobralensis or bivalve banks of Isognomon lusitanicum, represent prodelta and interdistributary bay deposits. Soft bottom substrata were also inhabited by the specialized regular sea-urchin Pseudocidans lusitanicus and occasionally by adapted corals which are interpreted in respect of functional morphology. The delta complex was prograding from west and north. It was biparted in N-S direction by a morphological and structural ridge, possibly caused by uprising salt diapirs. The delta not only outwedges towards south but also towards southeast due to another structural high along the Vila Franca fault system. This resulted in lowered subsidence rates, so that distribution of the eastern part of the delta was mainly determined by the heavily subsiding Arruda depo-center.
The superimposed, uppermost Kimmeridgian to early Lower Portlandian "Pteroceriano" formation is mainly restricted to this Arruda depo-center, only extending further north and west at its base, probably coinciding with the peak in global eustatic sea level highstand. The formation's lower part is mainly composed of low energetic marls and limestones, among which the nodular Arcomytilus limestones deserve special interest, since they indicate occasional very rapid deposition of lime mud followed by morphological adaptions of the bivalve Arcomytilus morrisi. Macrofaunal shell coquinas interpreted as storm layers are another important facies type.
Intensive subsidence in the Arruda depo-center caused large thicknesses and pronounced channelling of terrigenous clay, in contrast to the structurally higher block further east where low depositional rates led to oncoid formation and occasional establishment of coral facies. Heavy, though intermittent, subsidence in a wrench block directly bordering the Vila Franca fault system further east resulted in considerable thicknesses in this area. Contemporaneously, marginal marine siliciclastics (Santa Cruz member of the Bombarral formation, upper sandstone group) were deposited further west and north which only occasionally affected the area of ,"Pteroceriano" facies around its western and northern borders. This indicates the establishment of a clastic trap in the north and the continuing activity of the mentioned morphological elevations, now acting as clastic fences in the west and east. General shallowing in the upper part of the "Pteroceriano" formation led to a vast establishment of mud-rich coral patch reefs and associated high and low energy facies types. Particularly discussed here are the adaption of corals to fairly high background sedimentation and the systematic position of some algae (Marinella lugeoni, Solenopora cayeuxiformis n.sp., Lithocodium sp ). This development was restricted to the Arruda depo-center. Perfect sheltering from terrestrial clastics in the west, north and, probably, east (Bombarral formation) indicates once more highly potent clastic fences in form of now emerging elevations. High subsidence rates in the north resulted in trapping coarse clastics, only allowing terrigenous clay to pass which settled down in tranquil water settings between individual patch reefs. During deposition of the late Lower to final Portlandian Freixial formation, no clastic traps or fences were obvious, so that episodic hinterland uplift and minor sea level fluctuations resulted in rapid spreading and withdrawal of sand facies from and to the west, north and east. More basinwards, typical lagoonal limestones were deposited, dominated by foraminifers during times of slightly elevated salinities or by algae during more normal periods. The study area silted and sanded up completely towards the Cretaceous boundary.
The late Upper Jurassic Lusitanian Basin displays the typical character of a protoocean marginal basin, characterized by calcareous facies into which clastic wedges were prograding. Yet, basin configuration and symmetry of sediment arrangement differs from other Jurassic marginal basins of the young Northern Atlantic, thus pointing out the control of basin development by local parameters.