Rogério Rocha

Flat-pebble conglomerates: a local marker for Early Jurassic seismicity related to syn-rift tectonics in the Sesimbra area (Lusitanian Basin, Portugal)

Flat-pebble conglomerates have been identified in the Lower Toarcian (Levisoni Zone) carbonates of the Sesimbra region (30 km south of Lisboa, Portugal) and related to submarine mass movements. Their origin is explained through a three-stage model based on the comparative analysis of potential generating mechanisms taking into account timing and type of geodynamic evolution in the Lusitanian Basin: (a) differential lithification of thin carbonate and non-bioturbated horizons embedded within a more argillaceous matrix; (b) disruption by seismic shocks related to active extensional faulting and block tilting; and (c) gravity sliding mixing material resulting from broken lithified horizons. This sequential process originated flat-pebble conglomerates during early Jurassic phases of syn-rift evolution in the southern Lusitanian Basin. q 2001 Elsevier Science B.V. All rights reserved.

Palaeogeographical Evolution of the Lusitanian Basin (Portugal) During the Jurassic. Part I: The Tectonic Constraints and Sedimentary Response

In this paper, we present the general tectonic framework that conditioned the evolution of the Lusitanian Basin during the Jurassic, when most of the filling of the basin was accumulated. The rifting episodes and the evolution of the basin’s geometry are presented and their close relationship with the main basin-wide unconformities is discussed. The unconformity-bounded sequences are briefly presented as a basis for understanding the evolution of the sedimentary environments. This constitutes the basic geological information to construct the palaeogeographical maps presented in Part II of this paper (this volume).

A Record of Upper Hauterivian Climatic Variability in a Lagoonal Environment in the Lusitanian Basin

Using the example of the upper Hauterivian in the Lusitanian Basin, we demonstrate a climatic control of cyclic variations in the biological, mineralogical, chemical, and magnetic markers in a lagoonal environment. In a lagoonal setting, variations in detrital supply, controlled by climate changes, modify the water turbidity. During humid climatic conditions, the increase in water turbidity reduces light penetration and photosynthetic activity. The reduced precipitation of carbonates is probably due to the low concentration of cyanobacteria or unicellular green algae, and is associated with a decrease in micritization processes. Another consequence of the increase in detrital supply is the reduction in benthic foraminifera, with the development of infaunal forms in favour of epifaunal forms, and the proliferation of large agglutinated foraminifera such as Choffatella decipiens. During dry climatic conditions, this tendency is reversed and there is an enhanced contribution of Miliolidae. Thus, in this lagoonal setting, climatic variability (controlled by orbital parameters) modified carbonate production, the faunal assemblages, and the water chemistry.

Bacia Lusitânica ou Bacia Lusitaniana

For more than last sixty years, numerous denominations have been used in the Portuguese language to name the Atlantic-type sedimentary basin formed between the Upper Triassic and the Late Cretaceous, located in the central part of the western Iberian margin, and associated with the initial episodes of fragmentation of the Pangea in the North Atlantic; in recent years only two names have been used: “Bacia Lusitânica” and “Bacia Lusitaniana”. The present text defends this second position based on strictly geological / stratigraphic reasons. Although Paul Choffat, the “creator” of the “Lusitanian” stage did not defined the etymology of the the word (“Lusitaniano” in Portuguese), we at least consider that this stage is clearly linked to the existence, in the Basin, of stratigraphic units of this age. It is shown that the adjective “Lusitanian” is neither a foreign word or a mistranslation of terms in both English (Lusitanian) and French (Lusitanienne). The origin of the people (“Lusitanos”) and the territory (“Lusitânia”) from a linguistic, ethnographic, historical and geographical point of view is discussed in detail to show how the use of the term “Bacia Lusitânica” (or even “Bacia Lusitana”) for this basin is inappropriate. On the other hand, we defend that the designation of “Bacia Lusitaniana” is clearly correct, because it is clearly linked to the existence of the “Lusitaniano” (geological stage) a particularity of the basin itself; this represents also a well-deserved tribute to this extraordinary geologist, one of the precursors of the Portuguese geology.

Palaeogeographical Evolution of the Lusitanian Basin (Portugal) During the Jurassic. Part II: The Slow-to-Fast Transformations of Sedimentary Infilling

In this paper we present the first palaeogeographical maps of the Lusitanian Basin during the Jurassic. We also present the methodology used to produce these maps and discuss the close relationship of the temporal and spatial distributions of facies with the sequence of tectonic events in the basin (already discussed in Part I of this study, this volume). One of the main conclusions is that the main bordering N–S-trending faults were the main tectonic structures that conditioned the facies geometry but only during paroxysmal events; during those short intervals, “instant” modifications of the facies geometry occurred. The other main conclusion is that in the time intervals between the paroxysmal events, the structures that conditioned the facies distribution were the main transfer faults, oriented ENE–WSW, and the sedimentary environments gradually rotated from N–S to ENE–WSW.