Paulo Caetano

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.

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.