Seismic and Sedimentologic Features of Kimmeridgian Syn-Rift Siliciclastic and Carbonate Slope Deposits of the Lusitanian Basin, Portugal

P. M. Ellis, R. C. L. Wilson, P. M. Ellwood, R. R. Leinfelder

During the Kimmeridgian, the southern part of the Lusitanian basin immediately north of Lisbon became differentiated into three subbasins. The northern Bombaral subbasin is a salt withdrawal structure flanked by salt walls and pillows. To the south, two north-south-trending half-graben subbasins occur, bounded by westward-dipping faults. The Arruda subbasin is the eastern basin of this pair. The siliciclastic Abadia Formation occurs in all three subbasins, and a carbonate-dominated time-equivalent sequence occurs to the southwest.

Sedimentological studies of outcrops of the Abadia Formation, which is up to 1,000 m thick, reveal it was deposited in a southward-prograding siliciclastic slope system capped by shelf carbonates of the Amaral Formation. On modern seismic data shot in the Bombarral basin, southward-dipping tangential clinoforms characterize the top 500 m or so of the Abadia Formation, confirming the sedimentological model. When traced across the Montejunto salt high, the thickness of the clinoform seismic unit is unchanged. Beneath it in the Arruda subbasin, over 2,200 m of arkosic gravels and sandstones occur, deposited in a submarine fan environment. Thus in earliest Kimmeridgian times the Arruda subbasin subsided by over 2 km. This spectacular differential subsidence then ceased, permitting the Aba ia slope system to prograde across the subbasin.

The structural setting of the Kimmeridgian carbonate slope deposits exposed around the Late Cretaceous Sintra granite 20 km west of Lisbon cannot be defined as clearly because no seismic data is available. The Kimmeridgian sequence begins with the Ramalhao Formation (>400 m), which is dominated by contact metamorphosed shales. It is followed by the Mem Martins Formation (400 m), which consists of siliciclastic shales, carbonate muds, debris flows, and turbidites. This formation is overlain by a dominantly lime-mud sequence of the Farta Pao Formation. The sequence is interpreted as a highly aggradational low-angle slope/ramp system inclined southeastward, possible situated on the gently inclined side of a half-graben subbasin.

AAPG Search and Discovery Article #91032©1988 Mediterranean Basins Conference and Exhibition, Nice, France, 25-28 September 1988.