The Transversarium-Bifurcatus Zone boundary at Rocha (Peral area, East-CentraIAlgarve, Portugal)

Abstract

Key-words: Ammonites-Benthic macroinvertebrates - Biostratigraphy - Ecostratigraphy, Discontinuities - Upper Jurassic - Oxfordian - Algarve - South Portugal. This paper represents the first detailed study of the Transversarium-Bifurcatus Zone boundary in the Algarve. The boundary studied in the Peral area is associated with a stratigraphic discontinuity, whose hiatus partially affects the Transversarium-Bifurcatus Zones. A discontinuity was also recognized in the Bifurcatus-Bimammatum Zone boundary, which can be correlated with the traces of a Type II unconformity, which separates cycles 4.3 - 4.4 in HAQ et al. (1987), present on the South Iberian palaeomargin. An analysis was made of the ecostratigraphic evolution in the interval between the uppermost Transversarium Zone and the lower part of the Bimammatum Zone on the basis of the faunal spectra obtained. We conclude that ammonites are the most tolerant cephalopods to the ecological stress caused by the increase of inflows and the decrease of the shelf's ecospace. Benthic fauna decreased considerably in these conditions. A relatively abundant and diversified fauna of Dichotomoceras is noteworthy among the ammonites collected, as this genus was previously little known in the Algarve.