Planktonic foraminiferal biostratigraphy and paleoecology of Upper Cretaceous deposits from the Palmyride Region, Syria

Abstract

This study represents a detailed micropaleontological investigation of the composition and diversity of planktonic foraminiferal assemblages from the Upper Turonian to Maastrichtian interval of two deep exploration wells (Al Mahr-1 and Palmyra-1) in the Palmyride area of Syria. In combination with a litostratigraphic analysis, this detailed biostratigraphic study provided important new paleoecological and paleoclimatic interpretations and insights into the nature of deposition along the northern passive margin of Gondwana during the Late Cretaceous.

The investigated strata belong to three lithostratigraphic units (from base to top): the upper part of the Judea Formation (Upper Turonian–Earliest Santonian), the Soukhne Formation (Santonian–Lower Campanian), and the Shiranish Formation (Upper Campanian–Maastrichtian). The results represent the first detailed determination of planktonic foraminifera from the Palmyidra region. The presence of rich and diverse foraminiferal associations enabled the establishment of the following nine late Turonian to Maastrichtian biostratigraphic zones based on documented index-taxa and/or the entire microfossil assemblages: I) Dicarinella concavata Zone; II) Dicarinella asymetrica Zone; III) Globotruncanita elevaata Zone; IV) Contusotruncana plummerae Zone; V) Radotruncana calcarata Zone; VI) Globotruncanella havanensis Zone;VII) Pseudoguembelina palpebra Zone; VIII) Racemiguembelina fructicosa Zone; and IX) Abathomphalus mayaroensis Zone. The late Turonian to early Campanian foraminiferal assemblages (biozones I-IV) are highly dominated by opportunistic taxa (r-strategists) and suggest a generally fluctuating subtropical climate and deposition in outer shelf environments. The well-preserved and highly diversified late Campanian to Maastrichtian foraminiferal assemblages (biozones V-IX) imply the presence of a well-stratified water column, tropical to subtropical climate, and deposition in outer shelf to upper bathyal environments. A decrease in the number of planktonic species during the late Maastrichtian indicates cooling and deterioration of paleoceanographic conditions.

Common occurrence of phosphate grains in the Soukhne Formation (Santonian–Lower Campanian) present an important indicator of specific geological and paleoenvironmental conditions, such as oxygen deficiency, upwelling and transgression. These conditions support the interpretations of high speciation and domination of opportunistic planktonic foraminiferal taxa (heterohelicids and muricohedbergelids) in Biozone II Dicarinella asymetrica Zone.