Abstract
Campanian-Maastrichtian (C-M) sediments in the Kalâat Senan area (north-western Tunisia) include rich microfauna. Small benthic foraminifera (SBF) analysis at the Wadi Necham (ON) section reveals significant biostratigraphic and palaeoecological changes during the C-M transition. A progressive evolutionary trend within the Bolivinoides lineage allowed us to establish the late Campanian B. decoratus Interval Range Zone (IRZ) and two successive zones in the early Maastrichtian, namely the B. miliaris IRZ and the B. draco Total Range Zone (TRZ), which span the standard planktonic foraminifera Gansserina gansseri IRZ. Foraminiferal semiquantitative data from the C-M transition interval at the studied section yield high values of planktonic/benthic (P/B) ratios (92%-98%) suggesting an upper to middle bathyal depositional palaeoenvironment. Assemblages of such a deep environment comprise diversified epibenthic and endobenthic taxa indicating major changes across and above the C-M transition. During the late Campanian (Bolivinoides decoratus Zone) endobenthic morphogroups dominate (71%) the assemblages as compared to epibenthic morphogroups, which reach 29%. In contrast, the abundance of epibenthic morphogroups increases to 40% just above the C/M boundary (Bolivinoides miliaris Zone) and suggests cooler and more oxygenated conditions on the sea floor. Epibenthic and endobenthic abundances fluctuate in opposite trends (40%-60%) throughout the early Maastrichtian (B. draco TRZ). These variations are interpreted to reflect mesotrophic conditions intermittently interrupted by eutrophic shifts associated with the early Maastrichtian cooling event. The relative proportion of endobenthics increased (60%-80%) again towards the late Maastrichtian (upper part of the B. draco TRZ), showing a particular increase in EnA morphogroups prevailing in the SBF assemblages. Moreover, test morphology analyses of SBF assemblages allowed us to distinguish six epibenthic (M1 to M6) and seven endobenthic (M7 to M13) morphogroups based on their mode of coiling. This classification reveals the flourishing of EpC M4 and M5 morphogroups during the early Maastrichtian due to more oxygenated conditions on the sea floor.
Highlights
A total of 62 species of small benthic foraminifera are recognised in the studied succession revealing diversified
The present work is part of the debate on the C-M transition and boundary (C/M) when significant palaeoenvironmental changes occurred as recorded in the temporal distribution patterns of small benthic foraminifera in the Kalâat Senan area
The biostratigraphic results are consistent with the adapted planktonic foraminiferal subzonal scheme and the recommendations formally accepted at the Brussels symposium of 1995
Summary
The Campanian/Maastrichtian (C/M) boundary has long been identified with the last occurrence (LO) of Radotruncana calcarata (Cushman, 1946; Saïd, 1978; Salaj, 1980; Bellier, 1983; Robaszynski et al, 1984; Caron, 1985; Li and Keller, 1998b; Rami, 1998; Li et al, 1999). In Tunisia, several studies of the CampanianMaastrichtian (C-M) interval used taxonomic characteristics and biostratigraphy of different palaeontological groups to define the C/M boundary (e.g., ammonites, belemnites, inoceramids, planktonic and benthic foraminifera, Ostracoda) (Saïd, 1978; Salaj, 1980; Donze et al, 1982; Nederbragt, 1991; Negra, 1994; Rami, 1998; Robaszynski et al, 2000) Most of these studies placed the C/M boundary within the intermediate member of the Abiod Formation (Burollet, 1956), based on the LO of Radotruncana calcarata (Saïd, 1978; Salaj, 1980; Donze et al, 1982; Nederbragt, 1991; Negra, 1994; Rami, 1998). Robaszynski et al (2000) placed the boundary within their Archaeoglobitruncana kefiana? Zone, corresponding to the lower part of the standard Gansserina gansseri Zone (Caron, 1985) and coeval with the interval of the newly proposed Ncham member, which correlates with the uppermost limestone beds of the Abiod Formation (Burollet, 1956; Negra, 1994)
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