8th International Nannoplankton Association Conference


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Armando A. Scarparo Cunha:
Dry-wet climatic cycles in the Turonian sequences of the Sergipe (NE Brazialian continental margin) and Angola (SW African continental margin) Basins

The Turonian sedimentary sequences in the Sergipe Basin (NE Brazil) and in the Angola Basin (SW Africa) have a well-defined cyclicity pattern. In the Sergipe Basin, these are represented by thin, varve-like laminae deposits and limestone/marlstone couplets. DSDP 364 recovered two cores (#22 and #23) that show cyclic sedimentation of the nannofossil chalks and mudstones. In both areas, these cycles are believed to reflect changes in primary productivity in response to runoff variations driven by climatic changes.

In the Sergipe Basin, one PETROBRAS exploratory well (SE-1) and one outcrop section, 3m thick, located in the Votoratim Quarry, were studied. The dominant facies of the Votorantim Quarry is carbonate mudstones with alternate layers of light to dark grey colour, and thicknesses that usually vary from 40 to 70cm. Well SE-1 shows a varve-like facies, with beige colour and very fine, dark laminae. Six retrogradational-progradational long-term cycles were recognised in the Turonian stratigraphic sequence from gamma-ray and sonic logs of Well SE-1. These long-term cycles were controlled mainly by tectonic-eustatic events of second order. The shorter term was modulated by orbitally-induced climatic changes.

A spectral analysis (FFT) performed on the gamma-ray log and numerical data which reflect the colour variations of core #9 (1m) from Well SE-1, reveal a periodicity due to the presence of alternating organic matter-rich/organic matter-poor layers. The very fine, dark organic-rich layers are mainly composed of dinoflagellates and amorphous organic matter. It was not possible to perform spectral analyses on the data from the Votorantim Quarry section. However, the TOC, delta13C, CaCO3 and Eprolithus abundance follow cyclic patterns. The dark layers are characterised by a relatively higher TOC, heavier delta13C values, monospecific calcareous nannofossils assemblages (Eprolithus floralis), spherical radiolarians and abundance peaks of the benthic foraminifera, Turrispirillina subconica (Figure 1).

Spectral analyses performed on colour logs of cores #22 and #23 of DSDP Site 364 shows cycles that are interpreted as relating to the eccentricity (100ky), obliquity (41ky) and precessional cycles (23-19ky). Some dark layers are characterised by a higher TOC, lower carbonate content, and abundance peaks of Eprolithus eptapetalus (core #23) or Marthasterites furcatus (core #22). The pale limestone appears to have a higher proportion of well-preserved and high-diversity nannofossil assemblages. These two lithologies differ isotopically. Carbonates from the dark layers contain heavier carbon and oxygen isotope values. This shift towards heavier values is attributed to an increase in surface-water productivity (Figure 2).

In cores #22, #23, #24 and #25 of Site 364, the organic matter, carbonate and clay content fluctuations have been explained either by intermittent oceanic fertility or by dissolution of the fine carbonate components (nannofossils) during cyclic euxinic events (Melguen, 1978; Thierstein & Roth, 1991). The relative increase of Eprolithus in the dark layers and the shift of delta13C to heavier values could suggest more-intense phytoplankton activity. In this respect, the Turonian deposits of Site 364 resemble, in part, the ones observed in the Votorantim Quarry and Well SE-1. The increase in certain nannofossils (Eprolithus and Marthasterites), foraminifers (Turrispirillina subconica), radiolarians and dinoflagellates is associated with eutrophication caused by an increase in continental runoff. The organic matter variation, dominantly of phytoplankton origin, is in response to the continental influx which increases in the wet phases.

Limestone/marlstone bedding couplets, with periodic or quasiperiodic patterns, are inferred to result from variations in organic carbon, mud content and carbonate productivity linked to dry-wet climatic oscillations (Koutsoukos, 1989; Carmo & Pratt, 1999). Palaeoclimatic models have shown that dry-wet climatic changes are directly associated with short-term, orbital-climatically-driven cycles (Perlmutter & Matthews, 1989; Park & Oglesby, 1994).


Carmo, A.M. & Pratt, L.M. 1999. Deciphering Late Cretaceous subequatorial ocean-climate interactions in the Sergipe Basin, Brazil. In: E. Barrera & C.C. Johnson (Eds). Evolution of the Cretaceous ocean-climate system. GSA Special Paper, 332: 231-243.

Koutsoukos, E.A.M. 1989. Mid- to Late Cretaceous microbiostratigraphy, palaeoecology and palaeogeography of the Sergipe Basin, northeastern Brazil. Unpubl. PhD thesis, Polytechnic South West, Plymouth: 886pp.

Melguen, M. 1978. Facies evolution, carbonate dissolution cycles in sediments from the eastern South Atlantic (DSDP Leg 40) since the early Cretaceous. IRDSDP, 40: 981-1023.

Park, J. & Oglesby, R.J. 1994. The effect of orbital cycles on Late and Middle Cretaceous climate: a comparative general circulation model study. In: P.L. De Boer & D.G. Smith (Eds). Orbital Forcing and Cyclic Sequences. Special Publication, 19: 509-530.

Perlmutter, M.A. & Matthews, M. 1989. Global cyclostratigraphy - A model. In: T.A. Cross (Ed.). Quantitative Dynamic Stratigraphy: 233-260.

Thierstein, H.R. & Roth, P.H. 1991. Stable isotopic and carbonate cyclicity in lower Cretaceous deep-sea sediments: Dominance of diagenetic effects. Marine Geology, 97: 1-34.


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