HIGH-FREQUENCY OCEANOGRAPHIC AND CLIMATIC FLUCTUATIONS IN A SUBEQUATORIAL MID-CRETACEOUS BASIN, NORTHEASTERN BRAZIL

In the Sergipe Basin on the Brazilian Atlantic margin, 10 km of Mesozoic sedimentary strata occur in grabens separated by shallow banks underlain by extensionally faulted Precambrian rocks. Paleogeographic reconstructions place the Sergipe Basin about 12 degrees south of the mid-Cretaceous equator.

A carbonate sequence of 333 limestone-marlstone bedding couplets containing normal-marine micro- and macrofossils was measured and described from the Votorantim Quarry. Couplet deposition occurred in the late transgressive, maximum flooding, and early highstand systems tracts of the Cenomanian-Turonian tectonoeustatic cycle. High-resolution stratigraphic, geochemical, and chronologic analyses were used to analyze cyclic processes influencing deposition in the basin. Orbital forcing of climate was recorded in the couplets, with half-precessional cycles suggested by spectral powers at 12.1 kyr in the limestone data and 12.6 kyr in the marlstones. Spectral peaks representing precessional and obliquity cycles in the limestone data mimic accepted Pliocene-Pleistocene values for these cycles at 19, 23, and 40 kyr. Limestone beds record a direct linear response to insolation variation, while marlstone beds record a more complex, nonlinear response to insolation. We suggest continental amplification of low-latitiude oscillations in insolation through monsoonal strength and southward displacement of the Intertropical Convergence Zone (ITCZ). Bedding couplets, 2336 years in duration, are the lithostratigraphic response to a shift in balance between evaporation (limestone) and precipitation (marlstone) as the dominant control on sediment supply and coastal circulation in the basin. If the 2336 year oscillations of insolation displaced northeast tradewinds southward and resulted in precipitation of Tethyan moisture in the Sergipe Basin, then these data provide geological evidence of southward atmospheric heat transport under Cretaceous greenhouse conditions.