Further Paleogene and Cretaceous sediment cores from the Kilwa area of coastal Tanzania: Tanzania Drilling Project Sites 6-10

摘要

Initial results from scientific drilling in southern coastal Tanzania are described. In a field season in 2003, a total of five sites was drilled (mostly using continuous coring) by the Tanzania Drilling Project for paleoclimate studies. The sediments are predominantly marine clays and claystones deposited in an outer shelf or slope environment and often contain excellently preserved microfossils suitable for geochemical analysis. The studies reported here include summaries of the lithostratigraphy, biostratigraphy (planktonic foraminifers, calcareous nannofossils, benthic foraminifers, and palynology) and organic geochemistry. TDP Site 6 was drilled near Kilwa Masoko (UTM 37L 555752, 9014922), 350 m to the south-east of a previous site, TDP Site 1. The top 59.58 m, which was mostly drilled without coring, consists of an Oligocene clay formation belonging to nannofossil Zone NP23. The rest of the hole, to a total depth of 61.25 m, consists of a fault zone in which the Oligocene sediments are intermixed with middle Eocene clays of planktonic foraminifer Zone E9 and nannofossil Subzone NP15b. TDP Site 7 consists of two holes (Hole TDP7A: UTM 37L 547126, 9030142; Hole TDP7B: UTM 37L 0547130, 9030140) drilled just 5 m apart at Kwamatola, a creek to the south of Kilwa Kivinje. Underneath approximately 20 m of unconsolidated sands and gravels, claystones and siltstones were recovered to a total depth of 128.00 m. The site spans lower Eocene planktonic foraminifer Zones El, E2 and E3 and nannofossil Subzones NP 9b and NP10. The bottom of Hole TDP7B approaches the Paleocene-Eocene boundary but no unambiguously Paleocene sediments were recovered. TDP Site 8 was drilled to the south-east of Singino Hill (UTM 37L 548033, 9025811). Below a covering of surface gravels, it yielded predominantly dark greenish-grey claystones to a total depth of 22.95 m. The sediments are from the lower Eocene and span the boundary between planktonic foraminifer Zones E3 and E4 and fall within nannofossil Zone NP10. TDP Site 9 was drilled near Nangurukuru junction (UTM 37L 538987, 9027049). It yielded predominantly dark greenish-grey silt-stones to a total depth of 88.80 m. Thin siliciclastic beds with trace fossils of the Nereites ichnofacies are common in the lower third of the site. These sediments span the Campanian-Maastrichtian stage boundary and are assigned to the Globotruncanella havanensis to Gansserina gansseri planktonic foraminifer zones and nannofossil Subzones UC15e~(TP) to UC17. TDP Site 10 was drilled to the west of Singino Hill, adjacent to the main Kilwa road (UTM 37L 0541243, 9028791). Below a superficial covering of gravel, it yielded predominantly dark greenish-grey silty claystones to a total depth of 100.80m. These sediments are assigned to Paleocene planktonic foraminifer Zone P4c-P5 and nannofossil Subzone NP9b. Organic geochemical analyses from all the sites revealed biomarkers of predominantly terrestrial origin and collectively indicate an unusually low degree of thermal maturity for the area, suggesting shallow maximum burial depths. Traces of migrated hydrocarbons were found in TDP Sites 7 and 10.