Southeastern Tanzania depositional environments, marine and terrestrial links, and exceptional microfossil preservation in the warm Turonian

Abstract

Sediment cores from southeastern Tanzania contain exceptionally well-preserved calcareous and organic-walled microfossils in numerous samples spanning from the Aptian to the Miocene. The unusually high quality of preservation is commonly attributed to shallow burial and high clay content of the host sediments. However, such attributes apply to many deposits that are not characterized by exceptional preservation, and, thus, invoking only grain size and burial depth is clearly an incomplete explanation for the exceptional microfossil preservation. In an attempt to better characterize additional factors that were important in creating the Tanzanian microfossil lagerstätte, we integrated a wide range of paleontological, geochemical, and sedimentological observations to constrain the paleoenvironmental and early diagenetic conditions of a Turonian interval where exceptional preservation is common. Planktic microfossil assemblages suggest that openocean surface-water conditions prevailed at the site, but, despite excellent organic matter preservation, marine biomarkers are rare. The sediments are dominantly composed of terrigenous silts and clays and terrestrially derived organic matter. Paleontological and geochemical observations record a remarkably stable interval dominated by excellent preservation through the Lower-Middle Turonian. During the Middle-Upper Turonian, though, the preservation quality declines, and this is associated with notable shifts in foraminiferal assemblages, palynological species diversity, carbon and sulfur isotope compositions, and biomarker distributions. The integrated data suggest that an increase in subsurface microbial activity and associated changes in pore-water chemistry were important proximate variables that led to the decline in the quality of micro fossil preservation up section within these shallowly buried, Turonian silty claystones from Tanzania.

Correlation of Turonian continental margin and deep-sea sequences in the subtropical Indian Ocean sediments by integrated planktonic foraminiferal and calcareous nannofossil biostratigraphy

“…The observed changes in planktonic and benthonic foraminiferal assemblages, calcareous nannofossil assemblages, and lithology across this interval suggest that a major shift in the depositional environment must have occurred during the missing time interval. The timing of this event approximately corresponds with a "major cycle boundary" identified as "KTu4" at 91.8 Ma within the upper Turonian M. schneegansi Zone in the Cretaceous sea level curve of Haq (2014), but the estimated sea level rise of 70 m is not supported by lithologic and biostratigraphic observations (Haynes et al 2016). Problems of correlation of this sequence boundary and uncertainty whether this is a global eustatic event are discussed in Haq and Huber (2016).…”

“…were probably emplaced during more oxygenated offshore bottom current transport (Jiménez Berrocoso et al 2015). Studies of the organic biomarker composition of the Cretaceous and Paleogene sediments indicates the organic matter is thermally immature and is mostly composed of terrestrially derived material with very little contribution by marine sources (van Dongen et al 2006, Haynes et al 2016. The excellent microfossil preservation has been attributed to shallow burial of impermeable, clay-rich sediments (Pearson et al 2004) and reduced microbial ac-tivity in the sediment column during deposition and early diagenesis (Haynes et al 2016).…”

“…In Turonian samples planktonic foraminifera generally occur in few (5-10 %) to common (10-30 %) abundance relative to total foraminiferal contentat all sites, with planktonic abundance ranging from between 30% to 80% and averaging ~50 % of the total foraminiferal assemblage (Haynes et al 2015(Haynes et al , 2016 Berrocoso et al (2010Berrocoso et al ( , 2015 is discovery of rare H. helvetica specimens in samples from as much as 86 m below the originally reported levels, resulting in thinning or elimination of the lower Turonian W. archaeocretacea Zone at most sites. Difficulty with reliable identification of the base of the H. helvetica Zone, due to morphologic variability of the nominate taxon and rarity of sensu stricto forms in its early range (Huber and Petrizzo 2014), results in a tentative assignment of the W. archaeocretacea Zone at six of the 11 drill sites (Fig.…”

“…Carbon isotope data from single species analyses of benthic and planktonic foraminifera from Sites 22 and 31 (Mac -Leod et al 2013) and bulk carbonate and organic carbon analyses from these and the other Turonian sites (Jiménez Berrocoso et al 2010 show no relative δ 13 C enrichment that correlates with the OAE2 interval. Samples are tentatively assigned to the W. archaeocretacea Zone at sites 24, 26, 30, and 36 because of the extreme rarity of H. helvetica Zone in its lowermost range (Huber and Petrizzo 2016). Absence of markers for the stratigraphic interval spanning all of calcareous nannofossil Zone CC10 and from the planktonic foraminiferal Thalmanninella reicheli through the W. archaeocretacea Zone (Ando et al 2015) demonstrates that at least 2.7 m.y.…”

“…This period of stasis among the calcareous planktonic assemblages is accompanied by high stability in the biserial planktonic foraminiferal assemblages (Haynes et al 2015;Fig. 6), benthonic foraminiferal assemblages (Wendler et al 2016), palynomorphs (Haynes et al 2016), organic carbon biomarkers (Haynes et al 2016), sediment grain size (Wendler et al 2016), and δ 18 O and δ 13 C signatures of benthonic and planktonic foraminifera (MacLeod et al 2013. It is puzzling that several marginotruncanid species that have been identified within the W. archaeocretacea Zone and H. helvetica Zone at Hole 762C and elsewhere (e. g., Gubbio, Italy: Premoli Silva and Sliter 1995, Tunisia: Robaszynski et al 1990, Kerguelen Plateau: Petrizzo 2001southeast France: Falzoni et al 2016) are absent from the TDP sites.…”