LATE CENOZOIC FORAMINIFERAL PALEOBATHYMETRIC ESTIMATES FOR THE TIBURON BASIN, NORTHERN CHILE

A total of 22 marine sediment samples were collected from two sections at Cuenca del Tiburon (23º 18.212’ S, 70º 29.378’ W) on the Mejillones Peninsula of northern Chile during the summer of 2001. Late Cenozoic sections of the Tiburon Basin have been described to a very limited extent, and we provide new information regarding paleobathymetric fluctuations along western South America during the late Miocene/Pliocene. The samples collected from these sections were processed for planktonic and benthic foraminifera using standard processing techniques with sample splits reserved for analysis of larger fossil specimens. Statistical analyses of the benthic foraminiferal data were used to determine paleobathymetry and depositional environments at the two localities. Planktic foraminifera will be used in conjunction with paleomagnetic data to obtain a more precise age of the sections sampled.

Results of the statistical analyses on the benthic foraminifera from the eastern section (T1) and the western section (T2) of the Tiburon Basin indicate a shallow marine, upper middle bathyal to upper bathyal environment that is no deeper than 1000 m. This depth is indicated by dominant species such as Buccella peruviana, Hanzawaia concentrica, and Hansenisca rotundimargo. The T1 section is lower stratigraphically, and consists predominantly of fine grained sands between massive diatomaceous sections, which indicate periods of high productivity. Whale fossils are also very abundant in this section, while shell fragments are generally uncommon. The T2 section to the west contains predominantly medium to coarse sediments, dominated by volcaniclastic sands, abundant in gastropods and other shell fragments, as well as some ash layers. An increase in coarse terrigenous sediments and shallow water foraminifera in section T2 indicates a depositional environment of increased shallowing towards exposed Jurassic volcanics further to the west. Planktonic foraminifera, along with a 2.8 My paleomagnetic age on an ash layer within the sections, date the Tiburon sections at approximately Middle/Late Pliocene (N19-N21).