CONCLUSIONS

The primary objective of this paper has been to refine middle Miocene–Quaternary nannofossil biostratigraphy of Site 1256 in the Guatemala Basin on Cocos plate crust and compare the data with those from other sites in this basin.

The middle Miocene–Quaternary sedimentary sequence at Site 1256 appears to be complete in terms of the calcareous nannofossil assemblage succession. A total of 16 zones/combined zones are recognized, and nearly all zonal boundaries are located within a quarter section of a core.
Age estimates for two nannofossil datums, 7.18 Ma for the FO and 6.32 Ma for the LO of R. rotaria, respectively, were derived from biostratigraphy and magnetostratigraphy.
Linear sedimentation rates were calculated using 28 nannofossil age estimates. Rates are high in the middle Miocene, decrease drastically in the lower upper Miocene, recover somewhat in the uppermost Miocene, and reach the lowest point in the Pliocene, but they increase thereafter. This pattern of variations is similar to that observed at nearby ODP sites in the Guatemala Basin. The middle–upper Miocene carbonate crash was observed. The crash is believed to have been caused by enhanced dissolution that resulted from lowered productivity as well as the initiation of NADW and the constriction of the Panama Gateway. These similar sedimentary patterns, together with the close correlation of epoch boundaries at Site 1256 to those published for other nearby Deep Sea Drilling Project/ODP sites, suggest virtually no intraplate variations on the Cocos plate in terms of depositional history.
Reworked nannofossils, together with sedimentation changes indicated by significant color change, were used to investigate redeposition events, and three episodes were recognized. The Messinian sea level drop and increased seismic activity may have contributed to intensified bottom current activity. Turbidity currents likely produced these episodes of redeposition as indicated by a sharp change in sediment color and a sharp reduction in bioturbation.