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Paleontology and biostratigraphy

At Site U1381, core catcher samples from sedimentary layers in the upper part of Hole U1381A were analyzed for calcareous nannofossils and foraminifers. The calcareous nannofossils observed at this site provided a significant biostratigraphic control of the cored sediment sequence above the basalts of the Cocos Ridge. Based on microfossil biostratigraphy, the sedimentary layers are tentatively divided into an upper part, of Pleistocene age, and a lower part, of middle Miocene age. Thus, the sediments just above the basalts are tentatively estimated to be of middle Miocene age and younger than 16 Ma. The zonation of planktonic foraminifers is approximately concordant with that of the calcareous nannofossils.

Diverse calcareous nannofossil assemblages ranging from the Pleistocene to the lower middle Miocene are present in Samples 334-U1381A-2R-CC (3.69 mbsf) through 11R-CC (95.5 mbsf). Two different environments are represented in this section by nannofossil abundance and preservation. The upper interval, spanning from Sample 2R-CC (3.69 mbsf) through 6R-CC (49.62 mbsf), represents a hemipelagic environment mixed with terrigenous material. The lower interval, a silicic to calcareous ooze spanning from Sample 7R-CC (53.86 mbsf) through Sample 11R-CC (95.5 mbsf), represents a pelagic environment. The presence, abundance, and preservation of the calcareous nannofossils recovered from Hole U1381A are reported in a range distribution chart (Table T2).

Samples 334-U1381A-2R-CC through 4R-CC cannot be biostratigraphically zoned because of poor preservation and the lack of zonal markers. Though undetermined, the assemblages are characteristic of the lower Pleistocene Zones NN20-NN19 and contain Gephryocapsa oceanica, Gephryocapsa caribbeanica, Helicosphaera carteri, and Calcidiscus leptoporus.

Sample 334-U1381A-5R-CC (49.52 mbsf) is tentatively assigned to nannofossil Zone NN19 based on the occurrence of Pseudoemiliania lacunosa and the absence of Discoaster brouweri. However, the top boundary, defined by the last occurrence of P. lacunosa, is undetermined.

Sample 334-1381A-6R-CC (49.62 mbsf) contains a diverse nannofossil assemblage of mixed ages, ranging from Pleistocene Zone NN19 into the lower to middle Miocene. The discoasters are poorly to moderately preserved, whereas the placoliths exhibit moderate to good preservation. The condition of the discoasters and the rarity or lack of biostratigraphic markers prevents the further delineation of the Pliocene and Miocene zones. The diverse assemblage is dominated by Pleistocene species, including G. oceanica, G. caribbeanica, H. carteri, and C. leptoporus. Also present, but rare to a few in abundance, are Miocene species including Discoaster bellus, Discoaster exilis, Discoaster quinqueramus, Discoaster variabilis, and unidentifiable five- and six-rayed discoasters.

Samples 334-U1381A-7R-CC (53.86 mbsf) through 11R-CC (95.5 mbsf) are assigned to middle Miocene Zone NN5 based on the occurrence of Helicosphaera heteromorphus and the absence of Helicosphaera ampliaperta. The top and bottom of this zone cannot be constrained because of the uncertainty of the last occurrences of the biostratigraphic markers. Typical species found in the samples include Sphenolithus heteromorphus, Sphenolithus moriformis, C. leptoporus, Cyclicargolithus floridanus, Coccolithus miopelagicus, D. exilis, D. variabilis, Discoaster deflandrei, and Reticulofenestra pseudoumbilicus.


Planktonic foraminifers were analyzed in nine core catcher samples (Table T3). Foraminifers are abundant to common in the sediment of Hole U1381A. Preservation is good to moderate. Fragmentation of foraminifers caused by carbonate dissolution is observed in the samples of sediment from lower bathyal depths. Planktonic foraminifers, abundant to common in this hole, are much more abundant than benthic foraminifers. These trends are different from the trends observed in the cored sediment of the other sites. Similar to the observed nannofossil communities in this hole, the foraminiferal assemblages of the uppermost part of the sediment sequence are quite different from those of the lower parts. This is either caused by a hiatus or by very low sedimentation rates. The upper sediment sequence (3.69–49.62 mbsf) contains a foraminiferal community characteristic of a tropical fauna (Globigerinoides quadrilobatus [Globigerinoides sacculifer], Globigerinoides ruber, Orbulina universa, Globorotalia menardii, and Neogloboquadrina dutertrei). Sample 334-U1381A-3R-CC (13.34 mbsf) contains pink G. ruber and is assigned to the Pleistocene (older than 0.12 Ma). Sample 6R-CC contains sinistrally coiling Pulletiatina and is older than 0.8 Ma but younger than 6.4 Ma. From 53.86 (Sample 7R-CC) to 95.55 mbsf (Sample 10R-CC) the planktonic foraminiferal assemblages are composed of Dentoglobigerina altispira, G. quadrilobatus, Globigerinoides obliquus, Globoquadrina dehiscens, Globorotalia peripheroronda, Globorotalia peripheroacuta, Paragloborotalia siakensis, and Orbulina suturalis. This sequence is tentatively assigned to planktonic foraminiferal Zone M7 (14 Ma). However, the occurrence of Praeorbulina circularis in Sample 10R-CC (95.55mbsf) may be a sign that the sediments just above the basement basalt are much older (either M5 or M6).