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doi:10.2204/iodp.proc.329.108.2011

Paleontology and biostratigraphy

A ~70 m thick section of predominantly zeolitic metalliferous pelagic clay was recovered at Site U1370. The section also contained a short interval (30 cm in Hole U1370F to 2.9 m in Hole U1370D) of nannofossil ooze at ~62 mbsf (Fig. F13; see “Lithostratigraphy”). Site U1370 is on basement located within magnetic polarity Chron 33n, which implies a crustal age between 73.6 and 79.5 Ma (Gradstein et al., 2004). Based on a tectonic reconstruction of the region by Larson et al. (2002), the crust was accreted along the Pacific-Phoenix spreading center at ~75 Ma.

Three samples taken from the distinct layer of nannofossil ooze found in Hole U1370D (see “Lithostratigraphy”) were examined for paleontological and biostratigraphic purposes. Sample 329-U1370D-7H-6, 97–99 cm, just below the sharp contact with the overlying pelagic clay; Sample 7H-CC, from the middle of the layer; and Sample 8H-1, 147–149 cm, from the base. The samples were wet-sieved using a 38 µm mesh sieve to ensure the capture of smaller size planktonic foraminifers (see “Paleontology and biostratigraphy” in the “Methods” chapter [Expedition 329 Scientists, 2011a]). The silt- and clay-size fractions (<38 µm) were examined in smear slide samples (see “Lithostratigraphy”).

The biogenic component of the >38 µm sediment size fraction of the samples taken from the calcareous ooze interval is dominated by planktonic and benthic foraminifers with significantly lesser amounts of ostracods (Tables T2, T3). The <38 µm sediment size fraction is dominated by calcareous nannofossils (see “Lithostratigraphy”). Planktonic and benthic foraminifers are generally well preserved, although a fraction of the benthic specimens are abraded or show signs of dissolution. Preliminary results from postexpedition studies indicate that dissolution is more pervasive in the pelagic clay below and above the nannofossil ooze. No planktonic foraminifers were found in the pelagic clay immediately above the sharp contact with the underlying ooze, and calcareous benthic foraminifers are quickly replaced uphole by simple agglutinated forms.

Planktonic foraminifers

The overall planktonic foraminiferal assemblage is composed of Chiloguembelina midwayensis, Chiloguembelina morsei, Eoglobigerina edita, Eoglobigerina eobulloides, Guembelitria cretacea, Parasubbotina pseudobulloides, Parasubbotina aff. pseudobulloides, Subbotina triloculinoides, and Subbotina trivialis, among others (Table T2). Planktonic foraminiferal taxonomic concepts follow Olsson et al. (1999).

Because of time limitations for shipboard micropaleontological analyses during Expedition 329, it was possible to produce only a preliminary planktonic foraminifer biostratigraphic control for the calcareous ooze interval recovered at Site U1370. The uppermost Sample 329-U1370D-7H-6, 97–99 cm, was provisionally assigned to lower Paleocene planktonic foraminiferal Zone P1b (~64.3 Ma) based on the presence of S. triloculinoides and P. pseudobulloides and the absence of distinctive markers of Zone P1c (Olsson et al., 1999; Wade et al., 2011). Basal Sample 8H-1, 147–149 cm, was provisionally assigned to lower Paleocene planktonic foraminiferal Zone P1a (64.97–64.8 Ma) based on the common occurrence of G. cretacea, P. aff. Pseudobulloides, and C. morsei and the absence of characteristic Zone P1b markers (Olsson et al., 1999; Wade et al., 2011). More detailed postexpedition shore-based analyses using scanning electron microscopy will be carried out to check the zonal assignments and better characterize the planktonic foraminiferal assemblages.

Calcareous nannofossils

Calcareous nannofossils were examined after the expedition by Dr. John Firth (IODP-Texas A&M University) in a few samples taken just above, below, and within the calcareous sedimentary interval found in Cores 329-U1370D-7H and 8H (lithologic Unit II [nannofossil ooze]; see “Lithostratigraphy”).

Sample 329-U1370D-7H-6, 75–79 cm, is composed primarily of red clay ~15 cm above the top of the calcareous ooze. This sample contains moderate to poorly preserved rare specimens of Watzneuria sp., Quadrum cf. Q. gothicum, Prediscosphaera sp., and Eiffellithus sp. These taxa are all Cretaceous taxa and do not range above the Cretaceous/Tertiary boundary. In addition, one specimen of Discoaster sp., with six slender rays, rounded tips, and very small knobs on the sides of the rays, occurs in this sample. This form of Discoaster only occurs in sediments of Eocene or younger age. Its presence in this sample could either be from contamination of the core surface or it could indicate an Eocene sedimentary age.

Sample 329-U1370D-7H-7, 37–41 cm, is within the calcareous ooze and contains abundant nannofossils. The assemblage is dominated by elliptical, mostly dark-shielded (both in cross-polarized light and in phase contrast) placoliths of small to medium size (3 or 4 µm up to ~10 µm). Many specimens of placoliths show some kind of internal structures either crossing or filling the central area. The preservation of nannofossil in this sample is moderate to poor, which obscures the central structures in most specimens. Therefore identification of taxa is limited in most cases to the Genus level, or even the Subfamily or Family level. The following species are observed: Markalius inversus (rare), Coccolithus pelagicus s.l. (common to abundant?, small to medium sized), Cruciplacolithus edwardsii (few?), and Cruciplacolithus sp. cf. C. tenuis (medium sized, very rare?). The following genera are observed: Thoracosphaera spp. (rare), Reinhardtites sp. (rare to few?), Eiffellithus sp. (rare to few?), and Cruciplacolithus spp. (common to abundant?). The following Family/Subfamilies are observed: Podorhabdaceae/Podorhabdoideae (few to common?), Podorhabdaceae/Retecapsoideae (few to common?, possibly including the genera Stradneria and Cretarhabdus). Other than M. inversus, no other taxa from the Family Ellipsagelosphaeraceae, such as Watzneuria, were observed. This sample, therefore, appears to correspond to the upper part of Martini’s (1971) Zone NP1 or possibly the base of Zone NP2, based on the occurrence of C. edwardsii and Cruciplacolithus cf. C. tenuis (one specimen). The occurrence of Cretaceous placoliths along with lower Paleocene placoliths indicates that the former have been reworked.

The assemblage in Samples 329-U1370D-7H-6, 90 and 130 cm, near the top of the calcareous ooze, is similar to that in Sample 7H-7, 37–41 cm, and does not show any taxa of younger age than in Sample 7H-7, 37–41 cm.

Sample 329-U1370D-8H-1, 67–71 cm, near the base of the calcareous ooze, was studied using scanning electron microscopy. The increased image resolution confirmed light microscope observations from the other calcareous ooze samples, specifically that the preservation of nannofossils is poor, with considerable overgrowth and dissolution/breakage of specimens. The assemblage is also similar to that from Samples 329-U1370D-7H-6, 90 and 130 cm, and Sample 7H-7, 37–41 cm. Common Cruciplacolithus specimens of early Paleocene age are accompanied by rare to common, poorly preserved specimens of Cretaceous age. In addition to Cretaceous taxa listed above, this sample also contained specimens of Rhagodiscus cf. R. angustus, Eiffellithus cf. E. eximius, possible Teichorhabdus ethmose, and Cribrosphaerella cf. C. daniae. Very small (3 µm) specimens of Biscutum sp. also occur, which have been recorded in both late Maastrichtian and early Paleocene age sediments elsewhere.

Sample 329-U1370D-8H-5, 18–25 cm, is within the red clay below the calcareous ooze and contains very rare specimens of Cretaceous nannofossils, such as Watzneuria.

These results indicate that the calcareous ooze of Core 329-U1370D-7H and 8H is all of early Paleocene age, approximately upper Zone NP1 or basal Zone NP2 (Martini, 1971). Cretaceous taxa within the ooze all have stratigraphic ranges into the Maastrichtian, except for Eiffellithus cf. E. eximius, which has a highest occurrence in the upper Campanian. However, the red clay above the calcareous ooze (Sample 7H-6, 75–79 cm) contains rare Cretaceous taxa that were not observed within the calcareous ooze. We interpret these specimens to be reworked based on the lower Paleocene nannofossils recovered within the calcareous ooze and based on the single specimen of Discoaster in that sample, which may indicate a possible Eocene age.

Benthic foraminifers

Benthic foraminifers are abundant and moderately well preserved throughout the calcareous nannofossil ooze at the base of the Hole U1370D. Benthic foraminiferal assemblages from this interval are dominated by large, heavily calcified Paleocene to Eocene taxa. More than 23 species were identified (Table T3). The most common taxa are Aragonia ouezzanensis, Aragonia velascoensis, Dorothia trochoides, Nuttalides truempyi, Lenticulina spp., Oridorsalis umbonatus, and Stilostomella spp. Benthic foraminiferal taxonomic concepts follow Tjalsma and Lohman (1983).

Ostracods

One as yet unidentified ostracod specimen was found in Sample 329-U1370D-7H-CC.

Paleoenvironmental interpretation

The nannofossil ooze found at Site U1370 was deposited during lower Paleocene foraminiferal Zone P1. The ooze’s occurrence in this deep-sea clay sequence is attributed to deepening of the calcite compensation depth and lysocline during the interval of decreased planktonic carbonate precipitation that followed the end-Cretaceous mass extinction (D’Hondt, 2005).

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