Previous   |   Next

doi:10.2204/iodp.proc.324.105.2010

Paleontology

Sediments from Site U1348 (Cores 324-U1348A-1W through 10R) are pelagic in origin and dominated by chert-rich lithologies (stratigraphic Unit I; see "Sedimentology"). Their uniqueness is highlighted by two intervals of undisrupted nannofossil oozes recovered in Cores 2R and 10R (140 and 22 cm thick, respectively) that yielded well-preserved, abundant foraminifers. Cherts from Cores 3R through 9R are characterized by loosely attached pinkish oozes or chalks, which are also generally suited for separation of foraminifers with the absence of radiolarians. Sandy to clayey sediments underneath Core 10R (Unit II) are barren of calcareous microfossils.

The initially high abundance, diversity, and preservation state of calcareous nannofossils in Unit I (upper part) progressively decrease with increasing burial depth. Foraminifer abundance and diversity are high in the two narrow ooze intervals (see above) with good preservation. The other examined levels also show relatively high foraminifer abundance throughout, despite limitation of the available material. Both calcareous microfossil groups give ages of mid- to Late Cretaceous for Unit I (except for Cores 324-U1348A-1W and 2R [topmost <8 cm]; see below). In particular, a series of short-lived bioevents that are correlative with standard biochronology are recorded by planktonic foraminifers, ranging in age from the early Aptian to early Campanian (120–80 Ma). Taking account of generally poor preservation of calcareous nannofossils, the age model for this site depends solely on planktonic foraminifers (Fig. F23). Benthic foraminifers show changes in abundance and diversity upsection that can be interpreted in terms of the subsidence history of Tamu Massif from the upper to lower bathyal depth.

In light of planktonic foraminifer biochronology as well as calcareous nannofossil ages (Subzone NC7a–Zone CC16), the numerical age of Unit I is determined as ~120–80 Ma. The underlying Unit II and the basement section further below at Site U1348 are dated at older than 120 Ma.

It is noteworthy that the gray ooze loosely inserted in the topmost part of Core 324-U1348A-2R is rich in Cenozoic calcareous and siliceous microfossils. In addition to several species of the calcareous nannofossil genus Discoaster (Zones NN6–NN10; Miocene) (Shipboard Scientific Party, 2002a) and a poorly preserved globorotalid species of planktonic foraminifer (Zones N21–N22; Pliocene–Pleistocene) (Shipboard Scientific Party, 2002a), Cenozoic representatives of radiolarians, diatoms, and silicoflagelates are found, including the age-diagnostic silicoflagellate taxon Dictyocha pulchella (base of the Naviculopsis ponticula Zone to the middle of the Dictyocha fibula Zone; middle–late Miocene) (Bukry, 1981). Because the gray Cenozoic ooze is disrupted and shows an irregular and mixed contact with the pale yellow Campanian ooze, it is uncertain if the Cenozoic portion unconformably overlies the Cretaceous or is an artifact induced by the rotary core barrel (RCB) drilling disturbance. In any case, it is likely that Cenozoic sediments are present in the proximity of Core 324-U1348A-2R, and the estimated range of hiatus(?) is ~60 m.y. (Campanian to Miocene).

Calcareous nannofossils

Calcareous nannofossils in the sediments of Site U1348 are good to moderately preserved and occur in frequent abundance in Cores 324-U1348A-1W and 2R (ooze at the top of stratigraphic Unit I). Downhole, their preservation declines to moderate to poor, and the abundance decreases to rare. The sediments recovered below Core 10R are barren of calcareous nannofossils.

Cenozoic (Miocene) calcareous nannofossil assemblages occur in the uppermost part of Unit I (Section 324-U1348A-1W-1 through interval 2R-1, 6 cm). The presence of Discoaster challengeri, Discoaster druggii, Discoaster deflandrei, and Discoaster brouweri together with Reticulofenestra pseudoumbilicus in Sample 324-U1348A-1W-1, 0–6 cm, indicates Zones NN6–NN8 (lower to middle Miocene) (Table T2); co-occurrence of a few Upper Cretaceous specimens (e.g., Uniplanarius gothicus) is most likely contamination. Sample 324-U1348A-2R-1, 1 cm, contains Amaurolithus delicatus next to Calcidiscus macintyrei, D. challengeri, and D. brouweri and is therefore correlated to Zones NN11–NN14 (middle Miocene). Sample 324-U1348A-2R-1, 2 cm, displays nearly the same assemblage (Zones NN6–NN16) (Table T2) as Sample 324-U1348A-1W-1, 0–6 cm, except for the additional occurrence of C. macintyrei.

The change in color from light gray to pale yellow around the top ~6 cm of Section 324-U1348A-2R-1 marks the difference between Cenozoic and Cretaceous oozes. The presence of Arkhangelskiella cymbiformis in Sample 324-U1348A-2R-1, 6 cm, and of the pale yellow ooze together with Uniplanarius gothicus, Eiffellithus eximius, and Reinhardites levis is assignable to Zones UC13–CC24 (Santonian to Maastrichtian) (Table T2). The five additional samples taken from the pale yellow ooze of Sections 324-U1348A-2R-1 through 2R-CC represent a moderately to poorly preserved assemblage and cover the stratigraphic range from the Coniacian to Maastrichtian, as indicated by the presence of such species as E. eximius and U. gothicus.

Samples taken from Cores 324-U1348A-3R through 6R display rare specimens of poorly preserved Watznaueriaceae. Exceptionally, Sample 324-U1348A-4R-1, 0–1 cm (chert-encrusting chalk), illustrates the Santonian Stage (Zones CC14–CC16) by the presence of Eprolithus floralis and Micula decussata (Table T2). This determination may be compromised by the co-occurrence of a few specimens of U. gothicus and Micula staurophora of Zones CC17–CC24 (Campanian to Maastrichtian), but they are regarded as contaminants from the upper levels primarily because of their rare abundance.

Between Cores 324-U1348A-7R and 10R, the preservation of calcareous nannofossils is poor and the abundance is rare. The presence of E. floralis is the sole indicator that allows assignment of this interval to Subzone NC7a–Zone CC16 (Aptian to Santonian). In Sample 324-U1348A-9R-1, 19–20 cm (Table T2), Rhagodiscus robustus is found together with Cretarhabdus striatus, which roughly points to the range between Aptian and Albian Stages.

Foraminifers

Planktonic foraminifers

The planktonic foraminifer record of Site U1348 (stratigraphic Unit I) is marked by the stratigraphically continuous occurrences of typical mid- to Upper Cretaceous (lower Aptian–lower Campanian) assemblages (Table T3; Fig. F23). Despite limited availability of sediments at most examined levels (with notable exceptions of Cores 324-U1348A-2R and 10R), foraminifer abundance and diversity are generally high. Two samples from Cores 8R and 9R show significantly limited occurrences of planktonic foraminifers, and instead, the benthic foraminifer abundance is relatively high. The preservation is generally good throughout the examined section, though occasionally is poor owing to replacement by silica (Table T3).

Two pale yellow ooze samples from Core 324-U1348A-2R are correlated to the Santonian–lower Campanian interval based on well-preserved diverse planktonic foraminifer assemblages. In particular, Sample 324-U1348A-2R-CC, 12–15 cm, falls within the Dicarinella asymetrica Zone based on the abundant occurrence of the nominal taxon, together with abundant Contusotruncana fornicata, Marginotruncana sinuosa, Marginotruncana pseudolinneiana, and Sigalia deflaensis. Sample 324-U1348A-2R-1, 9–10 cm, above, is marked by rare to few C. fornicata, Globotruncana linneiana, Globotruncana arca, and Globotruncanita stuartiformis. The absence of D. asymetrica implies that this level is above the top of the D. asymetrica Zone and therefore is the lower Campanian.

The Turonian Stage is recognized for two samples (324-U1348A-3R-1, 0–1 cm, and 4R-1, 0–1 cm). The latter sample is marked by good planktonic foraminifer preservation and yields rare Helvetoglobotruncana helvetica (nominal taxon of the Hv. helvetica Zone) together with abundant Whiteinella aprica as well as few Whiteinella archaeocretacea and Whiteinella baltica. Noteworthy is the high abundance of biserial heterohelicids, up to 36% in total planktonics (N = 192; >150 µm), which is ascribed to the so-called "Heterohelix shift" (Ando et al., 2009b, and references therein). In contrast, Sample 324-U1348A-3R-1, 0–1 cm, represents poor preservation of planktonic foraminifers, as all retrieved specimens are replaced by silica and the sample does not include the primary zonal marker species. Yet, few to common occurrences of marginotruncanids (M. sinuosa, M. pseudolinneiana, and Marginotruncana renzi) together with the absence of Hv. helvetica or Dicarinella concavata are indicative of the upper Turonian.

Sample 324-U1348A-5R-1, 0–1 cm, despite limited recovery of planktonic foraminifers, represents the presence of zonal marker species Ticinella primula, which represents the middle–upper Albian interval. Further, the presence of rare Hedbergella wondersi points to the upper Albian, as this taxon occurs within the Rotalipora ticinensis Zone in the western North Atlantic (Petrizzo and Huber, 2006).

Two samples (324-U1348A-6R-1, 0–1 cm, and 7R-1, 6–7 cm) are correlated to the middle Albian, as they are marked by the dominance of zonal marker T. primula. Of these, Sample 324-U1348A-6R-1, 0–1 cm, is also marked by the common occurrences of Ticinella madecassiana and Ticinella roberti, and this observation allows this level to be placed in the upper part of the T. primula Zone. Sample 324-U1348A-7R-1, 6–7 cm, below, is dated as the lower part of the T. primula Zone, as judged from the somewhat smaller size and less pronounced supplementary apertures in T. primula, as well as the abundant occurrence of Hedbergella rischi (sensu Bellier and Moullade, 2002).

Downhole, the planktonic foraminifer records are interrupted by sparse occurrences in Cores 324-U1348A-8R and 9R but are again marked by the common occurrence with good preservation at the top of Core 10R (~22 cm thick white ooze), which can be designated as the lower Aptian. Representative taxa in the examined two levels (Samples 324-U1348A-10R-1, 1–2 cm, and 10R-1, 20–21 cm) are Pseudoschackoina saundersi, Globigerinelloides aptiensis, Globigerinelloides maridalensis, and Praehedbergella gorbachikae (Boudagher-Fadel et al., 1997; Verga and Premoli Silva, 2003a, 2003b, 2005). Taking account of the lack of large multichambered (>7 chambers) middle to late Aptian representatives of the genus Globigerinelloides (i.e., Globigerinelloides ferreolensis and Globigerinelloides algerianus), which were reported from the same central Pacific domain (Deep Sea Drilling Project Site 463, Mid-Pacific Mountains; Ando et al., 2008), it is most reasonable to consider that the ooze sediments of Core 324-U1348A-10R fall within the Leupoldina cabri Zone.

Benthic foraminifers

Two modes of benthic foraminifer occurrences are observed in stratigraphic Unit I in terms of abundance and diversity: (1) low-abundance assemblage through Cores 324-U1348A-2R through 8R and (2) high-abundance, high-diversity assemblage in Cores 9R through 10R (Table T4). The faunal elements are interpreted primarily as reflecting the subsidence history of Tamu Massif from the upper to lower bathyal depth.

In the low-abundance assemblage of Cores 324-U1348A-2R through 8R, benthic foraminifers constitute 2%–6% of total foraminifer counts with moderate diversity. With specific reference to two Santonian–Campanian samples from Core 2R, Aragonia and Sliteria are the dominant elements (cf. Widmark, 1997). These genera are the representatives of the middle to lower bathyal setting (Nyong and Olsson, 1984 [Sliteria reported as Conorbina]). Although the habitat of Aragonia may extend to the abyssal depth (Sliter, 1977), it is notable that agglutinated benthic foraminifers typical of the abyssal setting (e.g., Nyong and Olsson, 1984; Sikora and Olsson, 1991; Holbourn et al., 2001) are lacking, thereby allowing the estimation of middle–lower bathyal depth (<2500 mbsl) for these levels.

In another set of the low-abundance benthic assemblage in Cores 324-U1348A-4R through 8R (Albian–Turonian), calcareous trochospiral genera Gyroidinoides and Osangularia are the most common faunal elements, both of which are ubiquitous in the bathyal setting. Of these, Gyroidinoides has been revealed to dominate in the upper–middle bathyal depth (Sliter and Baker, 1972; Nyong and Olsson, 1984; Sikora and Olsson, 1991; Ando et al., 2009a). Additionally, the neritic to upper bathyal elements (e.g., nodosariids) are lacking. In summary, the reasonable bathymetric range for this assemblage would be the middle bathyal (500–1500 mbsl).

The Aptian benthic foraminifer assemblage in Cores 324-U1348A-9R and 10R represents a peculiar mode of occurrence, exhibiting significantly high abundance (49%–90% in total foraminifer counts) and high diversity. Calcareous trochospiral taxa are dominated by Gyroidinoides and Osangularia, together with small Gavelinella. Large-sized agglutinated forms of Dorothia and Gaudryina are also prevalent. Such faunal composition is reminiscent of the upper to middle bathyal setting (Sliter and Baker, 1972). Further, this assemblage is marked by the entries of some nodosariid taxa (Laevidentalina, Lenticulina, and Saracenaria) indicative of the upper bathyal setting. It is most reasonable that these observations are translated as indicating the upper to middle bathyal depth (200–1500 mbsl).

Top of page   |   Previous   |   Next