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doi:10.2204/iodp.proc.342.104.2014

Biostratigraphy

Coring at Site U1403 recovered a 265 m thick sequence of Quaternary–upper Campanian clay, radiolarian clay, and nannofossil ooze with minor chert layers. Clay and radiolarian clay are the dominant lithologies in the upper 119 mbsf, but nannofossils and planktonic foraminifers are present in the uppermost two sections of Hole U1403A (0.00–2.21 mbsf). Lithostratigraphic Units IV and V, below 119.14 mbsf, are predominantly composed of nannofossil ooze with clay-rich layers that are sometimes noncalcareous. Nannofossils and planktonic foraminifers indicate that the uppermost 2 m is Quaternary in age (Zones NN19–NN20 and Pt1). Rare to common spumellarian radiolarians are present in the upper 67 m, but they are poorly preserved and not age diagnostic. We were therefore unable to provide definitive ages for 2.20–66.91 mbsf in Hole U1403A. Below 66.91 mbsf, first radiolarians and then radiolarians and nannofossils provide biostratigraphic control, indicating middle Eocene to late Campanian–aged sediment.

An integrated calcareous and siliceous microfossil biozonation is shown in Figure F15. Nannofossil datum and zonal determinations agree well with the radiolarian biostratigraphy. An age-depth plot including biostratigraphic and paleomagnetic datums is shown in Figure F16.

Radiolarians are well preserved in the Eocene sediment recovered at Site U1403 and provide a coherent high-resolution biochronology. Calcareous nannofossils are abundant and poorly to well preserved when present. Planktonic foraminifers are absent through much of the sequence but occur in several samples in the lower Eocene, lower Paleocene, and Upper Cretaceous nannofossil ooze lithologies. Benthic foraminifers are present in the lower Eocene to lower Paleocene sediment and are abundant in Campanian to Maastrichtian sediment at Site U1403. In general, these benthic foraminifers indicate lower bathyal to abyssal paleodepths. A summary of calcareous and siliceous microfossil abundance and preservation is given in Figure F17.

Calcareous nannofossils

Calcareous nannofossil biostratigraphy is based on analysis of core catcher samples from both holes and from additional working-half section samples from Hole U1403A for the Paleogene and Hole U1403B for the Cretaceous. Depth positions and age estimates of biostratigraphic marker events are shown in Table T3. Calcareous nannofossil occurrence data are shown in Table T4. Note that the distribution chart is based on shipboard study only and is, therefore, biased toward age-diagnostic species.

At Site U1403, the preservation of calcareous nannofossils varies from poor to good. Dissolved assemblages characterize the uppermost lower Eocene sediment, whereas Cretaceous and Paleocene calcareous nannofossils show moderate to good preservation (Fig. F17).

The uppermost interval, from Samples 342-U1403A-1H-1, 45 cm, to 1H-2, 70 cm (0.45–2.2 mbsf), contains abundant nannofossils indicative of upper Quaternary Zones NN21/20–NN18. From Samples 1H-3, 70 cm, to 13H-6, 25 cm (3.7–117.93 mbsf), the sediment is noncalcareous and barren of nannofossils.

The interval from Samples 342-U1403A-13H-6, 100 cm, to 21X-5, 28 cm (118.68–181.68 mbsf), is largely nannofossiliferous. Nannofossil Zone NP14 is recognized using the base of Discoaster sublodoensis in Sample 15H-4, 75 cm (134.55 mbsf). The base of Nannotetrina cristata in Sample 15H-1, 39 cm (129.69 mbsf), is used to informally divide Zone NP14 because Blackites inflatus, which approximates the lower/middle Eocene boundary, is not present at Site U1403. The top of Tribrachiatus orthostylus in Sample 342-U1403A-17H-CC (147.8 mbsf), base of Discoaster lodoensis in Sample 18X-CC (148.76 mbsf), top of Tribrachiatus contortus in Sample 19X-4, 38 cm (160.88 mbsf), and base of Tribrachiatus bramlettei in Sample 21X-2, 28 cm (177.18 mbsf), delineate lower Eocene Zones NP13–NP10. The additional biohorizons of the base of Discoaster diastypus, base of T. contortus, and base of T. orthostylus provide further support for the identification of Zone NP10. The base of the Rhomboaster spp. biohorizon, which occurs in Sample 342-U1403A-21X-5W, 28 cm (181.68 mbsf), is close to, or coincident with, the base of the Eocene and is also used by some authors to mark the base of Subzone NP9b. Within Subzone NP9b we also observed the top of Fasciculithus spp., which is found to occur 300 k.y. after the Paleocene/Eocene boundary (e.g., Raffi et al., 2005). The presence of Discoaster araneus, just above the base Rhomboaster datum level, indicates the presence of PETM-equivalent sediment. Below the base Rhomboaster spp. level, in the interval from Samples 342-U1403A-21X-5, 83 cm, to 22X-CC (182.23–188.66 mbsf), the sediment is red clay and barren of calcareous nannofossils.

Calcareous nannofossils reappear from Sample 342-U1403A-23X-4, 139 cm (200.59 mbsf), and indicate a late Danian age (Zone NP4) based on the presence of Fasciculithus magnus (Sample 25X-1, 115 cm; 210.85 mbsf) and absence of Fasciculithus tympaniformis. The base of Chiasmolithus danicus in Sample 26X-1, 20 cm (214.7 mbsf), the base of Cruciplacolithus intermedius in Sample 26X-3, 84 cm (218.34 mbsf), and coincident bases of Biantholithus sparsus and the calcisphere acme in Sample 26X-5, 9 cm (220.59 mbsf), delineate the lower Danian Zones NP3–NP1 and the Cretaceous/Paleogene boundary. The Zone NP2 assemblage is characterized by abundant Prinsius tenuiculus and the base of Coccolithus pelagicus. Immediately above the K/Pg boundary (Samples 342-U1403A-26X-5, 3 cm, to 26X-5W, 9 cm; 220.53–220.59 mbsf), the nannofossil assemblages are of low diversity and dominated by calcisphere carbonate and several Cretaceous mass extinction survivor species, including Cyclagelosphaera reinhardtii, Markalius inversus, and Zeugrhabdotus sigmoides. Samples immediately below the boundary contain abundant and highly diverse uppermost Maastrichtian nannofossil assemblages.

The Maastrichtian Zones/Subzones UC20d through UC20a/19 are recognized using the base of Micula prinsii in Sample 342-U1403A-26X-CC (224.16 mbsf), the base of Ceratolithoides kamptneri in Sample 342-U1403B-28X-5, 50 cm (223.1 mbsf), the base of Micula murus in Sample 342-U1403B-30X-4, 40 cm (240.9 mbsf), and the top of Reinhardtites levis in Sample 342-U1403B-31X-3, 52 cm (249.22 mbsf). We are unable to differentiate Subzone UC20a because of the rare and sporadic distribution of Lithraphidites quadratus in the lower part of its range. The presence of common Cribrocorona echinus in Sample 342-U1403B-31X-3W, 52 cm (249.22 mbsf), is suggestive of Zones UC17–UC18, and its top occurs just prior to the mid-Maastrichtian event in the Pacific (Lees and Bown, 2005). The absence of definitive Tranolithus orionatus (base of Zone UC18 marker species) lower in the section indicates that this species may not be present at Site U1403. The coincident tops of Uniplanarius trifidus and Broinsonia parca constricta in Sample 342-U1403B-31X-4, 18 cm (250.38 mbsf), indicate an age no younger than the Zone UC17/UC16 boundary. As these two events have been estimated as lying 0.71 m.y. apart, the coincident tops recorded here suggest the presence of a hiatus or stratigraphic condensation at this level. These two taxa are present to just above the lowermost sediment in the hole (Sample 342-U1403B-32X-3, 28 cm; 258.68 mbsf), indicating that this lowermost sediment is of late Campanian–earliest Maastrichtian age (Zones/Subzones UC16–UC15dT). The deepest core catcher sample, 342-U1403B-32X-CC, is barren of nannofossils.

Radiolarians

Radiolarian biostratigraphy is based on analysis of all core catcher samples from Hole U1403A, selected core catcher samples from the upper part of Hole U1403B (Cores 342-U1493B-1H through 8H), and additional section-half samples from Hole U1403B for the upper part of the hole (Cores 1H through 10H), the Paleocene–Eocene transition (Core 23X), and the deepest Cretaceous core (Core 32X). Radiolarians occur sporadically and are poorly preserved in the undated upper part of Site U1403. They are abundant and well preserved through middle and lower Eocene, common and relatively poorly preserved through the Paleocene–Eocene transition, and rare and poorly preserved in the lower Paleocene and Cretaceous. Depth positions and age estimates of biostratigraphic marker events are shown in Table T5, and the radiolarian distribution is shown in Table T6. Note that the distribution chart is based on shipboard study and is biased toward age-diagnostic species and distinctive morphotypes.

The uppermost interval from Samples 342-U1403A-1H-CC to 5H-CC (5.85–43.84 mbsf) is barren or contains only rare and poorly preserved radiolarians. The underlying interval from Samples 6H-CC through 7H-CC (53.69–63.18 mbsf) contains common spumellarian radiolarians, which are poorly preserved and not age diagnostic. In the lower part of the correlated interval in Hole U1403B, rare, poorly preserved nassellarian radiolarians are tentatively referred to the genera Podocyrtis and Theocyrtis and suggest an Eocene age for the interval from 342-U1403B-8H-2, 66–68 cm, through 8H-CC (56.07–63.97 mbsf).

Radiolarians are abundant and generally well preserved in Hole U1403A from Samples 342-U1403A-8H-CC through 19X-CC (73.01–165.65 mbsf) and are assigned to Eocene radiolarian Zones RP12–RP9. The radiolarian assemblages are diverse and have many taxa in common with the low-latitude assemblages that form the basis of the Cenozoic radiolarian zonation. The top of this radiolarian-rich interval is bracketed in Hole U1403B by Samples 342-U1403B-8H-CC and 9H-3, 50–52 cm (63.97–66.91 mbsf). The radiolarian assemblage in the latter sample is assigned to Zone RP13 based on the base of Eusyringium fistuligerum. Samples 342-U1403B-10H-2, 120–122 cm, and 10H-5, 50–52 cm (75.56–79.26 mbsf), and the interval in Hole U1403A from Samples 342-U1403A-8H-CC through 11H-CC (73.01–101.15 mbsf) are correlated with Zone RP12 based on the presence of Eusyringium lagena. The evolutionary transition (or faunal crossover) from Thyrsocyrtis tensa to Thyrsocyrtis triacantha occurs in the lower part of this zone in Sample 342-U1403A-10H-CC (91.91 mbsf). Other key bioevents recorded in the zone include the bases of Podocyrtis diamesa, Theocotyle nigriniae, and Thyrsocyrtis hirsuta.

Identification of the two underlying zones is based on single samples in Hole U1403A. Zone RP11 is identified by the base of Dictyoprora mongolfieri in Sample 342-U1403A-8H-CC (110.87 mbsf), and Zone RP10 is identified by the faunal crossover from Theocotyle nigriniae to Theocotyle cryptocephala mongolfieri in Sample 342-U1403A-13H-CC (120.23 mbsf).

The interval in Hole U1403A from Samples 342-U1403A-14H-CC to 18X-CC (129.77–148.77 mbsf) is correlated with Zone RP9 using the base of Lychnocanoma bellum in the lowermost sample. Other key bioevents recorded in this zone include the bases of Podocyrtis diamesa and Thyrsocyrtis rhizodon.

Sample 342-U1403A-19X-CC (165.63 mbsf) is correlated with Zone RP8 using the base of T. hirsuta.

Cores 342-U1403A-20X and 21X contain common to abundant radiolarians of moderate to poor preservation. Samples 342-U1403A-20X-CC and 21X-CC (175.45–185.14 mbsf) are correlated with the Eocene part of Zone RP7 based on the base of both Podocyrtis papalis and Theocorys? phyzella. These two bioevents occur within the PETM at Ocean Drilling Program (ODP) Site 1051, Blake Nose (Sanfilippo and Blome, 2001), and at Mead Stream, New Zealand (Hollis et al., 2005). Other key bioevents that occur within this interval include the bases of Bekoma bidartensis (zonal marker), Amphicraspedum prolixum gr., and Calocyclas castum and the top of Amphisphaera goruna.

Radiolarians are common to few but poorly preserved in Cores 342-U1403A-22X through 25X. Samples 342-U1403A-22X-CC and 23X-CC (188.69–204.32 mbsf) are assigned to Zones RP5–RP6 based on the base of Buryella tetradica in the lowermost sample. Samples from Cores 24X and 25X lack age-diagnostic species.

Radiolarians are rare and generally poorly preserved in Cores 342-U1403A-26X through 28X (224.21–242.3 mbsf) and in the lowermost core catcher sample from Hole U1403B (342-U1403B-32X-CC; 261.9 mbsf). Further higher resolution sampling of this interval may yield age-diagnostic species, especially in the Cretaceous where nassellarians have been noted in the foraminifer samples.

Planktonic foraminifers

Core catchers were sampled principally from Hole U1403A, and additional Cretaceous samples were taken in Hole U1403B from intervals of light-colored sediment. Samples were, for the most part, barren of planktonic foraminifers except for thin carbonate-rich intervals in the lower Eocene and lower Paleo-cene. The Cretaceous samples consistently contain rare to few planktonic foraminifer specimens. Preservation is poor except in samples from Cores 342-U1403B-29X through 31X (234.66–251.90 mbsf). Thus, planktonic foraminifers are of limited use in the development of a biostratigraphy and for correlation between the two holes at this site. Depth positions and age estimates of biostratigraphic marker events identified are shown in Table T7. Abundances and preservation are shown in Table T8 and Figure F17. When recovered, planktonic foraminifer assemblages show evidence of dissolution, are moderately to poorly preserved, and have relatively low species diversities (typically ~3–6 species in the lower Paleogene and ~1–3 in the Upper Cretaceous). The sporadic occurrence of planktonic foraminifers at Site U1403 is the result of dissolution below or close to the CCD.

In Core 342-U1403A-20X, a poorly preserved lower Eocene–upper Paleocene assemblage was recovered containing the marker species Acarinina coalingensis, Acarinina soldadoensis, Morozovella subbotinae, and Subbotina velascoensis, indicating Zone E4 to Paleo-cene P4c (Sections 342-U1403A-20X-1 to 20X-5; 166.49–172.57 mbsf) and Zones E2–P5 (Sections 20X-6 to 20X-CC; 173.96–175.45 mbsf). Chiloguembelina wilcoxensis is a good biostratigraphic marker for the lower Eocene at Site U1403 and occurs in Samples 342-U1403A-20X-1, 78–80 cm, to 20X-CC (166.49–175.45 mbsf). Further differentiation of Eocene biozones was hindered by the absence of key taxa.

Samples 342-U1403A-23X-6, 48–50 cm, to 23X-7, 15–17 cm (202.59–203.66 mbsf), contain a poorly preserved early Paleocene assemblage. The presence of Parasubbotina varianta, Praemurica inconstans, Morozovella angulata, and Subbotina triloculinoides enables the identification of Zone P4 to Subzone P1c.

The nannofossil ooze recovered in Sections 342-U1403A-26X-1 through 26X-5 (214.5–222 mbsf) contains a good to very well preserved assemblage of lowest Paleocene planktonic foraminifers. Species identified in the assemblage include Chiloguembelina morsei, Eoglobigerina edita, Eoglobigerina eobulloides, Praemurica pseudoinconstans, Praemurica taurica, and Subbotina trivalis. This assemblage is assigned to planktonic foraminifer Zone Pα to Subzone P1b, consistent with the nannofossil results for this interval.

The K/Pg boundary was recovered in both holes, toward the top of Sections 342-U1403A-26X-5 and 342-U1403B-28X-1 (220.5–235.4 mbsf). Above the K/Pg boundary, Sample 342-U1403A-26X-5, 3–4 cm (220.54 mbsf), contains well-preserved lowest Paleocene faunas (E. eobulloides and E. edita) associated with reworked upper Maastrichtian specimens. However, the samples below the K/Pg boundary (Sample 342-U1403A-26X-5W, 22–24 cm; 220.73 mbsf) include only small specimens of hedbergellids. Cores 342-U1402B-29X through 30X (226.3–245.7 mbsf) contain well-preserved assemblages consisting of Contusotruncana (C. contusa, C. fornicata, and C. patelliformis), Globotruncana (G. bulloides, G. linneana, and G. ventricosa), and Globotruncanita (G. stuarti and G. stuartiformis) with some Heterohelix species (H. globulosa and H. navarroensis). The assemblage includes tropical Maastrichtian–Campanian faunas of C. contusa, Gansserina gansseri, and Globotruncana aegyptiaca. However, the Maastrichtian marker species Abathomphalus mayaroensis is absent. The co-occurrence of C. contusa, G. linneana, G. bulloides, and G. ventricosa suggests an early Maastrichtian age.

In other samples from Cores 342-U1403A-26X through 28X (214.5–253.3 mbsf) and Core 342-U1403B-32X (255.4–261.91 mbsf), the planktonic foraminifer assemblage is impoverished and poorly to moderately preserved. Typical constituents of the assemblage are Hedbergella cf. H. monmouthensis, Hedbergella holmdelensis, Globotruncanella petaloidea, and H. navarroensis. Because many of the marker species are absent (e.g., Abathomphalus mayaroensis, Pseudoguembelina hariaensis, Pseudoguembelina palpebra, and Racemiguembelina fructicosa), differentiation of individual biozones is not possible for the Maastrichtian and Campanian.

Benthic foraminifers

Benthic foraminifers were examined semiquantitatively in core catcher samples from Hole U1403A and from core catcher samples taken in the Cretaceous section of Hole U1403B (Cores 342-U1403B-29X through 32X). Additional samples taken from selected sections in Holes U1403A and U1403B were examined for preservation and relative abundance of total benthic foraminifers and individual morphogroups. Benthic foraminifers at this site are rare relative to total sediment particles >150 µm in the lower Eocene to lower Paleocene but become dominant throughout the Campanian to Maastrichtian. Faunas generally suggest a lower bathyal to abyssal paleodepth for this site (van Morkhoven et al., 1986). Preservation of foraminifer tests is generally moderate to good (Fig. F17). The occurrences of benthic foraminifers at this site are shown in Tables T9 and T10.

The uppermost sample (342-U1403A-1H-CC; 5.83 mbsf) yields only a few benthic foraminifer specimens including Cibicides sp., Nuttallides decorata?, and Pullenia subcarinata, most probably a result of dissolution below the CCD.

Lithostratigraphic Units II and III are predominantly barren of benthic foraminifers. The benthic foraminifer fauna in Samples 342-U1403A-3H-CC, 4H-CC, 6H-CC, 12H-CC, and 13H-CC are characterized by agglutinated taxa (mainly Bathysiphon sp.) and a few poorly preserved calcareous specimens in Samples 342-U1403A-12H-CC and 13H-CC.

Samples 342-U1403A-13H-CC through 20X-CC (120.33–175.45 mbsf) are characterized by Eocene assemblages that are mainly dominated by calcareous taxa, such as Abysammina poagi, Anomalina spissiformis, Cibicidoides mexicanus, Cibicidoides praemundulus, and Nuttallides truempyi. Preservation of these assemblages is poor to moderate but improves significantly in Samples 342-U1403A-19X-CC and 20X-CC. Two Eocene samples studied are barren of benthic foraminifers (Samples 14H-CC and 17H-CC).

The underlying Samples 342-U1403A-21X-CC and 22X-CC contain only sporadic benthic foraminifers. Agglutinated forms, such as Bathysiphon sp., Hormonisella sp., Glomospira sp., and Saccammina sp., are found together with some poorly preserved calcareous forms.

Paleocene to Campanian Samples 342-U1403A-23X-CC through 28X-CC and 342-U1403B-29X-CC through 32X-CC contain diverse calcareous benthic foraminifer assemblages. Diversity increases downhole and peaks in the Maastrichtian to Campanian. The assemblages are dominated by Gavelinella beccariiformis and N. truempyi. Abundant calcareous taxa are typically Anomalina praeacuta, Aragonia velascoensis, Gyroidinoides globosus, Lenticulina cf. adenalensis, and Nuttallinella florealis. Agglutinated forms identified include abundant Dorothia trochoides and Gaudryina pyramidata, whereas Bathysiphon sp., Rhizammina sp., and Rhabdammina sp. are subordinate. Preservation of these forms is moderate to good. Benthic foraminifers are the dominant component in the >150 µm fraction in the Cretaceous sediment at Site U1403.

The benthic foraminifer species Reussella szajnochae is found in Samples 342-U1403B-31X-4, 89–91 cm, through 32X-CC (251.10–261.89 mbsf). R. szajnochae is considered to be a low-latitude taxon that favors warm bottom water (Widmark, 1995) and is hypothesized to disappear during a shift in ocean circulation during the early Maastrichtian, which marks the first appearance of colder southern-component water masses. In Hole U1403B, the top of R. szajnochae at Sample 342-U1403B-31X-4, 89–91 cm (251.10 mbsf), is coincident with the last occurrence of inoceramid prisms in Sample 342-U1403B-31X-4, 89–91 cm (251.10 mbsf). This observation might indicate cooling in North Atlantic bottom water.

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