Previous   |   Next

doi:10.2204/iodp.proc.313.104.2010

Paleontology

Middle and lower Miocene sections were identified from calcareous nannofossils, planktonic foraminifers, and dinocysts (Fig. F22) and integrated with Sr isotope stratigraphy to establish a chronostratigraphic framework for Hole M0028A (Fig. F23). Calcareous microfossil occurrences ranged from absent to abundant, and preservation ranged from poor to excellent, with the best preserved, most abundant specimens and most diverse assemblages often occurring in finer grained sediments. In general, organic microfossils were abundant and well preserved in fine-grained sediments. Abundant reworking of Paleogene material made assigning ages to some intervals difficult, but there is generally good agreement between the planktonic microfossil groups and ages based on Sr isotopes. As in Hole M0027A, barren intervals coincided with coarse-grained sediments, diminishing biostratigraphic age control within intervals of the lower Miocene.

Paleobathymetry and paleoenvironments were determined from benthic foraminifers, dinocysts, and terrigenous palynomorphs. Paleodepths varied through these sections, ranging from inner to middle neritic (0–100 mbsf). In several sequences, paleobathymetric fluctuations indicated shallowing-upward successions that occurred within a sequence stratigraphic framework. In general, good correlation was found between lithostratigraphic units and benthic foraminifer biofacies. Similarly, benthic foraminifer water depth estimates and palynological estimates of proximity to shoreline were consistent. Palynological data support previous reconstructions of the warm, humid early Neogene climate.

Biostratigraphy

Calcareous nannofossils

A total of 56 samples from Hole M0028A were examined for calcareous nannofossil biostratigraphy. Sample spacing was generally one sample every three cores (approximately every 10 m), with fewer samples taken from sandy or disturbed intervals. The total abundance of calcareous nannofossils within samples ranged from barren to abundant. Preservation varied significantly throughout the hole, ranging from poor to good. Most samples contained moderately to moderately well preserved calcareous nannofossils, with rare horizons of poor preservation. Members of the Noelaerhabdaceae family dominate the assemblages throughout the hole. Reworked Paleogene material is also prevalent, particularly in Cores 313-M0028A-79R through 113R (414.66–519.96 mbsf).

Horizons recovered in Hole M0028A contain middle and lower Miocene assemblages (Table T4; Fig. F24). Two samples from the uppermost cores in this hole (Samples 313-M0028A-2R-1, 120 cm, and 5R-1, 43 cm; 224.53–232.91 mbsf) contain common numbers of calcareous nannofossils typical of Miocene sediments but no biostratigraphic marker taxa. The questionable presence of Helicosphaera walbersdorfensis could suggest Martini's (1971) Zone NN7 (11.3–11.8 Ma) or somewhat older; however, the range of this species is not well dated in the literature. A questionable specimen of Calcidiscus premacintyrei, whose last occurrence (LO) is dated to 12.65 Ma, within Zone NN6, was found in Sample 313-M0028A-8R-1, 45 cm (243.08 mbsf). Because there is no evidence of a significant hiatus between Cores 313-M0028A-5R and 8R (232.91–243.08 mbsf), the uppermost part of the hole is likely assignable to Zone NN6.

The LO of Sphenolithus heteromorphus is found in Sample 313-M0028A-9R-1, 145.5 cm (246.135 mbsf), although this may not be the actual extinction (13.6 Ma) of this species, suggesting that these sediments are somewhat older than 13.6 Ma. The interval from this sample to Sample 313-M0028A-27R-2, 43.5 cm (292.105 mbsf), is assigned to Zone NN5 (13.6–15.6 Ma). S. heteromorphus is only sporadically present throughout this section; however, calcareous nannofossils were not as abundant in these horizons because of the presence of coarser grained sediment.

The LO of Helicosphaera ampliaperta (15.6 Ma) occurs in Sample 313-M0028A-30R-1, 24 cm (297.37 mbsf). There is no obvious evidence of a hiatus above this sample, so this may represent the extinction of H. ampliaperta. The interval from Core 313-M0028A-30R to Sample 313-M0028A-113R-1, 94 cm (297.37–519.96 mbsf), is assigned to Zone NN4 (15.6–18.3 Ma) based on the co-occurrence of H. ampliaperta and S. heteromorphus and the absence of Sphenolithus belemnos. Within this zone, the last common occurrence (LcO) of Discoaster deflandrei (~16.21 Ma) is found in Sample 313-M0028A-58R-1, 95 cm (364.74 mbsf). This biohorizon is below a package of coarser grained material that was not sampled, so this datum may be somewhat depressed in this hole.

A zone barren of nannofossils occurs in Cores 313-M0028A-118R through 144R (530.84–590.88 mbsf), making it impossible to assign an age to these horizons. The first fossiliferous sample below this zone (Sample 313-M0028A-152R-1, 4 cm; 610.56 mbsf) contains an abundant but poorly preserved lower Miocene assemblage. Because this sample is from the base of a coarser grained unit, the calcareous nannofossil assemblage is likely reworked from below. A presumed in-place assemblage occurs in fine-grained sediment in the next section (Sample 313-M0028A-152R-2, 17 cm; 612.20 mbsf). The assemblage in this sample is older than the first occurrences (FOs) of S. heteromorphus (18.2 Ma) and H. ampliaperta (~21.0 Ma). Triquetrorhabdulus carinatus, the LO of which marks the base of Zone NN3 (19.6 Ma), is questionably present in this sample. Furthermore, the assemblage must be above the LcO of T. carinatus (22.9 Ma), which is near the base of Zone NN2. This section is also above a secondary event, the crossover in abundance of Helicosphaera euphratis with Helicosphaera carteri, which is tentatively dated to 21.5 Ma. Thus, samples from Cores 313-M0028A-152R through 154R (610.56–618.64 mbsf) are assigned to Zone NN2 and most likely the middle part of the zone, at ~21–21.5 Ma.

The next sample examined (Sample 313-M0028A-156R-1, 19 cm; 622.91 mbsf) contains frequent poorly preserved calcareous nannofossils. Cores 313-M0028A-158R through 168R (629.82–660.63 mbsf) are barren of calcareous nannofossils. The overlying fossiliferous and barren intervals are assigned to lithologic Unit VI, which consists of pale brown silty clay and clayey silts. The only obvious change from the top of this unit within Core 313-M0028A-152R (611.60 mbsf) to the zone barren of nannofossils beginning in Core 158R (629.82 mbsf) is a progressive decrease in the amount of pyrite and concretions (see "Lithostratigraphy"). The presence of carbonate in samples from the barren interval suggests that calcareous nannoplankton were living in the overlying water column during deposition of these sediments, but diagenetic processes significantly altered the assemblage after deposition. Calcareous nannofossils are again present in sediments from the bottom of the hole (Samples 313-M0028A-169R-1, 130.5 cm, and 170R-4, 37 cm; 663.675–668.47 mbsf), and the assemblage is very similar to that found in the overlying Zone NN2 horizons. Thus, these sediments are also assigned to mid?-Zone NN2.

Planktonic foraminifers

A total of 49 samples were studied for planktonic foraminifers (Fig. F25; Table T5). Of these, 31 were barren; however, planktonic foraminifers were well preserved in the remaining 18 samples. Planktonic foraminifer biostratigraphy indicates that Hole M0028A spans the lower to middle Miocene (Burdigalian–Serravalian), although sediment below Sample 313-M0028A-116R-CC, 0–4 cm (527.46 mbsf), was mostly barren of planktonic foraminifers. The absence of key marker taxa (e.g., Fohsella peripheroacuta, Praeorbulina sicana, Orbulina spp., Praeorbulina glomerosa, Catapsydrax dissimilis, and Globigerinatella insueta) made identification of Miocene planktonic foraminifer zones difficult, resulting in intervals assigned to undifferentiated zones.

The first biostratigraphically useful taxa are found in Sample 313-M0028A-4R-CC (232.80 mbsf) and include Paragloborotalia mayeri (LO 11.4 Ma), a questionable Globorotalia archeomenardii (LO 13.9 Ma), and Globorotalia praemenardii (FO 14.2 Ma). The co-occurrence of the latter two species yields a middle Miocene (Serravalian) age of 13.9–14.2 Ma (Zone M7/N10) for this sample. The interval between Samples 313-M0028A-6R-CC and 27R-CC (238.98–293.50 mbsf) contains an assemblage consisting of Globigerinoides sacculifer, Globoquadrina baroemoenensis, Globoquadrina venezuelana, Globorotalia bella, G. archeomenardii, and P. mayeri. The absence of G. praemenardii (FO 14.2 Ma) and Catapysdrax stainforthi (LO ~16.4 Ma) suggests a middle Miocene undifferentiated Zone M5–M7/N8–N10 (Langhian–Serravallian) for these sediments, although because of the absence of key taxa, this zonal assignment is based on the background assemblage, making it tentative.

C. stainforthi occurs in Sample 313-M0028A-29R-CC, 14–16 cm (299.72 mbsf). Although this species is not a primary marker taxon, Bolli et al. (1985) indicate that it ranges from M2/N5 to M4/N7, suggesting this sample is lower Miocene (upper Burdigalian) and older than 16.4 Ma. The interval from this sample to Sample 313-M0028A-38R-CC, 0–2 cm (323.92 mbsf), also contains Globorotalia praescitula, the FO of which falls within Zone M3/N6 at 18.5 Ma. Thus, this interval can be assigned to undifferentiated Zone M3–M4/N6–N7. Samples between Cores 313-M0028A-40R and 79R (329.80–415.65 mbsf) were barren of planktonic foraminifers and are not zoned.

Globorotalia semivera occurs in Sample 313-M0028A-82R-CC, 4–6 cm (426.74 mbsf). This species has a LO of 17.3 Ma, which approximates the base of Zone M4/N7. The interval from this sample to Sample 313-M0028A-116R-CC, 0–4 cm (527.46 mbsf), also contains G. praescitula (FO 18.5 Ma), as well as a general assemblage including Globigerinoides altiapertura and Globigerinoides primordius. Thus, this interval (426.74–527.46 mbsf) is assigned to lower Miocene (mid-Burdigalian) Zone M3/N6. Samples below Core 313-M0028A-116R (527.46 mbsf) were either barren or contained a sparse assemblage with no biostratigraphically useful taxa. Therefore, the interval below this sample to the base of the hole in Sample 313-M0028A-170R-CC, 6–8 cm (668.63 mbsf), is not zoned.

Dinocysts

Following the zonation of de Verteuil and Norris (1996), Samples 313-M0028A-9R-CC, 12–14 cm, to 11R-CC, 10–12 cm (246.98–254.18 mbsf), are assigned to upper Serravalian dinocyst Zone DN7 based on the presence of Habibacysta tectata together with Erymnodinium delectabile, Cannosphaeropsis passio, Selenopemphix dionaeacysta, and Cyclopsiella granosa. Because the FO of E. delectabile has been demonstrated to occur in older sediments from the North Sea, a more conservative assignment of Zones DN6–DN7 is recommended (Fig. F26; Table T6). Sample 313-M0028A-27R-CC (293.50 mbsf) is assigned to Zone DN5 because of the presence of Batiacasphaera sphaerica, Labyrinthodinium truncatum modicum, Trinovatedinium papulum, Cerebrocysta poulsenii, Heteraulacacysta campanula, and Systematophora placantha. The presence of H. tectata and absence of Apteodinium tectatum suggest a maximum age of 14.2 Ma for this sample, restricting it to the early Serravalian.

The occurrence of A. tectatum, Cousteaudinium aubryae, Lingulodinium multivirgatum, and L. truncatum modicum constrains the age of Samples 313-M0028A-29R-CC and 30R-CC (299.74–299.21 mbsf) to Zone DN4 (latest Burdigalian–Langhian; ~16.8–15.2 Ma). Sample 313-M0028A-34R-CC (311.43 mbsf) contains only relatively long ranging taxa, but Samples 37R-CC through 94R-CC (320.69–464.10 mbsf) are assigned to undifferentiated Zones DN2–DN3 (late Aquitanian–Burdigalian; ~22.2–16.8 Ma) based on the occurrence of L. multivirgatum with Apteodinium spiridoides, Distatodinium paradosum, A. tectatum, and C. aubryae. Sample 313-M0028A-97R-CC, 9–11 cm (473.35 mbsf), can be assigned to Zone DN3 based on the co-occurrence of L. multivirgatum and Sumatradinium druggii, thus constraining the age of the overlying samples (up to 311.52 mbsf) to the Burdigalian.

Sample 313-M0028A-100R-CC, 6–8 cm (482.53 mbsf), contains only relatively long ranging taxa, and reworking is pervasive in Sample 102R-CC (487.68 mbsf), making age assignment difficult. Samples 313-M0028A-104R-CC to 169R-CC, 16–18 cm (494.53–664.50 mbsf), are assigned to Zone DN2, which spans 22.2–19.1 Ma (late Aquitanian–early Burdigalian) based on the common presence of dinocysts typical of the lower Miocene (e.g., Sumatradinium hamulatum, Tityrosphaeridium cantharellum, Cribroperidinium tenuitabulatum, and Cerebrocysta satchelliae). Substantial reworking of Paleogene cysts is also noted in Samples 313-M0028A-167R-CC, 13–15 cm (659.49 mbsf), and 169R-CC, 16–18 cm (664.50 mbsf), so age assignment relied only on FOs. Sample 313-M0028A-170R-CC, 6–8 cm (668.63 mbsf), is very rich in amorphous organic matter, suggesting a distal depositional environment, but it also contains a very sparse dinocyst flora and cannot confidently be assigned an age.

Paleoenvironment

Benthic foraminifers

Core catcher (CC) samples and samples from within cores were examined for benthic foraminifers from Samples 313-M0028A-2R-3, 30–32 cm (226.63 mbsf), through 170R-CC (668.63 mbsf) (Fig. F27; Table T7). Benthic foraminifer abundances range from absent to abundant, and preservation ranges from poor to excellent. Poorly preserved specimens likely were reworked and were not used in paleodepth estimates. Barren intervals occur throughout the section and may indicate nearshore/nonmarine environments, substantial downslope transport, or dissolution. It should be noted that the low-resolution sampling interval did not bracket all lithologic changes or unconformities. Within the sample resolution used here, benthic foraminifer biofacies groupings correlate well with lithostratigraphic units. Preliminary results presented here suggest that benthic foraminifer biofacies changes in Hole M0028A indicate that paleobathymetric fluctuations occur within a sequence stratigraphic framework, with several sequences showing a shallowing-upward succession. Detailed, higher resolution postcruise studies will elaborate on this.

Benthic foraminifer faunas in Samples 313-M0028A-2R-3, 30–32 (226.63 mbsf), to 38R-CC, 0–2 cm (323.92 mbsf), indicate that paleowater depths varied within the inner to middle neritic zones (0–100 m), although several samples yielded no foraminifers (Fig. F27). Assemblages within this section indicate water depths that ranged from 10–25 m (dominated by Hanzawaia concentrica/Hanzawaia hughesi) to 25–50 m (dominated by Nonionella pizarrensis/Nonionella miocenica stella) to 50–75 m (characterized by elevated abundances of Buliminella gracilis), to as deep as 100 m (indicated by increases in Uvigerina juncea). Samples containing more abundant B. gracilis indicate slightly shallower paleodepths (50–80 m) than samples containing more abundant U. juncea (75–100 m), based on faunal studies from New Jersey (USA) coastal plain boreholes (Miller et al., 1997). Other rare to common species found in this section include Buliminella-Uvigerina–dominated faunas, typically including Bolivina spp., Buliminella elegantissima, Cancris sagra, Cassidulinoides spp., Cibicidoides spp., Dentalina spp., Fissurina spp., H. concentrica/H. hughesi, Lenticulina americana, Lenticulina spp., Nonion sp., Pararotalia sp., Plectofrondicularia spp., Rectuvigerina lamelata, Stilostomella spp., Textularia spp., and agglutinated foraminifers.

Samples 313-M0028A-40R-CC (329.80 mbsf) to 91R-CC, 9–11 cm (453.89 mbsf), were barren or contained only one or two specimens of foraminifers, with one exception; no paleodepth could be determined for these samples (Fig. F27). Based on rare but well-preserved foraminifers (primarily B. gracilis), the paleodepth for Sample 313-M0028A-82R-CC, 4–6 cm (426.74 mbsf), is estimated at 50–75 m.

Samples 313-M0028A-94R-CC, 0–2 cm (464.10 mbsf), through 107R-CC, 0–3 cm (503.78 mbsf), contain benthic foraminifer faunas that indicate paleowater depths in the inner to middle neritic zones (0–100 m), although several samples yielded no foraminifers (Fig. F27). This interval is underlain by another that is mostly barren of benthic foraminifers from Samples 313-M0028A-110R-CC, 0–2 cm (512.72 mbsf), to 167R-CC, 13–15 cm (659.49 mbsf). Foraminifers in Sample 313-M0028A-152R-CC, 151–153 cm (613.54 mbsf), indicate a possible paleodepth of 10–25 m, but specimens are too sparse to determine this with certainty. The lowermost two samples examined, Samples 313-M0028A-169R-CC, 16–18 cm (664.50 mbsf), and 170R-CC, 6–8 cm (668.63 mbsf), contain B. gracilis and a few U. juncea, indicating a ~50–80 m paleodepth.

Terrestrial palynomorphs and palynofacies

Sediments from 23 samples spanning the interval between Sample 313-M0028A-2R-3, 30–32 cm (226.63 mbsf), and 170R-CC, 6–8 cm (668.63 mbsf), were analyzed for palynomorphs (Tables T6, T8; Fig. F28). All samples contain numerous well-preserved pollen grains and other palynomorphs. Palynomorph identification was frequently hampered by accumulations of pyrite grains inside the palynomorphs. Nevertheless, pollen could generally be assigned to major groups: arboreal/herbal nonsaccate, grass/sedge, and bisaccate pollen. Furthermore, frequent pollen types (e.g., oak, hickory, and linden pollen) were identified to genus level to distinguish dominant vegetation types. In the following, the total content of nonsaccate pollen and pollen with small sacci is used as the reference sum (thus, combined small saccate and nonsaccate pollen are always 100%), and percentages of other palynomorphs are related to this sum. Ages mentioned are in accordance with the preliminary age model based on organic-walled dinoflagellate cysts (see "Dinocysts").

Serravallian and late Burdigalian (~225 to ~320 mbsf)

Eight samples from the Serravallian to uppermost Budigalian were examined between Samples 313-M0028A-2R-3, 30–32 cm, and 37R-CC (226.63–320.69 mbsf). All samples show high percentages of nonsaccate arboreal pollen (mainly oak and hickory). The early Serravallian obviously witnessed a spreading of hickories, as indicated by pollen percentages >20% (Fig. F28), taking into account that hickory pollen is generally underrepresented compared to oak and other arboreal pollen. This indicates that the climate was probably humid and warm. No samples between Samples 313-M0028A-37R-CC and 81R-CC, 9–11 cm (320.69–423.08 mbsf), were analyzed during the Onshore Science Party (OSP).

Early Burdigalian–late Burdigalian transition (~423 to ~513 mbsf)

The presumed transition from the lower to upper Burdigalian is reflected in 10 palynologically analyzed samples from Cores 313-M0028A-81R through 110R (423.08–512.72 mbsf). This interval is characterized by higher percentages of bisaccates in Core 313-M0028A-97R (473.35 mbsf) and above and lower percentages below that (i.e., Core 100R and below; 482.53–512.72 mbsf) (Fig. F28). A similar signal is also present in the foraminifer test lining percentages, with generally higher percentages between 423.08 and 473.35 mbsf and lower percentages (<5%) between 482.53 and 523.72 mbsf. Dinocyst percentages increase uphole over the entire interval. This indicates that Hole M0028A was probably closer to the shoreline during the late early Burdigalian. The position of Hole M0028A presumably became more distal until the early late Burdigalian; however, fluctuations in the foraminifer test linings indicate that this overall trend was interrupted by several shorter term sea level changes. In general, this interval was climatically stable, but slight shifts in the hickory/oak ratio indicate minor changes in humidity. Samples between Cores 313-M0028A-110R and 152R (512.72–613.54 mbsf) could not be analyzed during the OSP.

Late Aquitanian to early Burdigalian (~615 to ~670 mbsf)

Two samples (Samples 313-M0028A-152R-CC, 151–153 cm, and 155R-CC, 11–13 cm; 613.54–622.86 mbsf) are characterized by very high percentages of bisaccate pollen (>100%). Preliminary results from the comparison of different pine taxa indicate that these high percentages are at least partly caused by an increased distance between Hole M0028A and the shoreline, rather than by shifts in climate conditions. This hypothesis is also supported by relatively high amounts of dinocysts (>20%) and foraminifer test linings (5%–20%) in these samples (Fig. F28). Slight changes in climate conditions are indicated by high amounts of pollen from western and eastern hemlock (Tsuga) species, which, with one exception, are not found in other samples from Hole M0028A. The presence of western hemlock species in combination with lower amounts of hickory pollen points to especially moist conditions during this interval (Fig. F29).

Three samples between Samples 313-M0028A-167R-CC, 13–15 cm, and 170R-CC, 6–8 cm (659.49–668.63 mbsf), reflect the probable transition between the upper Aquitanian and the lowermost Burdigalian. This interval probably witnessed a change to warmer and dryer conditions, as indicated by a strong increase in hickory pollen between ~670 and ~660 mbsf, where pollen of other trees (e.g., oaks) decreased (Fig. F28). The hinterland was probably inhabited by hickory-oak forests, with hickory as the dominant taxon (Fig. F29). This is different from the late Burdigalian, where oaks were probably dominant in the hickory-oak forests (Fig. F29).

Top of page   |   Previous   |   Next