BIOSTRATIGRAPHY

The two holes at Site 1260 yielded primarily Eocene–mid-Cretaceous marine sediments that contain planktonic foraminifers, calcareous nannofossils, and radiolarians in abundances and states of preservation that vary widely with lithology and sediment induration. Shipboard examination of these microfossil groups in core catcher samples and additional samples in the cores permitted zonal or stage assignments to be made for the entire sequence. Datum levels are summarized in Figure F10 and in Tables T3, T4, T5, T6, T7, and T8.

A thin (10 cm) veneer of Pleistocene ooze at the top of the section overlies ~35 m of lower Oligocene calcareous ooze. The Oligocene interval from Cores 207-1260A-2R to 5R shows complicated ages, in part inverted by extensive reworking and slumping. A middle Eocene nannofossil radiolarian chalk from Cores 207-1260A-6R to 20R contains abundant and well-preserved radiolarians. Planktonic foraminifers are poorly preserved from Cores 207-1260A-11R to 23R because of dissolution. The P/E boundary is in Sections 207-1260A-30R-7 and 207-1260B-17R-7.

The K/T boundary ejecta layer (~1.8 cm thick) is recognized in Sections 207-1260A-36R-4 and 207-1260B-23R-3. The late Maastrichtian nannofossil Micula murus was identified immediately below the ejecta horizon; overlying strata contain the Danian planktonic foraminifers from Zone P.

A distinctive condensed glauconite-rich horizon visible in Core 207-1260A-42R separates the Campanian chalk from the ~93-m-thick black shales below (lithostratigraphic Unit IV) that compose the lower part of OAEs 3 and 2. Site 1260 provides a nearly continuous Coniacian–Cenomanian black shale record. Calcareous microfossils are generally present throughout the black shales, and foraminifers range in preservation from very poor to glassy. Clayey quartz siltstones of lithostratigraphic Unit V are early Albian in age, and microfossils are rare.

The two holes drilled at Site 1260 obtained primarily middle Eocene–early Albian deposits under a thin veneer of Pleistocene and slumped sediments of early Oligocene age. These contain, apart from the Turonian–Albian interval, generally common to abundant calcareous nannofossils of moderate to good preservation. The observations made allow for zonal or stage assignments that are summarized in Figure F10 and Tables T3 and T7. Core catcher samples were examined for all holes and supplemented as necessary by samples in the cores to further refinement of the zonal assignments. In the more detailed description to follow, the assemblages and ages pertain to Hole 1260A unless noted otherwise.

Sample 207-1260A-1R-CC contains a Pleistocene nannofossil assemblage (Zone NN21 of the Martini, 1971 scheme) including Gephyrocapsa oceanica, Emiliania huxleyi, and Pontosphaera indooceanica. Samples 207-1260A-2R-CC to 4R-CC are of early Oligocene age (Zones NP23–NP21) as indicated by the presence of Sphenolithus distentus, Reticulofenestra umbilica, Reticulofenestra hillae, Discoaster tani, Isthmolithus recurvus, and Sphenolithus predistentus. Samples 207-1260A-2R-CC, 3R-CC, and 4R-CC are assigned to Zone NP23, but intervening Samples 3R-2, 25 cm, to 3R-3, 25 cm, produce an older NP21 Zonal assignment. These partly inverted ages indicate some major slumping.

Sample 207-1260A-5R-CC contains an upper Eocene assemblage (Zone NP19/NP20) that includes Sphenolithus pseudoradians, Discoaster barbadiensis, Discoaster saipanensis, and D. tani. Chiasmolithus grandis, Chiasmolithus solitus, and R. umbilica in Sample 207-1260A-6R-CC are of late middle Eocene age (Zone NP17). The subjacent Samples 207-1260A-7R-CC to 13R-CC have a middle Eocene age (NP16), yielding C. grandis, Sphenolithus furcatus, R. umbilica, Discoaster lodoensis, Ericsonia formosa, and Sphenolithus radians. Based on the presence of C. grandis, Nannotetrina fulgens, and Chiasmolithus gigas, Samples 207-1260A-14R-CC to 21R-2, 71 cm, have been assigned a middle Eocene age (Zone NP15). The underlying Samples 207-1260A-22R-CC to 23R-CC are of early middle Eocene age (Zone NP14) based on the presence of Discoaster sublodoensis and Rhabdosphaera inflata. Sample 207-1260A-24R-CC contains D. barbadiensis, D. sublodoensis, D. lodoensis, and S. radians and is thus of earliest middle Eocene age (Zone NP14).

Samples 207-1260A-25R-CC to 26R-CC yielded Sphenolithus conspicuus, E. formosa, and S. radians, giving an early Eocene age (Zone NP12). Abundant Neochiastozygus junctus and Zygrhablithus bijugatus and the absence of Fasciculithus spp. in Samples 207-1260A-27R-CC to 207-1260B-29R-CC support an earliest Eocene age above the P/E boundary (upper part of Zone NP9). An assemblage with common Fasciculithus spp. and Discoaster multiradiatus in Samples 207-1260A-30R-CC to 32R-CC assigns this interval a latest Paleocene age (NP9 lower part). A nannofossil assemblage consisting of Discoaster mohleri, Heliolithus riedelii, and Fasciculithus tympaniformis in the absence of D. multiradiatus characterizes Sample 207-1260A-33R-CC as late Paleocene in age (Zone NP8). A late Paleocene age (Zone NP7) is also assigned to Samples 207-1260A-34R-CC based on the presence of Chiasmolithus consuetus and Ellipsolithus macellus along with common D. mohleri. Ellipsolithus macellus, however, is absent in Sample 207-1260A-35R-CC, but Cruciplacolithus primus and Cruciplacolithus tenuis (up to 10 µm long) denote Zone NP3.

Samples 207-1260A-36R-CC to 37R-CC contain a rich nannoflora of late Maastrichtian age, including M. murus (Zones CC25–CC26). Samples 207-1260A-38R-CC to 39-CC yield a nannofossil assemblage of Maastrichtian age (Zones CC25–CC23 of the Sissingh, 1977 scheme) including Reinhardtites levis, Ceratolithus aculeus, and Arkhangelskiella cymbiformis. Samples 207-1260A-40R-CC to 41R-CC contain an early Maastrichtian to late Campanian assemblage (Zone CC23), including Uniplanarius trifidum, R. levis, A. cymbiformis, and Uniplanarius sissinghi.

The bottom of subjacent Samples 207-1260A-42-CC to 43R-2, 109–111 cm, yielded Eprolithus octopetalus, Quadrum gartneri, and Eiffellithus eximius, allowing a Turonian age assignment (Zone CC12). Samples 207-1260A-44-CC to 46R-CC have an early Turonian age (Zone CC11), indicated by the co-occurrence of E. eximius and E. octopetalus. The underlying succession from Samples 207-1260A-47R-7, 0–4 cm, to 52R-CC is of Cenomanian–latest Albian age (Zone CC10) since Eiffellithus turriseiffelii and Axopodorhabdus albianus are present. A section of Albian age was encountered in Samples 207-1260A-53R-CC to 54R-CC (bottom of hole). Eiffellithus spp. is absent, whereas Prediscosphaera columnata, Rhagodiscus angustus, Microstaurus chiastus, Eprolithus floralis, Eprolithus apertior, Radiolithus planus, and occasional Watznauria britannica and Heliolithus trabeculatus range downhole.

Planktonic foraminifer biostratigraphy at Site 1260 was based upon a combination of core catchers and samples taken in every section in Hole 1260A and selected sections in Hole 1260B. Zonal assignments are summarized in Figure F10 and Tables T4, T5, and T8. Pleistocene planktonic foraminifer Subzone PT1b through lower Albian Zone KS13 were identified in Hole 1260A. Significant breaks in the biozonation occur in the middle Oligocene–Pleistocene, the middle Eocene–upper Eocene, the lower/middle Eocene boundary, and the Coniacian–Campanian. A significant hiatus may also occur around the Albian/Cenomanian boundary. Planktonic foraminifers are present in nearly all samples but vary widely in preservation and abundance. Preservation is best in clay-rich parts of the Albian–Coniacian sequence, although foraminifers are difficult to extract and completely clean in the "black shales." Many samples from the black shales required three or more washing/drying cycles as well as heating in a mixture of Calgon and peroxide to produce large numbers of identifiable specimens. Foraminifers in the light-colored bands in the organic-rich claystones are frequently filled with calcite spar. Good preservation is found in the Pleistocene, Oligocene, and middle–upper Eocene section, whereas preservation is mostly moderate or poor in the chalk of the lower Eocene, Paleocene, and Campanian–Maastrichtian sequence. Some clay-rich chalks in the Paleocene and Maastrichtian contain outwardly well-preserved foraminifers that are filled with bladelike crystals of calcite. Foraminifers in the lower Danian (Zones P3–P) are strongly fragmented and recrystallized.

Sample 207-1260A-1R-1, 0–2 cm, contains a mixture of Pleistocene or Holocene planktonic foraminifers such as Orbulina universa, Globigerinoides sacculifer, Globorotalia truncatulinoides, Globorotalia menardii, and Globigerinoides ruber.

Yellow and green calcareous ooze between Samples 207-1260A-2R-CC and 4R-CC contains a lower Oligocene section complicated by extensive reworking or slumping. Sample 207-1260A-2R-CC includes typical species of late early Oligocene age, including Globigerina angulosuturalis. This species is absent from the rest of the Oligocene section, and the presence of Pseudohastigerina spp. in all sections between Samples 207-1260A-3R-1, 50–54 cm, and 4R-2, 50–54 cm, suggests lowermost Oligocene Zone P18. However, Sample 207-1260A-3R-2, 50–54 cm, contains a diverse upper Eocene foraminifer assemblage with Turborotalia cunialensis and Hantkenina alabamensis, indicating Zone P16. Age reversals between Oligocene Zones P18, P19, and P20 were noted in Core 207-1260A-4R and suggest that the Oligocene sequence is not in original stratigraphic order. The absence of distinctive Oligocene species in the Eocene assemblage from Sample 207-1260A-3R-2, 50–54 cm, suggests reworking may be the result of slumps rather than winnowing. However, the foraminifer markers for Oligocene Zones P20, P19, and P18 are all last occurrences (LOs), which make it hard to distinguish wholesale slumping from reworking of distinctive species. Indeed, the absence of large-scale reworking of Eocene species of calcareous nannofossils into Oligocene assemblages (see "Calcareous Nannofossils") suggests that although there may have been extensive redeposition of older sediments into the Oligocene this need not always have been produced by slumping.

Sample 207-1260A-5R-CC contains a well-preserved upper Eocene foraminifer fauna with Subbotina gortani, H. alabamensis, Turborotalia pomeroli, and Turborotalia cerroazulensis. The upper Eocene sequence must be very thin in Hole 1260A because Sample 207-1260A-6R-1, 50–54 cm, contains a middle Eocene foraminifer fauna from Zone P13. Common species in Zone P13 include Orbulinoides beckmanni, T. cerroazulensis, Morozovella spinulosa, Globigerinatheka rubriformis, Acarinina rohri, and Catapsydrax africana. Specimens of O. beckmanni in Core 207-1260A-6R have a final chamber that envelops at least half of the test and includes large nearly spherical forms. There has been much confusion over the definition of O. beckmanni; Saito (1962) chose his holotype from an assemblage containing nearly perfectly spherical forms, but he selected a type specimen more closely resembling a large example of Globigerinatheka kugleri than the spherical forms that most workers have described as O. beckmanni. Hence, if we follow the type specimen to define the Zone P13 interval, the zone extends downward to Sample 207-1260A-8R-1, 50–54 cm, at which point O. beckmanni mainly consists of large specimens that closely resemble the holotype with three nearly equally sized chambers in the final whorl and relatively deep sutures between the chambers.

An expanded record of middle Eocene Zones P12–P10 is present between Samples 207-1260A-8R-2, 50–54 cm, and 24R-CC. Samples from Zones P12 and P11 are very rich in well-preserved radiolarians, which in some cases hinder observations of foraminiferal zone species. Zone P12 has been recognized between Samples 207-1260A-8R-2, 50–54 cm, and 14R-CC. Foraminifer assemblages are dominated by A. rohri, Acarinina bullbrooki, M. spinulosa, T. pomeroli, and G. kugleri. Guembelitrioides nuttali has its LO near the middle of Zone P12 in Sample 207-1260A-10R-CC. Zone P11 is recognized by the LO of Morozovella aragonensis in Sample 207-1260A-15R-1, 50–54 cm. In addition, Zone P11 faunas include G. kugleri, A. rohri, Igorina broedermanni, M. spinulosa, Subbotina boweri, Turborotalia possagnoensis, and Globigerinatheka index. The Zone P10/P11 boundary is difficult to distinguish precisely owing to the rare occurrence of G. kugleri. Single specimens of globigerinathikids are found in Samples 207-1260A-20R-CC and 21R-CC, and these resemble G. index with a single aperture and incised sutures. We draw the Zone P9/P10 contact between Samples 207-1260A-19R-CC and 20R-CC based upon the lowest common occurrence of globigerinathikids. Poor preservation also prevents us from clearly delineating this boundary in the core catchers, so, in contrast to the rest of the cored sequence, we do not attempt a refined estimate of the boundary location using samples from in the adjacent sections. The base of Zone P10 is approximated in Sample 207-1260A-24R-CC based upon the first occurrence (FO) of G. nuttali. Other species characteristic of Zone P10 include Acarinina aspensis, Acarinina pentacamerata, M. aragonensis, and Globigerina lozanoi. Unlike Site 1258, we do not find clavigerinellids in our samples from Zone P10, but we did find "Hastigerina" bolivariana in Sample 207-1260A-23R-CC.

An unconformity evidently separates Cores 207-1260A-24R (Zone P10) from Sample 207-1260A-25R-1, 50–54 cm (Zone P8). Zone P8 is between Samples 207-1260A-25R-1, 50–54 cm, and 25R-4, 50–54 cm, and is characterized by M. aragonensis, G. lozanoi, Acarinina quetra, Acarinina soldadoensis, and I. broedermanni. Zone P7 has a similar foraminifer fauna but includes Morozovella formosa and Globigerina praecentralis between Samples 207-1260A-25R-5, 50–54 cm, and 26R-CC.

The evolution of M. aragonensis with six chambers in the last whorl from Morozovella lensiformis with four or five chambers in the last whorl was used to recognize the Zone P6/P7 boundary in Sample 207-1260A-27R-1, 50–54 cm. The FO of M. formosa could not be determined precisely owing to extremely poor preservation in most of Core 207-1260A-27R, but specimens of M. formosa with six chambers in the last whorl were observed in Sample 207-1260A-27R-3, 50–54 cm, suggesting that this sample belongs to Subzone P6b. Large specimens of Chiloguembelina wilcoxensis and Morozovella gracilis were observed from the top of Core 207-1260A-27R to Sample 207-1260A-29R-CC along with typical species in Subzone P6a, such as Acarinina wilcoxensis, Morozovella subbotinae, Morozovella marginodentata, and Morozovella aequa.

The top of Zone P5 is observed in Sample 207-1260A-30R-1, 50–54 cm, at the last appearance of Morozovella acuta, Morozovella velascoensis, and Morozovella occlusa. This sample also contains inflated specimens of Pseudohastigerina wilcoxensis as well as M. subbotinae and A. soldadoensis. The P/E boundary appears at 74 cm in Section 207-1260A-30R-7 and 69 cm in Section 207-1260B-17R-7. Samples 207-1260A-30R-6, 50–54 cm, and 30R-8, 50–54 cm, span the boundary and contain very poorly preserved foraminifers. We are not able to identify any representatives of the excursion fauna of planktonic foraminifers that are characteristic of the Paleocene–Eocene Thermal Maximum. However, Sample 207-1260A-30R-CC contains the benthic foraminifer Aragonia velascoensis, a species that became extinct at the P/E boundary. The LO of Globanomalina pseudomenardii (total range marker for Zone P4) occurs between Samples 207-1260A-32R-1, 50–53 cm, and 32R-3, 45–48 cm, and defines the lower limit of Zone P5.

Zone P4 is characterized by abundant M. velascoensis, A. soldadoensis, Igorina albeari, Subbotina velascoensis, and Subbotina triangularis. The bottom of Zone P4 is found between Samples 207-1260A-35R-5, 50–54 cm, and 35R-6, 50–54 cm, based upon the first appearance of Acarinina coalingensis and Acarinina subspherica, which are known to make their initial appearance at the same time as the zone marker G. pseudomenardii. Sample 207-1260A-35R-5, 50–54 cm, contains abundant benthic foraminifers and a dissolved planktonic foraminifer assemblage that includes reworked Cretaceous species such as Globotruncana arca and Heterohelix globulosa. The underlying strata in Samples 207-1260A-35R-6, 50–54 cm, and 35R-7, 18–22 cm, include highly recrystallized and overgrown foraminifers such as Morozovella angulata, Morozovella praeangulata, and Parasubbotina varianta that suggest Subzone P3a.

The lower Danian is in a highly condensed interval in Cores 207-1260A-35R and 36R. Samples 207-1260A-35R-CC to 36R-3, 28–29 cm, contain species typical of Zone P2, including Praemurica uncinata, M. praeangulata, Parasubbotina pseudobulloides, and Subbotina triloculinoides. In contrast, Sample 207-1260A-36R-4, 0–1 cm, represents Subzone P1c, as it contains Praemurica pseudoinconstans, Praemurica inconstans, and Praemurica taurica. Sample 207-1260A-36R-4, 80–81 cm, includes extremely small specimens of Woodringina claytonensis, Guembelitria cretacea, Eoglobigerina eobulloides, and Parvularugoglobigerina eugubina that distinguish Zone P. The K/T boundary is believed to occur in interval 207-1260A-36R-4, 93 cm, based upon a spherule bed at this level.

The Maastrichtian sequence extends between Samples 207-1260A-36R-CC and 38R-CC. Assemblages belonging to Zone KS31 (defined by the total range of Abathomphalus mayaroensis) are present between Samples 207-1260A-36R-CC and 38R-1, 49–52 cm. Accompanying species include Gansserina gansseri, Rosita contusa, Rugoglobigerina rugosa, Rugoglobigerina rotundata, and Pseudoguembelina palpebra. Subzone KS30a (the Raceiguembilina fructicosa/R. contusa Zone) is identified in Samples 207-1260A-38R-2, 49–53 cm, to 38R-6, 49–52 cm. Species typical of Subzone KS30a include R. contusa, Abathomphalus intermedius, and Globotruncana aegyptiaca in addition to those taxa in Zone KS31. Subzone KS30b (the G. gansseri Zone) is suggested in Sample 207-1260A-38R-CC by the absence of R. contusa. The other marker for Subzone KS30a, R. fructicosa, is very rare in the Maastrichtian in Site 1260 and is not considered a reliable datum marker on Demerara Rise. Samples 207-1260A-39R-CC to 41R-CC are difficult to assign to a zone with confidence. However, the absence of G. gansseri and the presence of G. aegyptiaca suggests these samples belong to Campanian Zone KS29. Common species include Rosita plummerae, Rosita fornicata, Rugotruncana subcircumnodifer, Globotruncanita stuartiformis, and Globotruncana linneana. Samples 207-1260A-41R-CC to 42R-4, 0–1 cm, are barren of planktonic foraminifers.

A distinctive glauconitic-rich interval separates the green calcareous claystone of Sample 207-1260A-42R-4, 0–1 cm, from the brown claystone of Sample 207-1260A-42R-CC. The presence of Marginotruncana pseudolinneana, Dicarinella hagni, Archaeoglobigerina cretacea, and Hastigerinoides watersi from the base of Section 207-1260A-43R-2 suggests a Coniacian age for the assemblage (Zone KS23). Sample 207-1260A-42R-CC and several dark-colored shales in Core 207-1260A-43R yield well-preserved glassy foraminifers that were not age diagnostic and included Hedbergella delrioensis, Whiteinella baltica, H. globulosa, and Heterohelix moremani. Similar non–age diagnostic assemblages were present in Cores 207-1260A-44R, 45R, and 46R. Sample 207-1260A-46R-CC was the only sample in the black shale sequence that yielded no foraminifers at all.

The location of the Cenomanian/Turonian (C/T) boundary can be only approximated on the basis of planktonic foraminifers. The C/T boundary may lie above Sample 207-1260A-47R-7, 0–2 cm, since this sample contains Globigerinoides caseyi, a species that we have observed only in samples of Cenomanian age from other sites on Demerara Rise. The first distinctive Cenomanian species are found in Sample 207-1260A-49R-2, 42–44 cm, where Rotalipora globotruncanoides is present together with H. delrioensis and Hedbergella planispira. Preservation of foraminifers in Cores 207-1260A-48R and 49R is generally very poor because the shells are filled with calcite spar and cemented together with blocky calcite. In contrast, samples in Cores 207-1260A-50R and 51R are mostly well preserved with glassy shells. Specimens of R. globotruncanoides and Rotalipora appenninica were intermittent between Samples 207-1260A-50R-CC and 52R-2, 99–100 cm, and provide our lowest definite occurrence of Cenomanian assemblages at Site 1260. Well-preserved foraminifers including H. delrioensis, H. moremani, H. planispira, and Globigerinelloides spp. were present between Samples 207-1260A-52R-3, 115–116 cm, and 52R-CC. These samples are consistent with either a Cenomanian or a late Albian age.

The interval between Samples 207-1260A-52R-1, 115–117 cm, and 54R-CC contains a sparse fauna with Ticinella primula, Globigerinelloides bentonensis, benthic foraminifers, and ostracodes that represent lower Albian Zone KS13. Ticinellids are present continuously between Samples 207-1260B-41R-CC and 45R-CC. Sample 207-1260B-46R-CC is barren of planktonic foraminifers but contains a similar benthic foraminifer and ostracode assemblage to that observed in the overlying Albian samples.

Core catcher samples were processed and examined systematically for radiolarians from Hole 1260A. Identifiable radiolarians are present mainly in the middle Eocene interval, and the observations allow for zonal assignments summarized in Figure F10 and Table T6. The highest sample to yield age-diagnostic radiolarians, Sample 207-1260A-6R-CC, is assigned to Zone RP16 based on the co-occurrence of Podocyrtis goetheana (marker species of the base of Zone RP16) and Sethochyrtis triconiscus (which last occurs in the middle part of Zone RP16).

Samples 207-1260A-7R-CC and 8R-CC are assigned to Zone RP15 based on abundant Podocyrtis chalara. As Podocyrtis mitra is abundant in Sample 207-1260A-9R-CC, whereas P. chalara is absent, the boundary between Zones RP15 and RP14 is placed in Core 207-1260A-9R. Abundant P. mitra in Samples 207-1260A-9R-CC through 12R-CC allows their assignment to Zone RP14.

The marker species of Zone RP13 (Podocyrtis ampla) is abundant in Samples 207-1260A-13R-CC and 14R-CC. As abundant specimens of Podocyrtis phyxis are present in Sample 207-1260A-15R-CC, the boundary between Zones RP13 and RP12 is placed in Core 207-1260A-15R. Although the marker species of the base of Zone RP12 (Eusyringium lagena) is observed only in Sample 207-1260A-16R-CC, Samples 207-1260A-16R-CC to 19R-CC are all assigned to Zone RP12 based on abundant Thyrsocyrtis triacantha, Podocyrtis diamesa, and Theocotyle conica. Rare specimens of T. triacantha are observed in Section 207-1260A-20R-4 (top) together with abundant specimens of Thyrsocyrtis tensa, Thyrsocyrtis robusta, and Dictyoprora mongolfieri, which allows assignment of this sample to the lowermost part of Zone RP12 or to RP11. Therefore, the boundary between Zones RP12 and RP11 is tentatively placed in Core 207-1260A-20R. Based on T. robusta and T. tensa in Samples 207-1260A-21R-2, 71–76 cm, and 22R-CC, this interval is regarded as a part of Zone RP11 or upper Zone RP10. Cores 207-1260A-29R through 32R may be assigned to Zones RP5–RP8, based on the presence of species Buryella tetradica. Radiolarians are common and relatively well preserved again at the Campanian interval. The genera Amphipyndax and Dictyomitra dominate the assemblage of Sample 207-1260A-40R-CC. Possible Amphipyndax tylotus in this sample may argue for a late Campanian or younger age.