Calcareous nannofossil, planktonic foraminifer, and diatom biostratigraphies indicate that the ~288-m sediment succession recovered at Site 1242 consists mainly of an expanded, continuous Pleistocene-upper Pliocene sequence (Table T10; Figs. F22, F29). A major unconformity spanning ~3 to 12 Ma and corresponding to a marked change in color and lithology is identified by all three microfossil groups between 280.6 and 284.1 mcd. Calcareous nannofossils and planktonic foraminifers at the base of the hole suggest an age between ~12 and 13 Ma.
Calcareous nannofossils are less abundant than at Site 1241 and moderately well preserved with frequent indications of dissolution. Planktonic foraminifers are abundant to common and moderately well preserved down to ~70 mcd then become rarer and poorly preserved, except below the unconformity. The proportion of benthic foraminifers increases markedly below ~110 mcd. Marked changes in the relative proportions of benthic foraminiferal species within the Pleistocene-late Pliocene assemblage probably indicate variations in carbon fluxes at the seafloor. Diatoms are generally rare and moderately to poorly preserved. However, abundant Ethmodiscus fragments in Sections 202-1242A-21X-3 through 21X-7 (~208-215 mcd) form distinct thin, dark layers.
The abundance of calcareous nannofossils is notably lower at Site 1242 than at Site 1241, largely because of dilution by clay and some silt-sized detrital grains. Preservation of nannofossils is generally moderate, with frequent indications of dissolution but with minimal overgrowth (Table T11; Fig. F22).
Emiliania huxleyi was found from the mudline sample (Sample 202-1242A-1H-1, 0 cm) to Sample 2H-CC, 16 cm (Table T11). This documents the recovery of the 0- to 0.26-Ma interval at Site 1242. Most of the Pleistocene-late Pliocene nannofossil markers have been determined (Table T10). A few events, such as the last occurrences (LOs) of Calcidiscus macintyrei and Discoaster pentaradiatus, have not been located largely because of the lower abundance of these index species and/or inadequate shipboard sample resolution.
Calcareous nannofossil biostratigraphy documents a relatively continuous sequence that accumulated rapidly (~100 m/m.y.) down to ~2.5 Ma. A marked change in color and lithology occurs in Sample 202-1242B-27X-2, 33 cm (280.63 mcd) (see "Lithostratigraphy"), where a hiatus representing ~3-12 Ma is detected between Samples 27X-CC, 1 cm (280.6 mcd), and 27X-4, 75 cm (284.1 mcd), based on the presence of a late Pliocene assemblage characterized by D. pentaradiatus and Discoaster brouweri in the upper sample and the presence of the late middle Miocene index species Coccolithus miopelagicus and Coronocyclus nitescens in the lower sample.
Planktonic foraminifers are abundant to common in the upper part of Hole 1242A sediments (mudline to Sample 202-1242A-7H-CC; 0-69.28 mcd). Abundance decreases markedly downhole, and planktonic foraminifers are rare in samples from the lower part of Hole 1242A (Samples 202-1242A-16H-CC through 26X-CC; 164.3-272.5 mcd), except in Sample 27X-CC (280.9 mcd), which is below a major unconformity. Preservation is moderate to good down to Samples 202-1242A-10H-CC (99.2 mcd) but deteriorates markedly at greater depths, particularly between Samples 21X-CC and 26X-CC (215.2-272.5 mcd), where glauconite is commonly present in the >63-µm residue (Table T12).
The Pleistocene-late Pliocene assemblage is characterized by Globorotalia menardii, Globorotalia tumida, Globigerinoides sacculifer, Globigerinoides ruber, Globigerinoides trilobus, Neogloboquadrina acostaensis, Neogloboquadrina dutertrei, and Orbulina universa. A preliminary biostratigraphy was established for the Pleistocene-upper Pliocene interval. However, some of the standard marker species were not found during shipboard studies, and the biostratigraphy is not well constrained for the lower part of Hole 1242A (Samples 202-1242A-21X-CC and 26X-CC; 215.2-272.5 mcd) because of poor preservation and the scarcity of planktonic foraminifers.
The LO of G. ruber (pink), between Samples 202-1242A-1H-CC and 2H-CC (5.00-17.87 mcd), indicates an age younger than 0.12 Ma for the overlying interval. The first occurrence (FO) of G. ruber (pink) (0.40 Ma) can be placed between Samples 202-1242A-4H-CC and 5H-CC (40.00-49.52 mcd). However, the FO of Globorotalia flexuosa (0.40 Ma) appears to occur later, between Samples 202-1242A-3H-CC and 4H-CC (28.91-40.00 mcd), which probably reflects poor preservation, as severe dissolution damage was noted in particular for the species G. tumida and G. flexuosa. The standard marker Globorotalia tosaensis is extremely rare at Site 1242 and could not be used to define the boundary between the Subzones Pt1a and Pt1b of Berggren et al. (1995). The LO of representatives of the benthic genus Stilostomella (~0.65 Ma), which occurs between Samples 202-1242A-8H-CC and 9H-CC (80.13-88.94 mcd), provides an alternative datum to approximate this boundary. However, this extinction event was probably diachronous at low and high latitudes (Hayward, 2001), and detailed shore-based studies from Site 1242 and from more southerly Leg 202 sites are needed to clarify the timing of this event in the eastern Pacific. The species Globigerinoides fistulosus, which defines the base of the Pleistocene/Pliocene boundary, was not observed in our samples. The LO of Globigerinoides extremus (1.77 Ma), between Samples 202-1242A-21X-CC and 22X-CC (215.2-227.6 mcd), provides an alternative datum to approximate this boundary.
Planktonic foraminifers indicate a major unconformity (~3-12 Ma), which corresponds to an abrupt lithologic change (see "Lithostratigraphy") in the core catcher of Core 202-1242A-27X. The presence of the typical middle Miocene species Globorotalia fohsi s.l., Globorotalia panda, and Globorotalia praemenardii in Sample 202-1242A-27X-CC (280.9 mcd), below the unconformity, indicates an age between 11.9 and 13.42 Ma for the base of Hole 1242A.
The percentage of benthic foraminifers relative to total foraminifers is, initially, relatively low (mainly ~10% or less) in samples from the upper part of Hole 1242A (mudline to Sample 202-1242A-11H-CC; 0-109.4 mcd) but increases markedly downhole, reaching ~99% between Samples 23X-CC and 25X-CC (237.5-260.4 mcd). Benthic foraminifers overall show better preservation than planktonic foraminifers, but preservation deteriorates downhole, particularly between Samples 202-1242A-21X-CC and 26X-CC (215.23-268.99 mcd).
The relatively diverse but sparse Pleistocene-late Pliocene assemblage in Samples 202-1242A-1H-CC through 21H-CC typically contains Bolivina seminuda, Bulimina mexicana, Bulimina aculeata, Eggerrella bradyi, Eubulimina exilis, Globocassidulina subglobosa, Globobulimina affinis, Globobulimina pyrula, Gyroidinoides soldanii, Gyroidinoides orbicularis, Laticarinina pauperata, Melonis affinis, Melonis pompilioides, Planulina wuellerstorfi, Plectofrondicularia vaughani, Pleurostomella brevis, Pullenia bulloides, Stilostomella abyssorum, Stilostomella subspinosa, and Uvigerina proboscidea. Marked variations in the proportions of Bolivina, Bulimina, Globobulimina, and Uvigerina may relate to changes in productivity, oxygenation, or circulation. However, it was not possible to evaluate from shipboard study to what extent these changes were affected by diagenesis.
All core catcher samples from Hole 1242A and smear slides from some additional layers of the split cores were analyzed. Few to common diatoms of moderate preservation were found in the mudline sample (Sample 202-1242A-1H-1, 0 cm), Sample 1H-CC, and Samples 6H-CC to 14H-CC (59.5-141.2 mcd). Only traces of diatoms were observed in Samples 202-1242A-2H-CC through 5H-CC (17.9-49.5 mcd) and Sample 15H-CC (151.4 mcd). Abundant Ethmodiscus fragments were found in Sections 202-1242A-21X-3 through 21X-7 (~208-215 mcd), which formed distinct thin dark layers (see "Lithostratigraphy"). Below the obvious hiatus at 280.63 mcd, Sample 202-1242A-27X-CC (280.9 mcd) contained a middle-late Miocene diatom assemblage dominated by Thalassiothrix (Table T13).
Although diatoms are generally rare and poorly preserved at Site 1242, a total of three mid-Pleistocene diatom events were recognized (Table T10). In addition, the presence of middle Miocene diatoms Craspedodiscus coscinodiscus and Actinocyclus ingens and the absence of Coscinodiscus lewisianus in Sample 202-1242A-27X-CC allow a tentative placement of this sample into the Coscinodiscus gigas var. diorama Zone (12.18-12.93 Ma), although A. ingens is rare and only fragments of C. coscinodiscus have been found.